Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas

TL;DR

This study precisely measures the thermodynamics of a strongly interacting Fermi gas to identify the superfluid phase transition, revealing a lambda-like heat capacity feature at the critical temperature, serving as a benchmark for many-body theories.

Contribution

First direct thermodynamic observation of the superfluid transition in a spin-balanced atomic Fermi gas using high-precision measurements without external calibration.

Findings

Superfluid transition observed in compressibility, chemical potential, entropy, and heat capacity.

Heat capacity exhibits a lambda-like peak at the critical temperature T_c/T_F = 0.167(13).

Provides a universal thermodynamic benchmark for strongly interacting fermions.

Abstract

We have observed the superfluid phase transition in a strongly interacting Fermi gas via high-precision measurements of the local compressibility, density and pressure down to near-zero entropy. Our data completely determine the universal thermodynamics of strongly interacting fermions without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity. In particular, the heat capacity displays a characteristic lambda-like feature at the critical temperature of $T_c/T_F = 0.167(13)$. This is the first clear thermodynamic signature of the superfluid transition in a spin-balanced atomic Fermi gas. Our measurements provide a benchmark for many-body theories on strongly interacting fermions, relevant for problems ranging from high-temperature superconductivity to the equation of state of neutron stars.