Fate of transient order parameter domain walls in ultrafast experiments

TL;DR

This paper investigates how transient domain walls created by ultrafast optical pulses decay in materials with spontaneous symmetry breaking, revealing a two-stage process involving fluctuation-driven decay and defect coarsening.

Contribution

It provides a theoretical analysis of the decay dynamics of transient order parameter domain walls, including effects of symmetry and dimensionality, in ultrafast experimental conditions.

Findings

Domain walls decay via unstable fluctuations converting them into defect-rich interfaces.

Topological defect coarsening leads to diffusive growth of correlation length in 3D interfaces.

Crossover from diffusive to sub-diffusive behavior occurs in 2D interfaces with weak Z2 symmetry-breaking.

Abstract

In ultrafast experiments, an optical pump pulse often generates transient domain walls of the order parameter in materials with spontaneous symmetry breaking, due to either a finite penetration depth of light on a three-dimensional (3D) material, or a finite spot size on a two-dimensional (2D) material. We show that the domain wall decays due to unstable order parameter fluctuations. We study a generic system with $U(1)$-symmetric order, and those with an additional weak $Z_2$ ($U(1)$-symmetry-breaking) term, representing the charge-density-wave (CDW) orders in recent experiments. During the first stage of the decay dynamics, exponentially growing thermal fluctuations convert the domain wall into an interface with randomly distributed topological defects. In the second stage, the topological defects undergo a coarsening dynamics within the interface. For a 2D interface in a 3D system, the coarsening dynamics leads to a diffusive growth of the correlation length. For a one-dimensional (1D) interface in a 2D system with the weak $Z_2$ term, the correlation-length growth shows a crossover from diffusive to sub-diffusive behavior.