Quantum phase transitions and the H–T phase diagram of a multisublattice Van Vleck antiferromagnet

Abstract

The magnetic ordering of an easy-plane Van Vleck magnet with a large anisotropy of the easy-plane type and an antiferromagnetic exchange interaction between spins is considered in the case when the ground state of the integer-spin ions is a singlet and the antiferromagnetic phase is realized only in an external magnetic field. It is found that the phase transformation of the singlet (paramagnetic) phase to the antiferromagnetic phase is a quantum phase transition belonging to the magnetic phase transitions of the displacive type. It is shown that the Landau theory of phase transitions is applicable to such a transformation, with the order parameter being the polarization of the singlet state. Nonstandard manifestations of the paraprocess converting the second-order magnetic phase transition of the displacive type to a first-order transition are noted. The phase diagram is constructed; it differs from those of classical antiferromagnets in that the antiferromagnetic phase on it is surrounded only by the paramagnetic phase.