Graduate Thesis Or Dissertation
Conventional and Ultrafast Pump-Probe Time-Resolved Raman Spectroscopy of Strongly Correlated Systems Public Deposited
Raman scattering has become an invaluable tool for the study of strongly-correlated systems because it can directly probe phonons, magnetic excitations, and electronic excitations. The extension of Raman scattering to the time domain by using the pump-probe technique allows us to study the femtosecond dynamics under a non-equilibrium condition. Time-resolved Raman scattering thus is able to disentangle different fundamental interactions that are difficult to distinguish in the energy domain by their different temporal evolution. In this thesis we show the development of time-resolved Raman spectroscopy and its applications to investigate non-equilibrium dynamics in novel materials.
The first part of this thesis is devoted to using large-shift Raman spectroscopy to study the electronic structure of Sr₂IrO₄, a spin-orbit-induced Mott insulator. We found two high-energy excitations of the d-shell multiplet at 690 meV and 680 meV with A1g and B1g symmetry respectively. We show that both pseudospin-flip and non-pseudosin-flip dd electronic transitions are Raman active, but only the latter are observed.
The second part is devoted to the study of the time dynamics of electron-hole excitations as well as the G-phonon in graphite after an excitation by an intense laser pulse. We found that the increase of the G-phonon population occurs with a delay of ∼65 fs in contradiction with the two-temperature model. This time-delay is also evidenced by the absence of the so-called self-pumping for G phonons. It decreases with increased pump fluence. We show that these observations imply a new relaxation pathway: Instead of hot carriers transferring energy to G-phonons directly, the energy is first transferred to optical phonons near the zone boundary K-points, which then decay into G-phonons via phonon-phonon scattering.
In the third part we study magnetic dynamics in insulating YBa₂Cu₃O₆₊ₓ using time-resolved Raman spectroscopy. We observed ultrafast melting of the magnetic order induced by an ultrashort pulse as evidenced by suppression of the two-magnon Raman peak. The melting time is faster than our time resolution (<90 fs). The recovery time for the magnetic order is 1 ps. The melting of the magnetic order is attributed to the process where photo-excited electron hole pairs relax into local antiferromagnetic excitations.
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