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A. Pustogow:
"Thirty-Year Anniversary of κ-(BEDT-TTF)2Cu2(CN)3: Reconciling the Spin Gap in a Spin-Liquid Candidate";
Solids, 3 (2022), S. 93 - 110.

In 1991, the Argonne group led by Jack Williams [Inorg. Chem. 1991, 30, 2586-2588] reported the first synthesis of k-(BEDT-TTF)2Cu2(CN)3. Although, originally, the focus was on the superconducting properties under pressure, this frustrated Mott insulator has been the most promising quantum-spin-liquid candidate for almost two decades, widely believed to host gapless spin excitations down to T = 0. The recent observation of a spin gap by Miksch et al. [Science 2021, 372, 276-279.] rules out a gapless spin liquid with itinerant spinons and puts severe constraints on the magnetic ground state. This review evaluates magnetic, thermal transport, and structural anomalies around T* = 6 K. The opening of a spin gap yields a rapid drop of spin susceptibility, NMR Knight shift, spin-lattice relaxation rate, and m-SR spin fluctuation rate, but is often concealed by impurity spins. The concomitant structural transition at T? manifests in thermal expansion, THz phonons and ⁶³Cu NQR relaxation. Based on the field dependence of T*, a critical field of 30-60 T is estimated for the underlying spin-singlet state. Overall, the physical properties are remarkably similar to those of spin-Peierls compounds. Thus, a strong case is made that the `6K anomaly´ in k-(BEDT-TTF)2-Cu2(CN)3 is the transition to a valence-bond-solid state and it is suggested that such a scenario is rather the rule than the exception in materials with strong magnetic frustration.

quantum spin liquids, Mott insulators, metal-insulator transitions, valence bond solid, spin-Peierls

http://dx.doi.org/10.3390/solids3010007