In laboratories around the world, heat flow in crystals has suddenly begun to deflect under the influence of a magnetic field — and this has happened not in exotic materials, but in the most ordinary semiconductors and insulators, such as silicon, germanium, and magnesium oxide.
This concerns the phonon thermal Hall effect: quasiparticles that describe vibrations of the atomic lattice, under the influence of a magnetic field, create a transverse heat current. It was previously believed that such behavior was characteristic only of materials with magnetic or topological features, where spins or special excitations play a role.
New measurements have shown that the effect manifests universally — in a broad class of nonmagnetic crystals. Moreover, the magnitude of the transverse thermal conductivity scales with the square of the longitudinal one, which points to a direct interaction of atomic vibrations with the magnetic field, without the mediation of other quasiparticles.
This discovery forces a reconsideration of basic ideas about how phonons behave in a magnetic field. If previously thermal transport was considered mainly scalar and insensitive to magnetic influences in ordinary materials, now a new degree of freedom must be taken into account — the elliptical polarization of phonons and their interaction with an external field.
For practical applications, this means that in future thermoelectric devices and cooling systems it will be possible to control heat flow not only by a temperature gradient but also by a weak magnetic field. Such materials are already used in electronics, and the effect could influence the calculation of the thermal regimes of chips.
The experiments were conducted on high-quality samples, where the contribution of charge carriers was minimal, which made it possible to isolate a purely phonon mechanism. The results are consistent with theoretical models predicting the universality of the phenomenon for crystals with discrete symmetry.
Thus, the familiar picture of condensed matter, in which phonons were considered "simple" carriers of heat, becomes more complex — and opens the way to new methods of controlling heat flows at the microscopic level.
