Insulator Defies Expectations with 2% Thermal Hall Effect, Paving the Way for New Devices

Researchers have demonstrated a drastically enhanced thermal Hall effect within the topological insulator TlBiSbTe₂, revealing a novel mechanism for heat transport. Rohit Sharma, Yongjian Wang, and Yoichi Ando, all from the II. Physikalisches Institut, Universit at zu K oln, alongside Achim Rosch and Thomas Lorenz et al., pinpoint the origin of this effect to ‘charge puddles’ , locally conducting regions induced by charged impurities within the material. Their findings show a thermal Hall ratio exceeding 2%, an unprecedented value for a non-magnetic substance, and establish that electron-phonon coupling within these charge puddles imprints a substantial thermal Hall effect onto phonons, thereby explaining both the magnitude and magnetic field dependence of the phenomenon. This work provides crucial insight into manipulating heat flow at the microscopic level and opens avenues for developing innovative thermal technologies.

Charge puddle formation explains enhanced thermal Hall effect in TlBi0.15Sb0.85Te2

Researchers have experimentally observed a significantly enhanced thermal Hall effect within the insulating material TlBi0.15Sb0.85Te2. Despite heat being primarily carried by phonons, the application of moderate magnetic fields generates a thermal Hall ratio exceeding 2%, an unprecedented value for a nonmagnetic substance.
This substantial ratio indicates an unexpectedly strong transverse heat current induced by the magnetic field. The transverse thermal conductivity exhibits a distinct peak at magnetic fields of only a few Tesla, providing crucial insight into the underlying mechanisms driving this phenomenon. This characteristic field dependence has enabled the identification of the microscopic origin of the thermal Hall effect within this specific system.

The research demonstrates that small concentrations of charged impurities create localized conducting regions, termed charge puddles, within the insulating bulk material. These charge puddles, through their interaction with phonons via electron-phonon coupling, effectively transfer a substantial thermal Hall effect onto the phonons.

This process accounts for both the magnitude of the observed effect and its unique dependence on the applied magnetic field. The study reveals a thermal Hall ratio reaching approximately 2% for magnetic fields between 2 and 8 Tesla, sustained across a temperature range of 50 to 150 Kelvin. This contrasts sharply with previously observed phonon-related thermal Hall effects, which typically exhibit linear field dependence and Hall ratios of around 10−3.

The formation of charge puddles is attributed to unavoidable charged defects introduced during material growth, which, despite being present in only a few parts per million, significantly alter the material’s bulk properties due to its high dielectric constant of approximately 102. These puddles, acting as locally doped regions within the insulating matrix, are responsible for imprinting the observed thermal Hall effect onto the phonon population.

Thermal Hall effect determination via steady-state heat transport measurements

TlBi₀.₁₅Sb₀.₈₅Te₂ was investigated using a bespoke thermal transport measurement setup to reveal an exceptionally large thermal Hall effect. Heat transport within the topological insulator material was measured using a standard steady-state method, applying a temperature gradient along a bar-shaped sample measuring 3mm x 1mm x 1mm.

Precise temperature control was maintained via miniature heaters and thermometers affixed to the sample, allowing for accurate determination of thermal conductivity components. Measurements were performed in magnetic fields up to 9 Tesla, generated by a superconducting magnet, and over a temperature range of 50 to 150 Kelvin, achieved using a cryostat.

The transverse thermal conductivity, κxy, was determined by measuring the temperature difference along the sample width in the presence of a magnetic field applied perpendicular to the temperature gradient. This setup enabled the observation of a pronounced maximum in κxy at fields of a few Tesla, with a corresponding thermal Hall ratio, κxy/κxx, exceeding 2%, an unprecedented value for a nonmagnetic material.

The magnetic field dependence of the thermal Hall effect was meticulously mapped to discern the underlying microscopic origin of the phenomenon. Analysis of the data suggests that small densities of charged impurities within the TlBi₀.₁₅Sb₀.₈₅Te₂ create locally conducting regions termed charge puddles.

These charge puddles, acting as scattering centres, couple to phonons via electron-phonon interactions, imprinting a substantial thermal Hall effect onto the phonon population. This coupling accounts for both the magnitude and the observed non-monotonic magnetic field dependence of the thermal Hall effect, providing a novel explanation for phonon-driven thermal transport in topological insulators.

Enhanced Thermal Hall effect and charge carrier dynamics in TlBi0.15Sb0.85Te2 single crystals

Thermal Hall ratios in TlBi0.15Sb0.85Te2 samples reached above 2%, an unprecedented value for a nonmagnetic material. Measurements of transverse thermal conductivity revealed a pronounced maximum in fields of a few Tesla for both samples, S1 and S2. The study details how these observations stem from a novel mechanism involving charged impurities and the formation of charge puddles within the insulating matrix.

Longitudinal electrical resistivities, ρxx, displayed a metal-to-insulator-like transition around 80 K for sample S1, while sample S2 exhibited slight semiconducting behavior below 100 K, with absolute values of ρxx lower than previously reported for related materials. Thermal conductivities, κxx, showed weak phononic maxima around 10, 20 K, similar to those observed in BiSbTeSe2-type materials.

Estimates using the Wiedemann-Franz law indicated that the charge-carrier contribution to heat transport remained below 10% of the lower κxx value for sample S1, confirming phonon-dominated longitudinal heat transport. A small overall decrease in κxx was observed with increasing magnetic field. Thermal Hall conductivities, κxy, exhibited broad maxima as a function of temperature, broadening with increasing field and shifting to higher temperatures.

Specifically, κxy reached approximately 50mW/Km at moderate magnetic fields between 2 and 8 Tesla over a temperature range exceeding 100 K. The peaks in κxy and κxx occurred at different temperatures, distinguishing this behaviour from other phonon-dominated compounds where peaks typically align. This research identifies that the observed thermal Hall effect originates from the Lorentz force acting on mobile charge carriers within localized charge puddles, imprinting a large thermal Hall effect onto the phonons.

Charge puddle induced thermal Hall effect in TlBi0.15Sb0.85Te2

Scientists have observed a significantly enhanced thermal Hall effect in the insulating material TlBi₀.₁₅Sb₀.₈₅Te₂. This effect, where heat flows perpendicularly to a temperature gradient, typically requires magnetic fields to induce a measurable response, but this material exhibits a thermal Hall ratio exceeding 2% even with moderate magnetic fields.

The transverse thermal conductivity reaches a maximum at fields of a few Tesla, providing key insights into the underlying mechanisms. The origin of this substantial thermal Hall effect lies in the presence of charge puddles within the insulator. These puddles, formed by charged impurities, act as locally conducting regions and couple to phonons, imprinting a large thermal Hall effect onto the heat-carrying vibrations.

Experiments confirm that changes in temperature affect the density of these puddles, and contributions from bulk electrical transport are negligible at lower temperatures. This research establishes TlBi₀.₁₅Sb₀.₈₅Te₂ as a material with an exceptionally large thermal Hall effect, where the microscopic cause can be identified with reasonable certainty.

The authors acknowledge that the observed mechanism may not be universally applicable to all materials. While the large dielectric constant of bismuth-based compounds facilitates the formation of well-conducting charge puddles, the impact of charged impurities on the thermal Hall effect in other systems remains an open question.

Future research should investigate whether this puddle-induced effect is specific to certain materials or represents a more general pathway for generating substantial phonon thermal Hall effects in insulators, potentially extending to regimes where well-defined puddles do not form. Exploring the scattering of phonons from dynamical defects in such scenarios presents a compelling theoretical challenge.