Quantum Criticality at the Superconductor-Insulator Transition Probed by the Nernst Effect

Abstract

The superconductor-insulator transition (SIT) is an excellent example of 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 studies of the SIT have concentrated on transport properties and tunneling experiments that 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 α_{xy}, which is the thermodynamic quantity extracted from these measurements, follows quantum critical scaling with critical exponents ν∼0.7 and z∼1. These exponents are consistent with a clean X-Y model in 2+1 dimensions.

Universal bound to the amplitude of the vortex Nernst signal in superconductors

“…The Nernst peak evolves concomitantly with the resistive transition with increasing magnetic field. The magnitude of the vortex Nernst signal observed here is comparable to other two-dimensional amorphous films studied before, such as NbSi [27] or InO x [28,33]. Note also that the Nernst signal in MoGe is only slightly lower than in Nb: STO.…”

AC Measurement of the Nernst effect of thin films at low temperatures

“…In the case of superconductors which are characterized by an effective particle-hole symmetry, the Nernst coefficient can be expressed as a product of the resistivity and the transverse Peltier coefficient ν = ρ xx • α xy . During the past few years the Nernst effect has been shown to be a very effective tool to study vortex motion in the fluctuation regime both above and below T C [3][4][5][6][7][8].…”

“…An example is a thin film of amorphous indium oxide a-InO x in which evidence for vortices motion has been detected in the insulator [9][10][11][12][13]. Indeed, significant Nernst coefficients have been measured in both the superconducting and the insulating phases [8].…”

Universal Voltage Fluctuations in Disordered Superconductors

“…In this letter we describe measurements of spontaneous voltage in amorphous indium oxide (a−InO) films where the disorder is tuned so the samples are close to the superconductor-insulator transition (SIT). These samples, despite being morphologically uniform, have been shown to include "emergent granularity" in the form of superconducting puddles embedded in an insulating matrix [19][20][21][22][23][24][25][26][27][28][29], hence, they are perfect candidates for detection of AC effect signatures. For these films we find reproducible voltage fluctuations as a function of magnetic field amplitude, field orientation, and gate voltage.…”