Thermal Hall Effects refer to a class of phenomena in which thermal current carried by elementary excitations of a material is deflected by applying magnetic field. Magnons, as magnetic excitations have been predicted to show such behaviors when both magnetic field and temperature gradient are present. This has been just observed in Lu2V2O7, whose V element is magnetic and magnetic ordering occurs below about 100K[1].
[1]Science 329, 297 (2010)
The Hall effect usually occurs in conductors when the Lorentz force acts on a charge current in the presence of a perpendicular magnetic field. Neutral quasi-particles such as phonons and spins can, however, carry heat current and potentially exhibit the thermal Hall effect without resorting to the Lorentz force. We report experimental evidence for the anomalous thermal Hall effect caused by spin excitations (magnons) in an insulating ferromagnet with a pyrochlore lattice structure. Our theoretical analysis indicates that the propagation of the spin waves is
influenced by the Dzyaloshinskii-Moriya spin-orbit interaction, which plays the role of the vector potential, much as in the intrinsic anomalous Hall effect in metallic ferromagnets.
Thermal current is always interesting to me. It contains some very basic elements of the more general non-equilibrium thermodynamics. The concept of temperature is valid and thus used. And all other thermodynamic variables are also used. But now these variables become local and time-dependent. They shall change and induce currents. These currents will eventually drive the system to some equilibrium in isolation. These currents are essentially of diffusive and random character.
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