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This paper presents a study of the magnetically ordered (non-singlet) state of the spin tube with a frustrating exchange interaction. The authors use the path-integral formulation for spin systems. They calculate the free energy of a system whose profiles have the shape of double holes. Исследование выполнено при финансовой поддержке РФФИ в рамках научного проекта № 20-02-00213.

An analysis of the energy spectrum and magnetization curve of the two-dimensional organic antiferromagnet F2PNNNO with a spin-one dimerized structure shows that the behavior of the compound in an external magnetic field can be explained within a lattice boson model with an extended Pauli exclusion principle, i.e.no more than two bosons per dimer. The unusual magnetization curve observed experimentally in the compound reflects a sequence of phase transitions intrinsic for a lattice boson system with strong on-site and inter-site repulsions due to a tuning of magnon density by the applied magnetic field. © 2010 IOP Publishing Ltd. We would like to thank T Sakai, J Kishine and N V Baranov for discussions. VES would like to acknowledge the support of the US Civilian Research & Development Foundation (CRDF) and the Ministry of Education and Science of the Russian Federation (MinES) under the ‘Basic Research and Higher Education’ (BRHE) program.

We present a quantum cluster solver for the spin-S Heisenberg model on a twodimensional lattice. The formalism is based on the real-space renormalization procedure and uses the lattice point group-theoretical analysis and non-Abelian SU(2) spin symmetry technique. The exact diagonalization procedure is used twice at each renormalization group step. The method is applied to the spinhalf antiferromagnet on a square lattice, and a calculation of local observables is demonstrated. A symmetry-based truncation procedure is suggested and verified numerically © 2010 IOP Publishing Ltd.

We present a study of a new class of exact solutions having a form of spiral vortices for an isotropic two-dimensional Heisenberg ferromagnet using a continuum theory and direct numerical simulations of the spin system on a square lattice. We find their features issued from the conservation laws and describe their interaction. Reasons behind the formation of the proper spin configurations on a square lattice are investigated., accepted in Phys. Rev. B

The real-space renormalization group approach is applied to study critical temperatures of a system consisting of interacting spin chains of spin S = 1, with an inner antiferromagnetic exchange, which form a honeycomb crystal lattice. Using the anisotropic Heisenberg model we calculate the critical temperature as a function of the anisotropic parameter and the ratio of the interchain and intrachain interactions. A comparison our results with those obtained from RSRG calculations for the same model of spin-1/2 on a square lattice is given. © 2005 IOP Publishing Ltd.