Single-particle excitations in the BCS-BEC crossover region II: Broad Feshbach resonance

TL;DR

This paper investigates single-particle excitations in the BCS-BEC crossover with a broad Feshbach resonance, revealing that the order parameter suppresses low-energy bound states, aligning theoretical rf-spectrum peaks with experimental observations.

Contribution

It extends previous formulations to the broad resonance case, showing spatially spread profiles and the suppression of Andreev bound states, matching recent experimental results.

Findings

Density and order parameter profiles extend to the Thomas-Fermi radius.

Rf-spectrum peak occurs at the central superfluid order parameter.

LDA provides a good approximation for rf-tunneling spectra.

Abstract

We apply the formulation developed in a recent paper [Y. Ohashi and A. Griffin, Phys. Rev. A {\bf 72}, 013601, (2005)] for single-particle excitations in the BCS-BEC crossover to the case of a broad Feshbach resonance. At T=0, we solve the Bogoliubov-de Gennes coupled equations taking into account a Bose condensate of bound states (molecules). In the case of a broad resonance, the density profile $n(r)$, as well as the profile of the superfluid order parameter ${\tilde \Delta}(r)$, are spatially spread out to the Thomas-Fermi radius, even in the crossover region. This order parameter ${\tilde \Delta}(r)$ suppresses the effects of low-energy Andreev bound states on the rf-tunneling current. As a result, the peak energy in the rf-spectrum is found to occur at an energy equal to the superfluid order parameter ${\tilde \Delta}(r=0)$ at the center of the trap, in contrast to the case of a narrow resonance, and in agreement with recent measurements. The LDA is found to give a good approximation for the rf-tunneling spectrum.