Importance of van der Waals interactions and cation-anion coupling in an organic quantum spin liquid

The Mott insulator beta'-EtMe3Sb[Pd(dmit)(2)](2) belongs to a class of charge transfer solids with highly frustrated triangular lattice of S = 1/2 molecular dimers and a quantum-spin-liquid ground state. Our experimental and ab initio theoretical studies show the fingerprints of strong correlations and disorder, the important role of cation-dimer bonding in charge redistribution, no sign of intra-and interdimer dipoles, and the decisive van der Waals contribution to interdimer interactions and the ground-state structure. The latter consists of quasidegenerate electronic states related to the different configurations of cation moieties, which permit two different equally probable orientations. Upon reducing the temperature, the low-energy excitations slow down, indicating glassy signatures as the cation motion freezes out.