下地幔方镁铁矿自旋转变的研究进展

压力引起的铁的电子自旋转变发生在下地幔的主要矿物中,这种转变对下地幔矿物的力学、物理学和化学等性质都会产生重要影响,因此在下地幔温度、压力条件下开展下地幔主要矿物相的自旋转变研究对下地幔地球物理学、地球化学和地球动力学等模型的约束具有重要意义。作为下地幔矿物质量分数第2的方镁铁矿,前人对其自旋转变以及这种转变对它的密度、声速、弹性性质、铁的分配、辐射热传导和电导率等的影响有广泛研究。本文旨在对方镁铁矿的自旋转变的主要研究进展进行总结和评述,并对未来该研究的发展趋势进行展望。

多铁材料SmFe1-xCrxO3陶瓷的介电特性及磁相变的研究

采用溶胶-凝胶法制备SmFe1-xCrxO3(x=0~0.3)样品,研究Cr^3+掺杂对SmFeO3的晶体结构、介电特性及磁相变温度的影响。研究表明,所有样品的主衍射峰与SmFeO3标准图谱相吻合且具有良好的晶体结构,Cr^3+掺杂使其晶胞体积收缩,导致主衍射峰向衍射角(θ)较大的方向移动。SmFeO3的介电常数(εr)随着掺杂量(x)的增加逐渐减小,介电损耗(tanδ)随着Cr^3+的增加而增大,介电常数(εr)随频率(f)和掺杂量(x)的增加而逐渐减小的原因是由IBLC微观机制和偶极子取向极化机制两者共同作用的结果,其介电损耗主要是电导电流产生的损耗起主导作用,并发现SmFe1-xCrxO3的介电损耗峰随掺杂量的增加向高频方向迁移。SmFeO3的蜂腰状M-H随着Cr^3+的掺杂而消失且磁性也有所减弱,其自旋重组相变温度(TSR)和反铁磁相变温度(TN)随着Cr^3+的增加由原来的462,687K降低到428和536K。这主要是Cr^3+掺杂使SmFeO3晶格发生畸变,形成Fe^3+-O^2--Cr^3+磁交换作用,破坏了原有Fe^3+-O^2--Fe^3+反铁磁有序排列,降低了其G型反铁磁结构的稳定性,在宏观上表现为其铁磁性的减弱和磁相变温度TSR和TN的降低,进一步探讨了Cr^3+掺杂对SFO的介电性及铁磁性影响的微观机理,使其在自旋电子器件、磁制冷领域及磁性开关器件等方面的应用成为可能。

Ca^(2＋)掺杂对SmFeO_3的介电、铁磁特性及磁相变温度的影响

采用固相反应法制备Sm_(1-x)Ca_xFeO_3(x=0,0.1,0.2,0.3)样品,研究Ca^(2+)掺杂对SmFeO_3介电性能、铁磁性及磁相变温度的影响.X射线衍射图谱分析表明:所有样品的主衍射峰与SmFe03相符合且具有良好的晶体结构.随着x的增加,SmFeO_3样品的晶粒尺寸由原来的0.5μm逐渐增大到2μm.当f=1 kHz时,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的ε_r分别是SmFe03的5倍、3倍和2.6倍,而tgσ增大一个数量级.在3T磁场作用下,SmFe03样品的M-H呈线性,随着x的增加,M-H逐渐趋向饱和,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的M_r分别是SmFeO_3的20倍、31倍和68倍.X射线光电子能谱分析表明:Fe^(2+)和Fe3+共存于Sm_(1-x)Ca_xFeO_3样品中,Fe^(2+)/Fe^(3+)比例随着x的增加而增大,证明Ca^(2+)掺杂增加了Fe^(2+)的含量,形成Fe^(2+)—O^(2-)—Fe^(3+)超交换作用,增强SmFe03的铁磁特性.测量了Sm_(1-x)Ca_xFeO_3样品在外加磁场为1000 Oe(1 Oe=79.5775 A/m)的M-T变化关系,观测到其自旋重组温度(T_(SR))和尼尔温度(T_N)分别为438 K和687 K,发现SmFe03样品的T_(SR)和T_N均随着x的增加向低温方向移动,当x=0.3时,自旋重组现象消失.这主要是SmFeO_3样品磁结构的稳定性和Fe^(3+)—O^(2-)—Fe^(3+)及Sm^(3+)—O^(2-)—Fe^(3+)超交换三者共同作用的结果.

高温高压下(Mg,Fe)SiO_3钙钛矿的相稳定性

在冲击波物理实验测量高温高压下（Mg，Fe）SiO3钙钛矿的声速结果的基础之上，发现了在70～85GPa压力范围内发生了声速的“软化”现象．利用沃尔什热力学-冲击压缩线方法和Mie-Grüneisen物态方程等熵线方法计算了声速测量实验中的冲击温度，并计算了冲击压缩下70～85OPa压力范围内的相变边界，然后用电子自旋相变的自由能变化估算了这个压力范围内的相变边界，发现两种方法计算得到的相变边界相同，从而证实这种声速“软化”现象是由（Mg，Fe）SiO3钙钛矿的晶格畸变引起的，没有发生（Mg，Fe）SiO3钙钛矿向氧化物（Mg，h）O＋SiO2的分解相变、

Cr取代亚铁磁性Mn_2Sb_(0.95)In_(0.05)合金的磁及磁热性能

磁制冷是一种利用材料的磁热效应进行制冷的新型制冷技术,相比于传统气体压缩制冷,因其绿色环保、高效节能等优点而备受关注。在众多磁相变合金材料中,人们对Mn_2Sb基亚铁磁相变合金研究甚少。文章研究了Cr取代Mn后亚铁磁性Mn_(2-x)Cr_xSb_(0.95)In_(0.05)(x=0.05,0.09,0.13)合金的磁性和磁热性能。室温XRD数据表明合金在室温附近以四角Cu_2Sb型结构为主相。由于反铁磁中有高磁响应,因此从XRD图谱中能观察到少量的铁磁MnSb杂相。随着温度的降低,在这些合金中,发生了亚铁磁到反铁磁的一级磁致弹性转变。同时,在亚铁磁区域观察到两个自旋重新取向转变。由于反铁磁-亚铁磁的转变过程中磁化强度突变,使得在Mn_(1.91)Cr_(0.09)Sb_(0.95)In_(0.05)合金中在0~10 k Oe的磁场变化中获得高达1.63 J/kg·K的大磁熵变。目前的研究可能有助于研究和开发新的磁性冷材料。

Phase Diagram for Ashkin-Teller Model on Bethe Lattice

Using- the recursion method, we study the phase transitions of theAshkin-Teller model on the Bethe lattice, restricting ourselves to the case of ferromagneticinteractions. The isotropic Ashkin-Teller model and the anisotropic one are respectivelyinvestigated, and exact expressions for the free energy and the magnetization are obtained. It canbe found that each of the three varieties of phase diagrams, for the anisotropic Ashkin-Tellermodel, consists of four phases, i.e., the fully disordered paramagnetic phase Para, the fullyordered ferromagnetic phase Ferro, and two partially ordered ferromagnetic phases 【 σ 】 and 【 σs】, while the phase diagram, for the isotropic Ashkin-Teller model, contains three phases, i.e., thefully disordered paramagnetic phase Para, the fully ordered ferromagnetic phase Baxter Phase, andthe partially ordered ferromagnetic phase 【 σs 】.

Entanglement Entropy Signature of Quantum Phase Transitions in a Multiple Spin Interactions Model

Through the Jordan Wigner transformation, the entanglement entropy and ground state phase diagrams of exactly solvable spin model with alternating and multiple spin exchange interactions are investigated by means of Green＇s function theory. In the absence of four-spin interactions, the ground state presents plentiful quantum phases due to the multiple spin interactions and magnetic fields. It is shown that the two-site entanglement entropy is a good indicator of quantum phase transition （QPT）. In addition, the alternating interactions can destroy the magnetization plateau and wash out the spin-gap of low-lying excitations. However, in the presence of four-spin interactions, apart from the second order QPTs, the system manifests the first order OPT at the tricritical point and an additional new phase called ＂spin waves＂, which is due to the collapse of the continuous tower-like low-lying excitations modulated by the four-spin interactions for large three-spin couplings.

Magnetic Properties of Spin-1/2 Ising Superlattice

Using the effective-field theory we studied the magnetic properties of a spin-1/2 Ising supedattice, which consist of three different ferromagnet materials. The magnetic behavior of this superlattice is examined. The critical temperature and the compensation temprature of the system are studied as a function of the exchange interactions between the nearest-neiboring spins across the interface and in the intraface. Temperature dependence of magenetizations is also given.

Spin—Polarized States of Nuclear Matter

The equations of state of spin-polarized nuclear matter and pure neutron matter are studied in the framework of the Brueckner–Hartree–Fock theory including a three-body force. The energy per nucleon EA(δ) calculated in the full range of spin polarization for symmetric nuclear matter and pure neutron matter fulfills a parabolic law. In both the cases the spin-symmetry energy is calculated as a function of the baryonic density along with the related quantities such as the magnetic susceptibility and the Landau parameter G0. The main effect of the three-body force is to strongly reduce the degenerate Fermi gas magnetic susceptibility even more than the value with only two-body force. The equation of state is monotonically increasing with the density for all spin-aligned configurations studied here so that no any signature is found for a spontaneous transition to a ferromagnetic state.

Peculiar Quantum Phase Transitions and Hidden Supersymmetry in a Lipkin-Meshkov-Glick Model

In this paper we theoretically report an unconventional quantum phase transition of a simple Lipkin- Meshkow-Glick model： an interacting collective spin system without external magnetic field. It is shown that this model with integer-spin can exhibit a flrst-order quantum phase transition between different disordered phases, and more intriguingly, possesses a hidden supersymmetry at the critical point. However, for half-integer spin we predict another flrst-order quantum phase transition between two different long-range-ordered phases with a vanishing energy gap, which is induced by the destructive topological quantum interference between the intanton and anti-instanton tunneling paths and accompanies spontaneously breaking of supersymmetry at the same critical point. We also show that, when the total spin-value varies from half-integer to integer this model can exhibit an abrupt variation of Berry phase from π to zero.

Physical Problems of Quantum Calculation： A Novel Approach

The manuscript deals with the possibility of application of collective behavior of quantum particles to realize the quantum calculation procedure. The above collective behavior is likely resulted from interelectron correlations, characteristic for strongly correlated systems containing atoms with unoccupied 3d-, 4f- and 5f- shells. Among such systems can be the heterospin systems, complexes of paramagnetic ions of transition metals with organic radicals, because for such objects, spin-spin interaction between unpaired electron spins of different paramagnetic centers is typical. To apply the aforementioned possibility for the organization of real quantum calculations, it is necessary to synthesize such paramagnetic molecules （paramagnetic clusters）, where the entangled states will be realized naturally by self-organization of atoms incorporated in these molecules, i.e., without additional external effect of q-bits on the system. The specified self-organization may be due to intramolecular processes and, in particular, intramolecular rearrangement called valence tautomerism, which leads to heterogeneous magnetic states, i.e., to phase layering in paramagnetic cluster owing to interelectron correlations. The states realized during the phase layering can be used for coding the digits. Since such states correspond to specific structures of para-magnetic molecule, they can exist as much as long under certain conditions. In turn, it means that the account of the interelectron correlations, which take place in strongly correlated compounds, allows （at least, in principle） one to create elementary quantum bit of the information capable of modeling the elementary logical operations. Creation of a network of such quantum bits combined in a certain sequence should be considered as a practical step on a way to experimental realization of the idea of quantum computer creation. The group consisting of three quantum points can make the basis of quantum computer. In such a gate, quantum points can be connected via the interaction modeled by spin-spin interaction, characteristic for ABX system in NMR spectroscopy. The tunnel effect, which can be easily realized and controlled, can act as an indicator of bonding in such a block. The calculation procedure can be organized assuming that the initial state of the group corresponds to 1. Infringement of such a state indicates to zero （or, on the contrary）. Thus, the calculation in the binary system becomes organized. The creation of a network on the basis of combination of such processors in certain sequence should be considered as a practical step on a way to experimental realization of the idea of the quantum computer creation.

自旋-Peierls化合物GeCuO3电子结构的第一性原理研究

采用平面波赝势方法对自旋-Peierls化合物GeCuO3的电子结构进行了第一性原理研究.计算结果表明:Cu2+的3d轨道自由度被冻结,未配对电子填充dx2-y2轨道.自旋向上和向下的dx2-y2轨道间的交换劈裂导致了体系的绝缘性.费米能级附近的Cu3d态与O(2)2p态存在很强的杂化作用,GeCuO3属于共价绝缘体.这种强共价性使得Cu2+的自旋磁矩偏离理想值,并且使得体系在Cu—O(2)—Cu键角接近90°情况下形成稳定的一维反铁磁(AFM)有序.计算得到GeCuO3不同自旋态的总能量,采用Noodleman的对称性破缺方法拟合出的自旋交换耦合常数表明GeCuO3沿c轴的一维AFM作用非常强,这是导致该体系在低温发生自旋-Peierls相变的根本原因.

No Spin-Localization Phase Transition in the Spin-Boson Model without Local Field

We explore the spin-boson model in a special case, i.e., with zero local field. In contrast to previous studies, we find no possibility for quantum phase transition （QPT） happening between the localized and delocalized phases, and the behavior of the model can be fully characterized by the even or odd parity as well as the parity breaking, instead of the QPT, owned by the ground state of the system. The parity breaking mentioned in our case is completely different from the spontaneously broken symmetry relevant to the conventionally defined QPT in previous studies. Our analytical treatment about the eigensolution of the ground state of the model presents for the first time a rigorous proof of no- degeneracy for the ground state of the model, which is independent of the bath type, the degrees of freedom of the bath and the calculation precision. We argue that the QPT mentioned previously appears due to incorrect employment of the ground state of the model and/or unreasonable treatment of the infrared divergence existing in the spectral functions for Ohmic and sub-Ohmic dissipations.

Probing Berezinskii-Kosterlitz-Thouless Phase Transition of Spin-Half XXZ Chain by Quantum Fisher Information

The Berezinskii-Kosterlitz-Thouless phase transition of spin-1/2 XXZ chain is reinvestigated by the quantum Fisher information.Quantum Fisher informations of the whole N sites and the partial N/3 sites display remarkably similar behaviors near the critical point.The critical exponent of quantum Fisher information is obtained as β=0.47,which is consistent with the results obtained by the concurrence and quantum discord.

Quantum Correlations in Infinite XY Spin-1/2 Chains and Quantum Phase Transition

We examine the ability of quantum discord (QD) and entanglements (concurrence, EoF and negativity) to detect the critical points associated to quantum phase transitions (QPTs) for XY models, i.e., the isotropic XY model with three-spin interactions at zero temperature, and the anisotropic XY model in a transverse magnetic field h at finite temperatures. For the case of zero temperature, we found that both entanglements and QD can spotlight the critical points of QPTs for these two models. Moreover, QD versus distance M exhibits the long-range behavior of quantum correlation for the anisotropic XY model, while entanglement is short-ranged. For the case of finite temperatures, we found that negativity has the same behaviors with concurrence at or near transition points. Moreover, QD for the anisotropic XY model can increase with temperature even in the absence of a magnetic field.

Unveiling a critical stripy state in the triangular-lattice SU(4)spin-orbital model

The simplest spin-orbital model can host a nematic spin-orbital liquid state on the triangular lattice.We provide clear evidence that the ground state of the SU(4)Kugel-Khomskii model on the triangular lattice can be well described by a‘‘single”Gutzwiller projected wave function with an emergent parton Fermi surface,despite it exhibits strong finite-size effect in quasi-one-dimensional cylinders.The finite-size effect can be resolved by the fact that the parton Fermi surface consists of open orbits in the reciprocal space.Thereby,a stripy liquid state is expected in the two-dimensional limit,which preserves the SU(4)symmetry while breaks the translational symmetry by doubling the unit cell along one of the lattice vector directions.It is indicative that these stripes are critical and the central charge is c=3,in agreement with the SU(4)1Wess-Zumino-Witten conformal field theory.All these results are consistent with the Lieb-Schultz-Mattis-Oshikawa-Hastings theorem.