Kondo effect in a spinon metal

The Kondo screening of localized impurities possessing magnetic moments is traditionally associated with exchange scattering of conduction electrons. This nontrivial display of many-body physics is common to ordinary metals and was recently found also in various more complex conducting systems, such as quantum dots, graphene, and topological insulators. Moreover, the Kondo effect was theoretically predicted for certain electric insulators in which emergent fractional magnetic excitations can effectively take over the role of itinerant electrons in screening the impurities’ magnetic moments [1-4].

Indeed, our experimental study of the kagome lattice antiferromagnet Zn-brochantite, ZnCu₃(OH)₆SO₄, with a spin-liquid ground state has finally confirmed these theoretical predictions [5]. Although in this material the charge degrees of freedom are frozen, an effect conceptually identical to the traditional Kondo effect takes place. This is possible because the magnetic spinon excitations of Zn-brochantite behave like quasi-free fermions with a Fermi surface and thus, regarding their interactions with localized impurities, closely resemble itinerant electrons in a metal. The observed Kondo response is, however, to a certain degree modified from the ordinary case, which is attributed to spinon-spinon interactions. The discovered spinon-based Kondo effect thus provides a prominent platform for characterizing enigmatic spin-liquid states through impurities acting as in situ local probes of the host magnetic state.

[1] A. Kolezhuk, S. Sachdev, R. R. Biswas, and P. Chen, Phys. Rev. B 74, 165114 (2006).
[2] P. Ribeiro and P. A. Lee, Phys. Rev. B 83, 235119 (2011).
[3] K. Dhochak, R. Shankar, and V. Tripathi, Phys. Rev. Lett. 105, 117201 (2010).
[4] M. Vojta, A. K. Mitchell, and F. Zschocke, Phys. Rev. Lett. 117, 037202 (2016).
[5] M. Gomilsek, R. Zitko, M. Klanjsek, M. Pregelj, C. Baines, Y. Li, Q. M. Zhang, and A. Zorko, Nat. Phys. 15, 754 (2019).