Theory of quantum phase transitions in dimerized antiferromagnets

The quantum phase transition induced by an external magnetic field in a dimerized spin crystal from its singlet ground state to an antiferromagnetic ordered state is examined under the assumption that the exchange interaction is antiferromagnetic, both within dimers and between them. It is shown that during this transition the magnetic polarization of the ions within a dimer and, therefore, of the dimers as a whole, is critical. The orientations and projections of the spin vectors in the magnetically ordered phase depend on the applied magnetic field, while the magnetic susceptibility of this essentially quantum phase remains (similarly to the case of a classical Neel antiferromagnet) constant as the spins of the magnetic sublattices undergo reorientation. (c) 2010 American Institute of Physics. [doi:10.1063/1.3490658]