We discuss our experiments that apply ultrafast electron diffraction (UED) to study structural dynamics of the phase transition in single crystal tantalum ditelluride, TaTe2, a quasi-2D quantum material which exhibits a trimer superstructure at cryogenic temperatures. Intense near-infrared (NIR) pulses at 1030 nm are employed to quench the low temperature, atomically ordered state and the process is captured by ultrashort bunches of electrons as a function of pump-probe time delay. The diffraction signatures of the trimer superstructure recover on picosecond time scales. These measurements of TaTe2 underscore moreover the applicability of the HiRES UED beamline at Lawrence Berkeley National Laboratory (LBNL) to probe ultrafast structural dynamics of complex materials.
The correlated polar semimetal Ca3Ru2O7 exhibits a rich phase diagram including two magnetic transitions (TN =56 K and TC =48 K) with the appearance of an insulating-like pseudogap (at TC ). In addition, there is a crossover back to metallic behavior at T∗=30 K, the origin of which is still under debate. We utilized ultrafast optical pump optical probe spectroscopy to investigate quasi- particle dynamics as a function of temperature in this enigmatic quantum material. n conjunction with density functional theory, our experimental results synergistically reveal the origin of the T-dependent pseudogap. Further, our data and analysis indicate that the T∗ emerges as a natural consequence of T-dependent gapping out of carriers, and does not correspond to a separate electronic transition. Our results highlight the value of low fluence ultrafast optics as a sensitive probe of low energy electronic structure, thermodynamic parameters, and transport properties of Ruddlesden-Popper ruthenates.
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