Phonon-Dominated Thermal Transport and Large Violation of the Wiedemann-Franz Law in Topological Semimetal CoSi

TL;DR

This study reveals that in the topological semimetal CoSi, thermal transport is dominated by phonons, leading to a significant violation of the Wiedemann-Franz law due to bipolar diffusion and large lattice thermal conductivity.

Contribution

It uncovers the unusual dominance of phonon thermal transport and the large violation of the WF law in CoSi, highlighting the role of topological band effects and bipolar diffusion.

Findings

Electronic Lorenz number exceeds L0 by ~40%.

Lattice thermal conductivity becomes dominant below room temperature.

Bipolar diffusion enhances heat current due to electron-hole compensation.

Abstract

The Wiedemann-Franz (WF) law, relating the electronic thermal conductivity ($\kappa_{\rm e}$) to the electrical conductivity, is vital in numerous applications such as in the design of thermoelectric materials and in the experimental determination of the lattice thermal conductivity ($\kappa_{\rm L}$). While the WF law is generally robust, violations are frequently observed, typically manifesting in a reduced Lorenz number ($L$) relative to the Sommerfeld value ($L_0$) due to inelastic scattering. Here, we report a pronounced departure from the WF law in the topological semimetal CoSi, where the electronic Lorenz number ($L_{\rm e}$) instead rises up to $\sim40\%$ above $L_0$. We demonstrate that this anomaly arises from strong bipolar diffusive transport, enabled by topological band-induced electron-hole compensation, which allows electrons and holes to flow cooperatively and additively enhance the heat current. Concurrently, we unveil that the lattice contribution to thermal conductivity is anomalously large and becomes the dominant component below room temperature. As a result, if $\kappa_{\rm L}$ is assumed negligible -- as conventional in metals, the resulting $L$ from the total thermal conductivity ($\kappa_{\rm tot}=\kappa_{\rm L}+\kappa_{\rm e}$) deviates from $L_0$ by more than a factor of three. Our work provides deeper insight into the unconventional thermal transport physics in topological semimetals.