自旋为1/2的双层平方晶格阻挫模型的基态相变

基于线性自旋波理论,研究了二维各向异性双层平方晶格阻挫模型的基态性质.探讨了各向异性和自旋间交换作用对基态相图及系统可能发生相变的影响.结果显示:对于各向同性,当层间耦合相互作用J_c≥0.21或J_c≤-0.175时,Nèel态(NS)和Collinear态(CS)能共存;对于各向异性,NS和CS也可以共存.对弱的各向异性,NS更稳定;对于强的各向异性,系统将发生由NS向CS转换的一阶相变.     

磁性物质交换Hamiltonian中两项的竞争

对于一个N电子体系, 正确的交换Hamilton应该由两项组成，为H_ex=-2A1ii·s_j-2A₂ii·sj，而不是以往的铁磁学理论使用的H_ex=-2Aii ·s_j （其中A为A₁与A₂的代数和， A₁>0, A₂<0）, 以往的理论使用了一个不合理的交换Hamiltonian量.-2A₁ii·s_j与-2A₂ii< /sub>·s_j在数学上是同类项，但是在物理上不是 同类项，它们有不同的本征态和本征值.根据量子力学中的态叠加原理，这个电子系统的本 征态矢为X〉=1A²₁+A²₂(A₁ 1〉+A₂‖2〉),其中Dirac符号1〉表示系统所有电子 的自旋平行排列时的态（简称平行自旋态）矢量，2〉表示系统所有电子或最近邻电子的自 旋反平行排列时的态（简称反平行自旋态）矢量，H_ex的本征值（即系统的 交换能） 为E=-Nz（A₁-A₂）-2NzA₂²A₁ +A₂=-Nz（A₂-A₁）-2NzA²₁A< sub>1+A₂,其中z为最近邻电子数.当A₂=0时，X〉=1〉，E =-A₁, 系统具有Wei ss 铁磁性；当A₁ =0 时，X〉=2〉，E =-A₂，系统具有Neel 反铁磁性；当A₁ =A₂（即A=0）时，X〉=12 (1〉+2〉)，E=-A₁，系统处于自旋玻璃（spin glass）态；当A₁>A ₂时，X〉=1A²₁+A²₂［(A₁-A₂)1〉+A₂(1〉+2〉)］,平行自旋态与自旋 玻璃态共存；当A₁2时，X〉=1A²₁+A2₂［(A₂-A₁)2〉+A₁( 1〉+2〉)］，反平行自旋态与自旋玻 璃态共存.与原来理论中的Weiss铁磁态或Neel反铁磁态相比，平行自旋态与自旋玻璃态共存 或反平行自旋态与自旋玻璃态共存使系统的交换能降低.自旋玻璃态中电子自旋之间取向的 随机性或无序性是由交换Hamiltonian中-2A₁iis_j与-2A₂ii·s_j之间的竞争引起的，不是热运 动引起的. 关键词： 交换哈密顿量 铁磁态 反铁磁态 自旋玻璃态     

化合物HoMn6Sn6的磁晶各向异性及自旋重取向相变研究

采用交换相互作用的分子场理论模型对金属间化合物HoMn₆Sn₆的自旋重取向相变进行了研究. 从理论上计算了HoMn₆Sn₆的易磁化方向以及Ho和Mn离子磁矩与c轴夹角随温度的变化. 基于单离子模型计算了Ho离子的一阶和二阶磁晶各向异性常数K_1R和K_2R随温度的变化. 研究表明，为了很好描述该化合物的自旋重取向相变，必须考虑Ho离子的四阶晶场项及相应的二阶磁晶各向异性常数K_2R，K_2R与K_1R和Mn离子磁晶各向异性常数K_1t之间的相互竞争是导致HoMn₆Sn₆自旋重取向相变的重要因素. 关键词： 稀土-过渡族金属间化合物 自旋重取向 磁晶各向异性     

Mn位W掺杂对La0.3Ca0.7MnO3体系磁结构的影响

通过对La_0.3Ca_0.7Mn_1-xW_xO₃(x=0.00，0.04，0.08，0.12，0.15)多晶样品M-T曲线、M-H曲线及ESR谱的测量，研究了Mn位W掺杂对电荷有序体系La_0.3Ca_0.7MnO₃磁结构的影响.结果表明，当掺杂量为0.00≤x≤0.08时，体系存在电荷有序(CO)相，AFM/CO态共存于相变温度以下，电荷有序温度T_CO随着W掺杂量的增加而增加；x＝0.04时，样品在低温下为FM相与AFM/CO相共存，在CO相建立前、后均有FM从PM中分离出来；当x≥0.12时，CO态融化，在极低温度下存在顺磁-铁磁(PM-FM)相变. 关键词： 磁结构 电荷有序 融化 Mn位掺杂     

用Monte-Carlo方法计算L12结构的有序度

本文应用Monte-Carlo统计物理计算方法,计算了L1₂结构有序度随温度的变化。计算中考虑了最近邻有序相互作用J₁和次近邻有序相互作用J₂=0,-0.3J₁,0.3J₁三种情况。计算结果显示存在有序无序一级相变,临界温度T_c与准化学方法等理论结果相符,同时发现次近邻有序相互作用J₂敏感地影响到有序无序转变温度T_c,为间隙原子对L1₂结构金属间化合物塑性的作用提供了一种可能的解释。 关键词：     

Heusler合金NiCoMnSn中的磁场驱动马氏体相变、超自旋玻璃和交换偏置

合成了一系列Ni_50-xCo_xMn₃₉Sn₁₁ (8≤x≤10) 样品, 并对它们的结构和磁性进行了研究. 发现随Co含量的增加, 样品的饱和磁化强度逐渐增强, 并在Ni₄₂Co₈Mn₃₉Sn₁₁中实现了磁场诱发马氏体相变. 另外, 在Co大于8.0的成分中探测到了超自旋玻璃, 并且观察到交换偏置现象. 证实了超自旋玻璃的马氏体相和铁磁奥氏体母相共存, 这也是产生交换偏置的原因.我们猜测超自旋玻璃的形成可能是来源于Mn-Mn团簇的存在, 这和之前报道的Mn₂Ni_1.6Sn_0.4 的结果相一致^[1]. 关键词： NiCoMnSn Heusler合金 超自旋玻璃 交换偏置     

金属间化合物DyMn2Ge2的自发磁相变和场诱导的磁相变

在10—800K的温度范围内用X射线衍射方法测量了DyMn₂Ge₂化合物的晶格常数与温度的变化关系,观察到高温时DyMn₂Ge₂由顺磁状态到反铁磁状态的自发磁相变伴随着晶格常数a的负的磁弹性异常现象.在4.2K—200K的温度范围内测量了DyMn₂Ge₂的交流磁化率.在交换相互作用的分子场模型近似下,从理论上分析讨论了DyMn₂Ge₂的低温自发磁相变和场诱导的磁相变.计算了DyMn₂Ge₂单晶的磁化强度与温度的变化关系以及不同温度下外磁场沿晶轴c方向时的磁化曲线.理论分析和计算结果表明,温度低于33K时在DyMn₂Ge₂中观察到的场诱导的一级磁相变为由亚铁磁状态(Fi)到中间态(IS)相变. 关键词： 稀土-过渡族金属间化合物 磁结构 磁相变     

Co掺杂对铁磁金属La0.8Sr0.2MnO3磁电阻影响机理

通过对La_0.8Sr_0.2Mn_1-yCo_yO₃(y≤02)饱和磁矩和输运的测量,研究了Co对La_0.8Sr_0.2MnO₃的磁电阻影响机制.结果表明,在La_0.8Sr_0.2Mn_1-yCo_yO₃（y≤02）中Co³⁺离子是低自旋态.由于Mn³⁺—O—Co³⁺—O—Mn³⁺类型的磁交换与Mn³⁺-Mn⁴⁺离子间双交换作用相比较弱,Curie温度T_C附近的磁电阻随着Co掺杂量的增加而降低.与此相反,由于Co²⁺离子与e_g巡游电子的反铁磁交换耦合作用,低温区间的磁电阻随着Co掺杂量的增加而升高. 关键词： 低自旋 磁电阻 磁交换作用     

Nd-Fe-B/FeCo多层纳米复合膜的结构和磁性

制备了Nd₂₈Fe₆₆B₆/Fe₅₀Co₅₀多层纳米复合磁性薄膜，对溅射态和650℃退火处理15 min试样的相成分分析和微结构的观察显示，溅射态薄膜呈非晶态，经650℃退火处理15 min后，薄膜主要相成分为硬磁性Nd₂Fe₁₄B相和软磁性相FeCo(110)相.Nd₂Fe₁₄B相呈柱状，其易磁化c轴垂直于膜面，尺寸约10 nm.在硬磁性相和软磁性相之间存在少量富Nd相和非晶态，富Nd相大小约7 nm.磁性测量和分析表明，1)该系列薄膜退火态具有垂直于膜面的磁晶各向异性.2)对于固定厚度(10 nm)层Nd-Fe-B和不同厚度(t_FeCo=1—100 nm)层FeCo多层纳米复合膜,剩磁随软磁相FeCo 厚度的增加快速增加，而矫顽力则减小.当t_FeCo=5 nm时，最大磁能积达到200 kJ/m³. 3)硬磁相Nd-Fe-B层和软磁相FeCo层之间交换耦合导致剩磁和磁能积增强. 关键词： Nd-Fe-B/FeCo多层纳米复合膜 交换耦合 磁各向异性     

K2NiF4型Sr2CrO4的磁电性质研究

以SrO和CrO₂为原料, 在高温高压的条件下直接反应生成纯相的K₂NiF₄结构的Sr₂CrO₄多晶样品. 结构用粉末X射线衍射及GSAS精修表征. 磁化率测试显示样品存在一个弱的反铁磁相变, 奈尔温度为T_N=95 K. 在奈尔温度以上, 磁化率随温度的变化遵循居里-外斯定律. 对样品进行了电阻测试, 结果显示了样品的绝缘特性.     

Possible phase transition of anisotropic frustrated Heisenberg model at finite temperature

The frustrated spin-1/2 J_1a–J_1b–J₂ antiferromagnet with anisotropy on the two-dimensional square lattice was investigated, where the parameters J_1aand J_1b represent the nearest neighbor exchanges and along the x and y directions, respectively. J₂ represents the next-nearest neighbor exchange. The anisotropy includes the spatial and exchange anisotropies. Using the double-time Green’s function method, the effects of the interplay of exchanges and anisotropy on the possible phase transition of the Néel state and stripe state were discussed. Our results indicated that, in the case of anisotropic parameter 0≤η<1, the Néel and stripe states can exist and have the same critical temperature as long as J₂ = J_1b/2. Under such parameters, a first-order phase transformation between the Néel and stripe states can occur below the critical point. For J₂ ≠J_1b/2, our results indicate that the Néel and stripe states can also exist, while their critical temperatures differ. When J₂>J_1b/2, a first-order phase transformation between the two states may also occur. However, for J₂<J_1b/2, the Néel state is always more stable than the stripe state.     

Electronic and magnetic properties of single-layer and double-layer VX_2(X=Cl,Br) under biaxial stress

First-principles calculations and Monte Carlo simulations reveal that single-layer and double-layer VX_2(X = Cl, Br)can be tuned from antiferromagnetic(AFM) semiconductors to ferromagnetic(FM) state when biaxial tensile stress is applied. Their ground states are all T phase. The biaxial tensile stress at the phase transition point of the double-layer VX_2 is larger than that of the single-layer VX_2. The direct band gaps can be also manipulated by biaxial tensile stress as they increases with increasing tensile stress to a critical point and then decreases. The Néel temperature(TN) of double-layer VX_2 are higher than that of single-layer. As the stress increases, the TNof all materials tend to increase. The magnetic moment increases with the increase of biaxial tensile stress, and which become insensitive to stress after the phase transition points.Our research provides a method to control the electronic and magnetic properties of VX_2 by stress, and the single-layer and double-layer VX_2 may have potential applications in nano spintronic devices.     

Deconfined criticality: generic first-order transition in the SU(2) symmetry case

Monte Carlo simulations of the SU(2)-symmetric deconfined critical point action reveal strong violations of scale invariance for the deconfinement transition. We find compelling evidence that the generic runaway renormalization flow of the gauge coupling is to a weak first-order transition, similar to the case of U(1) x U(1) symmetry. Our results imply that recent numeric studies of the Nèel antiferromagnet to valence bond solid quantum phase transition in SU(2)-symmetric models were not accurate enough in determining the nature of the transition.     

铁基超导体的量子临界行为

铁基超导体呈现丰富的电子相图, 各种有序态相互交叠. 本文主要介绍利用核磁共振手段在空穴型和电子型掺杂的BaFe₂As₂以及LaFeAsO_1-xF_x这三种具有代表性的铁基超导体中探测到的反铁磁序与超导序的微观共存、量子临界点和量子临界行为. 实验发现, 无论在空穴型还是电子型掺杂的铁基超导体中, 反铁磁相变温度都随着掺杂被抑制, 并最终在某个掺杂量降到零温而形成量子临界点. 在反铁磁转变温度之上存在结构相变, 其转变温度也随着掺杂而降低. 核磁共振谱证实结构相变也形成一个量子临界点. 本文介绍核磁共振及输运测量揭示的这两种量子临界点附近存在的量子临界行为, 共存态下奇异的超导性质等.     

Emergent O(4) symmetry at the phase transition from plaquette-singlet to antiferromagnetic order in quasi-two-dimensional quantum magnets

Recent experiments [Guo et al., Phys. Rev. Lett. 124 206602(2020)] on thermodynamic properties of the frustrated layered quantum magnet SrCu_2(BO_3)_2-the Shastry–Sutherland material-have provided strong evidence for a lowtemperature phase transition between plaquette-singlet and antiferromagnetic order as a function of pressure. Further motivated by the recently discovered unusual first-order quantum phase transition with an apparent emergent O(4) symmetry of the antiferromagnetic and plaquette-singlet order parameters in a two-dimensional "checkerboard J-Q" quantum spin model[Zhao et al., Nat. Phys. 15 678(2019)], we here study the same model in the presence of weak inter-layer couplings. Our focus is on the evolution of the emergent symmetry as the system crosses over from two to three dimensions and the phase transition extends from strictly zero temperature in two dimensions up to finite temperature as expected in SrCu_2(BO_3)_2.Using quantum Monte Carlo simulations, we map out the phase boundaries of the plaquette-singlet and antiferromagnetic phases, with particular focus on the triple point where these two ordered phases meet the paramagnetic phase for given strength of the inter-layer coupling. All transitions are first-order in the neighborhood of the triple point. We show that the emergent O(4) symmetry of the coexistence state breaks down clearly when the interlayer coupling becomes sufficiently large, but for a weak coupling, of the magnitude expected experimentally, the enlarged symmetry can still be observed at the triple point up to significant length scales. Thus, it is likely that the plaquette-singlet to antiferromagnetic transition in SrCu_2(BO_3)_2 exhibits remnants of emergent O(4) symmetry, which should be observable due to additional weakly gapped Goldstone modes.     

Multipartite Entanglement of a Tetrahedron Lattice

Three-dimensional Heisenberg model in the form of a tetrahedron lattice is investigated. The concurrence and multipartite entanglement are calculated through 2-concurrence C and 4-concurrence C4. The concurrence C and multipartite entanglement G4 depend on different coupling strengths Ji and are decreased when the temperature T is increased. For a symmetric tetrahedron lattice, the concurrence C is symmetric about J1 when J~ is negative while the multipartite entanglement G4 is symmetric about J1 when J2 〈 2. For a regular tetrahedron lattice, the concurrence G of ground state is 1/3 for ferromagnetic case while G = 0 for antiferromagnetic ca.se. However, there is no multipartitc entanglement since C4=0 in a regular tetrahedron lattice. The external magnetic field 13 can increase the maximum value of the concurrence GB and induce two or three peaks in Cn. There is a peak in the multipartite entanglement G4 B when G4B is varied as a function of the temperature T. This peak is mainly induced by the magnetic field B.     

FeSe基超导单晶与薄膜研究新进展:自旋向列序、电子相分离及高临界参数

FeSe基超导体的超导临界温度可大范围调控,物理现象丰富,是非常规超导机理研究的热点.由于较高的超导临界参数及易于加工等特点,FeSe基超导体在超导应用开发方面也日益受到重视.大尺寸高质量的单晶和薄膜形态的FeSe基超导材料,对于相关基础科学研究和应用开发都极为重要.作者近年来先后开发和发明了水热离子交换（ion-exchange）、离子脱插（ion-deintercalation）、基底辅助水热外延生长方法,成功解决了二元FeSe和插层（Li,Fe）OHFeSe超导体高质量单晶和薄膜的生长和物性调控难题.进而在相关物理问题的研究中取得新进展,包括发现二元FeSe中自旋向列序与超导电性密切相关,观测到（Li,Fe）OHFeSe中的电子相分离现象.此外,（Li,Fe）OHFeSe超导薄膜呈现很高的超导临界电流密度和上临界磁场,其应用前景值得关注.     

Ba1-xKxFe2As2单晶(Tc=38.5K)磁通钉扎力与钉扎机理研究

铁基超导体是在2008年由Hosono发现的一种新型超导材料, 由于其具有上临界场高、各向异性小、临界电流密度大等优点, 在世界范围内引起了广泛关注. 以Ba_1-xK_xFe₂As₂为代表的FeAs-122系超导体具有结构简单、合成温度低、单晶容易制备等优点, 是物理学家和材料学家关注的焦点. 本工作在获得最优化掺杂的Ba_1-xK_xFe₂As₂单晶(T_c = 38.5 K)基础上, 通过分析其在不同磁场条件下电阻温度变化关系、不同温度条件下的磁滞回线等数据, 系统的研究了Ba_1-xK_xFe₂As₂单晶磁通钉扎力和磁通钉扎机理. 研究发现Ba_1-xK_xFe₂As₂超导体具有非常高的磁通钉扎势, 其中9 T的外场条件下, 其在H//c轴和H//ab面的钉扎势分别为5800 K和8100 K, 展示出良好的应用前景; 通过进一步分析发现, 其磁通钉扎机理应是由于晶格内部的小尺寸缺陷引起的电子平均自由程变化而导致的δl钉扎.     

掺杂Y2O3和BaCeO3提高MOD-YBCO超导性能的研究

本文通过在前驱液中添加过量钇盐和铈的有机盐,采用三氟乙酸盐-金属有机沉积法(TFA-MOD) 在铝酸镧单晶基体上制备了含有纳米氧化钇和纳米铈酸钡的YBCO薄膜. 与纯YBCO薄膜相比,掺杂Y₂O₃/BaCeO₃的YBCO膜的临界转变温度几乎保持不变,为91 K左右. 而掺杂Y₂O₃/BaCeO₃的YBCO膜的临界电流密度达到5.0 MA/cm² (77 K, 0T), 是纯YBCO膜临界电流密度的1.5倍.薄膜中的Y₂O₃和BaCeO₃可能在YBCO内部起到了 有效的钉扎磁通作用. 关键词： 钇钡铜氧薄膜 2O₃和纳米BaCeO₃')" href="#">纳米Y₂O₃和纳米BaCeO₃ 磁通钉扎 三氟乙酸盐-金属有机沉积     

Kinetic Phase Transition in A2 ＋ 2B2 → 2B2A Reaction System with Particle Diffusion

A lattice gas model is presented for the A2 ＋2B2 → 2B2A reaction system with particle diffusion in two dimensions. In the model, B2 dissociates in the random dimer-filling mechanism and A2 dissociates in the end-on dimer filling mechanism. A reactive window appears and the system exhibits a continuous phase transition from a reactive state to a ＂B ＋ vacancy＂ covered state with infinitely many absorbing states. When the diffusion of particle B is considered, there are only two absorbing states. It is found that the critical behavior of the continuous phase transition changes from the directed percolation （DP） class to the pair contact process with diffusion （PCPD） class.