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

摘要

We have observed the superfluid phase transition in a strongly interactingFermi gas via high-precision measurements of the local compressibility, densityand pressure down to near-zero entropy. Our data completely determine theuniversal thermodynamics of strongly interacting fermions without any fit orexternal thermometer. The onset of superfluidity is observed in thecompressibility, the chemical potential, the entropy, and the heat capacity. Inparticular, the heat capacity displays a characteristic lambda-like feature atthe critical temperature of $T_c/T_F = 0.167(13)$. This is the first clearthermodynamic signature of the superfluid transition in a spin-balanced atomicFermi gas. Our measurements provide a benchmark for many-body theories onstrongly interacting fermions, relevant for problems ranging fromhigh-temperature superconductivity to the equation of state of neutron stars.