Ground state and low-temperature magnetism of the quasi-two-dimensional honeycomb compound InCu2/3V1/3O3

We report a combined In-115 nuclear quadrupole resonance, V-51 nuclear magnetic resonance, and muon spin-relaxation spectroscopic study of the low-temperature magnetic properties of InCu2/3V1/3O3, a quasitwo-dimensional (2D) compound comprising in the spin sector a honeycomb lattice of antiferromagnetically coupled spins S = 1/2 associated with Cu2+ ions. Despite substantial experimental and theoretical efforts, the ground state of this material has not been ultimately identified. In particular, two characteristic temperatures of about 40 and 20 K manifesting themselves as anomalies in different magnetic measurements are discussed controversially. A combined analysis of the experimental data complemented with theoretical calculations of exchange constants enabled us to identify, below 39 K, an "intermediate" quasi-2D static spin state. This spin state is characterized by a staggered magnetization with a temperature evolution that agrees with the predictions for the 2D XY model. We observe that this state gradually transforms at 15 K into a fully developed 3D antiferromagnetic Neel state. We ascribe such an extended quasi-2D static regime to an effective magnetic decoupling of the honeycomb planes due to a strong frustration of the interlayer exchange interactions, which inhibits long-range spin-spin correlations across the planes. Interestingly, we find indications of the topological Berezinsky-Kosterlitz-Thouless transition in the quasi-2D static state of the honeycomb spin-1/2 planes of InCu2/3V1/3O3.