5d过渡金属氧化物中的奇异量子物性研究

相比于3d和4d过渡金属元素, 5d过渡金属元素既具有很强的自旋轨道耦合相互作用, 同时它们的电子关联作用也不可忽略. 因而5d过渡金属氧化物体系具有许多奇异的量子特性. 这篇综述主要介绍我们在5d过渡金属氧化物中的一些理论进展. 首先介绍烧绿石结构铱氧化物(A₂Ir₂O₇, A=Y或稀土元素)中的Weyl拓扑半金属性. 我们确定出A₂Ir₂O₇这一类具有阻挫结构材料的磁基态, 并预言其是Weyl半金属; 其Weyl 点受到拓扑保护而稳定, 而且它的表面态在费米能级形成特别的费米弧. 其次预言尖晶石结构锇氧化物(AOs₂O₄, A=Ca, Sr)是具有奇异磁电响应的Axion绝缘体; 然后分析了电子关联、自旋轨道耦合对钙钛矿结构的锇氧化物(NaOsO₃)的影响, 并成功定出它的基态磁构型, 最终确定其为Slater绝缘体. 最后介绍了LiOsO₃中铁电金属性的成因.     

5d过渡金属氧化物—Weyl半金属奇异量子物性研究

绝缘体系统的拓扑量子行为是当前物理学领域的焦点。那么没有能隙的金属体系是否也有拓扑非平庸行为呢?答案是肯定的,5d过渡金属元素既有着大的自旋轨道耦合,又有着不可忽略的电子关联作用,由其构成的材料体系因而具有奇异的量子物性。本文先简介人们最近关注的几类5d材料,着重讨论烧绿石结构铱(Ir)氧化物(A2Ir2O7,其中A=Y或稀土元素)。我们通过第一性原理计算结合有效模型成功地预言了这一大类磁阻挫材料的基态磁构型,并预言A2Ir2O7是Weyl半金属。我们发现和拓扑绝缘体一样,作为一种新型拓扑量子态,Weyl半金属态也具有其拓扑性质:Weyl点是受拓扑保护稳定的;Weyl半金属有着受拓扑保护的表面态,即非闭合的费米面(Fermi arc);它对外场的响应也由其拓扑性质决定(只与Weyl点的位置有关,和能带的细节无关)。我们进一步预言尖晶石结构锇(Os)氧化物(AOs2O4,其中A=Ca或Sr)是有着特别磁电响应的Axion绝缘体。最后我们讨论了电子关联、自旋轨道耦合、磁序结构对钙态矿结构锇氧化物的影响,进一步确定NaOsO3是由磁序导致金属–绝缘体相变的Slater绝缘体。     

阻挫诱导的亚铁磁性Heisenberg系统中的量子相变

利用密度矩阵重整化群(DMRG)方法研究磁性阻挫对一种S=1/2准一维反铁磁自旋链但却具有亚铁磁性的Heisenberg系统基态的影响.计算了单个晶胞的基态能、自旋关联函数以及自旋能隙.研究表明这种Heisenberg自旋系统的基态随着阻挫α的增强将从磁有序相变化到自旋无序相,并且伴随着自旋能隙的出现,量子相变点为α≈0.412.同时线形链上格点间自旋长程关联值的计算结果表明在磁有序区间体系的磁有序性质随着α的增强而减弱,阻挫在0≤α< 关键词： 准一维反铁磁自旋链 亚铁磁性 密度矩阵重整化群 自旋能隙     

反对称Spin-1/2阻挫钻石链的基态和磁化行为研究

对含有次近邻节点自旋交换耦合的自旋-1/2伊辛-海森伯钻石链体系进行了研究,利用矩阵对角化和传递矩阵方法对基态磁相和宏观热力学量进行了严格求解,重点探讨了所有交换耦合均为反铁磁耦合时,体系节点伊辛自旋间次近邻相互作用的影响.研究结果表明次近邻节点伊辛自旋存在反铁磁耦合时会增强系统的阻挫效应,引入破坏平移对称性的经典亚铁磁相,使基态呈现出上上上下上上的自旋构型以及磁化曲线新颖的2/3磁化平台,丰富了体系的基态相图和宏观磁性行为.     

阻挫诱导的亚铁磁性Heisenberg系统中的量子相变

利用密度矩阵重整化群(DMRG)方法研究磁性阻挫对一种S=1/2准一维反铁磁自旋链但却具有亚铁磁性的Heisenberg系统基态的影响.计算了单个晶胞的基态能、自旋关联函数以及自旋能隙.研究表明这种Heisenberg自旋系统的基态随着阻挫α的增强将从磁有序相变化到自旋无序相，并且伴随着自旋能隙的出现，量子相变点为α≈0.412.同时线形链上格点间自旋长程关联值的计算结果表明在磁有序区间体系的磁有序性质随着α的增强而减弱，阻挫在0≤α<     

La_(0.4)Ca_(0.6)MnO_3中Mn-位Fe和Cr掺杂对磁性质的影响

研究了Fe和Cr掺杂对La0.4Ca0.6Mn O3中电荷有序反铁磁基态的调控作用.磁性质的测量结果表明,两种离子掺杂均能有效抑制原型样品中的长程电荷有序相,但是Fe离子掺杂样品均具有反铁磁的基态,而Cr掺杂样品中则出现了显著的铁磁性.结合电输运测量结果显示,Cr掺杂引起的铁磁态同时具有金属性,表明其中是电子双交换作用占主导.对比两种掺杂离子的电子结构发现,Cr离子空的e g电子轨道促进了电子双交换作用,而Fe掺杂则只是引入了不同的自旋交换作用,导致自旋无序.     

La$lt;sub$gt;0.4$lt;/sub$gt;Ca$lt;sub$gt;0.6$lt;/sub$gt;MnO$lt;sub$gt;3$lt;/sub$gt;中Mn-位Fe和Cr掺杂对磁性质的影响

研究了Fe和Cr掺杂对La_0.4Ca_0.6MnO₃ 中电荷有序反铁磁基态的调控作用. 磁性质的测量结果表明, 两种离子掺杂均能有效抑制原型样品中的长程电荷有序相, 但是Fe离子掺杂样品均具有反铁磁的基态, 而Cr掺杂样品中则出现了显著的铁磁性. 结合电输运测量结果显示, Cr掺杂引起的铁磁态同时具有金属性, 表明其中是电子双交换作用占主导. 对比两种掺杂离子的电子结构发现, Cr离子空的e_g电子轨道促进了电子双交换作用, 而Fe掺杂则只是引入了不同的自旋交换作用, 导致自旋无序. 关键词： 磁性氧化物 反铁磁     

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.     

Pb掺杂对Cd_2Ru_2O_7反常金属态的调控

具有烧绿石结构的Cd_2Ru_2O_7在形成长程反铁磁序的同时进入反常的金属态.采用高压高温方法制备了一系列Pb掺杂的Cd_(2-x)Pb_xRu_2O_7(0≤x≤2)多晶样品,并系统研究了其晶体结构和电阻率、磁化率、热电势等物理性质.尽管Pb_2Ru_2O_7是泡利顺磁金属,但少量Pb~(2+)掺杂的样品Cd1.8Pb0.2Ru2O7却呈现出明显的金属-绝缘体转变,与施加静水压和少量Ca~(2+)掺杂的效果类似.通过与类似的烧绿石Ru~(5+)氧化物进行对比,提出Cd_2Ru_2O_7中的Ru~(5+)-4d~3电子态恰好处于巡游到局域过渡的区域,少量Pb~(2+)掺杂造成的晶格无序增强了电子的局域性,使得形成反铁磁序的同时伴随出现了金属-绝缘体转变.这表明具有烧绿石结构的Ru~(5+)氧化物是研究巡游-局域电子转变的理想材料体系.     

自旋-1/2量子罗盘链的量子相与相变

基于矩阵乘积态表述的无限时间演化块算法,研究了具有x,y,z三个自旋方向的轨道自由度和轨道序竞争的量子罗盘自旋链模型.为了刻画该模型的量子相和相变,计算了基态能量、局域序参量、弦关联序参量、临界指数、冯诺依曼熵、有限纠缠标度和中心荷.结果表明:该量子基态相图由条纹反铁磁相、反铁磁相、单调奇数Haldane相和振荡奇数Haldane相构成.从条纹反铁磁相到反铁磁相,以及从单调奇数Haldane相到振荡奇数Haldane相发生了非连续相变;从振荡奇数Haldane相到条纹反铁磁相,以及从反铁磁相到单调奇数Haldane相发生了连续相变;连续相变线和非连续相变线的交点是多临界点.此外,连续相变点处的临界指数β=1/8和中心荷c=1/2表明连续相变的普适类属于Ising类.由此揭示了该模型量子基态相图的本性,对今后研究更高自旋以及更为复杂轨道序竞争的量子罗盘链模型的量子相与相变具有一定借鉴与参考意义.     

合金团簇(FeCr)n中的非共线磁序和自旋轨道耦合效应

利用密度泛函理论对合金团簇(FeCr)_n (n≤6)的几何结构、稳定性和磁性进行了系统的研究. 研究结果表明, 对n≤3的合金团簇, 其基态具有共线的反铁磁序; 而对于n≥4 的合金团簇, 其基态具有非共线磁序, 因此在n=4时体系发生了共线磁序向非共线磁序的“相变”. 此外, 虽然3d过渡金属原子中电子的自旋轨道耦合效应比较弱, 但计算结果表明对于某些小尺寸的合金团簇其轨道磁矩不能忽略. 对非共线磁性团簇的成键性质以及产生磁序“相变”的物理起源进行了详细讨论. 关键词： n合金团簇')" href="#">(FeCr)_n合金团簇 密度泛函理论 非共线磁序 自旋轨道耦合效应     

Photoinduced phenomena in correlated electron systems with multi-degrees of freedom

Photo-induced electronic dynamics in strongly correlated electron system with spin-charge coupling are studied. Motivated from recent optical pump-probe experiments in perovskite manganites and cobaltites, the double-exchange system and the spin-state transition system are theoretically analyzed. First, we focus on the double-exchange interaction in photo-excited state. The time evolutions for the spin and charge structures are examined as functions of the pump photon amplitudes. In the weak-pumping case, the initial charge-ordered antiferromagnetic insulating state tends to become the ferromagnetic metallic state. On the other hand, in the strong-pumping case, the initial antiferromagnetic correlation and insulating character are enhanced after photoirradiation. This unexpected photo-excited state is detectable by the ultrafast optical measurement. Second, in the spin-state transition system, the two-band Hubbard model is analyzed by the time-dependent mean-field approximation. By introducing photons in the low-spin band insulator, high-spin states are generated through the electron-hole pair annihilation process which is governed by the electron band width. We also take into account the relativistic spin-orbit interaction, and compare the results with and without the spin-orbit interaction.     

Pressure induced magnetic and semiconductor–metal phase transitions in Cr_2MoO_6

We investigate magnetic ordering and electronic structures of Cr_2MoO_6under hydrostatic pressure. To overcome the band gap problem, the modified Becke and Johnson exchange potential is used to investigate the electronic structures of Cr_2MoO_6. The insulating nature at the experimental crystal structure is produced, with a band gap of 1.04 eV, and the magnetic moment of the Cr atom is 2.50 μB, compared to an experimental value of about 2.47 μB. The calculated results show that an antiferromagnetic inter-bilayer coupling–ferromagnetic intra-bilayer coupling to a ferromagnetic inter-bilayer coupling–antiferromagnetic intra-bilayer coupling phase transition is produced with the pressure increasing. The magnetic phase transition is simultaneously accompanied by a semiconductor–metal phase transition. The magnetic phase transition can be explained by the Mo–O hybridization strength, and ferromagnetic coupling between two Cr atoms can be understood by empty Mo-d bands perturbing the nearest O-p orbital.     

Topological magnetic insulators with corundum structure

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap are protected by time-reversal symmetry. When a proper kind of antiferromagnetic long-range order is established in a topological insulator, the system supports axionic excitations. In this Letter, we study theoretically the electronic states in a transition metal oxide of corundum structure, in which both spin-orbit interaction and electron-electron interaction play crucial roles. A tight-binding model analysis predicts that materials with this structure can be strong topological insulators. Because of the electron correlation, an antiferromagnetic order may develop, giving rise to a topological magnetic insulator phase with axionic excitations.     

Spin and orbital moments in Upt3

The local spin-density functional approximation is used to calculate the electronic structure of UPt₃ in assumed non-magnetic, ferromagnetic and antiferromagnetic ground states. When spin-orbit coupling is included it is found to induce orbital moments which to a large extend compensate the spin moments of the initially magnetic ground states.     

Magnetic and orbital ordering in the spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.     

Quantum size effects on the (0001) surface of double hexagonal close packed americium

Electronic structures of double hexagonal close-packed americium and the (0001) surface have been studied via full-potential all-electron density-functional calculations with a mixed APW+lo/LAPW basis. The electronic and geometric properties of bulk dhcp Am as well as quantum size effects in the surface energies and the work functions of the dhcp Am (0001) ultra thin films up to seven layers have been examined at nonmagnetic, ferromagnetic, and antiferromagnetic configurations with and without spin orbit coupling. The anti-ferromagnetic state including spin-orbit coupling is found to be the ground state of dhcp Am with the 5f electrons primarily localized. Our results show that both magnetic configurations and spin-orbit coupling play important roles in determining the equilibrium lattice constant, the bulk modulus as well as the localized feature of 5f electrons for dhcp Am. Our calculated equilibrium lattice constant and bulk modulus at the ground state are in good agreement with the experimental values respectively. The work function of dhcp Am (0001) 7-layer surface at the ground state is predicted to be 2.90 eV. The surface energy for dhcp Am (0001) semi-infinite surface energy at the ground state is predicted to be 0.84 J/m². Quantum size effects are found to be more pronounced in work functions than in surface energies.     

Mott physics and topological phase transition in correlated dirac fermions

We investigate the interplay between the strong correlation and the spin-orbit coupling in the Kane-Mele-Hubbard model and obtain the qualitative phase diagram via the variational cluster approach. We identify, through an increase of the Hubbard U, the transition from the topological band insulator to either the spin liquid phase or the easy-plane antiferromagnetic insulating phase, depending on the strength of the spin-orbit coupling. A nontrivial evolution of the bulk bands in the topological quantum phase transition is also demonstrated.     

The effects of Dresselhaus and Rashba spin-orbit interactions on the electron tunneling in a non-magnetic heterostructure

We theoretically investigate the electron transport properties in a non-magnetic heterostructure with both Dresselhaus and Rashba spin-orbit interactions. The detailed-numerical results show that (1) the large spin polarization can be achieved due to Dresselhaus and Rashba spin-orbit couplings induced splitting of the resonant level, although the magnetic field is zero in such a structure, (2) the Rashba spin-orbit coupling plays a greater role on the spin polarization than the Dresselhaus spin-orbit interaction does, and (3) the transmission probability and the spin polarization both periodically change with the increase of the well width.     

3d过渡金属掺杂对Cd12O12纳米线电子和磁性能的影响

采用基于密度泛函理论的第一性原理计算方法,系统研究了3d过渡金属元素(Sc、Ti、Cr、Mn、Co、Cu和Zn)掺杂Cd12O12纳米线的几何结构,电子结构和磁性。结果表明：所有掺杂体系均是热力学稳定的;掺杂Ti或Zn时体系保留了原有的非磁半导体特性,掺杂Mn、Co或Cu时能够实现磁性半导体态,而在掺杂Sc（Cr）时体系转变为非磁性金属态（磁性金属态）。研究结果表明,掺杂3d过渡金属元素的Cd12O12纳米线在电子、光电和自旋电子学领域具有潜在的应用价值。