Zhenjie Yan, Parth B. Patel, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck, Martin W. Zwierlein
Phys. Rev. Lett. 122, 093401 (2019)
See Viewpoint by Pietro Massignan: From Quantum Quasiparticles to a Classical Gas
We study the thermal evolution of a highly spin-imbalanced, homogeneous Fermi gas with unitarity limited interactions, from a Fermi liquid of polarons at low temperatures to a classical Boltzmann gas at high temperatures. Radio-frequency spectroscopy gives access to the energy, lifetime and the short-range correlations of Fermi polarons at low temperatures T. In this regime we observe a characteristic $\propto T^2$ dependence of the spectral width, corresponding to the quasiparticle decay rate expected for a Fermi liquid. At high T the spectral width decreases again towards the scattering rate of the classical, unitary Boltzmann gas, $\propto T^{1/2}$. In the transition region between the quantum degenerate and classical regime, the spectral width attains its maximum, on the scale of the Fermi energy, indicating the breakdown of a quasiparticle description. Density measurements in a harmonic trap directly reveal the majority dressing cloud surrounding the minority spins, and yield the compressibility along with the effective mass of Fermi polarons.