Development and Utilization of ARPES to Study Strong Spin-Orbit Coupled Materials

Justin Andrew Waugh, University of Colorado at Boulder

Abstract

Angle-resolved photoemission spectroscopy (ARPES) is an invaluable tool for observing the electronic structure, low-energy electron dynamics and in some cases, the orbital wavefunction of many solid state systems. Because of this, it is ideally suited to study strong spin-orbit-coupled materials, where the spin and orbital degrees of freedom can be directly measured. Here we present both improvements to ARPES as an experimental tool as well as measurements and calculations for multiple spin-orbit coupled materials.

We first present details of an improvement to the measurement scheme of ARPES, replacing the common analog detection scheme with a fast digital pulse-counting setup. This greatly improves counting linearity and allows for event discrimination to reject noise.

Next we present measurements on the prototypical topological insulator (TI) Bi2Se3. Topological insulators are new states of matter with symmetry-protected Dirac-like metallic states at their surfaces. Such Dirac states are also expected to exist at the interface between a TI and a non-topological material. Especially important interfaces are those between a superconductor and a TI as these are expected to become platforms for the creation of Majorana quasiparticles. Here we use ARPES to present the first direct studies of the buried interface between a metal and a TI, showing the Dirac-like states at the buried interface of superconducting Nb-Bi2Se3.

Our recent ARPES measurements on strong spin-orbit coupled materials have shown an in-plane orbital texture switch at their respective Dirac points. This feature has also been demonstrated in a few materials (Bi2Se3, Bi2Te3, and BiTeI) though DFT calculations. Here we present a minimal orbital-derived tight binding model to calculate the electron wave-function in a two-dimensional crystal lattice. We show that the orbital components of the wave-function demonstrate an orbital-texture switch in addition to the usual spin switch seen in spin polarized bands. Using our model we show that this feature is ubiquitous and to be expected in many real systems.

Last, we will discuss a new class of spin-orbit coupled materials. Spin polarized bands are traditionally expected to exist only under global inversion bulk symmetry breaking. LaBiOS2 has a centrosymmetric structure thus expected to have no spin polarization. However, it is predicted to have a local inversion-asymmetric structure, leading to spin polarization localized on the individual BiS2 sublayers. We measured ARPES and spin ARPES on the inversion-symmetric bulk crystal LaBiOS2 in search of such a hidden spin polarization. Our measurements show the band structure to have qualitative agreement with DFT calculations.