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 ∼ = 140 K, including a second-order magnetic phase transition (PM − → AFM) and a first-order structural phase transition, accompanied by C-type spin order and G-type orbital order. 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 ∼ = 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.
We investigate quasiparticle relaxation dynamics in URu2−xFexSi2 single crystals using ultrafast optical-pump optical-probe (OPOP) spectroscopy as a function of temperature and Fe substitution (x), crossing from the hidden order (HO) phase (x = 0) to the large moment antiferromagnet (LMAFM) phase (x = 0.12). At low temperature, the dynamics for x = 0 and x = 0.12 are consistent with the low energy electronic structure of the HO and LMAFM phases that emerge from the high temperature paramagnetic (PM) phase. In contrast, near the bicritical point separating HO and LMAFM (x = 0.1), two transitions occur over a narrow temperature range (from 15.5 -17.5 K). A PM to HO transition occurs at an intermediate temperature followed by a transition to the LMAFM phase at lower temperature. While the data at low temperatures are consistent with the expected coexistence of LMAFM and HO, the data in the intermediate temperature phase are not, and instead suggest the possibility of an unexpected coexistence of HO and PM. Additionally, the dynamics in the PM phase reflect the presence of a hybridization gap as well as strongly interacting spin and charge degrees of freedom. OPOP yields insights into meV-scale electrodynamics with sub-Kelvin temperature resolution, providing a complementary approach to study low energy electronic structure in quantum materials.
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