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Large thermal Hall conductivity of neutral spin excitations in a frustrated quantum magnet

Science  03 Apr 2015:
Vol. 348, Issue 6230, pp. 106-109
DOI: 10.1126/science.1257340

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Probing the nature of an exotic magnet

To minimize their energy, materials with magnetic interactions tend to become ordered at low temperatures. However, if the magnetism is frustrated (for example, if the geometry of the crystal lattice gets in the way of minimizing the energy), the material may not reach an ordered state even at very low temperatures. Hirschberger et al. studied the excitations of such a system—the pyrochlore compound Tb2Ti2O7—using thermal transport measurements. Thermal conductivity at very low temperatures resembled that of a disordered metal; a puzzling finding in an electrically insulating transparent material.

Science, this issue p. 106

Abstract

In frustrated quantum magnets, long-range magnetic order fails to develop despite a large exchange coupling between the spins. In contrast to the magnons in conventional magnets, their spin excitations are poorly understood. Here, we show that the thermal Hall conductivity κxy provides a powerful probe of spin excitations in the “quantum spin ice” pyrochlore Tb2Ti2O7. The thermal Hall response is large, even though the material is transparent. The Hall response arises from spin excitations with specific characteristics that distinguish them from magnons. At low temperature (<1 kelvin), the thermal conductivity resembles that of a dirty metal. Using the Hall angle, we construct a phase diagram showing how the excitations are suppressed by a magnetic field.

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