Structural domain and finite-size effects of the antiferromagnetic S=1/2 honeycomb lattice in InCu2/3V1/3O3

The compound InCu(2/3)V(1/3)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 Cu(2+) (S=1/2) ions on a honeycomb lattice with nonmagnetic 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 A 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 Neel-type ground state on this hitherto rarely studied lattice geometry in InCu(2/3)V(1/3)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.