Magnetically doped topological insulators (TIs) attract a great deal of interest for both fundamental scientific studies and potential applications. These systems are promising for the realization of quantum anomalous Hall effect, and more generally, for the potentially controllable magnetism, which can underlay many useful technological applications. Here, we reveal the first photoinduced magnetization dynamics in a 40 nm thin film of magnetically doped TIs, CrxBi2-xTe3 with x=0.29 using femtosecond time-resolved magneto-optical Kerr effect (MOKE) spectroscopy. The ultrafast spin dynamics of the sample at low temperature 5 K is characterized by two demagnetization dynamics, attributed to spin-spin (~0.5 ps) and spin-phonon (~10 ps) scatterings, respectively, followed by a slow recovery process with 100s of ps time scale. While it gets faster at elevated temperature and finally vanishes above ~ 90 K, much higher than the reported Curie temperature Tc~23.8 K, due to strong Van Vleck magnetic susceptibility in the magnetically doped TIs system, distinct from the case of GaMnAs. In addition to providing implications for deeper understanding of spin dynamics in magnetically doped TI systems, the study will potentially benefit the development of magnetic TIs-based spintronic devices.
How photoexcitations evolve in time into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaic and optoelectronic technologies. Until now, the initial quantum dynamics following photoexcitation remains elusive in the organometal halide perovskite system. Here we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, unbound carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot state cooling processes. The terahertz transport with rather long dephasing time and scattering processes due to discrete terahertz phonons in perovskites are distinct from conventional photovoltaic materials. In addition to providing implications for ultrafast coherent transport, these results break ground for a perovskite-based device paradigm for terahertz and coherent optoelectronics.
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