Direct observation of the Fermi surface in an ultracold atomic gas

TL;DR

This paper demonstrates the direct measurement of the Fermi surface in a nearly ideal ultracold atomic gas by selectively probing the central region of a trapped gas, overcoming density inhomogeneity effects.

Contribution

It introduces a method to observe the Fermi surface in a trapped ultracold gas by focusing on the central atoms, enabling direct momentum space measurement.

Findings

Successfully measured the Fermi surface in a trapped ultracold gas.

Determined the density and temperature from the Fermi surface features.

Showed that selective probing mitigates density variation effects.

Abstract

The ideal (i.e. noninteracting), homogeneous Fermi gas, with its characteristic sharp Fermi surface in the momentum distribution, is a fundamental concept relevant to the behavior of many systems. With trapped Fermi gases of ultracold atoms, one can realize and probe a nearly ideal Fermi gas, however these systems have a nonuniform density due to the confining potential. We show that the effect of the density variation, which typically washes out any semblance of a Fermi surface step in the momentum distribution, can be mitigated by selectively probing atoms near the center of a trapped gas. With this approach, we have directly measured a Fermi surface in momentum space for a nearly ideal gas, where the average density and temperature of the probed portion of the gas can be determined from the location and sharpness of the Fermi surface.