UC Santa Barbara

This dissertation explores epitaxial growth and modifications to the electronic band structure of topological semimetal materials through heteroepitaxy, biaxial strain, and reduced dimensionality. High-quality thin films are grown via molecular beam epitaxy (MBE) and studied using a combination of angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT), and low-temperature magnetotransport.

Recent predictions of topological phases and observations of extremely large magnetoresistance in the class of rare-earth monopnictides, and specifically GdSb, have opened up a new research front aimed at studying the interplay between magnetoresistance, topology, and magnetic ordering. The first part of the dissertation focuses on magnetoresistance and band topology evolution in lattice-matched and biaxially strained GdSb (001) thin films. Lattice-matched GdSb films show a mobility and carrier concentration imbalance, deviating from the commonly assumed compensated charge carrier densities seen in bulk rare-earth monopnictides. Next, we established a clear connection between biaxial strain in GdSb films and the affected band dispersions based on their orbital composition. As biaxial strain is tuned from tensile to compressive strain, the gap between the hole and the electron bands dispersed along [001] decreases.

The second part of this dissertation reports the first ARPES investigation conclusively assigning the topological character of bismuth as a trivial ℤ2 state by studying ultrathin Bi (111) films grown on InSb (111)B. Bismuth films hold promise for potential applications in spintronic devices and topological one-dimensional edge transport. Yet synthesizing high-quality, wafer-scale ultrathin bismuth films on non-metallic substrates remains challenging. We achieved large-area Bi (111) films with a single epitaxial domain orientation and mapped the dispersion of surface states and quantum well states. Strong film-substrate interactions were found to promote epitaxial stabilization of the (111) orientation and lead to inversion symmetry breaking. Our results demonstrate that interfacial bonds prevent the semimetal-to-semiconductor transition predicted for freestanding bismuth layers, highlighting the importance of controlled functionalization and surface passivation in two-dimensional materials.

Finally, the growth parameters of biaxially strained LuPtBi films on InSb were explored. The half-Heusler compound LuPtBi belongs to a unique group of superconductors with a topologically nontrivial band structure, and very low carrier densities (<10^20 cm^-3). A suitable growth window could not be identified for (111) oriented LuPtBi. Room-temperature nucleation of LuPtBi on LuPtSb/InSb (001) allowed higher incorporation of Bi in LuPtBi and produced smooth, coherently strained films.