Abstract
In 1991 the layered organic compound κ-(BEDT-TTF)2Cu2(CN)3 with a triangular lattice was synthesized for the first time. 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 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 μ-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 κ-(BEDT-TTF)2Cu2(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.
Highlights
Triangular lattices have been realized in organic compounds [6,7] and YbMgGaO4 [8–10], whereas kagome symmetry is studied in Herbertsmithite (ZnCu3 (OH)6 Cl2 ) and related systems [11,12], as well as optical lattices of Rydberg atoms [13]
It will be shown that the low-temperature spin degrees of freedom probed by (Section 2.2.1) χ, (Section 2.2.2) T1−1, (Section 2.2.3) thermal transport, (Section 2.2.4) Nuclear magnetic resonance (NMR) Knight shift and μ-SR are in excellent agreement with a singlet ground state and paramagnetic impurities embedded in a spin-gapped matrix—as put forward in Ref. [82]
Already small amounts of disorder and defects are sufficient to conceal the intrinsic response of valence electrons in various magnetic probes [82], precluding a proper identification of the magnetic ground state in the past
Summary
The spins of neighboring sites form a singlet state, which is usually accompanied by a Peierls-like lattice distortion Such nonmagnetic phases occur close to t0 /t ≈ 1, e.g., in EtMe3 P[Pd(dmit)2 ]2 [68–71], and, in particular, for quasi-1D (t0 /t > 1) exchange interactions, like for κ-(BEDT-TTF) B(CN) with t0 /t > 1.4 [72,73,77]. The following sections summarize structural and magnetic properties reported over the last two decades, leading to the conclusion that κ-(BEDT-TTF) Cu2 (CN) does not exhibit a gapless QSL state, but rather a nonmagnetic VBS phase. = 6 K associated with the well known ‘6K anomaly’ [26,81] is apparent in a broad set of experimental results, including thermodynamic, structural, ultrasonic, optical, and magnetic probes Since such anomalies occur in κ-(BEDT-TTF) Cu2 (CN) , a similar scenario is suggested for related QSL candidates.
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