Relationship between Fermi-Surface Warping and Out-of-Plane Spin Polarization in Topological Insulators: a View from Spin-Resolved ARPES

TL;DR

This study investigates the link between Fermi-surface warping and out-of-plane spin polarization in topological insulators using spin-resolved ARPES, revealing a systematic correlation and suggesting many-body effects influence the observed polarization.

Contribution

It provides a quantitative analysis of warping and spin polarization across multiple topological insulators, establishing a universal relationship and highlighting deviations from theoretical predictions.

Findings

Out-of-plane spin polarization correlates with warping strength.

Observed Pz is half of theoretical predictions under strong warping.

Data offers empirical guidance for tuning spin polarization.

Abstract

We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)2Te4 (Pb124) and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators (Pb124, Bi2Te3, Bi2Se3, and TlBiSe2) and elucidated that the out-of-plane spin polarization Pz is systematically correlated with the warping strength. However, the magnitude of Pz is found to be only half of that expected from the kp theory when the warping is strong, which points to the possible role of many-body effects. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators.