Influence of spin and orbital fluctuations on Mott-Hubbard exciton dynamics in LaVO${}_3$ Thin Films

Abstract

Recent optical conductivity measurements reveal the presence of Hubbard excitons in certain Mott insulators. In light of these results, it is important to revisit the dynamics of these materials to account for excitonic correlations. We investigate time-resolved excitation and relaxation dynamics as a function of temperature in perovskite-type LaVO${}_3$ thin films using ultrafast optical pump-probe spectroscopy. LaVO${}_3$ undergoes a series of phase transitions at roughly the same critical temperature $T_C\cong 140K$, including a second-order magnetic phase transition (PM $\to$ AFM) and a first-order structural phase transition, accompanied by \textit{C}-type spin order (SO) and \textit{G}-type orbital order (OO). Ultrafast optical pump-probe spectroscopy at 1.6 eV monitors changes in the spectral weight of the Hubbard exciton resonance which serves as a sensitive reporter of spin and orbital fluctuation dynamics. We observe dramatic slowing down of the spin, and orbital dynamics in the vicinity of $T_C\cong 140$ K, reminiscent of a second-order phase transition, despite the (weakly) first-order nature of the transition. We emphasize that since it is spectral weight changes that are probed, the measured dynamics are not reflective of conventional exciton generation and recombination, but are related to the dynamics of Hubbard exciton formation in the presence of a fluctuating many-body environment.

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