Nonequilibrium charge-density-wave order beyond the thermal limit

Julian Maklar ,G Ingold,M Porer,P Beaud,U Staub,I R Fisher,Martin Kubli ,Philip Walmsley ,Vincent Esposito ,Christopher W Nicholson ,Ralph Ernstorfer ,Yoav William Windsor ,J Rittmann,Steven L Johnson ,Matteo Savoini ,Elsa Abreu ,D Leuenberger,Martin Wolf ,Michele Puppin ,Laurenz Rettig

doi:10.1038/s41467-021-22778-w

Abstract

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. Recent discoveries, such as the optical enhancement of the critical temperature in certain superconductors and the photo-stabilization of hidden phases, have turned this field into an important research frontier. Here, we demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. A complete description of the system’s coherent and incoherent order-parameter dynamics is given by a time-dependent Ginzburg-Landau framework, providing access to the transient potential energy surfaces.

Highlights

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium
In close analogy to superconductivity, the formation of a CDW broken symmetry ground state can be described by an effective mean field that serves as an order parameter, which is governed in equilibrium by a static free energy surface
While mean field theory captures the phase transition on a qualitative level, thermal lattice fluctuations reduce the critical temperature Tc of longrange 3D order significantly below the predicted mean field value TMF1,2. It is of strong interest how our understanding of phase transitions in the adiabatic limit can be adapted to a nonequilibrium, dynamical setting induced by an impulsive excitation[11,23,24,25,26,27]

Summary

Introduction

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. While mean field theory captures the phase transition on a qualitative level, thermal lattice fluctuations reduce the critical temperature Tc of longrange 3D order significantly below the predicted mean field value TMF1,2 It is of strong interest how our understanding of phase transitions in the adiabatic limit can be adapted to a nonequilibrium, dynamical setting induced by an impulsive excitation[11,23,24,25,26,27]. Using time- and angle-resolved photoemission spectroscopy (trARPES) in combination with time-resolved X-ray diffraction (trXRD), schematically depicted, we extract the amplitude of the electronic and structural order parameters and the electronic temperature as functions of pump-probe delay t This reveals CDW formation at electronic temperatures substantially above the thermal critical temperature. We model the orderparameter dynamics in a time-dependent Ginzburg–Landau framework, which further supports the scenario of a nonthermal stabilization of the CDW order

Methods

Results

Conclusion