Condensed Matter Special Seminar - "Thermal and Thermoelectric Probes of Quantum States"

Conventional electrical transport is often the first measurement used to characterize a new quantum material. Thermal and thermoelectric transport, however, can reveal information that is inaccessible to charge transport alone. In this talk, I will present two examples that demonstrate the distinct capabilities of these probes.

First, in the Weyl semimetal NbP, planar thermoelectric transport reveals a field-activated, butterfly-like angular dependence of the Seebeck response. Conventional angular magnetotransport in high-mobility semimetals can be obscured by current-path inhomogeneity. Through the Mott relation, the thermoelectric response is governed by the energy derivative of the electrical conductivity near the Fermi level, making it particularly sensitive to small Fermi pockets, low Fermi energies, and subtle field-induced changes in multiband transport weights. Combined with angle-dependent quantum oscillations and density-functional-theory calculations, our results show that the butterfly-like response arises from anisotropic Fermi-surface geometry together with electron–hole thermoelectric compensation. These findings establish planar thermoelectricity as a symmetry-resolved probe of hidden electronic anisotropy in topological semimetals.

Second, in the kagome Mott insulator YCu₃(OH)₆Br₂[Br_x(OH)_1-x](YCOB), ultrasensitive high-field thermal-transport measurements reveal a finite residual linear term in the thermal conductivity despite the absence of electrical conduction, providing evidence for mobile, gapless, charge-neutral excitations. Together with field-dependent specific-heat measurements, these observations support a field-induced quantum-spin-liquid regime associated with the one-ninth magnetization plateau. I will also discuss the development of a high-field, millikelvin thermal-transport platform at the National High Magnetic Field Laboratory that enabled these measurements.

Together, these studies demonstrate that thermal and thermoelectric transport provide a powerful window into emergent quantum matter, uncovering hidden electronic anisotropy and charge-neutral excitations that remain inaccessible to conventional electrical transport.