Spin-polarization of topological crystalline and normal insulator Pb$_{1-x}$Sn$_x$Se (111) epilayers probed by photoelectron spectroscopy

TL;DR

This study investigates the spin polarization and electronic structure of Pb$_{1-x}$Sn$_x$Se (111) epilayers, revealing persistent helical spin textures across topological and trivial phases, with implications for spintronic applications.

Contribution

It demonstrates that surface state spin polarization exists even in trivial insulators and explores how surface modifications induce band gap opening without magnetism.

Findings

Helical spin polarization reaches 30% in surface states.

Band gap opening can be induced by Sn content or surface metal deposition.

Surface states exhibit spin polarization even in trivial insulators.

Abstract

The helical spin texture on the surface of topological crystalline insulators (TCI) makes these materials attractive for application in spintronics. In this work, spin-polarization and electronic structure of surface states of (111)-oriented Pb$_{1-x}$Sn$_x$Se TCI epitaxial films are examined by angle -- as well as spin-resolved photoemission spectroscopy (SR-ARPES). High-quality epilayers with various Sn content are grown by the molecular beam epitaxy (MBE) method. Topological-normal insulator transition manifesting itself as band gap opening is observed. It is shown that the gap opening can be induced not only by changing the Sn content of the epilayer but also depositing a transition metal (TM) on its surface. In the latter case, the observed gaping of the surface states is caused by change in surface composition and not by magnetism. We also show that helical spin polarization is present not only for samples of topological composition but also for trivial ones (with an open band gap). The observed spin polarization reaches a value of 30 % for the in-plane spin component and is almost absent for the out-of-plane one. We believe that our work will pave the way for the application of surface states not only of topological but also normal insulators based on lead-tin chalcogenides in spin-charge conversion devices.