Ultrafast Quenching of the Exchange Interaction in a Mott Insulator

We investigate how fast and how effective photocarrier excitation can modify the exchange interaction $J_{\mathrm{ex}}$ in the prototype Mott-Hubbard insulator. We demonstrate an ultrafast quenching of $J_{\mathrm{ex}}$ both by evaluating exchange integrals from a time-dependent response formalism and by explicitly simulating laser-induced spin precession in an antiferromagnet that is canted by an external magnetic field. In both cases, the electron dynamics is obtained from nonequilibrium dynamical mean-field theory. We find that the modified $J_{\mathrm{ex}}$ emerges already within a few electron hopping times after the pulse, with a reduction that is comparable to the effect of chemical doping.

Figure 1

Bare exchange interaction as function of temperature for different values $U$, computed from the formula Eq. (4) (red open circles) and from the canted geometry Eq. (5) (blue solid disks). For large $U$ the calculations show excellent agreement with the analytical result $|J_{\mathrm{ex}}^a|=2t_0^2/U$ indicated with dashed lines.

Figure 2

Comparison of the nonequilibrium exchange interaction (open circles) in the quasistationary state after an interaction quench $\mathrm{\Delta }U$ in the Bethe lattice with the equilibrium exchange interaction of the chemically doped model (solid symbols) for $U=8$ and different temperatures. The inset shows the bare time-dependent exchange interaction (solid line) and staggered magnetization (dashed line) caused by the quench $U=4\to 8$.

Figure 3

Induced spin dynamics (top) and modification of the exchange interaction (bottom) caused by excitation with an electric field (hypercubic lattice, $U=8$).

Figure 4

Comparison of the nonequilibrium exchange interaction (red open circles) computed from the induced precession (Fig. 3), with the equilibrium exchange interaction in the chemically doped system (blue lines).