Tunable pairing with local spin-dependent Rydberg molecule potentials in an atomic Fermi superfluid

TL;DR

This paper investigates how spin-dependent Rydberg molecule potentials influence pairing, density, and polarization in atomic Fermi superfluids, revealing in-gap states, phase transitions, and novel bound states with potential experimental applications.

Contribution

It introduces a theoretical framework for understanding tunable spin-dependent Rydberg molecule effects in Fermi superfluids, highlighting new in-gap states and phase transition phenomena.

Findings

Emergence of Yu-Shiba-Rusinov states below the energy gap.

Transition from equal-population to population-imbalance with pairing strength.

Appearance of FFLO-like states and sign change in the gap function.

Abstract

We explore the energy spectrum and eigenstates of two-component atomic Fermi superfluids with tunable pairing interactions in the presence of spin-dependent ultra long-range Rydberg molecule (ULRM) potentials, within the Bogoliubov-de Gennes formalism. The attractive ULRM potentials lead to local density accumulation, while their difference results in a local polarization potential and induces the in-gap Yu-Shiba-Rusinov (YSR) states whose energies lie below the bulk energy gap. A transition from equal-population to population-imbalance occurs as the pairing strength falls below a critical value, accompanied by the emergence of local Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like states characterized by out-of-phase wave functions and lower energies compared to the YSR states. The negative contribution emanating from the FFLO-like states also causes a sign change in the gap function within the ULRM potentials. Depending on the Rydberg state generating the ULRM potentials, the transition towards population-imbalance can be on either the BCS or the Bose-Einstein condensation side of the Fermi superfluid. Additionally, spin-polarized bound states arise along with oscillatory ``clumpy states" to compensate for the local density difference. Finally, we discuss possible experimental realizations and measurements of the composite Rydberg atom-Fermi superfluid system.