Critical enhancement of thermopower in a chemically tuned polar semimetal MoTe2
Hideaki Sakai, Koji Ikeura, Mohammad Saeed Bahramy, Naoki Ogawa, Daisuke Hashizume, Jun Fujioka, Yoshinori Tokura, Shintaro Ishiwata
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−xNbxTe2 with 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.
Probe Raman, polar metal, transition metal dichalcogenides, semimetal, thermo, power, optical second-harmonic generation, polar structural transition, critical scattering, first-principles calculation