Resonantly-paired fermionic superfluids

TL;DR

This paper develops a controlled theory for narrow Feshbach resonances in ultracold Fermi gases, describing BCS-BEC crossover, phase diagrams, and topological superfluid phases, including p-wave pairing and non-Abelian states.

Contribution

It provides a quantitative, resonance-width-dependent framework for understanding fermionic superfluidity and topological phases in ultracold gases, extending previous models to narrow resonances and p-wave pairing.

Findings

Quantitative description of BCS-BEC crossover for narrow s-wave resonances.

Prediction of a rich phase diagram with multiple phase transitions in p-wave resonant gases.

Identification of topological p_x + i p_y superfluidity with non-Abelian excitations in 2D.

Abstract

We present a theory of a degenerate atomic Fermi gas, interacting through a narrow Feshbach resonance, whose position and therefore strength can be tuned experimentally, as demonstrated recently in ultracold trapped atomic gases. The distinguishing feature of the theory is that its accuracy is controlled by a dimensionless parameter proportional to the ratio of the width of the resonance to Fermi energy. The theory is therefore quantitatively accurate for a narrow Feshbach resonance. In the case of a narrow s-wave resonance, our analysis leads to a quantitative description of the crossover between a weakly-paired BCS superconductor of overlapping Cooper pairs and a strongly-paired molecular Bose-Einstein condensate of diatomic molecules. In the case of pairing via a p-wave resonance, that we show is always narrow for a sufficiently low density, we predict a detuning-temperature phase diagram, that in the course of a BCS-BEC crossover can exhibit a host of thermodynamically-distinct phases separated by quantum and classical phase transitions. For an intermediate strength of the dipolar anisotropy, the system exhibits a p_x + i p_y paired superfluidity that undergoes a topological phase transition between a weakly-coupled gapless ground state at large positive detuning and a strongly-paired fully-gapped molecular superfluid for a negative detuning. In two dimensions the former state is characterized by a Pfaffian ground state exhibiting topological order and non-Abelian vortex excitations familiar from fractional quantum Hall systems.