Upper-branch thermal Hall effect in quantum paramagnets

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Inspired by the persistent thermal Hall effects at finite temperatures in various quantum paramagnets, we explore the origin of the thermal Hall effects from the perspective of the upper branch parts by invoking dispersive and twisted crystal field excitations. It is shown that the upper branches of the local energy levels could hybridize and form dispersive bands. The observation is that, upon time-reversal symmetry breaking by the magnetic fields, these upper branch bands could acquire a Berry curvature distribution and contribute to the thermal Hall effect even in the paramagnetic regime. As a proof of principle, we consider the setting on kagomé lattice with one ground-state singlet and an excited doublet, and show this is indeed possible. We expect this effect to be universal and that it has no strong connection with the underlying lattice. Although the thermal Hall signal can be contributed from other sources such as phonons and their scattering in the actual materials, we discuss the application to the relevant quantum materials. Published by the American Physical Society 2024