Quantum phase transition in two-dimensional NbN superconducting thin films

TL;DR

This study explores the quantum phase transition in ultrathin NbN superconducting films, revealing a quantum Griffiths singularity and critical behavior near the superconductor-insulator transition, with implications for understanding 2D quantum criticality.

Contribution

It demonstrates the occurrence of quantum Griffiths singularity in highly crystalline 2D NbN films undergoing a superconductor-insulator transition, expanding understanding of quantum critical phenomena.

Findings

Quantum Griffiths singularity observed in NbN films.

Divergent dynamical critical exponent near quantum critical point.

Absence of field-driven anomalous metal state in these films.

Abstract

We systematically investigated the low-temperature transport properties of a series of NbN epitaxial films with thickness $t$ ranging from $\sim$2.0 to $\sim$4.0 nm. The films undergo a superconductor-insulator transition (SIT) with decreasing film thickness, and the critical sheet resistance for the SIT is close to the quantum resistance of Cooper pairs $h/4e^2$ (6.45 k$\Omega$). Besides the Berezinski-Koterlitz-Thouless transition, a magnetic-field-driven SIT is observed in those two-dimensional (2D) superconducting films (2.6 nm $\lesssim t \lesssim 4.0$ nm). Interestingly, it is found that the low-temperature magnetoresistance isotherms do not cross at a single fixed point but at a well-distinguished region for these superconducting films. The dynamical critical exponent obtained by analyzing these magnetoresistance isotherms is divergent as the quantum critical point is being approached. The behavior of the dynamical critical exponent, originating from quenched disorder at ultralow temperatures, provides direct evidence for the occurrence of quantum Griffiths singularity in the quantum phase transition process of the films. The field-driven anomalous metal (quantum metal) state does not appear in these films. Our results suggest that the quantum Griffiths singularity not only occurs in the highly crystalline 2D superconductors with superconductor-metal transition but also in those with SIT.