STM imaging of strongly correlated materials with spin-orbit coupling

Abstract

Most of the properties of electrons in conducting materials that we encounter in our everyday lives can be explained very neatly by considering them to be independent entities residing in the crystal lattice potential, as described by band theory of solids. However, the scenario gets very interesting when inter-electronic interactions and relativistic effects become strong enough, such that they can no longer be treated as small perturbations to the standard band structure. The situation gets even more intriguing for the surface states where the inversion symmetry is naturally lifted, leading to additional spin-orbit coupling. In thesis I study two quantum materials (PdCoO₂ and PdTe₂) where the interplay of spin orbit coupling (SOC) and inversion symmetry breaking (ISB) give rise to intriguing spin texture of the surface bands, which further leads to spin-selective scattering. Using Scanning Tunneling Microscopy and Spectroscopy, I sought to explore these spin-selective scattering processes by studying the quasi-particle interference (QPI) patterns at different energies in the vicinity of the Fermi-level. Among the two systems studied in the thesis, the CoO₂ terminated surface of metallic Delafossite PdCoO₂ shows an abnormally large Rashba spin splitting of the surface states. It is a Rashba system where the energy scale associated with the inversion symmetry breaking is much larger than the spin-orbit coupling. The surface states also exhibit signatures of Fermi-liquid like behaviour. Therefore this system allowed me to do a case study of the interplay of strong electronic correlations and spin-orbit coupling and thereby estimate the relative values of various intrinsic length scales in the system, which become significant from the point of view of application in device fabrication. I proceeded to study the influence of quantum confinement on the scattering processes and the related modulations in the local density of states. In course of these experiments, I determined the difference in the local barrier height of polar surface terminations, here the Pd and CoO₂ terminated surfaces of PdCoO₂, which may prove to be useful information for future researchers. I further went on to investigate the spin selective scattering processes in Dirac semi-metal PdTe₂ from analysis of quasiparticle interference. The surface states in this material have a complicated spin texture and this system also serves as the example case of my study, for the limit where the spin-orbit coupling is the dominant energy scale.