Kondo screening in a charge-insulating spinon metal [dataset]

The Kondo effect, an eminent manifestation of many-body physics in condensed matter, is traditionally explained as exchange scattering of conduction electrons on a spinful impurity in a metal. The resulting screening of the impurity's local moment by the electron Fermi sea is characterized by a Kondo temperature TK, below which the system enters a strongly-coupled regime. In recent years, this effect has found its realizations beyond the bulk-metal paradigm in many other conduction-electron systems, such as in quantum dots in semiconductor heterostructures and nanomaterials, in quantum point contacts, in graphene, in topological insulators, and was also predicted for three-dimensional Dirac and Weyl semimetals. Here we report on an experimental observation of the Kondo screening by charge-neutral quasiparticles. This occurs in a charge-insulating quantum spin liquid, where spinon excitations forming a Fermi surface take the role of conduction electrons. The observed impurity behaviour therefore bears a strong resemblance to the conventional case in a metal. The discovered spinon-based Kondo effect provides a prominent platform for characterising spin liquids, in the general context of utilizing impurities as in situ probes of host electron states, and offers a unique way to manipulate these enigmatic states.