title: Critical enhancement of thermopower in a chemically tuned polar semimetal MoTe2.
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 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. However, to date, a "ferroelectric"-like transition in metals has seldom been controlled, and hence, its possible impacts on transport phenomena remain unexplored. We report the discovery of anomalous enhancement of thermopower near the critical region between the polar and nonpolar metallic phases in 1T'-Mo1-xNbxTe2with a chemically tunable polar transition. It is unveiled from the first-principles calculations and magnetotransport measurements that charge transport with a strongly energy-dependent scattering rate critically evolves toward 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.
authors.affiliation: Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.; Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Hideaki
authors.initials: H
authors.lastname: Sakai
authors.affiliation: Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Ikeura
authors.affiliation: Quantum-Phase Electronics Center and Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.; RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan.
authors.forename: Mohammad Saeed
authors.initials: MS
authors.lastname: Bahramy
authors.affiliation: RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan.
authors.forename: Naoki
authors.initials: N
authors.lastname: Ogawa
authors.affiliation: RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Hashizume
authors.affiliation: Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.; Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Fujioka
authors.affiliation: Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.; RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan.
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Tokura
authors.affiliation: Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.; Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Shintaro
authors.initials: S
authors.lastname: Ishiwata
keywords: Polar metal; critical scattering; first-principles calculation; optical second-harmonic generation; polar structural transition; semimetal; thermopower; transition metal dichalcogenides