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 ψ­₀, whereas in the bosonic case, it is the phase φ that fluctuates. These mechanisms are not always easy to address in experiment. Whereas resistivity-vs-temperature curves show some qualitative differences, the other extensively used probe for the SIT, namely tunnelling experiment, is 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 on the insulating side of the SIT, the first of its kind, which proves the presence of vortex-like fluctuations even when the sample is not a superconductor. The dependence of the Nernst coefficient on disorder is in good qualitative agreement with a theoretical model developed for Josephson-coupled superconducting chains [1]. The underlying thermodynamic variable, the off diagonal Peltier coefficient α_xy, shows excellent scaling properties across the SIT, with critical exponent ν = 2/3 corresponding to the 3D-XY model. This indicates a weakly disordered 2D quantum system dominated by quantum fluctuations.

[1]    Y. Atzmon and E. Shimshoni, Phys. Rev. B - Condens. Matter Mater. Phys. 87, 1 (2013).