Text: Number 750 #1, October 18, 2005 by Phil Schewe and Ben Stein http://www.aip.org/pnu/2005/split/750-1.html Particles of Heat The phonon Hall effect, the acoustic equivalent of the electrical Hall effect, has been observed by physicists at the Max Planck Institut für Festkörperforschung (MPI) and the Centre National de la Recherche Scientifique (CNRS) in France. In the electrical Hall effect, when an electrical current (consisting of free electrons moving along a material sample) being driven by an electric field is subjected to an external magnetic field, the charge carriers will feel a force perpendicular to both the original current and the magnetic force, causing the electrical current to be deflected somewhat to the side. Thermal transport is a bit more complicated than electrical transport. A "current" of heat can consist of free electrons carrying thermal energy or it can consist of phonons, which are vibrations rippling through the lattice of atoms of the sample. Previously, some scientists believed that in the absence of free electrons, a magnetically induced deflection of heat could not be possible. The MPI-CNRS researchers felt, however, that a magnetic deflection of phonons was possible, and have now demonstrated it experimentally in insulating samples of Terbium Gallium Garnet (a material often used for its magneto optical properties) where no free charges are present. The sample was held at a temperature of 5 degrees Kelvin and was warmed at one side, creating the thermal equivalent of an applied voltage. Application of a magnetic field of a few Tesla led to an extremely small (smaller than one thousandth of a degree), yet detectable temperature difference. The same team of MPI-CNRS scientists in 1997 demonstrated a kind of "photon Hall effect": see PNU 349 Strohm et al., Physical Review Letters, 7 October 2005

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