Enhanced Thomson and Unusual Nernst Coefficients in 1T-TiSe2 Due to Bipolar Transport and CDW Phase Transition

Abstract

Thermoelectric coolers utilizing the Peltier effect have dominated the field of solid-state cooling but their efficiency is hindered by material limitations. Alternative routes based on the Thomson and Nernst effects offer new possibilities. Here, we present a comprehensive investigation of the thermoelectric properties of 1T-TiSe₂, focusing on these effects around the charge density wave transition (≈200 K). The abrupt Fermi surface reconstruction associated with this transition leads to an exceptional peak in the Thomson coefficient of 450 μV K⁻¹ at 184 K, surpassing the Seebeck coefficient. Furthermore, 1T-TiSe₂ exhibits a remarkably broad temperature range (170–400 K) with a Thomson coefficient exceeding 190 μV K⁻¹, a characteristic highly desirable for the development of practical Thomson coolers with extended operational ranges. Additionally, the Nernst coefficient exhibits an unusual temperature dependence, increasing with temperature in the normal phase, which we attribute to bipolar conduction effects. The combination of solid–solid pure electronic phase transition to a semimetallic phase with bipolar transport is identified as responsible for the unusual Nernst trend and the unusually large Thomson coefficient over a broad temperature range.