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S. I. Matveenko, M. Baranov, G. Shlyapnikov Rotons and their damping in elongated Bose-Einstein condensates,
Phys. Rev. A 106 13319 (2022-07-26),
http://dx.doi.org/10.1103/PhysRevA.106.013319 doi:10.1103/PhysRevA.106.013319 (ID: 720743)
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We discuss finite temperature damping of rotons in elongated Bose-condensed dipolar gases, which are in theThomas-Fermi regimein the tightly confined directions. The presence of many branches of excitations which can participate in the damping process, is crucial for the Landau damping and results in significant increase of the damping rate. It is found, however, that even rotons with energies close to the roton gap may remain fairly stable in systems with the roton gap as small as 1nK.
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M. Di Liberto, A. Kruckenhauser, P. Zoller, M. Baranov Topological phonons in arrays of ultracold dipolar particles,
Quantum 6 731 (2022-05-31),
http://dx.doi.org/10.22331/q-2022-06-07-731 doi:10.22331/q-2022-06-07-731 (ID: 720680)
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The notion of topology in physical systems is associated with the existence of a nonlocal ordering that is insensitive to a large class of perturbations. This brings robustness to the behaviour of the system and can serve as a ground for developing new fault-tolerant applications. We discuss how to design and study a large variety of topology-related phenomena for phonon-like collective modes in arrays of ultracold polarized dipolar particles. These modes are coherently propagating vibrational excitations, corresponding to oscillations of particles around their equilibrium positions, which exist in the regime where long-range interactions dominate over single-particle motion. We demonstrate that such systems offer a distinct and versatile tool to investigate topological effects that can be accessed by choosing the underlying crystal structure and by controlling the anisotropy of the interactions. Our results show that arrays of dipolar particles provide a promising unifying platform to investigate topological phenomena with phononic modes.
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V. Kuzmin, T. Zache, L. Pastori, A. Celi, M. Baranov, P. Zoller Probing infinite many-body quantum systems with finite size quantum simulators,
PRX Quantum 3 20304 (2022-04-06),
http://dx.doi.org/10.1103/PRXQuantum.3.020304 doi:10.1103/PRXQuantum.3.020304 (ID: 720681)
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Experimental studies of synthetic quantum matter are necessarily restricted to approximate ground states prepared on finite-size quantum simulators, which limits their reliability for strongly correlated systems, for instance in the vicinity of a quantum phase transition (QPT). Here, we propose a protocol that makes optimal use of a given finite system size by directly preparing, via coherent evolution with a local deformation of the system Hamiltonian, a part of the translation-invariant infinite-sized system as a mixed state representing the reduced density operator. For systems of free fermions in one and two spatial dimensions, we illustrate and explain the underlying physics, which consists of quasi-particle transport towards the system's boundaries while retaining the bulk "vacuum". For the example of a non-integrable extended Su-Schrieffer-Heeger model, we demonstrate that our protocol enables a more accurate study of QPTs.
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D. Yang, D. Vasilyev, C. Laflamme, M. Baranov, P. Zoller Quantum Scanning Microscope for Cold Atoms,
Phys. Rev. A 98 23852 (2018-08-27),
http://dx.doi.org/10.1103/PhysRevA.98.023852 doi:10.1103/PhysRevA.98.023852 (ID: 720027)
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We present a detailed theoretical description of an atomic scanning microscope in a cavity QED setup proposed in Phys. Rev. Lett. 120, 133601 (2018). The microscope continuously observes atomic densities with optical subwavelength resolution in a nondestructive way. The super-resolution is achieved by engineering an internal atomic dark state with a sharp spatial variation of population of a ground level dispersively coupled to the cavity field. Thus, the atomic position encoded in the internal state is revealed as a phase shift of the light reflected from the cavity in a homodyne experiment. Our theoretical description of the microscope operation is based on the stochastic master equation describing the conditional time evolution of the atomic system under continuous observation as a competition between dynamics induced by the Hamiltonian of the system, decoherence effects due to atomic spontaneous decay, and the measurement backaction. Within our approach we relate the observed homodyne current with a local atomic density, and discuss the emergence of a quantum nondemolition measurement regime allowing continuous observation of spatial densities of quantum motional eigenstates without measurement backaction in a single experimental run.
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D. Yang, C. Laflamme, D. Vasilyev, M. Baranov, P. Zoller Theory of a Quantum Scanning Microscope for Cold Atoms,
Phys. Rev. Lett. 120 133601 (2018-03-30),
http://dx.doi.org/10.1103/PhysRevLett.120.133601 doi:10.1103/PhysRevLett.120.133601 (ID: 720010)
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We propose and analyze a scanning microscope to monitor “live” the quantum dynamics of cold atoms in a cavity QED setup. The microscope measures the atomic density with subwavelength resolution via dispersive couplings to a cavity and homodyne detection within the framework of continuous measurement theory. We analyze two modes of operation. First, for a fixed focal point the microscope records the wave packet dynamics of atoms with time resolution set by the cavity lifetime. Second, a spatial scan of the microscope acts to map out the spatial density of stationary quantum states. Remarkably, in the latter case, for a good cavity limit, the microscope becomes an effective quantum nondemolition device, such that the spatial distribution of motional eigenstates can be measured backaction free in single scans, as an emergent quantum nondemolition measurement.
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Y. Wang, S. Subhankar, P. Bienias, M. Łącki, T. Tsui, M. Baranov, A. V. Gorshkov, P. Zoller, J. V. Porto, S. L. Rolston Dark state optical lattice with sub-wavelength spatial structure,
Phys. Rev. Lett. 120 83601 (2018-02-20),
http://dx.doi.org/10.1103/PhysRevLett.120.083601 doi:10.1103/PhysRevLett.120.083601 (ID: 719917)
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We report on the experimental realization of a conservative optical lattice for cold atoms with sub-wavelength spatial structure. The potential is based on the nonlinear optical response of three-level atoms in laser-dressed dark states, which is not constrained by the diffraction limit of the light generating the potential. The lattice consists of a 1D array of ultra-narrow barriers with widths less than 10~nm, well below the wavelength of the lattice light, physically realizing a Kronig-Penney potential. We study the band structure and dissipation of this lattice, and find good agreement with theoretical predictions. The observed lifetimes of atoms trapped in the lattice are as long as 60 ms, nearly 105 times the excited state lifetime, and could be further improved with more laser intensity. The potential is readily generalizable to higher dimension and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with sub-wavelength spacings.
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A. Mazloom Shahraki, B. Vermersch, M. Baranov, M. Dalmonte Adiabatic state preparation of stripe phases with strongly magnetic atoms,
Phys. Rev. A 96 33602 (2017-09-01),
http://dx.doi.org/10.1103/PhysRevA.96.033602 doi:10.1103/PhysRevA.96.033602 (ID: 719755)
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We propose a protocol for realizing the stripe phase in two spin models on a two-dimensional square lattice, which can be implemented with strongly magnetic atoms (Cr, Dy, Er, etc.) in optical lattices by encoding spin states into Zeeman sublevels of the ground state manifold. The protocol is tested with cluster-mean-field time-dependent variational ans\"atze, validated by comparison with exact results for small systems, which enable us to simulate the dynamics of systems with up to 64 sites during the state-preparation protocol. This allows, in particular, to estimate the time required for preparation of the stripe phase with high fidelity under real experimental conditions.
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J. Budich, A. Elben, M. Łącki, A. Sterdyniak, M. Baranov, P. Zoller Coupled Atomic Wires in a Synthetic Magnetic Field,
Phys. Rev. A 95 43632 (2017-04-24),
http://dx.doi.org/10.1103/PhysRevA.95.043632 doi:10.1103/PhysRevA.95.043632 (ID: 719753)
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We propose and study systems of coupled atomic wires in a perpendicular synthetic magnetic field as a platform to realize exotic phases of quantum matter. This includes (fractional) quantum Hall states in arrays of many wires inspired by the pioneering work [Kane et al. PRL {\bf{88}}, 036401 (2002)], as well as Meissner phases and Vortex phases in double-wires. With one continuous and one discrete spatial dimension, the proposed setup naturally complements recently realized discrete counterparts, i.e.~the Harper-Hofstadter model and the two leg flux ladder, respectively. We present both an in-depth theoretical study and a detailed experimental proposal to make the unique properties of the semi-continuous Harper-Hofstadter model accessible with cold atom experiments. For the minimal setup of a double-wire, we explore how a sub-wavelength spacing of the wires can be implemented. This construction increases the relevant energy scales by at least an order of magnitude compared to ordinary optical lattices, thus rendering subtle many-body phenomena such as Lifshitz transitions in Fermi gases observable in an experimentally realistic parameter regime. For arrays of many wires, we discuss the emergence of Chern bands with readily tunable flatness of the dispersion and show how fractional quantum Hall states can be stabilized in such systems. Using for the creation of optical potentials Laguerre-Gauss beams that carry orbital angular momentum, we detail how the coupled atomic wire setups can be realized in non-planar geometries such as cylinders, discs, and tori.
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C. Kraus, S. Diehl, P. Zoller, M. Baranov Preparing and probing atomic Majorana fermions and topological order in optical lattices,
New J. Phys. 14 113036 (2012-11-27),
http://dx.doi.org/10.1088/1367-2630/14/11/113036 doi:10.1088/1367-2630/14/11/113036 (ID: 717882)
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We introduce a one-dimensional system of fermionic atoms in an optical lattice whose phase diagram includes topological states of different symmetry classes. These states can be identified by their zero-energy edge modes which are Majorana fermions. We propose several universal methods of detecting the Majorana edge states, based on their genuine features: zero-energy, localized character of the wave functions, and induced non-local fermionic correlations.
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M. Cordin, P. Amann, A. Menzel, E. Bertel, M. Baranov, S. Diehl, J. Redinger, C. Franchini Cleavage surface of the BaFe2−xCoxAs2 and FeySe1−xTex superconductors: A combined STM plus LEED study,
Phys. Rev. B 86 167401 (2012-10-19),
http://dx.doi.org/10.1103/PhysRevB.86.167401 doi:10.1103/PhysRevB.86.167401 (ID: 718263)
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Massee et al. [ Phys. Rev. B 80 140507 (2009)] found on the cleavage planes of BaFe2−xCoxAs2 two different long-range ordered structures, i.e., a (2×1) phase present only after cleavage at low temperature and a √2×√2 phase observed after cleavage at room temperature. These results apply generally to 122 Fe-based superconductors, but have been discussed controversially [for a summary of the conflicting views, see Hoffman Rep. Prog. Phys. 74 124513 (2011)]. Here we support the interpretation of Massee et al. In addition, we argue that the existence of different long-range ordered structures corresponding to the same coverage in different temperature regimes is associated with the melting of a charge density wave and removal of an associated periodic lattice distortion (CDW/PLD) in the substrate as T is increased. At sufficiently low temperature the fluctuating CDW/PLD order parameter is stabilized by the adsorbate in a lock-in type mechanism. Accordingly, we interpret the surface structures observed on the 122 Fe pnictide surfaces as evidence for the presence of CDW fluctuations at low temperature, but with a wave vector differing from that of the antiferromagnetic spin-density fluctuations.
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C. Bardyn, M. Baranov, E. Rico Ortega, A. Imamoglu, P. Zoller, S. Diehl Majorana Modes in Driven-Dissipative Atomic Superfluids With Zero Chern Number,
Phys. Rev. Lett. 109 130402 (2012-09-25),
http://dx.doi.org/10.1103/PhysRevLett.109.130402 doi:10.1103/PhysRevLett.109.130402 (ID: 717881)
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We investigate dissipation-induced p-wave paired states of fermions in two dimensions and show that dissipation can break the bulk-edge correspondence present in Hamiltonian systems in a way that leads to the appearance of spatially separated Majorana zero modes in a phase with vanishing Chern number. We construct an explicit model of a dissipative vortex that traps a single of these modes and establish its topological origin by mapping it to a one-dimensional wire where we observe a non-equilibrium topological phase transition characterized by an abrupt change of a topological invariant (winding number). Engineered dissipation opens up possibilities for experimentally realizing such states with no Hamiltonian counterpart.
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M. Baranov, M. Dalmonte, G. Pupillo, P. Zoller Condensed Matter Theory of Dipolar Quantum Gases,
Chem. Rev. (2012-08-09),
http://dx.doi.org/10.1021/cr2003568 doi:10.1021/cr2003568 (ID: 718140)
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Recent experimental breakthroughs in trapping, cooling and controlling ultracold gases of polar molecules, magnetic and Rydberg atoms have paved the way toward the investigation of highly tunable quantum systems, where anisotropic, long-range dipolar interactions play a prominent role at the many-body level. In this article we review recent theoretical studies concerning the physics of such systems. Starting from a general discussion on interaction design techniques and microscopic Hamiltonians, we provide a summary of recent work focused on many-body properties of dipolar systems, including: weakly interacting Bose gases, weakly interacting Fermi gases, multilayer systems, strongly interacting dipolar gases and dipolar gases in 1D and quasi-1D geometries. Within each of these topics, purely dipolar effects and connections with experimental realizations are emphasized.
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L. Sieberer, M. Baranov Collective modes, stability, and superfluid transition of a quasi-two-dimensional dipolar Fermi gas,
Phys. Rev. A 84 063633 (2011-12-22),
http://dx.doi.org/10.1103/PhysRevA.84.063633 doi:10.1103/PhysRevA.84.063633 (ID: 717842)
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We examine collective modes, stability, and BCS pairing in a quasi-two-dimensional gas of dipolar fermions aligned by an external field. By using the (conserving) Hartree-Fock approximation, which treats direct and exchange interactions on an equal footing, we obtain the spectrum of single-particle excitations and long-wavelength collective modes (zero sound) in the normal phase. It appears that exchange interactions result in strong damping of zero sound when the tilting angle between the dipoles and the normal to the plane of confinement is below some critical value. In particular, zero sound cannot propagate if the dipoles are perpendicular to the plane of confinement. At intermediate coupling, we find unstable modes that can lead either to collapse of the system or to the formation of a density wave. The BCS transition to a superfluid phase, on the other hand, occurs at arbitrarily weak strengths of the dipole-dipole interaction, provided the tilting angle exceeds a critical value. We determine the critical temperature of the transition, taking into account many-body effects as well as virtual transitions to higher excited states in the confining potential, and discuss prospects of experimental observations.
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T. Grass, M. Baranov, M. Lewenstein Robustness of fractional quantum Hall states with dipolar atoms in artificial gauge fields,
Phys. Rev. A 84 043605 (2011-10-07),
http://dx.doi.org/10.1103/PhysRevA.84.043605 doi:10.1103/PhysRevA.84.043605 (ID: 717780)
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The robustness of fractional quantum Hall states is measured as the energy gap separating the Laughlin ground state from excitations. Using thermodynamic approximations for the correlation functions of the Laughlin state and the quasihole state, we evaluate the gap in a two-dimensional system of dipolar atoms exposed to an artificial gauge field. For Abelian fields, our results agree well with the results of exact diagonalization for small systems but indicate that the large value of the gap predicted [ Phys. Rev. Lett. 94 070404 (2005)] was overestimated. However, we are able to show that the small gap found in the Abelian scenario dramatically increases if we turn to non-Abelian fields squeezing the Landau levels.
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S. Diehl, E. Rico Ortega, M. Baranov, P. Zoller Topology by Dissipation in Atomic Quantum Wires,
Nature Phys. 7 971 (2011-10-02),
http://dx.doi.org/10.1038/nphys2106 doi:10.1038/nphys2106 (ID: 717686)
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Robust edge states and non-Abelian excitations are the trademark of topological states of matter, with promising applications such as "topologically protected" quantum memory and computing. While so far topological phases have been exclusively discussed in a Hamiltonian context, we show that such phases and the associated topological protection and phenomena also emerge in open quantum systems with engineered dissipation. The specific system studied here is a quantum wire of spinless atomic fermions in an optical lattice coupled to a bath. The key feature of the dissipative dynamics described by a Lindblad master equation is the existence of Majorana edge modes, representing a non-local decoherence free subspace. The isolation of the edge states is enforced by a dissipative gap in the p-wave paired bulk of the wire. We describe dissipative non-Abelian braiding operations within the Majorana subspace, and we illustrate the insensitivity to imperfections. Topological protection is granted by a nontrivial winding number of the system density matrix.
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I. Titvinidze, A. Privitera, S. Chang, S. Diehl, M. Baranov, A. J. Daley, W. Hofstetter Magnetism and domain formation in SU(3)-symmetric multi-species Fermi mixtures,
New J. Phys. 13 035013 (2011-03-16),
http://dx.doi.org/10.1088/1367-2630/13/3/035013 doi:10.1088/1367-2630/13/3/035013 (ID: 717395)
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We study the phase diagram of an SU(3)-symmetric mixture of three-component ultracold fermions with attractive interactions in an optical lattice, including the additional effect on the mixture of an effective three-body constraint induced by three-body losses. We address the properties of the system in $D \\geq 2$ by using dynamical mean-field theory and variational Monte Carlo techniques. The phase diagram of the model shows a strong interplay between magnetism and superfluidity. In the absence of the three-body constraint (no losses), the system undergoes a phase transition from a color superfluid phase to a trionic phase, which shows additional particle density modulations at half-filling. Away from the particle-hole symmetric point the color superfluid phase is always spontaneously magnetized, leading to the formation of different color superfluid domains in systems where the total number of particles of each species is conserved. This can be seen as the SU(3) symmetric realization of a more general tendency to phase-separation in three-component Fermi mixtures. The three-body constraint strongly disfavors the trionic phase, stabilizing a (fully magnetized) color superfluid also at strong coupling. With increasing temperature we observe a transition to a non-magnetized SU(3) Fermi liquid phase.
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M. Cordin, B. A. Lechner, P. Amann, A. Menzel, E. Bertel, C. Franchini, R. Zucca, J. Redinger, M. Baranov, S. Diehl Phase transitions driven by competing interactions in low-dimensional systems,
Europhysics Letters 92 26004 (2010-11-16),
http://dx.doi.org/10.1209/0295-5075/92/26004 doi:10.1209/0295-5075/92/26004 (ID: 717348)
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Variable-temperature scanning tunnelling microscopy is used to study an order-order phase transition in a virtually defect-free quasi–one-dimensional surface system. The phase transition is driven by competing electronic interactions. The phase diagram is captured by a modified Landau formalism containing a coupling term between two different subsystems. The extra term has the effect of a spontaneously generated field which drives the phase transition. The proposed formalism applies to a variety of problems, where competing interactions produce sometimes counter-intuitive ordering phenomena.
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A. Micheli, Z. Idziaszek, G. Pupillo, M. Baranov, P. Zoller, P. S. Julienne Universal rates for reactive ultracold polar molecules in reduced dimensions,
Phys. Rev. Lett. 105 073202 (2010-08-13),
http://dx.doi.org/10.1103/PhysRevLett.105.073202 doi:10.1103/PhysRevLett.105.073202 (ID: 717207)
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Analytic expressions describe universal elastic and reactive rates of quasi-two-dimensional and quasi-one-dimensional collisions of highly reactive ultracold molecules interacting by a van der Waals potential. Exact and approximate calculations for the example species of KRb show that stability and evaporative cooling can be realized for spin-polarized fermions at moderate dipole and trapping strength, whereas bosons or unlike fermions require significantly higher dipole or trapping strengths.
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S. Diehl, M. Baranov, A. J. Daley, P. Zoller Quantum Field Theory for the Three-Body Constrained Lattice Bose Gas -- Part II: Application to the Many-Body Problem,
Phys. Rev. B 82 064510 (2010-08-13),
http://dx.doi.org/10.1103/PhysRevB.82.064510 doi:10.1103/PhysRevB.82.064510 (ID: 716840)
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We analyze the ground state phase diagram of attractive lattice bosons, which are stabilized by a three-body onsite hardcore constraint. A salient feature of this model is an Ising type transition from a conventional atomic superfluid to a dimer superfluid with vanishing atomic condensate. The study builds on an exact mapping of the constrained model to a theory of coupled bosons with polynomial interactions, proposed in a related paper [11]. In this framework, we focus by analytical means on aspects of the phase diagram which are intimately connected to interactions, and are thus not accessible in a mean field plus spin wave approach. First, we determine shifts in the mean field phase border, which are most pronounced in the low density regime. Second, the investigation of the strong coupling limit reveals the existence of a new collective mode, which emerges as a consequence of enhanced symmetries in this regime. Third, we show that the Ising type phase transition, driven first order via the competition of long wavelength modes at generic fillings, terminates into a true Ising quantum critical point in the vicinity of half filling.
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S. Diehl, M. Baranov, A. J. Daley, P. Zoller Quantum Field Theory for the Three-Body Constrained Lattice Bose Gas -- Part I: Formal Developments,
Phys. Rev. B 82 064509 (2010-08-13),
http://dx.doi.org/10.1103/PhysRevB.82.064509 doi:10.1103/PhysRevB.82.064509 (ID: 716839)
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We develop a quantum field theoretical framework to analytically study the three-body constrained Bose-Hubbard model beyond mean field and non-interacting spin wave approximations. It is based on an exact mapping of the constrained model to a theory with two coupled bosonic degrees of freedom with polynomial interactions, which have a natural interpretation as single particles and two-particle states. The procedure can be seen as a proper quantization of the Gutzwiller mean field theory. The theory is conveniently evaluated in the framework of the quantum effective action, for which the usual symmetry principles are now supplemented with a ``constraint principle'' operative on short distances. We test the theory via investigation of scattering properties of few particles in the limit of vanishing density, and we address the complementary problem in the limit of maximum filling, where the low lying excitations are holes and di-holes on top of the constraint induced insulator. This is the first of a sequence of two papers. The application of the formalism to the many-body problem, which can be realized with atoms in optical lattices with strong three-body loss, is performed in a related work [13].
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S. Diehl, M. Baranov, A. J. Daley, P. Zoller Observability of Quantum Criticality and a Continuous Supersolid in Atomic Gases,
Phys. Rev. Lett. 104 165301 (2010-04-20),
http://dx.doi.org/10.1103/PhysRevLett.104.165301 doi:10.1103/PhysRevLett.104.165301 (ID: 716766)
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We analyze the Bose-Hubbard model with three-body onsite hardcore constraint, which stabilizes the system for an attractive interparticle interaction and allows, in particular, the formation of a superfluid phase of bosonic dimers. Our approach is based on an exact mapping of the constrained Hamiltonian to a theory of two coupled bosonic degrees of freedom. We demonstrate that the phase transition between atomic and dimer superfluidity is generically of the first order as a result of the Coleman-Weinberg phenomenon, while at unit filling we identify an Ising quantum critical point. At this filling, furthermore, a symmetry enhancement in the strong coupling limit leads to a continuous supersolid phase for deeply bound dimers, observable in experiments.
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M. Baranov, A. Micheli, S. Ronen, P. Zoller Bilayer superfluidity of fermionic polar molecules: many body effects,
Phys. Rev. A 83 043602 (2010-02-24),
http://dx.doi.org/10.1103/PhysRevA.83.043602 doi:10.1103/PhysRevA.83.043602 (ID: 717481)
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We study the BCS superfluid transition in a single-component fermionic gas of dipolar particles loaded in a tight bilayer trap, with the electric dipole moments polarized perpendicular to the layers. Based on the detailed analysis of the interlayer scattering, we calculate the critical temperature of the interlayer superfluid pairing transition when the layer separation is both smaller (dilute regime) and of the order or larger (dense regime) than the mean interparticle separation in each layer. Our calculations go beyond the standard BCS approach and include the many-body contributions resulting in the mass renormalization, as well as additional contributions to the pairing interaction. We find that the many-body effects have a pronounced effect on the critical temperature, and can either decrease (in the very dilute limit) or increase (in the dense and moderately dilute limits) the transition temperature as compared to the BCS approach.
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P. Amann, M. Cordin, C. Braun, B. A. Lechner, A. Menzel, E. Bertel, C. Franchini, R. Zucca, J. Redinger, M. Baranov, S. Diehl Electronically driven phase transitions in a quasi-one-dimensional adsorbate system,
Eur. Phys. J. B (2010-02-02),
http://dx.doi.org/10.1140/epjb/e2010-00026-5 doi:10.1140/epjb/e2010-00026-5 (ID: 717176)
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A quasi-1D system is prepared using the Pt(110) surface as a template. The electronic surface resonance structure is studied by angle-resolved photoemission spectroscopy for the clean surface as well as for different Bromine coverages. A Fermi surface mapping reveals saddle points at the Fermi level in the interior of the surface Brillouin zone. Correspondingly, a maximum in the static response function χ(q, 0) at the connecting vector q is expected. With 1/2Gx < q < 2/3Gx one observes indeed a 3-fold periodicity around defects and a 2-fold periodicity at low temperature for ΘBr = 0.5 ML. Cooling of a defect-free c(2×2)-Br/Pt(110) preparation counter-intuitively results in a loss of long-range order. Motivated by DFT calculations this is attributed to an anomalous order-order phase transition into the (2×1) phase accompanied by intense, strongly anisotropic fluctuations within a temperature range of ~200 K. The peculiar behaviour is rationalised in terms of a competition between inter-adsorbate repulsion and an adsorbate triggered 2kF interaction in the substrate.
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M. Baranov, C. Lobo, G. V. Shlyapnikov Superfluid pairing between fermions with unequal masses,
Phys. Rev. A 78 033620 (2008-09-29),
http://dx.doi.org/10.1103/PhysRevA.78.033620 doi:10.1103/PhysRevA.78.033620 (ID: 593993)
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We consider a superfluid state in a two-component gas of fermionic atoms with equal densities and unequal masses in the BCS limit. We develop a perturbation theory along the lines proposed by Gorkov and Melik-Barkhudarov and find that for a large difference in the masses of heavy ($M$) and light ($m$) atoms one has to take into account both the second-order and third-order contributions. The result for the critical temperature and order parameter is then quite different from the prediction of the simple BCS approach. Moreover, the small parameter of the theory turns out to be $(p_{F}|a|)/\hbar)\sqrt{M/m}\ll1$, where $p_{F}$ is the Fermi momentum, and $a$ the scattering length. Thus, for a large mass ratio $M/m$ the conventional perturbation theory requires significantly smaller Fermi momenta (densities) or scattering lengths than in the case of $M\sim m$, where the small parameter is $(p_{F}|a|)/\hbar)\ll1$. We show that 3-body scattering resonances appearing at a large mass ratio due to the presence of 3-body bound Efimov states do not influence the result, which in this sense becomes universal.
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M. Baranov, H. Fehrmann, M. Lewenstein Wigner Crystallization in Rapidly Rotating 2D Dipolar Fermi Gases,
Phys. Rev. Lett. 100 200402 (2008-05-20),
http://dx.doi.org/10.1103/PhysRevLett.100.200402 doi:10.1103/PhysRevLett.100.200402 (ID: 593996)
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We study the competition between the Wigner crystal and the Laughlin liquid states in an ultracold quasi-two-dimensional rapidly rotating polarized fermionic dipolar gas, and find that the Wigner crystal has a lower energy below a critical filling factor. We examine the quantum crystal to liquid transition for different confinements in the third direction. Our analysis of the phonon spectra of the Wigner crystal taking into account the phonon-phonon interactions also shows the stability of the Wigner crystal for sufficiently low filling factors (<1/7).
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M. Baranov Theoretical progress in many-body physics with ultracold dipolar gases,
Physics Reports 464 111 (2008-05-05),
http://dx.doi.org/10.1016/j.physrep.2008.04.007 doi:10.1016/j.physrep.2008.04.007 (ID: 610896)
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Recent experimental progress in trapping and cooling of molecular gases boosts interest in the interdisciplinary field of quantum gases with dominant dipole–dipole interactions. An unprecedented level of experimental control together with specific physical properties of dipole–dipole interaction provide a unique possibility to find new physical phenomena and practical applications.
In this review, recent achievements in theoretical studies of ultracold dipolar gases, both fermionic and bosonic, are presented. We focus our attention on many-body properties of such systems and discuss how the characteristic features of dipole–dipole interaction, long range and anisotropy, affect their collective behavior and result in novel macroscopic quantum phenomena. The consideration covers spatially homogeneous and trapped cases, and includes analysis of the properties of dipolar gases in both the mean-field regime (dipolar Bose–Einstein condensates and superfluid BCS pairing transition) and in the strongly correlated one (dipolar gases in optical lattices and in rotating traps).