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

TL;DR

This study reveals that van der Waals interactions and cation-anion coupling critically influence the electronic structure and ground state of a quantum spin liquid material, highlighting the importance of weak intermolecular forces and disorder effects.

Contribution

It combines experimental and ab initio theoretical approaches to demonstrate the significant role of van der Waals forces and cation configurations in the properties of a quantum spin liquid.

Findings

Van der Waals interactions are crucial for inter-dimer coupling.

Cation configurations lead to quasi-degenerate electronic states.

Glassy behavior emerges as cation motion freezes at low temperatures.

Abstract

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.