Researchers crack the unsolved case of phonon thermal Hall effect

China News Service, Wuhan, October 23 (Huang Shuangzi Shi Mengshi) Professor Li Liang's team from the National Center for Pulsed High Magnetic Field Science at Huazhong University of Science and Technology, together with scholars from the Ecole Supérieure de Physique et Chemique etiquette in Paris, France, confirmed for the first time from a thermodynamic perspective that the phonon thermal Hall effect originates from the intrinsic mechanism of the material, rather than external scattering caused by impurities or defects. This provides a key experimental criterion for the intrinsic-extrinsic debate that has plagued the academic community for 21 years.

The relevant results were published online in the international academic journal "Science" on October 23, Beijing time.

The so-called Hall effect refers to the deflection of electric current in a magnetic field, thereby generating a transverse voltage. This phenomenon only occurs for charged particles. The "phonon thermal Hall effect" means that in an insulator, heat is transferred through "phonons". When a magnetic field is applied, the heat flow will also be deflected laterally, resulting in a temperature difference.

According to common sense, phonons are uncharged and therefore cannot interact with the magnetic field and will not produce the thermal Hall effect. However, in 2005, researchers observed experimental signals suspected of phonon thermal Hall effect for the first time. Questions arose: Is this the inherent intrinsic behavior of phonons themselves, or is it the role of extrinsic scattering mechanisms such as impurities and crystal defects?

In order to solve the mystery, the research team relied on the National Pulsed High Magnetic Field Science Center of Huazhong University of Science and Technology to carry out precise measurements on multiple single crystal samples. The experiment found that among five samples with basically the same "road conditions", the maximum difference in heat flow deflection was nearly ten times, and the deflection size had nothing to do with the phonon mean free path, but was closely related to the magnetic anisotropy of the material - this shows that the deflection is not caused by impurities "by chance", but is an intrinsic property of the material itself.

Further analysis revealed that there are many intersections between the energy bands of magnons and phonons in the material. After the two are coupled, a new hybrid mode is formed, allowing the originally uncharged phonons to gain the "ability to sense" the magnetic field and thereby be deflected.

Li Liang said that the phonon Hall effect shows that "neutral particles" such as phonons also have a wonderful quantum world. This achievement expands the cognitive boundaries of the Hall effect family and prompts the physics community to re-examine the theoretical framework of the interaction between phonons and magnetic fields.