Fluctuation control of non-thermal orbital order

TL;DR

This paper investigates how non-thermal fluctuations influence the ultrafast control of orbital order in solids, demonstrating that laser-induced dynamics can be stabilized by entropic forces, enabling switching between different ordered states.

Contribution

It introduces a time-dependent Ginzburg-Landau model to analyze laser-driven orbital order dynamics, highlighting the role of non-thermal fluctuations in stabilizing phases and enabling order switching.

Findings

Remanent non-thermal fluctuations stabilize high-symmetry phase.

Laser protocols can switch order parameters between configurations.

Entropic forces play a crucial role in non-equilibrium orbital dynamics.

Abstract

Orbitally ordered states exhibit unique features which make them a promising platform for exploring the ultrafast dynamics of long-range order in solids: Their free energy typically has multiple discrete minima, and electric laser fields or selectively excited phonons can exert effective forces that may be used to steer the order parameter through these free energy landscapes. Moreover, their free energy strongly depends on fluctuations, and in some cases restoring forces close to a minimum are exclusively of entropic origin (order-by-disorder mechanisms). This can open pathways to control the dynamics of the order parameter via non-thermal fluctuations. In this work, we study the laser-induced non-equilibrium dynamics in a $120^\circ$ compass model, using time-dependent Ginzburg-Landau theory. We analyze protocols to switch the order parameter between equivalent configurations, with a focus on the interplay between the external force due to the driving field, and the non-thermal entropic forces. In particular, we find that remanent non-thermal fluctuations after some excitation can stabilize the high-symmetry phase even when the homogeneous potential has retrieved its low-temperature form, which facilitates laser-induced switching.