Structural domain and finite-size effects of the antiferromagnetic $\text{S}=1/2$ honeycomb lattice in ${\text{InCu}}_{2/3}{\text{V}}_{1/3}{\text{O}}_3$

The compound ${\text{InCu}}_{2/3}{\text{V}}_{1/3}{\text{O}}_3$ has been studied by spectroscopy in the IR, near-infrared, and visible regions. X-ray and neutron diffraction indicate two-dimensional (2D) structural order of magnetic ${\text{Cu}}^{2+}$ $\left(S=1/2\right)$ ions on a honeycomb lattice with nonmagnetic ${\text{V}}^{5+}$ ions complementing the hexagonal layer. Superstructure reflections observed by neutron diffraction reveal the presence of this 2D structural Cu/V order in domains of approximately $300\text{Å}$ with a disordered stacking along the $c$ axis. Specific-heat, thermal-expansion, and neutron-diffraction experiments gave no signs of a three-dimensional long-range magnetic order. Strong 2D magnetic correlations seem to persist down to low temperatures, providing evidence of an incipient Néel-type ground state on this hitherto rarely studied lattice geometry in ${\text{InCu}}_{2/3}{\text{V}}_{1/3}{\text{O}}_3$. Magnetic correlations in these structural domains are compared with continuous time quantum-Monte Carlo calculations of the specific heat and in particular of the spin-spin-correlation length.