Unconventional interaction between vortices in a polarized Fermi gas

Abstract

Recently, a homogeneous superfluid state with a single gapless Fermi surface was predicted to be the ground state of an ultracold Fermi gas with spin population imbalance in the regime of molecular Bose-Einstein condensation. We study vortices in this novel state using a symmetry-based effective field theory, which captures the low-energy physics of gapless fermions and superfluid phase fluctuations. This theory is applicable to all spin-imbalanced ultracold Fermi gases in the superfluid regime, regardless of whether the original fermion pairing interaction is weak or strong. We find a remarkable, unconventional form of the interaction between vortices. The presence of gapless fermions gives rise to a spatially oscillating potential, akin to the RKKY indirect-exchange interaction in non-magnetic metals. We compare the parameters of the effective theory to the experimentally measurable quantities and further discuss the conditions for the verification of the predicted new feature. Our study opens up an interesting question as to the nature of the vortex lattice resulting from the competition between the usual repulsive logarithmic (2D Coulomb) and predominantly attractive fermion-induced interactions.

Static and dynamic phases for magnetic vortex matter with attractive and repulsive interactions

“…We show that transport measurements provide a clear way to determine whether MI vortices exist in a sample, since the dynamic phase transition into inhomogeneous clump or stripe states produces a robust double peak in the differential conductivity. Our results are general and can be applied to other systems of particles with competing long range attraction and short range repulsion in the presence of a substrate, such as colloidal particles or vortices in Bose-Einstein condensates [36].…”

“…We show that transport measurements provide a clear way to determine whether MI vortices exist in a sample, since the dynamic phase transition into inhomogeneous clump or stripe states produces a robust double peak in the differential conductivity. Our results are general and can be applied to other systems of particles with competing long range attraction and short range repulsion in the presence of a substrate, such as colloidal particles or vortices in Bose-Einstein condensates [36].Simulation-We simulate a two-dimensional (2D) system of N v = 400 vortices and N p pinning sites with periodic boundary conditions in the x and y directions. The vortex dynamics are obtained by integrating the following equation of motion: η(dR i /dt) = F vv i +F vp i +F d +F T i , where η is the damping constant and R i is the position of vortex i.…”

Collective modes in asymmetric ultracold Fermi systems

“…The properties of vortices in the gapless region have been studied previously in [58,59] who have concentrated on the interaction between two vortices in this regime. The vortex core structure in imbalanced superfluids has been studied in [60] who have focused on the occupation number of particles that determine the "visibility" of vortices.…”

Phase diagram of two-dimensional fast-rotating ultracold fermionic atoms near unitarity

“…These are FFLO states in the presence of a vortex lattice. Typically, one Landau level is much more populated than the other ones, and the map of FFLO states is shown in Vortex lattice structural transitions within the same Landau level are also found in the saddle-point approximation because of complicated intervortex forces mediated by fermionic degrees of freedom [34]. We do not show these structural transitions because quantum fluctuations of the order parameter generate familiar logarithmic "Coulomb" forces between vortices (known from Landau-Ginzburg theory), which dominate at low densities and act to stabilize the hexagonal Abrikosov lattice if they occur in the lowest Landau level.…”