Unusual spin dynamics in topological insulators

TL;DR

This paper reveals that topological insulators exhibit an unusual, non-Lorentzian dynamic spin susceptibility due to strong spin-orbit coupling, affecting spin relaxation understanding and drawing parallels with graphene's optical properties.

Contribution

It demonstrates the non-Lorentzian form of DSS in topological insulators and Weyl semimetals, highlighting the impact of strong SOC on spin dynamics and relaxation.

Findings

DSS has a universal high-frequency form increasing as ω^{d-1} in certain topological systems.

Spin relaxation rates cannot be inferred from DSS in topological insulators.

Parallel behavior observed between DSS in topological insulators and optical conductivity in graphene.

Abstract

The dynamic spin susceptibility (DSS) has a ubiquitous Lorentzian form in conventional materials with weak spin orbit coupling, whose spectral width characterizes the spin relaxation rate. We show that DSS has an unusual non-Lorentzian form in topological insulators, which are characterized by strong SOC. At zero temperature, the high frequency part of DSS is universal and increases in certain directions as $\omega^{d-1}$ with $d=2$ and 3 for surface states and Weyl semimetals, respectively, while for helical edge states, the interactions renormalize the exponent as $d=2K-1$ with $K$ the Luttinger-liquid parameter. As a result, spin relaxation rate cannot be deduced from the DSS in contrast to the case of usual metals, which follows from the strongly entangled spin and charge degrees of freedom in these systems. These parallel with the optical conductivity of neutral graphene.