Topological Thermoelectricity in Metals

TL;DR

This paper demonstrates that certain topological phonon features in metals can lead to high thermoelectric efficiency by combining low thermal conductivity with good electrical transport, introducing a new class of topological thermoelectric metals.

Contribution

It provides theoretical and numerical evidence that triple point topological phonon crossings in metals create phases with enhanced thermoelectric properties, proposing new compounds like TaSb and TaBi.

Findings

Triple point phonon crossings suppress lattice thermal conductivity.

Topological fermionic excitations enhance electronic density of states.

Predicted compounds exhibit promising thermoelectric performance.

Abstract

Recently published discoveries of acoustic and optical mode inversion in the phonon spectrum of certain metals became the first realistic example of non-interacting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this work we provide a strong theoretical and numerical evidence that for a class of metallic compounds (known as triple point topological metals), the points in the phonon spectrum, at which three (two optical and one acoustic) phonon modes (bands) cross, represent a well-defined topological material phase, in which the hosting metals have very strong thermoelectric response. The triple point bosonic collective excitations appearing due to these topological phonon band-crossing points significantly suppress the lattice thermal conductivity, making such metals phonon-glass like. At the same time, the topological triple-point and Weyl fermionic quasiparticle excitations present in these metals yield good electrical transport (electron-crystal) and cause a local enhancement in the electronic density of states near the Fermi level, which considerably improves the thermopower. This combination of "phonon-glass" and "electron-crystal" is the key for high thermoelectric performance in metals. We call these materials topological thermoelectric metals and propose several newly predicted compounds for this phase (TaSb and TaBi). We hope that this work will lead researchers in physics and materials science to the detailed study of topological phonon phases in electronic materials, and the possibility of these phases to introduce novel and more efficient use of thermoelectric materials in many everyday technological applications.