Probing molecular spectral functions and unconventional pairing using Raman spectroscopy

TL;DR

This paper proposes a theoretical method using Raman spectroscopy to directly observe molecular spectral functions and unconventional pairing phenomena in ultracold Fermi gases, revealing rich physics including phase transitions and exotic states.

Contribution

It introduces a protocol for preparing and probing molecular states in ultracold gases, linking spectral functions to exotic quantum phases like the FFLO state.

Findings

Molecular spectral functions can be directly measured using Raman spectroscopy.

The spectral function serves as a precursor to the FFLO phase.

The protocol enables exploration of impurity-induced many-body phenomena.

Abstract

An impurity interacting with an ultracold Fermi gas can form either a polaron state or a dressed molecular state in which the impurity forms a bound state with one gas particle. This molecular state features rich physics, including a first-order transition to the polaron state and a negative effective mass at small interactions. However, these features have remained so far experimentally inaccessible. In this work we show theoretically how the molecular state can be directly prepared experimentally even in its excited state using state-of-the-art cold atom Raman spectroscopy techniques. Initializing the system in the ultra-strong coupling limit, where the binding energy of the molaron is much larger than the Fermi energy, our protocol maps out the momentum-dependent spectral function of the molecule. Using a diagrammatic approach we furthermore show that the molecular spectral function serves as a direct precursor of the elusive Fulde-Ferell-Larkin-Ovchinnikov phase, which is realized for a finite density of fermionic impurity particles. Our results pave the way to a systematic understanding of how composite particles form in quantum many-body environments and provide a basis to develop new schemes for the observation of exotic phases of quantum many-body systems.