Dynamic properties of magnets with spin S = 3/2 and non-Heisenberg isotropic interaction
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Dynamic Properties of Magnets with Spin S = 3/2 and NonHeisenberg Isotropic Interaction O. A. Kosmacheva, Yu. A. Fridmana*, E. G. Galkinab, and B. A. Ivanovc a
Vernadsky Taurida National University, pr. Akademika Vernadskogo 4, Crimea, Simferopol, 295007 Russia b Institute of Physics, National Academy of Sciences of Ukraine, Kiev, 03028 Ukraine cInstitute of Magnetism, National Academy of Sciences of Ukraine, Kiev, 03142 Ukraine *email: [email protected] Received August 4, 2014
Abstract—The dynamic properties of a magnet with magneticion spin of 3/2 and an isotropic spin interac tion of a general form have been investigated. Only four phase states can be realized in the system under con sideration at various relationships between the material parameters: the ferro and antiferromagnetic phases with saturated spin and the states with tensor order parameters, the nematic and antinematic ones. For these phases, the spontaneous symmetry breaking is determined by the octupole order parameter containing the mean values trilinear in spin operator components at a given site. The spectra of elementary excitations have been determined in all phases. Additional branches of excitations arise in all four phase states DOI: 10.1134/S1063776115010021
1. INTRODUCTION The ordering in spin systems is usually associated with the standard magnetic order for which the mean spins 〈Sn〉 at the sites are nonzero and form various magnetic structures (ferromagnets, antiferromagnets, etc.; see [1, 2]). The main property of magnetically ordered systems is symmetry breaking with respect to –〈Sn〉 when t –t. However, time reversal, 〈Sn〉 the possibility of the existence of a spinnematic state for which the mean spins at the sites 〈Sn〉 are zero, but the spontaneous symmetry breaking in the spin system is associated with the anisotropy of some higher spin projection correlators was pointed out fairly long ago [3]. The spinnematic state can arise from the correla tion of spins at various sites, such that the symmetry with respect to time reversal for the entire system is not broken [3]. Such states were probably detected for the lowdimensional LiCuVO4 magnet [4, 5]. The possi bility of the realization of nematic states through the existence of spin multipole order parameters including the products of the mean spin operator projections at the same site is no less interesting. Such order is attrib utable to nontrivial means of the form 〈 S α1 S α2 … S αn 〉; for spin S, it makes sense to consider n ≤ 2S. Here, n = 1 corresponds to the dipole order parameter, i.e., the mean spin 〈S〉, n = 2 corresponds to the quadrupole one, n = 3 corresponds to the octupole one, etc. An example of a spin nematic with quadrupole order for a system with spin S = 1 was considered in [3]. The problem of such (singlesite) spin nematics is closely related to the problem of quadrupole ordering and
peculiar quadrupole dynamics that has long been dis cussed in the literature (see, e.g., [6–14]). The nematic o
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