Evolution from BCS to BEC superfluidity in the presence of spin-orbit coupling

Han, Lin‐Hai; Melo, C. A. R. Sá de

doi:10.1103/physreva.85.011606

Cited by 95 publications

(112 citation statements)

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“…The successful realization of SOC in both Bose-Einstein condensate (BEC) [4,5] and Fermi gas [6,7] opens up a new avenue towards studying the rich physics of spin-orbit (SO) coupled ultracold atoms [8][9][10][11][12][13]. One of the important advances is that SOC was shown to have fundamental effects on the superfluidity of 3D [14][15][16][17][18][19] and 2D [19,20] continuous Fermi gases.On the other hand, the attractive Fermi gas subjected to an optical lattice [21,22] has made it possible to simulate the negative-U Hubbard model, a basic model for the superconductivity of many solid state materials [23]. In particular, the on-site attractions can induce deep bound states, which cause the conventional BCS-BEC crossover.…”

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confidence: 99%

Spin-orbit coupling effects on the superfluidity of a Fermi gas in an optical lattice

et al. 2013

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We investigate the superfluidity of attractive Fermi gas in a square optical lattice with spin-orbit coupling (SOC). We show that the system displays a variety of new filling-dependent features. At half filling, a quantum phase transition from a semimetal to a superfluid is found for large SOC. Close to half filling where the emerging Dirac cones governs the behaviors of the system, SOC tends to suppress the BCS superfluidity. Conversely, SOC can significantly enhance both the pairing gap and condensate fraction and lead to a new BCS-BEC crossover for small fillings. Moreover, we demonstrate that the superfluid fraction also exhibits many interesting phenomena compared with the spin-orbit coupled Fermi gas without lattice. The spin-orbit coupling (SOC) plays a central role in the investigation of novel topological states in solid state physics [1,2]. This has stimulated tremendous interests in creating artificial non-Abelian gauge fields in ultracold atom systems [3]. The successful realization of SOC in both Bose-Einstein condensate (BEC) [4,5] and Fermi gas [6,7] opens up a new avenue towards studying the rich physics of spin-orbit (SO) coupled ultracold atoms [8][9][10][11][12][13]. One of the important advances is that SOC was shown to have fundamental effects on the superfluidity of 3D [14][15][16][17][18][19] and 2D [19,20] continuous Fermi gases.On the other hand, the attractive Fermi gas subjected to an optical lattice [21,22] has made it possible to simulate the negative-U Hubbard model, a basic model for the superconductivity of many solid state materials [23]. In particular, the on-site attractions can induce deep bound states, which cause the conventional BCS-BEC crossover. Recently, SOC has been combined to optical lattices for repulsive ultracold gases and predicted to lead to many interesting phenomena [24][25][26]. Nevertheless, the superfluidity of SO coupled attractive Fermi gas in an optical lattice remains a new frontier to be explored.In this Letter, we study the Fermi gas subjected to a square optical lattice with SOC. Such a system can be described by a generalized negative-U Hubbard model. We show that, the combination of SOC and lattice can give rise to various new features that depend on the fillings. Remarkably, there develops a quantum phase transition (QPT) from a semimetal to a superfluid for large SOC at half filling, with the critical interaction U c /t ≃ 3.11 (t is the hopping amplitude). For close to half filling, we show that the emerging Dirac cones governs the behaviors of the system, which tends to suppress the BCS superfluidity. By contrary, SOC can significantly enhance both the pairing gap and condensate fraction and lead to a new BCS-BEC crossover for small fillings. Compared with the SO coupled Fermi gas without lattice, such opposite filling-dependent behavior of SOC is rather unique as it can only be induced in the lattice system. Furthermore, we investigate the superfluid fraction, which also exhibits many unusual characteristics in contrast to the continuous Fermi g...

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confidence: 99%

Spin-orbit coupling effects on the superfluidity of a Fermi gas in an optical lattice

et al. 2013

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“…In addition, these recent works on thermodynamic systems showed that, unlike the α = 0 limit where the unpolarized superfluid phase is gapped and polarized superfluid phase is gapless, α = 0 allows the possibility of having a gapped polarized superfluid phase up to a critical polarization, depending on the particular value of α [14][15][16][17][18][19]. Therefore, when α = 0, in contrast to the topologically trivial unpolarized and low-polarized superfluid phases, the polarized superfluid phase with sufficiently high polarization becomes topologically nontrivial, and has gapless quasiparticle/quasihole excitations.…”

Section: B Population-imbalanced Fermi Gasesmentioning

confidence: 99%

“…On the other hand, for population-imbalanced uniform systems, the BCS-BEC evolution is not a crossover, and quantum phase transitions are found between thermodynamically stable and topologically distinct gapped and gapless superfluid phases. These phases are distinguished in momentum space by their numbers of zero-energy points, rings or surfaces (depending on the type of spin-orbit coupling) in their quasiparticle/quasihole excitation spectrum [13][14][15][16][17][18][19].…”

Section: Introductionmentioning

confidence: 99%

Trapped Fermi gases with Rashba spin-orbit coupling in two dimensions

Iskin¹

2012

Phys. Rev. A

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We use the Bogoliubov-de Gennes formalism to analyze harmonically trapped Fermi gases with Rashbatype spin-orbit coupling in two dimensions. We consider both population-balanced and -imbalanced Fermi gases throughout the BCS-BEC evolution, and study the effects of spin-orbit coupling on the spontaneously induced countercirculating mass currents and the associated intrinsic angular momentum. In particular, we find that even a small spin-orbit coupling destabilizes Fulde-Ferrel-Larkin-Ovchinnikov (FFLO)-type spatially modulated superfluid phases as well as the phase-separated states against the polarized superfluid phase. We also show that the continuum of quasiparticle and quasihole excitation spectrum can be connected by zero, one or two discrete branches of interface modes, depending on the number of interfaces between a topologically trivial phase (e.g. locally unpolarized/low-polarized superfluid or spin-polarized normal) and a topologically nontrivial one (e.g. locally high-polarized superfluid) that may be present in a trapped system.

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“…The experimental realization of spin-orbit coupling (SOC) in ultracold atoms [1][2][3] has initiated substantial theoretical efforts to study its effects in 2D and 3D Fermi gases [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] . This is because SOC systems with a Zeeman field, which breaks the population balance, can lead to novel types of exotic superfluid phases that may support Majorana fermions.…”

Section: Introductionmentioning

confidence: 99%

Spin-orbit-coupled atomic Fermi gases in two-dimensional optical lattices in the presence of a Zeeman field

Koinov

Pahl

2017

Phys. Rev. A

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We investigate the single-particle and collective excitations of a Rashba spin-orbit coupled atomic Fermi gas with attractive interaction, loaded in a two dimensional (2D) square optical lattice, in the presence of an effective out-of-plane Zeeman field. Our numerical calculations show that the many body physics of the Bardeen-Cooper-Schrieffer (BCS) side is strongly modified compared to the Fermi gases in the free space. The physics behind this statement is in the fact that without a lattice structure, if the value of the Zeeman field does not exceed some threshold value, the minimum of the single-particle ground state energy is infinitely degenerate and occurs along a ring in (kx, ky) space. This reduces the effective dimensionality; the single-particle density of states is a constant at low energies, and the molecular pairing is strongly enhanced. In contrast to the continuum, in the 2D lattice case there are only four degenerate minima, and because of that, the spin-orbit coupling (SOC) in optical lattices gives rise to unusual properties entirely different from the continuum. For example, in the continuum, the pairing gap as well as the condensate fraction are strongly enhanced by the SOC strength on the BCS side. In the presence of lattice geometry the gap and the condensate fraction increase as a function of the SOC strength only at the small fillings and weak attraction limit. Moreover, we found that the speed of sound also exhibits different behavior: in the 3D and 2D continuum, the slope of the Goldstone mode decreases as a function of the SOC strength, while in the lattice case the speed of sound increases monotonically with SOC strength.

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Evolution from BCS to BEC superfluidity in the presence of spin-orbit coupling

Cited by 95 publications

References 38 publications

Spin-orbit coupling effects on the superfluidity of a Fermi gas in an optical lattice

Spin-orbit coupling effects on the superfluidity of a Fermi gas in an optical lattice

Trapped Fermi gases with Rashba spin-orbit coupling in two dimensions

Spin-orbit-coupled atomic Fermi gases in two-dimensional optical lattices in the presence of a Zeeman field

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