Ab initio phonon self-energies and fluctuation diagnostics of phonon anomalies: Lattice instabilities from Dirac pseudospin physics in transition metal dichalcogenides

TL;DR

This paper introduces an ab initio method to analyze phonon self-energies and their electronic origins, revealing that Dirac pseudospin textures influence charge-density waves in transition metal dichalcogenides like 1H-TaS2.

Contribution

It provides a novel ab initio framework for diagnosing phonon anomalies and identifies Dirac pseudospin textures as key to charge-density-wave phenomena in these materials.

Findings

Coupling between acoustic phonons and a low-energy metallic band causes phonon anomalies.

Dirac pseudospin textures control charge-density-wave physics.

Matrix-element effects are crucial in phonon mode softening.

Abstract

We present an ab initio approach for the calculation of phonon self-energies and their fluctuation diagnostics, which allows us to identify the electronic processes behind phonon anomalies. Application to the transition-metal-dichalcogenide monolayer 1H-TaS$_2$ reveals that coupling between the longitudinal-acoustic phonons and the electrons from an isolated low-energy metallic band is entirely responsible for phonon anomalies such as the mode softening and associated charge-density waves observed in this material. Our analysis allows us to distinguish between different mode-softening mechanisms including matrix-element effects, Fermi-surface nesting, and Van Hove scenarios. We find that matrix-element effects originating from a peculiar type of Dirac pseudospin textures control the charge-density-wave physics in 1H-TaS$_2$ and similar transition metal dichalcogenides.