Thermal conductivity of the kagome antiferromagnet herbertsmithite

As a close realization of the S = 1/2 kagome Heisenberg antiferromagnet, herbertsmithite (ZnCu₃(OH)₆Cl₂) is an emblematic quantum spin liquid candidate. We have measured the thermal conductivity of several protonated and deuterated single crystals of herbertsmithite, over a wide range of temperature (0.05 – 120 K) and magnetic field (0 – 15 T).

While the thermal conductivity is found to be independent of field above T = 15 K, we uncover two field-dependent regimes at lower temperatures. For 2 ≤ T ≤ 15 K, the thermal conductivity increases monotonically with the magnetic field. For T ≤ 2K, the field dependence is non monotonic and reaches a maximum value around 10 T. There is no residual linear term in the T = 0 limit, consistent with the absence of fermionic spinons.

We attribute the thermal conductivity mostly to phonons, most likely scattered by impurity spins, and discuss the possibility of a contribution from itinerant spin excitations.