Universal Relations for Fermions with Large Scattering Length

Precise Photoexcitation Measurement of Tan's Contact in the Entire BCS-BEC Crossover

We study two-body correlations in a spin-balanced ultracold harmonically trapped Fermi gas of ^{6}Li atoms in the crossover from the Bardeen-Cooper-Schrieffer (BCS) to the Bose-Einstein-Condensate (BEC) regime. For this, we precisely measure Tan's contact using a novel method based on photoexcitation of atomic pairs, which was recently proposed by Wang et al. [Photoexcitation measurement of Tan's contact for a strongly interacting Fermi gas, Phys. Rev. A 104, 063309 (2021).PLRAAN2469-992610.1103/PhysRevA.104.063309]. We map out the contact in the entire phase diagram of the BCS-BEC crossover for various temperatures and interaction strengths, probing regions in phase space that have not been investigated yet. Our measurements reach an uncertainty of ≈2% at the lowest temperatures and thus represent a precise quantitative benchmark. By comparison to our data, we localize the regions in phase space where theoretical predictions and interpolations give valid results. In regions where the contact is already well known we find excellent agreement with our measurements. Thus, our results demonstrate that photoinduced loss is a precise probe to measure quantum correlations in a strongly interacting Fermi gas.

Review of pseudogaps in strongly interacting Fermi gases

A central challenge in modern condensed matter physics is developing the tools for understanding nontrivial yet unordered states of matter. One important idea to emerge in this context is that of a 'pseudogap': the fact that under appropriate circumstances the normal state displays a suppression of the single particle spectral density near the Fermi level, reminiscent of the gaps seen in ordered states of matter. While these concepts arose in a solid state context, they are now being explored in cold gases. This article reviews the current experimental and theoretical understanding of the normal state of strongly interacting Fermi gases, with particular focus on the phenomonology which is traditionally associated with the pseudogap.