Sample Shape and Boundary Dependence of Measured Transverse Thermal Properties

Abstract: Despite increased interest in thermal Hall measurements for the analysis of insulating quantum materials, there remains large uncertainty in such measurements due to contact misalignment. In this paper we propose that sample geometry and uncertain boundary conditions may account for uncertainty in the measurement of $D_{xy}$ or $\kappa_{xy}$ as well. By running simple simulations in an open source finite-element solver, we demonstrate that measured $D_{xy}$ in a thermal Hall bar can be changed by a factor of order unity in samples with similar width and length. This geometric corrective factor depends on the distinction between uniform heat flow and constant temperature boundary couplings to a bath. Sample geometry and boundary conditions can be accounted for through simulation or by using rectangular samples to make thermal Hall measurements more reliable and reproducible when the amplitude of $\kappa_{xy}$ is important. Finally, we propose a contactless optical method for measuring $D_{xy}$ which is insensitive to the longitudinal diffusivity pollution caused by contact misalignment.