Spin Susceptibility Above the Superfluid Onset in Ultracold Fermi Gases

Abstract

Ultracold atomic Fermi gases can be tuned to interact strongly, where they display spectroscopic signatures above the superfluid transition reminiscent of the pseudogap in cuprates. However, the extent of the analogy can be questioned, since thermodynamic quantities in the low temperature spin-imbalanced normal state can be described successfully using Fermi liquid theory. Here we present spin susceptibility measurements across the interaction strength-temperature phase diagram using a novel radiofrequency technique with ultracold 6 Li gases. For all significant interaction strengths and at all temperatures we find the spin susceptibility is reduced compared with the equivalent value for a non-interacting Fermi gas, with the low temperature results consistent with previous studies. However, our measurements extend to higher temperatures, where we find that the reduction persists consistently with a mean-field scenario. At unitarity, we can use the local density approximation to extract the spin susceptibility for the uniform gas, which is well described by mean-field models at temperatures from the superfluid transition to the Fermi temperature.

Pairing and the spin susceptibility of the polarized unitary Fermi gas in the normal phase

“…1 (depicted by the red circle in the top panel) differs by roughly two standard deviations from the AFQMC results. Very recently, new experimental data for the spin susceptibility of the UFG became available [30]. In the corresponding study, consistency with a mean-field approach was found, suggesting Fermi-liquid-type behavior.…”

“…In spin- 1 2 systems with weak attractive interactions, for example, pairing of spin-up and spin-down particles at diametrically opposite points of the Fermi surface (i.e., Cooper pairing) typically leads to a superfluid phase at low enough temperatures, accompanied by the opening of an energy gap in the quasiparticle spectrum. Similar conclusions hold for strongly correlated systems, where pairing and superfluidity have been addressed thoroughly in theory [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] and experiment [20][21][22][23][24][25][26][27][28][29][30]. A central open question in this regard is whether signatures of strong pairing fluctuations survive in the normal, non-superfluid high-temperature phase, which is often referred to as a "pseudogap regime", obtaining its name from a (potentially strong) suppression of the single-particle density of states around the Fermi surface.…”

“…As mentioned in the main text, a recent experimental study [30] presented trap-averaged data for the spinsusceptibility of the spin-balanced UFG. For the bulk system, as studied here via periodic boundary conditions, also the integrated spin-susceptibility…”