ENERGY TRANSPORT IN THIN-FILM TOPOLOGICAL MATERIALS

Solid-state thin films are vastly involved in the advanced electronics industry and emerging condensed-matter physics research. The perspective of energy carriers plays a key role of understanding heat transfer physics in nanoscale thin films, also introduces numerous experimental and numerical tools. Topological materials are a new phase of condensed matter, with topologically nontrivial electron states emerged on whose boundary. The topological state electrons possess unique carrier dynamics and spin polarization, leading to unconventional energy transport performance and the possibility for controlled heat transfer, which is largely unexplored. Thin films three-dimensional topological insulators are a good platform to investigate topological surface states due to the large surface to bulk ratio.

In this doctoral work, optical, electrical, and thermal methods are incorporated to study the energy transport carried by topological surface state electrons. The optothermal micro-Raman thermometry method was applied to measure the thermal conductivity of topological insulator thin films. The surface states in sub-20-nm were found dominant in charge and energy transport, and an extraordinary large Lorenz number was identified. The thermal conductivity of Weyl semiconductor 2D-tellurium, another topological material, was similarly studied. A comprehensive calibration method was proposed to resolve the complication of temperature/strain Raman responses in the presence of thermal expansion during micro-Raman thermal conductivity measurement. Photocurrent under circular polarization incidence was studied to understand topological surface states transport characteristics under optical spin injection. The level of helicity-tunability of topological surface states was optimized and deep tuning of photo-thermoelectricity from surface states was observed. The helicity-dependent photoconductance was also discovered to be a competitive candidate for optical chirality detection, and single-device Stokes parameters analyses were implemented using a Bi₂Te₂Se phototransistor.Lastly, micro-Raman thermometry and fabrication techniques for vacuum or optical gating are combined to perform gate-dependent thermal conductivity measurement for nanometer thin films. Gate-dependent thermal conductivity of thin-film topological insulators is studied and the large contribution from topological surface state is found electrostatically tunable. The results lead to an implementation of solid-state electrostatic thermal transistors with large ON/OFF ratio and fast switching time. Temperature dependence of the gated-tunable heat transport is also studied for further understanding of the topological states in heat transport.

Thermal energy transport phenomena in topological materials are approaches from both directions of metrology and dynamic control. The studies demonstrate the intriguing behaviors of topological states and also indicate the implementation of optical and thermal devices with exotic functionalities.