A twisting mystery of electrons, vibrations and heat Premium

Scientists have observed the thermal Hall effect in insulators, leading to an important open question in condensed-matter physics.

High-school physics: When charged particles enter an area where there is a magnetic field, a force deflects them away from their path.

This is how the thermal Hall effect emerges in an electrical conductor: when you apply a temperature gradient in the material in one direction, another temperature gradient appears in a perpendicular direction in the presence of a magnetic field. The electrons in the material carry both electric charges and thermal energy, and the magnetic field deflects them, giving rise to the perpendicular gradient.

But scientists have observed the thermal Hall effect in insulators as well, especially terbium oxides, strontium titanate and twokinds of cuprates. Electrons in insulators aren’t involved in transferring heat or electricity, so what could explain this? This is an important open question in condensed-matter physics.

A leading candidate for an answer involves a particle called the phonon, and a paper published on December 21 advances a new idea: while phonons can’t be deflected by the magnetic field – they have no electric charge – they are affected by the electrons that are deflected by the magnetic field.

Subroto Mukerjee, an associate professor in the Department of Physics, Indian Institute of Science, called the study a “solid piece of work, for sure”. (He wasn’t involved in the study.)

What are phonons?

Technically, phonons aren’t particles; they’re quasiparticles – packets of energy that behave like particles in a system. A phonon is a quasiparticle of vibrational energy. When the grid of atoms that make up the material vibrates, it releases this energy, and physicists encapsulate it in the form of phonons.