Scanning tunneling microscopy and spectroscopy of topological materials with broken symmetry

Abstract

This dissertation focuses on the probing of physics governing the electronic and structural properties of topological materials. In three-dimensional topological insulators, the native substitutional defects result in a shift of the chemical potential into the conduction and valence bands. The added conduction channels obscure the physics of the topological surface states. Chemical tuning has been used previously to counteract this parasitic conductivity. However, many details of the process are not well understood and the conditions required to produce optimal samples are not yet well established. In the first project, scanning tunneling spectroscopy was used to observe Landau quantization in thin films of (Bi1-xSbx)2Te3. By combining nanoscale imaging and spectroscopy, the sensitivity of the chemical potential to the chemical composition and thin film growth conditions was studied. The results demonstrate the multi-dimensional parameter space required to obtain an intrinsic topological insulator and provide knowledge to optimize the electronic properties of topological materials. Magnetic doping was then added to induce ferromagnetism, which creates an energy gap in the surface states of (Bi1-xSbx)2Te3. Tunneling conductance spectroscopy was used to examine the correlation between the density of magnetic impurities and the size of the surface band gap. The results indicate that large concentrations of Cr create impurity states inside the gap that reduce the effective gap magnitude. Finally, Landau level spectroscopy was applied to the surface state of Pb1-xSnxSe where the signature of electron-phonon coupling was extracted from the surface state dispersion and was used to determine the mass enhancement factor.