Two-dimensional polaron spectroscopy of Fermi superfluids

TL;DR

This paper explores the use of two-dimensional polaron spectroscopy to analyze Fermi superfluids with impurities, revealing limitations in extracting quasiparticle gap information and discussing broader implications for ultracold gases and 2D materials.

Contribution

It applies a recent multidimensional spectroscopy protocol to Fermi superfluids, clarifying its limitations in probing quasiparticle gaps and extending discussion to 2D materials.

Findings

Spectroscopy response includes symmetric and asymmetric contributions.

Asymmetric part does not reliably indicate quasiparticle gap.

Multidimensional spectroscopy offers limited additional insight without incoherent processes.

Abstract

Multidimensional spectroscopy is becoming an increasingly popular tool and there is an ongoing effort to access electronic transitions and many-body dynamics in correlated materials. We apply the protocol recently proposed by Wang to extract two-dimensional polaron spectra in a Fermi superfluid with an impurity. The bath is descibed by a BCS ansatz and it assumed that the impurity can scatter at most one quasiparticle pair. The spectral response contains a symmetric contribution, which carries the same information as Ramsey spectra, and an asymmetric one. While {\it a priori} it may seem promising to probe the quasiparticle gap from the asymmetric contribution, we show explicitly that this is not the case and, in the absence of incoherent processes, multidimensional spectroscopy does not bring much additional information. Our calculation is suitable for 3D ultracold gases, but we discuss implications for exciton-polarons in 2D materials.