Generic features of the spectrum of trapped polarized fermions

TL;DR

This paper demonstrates that bimodal radio frequency spectra are a universal feature of strongly interacting trapped Fermi gases at intermediate temperatures, independent of superfluidity, and provides a simple model to interpret experimental line-shapes.

Contribution

It reveals the universal emergence of bimodal RF spectra in polarized Fermi gases and introduces a straightforward energetic model to analyze line-shapes considering final state interactions.

Findings

Bimodal spectra are universal at intermediate temperatures.

The spectral shape depends only on the equation of state.

A simple energetic model qualitatively matches experimental line-shapes.

Abstract

We show that bimodal radio frequency spectra universally arise at intermediate temperatures in models of strongly interacting trapped Fermi gases. The bimodality is independent of superfluidity or pseudogap physics, depending only on the functional form of the equation of state -- which is constrained by dimensional analysis at low temperatures and the virial expansion at high temperatures. In addition to these model independent results, we present a simple calculation of the radio frequency line-shape of a highly polarized Fermi gas which uses energetic considerations to include final state interactions. While this model only qualitatively captures the line-shapes observed in the experiments, it provides a conceptually clean and powerful technique for estimating the energy scales and how they vary with experimental parameters.