Composition and structure of the anion layer in the organic superconductor kappa'-(ET)(2)Cu(2)(CN)(3): optical study

Critical spin liquid versus valence-bond glass in a triangular-lattice organic antiferromagnet

In the quest for materials with unconventional quantum phases, the organic triangular-lattice antiferromagnet κ-(ET)₂Cu₂(CN)₃ has been extensively discussed as a quantum spin liquid (QSL) candidate. The description of its low temperature properties has become, however, a particularly challenging task. Recently, an intriguing quantum critical behaviour was suggested from low-temperature magnetic torque experiments. Here we highlight significant deviations of the experimental observations from a quantum critical scenario by performing a microscopic analysis of all anisotropic contributions, including Dzyaloshinskii–Moriya and multi-spin scalar chiral interactions. Instead, we show that disorder-induced spin defects provide a comprehensive explanation of the low-temperature properties. 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 and has important implications for the interpretation of magnetic torque, nuclear magnetic resonance, thermal transport and thermodynamic experiments.

The low temperature magnetic properties of organic Mott insulator κ-(ET)₂-Cu₂(CN)₃ a quantum spin liquid candidate is notoriously difficult to understand. Here, a model based on defect spins is introduced that allows to describe results of complementary experiments on the same theoretical footing.

Are formal oxidation states above one viable in cyclopentadienylcopper cyanides?

Recent experiments have led to the discovery of the thermally unstable organocopper compounds (η(3)-C(3)H(5))CuMe(2), [(η(3)-C(3)H(5))CuMe(3)](-), and CuMe (4)(-) in which the copper atom is in the +3 formal oxidation state. In a quest for more stable organocopper compounds with copper in formal oxidation states above one, the binuclear cyclopentadienylcopper cyanides Cp(2)Cu(2)(CN)(n) (Cp = η(5)-C(5)H(5); n = 1, 2, 3) have been studied using density functional theory (DFT). The lowest energy structures are found to have terminal Cp rings and bridging cyanide ligands up to a maximum of two bridges. Higher-energy Cp(2)Cu(2)(CN)(n) (n = 1, 2, 3) structures are found with bridging Cp rings. The Cp(2)Cu(2)(CN)(3) derivatives, with the copper atoms in an average +2.5 oxidation state, are clearly thermodynamically disfavored with respect to cyanogen loss. However, Cp(2)Cu(2)(CN)(2) and Cp(2)Cu(2)(CN), with the copper atoms in the average oxidation states +1.5 and +2, respectively, are predicted to have marginal viability. The prospects for the copper(II) derivative Cp(2)Cu(2)(CN)(2) contrast with that of the "simple" Cu(CN)(2), which is shown both experimentally and theoretically to be unstable with respect to cyanogen loss to give CuCN.

Frontiers of organic conductors and superconductors

We review the development of conductive organic molecular assemblies including organic metals, superconductors, single component conductors, conductive films, conductors with a switching function, and new spin state (quantum spin liquid state). We emphasize the importance of the ionicity phase diagram for a variety of charge transfer systems to provide a strategy for the development of functional organic solids (Mott insulator, semiconductor, superconductor, metal, complex isomer, neutral-ionic system, alignment of chemical potentials, etc.). For organic (super)conductors, the electronic dimensionality of the solids is a key parameter and can be designed based on the self-aggregation ability of a molecule. We present characteristic structural and physical properties of organic superconductors.