FFLO on the Horizon

Abstract

Physics Such counterintuitive phenomena as the flow of electrical current through a solid without any dissipation, or the zero viscosity of 3He at low temperatures, may be explained by the unlikely pairing of the electrons or He atoms (respectively) that constitute the flow. In the simplest case, pairing occurs between fermions of opposite spins and momenta, so that the total momentum of a pair is zero. Usually, the numbers of fermions of opposite spins is balanced, so that each up-spin has a down-spin partner; an exotic situation when this is not the case may lead to the formation of the so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO), state, in which the pair momentum is finite and the pair density is oscillating in space. It is expected that this state will be the most stable in a one-dimensional (1D) wire-like system. Liao et al. recreate this situation in an ultracold gas of 6Li confined to an array of 1D tubes. In line with theoretical predictions, the measured spin density profiles indicate that a core with unequal densities of up- and down-spins (a potential FFLO state) is flanked by either a fully paired or a fully spin-polarized state, depending on the overall degree of spin imbalance. Although not a direct demonstration of an FFLO state, the experiment may set the groundwork for such observations in the future. Nature 467 , 567 (2010).