Fluctuation control of nonthermal orbital order

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 nonthermal fluctuations. In this paper, we study the laser-induced nonequilibrium dynamics in a ${120}^{\ensuremath{\circ}}$ 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 nonthermal entropic forces. In particular, we find that remanent nonthermal 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.