Observation of a warped helical spin-texture in Bi$_2$Se$_3$ from circular dichroism angle-resolved photoemission spectroscopy

TL;DR

This study provides direct experimental evidence of a warped helical spin-texture in Bi$_2$Se$_3$ surface states through circular dichroism angle-resolved photoemission spectroscopy, revealing complex spin structures influenced by spin-orbit coupling.

Contribution

The paper introduces a novel rotational analysis technique in photoemission spectroscopy to observe the three-dimensional warped spin-texture in topological insulators, supported by a microscopic theory.

Findings

Circular dichroism modulations reveal warped spin-texture

The observed patterns match theoretical predictions

Spin-orbit coupling explains the dichroism behavior

Abstract

A differential coupling of topological surface states to left- versus right-circularly polarized light is the basis of many opto-spintronics applications of topological insulators. Here we report direct evidence of circular dichroism from the surface states of Bi$_2$Se$_3$ using a laser-based time-of-flight angle-resolved photoemission spectroscopy. By employing a novel sample rotational analysis, we resolve unusual modulations in the circular dichroism photoemission pattern as a function of both energy and momentum, which perfectly mimic the predicted but hitherto un-observed three-dimensional warped spin-texture of the surface states. By developing a microscopic theory of photoemission from topological surface states, we show that this correlation is a natural consequence of spin-orbit coupling. These results suggest that our technique may be a powerful probe of the spin-texture of spin-orbit coupled materials in general.