Valence-bond solid as the quantum ground state in honeycomb layered urusoviteCuAl(AsO4)O

Abstract

The synthetic mineral urusovite $\mathrm{CuAl}(\mathrm{As}{\mathrm{O}}_{4})\mathrm{O}$ was prepared through the wet chemistry route and characterized over a wide temperature range in terms of studies of magnetization, specific heat, and X-band electron spin resonance. The basic structural units of the compound are distorted square pyramids $\mathrm{Cu}{\mathrm{O}}_{5}$ assembled into corrugated honeycomb layers separated by $\mathrm{As}{\mathrm{O}}_{4}$ and $\mathrm{Al}{\mathrm{O}}_{4}$ tetrahedrons. Both thermodynamic and resonant measurements indicate that $\mathrm{CuAl}(\mathrm{As}{\mathrm{O}}_{4})\mathrm{O}$ is a spin gap system with a gap of $\ensuremath{\sim}350\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The electronic structure calculations performed within the framework of density functional theory suggest a weakly interacting dimer model with antiferromagnetic signs for both intradimer and interdimer superexchange interactions. This establishes the valence bond solid as the quantum ground state of the title compound. The pronounced discrepancy between experimental data and calculations within the weakly interacting dimer model at elevated temperatures is ascribed in part to the steep increase of the intradimer exchange interaction parameter driven by the thermal expansion effects.