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Electron-phonon coupling in the time domain : TR-ARPES studies by a cavity-based XUV laser Na, Mengxing


The electron-phonon interaction is ubiquitous in crystalline materials, leaving fingerprints on both physical and electronic properties. In the study of materials, angle-resolved photoemission spectroscopy (ARPES) can uniquely access the electronic band structure, and the electron interactions encoded within. However, disentangling the contributions from different degrees of freedom -- such as electron-electron and electron-phonon – can be very challenging. Extension of ARPES into the time domain via pump-probe spectroscopy allows one to access the electronic structure on an ultrafast timescale: this is advantageous as the intertwined interactions in equilibrium become separated in the time domain. In this thesis, we use time-resolved (TR)-ARPES to study the electron-phonon interaction in graphite. Specifically, we observe spectral features arising from the photoexcitation of electrons from the valence band to the conduction band, followed by quantized energy-loss processes corresponding to the emission of strongly-coupled optical phonons. The transfer of spectral weight from an identifiable initial state to a final state is the direct manifestation of a microscopic two-body scattering process from which we can extract the mode-projected electron-phonon matrix element. The spectral features observed in this study arise from the non-thermal (i.e. non-Fermi-Dirac) occupation of electrons. We use Boltzmann simulations to map out various regimes in graphite where non-thermal features arise. These non-thermal signatures are not unique to graphite but are ubiquitous in pump-probe experiments and intrinsically tied to the dominant scattering processes, their timescales, and corresponding bottlenecks. Our results were made possible by a custom-built state-of-the-art laser source featuring cavity-enhanced high-harmonic generation. The source has three key features: First, high photon energies capable of mapping the whole Brillouin zone of materials; second, a high repetition rate that minimizes the space-charge effect; and last, a balanced time and energy resolution capable of studying subtle spectral features. All three elements were crucial in discerning the spectral features related to electron-phonon scattering in graphite, the first observation of its kind. With these results, we demonstrate that the maturation of high-harmonic sources can now offer a tunable table-top source with unprecedented intensity, repetition rate and resolution.

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