Nernst effect measurements at the SIT

The superconductor-insulator transition (SIT) is a prototype of a quantum phase transition which is very versatile experimentally: varying a non-thermal tuning parameter such as disorder, thickness, composition, magnetic field or gate-voltage causes the system to switch from a superconductor to an insulator at zero temperature.

Unlike their classic counterparts, quantum phase transitions are governed by quantum fluctuations rather than thermal fluctuations. The direct experimental study of such fluctuations close to the SIT is rather challenging. So far research has mainly concentrated on dc resistivity based measurements such as transport and magnetoresistance and on global and local tunneling spectroscopy. These provide only limited information on the critical behavior.

In my talk I will describe an experiment designed to study the evolution of quantum fluctuations through the quantum critical point, i.e. the Nernst effect measurement.  The Nernst signal has been proven to be very effective in probing superconducting fluctuations and in particular phase-fluctuation induced by mobile vortices. In our experiments we find a significant Nernst signal in both the superconductor and the insulators sides of the transition which peaks close to criticality. I will discuss the significance of the results and their contribution to understanding the electronic processes in the vicinity of the quantum phase transition.