The thermal Hall effect of neutral spin excitations in quantum magnets

N. Phuan Ong,
Dept Physics, Princeton University


Can charge-neutral currents show a Hall effect? At the semiclassical level, the answer would appear to be no since the Lorentz force couples to the charge of the excitation. However, recent theories predict that spin excitations in Kagome and pyrochlore lattices should display a large thermal Hall conductivity because of the Berry curvature. We have tested these predictions in two quantum magnets. The local moments in the pyrochlore Tb2Ti2O7 do not order down to 50 mK despite a large exchange. Its ground state is widely believed to be a spin liquid or a quantum spin ice. The second material is the antiferromagnet called Cu(1-3, bdc) comprised of organic linkers sandwiched between Kagome sheets of Cu moments. It orders at 2 K. I will describe the thermal Hall conductivity in both systems and compare our results with theory.