Direct Measurement of the Out-of-Plane Spin Texture in the Dirac Cone Surface State of a Topological Insulator

TL;DR

This study directly measures the out-of-plane spin component in the surface state of Bi2Te3, revealing a smaller-than-expected polarization influenced by Fermi surface shape, challenging existing theoretical models.

Contribution

First direct measurement of out-of-plane spin component in a topological insulator's surface state, highlighting the role of Fermi surface deformation.

Findings

Out-of-plane spin polarization reaches 25% of in-plane in Bi2Te3.

Hexagonal Fermi surface deformation correlates with out-of-plane spin component.

Observed polarization is smaller than theoretical predictions.

Abstract

We have performed spin- and angle-resolved photoemission spectroscopy of Bi2Te3 and present the first direct evidence for the existence of the out-of-plane spin component on the surface state of a topological insulator. We found that the magnitude of the out-of-plane spin polarization on a hexagonally deformed Fermi surface (FS) of Bi2Te3 reaches maximally 25% of the in-plane counterpart while such a sizable out-of-plane spin component does not exist in the more circular FS of TlBiSe2, indicating that the hexagonal deformation of the FS is responsible for the deviation from the ideal helical spin texture. The observed out-of-plane polarization is much smaller than that expected from existing theory, suggesting that an additional ingredient is necessary for correctly understanding the surface spin polarization in Bi2Te3.