Ultrafast Spectroscopy of Quantum Materials at High Pressure and Low Temperatures

Abstract

The photo-induced carrier dynamics using ultrafast spectroscopy provides insights into understanding of electron-electron interactions, electron-phonon interactions, quasiparticle formation, carriers recombination, trapping of carriers, as well as nonlinear processes such as second harmonic generation. For a given material, extreme conditions such as high pressure or low temperature can tune these properties and provide insights into the microscopic processes of the phase transition. This thesis focuses on the application of ultrafast spectroscopy to quantum materials under high pressure and low temperature conditions. In-situ pressure-dependent ultrafast optical pump-optical probe spectroscopy of Dirac semimetal Cd3As2 reveals pressure-induced phase transitions at PC1 ∼3 GPa and PC2 ∼9 GPa. The interband and intraband relaxation processes contribute to the negative and positive differential reflectivity, respectively. The decrease in relaxation time beyond PC1 reflects stronger electron-phonon coupling, describing the semiconducting phase. A linear increase in the bandgap with pressure beyond PC1 measured from our data demonstrates the experimentally observed evolution of interband and intraband contributions. Ultrafast spectroscopy of another quantum material, excitonic insulator (EI) Ta2NiSe5 (TNSe) under pressure shows its three different phases: excitonic insulator (P<1 GPa), semiconductor (1<P<3 GPa), and semimetal (P>3 GPa). Application of pressure reduces the bandgap linearly in the EI phase, with a pressure coefficient of ~65 meV/GPa, closely agreeing with our first principle calculations. Low-temperature degenerate optical pump-probe spectroscopy of EI TNSe reveals carrier dynamics indicative of the Bose-Einstein Condensation regime and coherent phonons that demonstrate the structural transition at TC = 325 K. Ultrafast spectroscopy of coherent phonons demonstrates their time evolution and captures the birth of quasiparticle. It also provides evidence that photoexcited carriers cause the asymmetry in a few coherent phonons. Compared to Raman phonons, coherent phonons exhibit lower cubic anharmonicity. In the final part of the thesis, non-degenerate optical pump-probe spectroscopy of the TMDC heterostructure (HS) MoSe2/WSe2 at large twist angles and at high pump fluence (beyond the threshold for Mott transition) resulting in electron-hole plasma reveals additional relaxation channels in carrier recombination near the commensurate angles of 21.8° and 38.2°. This is qualitatively understood in terms of interlayer e-h recombination and non-radiative interlayer Auger recombination.