Higher measurement precision opens a new window into the quantum world

A team from HZB has developed a new measurement method that, for the first time, accurately detects tiny temperature differences of around 100 microKelvin in the thermal Hall effect. Previously, these temperature differences could not be measured quantitatively due to thermal noise.

Their study is published in Materials and design.

Using the famous terbium titanate as an example, the team demonstrated that the method gives very reliable results. The thermal Hall effect provides information about coherent multiparticle states in quantum materials based on their interaction with lattice vibrations (phonons).

The laws of quantum physics apply to all materials. However, these laws give rise to particularly unusual properties in so-called quantum materials. For example, magnetic fields or temperature changes can cause excitations, collective states or quasiparticles accompanied by phase transitions to exotic states.

This can be used in a variety of ways, provided it can be understood, managed and controlled. For example, in the future, information technologies capable of storing or processing data with minimal energy consumption.

The thermal Hall effect (THE) plays a key role in identifying exotic states in condensed matter. The effect is based on tiny transverse temperature differences that occur when a thermal current passes through a sample and a perpendicular magnetic field is applied.

In particular, the quantitative measurement of the thermal Hall effect makes it possible to separate exotic excitations from conventional behavior. The thermal Hall effect is observed in a variety of materials, including spin liquids, spin ice, the parent phases of high-temperature superconductors, and materials with strongly polar properties.

However, the thermal differences that occur perpendicular to the temperature gradient in the sample are extremely small: in typical millimeter-sized samples, they are in the microkelvin to millikelvin range. Until now, it has been difficult to detect these heat differences experimentally, because the heat introduced by the measuring electronics and sensors masked the effect.

A new sample holder

The team led by PD Dr. Klaus Habicht has now carried out pioneering work. Together with HZB sample environment specialists, they developed a new sample rod with a modular structure that can be inserted into various cryomagnets. The sample head measures the thermal Hall effect using capacitive thermometry.

This takes advantage of the temperature dependence of the capacitance of specially manufactured miniature capacitors. With this setup, the experts managed to significantly reduce heat transfer via sensors and electronics and mitigate interference signals and noise through several innovations.

To validate the measurement method, they analyzed a sample of terbium titanate, whose thermal conductivity in different crystal directions under a magnetic field is well known. The measured data were in excellent agreement with the literature.

Further improvement of the measurement method

“The ability to resolve temperature differences in the sub-millikelvin range greatly fascinates me and is a key to studying quantum materials in more detail,” says first author Dr Danny Kojda. “We have now jointly developed a sophisticated experimental model, clear measurement protocols and precise analysis procedures that enable high-resolution and reproducible measurements.”

Department head Klaus Habicht adds: “Our work also provides information on how to further improve the resolution of future instruments designed for low sample temperatures. I would like to thank everyone involved, especially the sample environment team. I hope that the experimental setup will be firmly integrated into the HZB infrastructure and that the proposed upgrades will be implemented.

Habicht’s group will now use thermal Hall effect measurements to study the topological properties of lattice vibrations or phonons in quantum materials.

“The microscopic mechanisms and physics of thermal Hall diffusion processes in ionic crystals are far from being fully understood. The exciting question is why electrically neutral quasiparticles in non-magnetic insulators are nonetheless deflected in the magnetic field,” explains Habicht. . With the new instrument, the team has now created the preconditions to answer this question.