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# Kondo screening in a charge-insulating spinon metal

## Abstract

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 metal1,2. 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 realizations beyond the bulk-metal paradigm in many other conduction-electron systems, such as in quantum dots in semiconductor heterostructures3,4 and nanomaterials5,6,7, in quantum point contacts8,9, in graphene10,11 and in topological insulators12, and has also been predicted for three-dimensional Dirac and Weyl semimetals13. Here, we report an experimental observation of 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 characterizing spin liquids in the general context of utilizing impurities as in situ probes of host electron states14,15, and offers a unique way to manipulate these enigmatic states.

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## Data availability

The data that support the findings of this study are available via https://doi.org/10.15128/r241687h44k or from the corresponding author upon reasonable request.

Journal peer review information: Nature Physics thanks Peter Baker and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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## Acknowledgements

The authors acknowledge discussions with T. Lancaster and D. Manevski. This work is partially based on experiments performed at the Swiss Muon Source SμS, Paul Scherrer Institute, Villigen, Switzerland. Financial support of the Slovenian Research Agency under programmes P1-0125 and P1-0044 and project no. J1-7259 is acknowledged. M.G. is grateful to EPSRC (UK) for financial support (grant no. EP/N024028/1). Q.M.Z. was supported by the Ministry of Science and Technology of China (2016YFA0300504 and 2017YFA0302904) and the NSF of China (11774419 and 11474357).

## Author information

A.Z. conceived, designed and supervised the project. M.G. and A.Z. performed the μSR measurements, with technical assistance from C.B., and analysed the data. R.Ž. carried out the NRG calculations. M.G. developed the percolation-theory-based model for the spin of impurity clusters. Y.L. and Q.M.Z. synthesized and characterized the sample. All authors discussed the results. A.Z. wrote the paper with feedback from all the authors.

### Competing interests

The authors declare no competing interests.

Correspondence to A. Zorko.

## Supplementary information

1. ### Supplementary Information

Supplementary Text, Supplementary Figs. 1–7 and Supplementary References.

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