TbCr₆Ge₆ Kagome Metal Demonstrates Breakdown of the Wiedemann–Franz Law and Thermal Hall Effect

The interplay between heat and electrical charge remains a fundamental question in condensed matter physics, and recent research explores this relationship in unconventional materials. Jhinkyu Choi, Mohan B. Neupane, L. H. Vilela-Leão, and colleagues at Purdue University, along with Arjun Unnikrishnan from both Purdue University and the Indian Institute of Science, and Syeda Neha Zaidi, investigate this connection within the kagome metal TbCr6Ge6. Their work demonstrates a significant violation of the Wiedemann-Franz law, a principle linking thermal and electrical conductivity, revealing a decoupling of heat and charge transport. This breakdown occurs alongside the emergence of a thermal Hall effect, indicating the presence of heat carried by charge-neutral excitations, and establishes TbCr6Ge6 as a promising material for studying and controlling the flow of heat and charge in novel ways.

Kagome Lattice Magnons and Thermal Transport

This research details investigations into the unusual thermal and electrical properties of a novel material, a kagome lattice compound, specifically YCr6Ge6. The study focuses on understanding how magnetic excitations, known as magnons, contribute to both heat and charge transport, deviating from conventional behavior. The material’s kagome lattice structure is crucial, as it supports unique magnetic properties and potentially exotic excitations like topological magnons. Researchers developed a theoretical framework, a two-band magnon theory, to explain the observed transport phenomena, suggesting a complex interplay between different magnon bands.

The team discovered that the material exhibits unusual thermal conductivity, potentially due to ballistic magnon transport, where heat is carried by magnons without scattering. They also observed a large magnetoresistance, a significant change in electrical resistance in the presence of a magnetic field, indicating a strong coupling between charge carriers and the magnetic order. This research suggests that magnons play a significant role in both thermal and electrical transport, extending beyond the typical understanding of heat conduction by phonons and charge conduction by electrons. The developed two-band magnon model provides a framework for understanding these transport properties.

High-quality single crystals of the material were successfully synthesized and structurally characterized. Researchers then investigated the material’s complex magnetic ordering using a range of experimental techniques, including measurements of magnetic properties and detailed thermal and electrical transport measurements performed in extremely cold environments. They also employed theoretical modeling based on the two-band magnon theory to interpret their findings.

TbCr6Ge6 Transport and Magnetoresistance Measurements

This study investigates thermal and charge transport in the kagome metal TbCr6Ge6, employing a comprehensive suite of techniques to probe the interplay between heat and charge carriers. Single crystals of TbCr6Ge6 were grown and characterized using a dilution refrigerator, allowing precise control of temperature. Electrical resistivity and magnetization were measured using a four-probe configuration, allowing researchers to isolate contributions from different transport regimes and correlate them with the magnetic state of the material. Background magnetoresistance was carefully subtracted from the raw data, revealing intrinsic contributions arising from spin-dependent scattering.

Researchers meticulously tracked the evolution of magnetization using both field and temperature sweeps, providing a quantitative reference for interpreting the resistivity data. Detailed fitting procedures were performed to extract key parameters from the resistivity data, allowing them to correlate the system’s magnetization with its electrical response. Analysis of the temperature dependence of resistivity revealed a transition from behavior typical of a simple metal at higher temperatures to a different behavior below the ferrimagnetic transition, signifying the importance of scattering from low-energy magnetic excitations. These measurements, spanning a temperature range of 2 to 20 Kelvin, demonstrate that TbCr6Ge6 maintains metallic behavior, with its resistivity strongly linked to both applied magnetic field and the evolution of magnetization. Researchers observed a significant breakdown of the Wiedemann-Franz law, a fundamental principle linking thermal and electrical conductivity, across the ferrimagnetic transition within the compound. Specifically, both longitudinal and transverse Lorenz ratios, quantifying the relationship between thermal and electrical conduction, deviate strongly from the expected value upon cooling. Following a partial recovery between 5 and 7 Kelvin, these Lorenz ratios are sharply suppressed to values well below the expected baseline, despite the material maintaining a metallic charge response.

This suppression indicates the presence of additional heat-carrying channels beyond the conventional quasiparticles typically responsible for thermal conduction. Further investigation revealed a pronounced suppression of both longitudinal and transverse Lorenz ratios at low temperatures, alongside a sign-changing transverse Lorenz ratio. This pattern provides clear evidence of decoupling between heat and charge transport, signaling substantial contributions from charge-neutral excitations. Detailed analysis of the material’s magnetic properties, including temperature-dependent susceptibility measurements, established a field-temperature phase diagram delineating regions of long-range order, short-range order, and a paramagnetic state. Measurements of electrical resistivity and magnetization further revealed a strong correlation between the low-field resistivity and the material’s magnetization, indicating that spin-dependent scattering plays a dominant role in the observed transport behavior. These findings establish TbCr6Ge6 as a tunable metallic system where exchange-driven ferrimagnetism governs both longitudinal and transverse thermal responses, enabling controlled departures from the Wiedemann-Franz law.

Magnetism Decouples Heat and Charge Carriers

This research demonstrates a significant departure from conventional thermal transport behavior in the kagome metal TbCr6Ge6, revealing a strong decoupling between heat and charge carriers. Scientists observed a breakdown of the Wiedemann-Franz law, which normally links electrical and thermal conductivity, across the material’s ferrimagnetic transition, with both longitudinal and transverse Lorenz ratios deviating substantially from expected values. Further investigation revealed a pronounced suppression of these ratios at low temperatures, alongside a sign-changing transverse Lorenz ratio, indicating the presence of charge-neutral heat carriers contributing to thermal transport. The findings establish TbCr6Ge6 as a tunable platform for studying the interplay between magnetism and thermal conduction, where the material’s ferrimagnetic state governs both longitudinal and transverse thermal responses. Researchers connected the observed behavior to low-energy spin excitations acting as an efficient channel for heat transport and scattering, evidenced by a crossover from paramagnetic to magnon-dominated scattering. The team also notes the existence of at least two contributions with opposite signs to the thermal Hall effect, suggesting complex topological properties of the thermal carriers.