Study of the ferromagnetic transition in a positionally frozen Heisenberg spin system

Using extensive Monte Carlo simulations with both particle and cluster orientational moves, in conjunction with finite size scaling and histogram reweighting techniques, we have determined the Curie temperature for two models of positionally frozen Heisenberg spin systems: a system with spatial correlations corresponding to a hard sphere fluid and a spatially random system. We find that the results for the positionally frozen hard sphere Heisenberg system are fairly similar to those previously obtained for the Heisenberg spin fluid and quantitatively agree with the mean field theory estimates. The random system undergoes the ferromagnetic transition at a higher temperature since the lack of core repulsion increases the spin correlations. In this case however the mean field theory overestimates by far the critical temperature.     

Thermal entanglement in the mixed three-spin XXZ Heisenberg model on a triangular cell

We numerically investigate the thermal entanglements of spins （1/2, 1） and spins （1/2, 1/2） in the three-mixed （1/2, 1, 1/2） anisotropic Heisenberg XXZ spin system on a simple triangular cell under an inhomogeneous magnetic field. We show that the external magnetic field induces strong plateau formation in the pairwise thermal entanglement for fixed parame-ters of the Hamiltonian in the cases of ferromagnetic and antiferromagnetic interactions. We also .observe an unexpected critical point at finite temperature in the thermal entanglement of spins （1/2, 1） for the antiferromagnetic case, while the entanglement of spins （1/2, 1） in the ferromagnetic case and the entanglement of spins （1/2, 1/2） in both ferromagnetic and antiferromagnetic cases almost decay exponentially to zero with increasing temperature. The critical point in the en-tanglement of spins （1/2, 1） in the antiferromagnetic case may be a signature of the quantum phase transition at finite temperature.     

Enhanced decoherence in the vicinity of a phase transition

We study the decoherence of a spin-1/2 induced by an environment which is on the verge of a continuous phase transition. We consider spin environments described by the ferromagnetic and antiferromagnetic Heisenberg models on a square lattice. As is well known, these two-dimensional systems undergo a continuous phase transition at zero temperature, where the spins order spontaneously. For weak coupling of the central spin to these baths, we find that as one approaches the transition temperature, critical fluctuations make the central spin decohere faster. Furthermore, the decoherence is maximal at zero temperature as signaled by the divergence of the Markovian decoherence rate.     

Torsion and entropy driven denaturation of DNA

A unified theory of the denaturation transition having torsion energy as the control parameter has been formulated here in the framework of the mapping of a DNA molecule onto a Heisenberg spin system. The torsion energy incorporates the torque, tension and temperature, the latter being associated with the twist angle. The denaturation transition can be mapped onto the quantum phase transition induced by a quench when the temperature effect is incorporated in the quench time and torsion takes the role of the external field. The denaturation transition occurs when the entanglement entropy of the spin system vanishes.     

海森堡XYZ自旋链系统的热纠缠与局域量子不确定性研究

研究了热平衡温度,自旋交换相互作用,Dzyaloshinskii-Moriya(DM)相互作用及外加非一致性磁场对两比特海森堡XYZ自旋链量子系统的热纠缠与局域量子不确定度的影响,对比分析了并发度量子纠缠与局域量子不确定度描述自旋链系统量子关联的差别.结果表明自旋链系统的量子纠缠在热平衡温度,DM相互作用及外加磁场的非一致性参数的变化情况下均会出现纠缠突然死亡的再生现象,而自旋链系统的局域量子不确定度随着这些参数呈连续变化现象.并且,自旋交换相互作用,DM相互作用及外加横向磁场作用强度较小时,他们的变化对自旋链系统的量子纠缠与局域量子不确定度的影响有着明显的差别.     

Electron spin resonance studies on quantum tunneling in spinel ferrite nanoparticles

The electron spin resonance (ESR) spectrometer, a very sensitive instrument with fast detecting window to explore quantum phase transitions for magnetic nanoparticles, was exploited to study the fascinating interplay between thermal and quantum fluctuations in the vicinity of a quantum critical point. We have measured ESR in ferrofluid samples containing nanosize particles of Fe₂O₃. The evolution of the ESR spectrum with temperature suggests that quantum tunneling of spins occurs in single domain magnetic particles in the low temperature regime. The effects of various microwave fields, particle sizes, and temperatures on the magnetic states of single domain spinel ferrite nanoparticles are investigated. We can consistently explain experimental data assuming that, as the temperature decreases, the spectrum changes from superparamagnetic (SPR) to blocked SPR and finally evolves quantum superparamagnetic behaviour as the temperature lowers down further. A nanoparticle system of a highly anisotropic magnetic material can be qualitatively specified by a simple quantum spin model, or by the Heisenberg model with strong easy-plane anisotropy.Received: 29 August 2003, Published online: 15 October 2003PACS: 76.30.-v Electron paramagnetic resonance and relaxation - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.) - 05.30.-d Quantum statistical mechanics - 75.50.Dd Nonmetallic ferromagnetic materials     

Monte Carlo study of low/high spin transitions

We study the finite temperature property of a model on two dimensional square lattices with two Ising spins at each lattice site by Monte Carlo simulations. When those Ising spins at a lattice site are parallel the site is said to be in the high-spin state (HS), while when they are antiparallel the site is said to be in the low-spin state (LS). Throughout the study, the energy of HS is presumed to be higher than that of LS. Two Ising spins at each site are added to form a total spin, which interacts with its nearest neighbour total spins via spin-spin couplings. The spin-phonon coupling also is introduced via harmonic springs between nearest neighbour sites with spring constants and equilibrium distances depending on the spin states of the sites involved. In this system, we investigate the feature of transitions between LS and HS (to be called low/high spin transition (LHST)) by varying the temperature. As for the ferromagnetic interaction between total spins, the second order phase transition: pure HSmixed state of HS and LS is possible to occur in a pure spin system, as is expected from mean field calculations. The role of lattice distortions by the change of lattice spacings is shown to be essential for LHST: pure LS(pure)HS. In the model investigated, there appears an indication of the strong first order phase transition which reveals a conspicuous hysteresis.     

On the Interplay of Magnetic and Molecular Forces in Curie–Weiss Ferrofluid Models

We consider a mean-field continuum model of classical particles in R ^d with Ising or Heisenberg spins. The interaction has two ingredients, a ferromagnetic spin coupling and a spin-independent molecular force. We show that a feedback between these forces gives rise to a first-order phase transition with simultaneous jumps of particle density and magnetization per particle, either at the threshold of ferromagnetic order or within the ferromagnetic region. If the direct particle interaction alone already implies a phase transition, then the additional spin coupling leads to an even richer phase diagram containing triple (or higher order) points.     

小粒子体系的自发磁化

从自旋波理论出发,通过直接求解Heisenberg模型,研究了低温下小粒子自旋体系的自发磁化规律。研究表明:极低温下自旋波被冻结;在任何温度范围内,Bloch T^3/2定律都不再严格成立。同时,本文还给出了小粒子中自旋受激反转几率随位置的分布,指出低温下不存在运动明显剧烈的表面层。 关键词：     

Influence of correlated spins on the superconducting transition temperature

Using a variational procedure, the depression of the superconducting transition temperature by a system of correlated moving spins is calculated. In particular, corrections to the Abrikosov-Gorkov theory in the neighbourhood of a magnetic phase transition are examined. Characteristic differences in the concentration dependence of the superconducting transition temperature are found in the cases of ferromagnetic and antiferromagnetic spin correlations.     

Chiral-glass transition in a diluted dipolar-interaction Heisenberg system

Recently, numerical simulations reveal that a spin-glass transition can occur in the three-dimensional diluted dipolar system. By defining the chirality of triple spins in a diluted dipolar Heisenberg spin glass, we study the chiral ordering in the system using parallel tempering algorithm and heat bath method. The finite-size scaling analysis reveals that the system undergoes a chiral-glass transition at finite temperature.     

A new method for calculation of the magnetic properties in one- and two-dimensional spin systems with anisotropic Heisenberg exchange

A new approximation method is proposed for the calculation of the magnetic susceptibility of one-dimensional assembly of spins and the critical temperature of two-dimensional one both with the anisotropic Heisenberg exchange. In a linear chain system, every spins are grouped into pairs of adjacent spins (pair-approximation) or clusters of adjacent three spins ((q+1)-approximation), and the partition function of the total spin system is approximated as a sum of products of the partitions functions for the pairs or the clusters. Then the partition function of the anisotropic Heisenberg spin system is shown to reduce into a form of the Ising spin system with modified coupling constants. The exact result for the Ising chain system enables us to obtain an analytical expression for the magnetic susceptibility of anisotropic Heisenberg chain system. The same approximations are also applied to two-dimensional lattices, and the critical temperatures of the square, triangular, and honeycomb lattices with anisotropic Heisenberg exchange are calculated as a function of anisotropy parameter. The results are compared with those of the existing theories and shown to be quite excellent.     

The magnetic phase transition in amorphous ferromagnets and in spin glasses

The magnetic phase transition in materials with exchange disorder (amorphous ferromagnets, spin glasses) is discussed. In the critical temperature range the behavior of amorphous ferromagnetic transition metal-metalloid glasses is found to be similar to the one derived for a three-dimensional homogeneous Heisenberg ferromagnet. The most prominent difference between disordered and homogeneous materials is manifested in a large temperature range of deviations from the mean field behavior beyond the critical region, as observed experimentally for the temperature dependence of the linear susceptibility of amorphous ferromagnets and of the nonlinear susceptibility of spin glasses. A molecular field theory with correlations in space and time is developed, which relates the deviations from the mean field behavior to the interplay between the temperature dependent thermal correlations in the spin system and the spatial fluctuations of the material. Application to dynamical processes (kinetic critical slowing down) is discussed.     

Effect of CDW and magnetic interactions on the electrons of the manganite systems

We address a model study which includes the co-existence of the charge density wave (CDW) and ferromagnetic interactions in order to explain the colossal magnetoresistance (CMR) in manganites. The Hamiltonian consists of the ferromagnetic Hund's rule exchange interaction between e_g and t_2g spins, Heisenberg core spin interactions and the CDW interaction present in the e_g band electrons. The core electron magnetization, induced e_g electron magnetization and the CDW gap are calculated using Zubarev's Green's function technique and determined self-consistently. The effect of core electron magnetization and the CDW interaction on the induced magnetization as well as on the occupation number in the different spin states of the e_g band electrons are investigated by varying the model parameters of the system like the CDW coupling, the exchange coupling, the Heisenberg coupling and the external field. It is observed that the induced magnetization exhibits re-entrant behaviour and exists within a narrow temperature range just below the Curie temperature. This unusual behaviour of the e_g band electrons will throw some new insights on the physical properties of the manganite systems.     

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the quantum dots and the itinerant holes in the semiconductor valence band makes possible hole transfer between the DMS quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.Received: 17 Mars 2003, Published online: 30 January 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.30.Ds Spin waves - 75.50.Dd Nonmetallic ferromagnetic materials - 75.50.Pp Magnetic semiconductors     

Analogue of surface reconstruction in a Heisenberg ferromagnet

When the exchange interaction between spins in the surface of a Heisenberg ferromagnet is antiferromagnetic in sign, the surface region may order in an anti-ferromagnetic manner, even though the bulk is ferromagnetic. We discuss the properties of a model which exhibits this behavior from a theoretical point of view. The properties of the model are examined to see if by this means one can understand some aspects of recent observations of spin polarized photoemission from the surface of EuO.     

On the dynamic nature of the freezing process in Ising spin glasses

The distribution of local moments in a two dimensional Ising spin glass with short range Gaussian interactions is investigated by Monte Carlo simulations. Below the freezing temperatureT _f , this distribution has a sharp peak at the saturated moment. The spins can clearly be characterized by a fractionq of frozen spins and 1-q of fast spins which are in thermal equilibrium. Just belowT _f the frozen spins appear in small clusters; the spin glass transition isnot a percolation process. Our results support the local and nonequilibrium character of the spin glass transition.q is related to the remanent magnetization (TRM), the linear response and the field cooled susceptibility. As a consequence magnetic resonance experiments should see, in addition to a broad background, a sharp line splitting whose position does not shift with temperature.     

Characterization of ferrimagnetic Heisenberg chains according to the constituent spins

The low-energy structure and the thermodynamic properties of ferrimagnetic Heisenberg chains of alternating spins S and s are investigated by the use of numerical tools as well as the spin-wave theory. The elementary excitations are calculated through an efficient quantum Monte Carlo technique featuring imaginary-time correlation functions and are characterized in terms of interacting spin waves. The thermal behavior is analyzed with particular emphasis on its ferromagnetic and antiferromagnetic dual aspect. The extensive numerical and analytic calculations lead to the classification of the one-dimensional ferrimagnetic behavior according to the constituent spins: the ferromagnetic (S>2s), antiferromagnetic (S<2s), and balanced (S=2s) ferrimagnetism. Received 27 August 1999 and Received in final form 15 November 1999     

Ising-type critical exponents of the fully frustrated spin-1/2 Heisenberg FM/AF square bilayer at a critical magnetic field

The fully frustrated spin-1/2 Heisenberg FM/AF square bilayer in a magnetic field with the ferromagnetic inter-dimer interaction and the antiferromagnetic intra-dimer interaction is explored by the use of localized many-magnon approach, which allows to connect the original purely quantum Heisenberg spin model on a square bilayer with the effective ferromagnetic Ising model on a simple square lattice. Magnetization and specific heat are investigated exactly at a field-driven phase transition from the singlet-dimer phase towards the fully saturated ferromagnetic phase, which changes from a discontinuous phase transition to a continuous one at a certain critical temperature. The mapping correspondence between the spin-1/2 Heisenberg FM/AF square bilayer and the ferromagnetic Ising square lattice suggests for this special critical point of the spin-1/2 Heisenberg FM/AF square bilayer critical exponents from the standard two-dimensional Ising universality class.