Quantum criticality at the superconductor insulator transition probed by the Nernst effect

TL;DR

This study uses the Nernst effect to directly probe quantum fluctuations near the superconductor-insulator transition, revealing critical scaling behavior consistent with a 2+1D XY model.

Contribution

It introduces a novel experimental approach using the Nernst effect to study quantum criticality at the SIT, providing direct thermodynamic evidence of critical scaling.

Findings

Large Nernst signal observed on both sides of the transition.

Critical exponents $ u \\sim 0.7$ and $z \\sim 1$ consistent with a 2+1D XY model.

Thermodynamic quantity $\\alpha_{xy}$ follows quantum critical scaling.

Abstract

The superconductor-insulator transition (SIT) is an excellent example for a quantum phase transition at zero temperature, dominated by quantum fluctuations. These are expected to be very prominent close to the quantum critical point. So far most of the experimental study of the SIT has concentrated on transport properties and tunneling experiments which provide indirect information on criticality close to the transition. Here we present an experiment uniquely designed to study the evolution of quantum fluctuations through the quantum critical point. We utilize the Nernst effect, which has been shown to be effective in probing superconducting fluctuation. We measure the Nernst coefficient in amorphous indium oxide films tuned through the SIT and find a large signal on both the superconducting and the insulating sides which peaks close to the critical point. The transverse Peltier coefficient, $\alpha_{xy}$ which is the thermodynamic quantity extracted from these measurements, follows quantum critical scaling with critical exponents $\nu \sim 0.7$ and $z \sim 1$ which is consistent with a clean XY model in 2+1 dimensions.