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

The Mott insulator \(\beta'\)-EtMe\(_3\)Sb[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, important role of cation-dimer bonding in charge redistribution, no sign of intra- and inter-dimer dipoles, and the decisive van der Waals contribution to inter-dimer interactions and the ground state structure. The latter consists of quasi-degenerate 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.