Quantifying quasiparticle chirality in a chiral topological semimetal

TL;DR

This study directly measures the electronic chirality in a chiral topological semimetal RhSi using spin-resolved photoemission, introducing a new energy-dependent metric that correlates with magneto-optical and transport properties.

Contribution

It introduces the normalized electron chirality density (NECD) as a novel, experimentally accessible metric for bulk electronic chirality in chiral topological materials.

Findings

NECD decreases from 1 at the Kramers-Weyl point to ~0.8 at 200 meV below.

Measured deviations from perfect spin-momentum locking are up to ~40°.

NECD predicts magneto-optical and transport responses such as the Edelstein effect.

Abstract

Recently, the projection of the electron's spin on its crystal momentum has been proposed as a metric to quantify electronic chirality of Bloch states in crystals, which is expected to affect a wide range of physical properties, such as magnetoelectric and optical responses. However, a direct experimental quantification of this chirality metric over an entire iso-energy surface has remained elusive. Here, we have used spin- and angle-resolved photoemission spectroscopy to directly probe the electronic chirality by measuring the bulk spin texture of Kramers-Weyl and Weyl cones in RhSi, a chiral topological semimetal with strong spin-orbit coupling (SOC). After quantifying the SOC splitting of Weyl cones, we determine their spin direction along different azimuthal angles to extract energy dependent the deviations (up to ~40{\deg}) from perfect parallel spin-momentum locking. From these deviations we define an energy-dependent normalized electron chirality density (NECD), a directly accessible metric of bulk electronic chirality. In RhSi, the NECD decreases from 1 at the Kramers-Weyl point to ~0.8 at ~200 meV below it. Finally, we show that this experimentally grounded NECD provides predictive power for magneto-optical and transport responses of chiral materials, exemplified by the longitudinal Edelstein effect.