轨道诱导Peierls相变材料MgTi_2O_4的电子自旋共振研究

用变温电子自旋共振手段(Electron Spin Resonance,ESR),对轨道诱导Peierls相变MgTi_2O_4体系进行了研究.研究发现,轨道诱导Peierls相变所伴随的自旋二聚相变对ESR谱产生了影响.在相变温度以上,MgTi_2O_4的磁性为顺磁行为.而在相变温度以下,ESR谱显示MgTi_2O_4的磁性偏离了顺磁行为.对ESR谱线的参数拟合结果显示,MgTi_2O_4在发生轨道诱导Peierls相变时,自旋耦合作用逐渐增强.这说明:自旋耦合作用的增强很有可能是导致相变的一个重要的因素.     

轨道序和巡游性对尖晶石结构钒氧化物AV2O4 (A = Mn,Fe, Co) 物性影响的研究

在尖晶石钒基氧化物AV_2O_4中,因为自旋阻挫、轨道序、巡游性等独特的内禀属性间存在着相互合作和竞争,所以该体系常常表现出复杂而有趣的物理现象。通过对A位磁性离子的调控,我们详细研究该体系中不同物性的起源,比如磁相变与结构相变的不同起因,局域和巡游电子的交叉行为等等。我们以Mn_(1-x)Co_x V_2O_4和Fe_(1-x)Co_x V_2O_4体系为研究对象,通过变温X射线衍射、磁化率、比热和中子散射等测试手段,结合第一性原理计算,对其物性起源进行了详细的研究。我们发现:(i)在低Co2+离子掺杂时,体系受到局域V~(3+)离子以及A位Fe~(2+)离子的轨道序作用,往往会在磁有序附近伴随着结构相变的出现,以至于弱化体系中钒离子独立形成的四面体造成的几何阻挫。这也说明Co~(2+)离子低掺杂下,该体系有着强的自旋–晶格耦合;(ii)在高Co~(2+)离子掺杂时,由于巡游性的增强,轨道序的弱化,JAB交换相互作用增强,体系也表现出明显的磁各向同性。因此磁相变温度向更高的温度移动,而结构相变温度向低温移动甚至消失。     

氰根桥联的Cu(Ⅱ)-Fe(Ⅲ)配合物穆斯堡尔谱研究

采用穆谱斯堡尔谱学方法研究了普鲁士蓝类配合物Cu3 [Fe(CN) 6]2 ·11 6H2 O。当测量温度从室温降至低温时 ,发现该配合物表现为从顺磁相转变为铁磁相 ,并且指出顺磁到铁磁的相变是由于穆斯堡尔谱的双线作用 ,即三价铁离子和二价铜离子磁耦合起主导作用。二价铁离子不参与磁耦合 ,磁相变点TC=18.1K。     

斯格明子相关的螺旋磁有序体系的临界行为

介绍了与斯格明子相关的螺旋磁有序体系的临界行为.首先阐述了连续相变中的临界现象、临界指数、标度律、普适性等概念;随后介绍了磁相变体系中几种临界指数的获得方法,包括直流磁性迭代法、磁熵变法;进而,分析了几类与斯格明子相关的螺旋磁有序体系的临界行为.MnSi是典型的斯格明子材料,临界指数显示其磁性行为符合三重临界行为.MnSi的临界行为揭示:外磁场可以抑制这一体系在零场下的一级相变,使其转变为二级相变,从而在螺旋磁有序、锥形磁有序、顺磁相的三相交汇点形成三重临界点.斯格明子体系FeGe和Cu_2OSeO_3的临界行为符合三维海森伯相互作用,表明它们的磁性行为主要是由近邻的各向同性的自旋耦合作用所决定;而Fe_(1-x)Co_xSi和新发现的斯格明子体系Fe_(1.5-x)Co_xRh_(0.5)MoN的临界行为显示Co掺杂可以有效地调制其中的磁性耦合.对螺旋磁有序体系的临界行为研究表明,尽管这些体系都表现出类似的斯格明子态,但是它们的磁性耦合机制却大不相同,并且其耦合机制可以受到外界手段的调制.最后,根据普适性原理和标度方程,阐述了一种构建磁场诱导相变体系在临界温度附近H-T相图的方法.     

金属表面Rashba自旋轨道耦合作用研究进展

自旋轨道耦合是电子自旋与轨道相互作用的桥梁, 它提供了利用外电场来调控电子的轨道运动、进而调控电子自旋状态的可能. 固体材料中有很多有趣的物理现象, 例如磁晶各向异性、自旋霍尔效应、拓扑绝缘体等, 都与自旋轨道耦合密切相关. 在表面/界面体系中, 由于结构反演不对称导致的自旋轨道耦合称为Rashba自旋轨道耦合, 它最早在半导体材料中获得研究, 并因其强度可由栅电压灵活调控而备受关注, 成为电控磁性的重要物理基础之一. 继半导体材料后, 金属表面成为具有Rashba自旋轨道耦合作用的又一主流体系. 本文以Au(111), Bi(111), Gd(0001)等为例综述了磁性与非磁性金属表面Rashba自旋轨道耦合的研究进展, 讨论了表面电势梯度、原子序数、表面态波函数的对称性, 以及表面态中轨道杂化等因素对金属表面Rashba自旋轨道耦合强度的影响. 在磁性金属表面, 同时存在Rashba自旋轨道耦合作用与磁交换作用, 通过Rashba自旋轨道耦合可能实现电场对磁性的调控. 最后, 阐述了外加电场和表面吸附等方法对金属表面Rashba自旋轨道耦合的调控. 基于密度泛函理论的第一性原理计算和角分辨光电子能谱测量是金属表面Rashba自旋轨道耦合的两大主要研究方法, 本文综述了这两方面的研究结果, 对金属表面Rashba自旋轨道耦合进行了深入全面的总结和分析.     

多层结构双自旋过滤隧道结中的电子输运特性

在转移矩阵方法及Mireles和Kirczenow的量子相干输运理论的基础上,研究了正常金属层/磁性半导体层/非磁绝缘层/磁性半导体层/正常金属层型双自旋过滤隧道结中Rashba自旋轨道耦合效应和自旋过滤效应对自旋相关输运的影响.讨论了隧穿磁电阻（TMR）、隧穿电导与各材料层厚度、Rashba自旋轨道耦合强度以及两磁性半导体中磁矩的相对夹角θ之间的关系.研究表明：含磁性半导体层的双自旋过滤隧道结由于磁性半导体层的自旋过滤效应和Rashba自旋轨道耦合作用可获得极大的TMR值.另外TMR和隧穿电导随着Rashba自旋轨道耦合强度的变化而振荡,振荡周期随Rashba自旋轨道耦合强度的增大逐渐减小. 关键词： 双自旋过滤隧道结 Rashba自旋轨道耦合 隧穿磁电阻 隧穿电导     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

La0.7-x(Sm,Er,Dy)xSr0.3MnO3体系的相分离和渗流相变

通过M-T曲线、M—H曲线、ESR曲线研究了La0.7-x(Sm,Er,Dy)xSr0．3MnO3体系的磁相变．从ESR信号显示,对Er和Dy掺杂在居里温度以上,顺磁相中分离出铁磁团簇,随温度降低呈现渗流相变．稀土锰氧化物的居里温度、宏观磁性和微观磁性不仅取决于A位稀土离子掺杂引起晶格的畸变,而且取决于A位离子掺杂引起的额外磁矩．     

中子散射技术在功能性材料中的应用

中子散射技术能够有效探测物质微观尺度的特性,尤其是在磁性探测方面具有独特的优势,因而在科学研究方面应用越来越广泛。文章结合作者的研究领域,介绍了中子散射技术在研究一些功能性材料中的应用及成果。包括铁电材料BiFeO3中自旋,声子的激发谱,热电材料PeTe中声子特殊的动态行为,以及磁性材料Mn1-xCoxV2O4中自旋波的特性,并分析了材料中自旋、电子、晶格、轨道的相互耦合作用。     

Nd0.7Sr0.3MnO3顺磁-铁磁转变的电子自旋共振谱

用电子自旋共振(ESR)在100～300K温区对Nd0.7Sr0.3MnO3的顺磁-铁磁相变进行了细致的观测.所得ESR信号显示,在居里温度附近温区,顺磁相中存在着铁磁团簇,铁磁相中也存在顺磁成分.巨磁电阻峰值出现在ESR谱形状变化急剧的温区.ESR线宽在顺磁态随温度升高而线性增大,在居里温度附近呈现了极小值.     

Electron spin resonance study of a CuIr2S4 single crystal

A spinel sulphide CuIr₂S₄ single crystal, which exhibits an orbitally induced Peierls phase transition at ～230?K, is investigated by electron spin resonance (ESR) spectroscopy. The phase transition is clearly manifested on the ESR spectra. It is suggested that the ESR signals are produced by a few non-dimerized Ir⁴⁺ ions. Moreover, an extra ESR spectrum appears at low temperature in addition to the paramagnetic ESR signals of Ir⁴⁺ ions, which is suggested to be caused by the Jahn–Teller effect of the non-dimerized Ir⁴⁺ ions. From the ESR results, it is found that the Jahn–Teller splitting energy ΔE _JT is much smaller than the spin-dimerization gap.     

Comparative ESR study of three (BEDT-TTF): ReO4 salts: An organic superconductor,a peierls metal and a semiconductor

We present electron spin resonance results on three different conducting phases of the organic conductor based on the planar molecule BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) and the anion ReO₄ in different stoichiometries. Both the paramagnetic susceptibility χ as well as the ESR-linewidth ΔH_pp may be used to classify the magnetic behaviour in terms of known behaviour of other organic metals. They indicate that the 2:1 phase, which under pressure becomes superconducting, has an anion-ordering assisted metal-insulator transition near 80 K and that the 3:2 phase has a Peierls transition at 88 K, below which two ESR lines are resolved. A third phase, which includes solvent in the crystal lattice, shows no evidence of a transition.     

Ti：Al2O3晶体的电子自旋共振谱研究

Ｔｉ：Ａｌ２Ｏ３晶体中的顺磁中心Ｔｉ＾３＋由于强烈的晶格自旋耦合而使其电子自旋共振吸收只有在液拟温度附近才能看到，本实验在液氮温度附近记录到由许多强的的吸收峰迭加于Ｔｉ＾３＋宽吸收线所组成的电子自旋共振谱，这些吸吸收峰被认为是Ａｌ２Ｏ３基质中的Ｆｅ＾３＋，Ｍｎ＾２＋，Ｃｒ＾３＋，Ｍｏ＾３＋，Ｆｅ＾２＋，Ｃｏ＾２＋等杂质的共振吸收及Ｔｉ＾２＋３Ａ２ｇ基态的双量子跃迁造成的。     

Spin singlet formation in MgTi2O4: evidence of a helical dimerization pattern

The transition-metal spinel MgTi2O4 undergoes a metal-insulator (M-I) transition on cooling below T(M-I)=260 K. A sharp reduction of the magnetic susceptibility below T(M-I) suggests the onset of a magnetic singlet state. Using high-resolution synchrotron and neutron powder diffraction, we have solved the low-temperature crystal structure of MgTi2O4, which is found to contain dimers with short Ti-Ti distances (the locations of the spin singlets) alternating with long bonds to form helices. Band structure calculations based on hybrid exchange density functional theory show that, at low temperatures, MgTi2O4 is an orbitally ordered band insulator.     

The micromagnetic study of the vanadate spinel MnV2O4

MnV₂O₄ exhibits a paramagnetic to ferrimagnetic transition at 57 K and shows significant magnetic hysteresis below 55 K. By performing detailed powder X-ray diffraction at the same temperature during cooling and warming sequences, it is found that the magnetic hysteresis observed here is owing to strains induced by the structural phase separation. The intensity of the electron spin resonance spectra shows unusual temperature dependence, which might be related to the phase separation induced by the structural transition. By performing a mean field analysis, we obtained the exchange energies among the different magnetic moments and qualitatively understood the micromagnetic properties.     

An electron spin resonance study of Eu doping effect in La_(4/3)Sr_(5/3)Mn_2O_7 single crystal

The effect of Eu3+ion doping in the La sites of single-crystal La4/3Sr5/3Mn2O7was investigated. Electron spin resonance(ESR) was applied to La4/3Sr5/3Mn2O7and(La0.8Eu0.2)4/3Sr5/3Mn2O7single crystals. A phase separation and phase transitions were observed from the ESR spectra data. Between 350 K and 300 K, both paramagnetic resonance(PMR)and anisotropic ferromagnetic resonance(FMR) lines were observed in the ab plane and the c axis direction, suggesting a coexistence of the paramagnetic(PM) phase and the ferromagnetic(FM) phase. The magnetization measurement reveals a spin-glass-like behavior in single-crystal(La0.8Eu0.2)4/3Sr5/3Mn2O7below the temperature of spin freezing Tf(～ 29.5 K).     

Peierls and spin-Peierls phase transitions in structurally unstable quasi-one-dimensional solids

The Peierls and spin-Peierls phase transitions are studied in solids in which a structural instability is already present. It is found that the presence of this intrinsic mode can increase considerably the critical temperature. For small values of the critical temperature, the transition is of the BCS-type, like the Peierls (or spin-Peierls) phase transition, but with an effective electron (or spin)-phonon coupling constant renormalized by the anharmonicity and by the instability of the phonon. Numerical results are also presented for larger critical temperatures. Then the BCS behaviour is no longer observed.     

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.     

Nuclear spin resonance of (129)Xe doped with O(2)

Spin-lattice relaxation of (129)Xe nuclei in solid natural xenon has been investigated in detail over a large range of paramagnetic O(2) impurity concentrations. Direct measurements of the ground state magnetic properties of the O(2) are difficult because the ESR (electron spin resonance) lines of O(2) are rather unstructured, but NMR measurements in the liquid helium temperature region (1.4-4 K) are very sensitive to the effective magnetic moments associated with the spin 1 Zeeman levels of the O(2) molecules and to the O(2) magnetic relaxation. From these measurements, the value of the D[Sz(2)-(1/3)S(2)] spin-Hamiltonian term of the triplet spin ground state of O(2) can be determined. The temperature and magnetic field dependence of the measured paramagnetic O(2)-induced excess line width of the (129)Xe NMR signal agree well with the theoretical model with the spin-Hamiltonian D=0.19 meV (2.3 K), and with the reasonable assumption that the E[S(x)(2)-S(y)(2)] spin-Hamiltonian term is close to 0 meV. An anomalous temperature dependence between 1.4 K and 4.2K of the (129)Xe spin-lattice relaxation rate, T(1n)(-1)(T), is also accounted for by our model. Using an independent determination of the true O(2) concentration in the Xe-O(2) solid, the effective spin lattice relaxation time (which will be seen to be transition dependent) of the O(2) at 2.3 K and 0.96 T is determined to be approximately 1.4 x 10(-8)s. The experimental results, taken together with the relaxation model, suggest routes for bringing highly spin-polarized (129)Xe from the low temperature condensed phase to higher temperatures without excessive depolarization.