Two-dimensional polaron spectroscopy of Fermi superfluids

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 [Phys. Rev. A 107, 013305 (2023)] to extract two-dimensional polaron spectra in a Fermi superfluid with an impurity. The bath is described 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 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 ultracold gases, but we discuss implications for exciton polarons in two-dimensional materials.