Real-time detection of critical slowing-down at the superconducting phase transition

TL;DR

This study uses time-resolved THz spectroscopy to observe critical slowing-down in superconducting NbN near its phase transition, linking experimental results with theoretical simulations to explore out-of-equilibrium critical phenomena.

Contribution

It provides the first real-time observation of critical slowing-down in a superconductor using ultrafast spectroscopy and theoretical modeling.

Findings

Superconductivity quenching time lengthens near the condensation energy.

Simulations reproduce the slowing-down behavior and relate it to free energy landscape flattening.

Demonstrates a non-equilibrium analog of critical slowing-down in superconductors.

Abstract

We employ optical pump-THz probe spectroscopy to chart the ultrafast superconductivity suppression in NbN over a broad range of excitation fluences. Our measurements uncover a pronounced lengthening of the superconductivity quenching time when the absorbed optical energy is close to the condensation energy of the superconductor, which constitutes a non-equilibrium analog of critical slowing-down on a timescale comparable to that of superconducting fluctuations. Time-dependent Ginzburg-Landau simulations reproduce this behavior and ascribe it to the flattening of the free energy landscape at the dynamical phase transition boundary. Our findings represent a direct observation in real time of slowed-down superconductivity dynamics in proximity to a critical point and open a pathway for investigating out-of-equilibrium critical phenomena with time-resolved THz spectroscopy.