Josephson effects in an interaction-asymmetric junction across the BCS-BEC crossover

TL;DR

This paper provides a comprehensive theoretical analysis of the Josephson effect in ultracold Fermi gases across the BCS-BEC crossover, revealing how the tunneling current varies with interaction strength and identifying the interaction-biased Riedel peak.

Contribution

It introduces a nonequilibrium Green's function approach to evaluate Josephson currents across the entire BCS-BEC crossover, including the asymmetric interaction case, and predicts the Riedel peak in strong-coupling regimes.

Findings

DC Josephson current varies with interaction strength.

A peak in tunneling current occurs when one side is BCS and the other BEC.

Demonstrates the Riedel peak in ultracold quantum gases.

Abstract

We theoretically study the Josephson effect in ultracold Fermi gases, where the two sides of the Josephson junction are independently tuned to different regions of the Bardeen-Cooper-Schrieffer (BCS)-Bose-Einstein condensation (BEC) crossover. Using the nonequilibrium Green's function approach combined with the tunnel Hamiltonian formalism, we evaluate the DC and AC Josephson currents throughout the entire crossover region. We calculate the DC Josephson current as a function of interaction strength by tuning both sides of the junction synchronously from the BCS to the BEC regimes, and give the asymptotic expression of the current in the deep BCS and BEC limits. We also study the AC Josephson junction through the interaction-asymmetric junction by fixing the interaction in one reservoir and tuning that of the other one. A peak of the tunneling current is found when one side is fixed in the BCS limit and the other side is tuned into the BEC regime, which corresponds to the interaction-biased Riedel peak. Our results indicate the competition between contributions of increasing pair spectral weight and decreasing chemical potential to Josephson tunneling throughout the BCS-BEC crossover, and demonstrate the realization of the Riedel peak in strong-coupling quantum gases.