Fulde-Ferrell Superfluids without Spin Imbalance in Driven Optical Lattices

Abstract

Spin-imbalanced ultracold Fermi gases have been widely studied recently as a platform for exploring the long-sought Fulde-Ferrell-Larkin-Ovchinnikov superfluid phases, but so far conclusive evidence has not been found. Here we propose to realize an Fulde-Ferrell (FF) superfluid without spin imbalance in a three-dimensional fermionic cold atom optical lattice, where s-and p-orbital bands of the lattice are coupled by another weak moving optical lattice. Such coupling leads to a spin-independent asymmetric Fermi surface, which, together with the s-wave scattering interaction between two spins, yields an FF type of superfluid pairing. Unlike traditional schemes, our proposal does not rely on the spin imbalance (or an equivalent Zeeman field) to induce the Fermi surface mismatch and provides a completely new route for realizing FF superfluids.PACS numbers: 03.75. Ss, 74.20.Fg The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, characterized by Cooper pairs with finite center-of-mass momenta [1,2], is a central concept for understanding many exotic phenomena in different physics branches [3]. A crucial ingredient for realizing FFLO states is a large Zeeman field that induces a Fermi surface mismatch of two paired spins [1,2]. In recent years, FFLO states have been extensively studied in ultracold Fermi gases, where the population imbalance between two atomic internal states (pseudo-spins) serves as an effective Zeeman field [4][5][6][7][8][9][10][11][12][13][14]. Despite the intrinsic advantages of cold atoms compared to their solid state counterparts, conclusive evidence of FFLO states has not been found yet because of various obstacles. For instance, a large Zeeman field suppresses the superfluid order parameter, leading to a very narrow parameter region for FFLO states in 2D or 3D which can be easily destroyed by thermodynamic fluctuations [4][5][6]. In 1D, the parameter region for FFLO states could be large, but the quantum fluctuation is strong [7,12,14]. The recently proposed schemes using spin-orbit coupling and in-plane Zeeman field in a 3D Fermi gas may potentially overcome these obstacles [15][16][17][18][19][20][21] in principle, but they face practical experimental issues such as the large spontaneous photon emission from the near-resonant Raman lasers [22][23][24][25][26][27][28][29][30][31] and the strong three-body loss at Feshbach resonance in the presence of spin-orbit coupling [23][24][25][26].In this Letter, we propose a new route for realizing FF superfluids in ultracold Fermi gases without involving population imbalance of two spin states that interact for generating Cooper pairing. Instead, we induce an asymmetric Fermi surface for the generation of FF states by other means and the populations of the two spins are fully equal. Our main results are the following: 1) We show that the s-and p x -orbital bands of a 3D static optical lattice can be coupled using a weak 1D moving optical lattice along the x direction, which can be generated by two counter-propagating lasers with the frequency differ...

Fulde–Ferrell superfluids in spinless ultracold Fermi gases

“…Due to the stringent conditions on materials, the evidence of the FF state in condensed matters is still pending. On the other hand, in the past few years, it opens an alternative way in synthesizing the FF superfluids in cold atoms, by taking advantages of anisotropic optical lattices [18], spin-dependent optical lattices [19], spin-orbital couplings (SOC) [20][21][22][23][24][25], periodic-driven optical lattices [26], multi-orbital interactions [27], the optical control of Feshbach resonances [28], and instantaneous spin imbalance via radiofrequency fields [29]. The series of investigations in cold atoms reveals that the FF superfluids can originate from the distortion of Fermi surfaces instead of large spin imbalance [15][16][17]29], which is expected to facilitate its observation in cold-atom experiments.…”

“…So far in searching FF superfluids in cold atoms, the earlier advances [18][19][20][21][22][23][24][25][26][27][28] bear similarities that they all focus on a spinful system. In these systems, the contact interaction between opposite pseudo-spin atoms plays the key role for the superfluid phases.…”

Synthetic-gauge-field-induced resonances and Fulde-Ferrell-Larkin-Ovchinnikov states in a one-dimensional optical lattice

“…Realization of the Tonks-Girardeau gas of hard-core bosons [14] and a quantum Newton's cradle [15] in one dimension (1D) and Kosterlitz-Thouless transition [16] in 2D are a few of such examples. But despite extensive theoretical [17][18][19][20][21] and experimental [22][23][24] efforts, a direct observation of an FFLO state still remains elusive. It is hindered by technical limitations in 3D and 2D [22,23,25], but the 1D Fermi gases with population imbalance [13,[26][27][28][29][30][31][32][33] are believed to be the most suitable candidates (with already an indirect observation reported in Ref.…”

Spin-momentum coupled Bose-Einstein condensates with lattice band pseudospins

“…We show that the moving lattice generates two types of coupling between s- and p -band pseudospins: a momentum-independent Rabi coupling (Ω σ x ) and SMC ( ασ x sin( q x d ), where q x is the quasimomentum and d the lattice period), with strengths of the same order. The coexistence of these two types of coupling leads to asymmetric FB band dispersions 38 . We investigate the FB band structures by measuring the quasimomentum of the BEC.…”