Dynamical Slowing-Down in an Ultrafast Transition

Abstract

Complex systems, which consist of a large number of interacting constituents , often exhibit universal behavior near a phase transition, such as a slowdown of certain dynamical observables. This phenomenon, known as critical slowing-down, is well studied in thermodynamic phase transitions but is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photoinduced phase transition in a model charge density wave (CDW) compound, LaTe 3. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be captured by generalizing the time-dependent Landau theory to a system far from equilibrium. Our understanding of dynamical slowing-down may offer insights into other general principles behind nonequilibrium transitions in many-body systems.