反铁磁链的自旋Peierls相变

通过费米-玻色变换将一维自旋1/2反铁磁链映射为连续玻色场形式,讨论了模型基态能量和激发态能量,证明了此系统存在自旋Peierls相变.并给出了模型基态能量密度与晶格畸变大小的关系.数值计算的结果验证了上述理论的正确性. 关键词：     

一维亚铁磁混合自旋链的磁特性的Monte Carlo模拟

基于一维自旋链模型，采用Monte Carlo方法对具有反铁磁相互作用的自旋一I和自旋-3／2交替混合的亚铁磁系统进行模拟计算，研究单离子各向异性对系统磁特性的影响．模拟结果发现：基态时系统的磁化强度在外加磁场作用下呈现出阶梯状，并获得了自旋-1和自旋-3／2交替混合的亚铁磁系统相图．最后，通过对系统能量和自旋组态的研究，解释了该系统中出现的阶梯效应和相图．     

一维钻石链反铁磁Ising模型磁化的模拟

采用Monte Carlo模拟方法对自旋为1/2的一维钻石链反铁磁Ising系统的磁化行为进行研究.在这个系统中,反铁磁的交换作用和三角结构导致存在自旋阻挫.重点研究不同的反铁磁自旋交换作用对系统磁行为和自旋构型的影响.模拟磁化曲线中M=M_S/3磁化平台,得到平台宽度随不同的自旋交换相互作用强弱的变化关系,以及出现磁化平台时格点的自旋构型;给出亚稳态存在的条件及亚稳态时的微观构型.研究磁滞回线随温度的变化关系.结果表明,随着温度的升高,磁滞回线逐渐减小,最终消失.     

铁磁/反铁磁双层薄膜自旋波共振谱

本文在一维海森堡模型的基础上,采用界面参数化方法,研究了铁磁/反铁磁双层薄膜中自旋波低温激发问题．重点讨论了表面各向异性对薄膜中自旋波共振谱的影响．结果表明：体系中的自旋波本征模存在共振行为,表面各向异性场对体模、完全禁闭模的共振谱影响较大,对界面模没有影响．     

铁磁/反铁磁双层薄膜自旋波共振谱

本文在一维海森堡模型的基础上,采用界面参数化方法,研究了铁磁/反铁磁双层薄膜中自旋波低温激发问题．重点讨论了表面各向异性对薄膜中自旋波共振谱的影响．结果表明：体系中的自旋波本征模存在共振行为,表面各向异性场对体模、完全禁闭模的共振谱影响较大,对界面模没有影响．     

铁磁/反铁磁双层薄膜中自旋波特性研究

本文在一维海森堡模型的基础上,采用界面参数化方法,研究了非周期性边界条件下,铁磁/反铁磁双层薄膜中自旋波低温激发问题.重点讨论了表面各项异性对薄膜中自旋波谱、色散关系的影响.结果表明:表面各向异性对铁磁层中自旋波谱影响较大,对色散关系影响甚微。     

二维Heisenberg反铁磁系统中的正则变换和自旋波

本文给出了二维Heisenberg反铁磁系统中的正则变换.在此变换下发现二维Heisenberg反铁磁模型在连续极限下是具有拓扑项的O(3)非线性σ模型.由于在选取正则变量时有很大的不确定性,因此还不能判定此系统是否确有拓扑项存在.最后对正则变换的物理意义进行了研究,发现它体现了系统自旋的集体激发,表明存在着一种自旋波.     

高温超导体中反铁磁自旋涨落对其自旋磁化率效应

本提出温超导体中存在反铁磁自旋涨落对其自旋磁化率效应，研究指出高温超导体中反铁磁自旋涨落对其自旋磁化率的效应引起自旋磁化率随温度单调变化。     

梯形自旋链的一种规范场论的形式

考虑无质量费米场和规范场的Pauli相互作用项的系统,在这一理论中耦合常数为无量纲量.该模型等价于自旋1/2反铁磁XXZ量子链.如在模型中引入费米场的味自由度,则可定性描述二条自旋链的梯形结构系统.在这一系统中不再存在无能隙激发. 关键词： 梯形结构 规范场 自旋链     

有限铁磁—反铁磁线链中的自旋波谱及其激发

本文研究了由交换作用耦合起来的有限长铁磁和反铁磁链组成的自旋系统。计算了系统的自旋波的波谱及其在微波场中的激发强度。在所得结果的基础上,讨论了具有反铁磁氧化层的铁磁薄膜的自旋波共振。阐明了等效地使用唯象普遍边界条件的可能性。根据理论,对等效的表面各向异性能作了估计,得到的数值与实验值很好地符合。     

Analytic evidence of the equivalence of the alternating Heisenberg spin chain to the mixed spin (1, 1/2) Heisenberg chain

We investigated the properties of the spin-1/2 ferromagnetic-antiferromagnetic-antiferromagnetic alternating Heisenberg chain using the spin-wave theory. The spin-wave excitation spectra, the sublattice magnetizations and the local bond energies of the model are calculated to be compared with the corresponding properties of the mixed spin (1, 1/2) chain for a range of α. The results demonstrate that all the properties show similar behaviours in the small α limit, so the properties of the mixed spin (1, 1/2) chain can be described using the spin-1/2 ferromagnetic-antiferromagnetic-antiferromagnetic alternating Heisenberg chain.     

Phase diagram of frustrated mixed-spin ladders in the strong-coupling limit

We study the ground-state properties of frustrated Heisenberg ferrimagnetic ladders with antiferromagnetic exchange interactions and two types of alternating sublattice spins. In the limit of strong rung couplings, we show that the mixed spin-1/2 and spin-1 ladders can be systematically mapped onto a spin-1/2 Heisenberg model with additional next-nearest-neighbor exchanges. The system is either in a ferrimagnetic state or in a critical spin-liquid state depending on the competition between the spin exchanges along the legs and the diagonal exchanges.     

Spin frustration-related behavior of mixed spin diamond chain

The magnetization of mixed spin-1/2, 1 Ising diamond chain with antiferromagnetic spin exchange interaction has been investigated by using the Monte Carlo simulation. Magnetization behaviors with different antiferromagnetic interactions are simulated under magnetic field at low temperature. The behaviors are studied in two different cases. In one case the absolute value of the interaction between two spin-1/2 atoms is the largest and in the other case it is not the largest. Multiple plateaus of magnetization M, such as plateaus at M=±1, M=0 and M=±0.566 are obtained. The plateaus at M=±0.566 correspond to two metastable states. The M=0 plateau may disappear at some specific values of spin exchange interactions. The spin configurations were checked when the plateaus appear. The dimmer and trimmer states are found.     

Thermodynamic properties of the mixed spin-1/2 and spin-1 Ising chain with both longitudinal and transverse single-ion anisotropies

The mixed spin-1/2 and spin-1 Ising chain with both longitudinal and transverse single-ion anisotropies is solved exactly by means of a mapping to the spin-1/2 Ising chains with alternating transverse fields and the Jordan-Wigner transformation. Within this scheme, the thermodynamic quantities of this model are rigorously determined by a recursion formula derived for the partition function based on the reduced spin-1/2 transverse Ising model. The corresponding thermodynamic properties are calculated and discussed.     

Finite-size scaling for correlations of quantum spin chains at criticality

We study the finite-size scaling behavior of two-point correlation functions of translationally invariant many-body systems at criticality. We propose an efficient method for calculating the two-point correlation functions in the thermodynamic limit from numerical data of finite systems. Our method is most effective when applied to a two-dimensional (classical) system which possesses a conformal invariance. By using this method with numerical data obtained from exact diagonalizations and Monte Carlo simulations, we study the spin-spin correlations of the quantum spin-1/2 and-3/2 antifierromagnetic chains. In particular, the logarithmic corrections to power-law decay of the correlation of the spin-1/2 isotropic Heisenberg antiferromagnetic chain are studied thoroughly. We clarify the cause of the discrepancy in previous calculations for the logarithmic corrections. Our result strongly supports the field-theoretic prediction based on the mappings to the Wess-Zumino-Witten nonlinear -model or the sine-Gordon model. We also treat logarithmic corrections and crossover phenomena in the spin-spin correlation of the spin-3/2 isotropic Heisenberg antiferromagnetic chain. Our results are consistent with the Affleck-Haldane prediction that the correlation of the spin-3/2 chain exhibits a crossover to the same asymptotic behavior as in the spin-1/2 chain.     

三量子位和四量子位Heisenberg XY链中的纠缠与自旋压缩(英文)

研究了三量子位和四量子位Heisenberg XY链中的基态纠缠与自旋压缩,给出了纠缠C和自旋压缩参数ξ2的解析表达式.结果表明,当外部磁场B大于某一临界值Bc时,纠缠与自旋压缩等价,即纠缠意味着自旋压缩,反之亦然.对三量子位情形,Bc=J[(4-3γ2)1/2-1];对四量子位情形,Bc可以通过数值方法进行求解.     

Magnetic properties of the spin S = 1/2 Heisenberg chain with hexamer modulation of exchange

We consider the spin-1/2 Heisenberg chain with alternating spin exchange in the presence of additional modulation of exchange on odd bonds with period 3. We study the ground state magnetic phase diagram of this hexamer spin chain in the limit of very strong antiferromagnetic (AF) exchange on odd bonds using the numerical Lanczos method and bosonization approach. In the limit of strong magnetic field commensurate with the dominating AF exchange, the model is mapped onto an effective XXZ Heisenberg chain in the presence of uniform and spatially modulated fields, which is studied using the standard continuum-limit bosonization approach. In the absence of additional hexamer modulation, the model undergoes a quantum phase transition from a gapped phase into the only one gapless Lüttinger liquid (LL) phase by increasing the magnetic field. In the presence of hexamer modulation, two new gapped phases are identified in the ground state at magnetization equal to [Formula: see text] and [Formula: see text] of the saturation value. These phases reveal themselves also in the magnetization curve as plateaus at corresponding values of magnetization. As a result, the magnetic phase diagram of the hexamer chain shows seven different quantum phases, four gapped and three gapless, and the system is characterized by six critical fields which mark quantum phase transitions between the ordered gapped and the LL gapless phases.     

Alternating-spin S = 3/2 and <Emphasis Type="Italic">σ</Emphasis> = 1/2 Heisenberg chain with isotropic three-body exchange interactions

The promotion of collinear classical spin configurations as well as the enhanced tendencytowards nearest-neighbor clustering of the quantum spins are typical features of thefrustrating isotropic three-body exchange interactions in Heisenberg spin systems. Basedon numerical density-matrix renormalization group calculations, we demonstrate that theseextra interactions in the Heisenberg chain constructed from alternating S = 3/2 and σ = 1/2 site spins can generate numerous specific quantum spinstates, including some partially-polarized ferrimagnetic states as well as adoubly-degenerate non-magnetic gapped phase. In the non-magnetic region of the phasediagram, the model describes a crossover between the spin-1 and spin-2 Haldane-typestates.     

Entanglement and quantum phase transition in alternating XY spin chain with next-nearest neighbouring interactions

By using the method of density-matrix renormalization-group to solve the different spin spin correlation functions, the nearest-neighbouring entanglement （NNE） and the next-nearest-neighbouring entanglement （NNNE） of one-dimensional alternating Heisenberg XY spin chain are investigated in the presence of alternating the-nearestneighbouring interaction of exchange couplings, external magnetic fields and the next-nearest neighbouring interaction. For a dimerised ferromagnetic spin chain, the NNNE appears only above a critical dimerized interaction, meanwhile, the dimerized interaction a effects a quantum phase transition point and improves the NNNE to a large extent. We also study the effect of ferromagnetic or antiferromagnetic next-nearest neighbouring （NNN） interaction on the dynamics of NNE and NNNE. The ferromagnetic NNN interaction increases and shrinks the NNE below and above a critical frustrated interaction respectively, while the antiferromagnetic NNN interaction always reduces the NNE. The antiferromagnetic NNN interaction results in a large value of NNNE compared with the case where the NNN interaction is ferromagnetic.