Spinon-Mediated Interactions and Transition in Herbertsmithite ZnCu₃(OH)₆Cl₂

Spin liquids are a state of matter characterised by a lack of magnetic order, large-scale entanglement, and fractionalised excitations. While numerous theoretical models for such states now exist, definitive experimental identification remains elusive. A promising new technique for categorically identifying spin liquids is spin noise spectroscopy.

This approach has already been successfully utilised to investigate emergent magnetic monopoles in the geometrically frustrated spin-ice Dy₂Ti₂O₇. Here we apply this technique to the S=1/2 kagome material Herbertsmithite ZnCu₃(OH)₆Cl₂. We observe intense low frequency spin noise between 1 mHz and 1 kHz generated by S=1/2 Cu2+ atoms substituted into interlayer sites. At T* = 260 mK, peaks in both the DC susceptibility and noise variance indicate a transition, below which the interlayer ‘witness spins’ show a slow ageing process over 10⁵ s as well as a 1/ω power spectral density. These are consistent with the RKKY interaction mediated by the quantum spin liquid within the kagome layers, hence the interlayer spins interact with and ‘witness’ the kagome layer providing a means to observe the physics of the spin liquid.