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ZhETF, Vol. 125, No. 3, p. 609 (March 2004)
(English translation - JETP, Vol. 98, No. 3, p. 538, March 2004 available online at )

Syromyatnikov A.V., Maleyev S.V.

Received: June 30, 2003

PACS: 75.10.Jm, 75.30.Kz, 75.40.Gb

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We suggest a new approach for description of the low-energy sector of the spin-\frac12 Heisenberg antiferromagnets (KAFs). We show that a lattice can be represented as a set of blocks containing 12 spins, having the form of stars and arranged in a triangular lattice. Each of these stars has two degenerate singlet ground states that can be considered in terms of pseudospin \frac12. Using symmetry consideration, we show that the KAF lower singlet band is made by the inter-star interaction from these degenerate states. We demonstrate that this band is described by the effective Hamiltonian of a magnet in the external magnetic field. The general form of this Hamiltonian is established. The Hamiltonian parameters are calculated up to the third order of the perturbation theory. The ground state energy calculated in the model considered is lower than those evaluated numerically in the previous finite clusters studies. A way of experimental verification of this picture using neutron scattering is discussed. It is shown that the approach presented cannot be directly extended to KAFs with larger spin values.

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