Phase Separation and Pair Condensation in a Spin-Imbalanced 2D Fermi Gas

Abstract

We study a two-component quasi-two-dimensional Fermi gas with imbalanced spin populations. We probe the gas at different interaction strengths and polarizations by measuring the density of each spin component in the trap and the pair momentum distribution after time of flight. For a wide range of experimental parameters, we observe in-trap phase separation characterized by the appearance of a spin-balanced core surrounded by a polarized gas. Our momentum space measurements indicate pair condensation in the imbalanced gas even for large polarizations where phase separation vanishes, pointing to the presence of a polarized pair condensate. Our observation of zero momentum pair condensates in 2D spin-imbalanced gases opens the way to explorations of more exotic superfluid phases that occupy a large part of the phase diagram in lower dimensions.Fermionic superfluids described by standard BardeenCooper-Schrieffer theory are momentum-space condensates of Cooper pairs of opposite spins. Imbalancing the chemical potentials of the two spin species disrupts the Cooper pairing mechanism and can give rise to many interesting scenarios. For a small difference in the chemical potentials, the Fermi gas remains a spin-balanced superfluid. As the chemical potential imbalance is increased, it eventually becomes comparable to the superfluid gap. At this point, known as the Clogston-Chandrasekhar limit [1], the gas becomes polarized but superfluidity may persist due to the presence of exotic superfluid phases such as the Sarma [2] or FFLO phase [3,4]. Eventually, for large enough chemical potential difference, superfluidity is completely destroyed. The stability of some exotic superfluid phases like FFLO is greatly enhanced by lowering the dimensionality of the gas [5,6].The search for exotic superfluids motivates our study of spin-imbalanced atomic Fermi gases in two dimensions. In addition, the 2D case becomes particularly interesting in the case of strong interactions [7][8][9][10]. In an atomic gas, Feshbach resonances enable tuning the interactions over a wide range and studying the effect of chemical potential imbalance beyond the described weak coupling BCS limit. Unlike the 1D case, exact solutions do not exist, and mean field models that do well in 3D fail in 2D due to the enhanced role of quantum fluctuations [11,12].Spin-imbalanced Fermi gases have been extensively studied both theoretically [13][14][15][16][17] and experimentally [18]. Experiments in 3D have observed vortex lattices in spin-imbalanced superfluids [19] as well as phase separation between the superfluid and normal phases in the trapped gas [20][21][22]. Subsequent experiments quantitatively mapped out the phase diagram of the 3D gas [23,24] and measured the equation of state of the imbalanced gas [25,26]. In 1D, phase separation was also observed, displaying an inverted phase profile in the trap compared to 3D [27]. Recent experiments have started to explore 2D Fermi gases [28][29][30][31][32][33][34][35][36][37], mostly focusing on the spin-balanced case...

Spin-imbalance in a 2D Fermi-Hubbard system

“…The global spin imbalance P = (N ↑ − N ↓ )/(N ↑ + N ↓ ) can be varied continuously from 0 to ≈ 0.9 by evaporating the gas in the presence of a magnetic gradient leading to preferential loss of one of the spin states. We work at a scattering length of 448(9) a 0 , where a 0 is the Bohr radius, obtained by adjusting a bias magnetic field in the vicinity of a broad Feshbach resonance centered at 690 G. The imbalanced mixture is prepared in a single layer (for details see [25]) and subsequently loaded adiabatically into a 2D square lattice of depth 10.5(3) E R , where t = h×450(25) Hz. Here, E R = h×14.66 kHz is the recoil energy.…”

Two-dimensional spin-imbalanced Fermi gases at nonzero temperature: Phase separation of a noncondensate

“…Note Added.-Near completion of the manuscript we learned of a recent preprint [9] which addressed experimentally many of the same findings as contained in our manuscript.…”

“…The former are applicable to the quasi-condensation regime discussed below, while the latter are closer to the regimes studied experimentally [8,9]. We consider an intermediate binding energy E F 1 / B = 0.75.…”

“…Their utility comes from their tunability [1] which allows the dimensionality, band structure, interaction strength, and spin imbalance to be freely varied. With these various parameters one can, in principle, simulate a number of important condensed matter systems ranging from preformed pair and related effects in the high T c cuprates [2-4] to intrinsic topological superfluids [5][6][7] and other exotic pairing states.In this paper we focus on recent experiments [8,9] of two-dimensional (2D) spin-imbalanced Fermi gases. These address the interesting conflict between the tendency towards enhanced pairing (which is associated with two dimensionality [10]), and spin imbalance which acts to greatly undermine pairing.…”

“…In this paper we focus on recent experiments [8,9] of two-dimensional (2D) spin-imbalanced Fermi gases. These address the interesting conflict between the tendency towards enhanced pairing (which is associated with two dimensionality [10]), and spin imbalance which acts to greatly undermine pairing.…”