Accessing the spin structure of buried electronic states

TL;DR

This paper demonstrates the use of spin- and angle-resolved photoemission spectroscopy (SARPES) with low-energy photons to directly measure the spin structure of electronic states at buried interfaces in topological insulators, advancing spin physics studies.

Contribution

It shows how low-energy SARPES can access and analyze the spin polarization of buried electronic states, specifically in a topological insulator film, revealing the spin texture of the interface state.

Findings

Successfully measured spin polarization of buried interface states

Modeled the complex spin signals from surface, bulk, and interface

Demonstrated direct determination of spin texture at buried interfaces

Abstract

In spin- and angle-resolved photoemission spectroscopy (SARPES) the energy-momentum dispersion of electronic states in crystalline solids is measured along with the spin direction of the photoemitted electrons. The technique therefore allows for mapping out a material's band structure in a spin resolved fashion. By conducting SARPES measurements using low-energy photons, the spin sensitivity of the technique can be combined an increased bulk probe depth, provided by the large electron inelastic mean-free path at these kinetic energies, to directly access the spin structure of electronic states at buried interfaces. Here, we demonstrate this capability by using SARPES to determine the spin polarization of photoelectrons emitted from a 6-nm-thick film of the topological insulator Bi$_2$Se$_3$ using photons with an energy of 8.5 eV. By modelling the expected spin structure in the film, we show that the complex spin polarization that is observed is the integrated spin signal from spin-polarized states at the surface, bulk and buried interface (bottom surface) of the topological-insulator film. Our results therefore allows us to directly determine the spin texture of the buried Dirac interface state. This capability is highly attractive for state-of-the art spectroscopic measurements of the spin-physics at play in quantum-material based or spintronic devices where spin-polarized interface states define the operational principle of the devices.