Crossovers and quantum phase transitions in two-band superfluids: The evolution from BCS to Bose pairing by tuning interactions and band offset

TL;DR

This paper demonstrates that two-band s-wave superfluids can undergo quantum phase transitions by tuning interactions and band offset, revealing multiple superfluid phases beyond the simple BCS-BEC crossover.

Contribution

It introduces the possibility of quantum phase transitions in two-band superfluids by tuning interband and intraband interactions, unlike single-band systems.

Findings

Identification of three distinct pairing regimes: BCS-BCS, BCS-BEC, BEC-BEC.

Discovery of quantum phase transitions between superfluid phases by tuning interband interactions.

Prediction of observable phenomena in Ytterbium isotopes.

Abstract

We show that in two-band $s$-wave superfluids it is possible to induce quantum phase transitions (QPTs) by tuning intraband and interband $s$-wave interactions, in sharp contrast to single-band $s$-wave superfluids, where only a crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensation (BEC) superfluidity occurs. For non-zero interband and attractive intraband interactions, we demonstrate that the ground state has always two interpenetrating superfluids possessing three spectroscopically distinct regions where pairing is qualitatively different: (I) BCS pairing in both bands (BCS-BCS), (II) BCS pairing in one band and BEC pairing in the other (BCS-BEC), and (III) Bose pairing in both bands (BEC-BEC). Furthermore, we show that by fine tuning the interband interactions to zero one can induce QPTs in the ground state between three distinct superfluid phases. There are two phases where only one band is superfluid ($S_1$ or $S_2$), and one phase where both bands are superfluid $(S_1 + S_2)$, a situation which is absent in one-band $s$-wave systems. Lastly, we suggest that these crossovers and QPTs may be observed in $^{173}$Yb and $^{87}$Sr.