Towards the optical control of resonantly bonded materials : an ultrafast X-ray study
Keywords: Applied Physics.

There has been growing interest in using ultrafast light pulses to drive materials into nonequilibrium states with novel properties. In my study, I focus on a specific class of materials, the resonantly bonded materials with various functional properties including ferroelectricity, high thermoelectric figure of merit, and large change of optical constants upon crystallization and amorphization (or phase change materials). More importantly, this class of materials hosts a number of structural phases that are sensitive to external parameters including temperature, pressure, and chemical doping. It will be very interesting to structurally probe these materials under photoexcitation and explore possible new functionalities. The large polarizability in resonantly bonded materials means pronounced coupling between phonons and electronic states. Therefore, on top of probing the structural dynamics, we want to understand the photoexcited interatomic forces that drive the atomic motion and quantify the electron-phonon coupling. Using time-resolved X-ray scattering, I demonstrate that SnSe, one of the IV-VI resonantly bonded compounds, hosts a novel photo-induced lattice instability associated with an orthorhombic distortion of the rock-salt structure. This lattice instability is distinct from the one associated with the high-temperature phase, providing a counterexample of the conventional wisdom that laser pump pulse serves as a heat dump. I show that the driving mechanism for this new lattice instability is related to the removal of valence electrons from the lone pair orbital. Furthermore, using non-zone-center measurements of time-resolved X-ray scattering, I investigate the microscopic details of the photoinduced lattice instability from the perspective of interatomic interactions. I infer the photoexcited interatomic forces from the phonon dispersion and identify a certain bond that is largely overlapped with the lone pair orbital as responsible for the observed photoinduced lattice instability. The conclusion is contrary to the consensus that in thermal equilibrium, the resonant bonding network of chalcogen p orbitals is the main origin of lattice instability. And indeed, the photoexcited phonon modes have a significantly longer lifetime, which means less anharmonicity of the lattice, than those in thermal equilibrium. The results have implications for optical control of the thermoelectric, ferroelectric, and topological properties of the monochalcogenides and related materials. More generally, the results emphasize the need for structural probes to reveal distinct atomic-scale dynamics that are otherwise too subtle or invisible in conventional ultrafast spectroscopies. In the thesis, I also show that by combining time-resolved X-ray diffraction with time-resolved ARPES (angular-resolved photoemission spectroscopy) on Bi2Te3 and Bi2Se3 (V2-VI3 resonantly bonded materials) I can extract state-specific specific deformation potentials (DPs) in these topological insulators which are quantification of electron-phonon coupling. The measured DPs are comparable to those known in non-topological semiconductors. We observe an order of magnitude larger A1g1 phonon- surface state electron DP in Bi2Te3 than Bi2Se3 and reproduce such result with density functional theory. Our results generally help understanding fundamental processes in topological insulators such as surface state transport and potentially electron-phonon-coupling mediated unconventional Cooper-pairing. In investigations of ultrafast dynamics, the methodology holds implications for optical control of matter and even ultrafast switching between topological phases