Concomitant modulated superfluidity in polarized Fermi gases

Abstract

Recent groundbreaking experiments studying the effects of spin polarization on pairing in unitary Fermi gases encountered mutual qualitative and quantitative discrepancies which seem to be a function of the confining geometry. Using novel numerical algorithms we study the solution space for a 3-dimensional fully self-consistent formulation of realistic systems with up to 10 5 atoms. A study of the three types of solutions obtained demonstrates a tendency towards metastability as the confining geometry is elongated. One of these solutions, which is consistent with Rice experiments at high trap aspect ratio, supports a state strikingly similar to the long sought Fulde-Ferrel-Larkin-Ovchinnikov state. Our study helps to resolve the long-standing controversy concerning the discrepancies between the findings from two different experimental groups and highlights the versatility of actual-size numerical calculations for investigating inhomogeneous fermionic superfluids. [3]. Primarily due to lack of sufficient experimental evidence, the issue remains largely unresolved even though it is central to many forms of matter such as superconductors, neutron stars and color superfluids in the quark-gluon plasma [3]. Ultra-cold samples of two component degenerate Fermi gases [6][7][8][9][10] have reenergized the debate because of their exquisite controllability.In this paper, we focus on apparently contradictory results on spin-imbalanced unitary Fermi gases from recent experiments between two leading groups [7][8][9][10]. In both experiments it was observed that, consistent with earlier predictions [1,2], the trapped superfliud responds to polarization by phase separating into an inner core with negligible polarization surrounded by a polarized outer shell. However, the Rice experiments [8,9] performed in cigar-shaped traps with total particle numbers N ∼ 10 5 observed a significant and unexpected deformation of the central superfluid core, indicating a clear violation of the local density approximation (LDA). In addition, these results also suggest a much higher superfluid to normal (Chandrasekhar-Clogston) transition than the MIT experiments [7,10] in which no deformations were observed. The excellent quantitative agreement with theory [11,12] for the MIT experiments conducted at much lower trap aspect ratio and with higher particle numbers N ∼ 10 6 , hints that there might be unexpected physics at work in the Rice experiment. In addition, the concurrence of experiments performed in Paris [13] with the MIT experiments also suggest a crucial role of the trapping geometry. This impasse has inspired speculation about the possible role of exotic phases such as the FFLO state in the observed discrepancies, and stirred much discussion and debate over the past few years by the cold atom community.The apparent contradiction between the Rice and MIT experiments reflects theoretical difficulties within trapped geometries: since the effective chemical potential (µ) varies in space, several phases may co-exist within a trapped sample. ...

Thermodynamics of Ultracold Fermi Gases

“…Using a quadrant photodiode, we measured the beam pointing position noise power spectrum S x (ν) (see Fig.2.8a). Gas heating ‡ or even possibly to metastable macroscopic states at low temperature [96]. The latter phenomenon may be at the origin of the discrepancy between the observations in Rice university [55,97] and in other groups [98,49,99] on spin-imbalanced Fermi gases (see Chapter 5), as suggested in [100].…”

Thermodynamics of Trapped Imbalanced Fermi Gases at Unitarity

“…This does, therefore, not explain the dramatic deformation seen in the experiment of Partridge et al [6]. An alternative explanation may be that there exists a metastable state with a deformed superfluid core [14]. At the end of this chapter we briefly discuss this possibility.…”

The BCS-BEC Crossover and the Unitary Fermi Gas

“…Note that the BdG and SLDA functionals have also been considered in larger trapped systems [139,140].…”