Pairing of Fermions with Unequal Effective Charges in an Artificial Magnetic Field

TL;DR

This paper explores how unequal effective charges in fermion pairs under artificial magnetic fields affect superconductivity, revealing drastic phase diagram changes, reentrant phases, and a pair breaking mechanism.

Contribution

It introduces the study of BCS pairing in fermions with unequal effective charges in artificial magnetic fields, highlighting novel phase behaviors and pair breaking effects.

Findings

No superconducting transition at certain field strengths.

Reentrant superconducting phases in magnetic field and temperature.

Charge imbalance suppresses critical temperature.

Abstract

Artificial magnetic fields (AMFs) created for ultra cold systems depend sensitively on the internal structure of the atoms. In a mixture, each component experiences a different AMF depending on its internal state. This enables the study of Bardeen-Cooper-Schrieffer pairing of fermions with unequal effective charges. In this Letter, we investigate the superconducting (SC) transition of a system formed by such pairs as a function of field strength. We consider a homogeneous two-component Fermi gas of unequal effective charges but equal densities with attractive interactions. We find that the phase diagram is altered drastically compared to the usual balanced charge case. First, for some AMFs there is no SC transition and isolated SC phases are formed, reflecting the discrete Landau level (LL) structure. SC phases become reentrant both in AMF and temperature. For extremely high fields where both components are confined to their lowest LLs, the effect of the charge imbalance is suppressed. Charge asymmetry reduces the critical temperature even in the low-field semiclassical regime. We discuss a pair breaking mechanism due to the unequal Lorentz forces acting on the components of the Cooper pairs to identify the underlying physics.