有限二维复式晶格电子结构的尺度效应

以二维复式晶格作为有限系统的集团模型,利用紧束缚近似,在考虑链间耦合及链端效应的情况下,计算了格点数分别为16、32、64、128时π电子在最近邻及次近邻跳跃集团的能谱和态密度.讨论了不同格点数和结构参数对态密度及带宽的影响.     

非闭环结构下有机磁性材料中的自旋密度波性质

研究了有机磁性聚合物ploy-BIPO链的封闭和非封闭结构对体系自旋密度（SDW)的影响.在开链情况下,自旋耦合与电子-电子相互作用对主链SDW的形成存在竞争,自旋耦合有利于SDW的形成;而电子-电子相互作用对SDW起到抑制作用.闭环下,没有得到这些结论. 关键词：     

自旋轨道耦合Su-Schrieffer-Heeger原子链系统的电子输运特性

在Su-Schrieffer-Heeger (SSH)原子链中,电子在胞内和胞间的跳跃依赖于其自旋时,即SSH原子链存在自旋轨道耦合作用时,存在不同缠绕数的非平庸拓扑边缘态.如何探测自旋轨道耦合SSH原子链不同缠绕数的边缘态是一个重要问题.本文在紧束缚近似下研究了自旋轨道耦合SSH原子链的非平庸拓扑边缘态性质及其零能附近的电子输运特性.研究发现四重和二重简并边缘态的缠绕数分别为2和1;并且仅当源极入射电子的自旋被极化(铁磁电极)时,自旋轨道耦合SSH原子链在零能附近的电子输运特性才能反映其边缘态的能谱特性.尤其是,随着自旋轨道耦合SSH原子链与左、右导线之间的耦合强度由弱到强改变,对于缠绕数为2的四重简并边缘态,入射电子在零能附近的透射峰数目将从4个变为0;而对于缠绕数为1的二重简并边缘态情形,其透射峰数目将从2个变为0.因此,在源极为铁磁电极的情形下,通过观察自旋轨道耦合SSH原子链在零能附近电子共振透射峰的数目随着其与左、右导线之间耦合强度的变化,来探测其不同缠绕数的边缘态.上述结果为基于电子输运特性探测自旋轨道耦合SSH原子链不同拓扑性质的边缘态提供了一种可选择的理论方案.     

半满Hubbard模型的新处理方法

从标准Hubbard模型出发,对半满狭带强关联系统,在讨论自旋为δ的电子运动时,忽略自旋为-δ的电子在格点间的跳跃,得到不对称哈密顿量,并运用平均场近似求得相应准粒子谱．在绝对零度时,与Green函数近似解作了比较;在有限温度时,讨论了从绝缘体到金属相变的可能．     

链间耦合对聚乙炔多链体系中电子极化子再激发态的影响

采用拓展紧束缚Su-Schrieffer-Heeger(SSH)模型,研究了链间耦合对反式聚乙炔多链体系中电子极化子再激发态的晶格位形、净电荷密度、局域能级波函数和态密度的影响.结果发现:对于两条链体系,当链间耦合很小(t⊥≤0.01 e V)时,注入到系统中的电子只会在第一条链上诱发产生一个晶格缺陷,形成电子极化子再激发态,这和单链体系是一致,而第二条链仍是二聚化基态.随着链间耦合的增大,第一条链上缺陷的局域度减少而第二条链上的缺陷局域度相应增加,直至两条链上的位形相同;对于多条链(5条链和6条链)体系,当耦合很小(t⊥≤0.05 e V)时,电子极化子再激发态也只会存在于一条链上,当链间耦合较强时,极化子再激发态会在链间层次性地扩展开来,并不会出现多条链位形相同;从两条链的能级图上可以看到随着链间耦合t⊥的增大,体系的带隙不断的增大和电子态密度显示的是完全吻合的,体系的导电性减弱.通过分析两条链体系在t⊥=0 e V和t⊥=0.1 e V的能级态密度,发现链间耦合越强,则中间局域能级的态密度越小,最后没有中间局域态.     

四端双量子点系统中的自旋和电荷能斯特效应

基于非平衡态格林函数方法,理论研究了与四个电极耦合的双量子点系统中的自旋和电荷能斯特效应,考虑了不同电极的磁动量结构和量子点内以及量子点间电子的库仑相互作用对热电效应的影响.结果表明铁磁端口中的磁化方向能够有效地调节能斯特效应:当电极1和电极3中的磁化方向反平行排列时,通过施加横向的温度梯度,系统中将会出现纯的自旋能斯特效应;当电极4从普通金属端口转变为铁磁金属端口时,将同时观测到电荷和自旋能斯特效应.研究发现,能斯特效应对于铁磁电极极化强度的依赖程度较弱,但对库仑排斥作用十分敏感.在量子点内和点间库仑排斥作用的影响下,自旋及电荷能斯特系数有望提高两个数量级.     

链间耦合对聚乙炔多链体系中电子极化子再激发态的影响

从拓展紧束缚模型出发,研究了链间耦合对反式聚乙炔多链体系中电子极化子再激发态的晶格位形、净电荷密度、局域能级波函数和态密度的影响。结果发现：对于两条链体系,当链间耦合很小（eV）时,注入到系统中的电子只会在第一条链上诱发产生一个晶格缺陷,形成电子极化子再激发态,这和单链体系是一致,而第二条链仍是二聚化基态。随着链间耦合的增大,第一条链上缺陷的局域度减少而第二条链上的缺陷局域度相应增加,直至两条链上的位形相同。对于多条链（5条链和6条链）体系,当耦合很小（0.05eV）时,电子极化子再激发态也只会存在于一条链上,当链间耦合较强时,极化子再激发态会在链间层次性地扩展开来,并不会出现多条链位形相同。从两条链的能级图上可以看到随着链间耦合的增大,体系的带隙不断的增大和电子态密度显示的是完全吻合的,体系的导电性减弱。通过分析两条链体系在eV和eV的能级态密度,发现链间耦合越强,则中间局域能级的态密度越小,最后没有中间局域态。     

Rashba自旋轨道耦合作用下电荷流散粒噪声与自旋极化的关系研究

根据存在自旋轨道耦合时基于散射理论的电流表达式和散粒噪声表达式,并利用自旋密度矩阵推导出沿自旋量子化坐标的自旋极化率表达式.解析计算了单通道的情况,发现自旋极化率和电荷流散粒噪声无关.由于多通道解析推导的困难,使用非平衡格林函数技巧,数值计算了包含自旋轨道耦合效应的纯净二维电子气的多通道情况.分别改变偏压、自旋轨道耦合系数、导体长度,研究了这三种不同条件下的自旋极化率与电荷流散粒噪声Fano因子的相关性.两者的相关性表明,相关性定量关系的建立可能为自旋极化的全电学检测提供新思路. 关键词： 散粒噪声 自旋极化 Rashba自旋轨道耦合 散射矩阵     

自旋轨道耦合作用的等效磁场

自旋与电荷一样,是电子的固有属性,电子的周期性轨道运动产生的磁场与电子的自旋磁矩相互作用,这种磁相互作用就是自旋轨道相互作用。在原子物理学中,这种自旋轨道作用会影响原子光谱的精细结构,然而教材中缺少自旋轨道耦合作用在二维半导体材料中的微观描述。本文将引入Rashba和Dresselhaus自旋轨道耦合作用的哈密顿量,研究单电子在无外场下二维平面内运动,讨论一种或者两种自旋轨道耦合的哈密顿量表示。通过自旋与等效磁场耦合的塞曼能量表示,本文计算了本征态下不同自旋轨道耦合作用下的等效磁场,从而有助于探索二维半导体材料中不同自旋轨道耦合作用下的物理特性。     

基于镜像电荷法探测液氦表面电子的自旋态

文献[Phys.Rev.A,2006,74:052338]的理论研究表明液氦上电子自旋具有长达100秒的相干时间,因此在实施量子信息处理方面具有很大的应用前景.然而,这一理论还未得到实验的证实.液氦上电子的自旋-轨道耦合可以为电子自旋探测提供一种可选的方法.理论上,电子自旋-轨道耦合可以由液氦膜下方的微电极电流产生.在微波驱动下,电子发生电偶极跃迁,导致电子距离液氦表面的平均高度发生改变.利用镜像电荷法[Phys.Rev.Lett.,2019,123:086801],电子的这种轨道运动可以被实验测量.建立基于液氦上电子-自旋轨道耦合的密度矩阵方程,并数值求解镜像电荷的振动行为.结果表明不同的自旋态将会引起不同的感应电流,这种差异提供了一种可选的电子自旋探测方法.     

The competition between CDW and SDW in quasi-one-dimensional organic magnetic polymer with interchain coupling interaction

Based on a theoretical model proposed for interchain-coupled quasi-one-dimensional organic magnetic polymer, the effects of the interchain couplings and electron–electron interactions on the charge density wave (CDW) and spin density wave (SDW) that exist in the system are studied. It is found that the amplitude of the SDW along the main chain will decrease with increasing of the oscillatory term of the interchain couplings in the system, which is unfavorable to the ferromagnetic ground state of the system. Moreover, with different interchain couplings, there will all exist a critical value of the inter-site electron–electron Coulomb repulsion, and at this value, the system will experience a transformation from strong SDW state to strong CDW one, which will weaken the mediating function of the antiferromagnetic SDW along the main chain. As a result, the ferromagnetic correlation intensity between the spins of the side radicals will be affected and consequently the stability of the ferromagnetic state in the system will be weakened.     

Role of the spin anisotropy of the interchain interaction in weakly coupled antiferromagnetic Heisenberg chains

In quasi-one-dimensional(q1D) quantum antiferromagnets, the complicated interplay of intrachain and interchain exchange couplings may give rise to rich phenomena. Motivated by recent progress on field-induced phase transitions in the q1D antiferromagnetic(AFM) compound YbAlO_3, we study the phase diagram of spin-1/2 Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations,and investigate the role of the spin anisotropy of the interchain coupling on the ground state of the system. We find that the Ising anisotropy of the interchain coupling can significantly enhance the longitudinal spin correlations and drive the system to an incommensurate AFM phase at intermediate magnetic fields, which is understood as a longitudinal spin density wave(LSDW). With increasing field, the ground state changes to a canted AFM order with transverse spin correlations. We further provide a global phase diagram showing how the competition between the LSDW and the canted AFM states is tuned by the Ising anisotropy of the interchain coupling.     

Dimensional crossover in alternating spin chains

The effect of antiferromagnetic interchain coupling in alternating spin chains is studied by means of spin wave (SW) theory and density matrix renormalization group (DMRG). Two limiting cases are investigated, the two-leg ladder and its two-dimensional (2D) generalization. For the 2D case, SW approximation predicts a smooth-dimensional crossover keeping the ground state ordered, whereas in the ladder case the DMRG results show a gapped ground state for any J>0. Furthermore, the behavior of the correlation functions closely resemble the uniform spin- ladder. However, for small J, the gap behaves quadratically as Δ0.6J². Similarly to uniform spin chains, it is conjectured an analogous spin gap behavior for an arbitrary number of mixed spin chains.     

Symmetry-breaking phase transition without a Peierls instability in conducting monoatomic chains

The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains of single atoms on a surface, is scrutinized for lattice instabilities predicted in the Peierls paradigm. By scanning tunneling microscopy and electron diffraction we reveal a second-order phase transition at 585 K. It leads to charge ordering with transversal and vertical displacements and complex interchain correlations. However, the structural phase transition is not accompanied by the electronic signatures of a charge density wave, thus precluding a Peierls instability as origin. Instead, this symmetry-breaking transition exhibits three-dimensional critical behavior. This reflects a dichotomy between the decoupled 1D electron system and the structural elements that interact via the substrate. Such substrate-mediated coupling between the wires thus appears to have been underestimated also in related chain systems.     

Quantum phase transition and von Neumann entropy of quasiperiodic Hubbard chains

The half-filled Hubbard chains with the Fibonacci and Harper modulating site potentials are studied in a selfconsistent mean-field approximation. A new order parameter is introduced to describe a charge density order. We also calculate the von Neumann entropy of the ground state. The results show that the von Neumann entropy can identify a CDW/SDW （charge density wave/spin density wave） transition for quasiperiodic models.     

Phase diagram of the frustrated spin ladder with the next nearest intrachain couplings

By the density matrix renormalization group method, the phase diagram of the frustrated spin-1/2 ladder with the next nearest intrachain and ferromagnetic interchain couplings is obtained. According to our results, the two-rung entanglement is shown to be more sensitive than order parameters for determining phase boundary in such a frustrated spin ladder model. Furthermore, the additional next nearest intrachain coupling is found to enhance the intermediate columnar dimerized phase evidently.     

镓原子链的结构稳定性和电子结构

使用基于密度泛函理论的第一原理赝势法,研究了Ga-原子链的结构稳定性和电子结构性质.结果表明,Ga原子可以形成线性链,平面之字型以及梯子型等一系列的一维链式结构．其中之字型结构最为稳定,其他结构均为亚稳的．通过第一原理计算的电子结构和JahnTeller 效应,讨论了这些结构的相对稳定性以及各链式结构的电子能带和电荷密度等性质．     

Competing periodicities in fractionally filled one-dimensional bands

We present a variable temperature scanning tunneling microscopy and spectroscopy study of the Si(553)-Au atomic chain reconstruction. This quasi-one-dimensional system undergoes at least two charge density wave (CDW) transitions, which can be attributed to electronic instabilities in the fractionally filled 1D bands of the high-symmetry phase. Upon cooling, Si(553)-Au first undergoes a single-band Peierls distortion, resulting in period doubling along the chains. This Peierls state is ultimately overcome by a competing x3 CDW, which is accompanied by a x2 periodicity in between the chains. These locked-in periodicities indicate small charge transfer between the nearly 1/2-filled and 1/4-filled bands. The presence and the mobility of atomic-scale dislocations in the x3 CDW state indicates the possibility of manipulating phase solitons carrying a (spin, charge) of (1/2, +/- e/3) or (0, +/-2e/3).