The study of a Superconductor–Insulator quantum phase transition using Nernst effect

The superconductor-insulator transition (SIT) is a quantum phase transition that has proved to be an interesting subject of study owing to the two very dissimilar microscopic mechanisms by which it can be brought about. The fermionic mechanism is dominated by amplitude fluctuations of the superconducting order parameter Ψ, whereas in the bosonic case, it is the phase φ that fluctuates. These mechanisms are not easy to address in experiment in some cases, and the usual probes for the SIT are sometimes not sensitive to it. A special case of interest is the SIT of amorphous Indium Oxide, which has never shown full conformity with either mechanism.
To probe this SIT, we used Nernst effect, which is generated by either fluctuating Cooper pairs (fermionic) or mobile vortices (bosonic) in the vicinity of the SIT. We find Nernst effect in the insulator which is in excellent agreement with a theoretical model developed for Josephson-coupled superconducting chains [1], giving support to a bosonic mechanism. The underlying thermodynamic variable, the off diagonal Peltier coefficient α_xy, is unique in the sense that it is generated by quantum fluctuations. It shows excellent scaling properties across the SIT, the scaling exponents lending further insight into the microscopic process of SIT.