Closed-channel contribution in the BCS-BEC crossover regime of an ultracold Fermi gas with an orbital Feshbach resonance

TL;DR

This paper theoretically analyzes the role of the closed channel in an ultracold Fermi gas with an orbital Feshbach resonance, finding its occupation remains small at the superfluid transition but increases with temperature, and proposes a method to handle deep bound states.

Contribution

It provides a detailed theoretical investigation of the closed-channel occupation in an OFR Fermi gas across the BCS-BEC crossover, including a new prescription to remove deep bound state effects.

Findings

Closed channel occupation is very small at T_c across the crossover.

Occupation increases with temperature above T_c, especially for stronger interactions.

A method is proposed to exclude effects of deep bound states in the NSR formalism.

Abstract

We theoretically investigate strong-coupling properties of an ultracold Fermi gas with an orbital Feshbach resonance (OFR). Including tunable pairing interaction associated with an OFR within the framework of the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink (NSR), we examine the occupation of the closed channel. We show that, although the importance of the closed channel is characteristic of the system with an OFR, the occupation number of the closed channel is found to actually be very small at the superfluid phase transition temperature $T_{\rm c}$, in the whole BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region, when we use the scattering parameters for an ultracold $^{173}$Yb Fermi gas. The occupation of the closed channel increases with increasing the temperature above $T_{\rm c}$, which is more remarkable for a stronger pairing interaction. We also present a prescription to remove effects of an experimentally inaccessible deep bound state from the NSR formalism, which we meet when we theoretically deal with a $^{173}$Yb Fermi gas with an OFR.