Dynamics in the nonequilibrium energy landscape of a frustrated Mott insulator

Abstract

In a Mott insulator, a laser pulse with frequency tuned to the gap scale can create a holon-doublon plasma, suppressing the magnetic moment ${\vec m}_i$ and destroying magnetic order. While this disruptive effect is well established experimentally on a square lattice, we investigate the effect of laser pumping on the triangular lattice, where geometric frustration leads to a richer set of ordering possibilities. We work with the Mott-Hubbard problem at a coupling where $120^{\circ}$ order is just stable and employ spatio-temporal mean field dynamics to study the pump response. Moderate pump amplitude just leads to the reduction of $120^{\circ}$ order, but at larger amplitude the suppression of $120^{\circ}$ order is followed by the appearance of `spiral order'. On the electronic side the density of `excited carriers' $n_{exc}$ in the upper Hubbard band increases monotonically with pump amplitude. We show that the long time ordering possibilities in the pumped system, e.g., the emergence of spiral order, can be inferred from a nonequilibrium `energy landscape'. We analyse the growth of spiral order by using an exact diagonalisation based Langevin equation on large lattices and discover that the new order can take $\sim 10^3-10^4$ times the electronic timescale to appear. The threefold combination, of mean field dynamics, landscape construction, and Langevin dynamics, readily generalises to the search for pump induced `hidden order' in other gapped systems.