Heat conduction in herbertsmithite: Field dependence at the onset of the quantum spin liquid regime

Thermal conductivity and thermal Hall effect measurements in the quantum kagome antiferromagnet herbertsmithite reveal that there is no significant contribution to the heat conduction from mobile spin excitations. A clear field dependence of the thermal conductivity is discovered to onset at a new temperature scale, attributed to the establishment of a quantum spin liquid and the gradual polarization of magnetic defects. Their magnetization profile is extracted and found incompatible with a Brillouin function, indicating that they are not isolated paramagnetic objects.