Quantum Hall calorimetry of quantum materials

• Applicant: Professor Dr. Erwann Bocquillon
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 572794977

Heat transport is a powerful tool in condensed matter physics, providing access to the dynamics of phonons, magnons, and exotic quasiparticles. In strongly correlated and low-dimensional systems, it reveals phenomena such as quantum coherence, fractionalization, and topological order that often escape conventional charge transport. However, existing thermal measurement methods suffer from key limitations, particularly at sub-kelvin temperatures, due to contact resistances, parasitic losses, and insufficient sensitivity of standard thermometers. This project proposes a universal, high-precision platform for thermal and thermoelectric transport measurements in quantum materials. We will develop hybrid calorimetric devices based on quantum Hall and quantum anomalous Hall systems which host edge states with quantized electrical and thermal conductance. In combination with a small metallic island, one can design hybrid devices functioning both as stable heaters and sensitive thermometers down to millikelvin temperatures. On the one hand, Joule heating can be locally generated in the metallic island by applying a current carried without dissipation in the edge channels. On the other hand, the thermal Johnson-Nyquist noise can be read to access the temperature of the island with millikelvin precision. Combining these elementary building blocks allows for four-terminal heat transport measurements, with compatibility across wide magnetic field range (0–12 T) at ultra-low temperature (10–100 mK). Our first target material is α-RuCl₃, a layered van der Waals material and leading candidate for realizing a Kitaev quantum spin liquid. Reports of a half-quantized thermal Hall effect suggest the presence of chiral Majorana edge modes, but these results remain controversial due to possible phonon contributions and sample variability. Our approach, using exfoliated α-RuCl₃ and precisely calibrated devices, aims to settle this debate by enabling clean, reproducible measurements in the low-temperature regime where phonon effects are suppressed. This work will both establish a novel measurement standard and explore fundamental questions in topological quantum matter.

• DFG Programme: Research Grants
• International Connection: Japan
• Partner Organisation: Japan Society for the Promotion of Science (JSPS)
• Cooperation Partner: Professor Masayuki Hashisaka, Ph.D.