Microscopic scales and mechanism of quantum phase transitions in two-dimensional superconducting systems

TL;DR

This paper investigates quantum phase transitions in two-dimensional superconductors, proposing a unifying pair-breaking mechanism and a microscopic length scale related to Cooper pairs, applicable across various materials and systems.

Contribution

It introduces a microscopic scale-based model for QPTs that extends beyond traditional critical exponent analysis, unifying understanding across different superconducting and non-superconducting systems.

Findings

Pair-breaking mechanism explains magnetic-field-driven QPTs in multiple materials.

The microscopic length scale corresponds to Cooper pair size.

Model applies to Josephson junction arrays and other non-superconducting systems.

Abstract

The superconducting ground state in many two-dimensional materials can be created or destroyed through quantum phase transitions (QPTs) controlled by non-thermal parameters such as carrier density or magnetic field. While various mechanisms for these QPTs have been proposed, it remains unclear which, if any, are applicable to a specific two-dimensional superconducting system. Here, we find that a pair-breaking mechanism which suppresses the Cooper pair density gives a unifying description of magnetic-field-driven QPTs in amorphous MoGe, Pb and TaN films, and the high-temperature superconductor La$_{1.92}$Sr$_{0.08}$CuO$_{4}$. This transition occurs within the superconducting subsystem and is masked by the dominant non-critical contribution of normal electrons. The discovery was enabled by the development of a QPT model that goes beyond the conventional determination of the critical exponents and incorporates into the analysis a microscopic length scale characterizing the transitions. We found that in the materials studied, and MoGe nanowires, this scale corresponds to the size of a Cooper pair. The model has also been successfully applied to QPTs in Josephson junction arrays and various non-superconducting materials. The observation that microscopic scales are encoded in the scaled experimental data of QPTs likely extends beyond equilibrium condensed matter physics and may reveal underlying principles of critical phenomena in a wide variety of systems.