镶嵌正方晶格上具有次近邻耦合作用Ising模型的临界性质

采用部分格点自旋消约变换,将镶嵌正方晶格上具有最近邻耦合作用K1和次近邻耦合作用K2的Ising模型变换成等效的具有最近邻、次近邻和四体耦合作用的正方Ising晶格.发现系统的临界点在(K1C,K2C)=(0.5125,0.2134),由此决定系统的临界温度,幷讨论了系统的普适性.     

镶嵌正方晶格上Gauss模型的相图

利用等效变换和自旋重标相结合的方法, 研究了镶嵌正方晶格上的Gauss模型. 研究 发现, 该系统可以变换为正方晶格上具有最近邻和次近邻相互作用的Gauss系统, 由此严格求得了镶嵌正方晶格上Gauss模型的临界温度, 得到了该系统的精确相图.     

次近邻相互作用对Heisenberg XX链纠缠影响的研究

研究了存在次近邻相互作用(耦合)时Heisenberg XX链的纠缠特性. 结果表明对近邻格点, 随着耦合常数J的变化, 次近邻相互作用的存在可能使其纠缠度增大或者减小; 而对次近邻格点, 引进次近邻相互作用却可以产生纠缠, 并且使其随着|J|的增大而增大. 近邻格点间纠缠存在的临界温度T_c随着J的增大而降低, 次近邻格点间纠缠存在的临界温度T_c随着|J|的增大而升高. 此外对纠缠W态的制备, 次近邻相互作用的存在还使得三量子位情形时W态产生的时刻改变, 而对于四量子位情形却没有影响.     

W(112)p(2×1)-O化学吸附系统有序-无序相变的重整化群理论分析

用位置空间的重整化群方法,讨论W(112)p(2×1)-O化学吸附系统的有序-无序相变。采用周期性集团的递推关系,进行了将格点组成单胞,再将单胞组成周期性集团的4×4重整化变换,考虑最近邻、次近邻氧原子之间的对相互作用以及三个氧原子之间和四个氧原子之间的相互作用,求出重整化群耦合参数K′_α和格点间相互作用参数K_α之间的关系,确定了固定点K_α^*求出不同覆盖度下的临界温度T_c,得到与实验结果相符合的W(112)p(2×1)-O相图。 关键词：     

具有四次幂交换作用的Heisenberg铁磁体的低温自旋波理论

本文讨论具有四次幂交换作用的Heisenberg铁磁体,用Малеев玻色变换,在低温区建立〈S_i^z〉的严格展式,正确地包括了动力学相互作用和运动学相互作用,得到自旋波谱和自发磁化的正确温度依赖关系。 关键词：     

匀强及非匀强磁场中Heisenberg XXX链的纠缠研究

研究了匀强及非匀强磁场中反铁磁体Heisenberg XXX链的近邻和次近邻纠缠.结果表明对基态情形,纠缠随磁场B变化呈现阶梯型结构,这可用来构建量子纠缠"放大器"或量子纠缠"开关".对有限温度情形,引进一非匀强磁场Bi=(-1)iB可以使近邻格点间纠缠在某些区域明显增大,而次近邻格点间纠缠则完全消失;同时引进非匀强磁场Bi=(-1)iB还可以使近邻格点纠缠的临界温度Tcn增大,且Tcn随B的增大而升高,这意味着我们可以通过调节B的大小而在任意温度下得到纠缠.     

具有Dzyaloshinskii-Moriya相互作用的四量子比特海森堡XXZ模型中的热纠缠

研究四量子比特海森堡XXZ模型中配对纠缠的性质,在该系统中引入了Dzyaloshinskii-Moriya (DM)相互作用,通过求解配对纠缠度来讨论最近邻和次近邻两量子比特之间的热纠缠性质. 研究结果表明:对于铁磁和反铁磁两种情形而言,次近邻两量子比特之间不存在配对热纠缠;但在最近邻两量子比特情况时,DM相互作用和各向异性参数Δ对配对热纠缠和临界温度T_c都具有重要的影响,且随着温度T的增加,配对纠缠度逐渐减小直至消失. 因此,选择和调整合适的DM相互作用和各向异性参数,可以有效地控制和提高配对热纠缠. 关键词： 配对纠缠 XXZ模型')" href="#">XXZ模型 Dzyaloshinskii-Moriya相互作用     

镶嵌正方晶格上Ising模型的近似解

采用格点自旋消约方法,将具有最近邻和次近邻耦合作用的镶嵌正方Ising晶格变换成等效的具有最近邻、次近邻和四体耦合作用的正方Ising晶格,得到系统近似解的临界点在K′C=0.4406868.结果表明:在相变点最近邻耦合作用K1和次近邻耦合作用K2之间满足一定关系.如果只计及镶嵌正方Ising晶格的最近邻耦合作用K1,则其严格解的临界点在K1C=0.7635.由此可以推断在正方格点间安放两个自旋的双镶嵌正方Ising晶格,在只计及最近邻耦合作用情况下,也是严格可解的.     

周期性调制稀释Ising体系的相变

利用关联有效场方法，研究了正弦调制稀释Ｉｓｉｎｇ铁磁体系在无横场和有横场下的相变，侧重于体系的临界温度随调制幅度的变化以及临界温度对横场的依赖关系．有趣的结论是：体系磁性原子浓度的这种非均匀性可以提高其临界温度．但是，调制幅度的进一步上升可以引起体系由整体磁有序向部分磁有序的跃变 关键词：     

反铁磁Ising自旋S=1三角格子上的基态研究

本文采用文献[1]中提出的费米变换方法,研究了与三态点阵气体模型等价的三角点阵上的近邻相互作用的具有五个参数的最一般的 S=1 lsing 反铁磁体,严格地求得了它的基态能量和丰富而复杂的零温相图.同时还利用四相点方程,将 k,l 平面分成25个小区域,它们分别对应于各个拓扑不等价的基态相图.     

Additional Long-Range Interactions on Diamond-Type Hierarchical Lattices

The Ising model with additional next-nearest neighbour (n.n.n.) interactions in generator of diamond-type hierarchical lattice is studied. It is found that phase transition occurs only when the nearest neighbour (n.n.) interaction K₁ and n.n.n. interaction K₂ fulfils a certain relation and the universal class of phase transition is not changed by the long-range interactions.     

Electron charge ordering in the extended Hubbard model

The Hubbard model on 3-dimensionalsc lattice with an attractive or repulsive on site interaction, augmented by a renulsive nearest neighbour interaction and its atomic limit are studied using the method of infrared bounds. Conditions upon interactions for the existence of a temperature driven phase transition to so called charge ordered phase are derived. A lower bound in terms of interactions for the corresponding critical temperature is calculated.     

Inelastic neutron scattering measurement of spin waves and magnetic interactions in NiO

The dispersion relation E(q) for spin waves in NiO, a Type II f.c.c. antiferromagnet, has been measured at 78°K. Values for the isotropic exchange interactions have been determined by fitting the Hamiltonian to the data. The next nearest neighbour interaction is found to dominate and the small nearest neighbour interaction is ferromagnetic in sign, in agreement with simple overlap theory.     

Ground-state properties of a 2-dimensional Ising-system with competing interactions

We analyze ground-state properties of a 2-dimensional spin –1/2 Ising System on a square lattice with interactions up to the third-next neighbour in the presence of a homogeneous magnetic field. The interactions may be competing or not. Our analysis is complete in the sense that we can give the ground-state energy for all combinations of the interaction parameters and magnetic field, i.e. we can calculate the magnetization and two-spin-correlation functions up to third-next neighbour.     

The influence of lateral interactions in the occurrence of ensemble effects in chemisorption and catalysis

Monte Carlo methods have been used to examine the influence of lateral interactions between adspecies at a metal surface on the poisoning of surface reactions. A simple model is employed in which the nearest neighbour interaction energy (B) between the poison species is allowed to vary between −5 and 10 kJ mol⁻¹. The variation in the maximum extent of chemisorption is evaluated for differing values of B and poison coverage. It is demonstrated that the common procedure used to determine critical ensemble sizes produces misleading results which are seriously in error when lateral forces of interaction are considered and the catalytic significance of the results is discussed.     

The Temperley-Lieb-Jones Algebra and Symmetry Group in Heisenberg Spin Models in One and Two Dimensions

We present a systematic method to construct the spin models of Heisenberg type in higher dimensions with nearesband non-nearest neighbour interactions. These models constructed in this way are of Temperley-Lieb-Jones (TLJ) algebraic structures and SU(2)-invariances. The TLJ algebra is generalized to adjusting the lattice spin models. The Hamiltonians of Heisenberg spin models in one dimension (including second nearest neighbour interactions) and in two-dimensional triangular lattice (with nearest interaction) are constructed explicitly. The hidden symmetries are shown to be the SU(2) group, and the terms in Ilamiltonian for different lattice cells are explicitly shown to be the representations of elements of (TLJ) algebra.     

Analytical solution of 1D Ising-like systems modified by weak long range interaction

It is well-known that 1D systems with only nearest neighbour interaction exhibit no phase transition. It is shown that the presence of a small long range interaction treated by the mean field approximation in addition to strong nearest neighbour interaction gives rise to hysteresis curves of large width. This situation is believed to exist in spin crossover systems where by the deformation of the spin changing molecules, an elastic coupling leads to a long range interaction, and strong bonding between the molecules in a chain compound leads to large values for nearest neighbour interaction constants. For this interaction scheme an analytical solution has been derived and the interplay between these two types of interaction is discussed on the basis of experimental data of the chain compound which exhibits a very large hysteresis of 50 K above RT at 370 K. The width and shape of the hysteresis loop depend on the balance between long and short range interaction. For short range interaction energies much larger than the transition temperature the hysteresis width is determined by the long range interaction alone. Received 26 November 1998     

Critical properties of the X-Y model near the lifshitz point

The Monte Carlo method is applied to a three-dimensional two-component spin model with nearest neighbour ferromagnetic interactions and next nearest neighbour antiferromagnetic interactions along one axis only (planar R-S model). The critical exponent of the order parameter is determined to be 0.20±0.02 very close to the Lifshitz point. The cross-over effect is discussed.     

Properties of magnetically dilute alloys

Recently it has been shown by analysing Mössbauer spectra of dilute alloys of iron in gold that the anisotropic part of the exchange interaction between iron near neighbours determines the direction of magnetisation of such iron moments at low temperatures. Estimates of the important interactions between moments, which are here identified as the isotropic and anisotropic parts of the near neighbour exchange, and the isotropic exchange between further neighbours via the conduction electrons, have been obtained from available experimental data. The near neighbour exchange causes spins within random clusters linked as near neighbours to align parallel (the isotropic part) and to prefer certain directions in the cluster (the anisotropic part). The further neighbour exchange is responsible for the cooperative magnetic transition shown in Mössbauer and small field susceptibility results. The final model leads to qualitative interpretations of the temperature and field dependence of the susceptibility, and is able to reproduce the hysteresis loops observed at low temperatures for samples cooled in different fields. The effects of the interactions on the entropy and specific heats are discussed, and it is shown that the linear specific heat contributions at low temperatures occur because of the anisotropic interactions.     

Critical temperature for a system with Heisenberg-like nearest neighbour and Ising-like next nearest neighbour interactions

A ferromagnetic system with $\text{S =}\text{1}\text{2}$, where the interactions between nearest neighbours are isotropic (having Heisenberg character) and those between the next nearest neighbours anisotropic (Ising-like), is investigated. Using the effective hamiltonian approach we find the change in the critical temperature due to the inclusion of next nearest neighbour interactions of different character. The change is greatest in two dimensions where the critical temperature is shifted up from its zero value for a system with the Ising-like interactions switched off. We also calculate the T_c for a system with both nearest and next nearest neighbours interactions of the Ising-type. The results for the two models are compared.