Pairing in a two-component ultracold Fermi gas: Phases with broken-space symmetries

Sedrakian, Armen; Mur-Petit, Jordi; Polls, A.; Müther, H.

doi:10.1103/physreva.72.013613

Cited by 104 publications

(97 citation statements)

References 61 publications

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“…Asymptotically, however, at arbitrary low temperature the 2d LO state is unstable to proliferation of dislocations [142]. The state that results from such dislocated superfluid smectic is either a "charge"-4 (paired Cooper pairs) [143] nematic superfluid [112,119] or a nematic (possibly "fractionalized"b) Fermi liquid [117,126], latter qualitatively the same as the deformed Fermi surface state [49].…”

Section: B Resultsmentioning

confidence: 99%

“…A nonzero species imbalance frustrates conventional BCS pairing of a two-species Fermi gas [38][39][40][41] and the associated BCS-BEC crossover [42][43][44][45][46], driving quantum phase transitions out of a paired superfluid to a variety of interesting possible ground states and thermodynamic phases [44,[46][47][48][49][50][51]. This rekindled considerable theoretical activity in the context of species-imbalanced resonant Fermi gases [4,.…”

Section: Imbalanced Resonant Atomic Gasesmentioning

confidence: 99%

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Fluctuations and phase transitions in Larkin-Ovchinnikov liquid-crystal states of a population-imbalanced resonant Fermi gas

Radzihovsky

2011

Phys. Rev. A

122

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Motivated by a realization of imbalanced Feshbach-resonant atomic Fermi gases, we formulate a low-energy theory of the Fulde-Ferrell and the Larkin-Ovchinnikov (LO) states and use it to analyze fluctuations, stability, and phase transitions in these enigmatic finite momentum-paired superfluids. Focusing on the unidirectional LO pair-density wave state, that spontaneously breaks the continuous rotational and translational symmetries, we show that it is characterized by two Goldstone modes, corresponding to a superfluid phase and a smectic phonon. Because of the liquidcrystalline "softness" of the latter, at finite temperature the 3d state is characterized by a vanishing LO order parameter, quasi-Bragg peaks in the structure and momentum distribution functions, and a "charge"-4, paired Cooper-pairs, off-diagonal-long-range order, with a superfluid-stiffness anisotropy that diverges near a transition into a nonsuperfluid state. In addition to conventional integer vortices and dislocations the LO superfluid smectic exhibits composite half-integer vortexdislocation defects. A proliferation of defects leads to a rich variety of descendant states, such as the "charge"-4 superfluid and Fermi-liquid nematics and topologically ordered nonsuperfluid states, that generically intervene between the LO state and the conventional superfluid and the polarized Fermi-liquid at low and high imbalance, respectively. The fermionic sector of the LO gapless superconductor is also quite unique, exhibiting a Fermi surface of Bogoliubov quasiparticles associated with the Andreev band of states, localized on the array of the LO domain-walls.

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Section: B Resultsmentioning

confidence: 99%

Section: Imbalanced Resonant Atomic Gasesmentioning

confidence: 99%

Fluctuations and phase transitions in Larkin-Ovchinnikov liquid-crystal states of a population-imbalanced resonant Fermi gas

Radzihovsky

2011

Phys. Rev. A

122

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“…The resulting state is either a "charge"-4 (four-fermion) superfluid or a nonsuperfluid nematic, depending on the relative energetics of aforementioned integer and half-integer vortexdislocation defects. The latter normal nematic state is a (complementarily described [22]) deformed Fermi surface state [27,28].…”

Section: Figmentioning

confidence: 99%

Quantum Liquid Crystals in an Imbalanced Fermi Gas: Fluctuations and Fractional Vortices in Larkin-Ovchinnikov States

2009

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We develop a low-energy model of a unidirectional Larkin-Ovchinnikov (LO) state. Because the underlying rotational and translational symmetries are broken spontaneously, this gapless superfluid is a smectic liquid crystal, that exhibits fluctuations that are qualitatively stronger than in a conventional superfluid, thus requiring a fully nonlinear description of its Goldstone modes. Consequently, at nonzero temperature the LO superfluid is an algebraic phase even in 3d. It exhibits half-integer vortex-dislocation defects, whose unbinding leads to transitions to a superfluid nematic and other phases. In 2d at nonzero temperature, the LO state is always unstable to a nematic superfluid. We expect this superfluid liquid-crystal phenomenology to be realizable in imbalanced resonant Fermi gases trapped isotropically.The tunability of interactions through Feshbach resonances has led to a realization of an s-wave paired superfluidity and BCS-BEC crossover [1,2,3], as well as promises of more exotic states such as gapless pwave[4] and periodic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) [5,6] superfluidity in strongly correlated degenerate alkali gases. The latter enigmatic state has been thoroughly explored within a BCS mean-field studies [7,8,9] and is expected to be realizable in a population-imbalanced (polarized) Feshbach resonant Fermi gas [10,11]. While recent experiments [12,13], have confirmed much of the predicted phenomenology of phase separation [11,14,15] in such systems, the FFLO states have so far eluded definitive observation.The simplest mean-field treatments [5,6,11] find that the FFLO type states are quite fragile, confined to a narrow range of polarization on the BCS side. However, motivated by earlier studies [7,8] and based on the finding of a negative domain-wall energy in an otherwise uniform singlet BCS superfluid [9,16], it has recently been argued that a more general periodic superfluid state that includes a larger set of collinear wavevectors may be significantly more stable. Much like a type-II superconductor undergoes a continuous transition into a vortex state at a lower-critical field H c1 , which is significantly below the thermodynamic field, here too, a Zeeman-field driven domain-wall nucleation (with the density increasing above the lower critical h c1 field) allows a continuous mechanism for a transition from a singlet paired superfluid to a LO-like periodic state [7,8,9,16].In this scenario the SF-LO transition is of a commensurate-incommensurate type as can be explicitly shown in one dimension (1d) [7,17]. The imposed species imbalance (excess of the majority fermionic atoms) can be continuously accommodated by the subgap states localized on the self-consistently induced domain-walls, with this picture resembling the doping of polyacetylene [18]. Such LO state can also be thought of as a periodically ordered micro-phase separation between the normal and BCS states, that thus naturally replaces the macro-phase separation ubiquitously found in the BEC-BCS detuning-polarization phase diag...

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“…These studies with two-component Fermi gas include the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover [1,2] with equal population two-component Fermi gas and the effects of population imbalance on the superfluid state [2,3,4,5,6,7,8,9]. For Fermi gas with population imbalance, various pairing scenarios are proposed: Fulde-Ferrel-Larkin-Ovchinnikov phase FFLO [10,11], breached pairing [12], phase separation [13] and pairing with deformed fermi surface [14].…”

Section: Introductionmentioning

confidence: 99%

Trap- and population-imbalanced two-component Fermi gas in the Bose-Einstein-condensate limit

Silotri

2010

Phys. Rev. A

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We study equal mass population imbalanced two-component atomic Fermi gas with unequal trap frequencies (ω ↑ = ω ↓ ) at zero temperature using the local density approximation (LDA). We consider the strongly attracting Bose-Einstein condensation (BEC) limit where polarized (gapless) superfluid is stable. The system exhibits shell structure: unpolarized SF→polarized SF→normal N. Compared to trap symmetric case, when the majority component is tightly confined the gapless superfluid shell grows in size leading to reduced threshold polarization to form polarized (gapless) superfluid core. In contrast, when the minority component is tightly confined, we find that the superfluid phase is dominated by unpolarized superfluid phase with gapless phase forming a narrow shell. The shell radii for various phases as a function of polarization at different values of trap asymmetry are presented and the features are explained using the phase diagram.

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Pairing in a two-component ultracold Fermi gas: Phases with broken-space symmetries

Cited by 104 publications

References 61 publications

Fluctuations and phase transitions in Larkin-Ovchinnikov liquid-crystal states of a population-imbalanced resonant Fermi gas

Fluctuations and phase transitions in Larkin-Ovchinnikov liquid-crystal states of a population-imbalanced resonant Fermi gas

Quantum Liquid Crystals in an Imbalanced Fermi Gas: Fluctuations and Fractional Vortices in Larkin-Ovchinnikov States

Trap- and population-imbalanced two-component Fermi gas in the Bose-Einstein-condensate limit

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