The discovery of a thermal version of the Hall effect in common semiconductors challenges our understanding of how magnetic fields and heat fluxes interact within solids.

In the late 19th century, two physicists independently discovered the Righi-Leduc, or thermal Hall, effect: In the presence of a perpendicular magnetic field, a longitudinal heat flux generates a transverse temperature gradient. In metals, the thermal Hall effect is tied to the more familiar electric Hall effect through the Wiedemann-Franz law, which states that electronic thermal conductivity divided by electrical conductivity is directly proportional to temperature.

In electrical insulators, lattice vibrations called phonons carry heat. Until the early 21st century, it was thought that phonons, despite being neutral, could still generate a nonzero thermal Hall signal, provided they are scattered by electron spins. However, since then many experiments worldwide have found that such a signal can be detected even in crystalline insulators with no unpaired electron spins. The decisive discovery was made last year by Xiaobo Jin of Fudan University in China and his colleagues, who found a phonon thermal Hall effect in two simple semiconducting materials: silicon and germanium [1]. Whereas the origin of the effect is hotly debated, its importance in challenging views of how magnetic fields and heat fluxes interact is undisputed.