Publications Rudolf GRIMM

2023

Articles

• C. Baroni, B. Huang, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun Mediated interactions between Fermi polarons and the role of impurity quantum statistics, Nature Phys. 20 (2023-10-26), http://dx.doi.org/10.1038/s41567-023-02248-4 doi:10.1038/s41567-023-02248-4 (ID: 721077) Toggle Abstract
  The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle, a polaron, is an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. As an inherent and important property such quasi-particles interact with each other via modulation of the medium. While quantum simulation experiments with ultracold atoms have significantly improved our understanding of individual polarons, the detection of their interactions has remained elusive in these systems. Here, we report the unambiguous observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two landmark predictions of Landau's Fermi-liquid theory: the shift of the polaron energy due to mediated interactions, linear in the concentration of impurities, and its sign inversion with impurity quantum statistics. For weak to moderate interactions between the impurities and the medium, we find excellent agreement with the static (zero-momentum and energy) predictions of Fermi-liquid theory. For stronger impurity-medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and molecule formation
• C. Baroni, B. Huang, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun Mediated interactions between Fermi polarons and the role of impurity quantum statistics, Nature Phys. (2023-10-26), http://dx.doi.org/10.1038/s41567-023-02248-4 doi:10.1038/s41567-023-02248-4 (ID: 721140) Toggle Abstract
  The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle is a polaron, formed by an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. Importantly, such quasi-particles interact with each other via the modulation of the medium. However, although quantum simulation experiments with ultracold atoms have substantially improved our understanding of individual polarons, the detection of their interactions has so far remained elusive. Here we report the observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two predictions of Landau’s Fermi-liquid theory: the shift in polaron energy due to mediated interactions, which is linear in the concentration of impurities; and its sign inversion with impurity quantum statistics. For weak-to-moderate interactions between the impurities and the medium, our results agree with the static predictions of Fermi-liquid theory. For stronger impurity–medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and dressed molecule formation.
• E. Soave, A. Canali, Z. Ye, M. Kreyer, E. Kirilov, R. Grimm Optically trapped Feshbach molecules of fermionic 161Dy and 40K, Phys. Rev. Research 5 33117 (2023-04-17), http://dx.doi.org/10.1103/PhysRevResearch.5.033117 doi:10.1103/PhysRevResearch.5.033117 (ID: 721079) Toggle Abstract
  We report on the preparation of a pure ultracold sample of bosonic DyK Feshbach molecules, composed of the fermionic isotopes 161Dy and 40K. By sweeping a magnetic field across a resonance located near 7.3 G, we generate up to 5000 molecules at a temperature of approximately 50 nK. To purify the sample from remaining atoms, we employ a Stern-Gerlach technique that relies on magnetic levitation of the molecules in a very weak optical dipole trap. With the trapped molecules we finally reach a high phase-space density of about 0.1. We investigate the magnetic field dependence of the molecular binding energy and the magnetic moment, refining our knowledge of the resonance parameters. We also demonstrate a peculiar anisotropic expansion effect observed when the molecules are released from the trap and expand freely in the magnetic levitation field. Furthermore, we identify an important lifetime limitation that is imposed by the 1064-nm infrared trap light itself and not by inelastic collisions. The light-induced decay rate is found to be proportional to the trap light intensity and the closed-channel fraction of the Feshbach molecule. These observations suggest a one-photon coupling to electronically excited states to limit the lifetime and point to the prospect of loss suppression by optimizing the wavelength of the trapping light. Our results offer crucial insights and experimental progress towards achieving quantum-degenerate samples of DyK molecules and novel superfluids based on mass-imbalanced fermion mixtures.

Preprints

• R. Grimm, C. Baroni Fermionic quantum mixtures with tunable interactions, (2023-11-02), http://dx.doi.org/10.48550/arXiv.2311.01220 doi:10.48550/arXiv.2311.01220 (ID: 721139) Toggle Abstract
  The topic of the present lecture notes are two-species quantum mixtures composed of a deeply degenerate Fermi gas and a second component, the latter being fermionic or bosonic. A key ingredient is the possibility to tune the s-wave interaction between the different species by means of magnetically controlled Feshbach resonances, which allow us to investigate regimes of strong interactions. In two case studies, we review our experiments on mixtures of 6Li fermions with fermionic 40K or bosonic 41K atoms and on mixtures of fermionic 161Dy with 40K atoms. We cover various topics of fermionic quantum many-body physics, ranging from impurity physics and quasiparticles over phase separation to the formation of ultracold molecules and progress towards novel superfluids.
• G. Anich, R. Grimm, E. Kirilov Comprehensive Characterization of a State-of-the-Art Apparatus for Cold Electromagnetic Dysprosium Dipoles, (2023-04-25), arXiv:2304.12844 arXiv:2304.12844 (ID: 721078) Toggle Abstract
  We developed a new advanced ultra-cold Dysprosium (Dy) apparatus, which incorporates a quantum gas microscope (QGM) with a resolution of a quarter micrometer. The QGM and the cooling and trapping regions are within the same vacuum glass vessel assuring simple atom transport between them. We demonstrate the essential experimental steps of laser and evaporative cooling, lattice loading, transporting and precise positioning of a cloud of the bosonic isotope 164 Dy at the QGM focal plane. Preliminary basic characterization of the QGM and future plans in enabling its full capacity are outlined. We also present a feasible platform for simulating complex spin models of quantum magnetism, such as XYZ model, by exploiting a set of closely spaced opposite parity levels in Dy with a large magnetic and electric dipole moment. We isolate a degenerate isospin-1/2 system, which possesses both magnetic and electric dipole-dipole coupling, containing Ising, exchange and spin-orbit terms. The last gives rise to a spin model with asymmetric tunable rates, dependable on the lattice geometry.

2022

Articles

• Zhuxiong Ye, Alberto Canali, Elisa Soave, Marian Kreyer, Yaakov Yudkin, Cornelis Ravensbergen, Emil Kirilov, Rudolf Grimm Observation of low-field Feshbach resonances between 161Dy and 40K, Phys. Rev. A 106 PhysRevA.106.043314 (2022-10-17), http://dx.doi.org/10.1103/PhysRevA.106.043314 doi:10.1103/PhysRevA.106.043314 (ID: 720848) Toggle Abstract
  We report on the observation of Feshbach resonances at low magnetic field strength (below 10 G) in the Fermi-Fermi mixture of 161Dy and 40K. We characterize five resonances by measurements of interspecies thermalization rates and molecular binding energies. As a case of particular interest for applications, we consider a resonance near 7.29 G, which combines accurate magnetic tunability and access to the universal regime of interactions with experimental simplicity. We show that lifetimes of a few 100 ms can be achieved for the optically trapped, resonantly interacting mixture. We also demonstrate the hydrodynamic expansion of the mixture in the strongly interacting regime and the formation of DyK Feshbach molecules. Our work opens up new experimental possibilities in view of mass-imbalanced superfluids and related phenomena.
• Elisa Soave, Vincent Corre, Cornelis Ravensbergen, Jeong Ho Han, Marian Kreyer, Emil Kirilov, Rudolf Grimm Low-field Feshbach resonances and three-body losses in a fermionic quantum gas of 161Dy, Ukr. J. Phys. 67 334 (2022-08-29), http://dx.doi.org/10.15407/ujpe67.5.334 doi:10.15407/ujpe67.5.334 (ID: 720847) Toggle Abstract
  We report on high-resolution Feshbach spectroscopy on a degenerate, spin-polarized Fermi gas of 161Dy atoms, measuring three-body recombination losses at low magnetic field. For field strength up to 1\,G, we identify as much as 44 resonance features and observe plateaus of very low losses. For four selected typical resonances, we study the dependence of the three-body recombination rate coefficient on the magnetic resonance detuning and on the temperature. We observe a strong suppression of losses with decreasing temperature already for small detunings from resonance. The characterization of complex behavior of three-body losses of fermionic 161Dy is important for future applications of this peculiar species in research on atomic quantum gases.

2021

Articles

• M. Kreyer, J. H. Han, C. Ravensbergen, V. Corre, E. Soave, E. Kirilov, R. Grimm Measurement of the dynamic polarizability of Dy atoms near the 626-nm intercombination line, Phys. Rev. A 104 33106 (2021-09-10), http://dx.doi.org/10.1103/PhysRevA.104.033106 doi:10.1103/PhysRevA.104.033106 (ID: 720639) Toggle Abstract
  We report on measurements of the anisotropic dynamical polarizability of Dy near the 626-nm intercombination line, employing modulation spectroscopy in a one-dimensional optical lattice. To eliminate large systematic uncertainties resulting from the limited knowledge of the spatial intensity distribution, we use K as a reference species with accurately known polarizability. This method can be applied independently of the sign of the polarizability, i.e. for both attractive and repulsive optical fields on both sides of a resonance. By variation of the laser polarization we extract the scalar and the tensorial part. To characterize the strength of the transition, we also derive the natural linewidth. We find our result in excellent agreement with literature values, which provide a sensitive benchmark for the accuracy of our method. In addition we demonstrate optical dipole trapping on the intercombination line, confirming the expected long lifetimes and low heating rates. This provides an additional tool to tailor optical potentials for Dy atoms and for the species-specific manipulation of atoms in the Dy-K mixture.
• I. Fritsche, C. Baroni, E. Dobler, E. Kirilov, B. Huang, R. Grimm, G. Bruun, P. Massignan Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture, Phys. Rev. A 103 53314 (2021-05-17), http://dx.doi.org/10.1103/PhysRevA.103.053314 doi:10.1103/PhysRevA.103.053314 (ID: 720640) Toggle Abstract
  We investigate the properties of a strongly interacting imbalanced mixture of bosonic 41K impurities immersed in a Fermi sea of ultracold 6Li atoms. This enables us to explore the Fermi polaron for large impurity concentrations including the case where they form a Bose-Einstein condensate. The system is characterized by means of radio-frequency injection spectroscopy for tunable interactions using an interspecies Feshbach resonance. We find that the energy of the Fermi polarons formed in the thermal fraction of the impurity cloud remains rather insensitive to the impurity concentration, even as we approach equal densities for both species. The apparent insensitivity to high concentration is consistent with the theoretical prediction, based on Landau's quasiparticle theory, of a weak effective interaction between the polarons. The condensed fraction of the bosonic 41K gas is much denser than its thermal component, which leads to a break-down of the Fermi polaron description. Instead, we observe a new branch in the radio-frequency spectrum with a small energy shift, which is consistent with the presence of Bose polarons formed by 6Li fermions inside the 41K condensate. A closer investigation of the behavior of the condensate by means of Rabi oscillation measurements support this observation, indicating that we have realized Fermi and Bose polarons, two fundamentally different quasiparticles, in one cloud.
• M. Pini, P. Pieri, G. Calvanese Strinati, R. Grimm Beyond-mean-field description of a trapped unitary Fermi gas with mass and population imbalance, Phys. Rev. A 103 23314 (2021-02-15), http://dx.doi.org/10.1103/PhysRevA.103.023314 doi:10.1103/PhysRevA.103.023314 (ID: 720561)

2020

Article

• C. Ravensbergen, E. Soave, V. Corre, M. Kreyer, B. Huang, E. Kirilov, R. Grimm Resonantly Interacting Fermi-Fermi Mixture of 161Dy and 40K, Phys. Rev. Lett. 124 203402 (2020-05-22), http://dx.doi.org/10.1103/PhysRevLett.124.203402 doi:10.1103/PhysRevLett.124.203402 (ID: 720519)

2019

Articles

• B. Huang, I. Fritsche, R. Lous, C. Baroni, J. T. Walraven, E. Kirilov, R. Grimm Breathing Mode of a BEC Repulsively Interacting with a Fermionic Reservoir, Phys. Rev. A 99 41602 (2019-04-03), http://dx.doi.org/10.1103/PhysRevA.99.041602 doi:10.1103/PhysRevA.99.041602 (ID: 720106) Toggle Abstract
  We investigate the fundamental breathing mode of a small-sized elongated Bose-Einstein condensate coupled to a large Fermi sea of atoms. Our observations show a dramatic shift of the breathing frequency when the mixture undergoes phase separation at strong interspecies repulsion. We find that the maximum frequency shift in the full phase-separation limit depends essentially on the atom number ratio of the components. We interpret the experimental observations within a model that assumes an adiabatic response of the Fermi sea, or within another model that considers single fermion trajectories for a fully phase-separated mixture. These two complementary models capture the observed features over the full range of interest.
• R. Grimm Efimov States in an Ultracold Gas: How it Happened in the Laboratory, Few-Body Syst. 60 23 (2019-03-23), http://dx.doi.org/10.1007/s00601-019-1495-y doi:10.1007/s00601-019-1495-y (ID: 720253)

2018

Articles

• C. Ravensbergen, V. Corre, E. Soave, M. Kreyer, E. Kirilov, R. Grimm Production of a degenerate Fermi-Fermi mixture of dysprosium and potassium atoms, Phys. Rev. A 98 63624 (2018-12-20), http://dx.doi.org/10.1103/PhysRevA.98.063624 doi:10.1103/PhysRevA.98.063624 (ID: 720087) Toggle Abstract
  We report on the realization of a mixture of fermionic 161^{161}Dy and fermionic 40^{40}K where both species are deep in the quantum-degenerate regime. Both components are spin-polarized in their absolute ground states, and the low temperatures are achieved by means of evaporative cooling of the dipolar dysprosium atoms together with sympathetic cooling of the potassium atoms. We describe the trapping and cooling methods, in particular the final evaporation stage, which leads to Fermi degeneracy of both species. Analyzing cross-species thermalization we obtain an estimate of the magnitude of the inter-species ss-wave scattering length at low magnetic field. We demonstrate magnetic levitation of the mixture as a tool to ensure spatial overlap of the two components. The properties of the Dy-K mixture make it a very promising candidate to explore the physics of strongly interacting mass-imbalanced Fermi-Fermi mixtures.
• R. Lous, I. Fritsche, M. Jag, F. Lehmann, E. Kirilov, B. Huang, R. Grimm Probing the Interface of a Phase-Separated State in a Repulsive Bose-Fermi Mixture, Phys. Rev. Lett. 120 243403 (2018-02-06), http://dx.doi.org/10.1103/PhysRevLett.120.243403 doi:10.1103/PhysRevLett.120.243403 (ID: 719979) Toggle Abstract
  We probe the interface between a phase-separated Bose-Fermi mixture consisting of a small BEC of 41K residing in a large Fermi sea of 6Li. We quantify the residual spatial overlap between the two components by measuring three-body recombination losses for variable strength of the interspecies repulsion. A comparison with a numerical mean-field model highlights the importance of the kinetic energy term for the condensed bosons in maintaining the thin interface far into the phase-separated regime. Our results demonstrate a corresponding smoothing of the phase transition in a system of finite size.
• C. Ravensbergen, V. Corre, E. Soave, M. Kreyer, S. Tzanova, E. Kirilov, R. Grimm Accurate Determination of the Dynamical Polarizability of Dysprosium, Phys. Rev. Lett. 120 223001 (2018-01-18), http://dx.doi.org/10.1103/PhysRevLett.120.223001 doi:10.1103/PhysRevLett.120.223001 (ID: 719964) Toggle Abstract
  We report a measurement of the dynamical polarizability of dysprosium atoms in their electronic ground state at the optical wavelength of 1064 nm, which is of particular interest for laser trapping experiments. Our method is based on collective oscillations in an optical dipole trap, and reaches unprecedented accuracy and precision by comparison with an alkali atom (potassium) as a reference species. We obtain values of 184.4(2.4) a.u. and 1.7(6) a.u. for the scalar and tensor polarizability, respectively. Our experiments have reached a level that permits meaningful tests of current theo- retical descriptions and provides valuable information for future experiments utilizing the intriguing properties of heavy lanthanide atoms.

2017

Articles

• L. Reichsöllner, A. Schindewolf, T. Takekoshi, R. Grimm, H. Nägerl Quantum engineering of a low-entropy gas of heteronuclear bosonic molecules in an optical lattice, Phys. Rev. Lett. 118 73201 (2017-02-17), http://dx.doi.org/10.1103/PhysRevLett.118.073201 doi:10.1103/PhysRevLett.118.073201 (ID: 719625) Toggle Abstract
  We demonstrate a generally applicable technique for mixing two-species quantum degenerate bosonic samples in the presence of an optical lattice, and we employ it to produce low-entropy samples of ultracold Rb87Cs133 Feshbach molecules with a lattice filling fraction exceeding 30%. Starting from two spatially separated Bose-Einstein condensates of Rb and Cs atoms, Rb-Cs atom pairs are efficiently produced by using the superfluid-to-Mott insulator quantum phase transition twice, first for the Cs sample, then for the Rb sample, after nulling the Rb-Cs interaction at a Feshbach resonance’s zero crossing. We form molecules out of atom pairs and characterize the mixing process in terms of sample overlap and mixing speed. The dense and ultracold sample of more than 5000 RbCs molecules is an ideal starting point for experiments in the context of quantum many-body physics with long-range dipolar interactions.
• R. Lous, I. Fritsche, M. Jag, B. Huang, R. Grimm Thermometry of a deeply degenerate Fermi gas with a Bose-Einstein condensate, Phys. Rev. A 95 53627 (2017-02-09), http://dx.doi.org/10.1103/PhysRevA.95.053627 doi:10.1103/PhysRevA.95.053627 (ID: 719754) Toggle Abstract
  We measure the temperature of a deeply degenerate Fermi gas, by using a weakly interacting sample of heavier bosonic atoms as a probe. This thermometry method relies on the thermalization between the two species and on the determination of the condensate fraction of the bosons. In our experimental implementation, a small sample of 41K atoms serves as the thermometer for a 6Li Fermi sea. We investigate the evaporative cooling of a 6Li spin mixture in a single-beam optical dipole trap and observe how the condensate fraction of the thermometry atoms depends on the final trap depth. From the condensate fraction, the temperature can be readily extracted. We show that the lowest temperature of 5.9(5)% of the Fermi temperature is obtained, when the decreasing trap depth closely approaches the Fermi energy. To understand the systematic effects that may in uence the results, we carefully investigate the role of the number of bosons and the thermalization dynamics between the two species. Our thermometry approach provides a conceptually simple, accurate, and general way to measure the temperature of deeply degenerate Fermi gases. Since the method is independent of the specific interaction conditions within the Fermi gas, it applies to both weakly and strongly interacting Fermi gases.

2016

Articles

• M. Jag, M. Cetina, R. Lous, R. Grimm, J. Levinsen, D. Petrov Lifetime of Feshbach dimers in a Fermi-Fermi mixture of 6Li and 40K, Phys. Rev. A 94 62706 (2016-12-27), http://dx.doi.org/10.1103/PhysRevA.94.062706 doi:10.1103/PhysRevA.94.062706 (ID: 719645) Toggle Abstract
  We present a joint experimental and theoretical investigation of the lifetime of weakly bound dimers formed near narrow interspecies Feshbach resonances in mass-imbalanced Fermi-Fermi systems, considering the specific example of a mixture of Li6 and K40 atoms. Our work addresses the central question of the increase in the stability of the dimers resulting from Pauli suppression of collisional losses, which is a well-known effect in mass-balanced fermionic systems near broad resonances. We present measurements of the spontaneous dissociation of dimers in dilute samples, and of the collisional losses in dense samples arising from both dimer-dimer processes and from atom-dimer processes. We find that all loss processes are suppressed close to the Feshbach resonance. Our general theoretical approach for fermionic mixtures near narrow Feshbach resonances provides predictions for the suppression of collisional decay as a function of the detuning from resonance, and we find excellent agreement with the experimental benchmarks provided by our Li6−K40 system. We finally present model calculations for other Feshbach-resonant Fermi-Fermi systems, which are of interest for experiments in the near future.
• M. Cetina, M. Jag, R. Lous, I. Fritsche, J. T. Walraven, R. Grimm, J. Levinsen, M. Parish, R. Schmidt, M. Knap, E. Demler Ultrafast many-body interferometry of impurities coupled to a Fermi sea, Science 354 99 (2016-10-07), http://dx.doi.org/10.1126/science.aaf5134 doi:10.1126/science.aaf5134 (ID: 719567) Toggle Abstract
  The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.

2015

Articles

• A. Frisch, M. Mark, K. Aikawa, S. Baier, R. Grimm, A. Petrov, S. Kotochigova, G. Quemener, M. Lepers, O. Dulieu, F. Ferlaino Ultracold polar molecules composed of strongly magnetic atoms, Phys. Rev. Lett. 115 203201 (2015-11-09), http://dx.doi.org/10.1103/PhysRevLett.115.203201 doi:10.1103/PhysRevLett.115.203201 (ID: 719228) Toggle Abstract
  In a combined experimental and theoretical effort, we demonstrate a novel type of dipolar system made of ultracold bosonic polar molecules with large magnetic dipole moments. Our polar molecules are formed in weakly bound Feshbach molecular states from a sample of strongly magnetic bosonic erbium atoms. We show that the ultracold magnetic molecules can carry very large dipole moments and we demonstrate how to create and characterize them, and how to change their orientation. Finally, we confirm that the relaxation rates of molecules in a quasi-two dimensional geometry can be reduced by using the anisotropy of the dipole-dipole interaction and that this reduction follows a universal dipolar behavior.
• M. Cetina, M. Jag, R. Lous, J. T. Walraven, R. Grimm, R. S. Christensen, G. Bruun Decoherence of Impurities in a Fermi Sea of Ultracold Atoms, Phys. Rev. Lett. 115 135302 (2015-05-05), http://dx.doi.org/10.1103/PhysRevLett.115.135302 doi:10.1103/PhysRevLett.115.135302 (ID: 719238) Toggle Abstract
  We investigate the decoherence of K-40 impurities interacting with a three-dimensional Fermi sea of Li-6 across an interspecies Feshbach resonance. The decoherence is measured as a function of the interaction strength and temperature using a spin-echo atom interferometry method. For weak to moderate interaction strengths, we interpret our measurements in terms of scattering of K quasiparticles by the Fermi sea and find very good agreement with a Fermi liquid calculation. For strong interactions, we observe significant enhancement of the decoherence rate, which is largely independent of temperature, pointing to behavior that is beyond the scattering of quasiparticles in the Fermi liquid picture.
• B. Huang, L. Sidorenkov, R. Grimm Finite-temperature effects on a triatomic Efimov resonance in ultracold cesium, Phys. Rev. A 91 63622 (2015-04-21), http://dx.doi.org/10.1103/PhysRevA.91.063622 doi:10.1103/PhysRevA.91.063622 (ID: 719230) Toggle Abstract
  We report a thorough investigation of finite-temperature effects on three-body recombination near a triatomic Efimov resonance in an ultracold gas of cesium atoms. Our measurements cover a wide range from a near-ideal realization of the zero-temperature limit to a strongly temperature-dominated regime. The experimental results are analyzed within a recently introduced theoretical model based on a universal zero-range theory. The temperature-induced shift of the resonance reveals a contribution that points to an energy dependence of the three-body parameter. We interpret this contribution in terms of the finite range of the van der Waals interaction in real atomic systems and we quantify it in an empirical way based on length scale arguments. A universal character of the corresponding resonance shift is suggested by observations related to other Efimov resonances and the comparison with a theoretical finite-temperature approach that explicitly takes the van der Waals interaction into account. Our findings are of importance for the precise determination of Efimov resonance positions from experiments at finite temperatures.

2014

Articles

• K. Aikawa, A. Frisch, M. Mark, S. Baier, R. Grimm, J. L. Bohn, D. S. Jin, G. Bruun, F. Ferlaino Anisotropic Relaxation Dynamics in a Dipolar Fermi Gas Driven Out of Equilibrium, Phys. Rev. Lett. 113 263201 (2014-12-23), http://dx.doi.org/10.1103/PhysRevLett.113.263201 doi:10.1103/PhysRevLett.113.263201 (ID: 718908) Toggle Abstract
  We report on the observation of a large anisotropy in the rethermalization dynamics of an ultracold dipolar Fermi gas driven out of equilibrium. Our system consists of an ultracold sample of strongly magnetic Er167 fermions, spin polarized in the lowest Zeeman sublevel. In this system, elastic collisions arise purely from universal dipolar scattering. Based on cross-dimensional rethermalization experiments, we observe a strong anisotropy of the scattering, which manifests itself in a large angular dependence of the thermal relaxation dynamics. Our result is in good agreement with recent theoretical predictions. Furthermore, we measure the rethermalization rate as a function of temperature for different angles and find that the suppression of collisions by Pauli blocking is not influenced by the dipole orientation.
• T. Takekoshi, L. Reichsöllner, A. Schindewolf, J. M. Hutson, C. Le Sueur, O. Dulieu, F. Ferlaino, R. Grimm, H. Nägerl Ultracold dense samples of dipolar RbCs molecules in the rovibrational and hyperfine ground state, Phys. Rev. Lett. 113 205301 (2014-11-12), http://dx.doi.org/10.1103/PhysRevLett.113.205301 doi:10.1103/PhysRevLett.113.205301 (ID: 718921) Toggle Abstract
  We produce ultracold dense trapped samples of Rb87Cs133 molecules in their rovibrational ground state, with full nuclear hyperfine state control, by stimulated Raman adiabatic passage (STIRAP) with efficiencies of 90%. We observe the onset of hyperfine-changing collisions when the magnetic field is ramped so that the molecules are no longer in the hyperfine ground state. A strong quadratic shift of the transition frequencies as a function of applied electric field shows the strongly dipolar character of the RbCs ground-state molecule. Our results open up the prospect of realizing stable bosonic dipolar quantum gases with ultracold molecules.
• B. Huang, K. O'Hara, R. Grimm, J. M. Hutson, D. Petrov Three-body parameter for Efimov states in 6Li, Phys. Rev. A 90 043636 (2014-08-30), http://dx.doi.org/10.1103/PhysRevA.90.043636 doi:10.1103/PhysRevA.90.043636 (ID: 718984) Toggle Abstract
  We present a state-of-the-art reanalysis of experimental results on Efimov resonances in the three-fermion system of 6Li. We discuss different definitions of the three-body parameter (3BP) for Efimov states and adopt a definition that excludes effects due to deviations from universal scaling for low-lying states. We develop a finite-temperature model for the case of three distinguishable fermions and apply it to the excited-state Efimov resonance to obtain the most accurate determination to date of the 3BP in an atomic three-body system. Our analysis of ground-state Efimov resonances in the same system yields values for the three-body parameter that are consistent with the excited-state result. Recent work has suggested that the reduced 3BP for atomic systems is a near-universal quantity, almost independent of the particular atom involved. However, the value of the 3BP obtained for Li6 is significantly (∼20%) different from that previously obtained from the excited-state resonance in Cs. The difference between these values poses a challenge for theory.
• A. Zenesini, B. Huang, M. Berninger, H. Nägerl, F. Ferlaino, R. Grimm Resonant atom-dimer collisions in cesium: Testing universality at positive scattering lengths, Phys. Rev. A 90 022704 (2014-08-11), http://dx.doi.org/10.1103/PhysRevA.90.022704 doi:10.1103/PhysRevA.90.022704 (ID: 718945) Toggle Abstract
  We study the collisional properties of an ultracold mixture of cesium atoms and dimers close to a Feshbach resonance near 550 G in the regime of positive s-wave scattering lengths. We observe an atom-dimer loss resonance that is related to Efimov's scenario of trimer states. The resonance is found at a value of the scattering length that is different from a previous observation at low magnetic fields. This indicates nonuniversal behavior of the Efimov spectrum for positive scattering lengths. We compare our observations with predictions from effective field theory and with a recent model based on the van der Waals interaction. We present additional measurements on pure atomic samples in order to check for the presence of a resonant loss feature related to an avalanche effect, as suggested by observations in other atomic species. We could not confirm the presence of such a feature.
• B. Huang, L. Sidorenkov, R. Grimm, J. M. Hutson Observation of the Second Triatomic Resonance in Efimov`s Scenario, Phys. Rev. Lett. 112 190401 (2014-05-12), http://dx.doi.org/10.1103/PhysRevLett.112.190401 doi:10.1103/PhysRevLett.112.190401 (ID: 718848) Toggle Abstract
  We report the observation of a three-body recombination resonance in an ultracold gas of cesium atoms at a very large negative value of the s-wave scattering length. The resonance is identified as the second triatomic Efimov resonance, which corresponds to the situation where the first excited Efimov state appears at the threshold of three free atoms. This observation, together with a finite-temperature analysis and the known first resonance, allows the most accurate demonstration to date of the discrete scaling behavior at the heart of Efimov physics. For the system of three identical bosons, we obtain a scaling factor of 21.0(1.3), close to the ideal value of 22.7.
• M. Jag, M. Zaccanti, M. Cetina, R. Lous, F. Schreck, R. Grimm, D. Petrov, J. Levinsen Observation of a Strong Atom-Dimer Attraction in a Mass-Imbalanced Fermi-Fermi Mixture, Phys. Rev. Lett. 112 075302 (2014-02-21), http://dx.doi.org/10.1103/PhysRevLett.112.075302 doi:10.1103/PhysRevLett.112.075302 (ID: 718661) Toggle Abstract
  We investigate a mixture of ultracold fermionic K40 atoms and weakly bound Li6K40 dimers on the repulsive side of a heteronuclear atomic Feshbach resonance. By radio-frequency spectroscopy we demonstrate that the normally repulsive atom-dimer interaction is turned into a strong attraction. The phenomenon can be understood as a three-body effect in which two heavy K40 fermions exchange the light Li6 atom, leading to attraction in odd partial-wave channels (mainly p wave). Our observations show that mass imbalance in a fermionic system can profoundly change the character of interactions as compared to the well-established mass-balanced case.
• K. Aikawa, A. Frisch, M. Mark, S. Baier, R. Grimm, F. Ferlaino Reaching Fermi Degeneracy via Universal Dipolar Scattering, Phys. Rev. Lett. 112 010404 (2014-01-06), http://dx.doi.org/10.1103/PhysRevLett.112.010404 doi:10.1103/PhysRevLett.112.010404 (ID: 718621) Toggle Abstract
  We report on the creation of a degenerate dipolar Fermi gas of erbium atoms. We force evaporative cooling in a fully spin-polarized sample down to temperatures as low as 0.2 times the Fermi temperature. The strong magnetic dipole-dipole interaction enables elastic collisions between identical fermions even in the zero-energy limit. The measured elastic scattering cross section agrees well with the predictions from the dipolar scattering theory, which follow a universal scaling law depending only on the dipole moment and on the atomic mass. Our approach to quantum degeneracy proceeds with very high cooling efficiency and provides large atomic densities, and it may be extended to various dipolar systems.

2013

Articles

• B. Pasquiou, A. Bayerle, S. Tzanova, S. Stellmer, J. Szczepkowski, M. Parigger, R. Grimm, F. Schreck Quantum degenerate mixtures of strontium and rubidium atoms, Phys. Rev. A 88 023601 (2013-08-01), http://dx.doi.org/10.1103/PhysRevA.88.023601 doi:10.1103/PhysRevA.88.023601 (ID: 718523) Toggle Abstract
  We report on the realization of quantum degenerate gas mixtures of the alkaline-earth-metal element strontium with the alkali-metal element rubidium. A key ingredient of our scheme is sympathetic cooling of Rb by Sr atoms that are continuously laser cooled on a narrow-linewidth transition. This versatile technique allows us to produce ultracold gas mixtures with a phase-space density of up to 0.06 for both elements. By further evaporative cooling we create double Bose-Einstein condensates of 87Rb with either 88Sr or 84Sr, reaching more than 1×105 condensed atoms per element for the 84Sr-87Rb mixture. These quantum gas mixtures constitute an important step towards the production of a quantum gas of polar, open-shell RbSr molecules.
• S. Stellmer, B. Pasquiou, R. Grimm, F. Schreck Laser cooling to quantum degeneracy, Phys. Rev. Lett. 110 263003 (2013-06-25), http://dx.doi.org/10.1103/PhysRevLett.110.263003 doi:10.1103/PhysRevLett.110.263003 (ID: 718365) Toggle Abstract
  We report on Bose-Einstein condensation (BEC) in a gas of strontium atoms, using laser cooling as the only cooling mechanism. The condensate is formed within a sample that is continuously Doppler cooled to below 1\muK on a narrow-linewidth transition. The critical phase-space density for BEC is reached in a central region of the sample, in which atoms are rendered transparent for laser cooling photons. The density in this region is enhanced by an additional dipole trap potential. Thermal equilibrium between the gas in this central region and the surrounding laser cooled part of the cloud is established by elastic collisions. Condensates of up to 10^5 atoms can be repeatedly formed on a timescale of 100ms, with prospects for the generation of a continuous atom laser.
• L. Sidorenkov, M. Tey, R. Grimm, Y. Hou, L. Pitaevskii, S. Stringari Second sound and the superfluid fraction in a Fermi gas with resonant interactions, Nature 498 78 (2013-06-06), http://dx.doi.org/10.1038/nature12136 doi:10.1038/nature12136 (ID: 718431) Toggle Abstract
  Superfluidity is a macroscopic quantum phenomenon occurring in systems as diverse as liquid helium and neutron stars. It occurs below a critical temperature and leads to peculiar behaviour such as frictionless flow, the formation of quantized vortices and quenching of the moment of inertia. Ultracold atomic gases offer control of interactions and external confinement, providing unique opportunities to explore superfluid phenomena. Many such (finite-temperature) phenomena can be explained in terms of a two-fluid mixture comprising a normal component, which behaves like an ordinary fluid, and a superfluid component with zero viscosity and zero entropy. The two-component nature of a superfluid is manifest in ‘second sound’, an entropy wave in which the superfluid and the non-superfluid components oscillate with opposite phases (as opposed to ordinary ‘first sound’, where they oscillate in phase). Here we report the observation of second sound in an ultracold Fermi gas with resonant interactions. The speed of second sound depends explicitly on the value of the superfluid fraction, a quantity that is sensitive to the spectrum of elementary excitations. Our measurements allow us to extract the temperature dependence of the superfluid fraction, a previously inaccessible quantity that will provide a benchmark for theories of strongly interacting quantum gases.
• E. R. Sanchez Guajardo, M. Tey, L. Sidorenkov, R. Grimm Higher-nodal collective modes in a resonantly interacting Fermi gas, Phys. Rev. A 87 063601 (2013-06-03), http://dx.doi.org/10.1103/PhysRevA.87.063601 doi:10.1103/PhysRevA.87.063601 (ID: 718494) Toggle Abstract
  We report on experimental investigations of longitudinal collective oscillations in a highly elongated, harmonically trapped two-component Fermi gas with resonantly tuned s-wave interactions (“unitary Fermi gas”). We focus on higher-nodal axial modes, which in contrast to the elementary modes have received little attention so far. We show how these modes can be efficiently excited using a resonant local excitation scheme and sensitively analyzed by a Fourier transformation of the detected time evolution of the axial density profile. We study the temperature dependence of the mode frequencies across the superfluid phase transition. The behavior is qualitatively different from the elementary modes, where the mode frequencies are independent of the temperature as long as the gas stays in the hydrodynamic regime. Our results are compared to theoretical predictions based on Landau's two-fluid theory and available experimental knowledge of the equation of state. The comparison shows excellent agreement and thus both represents a sensitive test for the validity of the theoretical approach and provides an independent test of the equation of state. The present results obtained on modes of first-sound character represent benchmarks for the observation of second-sound propagation and corresponding oscillation modes.
• A. Zenesini, B. Huang, M. Berninger, S. Besler, H. Nägerl, F. Ferlaino, R. Grimm, C. Greene, J. von Stecher Resonant five-body recombination in an ultracold gas of bosonic atoms, New J. Phys. 15 043040 (2013-04-22), http://dx.doi.org/10.1088/1367-2630/15/4/043040 doi:10.1088/1367-2630/15/4/043040 (ID: 718073) Toggle Abstract
  We combine theory and experiment to investigate five-body recombination in an ultracold gas of atomic cesium at negative scattering length. A refined theoretical model, in combination with extensive laboratory tunability of the interatomic interactions, enables the five-body resonant recombination rate to be calculated and measured. The position of the new observed recombination feature agrees with a recent theoretical prediction and supports the prediction of a family of universal cluster states at negative a that are tied to an Efimov trimer.
• M. Berninger, A. Zenesini, B. Huang, W. Harm, H. Nägerl, F. Ferlaino, R. Grimm, P. S. Julienne, J. M. Hutson Feshbach resonances, weakly bound molecular states, and coupled-channel potentials for cesium at high magnetic fields, Phys. Rev. A 87 032517 (2013-03-25), http://dx.doi.org/10.1103/PhysRevA.87.032517 doi:10.1103/PhysRevA.87.032517 (ID: 718341) Toggle Abstract
  We explore the scattering properties of ultracold ground-state Cs atoms at magnetic fields between 450 G (45 mT) and 1000 G. We identify 17 new Feshbach resonances, including two very broad ones near 549 G and 787 G. We measure the binding energies of several different dimer states by magnetic field modulation spectroscopy. We use least-squares fitting to these experimental results, together with previous measurements at lower field, to determine a new 6-parameter model of the long-range interaction potential, designated M2012. Coupled-channels calculations using M2012 provide an accurate mapping between the s-wave scattering length and the magnetic field over the entire range of fields considered. This mapping is crucial for experiments that rely on precise tuning of the scattering length, such as those on Efimov physics.
• M. Tey, L. Sidorenkov, E. R. Sanchez Guajardo, R. Grimm, M. J. Ku, M. W. Zwierlein, Y. Hou, L. Pitaevskii, S. Stringari Collective Modes in a Unitary Fermi Gas across the Superfluid Phase Transition, Phys. Rev. Lett. 110 055303 (2013-01-29), http://dx.doi.org/10.1103/PhysRevLett.110.055303 doi:10.1103/PhysRevLett.110.055303 (ID: 718270) Toggle Abstract
  We provide a joint theoretical and experimental investigation of the temperature dependence of the collective oscillations of first sound nature exhibited by a highly elongated harmonically trapped Fermi gas at unitarity, including the region below the critical temperature for superfluidity. Differently from the lowest axial breathing mode, the hydrodynamic frequencies of the higher-nodal excitations show a temperature dependence, which is calculated starting from Landau two-fluid theory and using the available experimental knowledge of the equation of state. The experimental results agree with high accuracy with the predictions of theory and provide the first evidence for the temperature dependence of the collective frequencies near the superfluid phase transition.
• S. Stellmer, R. Grimm, F. Schreck Production of quantum degenerate strontium gases, Phys. Rev. A 87 013611 (2013-01-14), http://dx.doi.org/10.1103/PhysRevA.87.013611 doi:10.1103/PhysRevA.87.013611 (ID: 718327) Toggle Abstract
  We report on an improved scheme to generate Bose-Einstein condensates (BECs) and degenerate Fermi gases of strontium. This scheme allows us to create quantum gases with higher atom number, a shorter time of the experimental cycle, or deeper quantum degeneracy than before. We create a BEC of 84Sr exceeding 107 atoms, which is a 30-fold improvement over previously reported experiments. We increase the atom number of 86Sr BECs to 2.5×104 (a fivefold improvement) and refine the generation of attractively interacting 88Sr BECs. We present a scheme to generate 84Sr BECs with a cycle time of 2 s. We create deeply degenerate 87Sr Fermi gases with T/TF as low as 0.10(1), where the number of populated nuclear spin states can be set to any value between one and ten. Furthermore, we report on a total of five different double-degenerate Bose-Bose and Bose-Fermi mixtures. These studies prepare an excellent starting point for applications of strontium quantum gases anticipated in the near future.

2012

Articles

• S. Stellmer, B. Pasquiou, R. Grimm, F. Schreck Creation of ultracold Sr2 molecules in the electronic ground state, Phys. Rev. Lett. 109 115302 (2012-09-13), http://dx.doi.org/10.1103/PhysRevLett.109.115302 doi:10.1103/PhysRevLett.109.115302 (ID: 718081) Toggle Abstract
  We report on the creation of ultracold 84Sr2 molecules in the electronic ground state. The molecules are formed from atom pairs on sites of an optical lattice using stimulated Raman adiabatic passage (STIRAP). We achieve a transfer efficiency of 30% and obtain 4x10^4 molecules with full control over the external and internal quantum state. STIRAP is performed near the narrow 1S0-3P1 intercombination transition, using a vibrational level of the 0u potential as intermediate state. In preparation of our molecule association scheme, we have determined the binding energies of the last vibrational levels of the 0u, 1u excited-state, and the 1\Sigma_g^+ ground-state potentials. Our work overcomes the previous limitation of STIRAP schemes to systems with Feshbach resonances, thereby establishing a route that is applicable to many systems beyond bi-alkalis.
• C. Kohstall, M. Zaccanti, M. Jag, A. Trenkwalder, P. Massignan, G. Bruun, F. Schreck, R. Grimm Metastability and Coherence of Repulsive Polarons in a Strongly Interacting Fermi Mixture, Nature 458 618 (2012-05-31), http://dx.doi.org/10.1038/nature11065 doi:10.1038/nature11065 (ID: 717828) Toggle Abstract
  Ultracold Fermi gases with tunable interactions provide a test bed for exploring the many-body physics of strongly interacting quantum systems.Over the past decade, experiments have investigated many intriguing phenomena, and precise measurements of groundstate properties have provided benchmarks for the development of theoretical descriptions. Metastable states in Fermi gases with strong repulsive interactions5–11 represent an exciting area of development. The realization of such systems is challenging, because a strong repulsive interaction in an atomic quantum gas implies the existence of a weakly bound molecular state, which makes the system intrinsically unstable against decay. Here we use radio-frequency spectroscopy to measure the complete excitation spectrum of fermionic 40K impurities resonantly interacting with a Fermi sea of 6Li atoms. In particular, we show that a well-defined quasiparticle exists for strongly repulsive interactions. We measure the energy and the lifetime of this ‘repulsive polaron’, and probe its coherence properties by measuring the quasiparticle residue. The results are well described by a theoretical approach that takes into account the finite effective range of the interaction in our system. We find that when the effective range is of the order of the interparticle spacing, there is a substantial increase in the lifetime of the quasiparticles. The existence of such a long-lived, metastable many-body state offers intriguing prospects for the creation of exotic quantum phases in ultracold, repulsively interacting Fermi gases.
• K. Aikawa, A. Frisch, M. Mark, S. Baier, A. Rietzler, R. Grimm, F. Ferlaino Bose-Einstein Condensation of Erbium, Phys. Rev. Lett. 108 210401 (2012-05-21), http://dx.doi.org/10.1103/PhysRevLett.108.210401 doi:10.1103/PhysRevLett.108.210401 (ID: 718048) Toggle Abstract
  We report on the achievement of Bose-Einstein condensation of erbium atoms and on the observation of magnetic Feshbach resonances at low magnetic field. By means of evaporative cooling in an optical dipole trap, we produce pure condensates of 168Er, containing up to 7×104 atoms. Feshbach spectroscopy reveals an extraordinary rich loss spectrum with six loss resonances already in a narrow magnetic-field range up to 3 G. Finally, we demonstrate the application of a low-field Feshbach resonance to produce a tunable dipolar Bose-Einstein condensate and we observe its characteristic d-wave collapse.
• A. Frisch, K. Aikawa, M. Mark, A. Rietzler, J. Schindler, E. Zupanic, R. Grimm, F. Ferlaino Narrow-line magneto-optical trap for erbium: Simple approach for a complex atom, Phys. Rev. A 85 051401 (2012-05-07), http://dx.doi.org/10.1103/PhysRevA.85.051401 doi:10.1103/PhysRevA.85.051401 (ID: 718027) Toggle Abstract
  We report on the experimental realization of a robust and efficient magneto-optical trap for erbium atoms, based on a narrow cooling transition at 583 nm. We observe up to N=2×108 atoms at a temperature of about T=15 μK. This simple scheme provides better starting conditions for direct loading of dipole traps as compared to approaches based on the strong cooling transition alone, or on a combination of a strong and a narrow kHz transition. Our results on Er point to a general, simple, and efficient approach to laser cool samples of other lanthanide atoms (Ho, Dy, and Tm) for the production of quantum-degenerate samples.
• T. Takekoshi, M. Debatin, R. Rameshan, F. Ferlaino, R. Grimm, H. Nägerl, C. Le Sueur, J. M. Hutson, P. S. Julienne, S. Kotochigova, E. Tiemann Towards the production of ultracold ground-state RbCs molecules: Feshbach resonances, weakly bound states, and the coupled-channel model, Phys. Rev. A 85 032506 (2012-03-05), http://dx.doi.org/10.1103/PhysRevA.85.032506 doi:10.1103/PhysRevA.85.032506 (ID: 718010) Toggle Abstract
  We have studied interspecies scattering in an ultracold mixture of 87Rb and 133Cs atoms, both in their lowest-energy spin states. The three-body loss signatures of 30 incoming s- and p-wave magnetic Feshbach resonances over the range 0 to 667 G have been cataloged. Magnetic field modulation spectroscopy was used to observe molecular states bound by up to 2.5 MHz×h. We have created RbCs Feshbach molecules using two of the resonances. Magnetic moment spectroscopy along the magnetoassociation pathway from 197 to 182 G gives results consistent with the observed and calculated dependence of the binding energy on magnetic field strength. We have set up a coupled-channel model of the interaction and have used direct least-squares fitting to refine its parameters to fit the experimental results from the Feshbach molecules, in addition to the Feshbach resonance positions and the spectroscopic results for deeply bound levels. The final model gives a good description of all the experimental results and predicts a large resonance near 790 G, which may be useful for tuning the interspecies scattering properties. Quantum numbers and vibrational wave functions from the model can also be used to choose optimal initial states of Feshbach molecules for their transfer to the rovibronic ground state using stimulated Raman adiabatic passage.

2011

Articles

• S. Stellmer, R. Grimm, F. Schreck Detection and manipulation of nuclear spin states in fermionic strontium, Phys. Rev. A 84 043611 (2011-10-10), http://dx.doi.org/10.1103/PhysRevA.84.043611 doi:10.1103/PhysRevA.84.043611 (ID: 717754) Toggle Abstract
  Fermionic 87Sr has a nuclear spin of I=9/2, higher than any other element with similar electronic structure. This large nuclear spin has many applications in quantum simulation and computation, for which preparation and detection of the spin state are requirements. For an ultracold 87Sr cloud, we show two complementary methods to characterize the spin-state mixture: optical Stern-Gerlach state separation and state-selective absorption imaging. We use these methods to optimize the preparation of a variety of spin-state mixtures by optical pumping and to measure an upper bound of the 87Sr spin relaxation rate.
• F. Ferlaino, A. Zenesini, M. Berninger, B. Huang, H. C. Nägerl, R. Grimm Efimov Resonances in Ultracold Quantum Gases, Few-Body Syst. 51 133 (2011-10-09), http://dx.doi.org/10.1007/s00601-011-0260-7 doi:10.1007/s00601-011-0260-7 (ID: 717751) Toggle Abstract
  Ultracold atomic gases have developed into prime systems for experimental studies of Efimov three-body physics and related few-body phenomena, which occur in the universal regime of resonant interactions. In the last few years, many important breakthroughs have been achieved, confirming basic predictions of universal few-body theory and deepening our understanding of such systems. We review the basic ideas along with the fast experimental developments of the field, focussing on ultracold cesium gases as a well-investigated model system. Triatomic Efimov resonances, atom-dimer Efimov resonances, and related four-body resonances are discussed as central observables. We also present some new observations of such resonances, supporting and complementing the set of available data.
• P. Pieri, A. Perali, G. C. Strinati, S. Riedl, M. J. Wright, A. Altmeyer, C. Kohstall, E. R. Sanchez Guajardo, J. Hecker Denschlag, R. Grimm Pairing-gap, pseudogap, and no-gap phases in the radio-frequency spectra of a trapped unitary 6Li gas, Phys. Rev. A 84 011608 (2011-07-25), http://dx.doi.org/10.1103/PhysRevA.84.011608 doi:10.1103/PhysRevA.84.011608 (ID: 717741) Toggle Abstract
  Radio frequency spectra of a trapped unitary 6Li gas are reported and analyzed in terms of a theoretical approach that includes both final-state and trap effects. The different strength of the final-state interaction across the trap is crucial for evidencing two main peaks associated with two distinct phases residing in different trap regions. These are the pairing-gap and pseudo-gap phases below the critical temperature Tc, which evolve into the pseudo-gap and no-gap phases above Tc. In this way, a long standing puzzle about the interpretation of rf spectra for 6Li in a trap is solved.
• M. Debatin, T. Takekoshi, R. Rameshan, L. Reichsöllner, F. Ferlaino, R. Grimm, R. Vexiau, N. Bouloufa, O. Dulieu, H. C. Nägerl Molecular spectroscopy for ground-state transfer of ultracold RbCs molecules, Phys. Chem. Chem. Phys. 13 18935 (2011-07-24), http://dx.doi.org/10.1039/C1CP21769K doi:10.1039/C1CP21769K (ID: 717719) Toggle Abstract
  We perform one- and two-photon high resolution spectroscopy on ultracold samples of RbCs Feshbach molecules with the aim to identify a suitable route for efficient ground-state transfer in the quantum-gas regime to produce quantum gases of dipolar RbCs ground-state molecules. One-photon loss spectroscopy allows us to probe deeply bound rovibrational levels of the mixed excited (A1{\Sigma}+ - b3{\Pi}0) 0+ molecular states. Two-photon dark state spectroscopy connects the initial Feshbach state to the rovibronic ground state. We determine the binding energy of the lowest rovibrational level |v"=0,J"=0> of the X1{\Sigma}+ ground state to be DX 0 = 3811.5755(16) 1/cm, a 300-fold improvement in accuracy with respect to previous data. We are now in the position to perform stimulated two-photon Raman transfer to the rovibronic ground state.
• C. Kohstall, S. Riedl, E. R. Sanchez Guajardo, L. Sidorenkov, J. Hecker Denschlag, R. Grimm Observation of interference between two molecular Bose-Einstein condensates, New J. Phys. 13 065027 (2011-06-28), http://dx.doi.org/10.1088/1367-2630/13/6/065027 doi:10.1088/1367-2630/13/6/065027 (ID: 717707) Toggle Abstract
  We have observed the interference between two Bose–Einstein condensates of weakly bound Feshbach molecules of fermionic 6Li atoms. Two condensates are prepared in a double-well trap and, after release from this trap, overlap in expansion. We detect a clear interference pattern that unambiguously demonstrates the de Broglie wavelength of molecules. We verify that only the condensate fraction shows interference. With increasing interaction strength, the pattern vanishes because elastic collisions during overlap remove particles from the condensate wave function. For strong interaction, the condensates do not penetrate each other as they collide hydrodynamically.
• M. Berninger, A. Zenesini, B. Huang, W. Harm, H. C. Nägerl, F. Ferlaino, R. Grimm, P. S. Julienne, J. M. Hutson Universality of the Three-Body Parameter for Efimov States in Ultracold Cesium, Phys. Rev. Lett. 107 120401 (2011-06-20), http://dx.doi.org/10.1103/PhysRevLett.107.120401 doi:10.1103/PhysRevLett.107.120401 (ID: 717706) Toggle Abstract
  We report on the observation of triatomic Efimov resonances in an ultracold gas of cesium atoms. Exploiting the wide tunability of interactions resulting from three broad Feshbach resonances in the same spin channel, we measure magnetic-field dependent three-body recombination loss. The positions of the loss resonances yield corresponding values for the three-body parameter, which in universal few-body physics is required to describe three-body phenomena and in particular to fix the spectrum of Efimov states. Our observations show a robust universal behavior with a three-body parameter that stays essentially constant.
• A. Lercher, T. Takekoshi, M. Debatin, B. Schuster, R. Rameshan, F. Ferlaino, R. Grimm, H. C. Nägerl Production of a dual-species Bose-Einstein condensate of Rb and Cs atoms, Eur. Phys. J. D 65 9 (2011-03-25), http://dx.doi.org/10.1140/epjd/e2011-20015-6 doi:10.1140/epjd/e2011-20015-6 (ID: 717717) Toggle Abstract
  We report the simultaneous production of Bose-Einstein condensates (BECs) of and atoms in separate optical traps. The two samples are mixed during laser cooling and loading but are separated by 400 μm for the final stage of evaporative cooling. This is done to avoid considerable interspecies three-body recombination, which causes heating and evaporative loss. We characterize the BEC production process, discuss limitations, and outline the use of the dual-species BEC in future experiments to produce rovibronic ground state molecules, including a scheme facilitated by the superfluid-to-Mott-insulator (SF-MI) phase transition.
• A. Trenkwalder, C. Kohstall, M. Zaccanti, D. Naik, A. Sidorov, F. Schreck, R. Grimm Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture, Phys. Rev. Lett. 106 115304 (2011-03-16), http://dx.doi.org/10.1103/PhysRevLett.106.115304 doi:10.1103/PhysRevLett.106.115304 (ID: 717708) Toggle Abstract
  We report on the expansion of an ultracold Fermi-Fermi mixture of 6Li and 40K under conditions of strong interactions controlled via an interspecies Feshbach resonance. We study the expansion of the mixture after release from the trap and, in a narrow magnetic-field range, we observe two phenomena related to hydrodynamic behavior. The common inversion of the aspect ratio is found to be accompanied by a collective effect where both species stick together and expand jointly despite of their widely different masses. Our work constitutes a major experimental step for a controlled investigation of the many-body physics of this novel strongly interacting quantum system.
• S. Riedl, E. R. Sanchez Guajardo, C. Kohstall, J. Hecker Denschlag, R. Grimm Superfluid quenching of the moment of inertia in a strongly interacting Fermi gas, New J. Phys. 13 035003 (2011-03-16), http://dx.doi.org/10.1088/1367-2630/13/3/035003 doi:10.1088/1367-2630/13/3/035003 (ID: 717721) Toggle Abstract
  We report on the observation of a quenched moment of inertia resulting from superfluidity in a strongly interacting Fermi gas. Our method is based on setting the hydrodynamic gas in slow rotation and determining its angular momentum by detecting the precession of a radial quadrupole excitation. The measurements distinguish between the superfluid and collisional origins of hydrodynamic behavior, and show the phase transition.
• D. Naik, A. Trenkwalder, C. Kohstall, F. M. Spiegelhalder, M. Zaccanti, G. Hendl, F. Schreck, R. Grimm, T. Hanna, P. S. Julienne Feshbach resonances in the 6Li-40K Fermi-Fermi mixture: Elastic versus inelastic interactions., Eur. Phys. J. D 65 65 (2011-01-14), http://dx.doi.org/10.1140/epjd/e2010-10591-2 doi:10.1140/epjd/e2010-10591-2 (ID: 717466) Toggle Abstract
  We present a detailed theoretical and experimental study of Feshbach resonances in the 6Li-40K mixture. Particular attention is given to the inelastic scattering properties, which have not been considered before. As an important example, we thoroughly investigate both elastic and inelastic scattering properties of a resonance that occurs near 155 G. Our theoretical predictions based on a coupled channels calculation are found in excellent agreement with the experimental results. We also present theoretical results on the molecular state that underlies the 155G resonance, in particular concerning its lifetime against spontaneous dissociation. We then present a survey of resonances in the system, fully characterizing the corresponding elastic and inelastic scattering properties. This provides the essential information to identify optimum resonances for applications relying on interaction control in this Fermi-Fermi mixture.

2010

Articles

• C. Strauss, T. Takekoshi, F. Lang, K. Winkler, R. Grimm, J. Hecker Denschlag Hyperfine, rotational, and vibrational structure of the a3Σu+ state of 87Rb2, Phys. Rev. A 82 052514 (2010-11-19), http://dx.doi.org/10.1103/PhysRevA.82.052514 doi:10.1103/PhysRevA.82.052514 (ID: 717716) Toggle Abstract
  We have performed high-resolution two-photon dark-state spectroscopy of an ultracold gas of 87Rb2 molecules in the a3Σu+ state at a magnetic field of about 1000 G. The vibrational ladder as well as the hyperfine and low-lying rotational structure are mapped out. Energy shifts in the spectrum are observed due to singlet-triplet mixing at binding energies as deep as a few hundred GHz×h. This information, together with data from other sources, is used to optimize the potentials of the a3Σu+ and X1Σg+ states in a coupled-channel model. We find that the hyperfine structure depends weakly on the vibrational level. This provides a possible explanation for inaccuracies in recent Feshbach resonance calculations.
• C. Chin, R. Grimm, P. S. Julienne, E. Tiesinga Feshbach Resonances in Ultracold Gases, Rev. Mod. Phys. 82 1286 (2010), http://dx.doi.org/10.1103/RevModPhys.82.1225 doi:10.1103/RevModPhys.82.1225 (ID: 660702) Toggle Abstract
  Feshbach resonances are the essential tool to control the interaction between atoms in ultracold quantum gases. They have found numerous experimental applications, opening up the way to important breakthroughs. This Review broadly covers the phenomenon of Feshbach resonances in ultracold gases and their main applications. This includes the theoretical background and models for the description of Feshbach resonances, the experimental methods to find and characterize the resonances, a discussion of the main properties of resonances in various atomic species and mixed-species systems, and an overview of key experiments with atomic Bose-Einstein condensates, degenerate Fermi gases, and ultracold molecules.
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• F. Ferlaino, S. Knoop, M. Berninger, M. Mark, H. C. Nägerl, R. Grimm Collisions of optically trapped ultracold cesium Feshbach molecules, Laser Phys. 20 31 (2010), http://dx.doi.org/10.1134/S1054660X0917006X doi:10.1134/S1054660X0917006X (ID: 717102) Toggle Abstract
  We study collisions in an optically trapped, pure sample of ultracold Cs$_2$ molecules in various internal states. The molecular gas is created by Feshbach association from a near-degenerate atomic gas, with adjustable temperatures in the nanokelvin range. We identify several narrow loss resonances, which point to the coupling to more complex molecular states and may be interpreted as Feshbach resonances in dimer-dimer interactions. Moreover, in some molecular states we observe a surprising temperature dependence in collisional loss. This shows that the situation cannot be understood in terms of the usual simple threshold behavior for inelastic two-body collisions. We interpret this observation as further evidence for a more complex molecular structure beyond the well-understood dimer physics.
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• F. Ferlaino, R. Grimm Forty years of Efimov physics: How a bizarre prediction turned into a hot topic, Physics 3 9 (2010), http://dx.doi.org/10.1103/Physics.3.9 doi:10.1103/Physics.3.9 (ID: 717106) Toggle Abstract
  A prediction that resonantly interacting particles can form weakly bound trimer states remained a mere theoretical oddity for more than three decades until tunable ultracold gases caused the field to explode, with enormous progress in just the last year.
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• F. Spiegelhalder, A. Trenkwalder, D. Naik, G. Kerner, E. Wille, G. Hendl, F. Schreck, R. Grimm All-optical production of a degenerate mixture of 6Li and 40K and creation of heteronuclear molecules, Phys. Rev. A 81 043637 (2010), http://dx.doi.org/10.1103/PhysRevA.81.043637 doi:10.1103/PhysRevA.81.043637 (ID: 717108) Toggle Abstract
  We present the essential experimental steps of our all-optical approach to prepare a double-degenerate Fermi-Fermi mixture of 6Li and 40K atoms, which then serves as a starting point for molecule formation. We first describe the optimized trap loading procedures, the internal-state preparation of the sample, and the combined evaporative and sympathetic cooling process. We then discuss the preparation of the sample near an interspecies Feshbach resonance, and we demonstrate the formation of heteronuclear molecules by a magnetic field ramp across the resonance.
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• S. Knoop, F. Ferlaino, M. Berninger, M. Mark, H. C. Nägerl, R. Grimm, J. P. D'Incao, B. Esry Magnetically Controlled Exchange Process in an Ultracold Atom-Dimer Mixture, Phys. Rev. Lett. 104 053201 (2010), http://dx.doi.org/10.1103/PhysRevLett.104.053201 doi:10.1103/PhysRevLett.104.053201 (ID: 717109) Toggle Abstract
  We report on the observation of an elementary exchange process in an optically trapped ultracold sample of atoms and Feshbach molecules. We can magnetically control the energetic nature of the process and tune it from endoergic to exoergic, enabling the observation of a pronounced threshold behavior. In contrast to relaxation to more deeply bound molecular states, the exchange process does not lead to trap loss. We find excellent agreement between our experimental observations and calculations based on the solutions of three-body Schrödinger equation in the adiabatic hyperspherical representation. The high efficiency of the exchange process is explained by the halo character of both the initial and final molecular states.
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• M. K. Tey, S. Stellmer, R. Grimm, F. Schreck Double-degenerate Bose-Fermi mixture of strontium, Phys. Rev. A 82 011608 (2010), http://dx.doi.org/10.1103/PhysRevA.82.011608 doi:10.1103/PhysRevA.82.011608 (ID: 717462) Toggle Abstract
  We report on the attainment of a spin-polarized Fermi sea of 87Sr in thermal contact with a Bose-Einstein condensate (BEC) of 84Sr. Interisotope collisions thermalize the fermions with the bosons during evaporative cooling. A degeneracy with T/TF=0.30(5) is reached for a 87Sr Fermi sea of 2×104 atoms together with an almost pure 84Sr BEC of 105 atoms.
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• S. Stellmer, M. K. Tey, R. Grimm, F. Schreck Bose-Einstein condensation of 86Sr., Phys. Rev. A 82 041602 (2010), http://dx.doi.org/10.1103/PhysRevA.82.041602 doi:10.1103/PhysRevA.82.041602 (ID: 717464) Toggle Abstract
  We report on the attainment of Bose-Einstein condensation of 86Sr. This isotope has a scattering length of about +800a0 and thus suffers from fast three-body losses. To avoid detrimental atom loss, evaporative cooling is performed at low densities around 3×1012 cm-3 in a large volume optical dipole trap. We obtain almost pure condensates of 5×103 atoms.
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2009

Articles

• F. Spiegelhalder, A. Trenkwalder, D. Naik, G. Hendl, F. Schreck, R. Grimm Collisional stability of 40K immersed in a strongly interacting Fermi gas of 6Li, Phys. Rev. Lett. 103 223203 (2009-11-25), http://dx.doi.org/10.1103/PhysRevLett.103.223203 doi:10.1103/PhysRevLett.103.223203 (ID: 717096) Toggle Abstract
  We investigate the collisional stability of a sample of 40K atoms immersed in a tunable spin mixture of 6Li atoms. In this three-component Fermi-Fermi mixture, we find very low loss rates in a wide range of interactions as long as molecule formation of 6Li is avoided. The stable fermionic mixture with two resonantly interacting spin states of one species together with another species is a promising system for a broad variety of phenomena in few- and many-body quantum physics.
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• S. Knoop, F. Ferlaino, M. Berninger, M. Mark, H. Nägerl, R. Grimm, J. P. D'Incao, B. Esry Magnetically Controlled Exchange Process in an Ultracold Atom-Dimer Mixture, Phys. Rev. Lett. 104 053201 (2009-11-10), http://dx.doi.org/10.1103/PhysRevLett.104.053201 doi:10.1103/PhysRevLett.104.053201 (ID: 717092) Toggle Abstract
  We report on the observation of an elementary exchange process in an optically trapped ultracold sample of atoms and Feshbach molecules. We can magnetically control the energetic nature of the process and tune it from endoergic to exoergic, enabling the observation of a pronounced threshold behavior. In contrast to relaxation to more deeply bound molecular states, the exchange process does not lead to trap loss. We find excellent agreement between our experimental observations and calculations based on the solutions of three-body Schr\"odinger equation in the adiabatic hyperspherical representation. The high efficiency of the exchange process is explained by the halo character of both the initial and final molecular states.
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• S. Stellmer, M. K. Tey, B. Huang, R. Grimm, F. Schreck Bose-Einstein Condensation of Strontium, Phys. Rev. Lett. 103 200401 (2009-11-09), http://dx.doi.org/10.1103/PhysRevLett.103.200401 doi:10.1103/PhysRevLett.103.200401 (ID: 717093) Toggle Abstract
  We report on the attainment of Bose-Einstein condensation with ultracold strontium atoms. We use the 84Sr isotope, which has a low natural abundance but offers excellent scattering properties for evaporative cooling. Accumulation in a metastable state using a magnetic-trap, narrowline cooling, and straightforward evaporative cooling in an optical trap lead to pure condensates containing 1.5x10^5 atoms. This puts 84Sr in a prime position for future experiments on quantum-degenerate gases involving atomic two-electron systems.
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• S. Riedl, E. R. Sanchez Guajardo, C. Kohstall, J. Hecker Denschlag, R. Grimm Superfluid quenching of the moment of inertia in a strongly interacting Fermi gas, E-print cond-mat 0907.3814 (2009-07-22), arXiv:0907.3814v1 arXiv:0907.3814v1 (ID: 717098) Toggle Abstract
  We report on the observation of a quenched moment of inertia as resulting from superfluidity in a strongly interacting Fermi gas. Our method is based on setting the hydrodynamic gas in slow rotation and determining its moment of inertia by detecting the precession of a radial quadrupole excitation. The measurements distinguish between the superfluid or collisional origin of hydrodynamic behavior, and show the phase transition.
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• S. Riedl, E. R. Sanchez Guajardo, C. Kohstall, J. Hecker Denschlag, R. Grimm Lifetime of angular momentum in a rotating strongly interacting Fermi gas, Phys. Rev. A 79 053628 (2009-05-15), http://dx.doi.org/10.1103/PhysRevA.79.053628 doi:10.1103/PhysRevA.79.053628 (ID: 717100) Toggle Abstract
  We investigate the lifetime of angular momentum in an ultracold strongly interacting Fermi gas, confined in a trap with controllable ellipticity. To determine the angular momentum we measure the precession of the radial quadrupole mode. We find that in the vicinity of a Feshbach resonance, the deeply hydrodynamic behavior in the normal phase leads to a very long lifetime of the angular momentum. Furthermore, we examine the dependence of the decay rate of the angular momentum on the ellipticity of the trapping potential and the interaction strength. The results are in general agreement with the theoretically expected behavior for a Boltzmann gas.
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• K. Pilch, A. D. Lange, A. Prantner, G. Kerner, F. Ferlaino, H. Nägerl, R. Grimm Observation of interspecies Feshbach resonances in an ultracold Rb-Cs mixture, Phys. Rev. A 79 042718 (2009-04-30), http://dx.doi.org/10.1103/PhysRevA.79.042718 doi:10.1103/PhysRevA.79.042718 (ID: 660704) Toggle Abstract
  We report on the observation of interspecies Feshbach resonances in an ultracold, optically trapped mixture of Rb and Cs atoms. In a magnetic field range up to 300 G we find 23 interspecies Feshbach resonances in the lowest spin channel and 2 resonances in a higher channel of the mixture. The extraordinarily rich Feshbach spectrum suggests the importance of different partial waves in both the open and closed channels of the scattering problem along with higher-order coupling mechanisms. Our results provide, on one hand, fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identify possible starting points for the association of ultracold heteronuclear RbCs molecules.
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• F. Ferlaino, S. Knoop, M. Berninger, W. Harm, J. P. D'Incao, H. Nägerl, R. Grimm Evidence for universal four-body states tied to an Efimov trimer, Phys. Rev. Lett. 102 140401 (2009-04-09), http://dx.doi.org/10.1103/PhysRevLett.102.140401 doi:10.1103/PhysRevLett.102.140401 (ID: 717103) Toggle Abstract
  We report on the measurement of four-body recombination rate coefficients in an atomic gas. Our results obtained with an ultracold sample of cesium atoms at negative scattering lengths show a resonant enhancement of losses and provide strong evidence for the existence of a pair of four-body states, which is strictly connected to Efimov trimers via universal relations. Our findings confirm recent theoretical predictions and demonstrate the enrichment of the Efimov scenario when a fourth particle is added to the generic three-body problem.
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• S. Knoop, F. Ferlaino, M. Mark, M. Berninger, H. Schöbel, H. Nägerl, R. Grimm Observation of an Efimov-like trimer resonance in ultracold atom-dimer scattering, Nature Phys. 5 227-230 (2009-02-09), http://dx.doi.org/10.1038/nphys1203 doi:10.1038/nphys1203 (ID: 717105) Toggle Abstract
  The field of few-body physics has originally been motivated by understanding nuclear matter. New model systems to experimentally explore few-body quantum systems can now be realized in ultracold gases with tunable interactions. Albeit the vastly different energy regimes of ultracold and nuclear matter (peV as compared to MeV), few-body phenomena are universal for near-resonant two-body interactions. Efimov states represent a paradigm for universal three-body states, and evidence for their existence has been obtained in measurements of three-body recombination in an ultracold gas of caesium atoms. Interacting samples of halo dimers can provide further information on universal few-body phenomena. Here we study interactions in an optically trapped mixture of such halo dimers with atoms, realized in a caesium gas at nanokelvin temperatures. We observe an atom-dimer scattering resonance, which we interpret as being due to a trimer state hitting the atom-dimer threshold. We discuss the close relation of this observation to Efimov's scenario, and in particular to atom-dimer Efimov resonances.
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• A. D. Lange, K. Pilch, A. Prantner, F. Ferlaino, B. Engeser, H. Nägerl, R. Grimm, C. Chin Determination of atomic scattering lengths from measurements of molecular binding energies near Feshbach resonances, Phys. Rev. A 79 013622 (2009-01-23), http://dx.doi.org/10.1103/PhysRevA.79.013622 doi:10.1103/PhysRevA.79.013622 (ID: 660710) Toggle Abstract
  We present an analytic model to calculate the atomic scattering length near a Feshbach resonance from data on the molecular binding energy. Our approach considers finite-range square-well potentials and can be applied near broad, narrow, or even overlapping Feshbach resonances. We test our model on Cs2 Feshbach molecules. We measure the binding energy using magnetic-field modulation spectroscopy in a range where one broad and two narrow Feshbach resonances overlap. From the data we accurately determine the Cs atomic scattering length and the positions and widths of two particular resonances.
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• F. Lang, C. Strauss, K. Winkler, T. Takekoshi, R. Grimm, J. Hecker Denschlag Dark state experiments with ultracold, deeply-bound triplet molecules, PhyDid A 142 271-282 (2009), http://dx.doi.org/10.1039/b818964a doi:10.1039/b818964a (ID: 660695) Toggle Abstract
  We examine dark quantum superposition states of weakly bound Rb2 Feshbach molecules and tightly bound triplet Rb2 molecules in the rovibrational ground state, created by subjecting a pure sample of Feshbach molecules in an optical lattice to a bichromatic Raman laser field. We analyze both experimentally and theoretically the creation and dynamics of these dark states. Coherent wavepacket oscillations of deeply bound molecules in lattice sites, as observed in one of our previous experiments, are suppressed due to laser-induced phase locking of molecular levels. This can be understood as the appearance of a novel multilevel dark state. In addition, the experimental methods developed help to determine important properties of our coupled atom / laser system.
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Proceeding

• S. Knoop, F. Ferlaino, M. Berninger, M. Mark, H. Nägerl, R. Grimm Observation of an Efimov resonance in an ultracold mixture of atoms and weakly bound dimers, J. Phys. Conf. Ser. 194 (2009-07-26), http://dx.doi.org/10.1088/1742-6596/194/1/012064 doi:10.1088/1742-6596/194/1/012064 (ID: 717097) Toggle Abstract
  We discuss our recent observation of an atom-dimer Efimov resonance in an ultracold mixture of Cs atoms and Cs_2 Feshbach molecules [Nature Phys. 5, 227 (2009)]. We review our experimental procedure and present additional data involving a non-universal g-wave dimer state, to contrast our previous results on the universal s-wave dimer. We resolve a seeming discrepancy when quantitatively comparing our experimental findings with theoretical results from effective field theory.
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2008

Articles

• F. Ferlaino, S. Knoop, M. Mark, M. Berninger, H. Schöbel, H. Nägerl, R. Grimm Collisions between tunable halo dimers: exploring an elementary four-body process with identical bosons, Phys. Rev. Lett. 101 023201 (2008-07-09), http://dx.doi.org/10.1103/PhysRevLett.101.023201 doi:10.1103/PhysRevLett.101.023201 (ID: 660508) Toggle Abstract
  We study inelastic collisions in a pure, trapped sample of Feshbach molecules made of bosonic cesium atoms in the quantum halo regime. We measure the relaxation rate coefficient for decay to lower-lying molecular states and study the dependence on scattering length and temperature. We identify a pronounced loss minimum with varying scattering length along with a further suppression of loss with decreasing temperature. Our observations provide insight into the physics of a few-body quantum system that consists of four identical bosons at large values of the two-body scattering length.
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• F. Lang, P. van der Straten, B. Brandstätter, G. Thalhammer, K. Winkler, P. S. Julienne, R. Grimm, J. Hecker Denschlag Cruising through molecular bound state manifolds with radio frequency, Nature Phys. 4 223 - 226 (2008), http://dx.doi.org/10.1038/nphys838 doi:10.1038/nphys838 (ID: 520799) Toggle Abstract
  The emerging field of ultracold molecules with their rich internal structure is currently attracting a lot of interest. Various methods have been developed to produce ultracold molecules in pre-set quantum states. For future experiments it will be important to efficiently transfer these molecules from their initial quantum state to other quantum states of interest. Optical Raman schemes are excellent tools for transfer, but can be involved in terms of equipment, laser stabilization and finding the right transitions. Here we demonstrate a very general and simple way for transfer of molecules from one quantum state to a neighboring quantum state with better than 99% efficiency. The scheme is based on Zeeman tuning the molecular state to avoided level crossings where radio-frequency transitions can then be carried out. By repeating this process at different crossings, molecules can be successively transported through a large manifold of quantum states. As an important spin-off of our experiments, we demonstrate a high-precision spectroscopy method for investigating level crossings.
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• S. Knoop, M. Mark, F. Ferlaino, J. Danzl, T. Kraemer, H. Nägerl, R. Grimm Metastable Feshbach Molecules in High Rotational States, Phys. Rev. Lett. 100 083002 (2008), http://dx.doi.org/10.1103/PhysRevLett.100.083002 doi:10.1103/PhysRevLett.100.083002 (ID: 559390) Toggle Abstract
  We experimentally demonstrate Cs2 Feshbach molecules well above the dissociation threshold, which are stable against spontaneous decay on the timescale of one second. An optically trapped sample of ultracold dimers is prepared in an l-wave state and magnetically tuned into a region with negative binding energy. The metastable character of these molecules arises from the large centrifugal barrier in combination with negligible coupling to states with low rotational angular momentum. A sharp onset of dissociation with increasing magnetic field is mediated by a crossing with a g-wave dimer state and facilitates dissociation on demand with a well defined energy.
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• E. Wille, F. Spiegelhalder, G. Kerner, D. Naik, A. Trenkwalder, G. Hendl, F. Schreck, R. Grimm, T. Tiecke, J. T. Walraven, S. J. Kokkelmans, E. Tiesinga, P. S. Julienne Exploring an Ultracold Fermi-Fermi Mixture: Interspecies Feshbach Resonances and Scattering Properties of 6Li and 40K, Phys. Rev. Lett. 100 053201 (2008), http://dx.doi.org/10.1103/PhysRevLett.100.053201 doi:10.1103/PhysRevLett.100.053201 (ID: 559392) Toggle Abstract
  We report on the observation of Feshbach resonances in an ultracold mixture of two fermionic species, 6Li and 40K. The experimental data are interpreted using a simple asymptotic bound state model and full coupled channels calculations. This unambiguously assigns the observed resonances in terms of various s- and p-wave molecular states and fully characterizes the ground-state scattering properties in any combination of spin states.
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• S. Riedl, E. R. Sanchez Guajardo, C. Kohstall, A. Altmeyer, M. J. Wright, J. Hecker Denschlag, R. Grimm, G. M. Bruun, H. Smith Collective oscillations of a Fermi gas in the unitarity limit: Temperature effects and the role of pair correlations, Phys. Rev. A 78 053609 (2008), http://dx.doi.org/10.1103/PhysRevA.78.053609 doi:10.1103/PhysRevA.78.053609 (ID: 660516) Toggle Abstract
  We present detailed measurements of the frequency and damping of three different collective modes in an ultracold trapped Fermi gas of 6Li atoms with resonantly tuned interactions. The measurements are carried out over a wide range of temperatures. We focus on the unitarity limit, where the scattering length is much greater than all other relevant length scales. The results are compared to theoretical calculations that take into account Pauli blocking and pair correlations in the normal state above the critical temperature for superfluidity. We show that these two effects nearly compensate each other and the behavior of the gas is close to that of a classical gas.
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• F. Lang, K. Winkler, C. Strauss, R. Grimm, J. Hecker Denschlag Ultracold Molecules in the Ro-Vibrational Triplet Ground State, Phys. Rev. Lett. 101 133005 (2008), http://dx.doi.org/10.1103/PhysRevLett.101.133005 doi:10.1103/PhysRevLett.101.133005 (ID: 660686) Toggle Abstract
  We report here on the production of an ultracold gas of tightly bound Rb2 triplet molecules in the rovibrational ground state, close to quantum degeneracy. This is achieved by optically transferring weakly bound Rb2 molecules to the absolute lowest level of the ground triplet potential with a transfer efficiency of about 90%. The transfer takes place in a 3D optical lattice which traps a sizeable fraction of the tightly bound molecules with a lifetime exceeding 200 ms.
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2007

Articles

• M. Mark, F. Ferlaino, S. Knoop, J. Danzl, T. Kraemer, C. Chin, H. Nägerl, R. Grimm Spectroscopy of ultracold, trapped cesium Feshbach molecules, Phys. Rev. A 76 042514 (2007-10-25), http://dx.doi.org/10.1103/PhysRevA.76.042514 doi:10.1103/PhysRevA.76.042514 (ID: 520794) Toggle Abstract
  We explore the rich internal structure of Cs_2 Feshbach molecules. Pure ultracold molecular samples are prepared in a CO_2-laser trap, and a multitude of weakly bound states is populated by elaborate magnetic-field ramping techniques. Our methods use different Feshbach resonances as input ports and various internal level crossings for controlled state transfer. We populate higher partial-wave states of up to eight units of rotational angular momentum (l-wave states). We investigate the molecular structure by measurements of the magnetic moments for various states. Avoided level crossings between different molecular states are characterized through the changes in magnetic moment and by a Landau-Zener tunneling method. Based on microwave spectroscopy, we present a precise measurement of the magnetic-field dependent binding energy of the weakly bound s-wave state that is responsible for the large background scattering length of Cs. This state is of particular interest because of its quantum-halo character.
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• A. Altmeyer, S. Riedl, M. J. Wright, C. Kohstall, J. Hecker Denschlag, R. Grimm Dynamics of a strongly interacting Fermi gas: the radial quadrupole mode, Phys. Rev. A 76 033610 (2007-09-18), http://dx.doi.org/10.1103/PhysRevA.76.033610 doi:10.1103/PhysRevA.76.033610 (ID: 520792) Toggle Abstract
  We report on measurements of an elementary surface mode in an ultracold, strongly interacting Fermi gas of 6Li atoms. The radial quadrupole mode allows us to probe hydrodynamic behavior in the BEC-BCS crossover without being influenced by changes in the equation of state. We examine frequency and damping of this mode, along with its expansion dynamics. In the unitarity limit and on the BEC side of the resonance, the observed frequencies agree with standard hydrodynamic theory. However, on the BCS side of the crossover, a striking down shift of the oscillation frequency is observed in the hydrodynamic regime as a precursor to an abrupt transition to collisionless behavior; this indicates coupling of the oscillation to fermionic pairs.
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• K. Winkler, F. Lang, G. Thalhammer, P. van der Straten, R. Grimm, J. Hecker Denschlag Coherent optical transfer of Feshbach molecules to a lower vibrational state, Phys. Rev. Lett. 98 043201 (2007-01-25), http://dx.doi.org/10.1103/PhysRevLett.98.043201 doi:10.1103/PhysRevLett.98.043201 (ID: 520783) Toggle Abstract
  Using the technique of stimulated Raman adiabatic passage (STIRAP) we have coherently transferred ultracold 87Rb2 Feshbach molecules into a more deeply bound vibrational quantum level. Our measurements indicate a high transfer efficiency of up to 87%. Because the molecules are held in an optical lattice with not more than a single molecule per lattice site, inelastic collisions between the molecules are suppressed and we observe long molecular lifetimes of about 1 s. Using STIRAP we have created quantum superpositions of the two molecular states and tested their coherence interferometrically. These results represent an important step towards Bose-Einstein condensation of molecules in the vibrational ground state.
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• A. Altmeyer, S. Riedl, C. Kohstall, M. J. Wright, R. Geursen, M. Bartenstein, C. Chin, J. Hecker Denschlag, R. Grimm Precision measurements of collective modes in the BEC-BCS crossover, Phys. Rev. Lett. 98 040401 (2007-01-22), http://dx.doi.org/10.1103/PhysRevLett.98.040401 doi:10.1103/PhysRevLett.98.040401 (ID: 520772) Toggle Abstract
  We report on precision measurements of the frequency of the radial compression mode in a strongly interacting, optically trapped Fermi gas of 6Li atoms. Our results allow for a test of theoretical predictions for the equation of state in the BEC-BCS crossover. We confirm recent quantum Monte Carlo results and rule out simple mean-field BCS theory. Our results show the long-sought beyond-mean-field effects in the strongly interacting Bose-Einstein condensation (BEC) regime.
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• R. Grimm Ultracold Fermi gases in the BEC-BCS crossover: a review from the Innsbruck perspective, QIPIRC (2007), (ID: 520785) Toggle Abstract
  A review of recent BEC-BCS crossover experiments in ultracold Fermi gases is given with particular emphasis on the work performed with lithium-6 at the University of Innsbruck.
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• M. Markiewisz, T. Kraemer, P. Waldburger, J. Herbig, C. Chin, H. Nägerl, R. Grimm 'Stückelberg interferometry' with ultracold molecules, Phys. Rev. Lett. 99 113201 (2007), http://dx.doi.org/10.1103/PhysRevLett.99.113201 doi:10.1103/PhysRevLett.99.113201 (ID: 520787) Toggle Abstract
  We report on the realization of a time-domain `St\"uckelberg interferometer', which is based on the internal state structure of ultracold Feshbach molecules. Two subsequent passages through a weak avoided crossing between two different orbital angular momentum states in combination with a variable hold time lead to high-contrast population oscillations. This allows for a precise determination of the energy difference between the two molecular states. We demonstrate a high degree of control over the interferometer dynamics. The interferometric scheme provides new possibilities for precision measurements with ultracold molecules.
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• M. J. Wright, A. Altmeyer, C. Kohstall, E. R. Sanchez Guajardo, J. Hecker Denschlag, R. Grimm, S. Riedl Finite-temperature collective dynamics of a Fermi gas in the BEC-BCS crossover, Phys. Rev. Lett. 99 150403 (2007), http://dx.doi.org/10.1103/PhysRevLett.99.150403 doi:10.1103/PhysRevLett.99.150403 (ID: 520795) Toggle Abstract
  We report on experimental studies on the collective behavior of a strongly interacting Fermi gas with tunable interactions and variable temperature. A scissors mode excitation in an elliptical trap is used to characterize the dynamics of the quantum gas in terms of hydrodynamic or near-collisionless behavior. We obtain a crossover phase diagram for collisional properties, showing a large region where a non-superfluid strongly interacting gas shows hydrodynamic behavior. In a narrow interaction regime on the BCS side of the crossover, we find a novel temperature-dependent damping peak, suggesting a relation to the superfluid phase transition.
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2006

Articles

• K. Winkler, G. Thalhammer, F. Lang, R. Grimm, J. Hecker Denschlag, A. J. Daley, A. Kantian, H. Büchler, P. Zoller Repulsively bound atom pairs in an optical lattice, Nature 441 853 (2006-06-15), http://dx.doi.org/10.1038/nature04918 doi:10.1038/nature04918 (ID: 371157) Toggle Abstract
  Throughout physics, stable composite objects are usually formed by way of attractive forces, which allow the constituents to lower their energy by binding together. Repulsive forces separate particles in free space. However, in a structured environment such as a periodic potential and in the absence of dissipation, stable composite objects can exist even for repulsive interactions. Here we report the observation of such an exotic bound state, which comprises a pair of ultracold rubidium atoms in an optical lattice. Consistent with our theoretical analysis, these repulsively bound pairs exhibit long lifetimes, even under conditions when they collide with one another. Signatures of the pairs are also recognized in the characteristic momentum distribution and through spectroscopic measurements. There is no analogue in traditional condensed matter systems of such repulsively bound pairs, owing to the presence of strong decay channels. Our results exemplify the strong correspondence between the optical lattice physics of ultracold bosonic atoms and the Bose–Hubbard model—a link that is vital for future applications of these systems to the study of strongly correlated condensed matter and to quantum information.
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• T. Kraemer, M. Markiewisz, P. Waldburger, J. G. Danzl, C. Chin, B. Engeser, A. D. Lange, K. Pilch, A. Jaakkola, H. Nägerl, R. Grimm Evidence for Efimov quantum states in an ultracold gas of caesium atoms, Nature 440 318 (2006-03-16), http://dx.doi.org/10.1038/nature04626 doi:10.1038/nature04626 (ID: 344003) Toggle Abstract
  Systems of three interacting particles are notorious for their complex physical behaviour. A landmark theoretical result in few-body quantum physics is Efimov's prediction1, 2 of a universal set of bound trimer states appearing for three identical bosons with a resonant two-body interaction. Counterintuitively, these states even exist in the absence of a corresponding two-body bound state. Since the formulation of Efimov's problem in the context of nuclear physics 35 years ago, it has attracted great interest in many areas of physics3, 4, 5, 6, 7, 8. However, the observation of Efimov quantum states has remained an elusive goal3, 5. Here we report the observation of an Efimov resonance in an ultracold gas of caesium atoms. The resonance occurs in the range of large negative two-body scattering lengths, arising from the coupling of three free atoms to an Efimov trimer. Experimentally, we observe its signature as a giant three-body recombination loss9, 10 when the strength of the two-body interaction is varied. We also detect a minimum9, 11, 12 in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point with which to explore the universal properties of resonantly interacting few-body systems7. While Feshbach resonances13, 14 have provided the key to control quantum-mechanical interactions on the two-body level, Efimov resonances connect ultracold matter15 to the world of few-body quantum phenomena.
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• G. Thalhammer, K. Winkler, F. Lang, S. Schmid, R. Grimm, J. Hecker Denschlag Long-lived Feshbach molecules in a 3D optical lattice, Phys. Rev. Lett. 96 050402 (2006-02-08), http://dx.doi.org/10.1103/PhysRevLett.96.050402 doi:10.1103/PhysRevLett.96.050402 (ID: 344005) Toggle Abstract
  We have created and trapped a pure sample of 87Rb2 Feshbach molecules in a three-dimensional optical lattice. Compared to previous experiments without a lattice, we find dramatic improvements such as long lifetimes of up to 700 ms and a near unit efficiency for converting tightly confined atom pairs into molecules. The lattice shields the trapped molecules from collisions and, thus, overcomes the problem of inelastic decay by vibrational quenching. Furthermore, we have developed an advanced purification scheme that removes residual atoms, resulting in a lattice in which individual sites are either empty or filled with a single molecule in the vibrational ground state of the lattice.
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• A. Altmeyer, S. Riedl, C. Kohstall, M. J. Wright, J. Hecker Denschlag, R. Grimm Note on "Collecitive Excitations of a Degenerate gas at the BEC-BCS Crossover",Phys. Rev. Lett. 92, 203201 (2004), (2006), arXiv:cond-mat/0611285 arXiv:cond-mat/0611285 (ID: 447172) Toggle Abstract
  We present a reinterpretation of our previous results on the radial compression mode of a degenerate quantum gas in the BEC-BCS crossover in [1]. We show that our former data are consistent with other experimental and theoretical work, when the ellipticity of the optical trapping potential in [1] is properly taken into account.
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Proceeding

• A. J. Daley, A. Kantian, H. Büchler, P. Zoller, K. Winkler, G. Thalhammer, F. Lang, R. Grimm, J. Hecker Denschlag Repulsively bound atom pairs: Overview, Simulations and Links, Proceedings of ICAP-2006 (2006-08-31), arXiv:cond-mat/0608721 arXiv:cond-mat/0608721 (ID: 387364) Toggle Abstract
  We review the basic physics of repulsively bound atom pairs in an optical lattice, which were recently observed in the laboratory, including the theory and the experimental implementation. We also briefly discuss related many-body numerical simulations, in which time-dependent Density Matrix Renormalisation Group (DMRG) methods are used to model the many-body physics of a collection of interacting pairs, and give a comparison of the single-particle quasimomentum distribution measured in the experiment and results from these simulations. We then give a short discussion of how these repulsively bound pairs relate to bound states in some other physical systems.

2005

Articles

• K. Winkler, G. Thalhammer, M. Theis, H. Ritsch, R. Grimm, J. Hecker Denschlag Atom-molecule dark states in a Bose-Einstein condensate, Phys. Rev. Lett. 95 063202 (2005-08-05), http://dx.doi.org/10.1103/PhysRevLett.95.063202 doi:10.1103/PhysRevLett.95.063202 (ID: 321809) Toggle Abstract
  We have created a dark quantum superposition state of a Rb Bose-Einstein condensate and a degenerate gas of Rb2 ground-state molecules in a specific rovibrational state using two-color photoassociation. As a signature for the decoupling of this coherent atom-molecule gas from the light field, we observe a striking suppression of photoassociation loss. In our experiment the maximal molecule population in the dark state is limited to about 100 Rb2 molecules due to laser induced decay. The experimental findings can be well described by a simple three mode model.
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• R. Grimm Low-temperature physics: A quantum revolution, Nature 435 1035 (2005-06-23), http://dx.doi.org/10.1038/4351035a doi:10.1038/4351035a (ID: 321811) Toggle Abstract
  Tiny quantum tornadoes observed in ultracold gases of fermionic atoms provide definitive evidence of superfluidity, and open up new vistas in the modelling of quantum many-body systems.
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• C. Chin, T. Kraemer, M. Markiewisz, J. Herbig, P. Waldburger, H. Nägerl, R. Grimm Observation of Feshbach-like resonances in collisions between ultracold molecules, Phys. Rev. Lett. 94 123201 (2005-04-01), http://dx.doi.org/10.1103/PhysRevLett.94.123201 doi:10.1103/PhysRevLett.94.123201 (ID: 321808) Toggle Abstract
  We observe magnetically tuned collision resonances for ultracold Cs2 molecules stored in a CO2-laser trap. By magnetically levitating the molecules against gravity, we precisely measure their magnetic moment. We find an avoided level crossing which allows us to transfer the molecules into another state. In the new state, two Feshbach-like collision resonances show up as strong inelastic loss features. We interpret these resonances as being induced by Cs4 bound states near the molecular scattering continuum. The tunability of the interactions between molecules opens up novel applications such as controlled chemical reactions and synthesis of ultracold complex molecules.
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• M. Bartenstein, A. Altmeyer, S. Riedl, R. Geursen, S. Jochim, C. Chin, J. Hecker Denschlag, R. Grimm, A. Simoni, E. Tiesinga, C. Williams, P. S. Julienne Precise determination of 6Li cold collision parameters by radio-frequency spectroscopy on weakly bound molecules, Phys. Rev. Lett. 94 103201 (2005-03-18), http://dx.doi.org/10.1103/PhysRevLett.94.103201 doi:10.1103/PhysRevLett.94.103201 (ID: 321681) Toggle Abstract
  We employ radio-frequency spectroscopy on weakly bound 6Li2 molecules to precisely determine the molecular binding energies and the energy splittings between molecular states for different magnetic fields. These measurements allow us to extract the interaction parameters of ultracold 6Li atoms based on a multichannel quantum scattering model. We determine the singlet and triplet scattering lengths to be as=45.167(8)a0 and at=-2140(18)a0 (1a0=0.052 917 7 nm), and the positions of the broad Feshbach resonances in the energetically lowest three s-wave scattering channels to be 83.41(15), 69.04(5), and 81.12(10) mT.
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• G. Thalhammer, M. Theis, K. Winkler, R. Grimm, J. Hecker Denschlag Inducing an optical Feshbach resonance via stimulated Raman coupling, Phys. Rev. A 71 033403 (2005-03-08), http://dx.doi.org/10.1103/PhysRevA.71.033403 doi:10.1103/PhysRevA.71.033403 (ID: 321743) Toggle Abstract
  We demonstrate a method of inducing an optical Feshbach resonance based on a coherent free-bound stimulated Raman transition. In our experiment atoms in a 87Rb Bose-Einstein condensate are exposed to two phase-locked Raman laser beams which couple pairs of colliding atoms to a molecular ground state. By controlling the power and relative detuning of the two laser beams, we can change the atomic scattering length considerably. The dependence of scattering length on these parameters is studied experimentally and modeled theoretically.
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• M. Markiewisz, T. Kraemer, J. Herbig, C. Chin, H. Nägerl, R. Grimm Efficient creation of molecules from a cesium Bose-Einstein condensate, 69 706-712 (2005-02-00), http://dx.doi.org/10.1209/epl/i2004-10427-7 doi:10.1209/epl/i2004-10427-7 (ID: 321805) Toggle Abstract
  We report a new scheme to create weakly bound $\chem{Cs_2}$ molecules from an atomic Bose-Einstein condensate. The method is based on switching the magnetic field to a narrow Feshbach resonance and yields a high atom-molecule conversion efficiency of more than 30%, a factor of three higher than obtained with conventional magnetic-field ramps. The $\chem{Cs_2}$ molecules are created in a single g-wave rotational quantum state. The observed dependence of the conversion efficiency on the magnetic field and atom density shows scattering processes beyond two-body coupling to occur in the vicinity of the
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2004

Articles

• T. Kraemer, J. Herbig, M. Markiewisz, T. Weber, C. Chin, H. Nägerl, R. Grimm Optimized production of a cesium Bose-Einstein condensate, Appl. Phys. B Las. Opt. 79 1013-1019 (2004-12-00), http://dx.doi.org/10.1007/s00340-004-1657-5 doi:10.1007/s00340-004-1657-5 (ID: 325399) Toggle Abstract
  We report on the optimized production of a Bose–Einstein condensate of cesium atoms using an optical trapping approach. Based on an improved trap loading and evaporation scheme we obtain more than 105 atoms in the condensed phase. To test the tunability of the interaction in the condensate we study the expansion of the condensate as a function of scattering length. We further excite strong oscillations of the trapped condensate by rapidly varying the interaction strength.
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• M. Bartenstein, A. Altmeyer, S. Riedl, S. Jochim, R. Geursen, C. Chin, J. Hecker Denschlag, R. Grimm Exploring the BEC-BCS crossover with an ultracold gas of 6Li atoms, (2004-12-00), arXiv:cond-mat/0412712v1 arXiv:cond-mat/0412712v1 (ID: 582557) Toggle Abstract
  We present an overview of our recent measurements on the crossover from a Bose-Einstein condensate of molecules to a Bardeen-Cooper-Schrieffer superfluid. The experiments are performed on a two-component spin-mixture of $^6$Li atoms, where a Fesh\-bach resonance serves as the experimental key to tune the s-wave scattering length and thus to explore the various interaction regimes. In the BEC-BCS crossover, we have characterized the interaction energy by measuring the size of the trapped gas, we have studied collective excitation modes, and we have observed the pairing gap. Our observations provide strong evidence for superfluidity in the strongly interacting Fermi gas.
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• M. Theis, G. Thalhammer, K. Winkler, M. Hellwig, G. Ruff, R. Grimm, J. Hecker Denschlag Tuning the scattering length with an optically induced Feshbach resonance, Phys. Rev. Lett. 93 123001 (2004-09-15), http://dx.doi.org/10.1103/PhysRevLett.93.123001 doi:10.1103/PhysRevLett.93.123001 (ID: 325401) Toggle Abstract
  We demonstrate optical tuning of the scattering length in a Bose-Einstein condensate as predicted by Fedichev et al. [Phys. Rev. Lett. 77, 2913 (1996)]. In our experiment, atoms in a 87Rb condensate are exposed to laser light which is tuned close to the transition frequency to an excited molecular state. By controlling the power and detuning of the laser beam we can change the atomic scattering length over a wide range. In view of laser-driven atomic losses, we use Bragg spectroscopy as a fast method to measure the scattering length of the atoms.
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• C. Chin, M. Bartenstein, A. Altmeyer, S. Riedl, S. Jochim, J. Hecker Denschlag, R. Grimm Observation of the Pairing Gap in a Strongly Interacting Fermi Gas, Science 305 1128-1130 (2004-08-20), http://dx.doi.org/10.1126/science.1100818 doi:10.1126/science.1100818 (ID: 325400) Toggle Abstract
  We studied fermionic pairing in an ultracold two-component gas of 6Li atoms by observing an energy gap in the radio-frequency excitation spectra. With control of the two-body interactions through a Feshbach resonance, we demonstrated the dependence of the pairing gap on coupling strength, temperature, and Fermi energy. The appearance of an energy gap with moderate evaporative cooling suggests that our full evaporation brought the strongly interacting system deep into a superfluid state.
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• M. Bartenstein, A. Altmeyer, S. Riedl, S. Jochim, C. Chin, J. Hecker Denschlag, R. Grimm Collective excitations of a degenerate gas at the BEC-BCS crossover, 92 203201 (2004-05-19), http://dx.doi.org/10.1103/PhysRevLett.92.203201 doi:10.1103/PhysRevLett.92.203201 (ID: 325403) Toggle Abstract
  We study collective excitation modes of a fermionic gas of 6Li atoms in the BEC-BCS crossover regime. While measurements of the axial compression mode in the cigar-shaped trap close to a Feshbach resonance confirm theoretical expectations, the radial compression mode shows surprising features. In the strongly interacting molecular BEC regime, we observe a negative frequency shift with increasing coupling strength. In the regime of a strongly interacting Fermi gas, an abrupt change in the collective excitation frequency occurs, which may be a signature for a transition from a superfluid to a collisionless phase.
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• C. Chin, R. Grimm Thermal equilibrium and efficient evaporation of an ultracold atom-molecule mixture, 69 033612 (2004-03-29), http://dx.doi.org/10.1103/PhysRevA.69.033612 doi:10.1103/PhysRevA.69.033612 (ID: 336838) Toggle Abstract
  We derive the equilibrium conditions for a thermal atom-molecule mixture near a Feshbach resonance. Under the assumption of low collisional loss, thermodynamical properties are calculated and compared to the measurements of a recent experiment on fermionic lithium experiment. We discuss and evaluate possible collision mechanisms which can lead to atom-molecule conversion. Finally, we propose a novel evaporative cooling scheme to efficiently cool the molecules toward Bose-Einstein condensation.
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• M. Bartenstein, A. Altmeyer, S. Riedl, S. Jochim, C. Chin, J. Hecker Denschlag, R. Grimm Crossover from a Molecular Bose-Einstein Condensate to a Degenerate Fermi Gas, Phys. Rev. Lett. 92 120401 (2004-03-23), http://dx.doi.org/10.1103/PhysRevLett.92.120401 doi:10.1103/PhysRevLett.92.120401 (ID: 325405) Toggle Abstract
  We demonstrate a reversible conversion of a 6Li2 molecular Bose-Einstein condensate to a degenerate Fermi gas of atoms by adiabatically crossing a Feshbach resonance. By optical in situ imaging, we observe a smooth change of the cloud size in the crossover regime. On the Feshbach resonance, the ensemble is strongly interacting and the measured cloud size is 75(7)% of the one of a noninteracting zero-temperature Fermi gas. The high condensate fraction of more than 90% and the adiabatic crossover suggest our Fermi gas to be cold enough to form a superfluid.
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• J. Hecker Denschlag, H. Nägerl, R. Grimm Moleküle am absoluten Nullpunkt: Ultrakalte Moleküle erobern die Welt der Quantgase, Physik Journal 3 33-39 (2004-03-00), URL (ID: 325404) Toggle Abstract
  Die erstmalige Erzeugung eines Bose-Einstein-Kondensats (BEC) eines schwach wechselwir-kenden Gases im Jahre 1995 begründete ein neues Forschungsfeld, das in rascher Folge fas-zinierende Erkenntnisse lieferte. Bis vor kurzem gelang es jedoch nur, Atome zu einem BEC zu kondensieren. Erst Ende 2003 entschied sich das Wettrennen mehrerer Arbeitsgruppen um das erste Bose-Einstein-Kondensat aus Molekülen. Aus fermionischen Atomen ließen sich nahe am absoluten Temperatur-Nullpunkt bosonische Molekül-BECs erzeugen. Dies eröffnet neue Perspektiven z. B. für die hochpräzise Molekül-spektroskopie, die Interferometrie mit Materie-wellen und das Verständnis der Supraleitung.
• D. Rychtarik, B. Engeser, M. Hammes, H. Nägerl, R. Grimm Crossover to 2D in a double-evanescent wave trap, J. Phys. IV France 116 241-245 (2004), doi:10.1051 doi:10.1051 (ID: 582581) Toggle Abstract
  We report the preparation of a dense gas of cesium atoms at the crossover to two-dimensionality in a highly anisotropic surface trap that is realized with two evanescent light waves. Temperatures as low as 100 nK are reached with 20.000 atoms at a phase-space density close to 0.1. The lowest quantum state in the tightly confined direction is populated by more than 60%. The system offers intriguing prospects for future experiments on degenerate quantum gases in two dimensions.

2003

Articles

• S. Jochim, M. Bartenstein, A. Altmeyer, G. Hendl, S. Riedl, C. Chin, J. Hecker Denschlag, R. Grimm Bose-Einstein Condensation of Molecules, Science 302 2103 (2003-12-19), http://dx.doi.org/10.1126/science.1093280 doi:10.1126/science.1093280 (ID: 336833) Toggle Abstract
  We report on the Bose-Einstein condensation of more than 105 Li2 molecules in an optical trap starting from a spin mixture of fermionic lithium atoms. During forced evaporative cooling, the molecules are formed by three-body recombination near a Feshbach resonance and finally condense in a long-lived thermal equilibrium state. We measured the characteristic frequency of a collective excitation mode and demonstrated the magnetic field–dependent mean field by controlled condensate spilling.
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• S. Jochim, M. Bartenstein, A. Altmeyer, G. Hendl, C. Chin, J. Hecker Denschlag, R. Grimm Pure Gas of Optically Trapped Molecules Created from Fermionic Atoms, Phys. Rev. Lett. 91 240402 (2003-12-08), http://dx.doi.org/10.1103/PhysRevLett.91.240402 doi:10.1103/PhysRevLett.91.240402 (ID: 336841) Toggle Abstract
  We report on the production of a pure sample of up to 3x10^5 optically trapped molecules from a Fermi gas of 6Li atoms. The dimers are formed by three-body recombination near a Feshbach resonance. For purification a Stern-Gerlach selection technique is used that efficiently removes all trapped atoms from the atom-molecule mixture. The behavior of the purified molecular sample shows a striking dependence on the applied magnetic field. For very weakly bound molecules near the Feshbach resonance, the gas exhibits a remarkable stability with respect to collisional decay.
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• T. Weber, J. Herbig, M. Markiewisz, H. Nägerl, R. Grimm Three-Body Recombination at Large Scattering Lengths in an Ultracold Atomic Gas, Phys. Rev. Lett. 91 123201 (2003-09-18), http://dx.doi.org/10.1103/PhysRevLett.91.123201 doi:10.1103/PhysRevLett.91.123201 (ID: 336869) Toggle Abstract
  We study three-body recombination in an optically trapped ultracold gas of cesium atoms with precise magnetic control of the s-wave scattering length a. At large positive values of a, we measure the dependence of the rate coefficient on a and confirm the theoretically predicted scaling proportional to a^4. Evidence of recombination heating indicates the formation of very weakly bound molecules in the last bound energy level.
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• J. Herbig, T. Kraemer, M. Markiewisz, T. Weber, C. Chin, H. Nägerl, R. Grimm Preparation of a Pure Molecular Quantum Gas, Science 301 1513 (2003-09-12), http://dx.doi.org/10.1126/science.1088876 doi:10.1126/science.1088876 (ID: 336839) Toggle Abstract
  An ultracold molecular quantum gas is created by application of a magnetic field sweep across a Feshbach resonance to a Bose-Einstein condensate of cesium atoms. The ability to separate the molecules from the atoms permits direct imaging of the pure molecular sample. Magnetic levitation enables study of the dynamics of the ensemble on extended time scales. We measured ultralow expansion energies in the range of a few nanokelvin for a sample of 3000 molecules. Our observations are consistent with the presence of a macroscopic molecular matter wave.
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• M. Hammes, D. Rychtarik, B. Engeser, H. Nägerl, R. Grimm Evanescent-Wave Trapping and Evaporative Cooling of an Atomic Gas at the Crossover to Two Dimensions, Phys. Rev. Lett. 90 173001 (2003-04-29), http://dx.doi.org/10.1103/PhysRevLett.90.173001 doi:10.1103/PhysRevLett.90.173001 (ID: 336871) Toggle Abstract
  A dense gas of cesium atoms at the crossover to two dimensions is prepared in a highly anisotropic surface trap that is realized with two evanescent light waves. Temperatures as low as 100 nK are reached with 20 000 atoms at a phase-space density close to 0.1. The lowest quantum state in the tightly confined direction is populated by more than 60%. The system provides atoms at a mean distance from the surface as low as 1 µm, and offers intriguing prospects for future experiments on degenerate quantum gases in two dimensions.
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• T. Weber, J. Herbig, M. Markiewisz, H. Nägerl, R. Grimm Bose-Einstein Condensation of Cesium, Science 299 235 (2003-01-10), http://dx.doi.org/10.1126/science.1079699 doi:10.1126/science.1079699 (ID: 336870) Toggle Abstract
  Bose-Einstein condensation of cesium atoms is achieved by evaporative cooling using optical trapping techniques. The ability to tune the interactions between the ultracold atoms by an external magnetic field is crucial to obtain the condensate and offers intriguing features for potential applications. We explore various regimes of condensate self-interaction (attractive, repulsive, and null interaction strength) and demonstrate properties of imploding, exploding, and non-interacting quantum matter.
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• D. Rychtarik, B. Engeser, H. Nägerl, R. Grimm Two-Dimensional Bose-Einstein Condensate in an Optical Surface Trap, Phys. Rev. Lett. 92 173003 (2003), http://dx.doi.org/10.1103/PhysRevLett.92.173003 doi:10.1103/PhysRevLett.92.173003 (ID: 336835) Toggle Abstract
  We report on the creation of a two-dimensional Bose-Einstein condensate of cesium atoms in a gravito-optical surface trap. The condensate is produced a few micrometer above a dielectric surface on an evanescent-wave atom mirror. After evaporative cooling by all-optical means, expansion measurements for the tightly confined vertical motion show energies well below the vibrational energy quantum. The presence of a condensate is observed in two independent ways by a magnetically induced collapse at negative scattering length and by measurements of the horizontal expansion.

2002

Articles

• S. Jochim, M. Bartenstein, G. Hendl, J. Hecker Denschlag, R. Grimm, A. Mosk, M. Weidemüller Magnetic Field Control of Elastic Scattering in a Cold Gas of Fermionic Lithium Atoms, Phys. Rev. Lett. 89 273202 (2002-12-18), http://dx.doi.org/10.1103/PhysRevLett.89.273202 doi:10.1103/PhysRevLett.89.273202 (ID: 337849) Toggle Abstract
  We study elastic collisions in an optically trapped spin mixture of fermionic lithium atoms in the presence of magnetic fields up to 1.5kG by measuring evaporative loss. Our experiments confirm the expected magnetic tunability of the scattering length by showing the main features of elastic scattering according to recent calculations. We measure the zero crossing of the scattering length that is associated with a predicted Feshbach resonance at 530(3)G. Beyond the resonance we observe the expected large cross section in the triplet scattering regime.
• M. Hammes, D. Rychtarik, H. Nägerl, R. Grimm Cold-atom gas at very high densities in an optical surface microtrap, Phys. Rev. A 66 051401(R) (2002-11-14), http://dx.doi.org/10.1103/PhysRevA.66.051401 doi:10.1103/PhysRevA.66.051401 (ID: 337852) Toggle Abstract
  An optical microtrap is realized on a dielectric surface by crossing a tightly focused laser beam with an horizontal evanescent-wave atom mirror. The nondissipative trap is loaded with $\sim$$10^5$ cesium atoms through elastic collisions from a cold reservoir provided by a large-volume optical surface trap. With an observed 300-fold local increase of the atomic number density approaching $10^{14}{\rm cm}^{-3}$, unprecedented conditions of cold atoms close to a surface are realized.
• M. Mudrich, S. Kraft, K. Singer, R. Grimm, M. Weidemüller Sympathetic Cooling with Two Atomic Species in an Optical Trap, Phys. Rev. Lett. 88 253001 (2002-06-07), http://dx.doi.org/10.1103/PhysRevLett.88.25300 doi:10.1103/PhysRevLett.88.25300 (ID: 337857) Toggle Abstract
  We simultaneously trap ultracold lithium and cesium atoms in an optical dipole trap formed by the focus of a CO$_2$ laser and study the exchange of thermal energy between the gases. The cesium gas, which is optically cooled to $20 \mu$K, efficiently decreases the temperature of the lithium gas through sympathetic cooling. The measured cross section for thermalizing $^{133}$Cs-$^7$Li collisions is $8 \times 10^{-12}$ cm$^2$, for both species in their lowest hyperfine ground state. Besides thermalization, we observe evaporation of lithium purely through elastic cesium-lithium collisions (sympathetic evaporation).
• R. Grimm Faszination von Quantenmaterie am absoluten Nullpunkt - Nobelpreis für Experimente zur Bose-Einstein-Kondensation, Mitteilungsblatt d. Österr. Physikal. Ges. Jänner 2002 (2002-01-00), (ID: 337855)

2001

Articles

• A. Mosk, S. Kraft, M. Mudrich, K. Singer, W. Wohlleben, R. Grimm, M. Weidemüller Mixture of ultracold lithium and cesium atoms in an optical dipole trap, Appl. Phys. B Las. Opt. 73 799 (2001-12-00), http://dx.doi.org/10.1007/s003400100743 doi:10.1007/s003400100743 (ID: 337867) Toggle Abstract
  We present the first simultaneous trapping of two different ultracold atomic species in a conservative trap. Lithium and cesium atoms are stored in an optical dipole trap formed by the focus of a CO$_2$ laser. Techniques for loading both species of atoms are discussed and observations of elastic and inelastic collisions between the two species are presented. A model for sympathetic cooling of two species with strongly different mass in the presence of slow evaporation is developed. From the observed Cs-induced evaporation of Li atoms we estimate a cross section for cold elastic Li-Cs collisions.
• A. Mosk, S. Jochim, H. Moritz, T. Elsässer, M. Weidemüller, R. Grimm Resonator-enhanced optical dipole trap for fermionic lithium atoms, Opt. Lett. 26 1837 (2001-07-21), URL (ID: 337869) Toggle Abstract
  We demonstrate a novel optical dipole trap which is based on the enhancement of the optical power density of a Nd:YAG laser beam in a resonator. The trap is particularly suited for experiments with ultracold gases, as it combines a potential depth of order 1 mK with storage times of several tens of seconds. We study the interactions in a gas of fermionic lithium atoms in our trap and observe the influence of spin-changing collisions and off-resonant photon scattering. A key element in reaching long storage times is an ultra-low noise laser. The dependence of the storage time on laser noise is investigated.
• R. Grimm Kristalliner Ionenstrahl im Mini-Speicherring, Physikalische Blätter 57 16 (2001), grimm2001a grimm2001a (ID: 337861)

Proceedings

• S. Kraft, M. Mudrich, K. Singer, R. Grimm, A. Mosk, M. Weidemüller Sympathetic cooling of lithium by laser-cooled cesium, Proceedings of ICOLS XV ICOLS (Snowbird, Utah (USA), 2001-06-10) URL (2001-09-21), oai:arXiv.org:physics/0109069 oai:arXiv.org:physics/0109069 (ID: 337862) Toggle Abstract
  We present first indications of sympathetic cooling between two neutral, optically trapped atomic species. Lithium and cesium atoms are simultaneously stored in an optical dipole trap formed by the focus of a CO$_2$ laser, and allowed to interact for a given period of time. The temperature of the lithium gas is found to decrease when in thermal contact with cold cesium. The timescale of thermalization yields an estimate for the Li-Cs cross-section.
• M. Hammes, D. Rychtarik, R. Grimm Evaporative cooling of cesium atoms in the gravito-optical surface trap, Proceedings of the Euroconference on Atom Optics and Interferometry Euroconference on Atom Optics and Interferometry (Cargèse, Corsica, France, 2000-07-26) 625 (2001), oai:arXiv.org:physics/0012058 oai:arXiv.org:physics/0012058 (ID: 337871) Toggle Abstract
  We report on cooling of an atomic cesium gas closely above an evanescent-wave. Our first evaporation experiments show a temperature reduction from 10muK down to 300nK along with a gain in phase-space density of almost two orders of magnitude. In a series of measurements of heating and spin depolarization an incoherent background of resonant photons in the evanescent-wave diode laser light was found to be the limiting factor at this stage.
• R. Grimm, M. Hammes, D. Rychtarik, T. Weber, J. Herbig, H. Nägerl, M. Mudrich, S. Kraft, K. Singer, A. Mosk, M. Weidemüller Evaporative cooling in optical dipole traps, QELS '01. Technical Digest QELS '01 (BALTIMORE, USA, 2001-05-09) 253 (2001), http://dx.doi.org/10.1109/QELS.2001.962208 doi:10.1109/QELS.2001.962208 (ID: 337876) Toggle Abstract
  Optical dipole traps operating with laser light very far from resonance can provide conservative trapping potentials with interesting new features for evaporation experiments. We explore the prospects of evaporative cooling in various trapping schemes.

2000

Article

• R. Grimm Publications before 2001, (2000-12-31), URL (ID: 337846) Toggle Abstract
  A list of 30 publications published before 2001.