Large thermal Hall conductivity contributed by phonons in a simple insulator

What is it about?

The thermal Hall effect is the thermal analog of the electrical Hall effect. Instead of a transverse voltage induced by a perpendicular magnetic field in the presence of an electric current, a transverse temperature difference is induced in the presence of a heat current. A major question to answer is: what kind of excitations can contribute to the thermal Hall effect? Electrons naturally give a contribution due to the Lorentz force which bends the trajectory of electrons in a magnetic field. For quite a long time, phonons are believed not to be able to generate a thermal Hall effect due to their lack of charge or spin, which are two properties held by electrons. However, thermal HallI effect contributed by phonons have been observed in more and more insulators in recent years. In this paper, we report a large thermal Hall conductivity in an insulating material Cu3TeO6, which is among the largest of any insulator so far.

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Photo by SpaceX on Unsplash

Photo by SpaceX on Unsplash

Why is it important?

First of all, this work tries to answer a fundamental question: How can phonons be bent by a magnetic field and produce a thermal Hall signal? Phonons are believed not to be able to generate a thermal Hall signal due to their lack of charge or spin. However, since the first discovery of a phonon thermal Hall effect in the paramagnetic insulator Tb3Ga5O12, much larger signals have been observed in several other families of insulators. Most of the insulators that exhibit a phonon Hall effect have some special feature, believed to be a key to the underlying mechanism. Here, our discovery of a large phonon thermal Hall conductivity in a simple material with none of the special features of the previous cases opens up the subject to a much broader question. Secondly, compared with electrical methods, which can detect charged excitations such as electrons, the thermal transport measurement, such as thermal conductivity and thermal Hall conductivity, is a powerful technique to detect charge-neutral excitations, such as lattice vibrations (phonons) and spin excitations, in many novel quantum materials. Understanding how charge-neutral excitations contribute to a thermal Hall effect is helpful in understanding the material itself.

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