The study entitled Thermography of the superfluid transition in a strongly interacting Fermi gas is a pioneering exploration of how entropy and heat propagate in a unitary Fermi system, carried out by Zhenjie Yan and colleagues. At its heart, the work sets out to answer a profound and long-standing question: how does one directly image and characterize the transformation of heat conduction when a system crosses from a normal fluid into a superfluid, a transition where entropy ceases to diffuse and instead begins to travel as a collective wave known as second sound. To do this, the researchers introduce a novel methodology they call thermography, which allows them to visualize with spatial resolution temperature changes at the sub-nanokelvin level by exploiting the sensitivity of radio-frequency spectra to local thermal fluctuations. This innovation turns a cloud of ultracold lithium-6 atoms into both the stage and the detector for one of the most elusive forms of collective quantum transport.
The conceptual foundation rests upon Landau’s two-fluid model, which predicts that in a superfluid, two coupled but distinct dynamical modes emerge. First sound corresponds to the familiar oscillation of density and pressure. Second sound, on the other hand, corresponds to oscillations of entropy and temperature, with only weak density variations. The theory predicts that the velocity of second sound is governed not by compressibility but by the thermodynamic properties of entropy and the relative densities of the normal and superfluid components.