Quantum Griffiths singularity in light-induced 2D superconducting phase on the surface of molecular Mott insulator

Quenched disorder can have profound effect upon quantum phase transition. In particular, the fluctuation of rare large regions that is in a phase distinct from the rest can modify the critical behavior near to the quantum criticality, leading to so-called quantum Griffiths singularity. The superconductor-insulator transition serves as a model for quantum phase transitions. Recent findings in certain two-dimensional (2D) superconductors have unveiled the presence of quantum Griffiths singularity. This singularity is characterized by anomalous scaling behavior and modification of quantum critical points due to the existence of local superconducting islands (rare regions). Field-effect devices, such as electric double layer transistors, have been increasingly employed to induce two-dimensional (2D) superconductivity. While a recently proposed method involves the utilization of a photoactive electric double layer system composed of a spiropyran (SP) monolayer.

In this study, we have successfully induced ideal 2D superconductivity on the surface of a bulk single crystal of an organic Mott insulator (κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl: κ-Cl) through the photochromic isomerization of an SP derivative. Within this 2D superconducting phases characterized by enhanced quantum fluctuation, we have discerned a clear and distinctive signature indicating presence of a quantum Griffiths singularity.