# Chiral Spin Mode on the Surface of a Topological Insulator

## TL;DR

This study uses polarization-resolved Raman spectroscopy to identify a transverse collective spin mode on the surface of a topological insulator, revealing a spin wave-like excitation influenced by spin-orbit coupling.

## Contribution

First observation and identification of a transverse collective spin mode on a topological insulator surface using Raman spectroscopy.

## Key findings

- Detected a sharp 150 meV peak in Raman spectra.
- Identified the peak as a transverse collective spin mode.
- Mode is analogous to a spin wave in a Fermi liquid.

## Abstract

Using polarization-resolved resonant Raman spectroscopy, we explore collective spin excitations of the chiral surface states in a three dimensional topological insulator, Bi$_2$Se$_3$. We observe a sharp peak at 150 meV in the pseudovector $A_2$ symmetry channel of the Raman spectra. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface plasmon in the charge sector of excitations, is analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field.