Second sound attenuation near quantum criticality

Second sound attenuation, a distinctive dissipative hydrodynamic phenomenon in a superfluid, is crucial for understanding superfluidity and elucidating critical phenomena. Here, we report the observation of second sound attenuation in a homogeneous Fermi gas of lithium-6 atoms at unitarity by performing Bragg spectroscopy with high energy resolution in the long-wavelength limit. We successfully obtained the temperature dependence of second sound diffusivity D-2 and thermal conductivity kappa. Furthermore, we observed a sudden rise-a precursor of critical divergence-in both D-2 and kappa at a temperature of about 0.95 superfluid transition temperature T-c. This suggests that the unitary Fermi gas has a much larger critical region than does liquid helium. Our results pave the way for determining the universal critical scaling functions near quantum criticality.