Quantum phase transitions in highly crystalline two-dimensional superconductors

TL;DR

This paper explores quantum phase transitions in highly crystalline 2D superconductors, revealing a transition from quantum metallic to Griffiths states driven by quantum fluctuations, contrasting with disordered systems.

Contribution

It provides new insights into quantum phase transitions in highly crystalline 2D superconductors, highlighting the role of quantum fluctuations and Griffiths singularity.

Findings

Quantum metallic state converts to Griffiths state at high magnetic fields.

Diverging dynamical critical exponent indicates Griffiths singularity.

Quantum fluctuations lead to superconducting puddles, unlike thermal fluctuations.

Abstract

Superconductor-insulator transition is one of the remarkable phenomena driven by quantum fluctuation in two-dimensional (2D) systems. Such a quantum phase transition (QPT) was investigated predominantly on highly disordered thin films with amorphous or granular structures using scaling law with constant exponents. Here, we provide a totally different view of QPT in highly crystalline 2D superconductors. According to the magneto-transport measurements in 2D superconducting ZrNCl and MoS2, we found that the quantum metallic state commonly observed at low magnetic fields is converted via the quantum Griffiths state to the weakly localized metal at high magnetic fields. The scaling behavior, characterized by the diverging dynamical critical exponent (Griffiths singularity), indicates that the quantum fluctuation manifests itself as superconducting puddles, in marked contrast with the thermal fluctuation. We suggest that an evolution from the quantum metallic to the quantum Griffiths state is generic nature in highly crystalline 2D superconductors with weak pinning potentials.