Yo Machida

When an ideal insulator is cooled, four regimes of thermal conductivity are expected to emerge one after another. Two of these, the Ziman and the Poiseuille, are hydrodynamic regimes in which collision among phonons are mostly normal. It has been difficult to observe them, save for a few insulators with high levels of isotopic and chemical purity. Our thermal transport measurements, covering four decades of temperatures between 0.1 K and 900 K, reveal that sapphire displays all four regimes, despite its isotopic impurity. In the Ziman regime, the thermal conductivity exponentially increases, attaining an amplitude as large as 35 000 W/Km. We show that the peak thermal conductivity of ultrapure, simple insulators, including diamond, silicon, and solid helium, is set by a universal scaling depending on isotopic purity. The thermal conductivity of sapphire is an order of magnitude higher than what is expected by this scaling. We argue that this may be caused by the proximity of optical and acoustic phonon modes, as a consequence of the large number of atoms in the primitive cell. Published by the American Physical Society 2025

Abstract The origin of phonon thermal Hall Effect (THE) observed in a variety of insulators is yet to be identified. Here, we report on the observation of a thermal Hall conductivity in a non-magnetic elemental insulator, with an amplitude exceeding what has been previously observed. In black phosphorus (BP), the longitudinal (κ_ii), and the transverse, κ_ij, thermal conductivities peak at the same temperature and at this peak temperature, the κ_ij/κ_jj/B is ≈ 10⁻⁴−10⁻³ T⁻¹. Both these features are shared by other insulators displaying THE, despite an absolute amplitude spreading over three orders of magnitude. The absence of correlation between the thermal Hall angle and the phonon mean-free-path imposes a severe constraint for theoretical scenarios of THE. We show that in BP a longitudinal and a transverse acoustic phonon mode anti-cross, facilitating wave-like transport across modes. The anisotropic charge distribution surrounding atomic bonds can pave the way for coupling between phonons and the magnetic field.

Abstract It has become common knowledge that phonons can generate thermal Hall effect in a wide variety of materials, although the underlying mechanism is still controversial. We study longitudinal κ_xx and transverse κ_xy thermal conductivity in Pr₂Ir₂O₇, which is a metallic analog of spin ice. Despite the presence of mobile charge carriers, we find that both κ_xx and κ_xy are dominated by phonons. A T/H scaling of κ_xx unambiguously reveals that longitudinal heat current is substantially impeded by resonant scattering of phonons on paramagnetic spins. Upon cooling, the resonant scattering is strongly affected by a development of spin ice correlation and κ_xx deviates from the scaling in an anisotropic way with respect to field directions. Strikingly, a set of the κ_xx and κ_xy data clearly shows that κ_xy correlates with κ_xx in its response to magnetic field including a success of the T/H scaling and its failure at low temperature. This remarkable correlation provides solid evidence that an indispensable role is played by spin-phonon scattering not only for hindering the longitudinal heat conduction, but also for generating the transverse response.