# Critical enhancement of thermopower in a chemically tuned polar semimetal MoTe$_{\bf 2}$

## TL;DR

This study uncovers a significant enhancement of thermopower in a chemically tuned polar semimetal near its phase transition, revealing how structural instability influences transport properties and suggesting new avenues for thermoelectric material development.

## Contribution

It demonstrates the first controlled observation of a ferroelectric-like transition in a metallic system and its impact on thermoelectric properties, combining first-principles calculations and experiments.

## Key findings

- Thermopower is anomalously enhanced near the polar-nonpolar phase boundary.
- Charge transport is critically affected by structural instability and polar fluctuations.
- The results suggest potential for improving thermoelectric efficiency through structural tuning.

## Abstract

Ferroelectrics with spontaneous electric polarization play an essential role in today's device engineering, such as capacitors and memories. Their physical properties are further enriched by suppressing the long-range polar order, as is exemplified by quantum paraelectrics with giant piezoelectric and dielectric responses at low temperatures. Likewise in metals, a polar lattice distortion has been theoretically predicted to give rise to various unusual physical properties. So far, however, a "ferroelectric"-like transition in metals has seldom been controlled and hence its possible impacts on transport phenomena remain unexplored. Here we report the discovery of anomalous enhancement of thermopower near the critical region between the polar and nonpolar metallic phases in 1T'-Mo$_{1-x}$Nb$_{x}$Te$_2$ with a chemically tunable polar transition. It is unveiled from the first-principles calculations and magnetotransport measurements that charge transport with strongly energy-dependent scattering rate critically evolves towards the boundary to the nonpolar phase, resulting in large cryogenic thermopower. Such a significant influence of the structural instability on transport phenomena might arise from the fluctuating or heterogeneous polar metallic states, which would pave a novel route to improving thermoelectric efficiency.