Factorized ground state for a general class of ferrimagnets

Progress of Theoretical Physics, 2012

The exact factorized ground state of a heterogeneous (ferrimagnetic) spin model which is composed of two spins (ρ, σ) has been presented in detail. The Hamiltonian is not necessarily translational invariant and the exchange couplings can be competing antiferromagnetic and ferromagnetic arbitrarily between different sublattices to build many practical models such as dimerized and tetramerized materials and ladder compounds. The condition to get a factorized ground state is investigated for non-frustrated spin models in the presence of a uniform and a staggered magnetic field. According to the lattice model structure we have categorized the spin models in two different classes and obtained their factorization conditions. The first class contains models in which their lattice structures do not provide a single uniform magnetic field to suppress the quantum correlations. Some of these models may have a factorized ground state in the presence of a uniform and a staggered magnetic field. However, in the second class there are several spin models in which their ground state could be factorized whether a staggered field is applied to the system or not. For the latter case, in the absence of a staggered field the factorizing uniform field is unique. However, the degrees of freedom for obtaining the factorization conditions are increased by adding a staggered magnetic field.

Physica Status Solidi (b), 1978

A system of "atoms", each one of which can be in one of two spin states, is considered. On the assumption that in one of the two atomic states a ferromagnetic interaction exists, the magnetic properties of the system are investigated. Various types of magnetic behaviour and phase transitions are found. Over a certain reasonable range of model parameters heat magnetization is obtained. CMCTeMa aTOMOB, HalrcAbIfi M 3 HOTOPbIX MOH(eT HaXOnMTbCR B OAHOM M 3 ABYX CnMHOBblX COCTOHHMfi, paCCMOTpeHa B IIpeAIlO.TlOH(eHMEl CylUeCTBOBalIEiH @eppOMarHMTHOrO B3aM-MoAeficTmn B OLIHOM ~3 BTMX COCTOFIHH~~. Mccnenoaaaa TeMnepaTypHan ~~B H C M M O C T~ HaMarHMqeHHocni M B O~M O M H~I~ THnM ( P~~O B M X nepexonoB. I I o~a a a~o , ~I T O B onpeneneH-HOM xwrepsane napaMeTpoB MonenH ciicTeMa o6nanaeT CBOPCTBOM ,,TennnoBofi MarHeTa-3aqm ' ' .

Physical Review B, 2013

We use a combination of numerical density matrix renormalization group (DMRG) calculations and several analytical approaches to comprehensively study a simplified model for a spatially anisotropic spin-1/2 triangular lattice Heisenberg antiferromagnet: the three-leg triangular spin tube (TST). The model is described by three Heisenberg chains, with exchange constant J, coupled antiferromagnetically with exchange constant J ′ along the diagonals of the ladder system, with periodic boundary conditions in the shorter direction. Here we determine the full phase diagram of this model as a function of both spatial anisotropy (between the isotropic and decoupled chain limits) and magnetic field. We find a rich phase diagram, which is remarkably dominated by quantum states -the phase corresponding to the classical ground state appears only in an exceedingly small region. Among the dominant phases generated by quantum effects are commensurate and incommensurate coplanar quasi-ordered states, which appear in the vicinity of the isotropic region for most fields, and in the high field region for most anisotropies. The coplanar states, while not classical ground states, can at least be understood semiclassically. Even more strikingly, the largest region of phase space is occupied by a spin density wave phase, which has incommensurate collinear correlations along the field. This phase has no semiclassical analog, and may be ascribed to enhanced one-dimensional fluctuations due to frustration. Cutting across the phase diagram is a magnetization plateau, with a gap to all excitations and "up up down" spin order, with a quantized magnetization equal to 1/3 of the saturation value. In the TST, this plateau extends almost but not quite to the decoupled chains limit. Most of the above features are expected to carry over to the two dimensional system, which we also discuss. At low field, a dimerized phase appears, which is particular to the one dimensional nature of the TST, and which can be understood from quantum Berry phase arguments. arXiv:1211.1676v2 [cond-mat.str-el]

Physical Review B, 2012

We study the finite-temperature spin dynamics of the paramagnetic phase of iron pnictides within an antiferromagnetic J1 − J2 Heisenberg model on a square lattice with a biquadratic coupling −K(Si • Sj) 2 between the nearest-neighbor spins. Our focus is on the paramagnetic phase in the parameter regime of this J1 − J2 − K model where the ground state is a (π, 0) collinear antiferromagnet. We treat the biquadratic interaction via a Hubbard-Stratonovich decomposition, and study the resulting effective quadratic-coupling model using both modified spin wave and Schwinger boson mean-field theories; the results for the spin dynamics derived from the two methods are very similar. We show that the spectral weight of dynamical structure factor S(q, ω) is peaked at ellipses in the momentum space at low excitation energies. With increasing energy, the elliptic features expand towards the zone boundary, and gradually split into two parts, forming a pattern around (π, π). Finally, the spectral weight is anisotropic, being larger along the major axis of the ellipse than along its minor axis. These characteristics of the dynamical structure factor are consistent with the recent measurements of the inelastic neutron scattering spectra on BaFe2As2 and SrFe2As2.

Physical Review B, 2021

We consider a multiparticle fermionic state to study the magnetization dynamics in an XX spin chain model. The advantage this state provides is the freedom in choosing the initial magnetization profile at each spin site. We characterize the t = 0 properties of this state, in particular, the probability for the state to have n fermions, fidelity, the two-point and the four-point correlators, and spin-spin correlations. The dynamics of the domain-wall state formed by considering the ground states with magnetizations m 0 and −m 0 in the left and right half of the chain, respectively, results in the growth of a flat plateau (m = 0) away from the origin. In contrast, the magnetization dynamics of the domain wall state initialized via the multiparticle state reveals the absence of such a region. Our studies of the particle-number fluctuation for the multiparticle state with all sites having identical magnetization reveals linear dependence with respect to the system size, as opposed to the ln l dependence which is obtained for the ground state with the same magnetization.

Physica B: Condensed Matter, 2018

An exhaustive ground-state analysis of extended two-dimensional (2D) correlated spin-electron model consisting of the Ising spins localized on nodal lattice sites and mobile electrons delocalized over pairs of decorating sites is performed within the framework of rigorous analytical calculations. The investigated model, defined on an arbitrary 2D doubly decorated lattice, takes into account the kinetic energy of mobile electrons, the nearest-neighbor Ising coupling between the localized spins and mobile electrons, the further-neighbor Ising coupling between the localized spins and the Zeeman energy. The ground-state phase diagrams are examined for a wide range of model parameters for both ferromagnetic as well as antiferromagnetic interaction between the nodal Ising spins and non-zero value of external magnetic field. It is found that non-zero values of further-neighbor interaction leads to a formation of new quantum states as a consequence of competition between all considered interaction terms. Moreover, the new quantum states are accompanied with different magnetic features and thus, several kinds of discontinuous field-driven phase transitions are observed.

The Journal of Multidisciplinary Research

In this research paper molecular mean-field theory (MMFT) has been investigated based on Gibbs-Bogoliubov free energy function of a ferrimagnetics mixed spin-3 and spin-5/2 Blume-Capel model with different magnetic crystal fields. The free energy of the proposed ferrimagnet has been evaluated depending on the trial Hamiltonian operator. Minimizing the free energy, one may induce characteristic features of the longitudinal magnetizations, phase transitions and spin compensation temperatures, in the ranges of low temperatures, respectively. In particular, we study the effect of crystal field domains on the critical phenomena for the proposed model. The sublattice magnetization dependence of free energy function has been discussed as well. Our results predict the existence of multiple spin compensation sites in the disordered Blume-Capel Ising system for a simple cubic lattice.

2014

The magnetic behaviours of a mixed Blume-Capel Ising ferrimagnetic system on a square lattice having spins 2 , in the absence and presence of an external magnetic field, are investigated, respectively. Our results which are examined have interesting features depending on higher positive values of anisotropy field. The longitudinal magnetic fields dependence of the spin compensation temperature is the essential substance of research. It is worth to note that in this model, the contribution of magnetic free energy to the thermodynamic stability of the mixed spin ferrimagnet with first nearest neighbour interaction is indicated.

Physical Review B, 2020

Elucidating the nature of spin excitations is important to understanding the mechanism of superconductivity in the iron pnictides. Motivated by recent inelastic neutron scattering measurements in the nearly 100% detwinned BaFe2As2, we study the spin dynamics of an S = 1 frustrated bilinear-biquadratic Heisenberg model in the antiferromagnetic phase with wavevector (π, 0). The biquadratic interactions are treated in a dynamical way using a flavor-wave theory in an SU (3) representation. Besides the dipolar spin wave (magnon) excitations, the biquadratic interactions give rise to quadrupolar excitations at high energies. We find that the quadrupolar wave significantly influences, in an energy dependent way, the anisotropy between the spin excitation spectra along the (π, 0) and (0, π) directions in the wave vector space. Our theoretical results capture the essential behavior of the spin dynamics measured in the antiferromagnetic phase of the detwinned BaFe2As2. More generally, our results underscore the importance of electron correlation effects for the microscopic physics of the iron pnictides.

The collective spin excitations in the unbounded 2D paramagnetic system with dipole interactions are studied. The model Hamiltonian includes Zeeman energy and dipole interaction energy, while the exchange vanishes. The system is placed into a constant uniform magnetic field which is orthogonal to the lattice plane. It provides the equilibrium state with spin ordering along the field direction, and the saturation is reached at zero temperature. We consider the deviations of spin magnetic moments from its equilibrium position along the external field. The Holstein-Primakoff representation is applied to spin operators in low-temperature approximation. When the interaction between the spin waves is negligible and only two-magnon terms are taken into account, the Hamiltonian diagonalisation is possible. We obtain the dispersion relation for spin waves in the square and hexagonal honeycomb lattice. Bose-Einstein statistics determine the average number of spin deviations, and total system magnetization. The lattice structure does not influence on magnetization at the long-wavelength limit. The dependencies of the relative magne-tization and longitudinal susceptibility on temperature and external field intensity are found. The internal energy and specific heat of the Bose gas of spin waves are calculated. The collective spin excitations play a significant role in the properties of the paramagnetic system at low temperature and strong external magnetic field.