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The organic triangular-lattice antiferromagnet κ-(ET)₂Cu₂(CN)₃ has been extensively discussed as a quantum spin liquid (QSL) candidate. Recently, an intriguing quantum critical behaviour was suggested from low-temperature B/T scaling of the magnetic torque experiments [1]. Through microscopic analysis of all anisotropic contributions, including Dzyaloshinskii-Moriya and multi-spin scalar chiral interactions, we highlight significant deviations of the experimental observations from a quantum critical scenario. Instead, we show that disorder-induced spin defects provide a comprehensive explanation of the low-temperature properties[2]. These spins are attributed to valence bond defects that emerge spontaneously as the QSL enters a valence bond glass phase at low temperature. This theoretical treatment is applicable to a general class of frustrated magnetic systems; similar scaling is also observed in the Kagome lattice antiferromagnet Herbertsmithite[3].

[1] Isono et al., Nature Communications 7, 13494 (2016).
[2] K. Riedl et al., arXiv: 1808.03868 (2018).
[3] J. S. Helton et al., PRL 104, 147201 (2010).