La0.67Sr0.08Na0.25MnO3的奇特输运性质及CMR效应

用固相反应法制备了La0.67Sr0.08Na0.25MnO3样品.通过磁化强度-温度(M-T)曲线、电阻率-温度(ρ-T)曲线以及ρ-T拟合曲线研究了样品的输运性质及庞磁电阻(colossal magnetoresistance,CMR)效应.结果表明,ρ-T曲线和磁电阻.温度(MR-T)曲线均出现双峰现象;高温峰是伴随顺磁-铁磁(PM-FM)相变出现绝缘体-金属(1-M)相变,低温峰是颗粒界面效应;两个绝缘相输运机理不同:较低温度下(248K＜T＜274K),P(T)符合极化子的可变程跃迁模型,而在更高温区(330K＜T＜374K),ρ(T)符合极化子近邻跃迁模型;两个类金属相输运机理也不同:在低温区(67K＜T＜186K),满足ρ-T2.5关系,输运机理是自旋波散射和电-磁子散射作用,而在高温区(292K＜T＜304K),满足ρ-T2关系,输运机理是单磁子散射作用.     

基于伊辛模型的Fe_(0.5)Mn_(0.1)Al_(0.4)合金磁化强度和磁熵变蒙特卡洛模拟

基于伊辛模型的单自旋反转蒙特卡洛算法,考虑了粒子间的最近邻以及次近邻相互作用,研究了无序Fe0.5Mn0.1Al0.4合金的磁化强度和磁熵变.首先,强调了粒子间的次近邻相关作用对体系的磁性和热力学性质的影响,明确了次近邻相互作用系数,证实了低温合金阻挫的存在;其次,研究了在相变温度处(不同磁场下)磁化强度随外加磁场(温度)的变化情况以及磁性粒子对磁化强度的贡献,发现反铁磁性粒子Mn在低温区对Fe0.5Mn0.1Al0.4合金的相变起了主要作用,而高温区体系的相变是由铁磁性粒子Fe贡献的;最后,分析了体系在相变温度处磁熵变数值随外加磁场的变化情况以及磁熵变在不同的外磁场下随温度的变化情况,当外加磁场H=0.14(a.u.)时,Mn粒子在冻结温度处的平均磁化强度为零,体系处于最无序的状态,对应的磁熵变ΔS(0.1,0.14)达到了正向最大值,极值的位置对应于体系的相变温度.     

交错跃迁Hofstadter梯子的量子流相

为玻色Hofstadter梯子模型引入交错跃迁,来扩展模型支持的量子流相.基于精确对角化和密度矩阵重整化群计算发现,无相互作用时,系统中包含横流相、涡旋相和纵流相;横流相来自均匀跃迁时Hofstadter梯子模型的Meissner相,纵流相是交错跃迁时才可见的流相.强相互作用极限下系统的超流区也包含横流相、纵流相和涡旋相,但存在更多的相变级数;超流区的横流相、纵流相之间存在相变但Mott区的不存在,把Mott区的"横、纵流相"称为Mott-均匀相,在Mott区只存在均匀相和涡旋相.跃迁的交错会压缩涡旋相存在的区域,使Mott区最终只剩下均匀相;跃迁的交错不仅能驱动Mott-超流相变,还使磁通的改变也能够驱动系统的Mott-超流相变.对这一系统的研究丰富了磁通系统中的量子流相,同时为研究拓扑流特性提供了模型支持.     

基于伊辛模型的Fe0.5Mn0.1Al0.4 合金磁化强度和磁熵变蒙特卡洛模拟

基于伊辛模型的单自旋反转蒙特卡洛算法,考虑了粒子间的最近邻以及次近邻相互作用,研究了无序 合金的磁化强度和磁熵变。首先,强调了粒子间的次近邻相关作用对体系的磁性和热力学性质的影响,明确了次近邻相互作用系数,证实了低温合金阻挫的存在;其次,研究了在相变温度处(不同磁场下)磁化强度随外加磁场（温度）的变化情况以及磁性粒子对磁化强度的贡献,发现反铁磁性粒子Mn在低温区对 合金的相变起了主要作用,而高温区体系的相变是由铁磁性粒子Fe贡献的;最后,分析了体系在相变温度处磁熵变数值随外加磁场的变化情况以及磁熵变在不同的外磁场下随温度的变化情况,当外加磁场h=0.14时,Mn粒子在冻结温度处的平均磁化强度为零,体系处于最无序的状态,对应的磁熵变 达到了正向最大值,极值的位置对应于体系的相变温度。     

La0.67Sr0.08Na0.25MnO3的奇特输运性质及CMR效应

用固相反应法制备了La_0.67Sr_0.08Na_0.25MnO₃样品.通过磁化强度-温度(M-T)曲线、电阻率-温度(ρ-T)曲线以及ρ-T拟合曲线研究了样品的输运性质及庞磁电阻(colossal magnetoresistance，CMR)效应.结果表明，ρ-T曲线和磁电阻-温度(MR-T)曲线均出现双峰现象；高温峰是伴随顺磁-铁磁(PM-FM)相变出现绝缘体-金属(I-M)相变，低温峰是颗粒界面效应；两个绝缘相输运机理不同：较低温度下(248K<T<274K)，ρ(T)符合极化子的可变程跃迁模型，而在更高温区(330K<T<374K)，ρ(T)符合极化子近邻跃迁模型；两个类金属相输运机理也不同：在低温区(67K<T<186K)，满足ρ-T^2.5关系，输运机理是自旋波散射和电-磁子散射作用，而在高温区(292K<T<304K)，满足ρ-T²关系，输运机理是单磁子散射作用. 关键词： 庞磁电阻 金属-绝缘体转变 晶界效应 输运行为     

二维各向异性SSH模型的拓扑性质研究

二维Su-Schrieffer-Heeger(SSH)模型是在拓扑物理领域受到广泛研究的一种模型,具有许多独特的物理性质.它属于高阶拓扑绝缘体,在第二条和第三条能带间会产生具有连续谱束缚态(bound states in the continuum,BICs)性质的角态.本文首先介绍了二维SSH模型的拓扑性质,在此基础上论证了第二条和第三条能带何时会在整个布里渊区上产生能隙.随后,计算了模型的电荷极化分布和电荷密度分布,证明了当x方向上胞内跃迁几率和胞间跃迁几率较大时,x方向的边缘电荷极化激发了y方向的边缘态,反之亦然.同时,边缘电荷极化激发了角上的异常填充,产生了具有良好局域性与鲁棒性的拓扑角态.最后,构建了一种声学谐振腔模型,并证明了该模型可以较好的模拟各向异性二维SSH模型的拓扑性质.     

自旋为1的双层平方晶格阻挫模型的相变

采用双时格林函数方法研究了自旋为1的双层平方晶格阻挫模型的相变行为.详细探讨了层间耦合相互作用J_c和单离子各向异性参数D对奈尔态(AF1)和共线态(AF2)之间相转换的影响.结果显示:只要参数J_c和D不同时为零,奈尔态和共线态在J₂=J₁/2(这里J₁和J₂分别描述的是系统自旋间最近邻和次近邻交换作用)时的相变温度相等,两个态共存.在低于相变点的温度范围内,AF1-AF2态之间可以发生相转换,其相变类型为一阶相变.当J₂≠J₁/2时,尽管AF1-AF2态有不同相变温度,但它们也可以共存.如果AF1(AF2)态的相变温度大,在低温,AF1(AF2)态更稳定;在高温,AF2(AF1)态更稳定;在中间温度范围内,AF1-AF2态之间也可以发生一阶相转换.     

Nd0.5Ca0.5Mn0.97Co0.03O3体系的超声研究

系统研究了Nd0.5Ca0.5Mn0.97Co0.03O3单相多晶样品在低温下的电磁性质和超声特性.超声测量结果表明,体系在低温下超声声速出现异常,超声声速从室温开始随着温度的降低逐渐减小,并在220K附近声速达到最小,其相对变化(ΔV/V)超过2%.之后,随着温度的进一步降低,声速急剧硬化,其相对变化达到14.5%.分析表明这是由于体系中电-声子相互作用导致的电荷有序态转变的结果,该电声子耦合来源于Mn3 的JahnTeller效应.同时,该体系在100K左右出现了金属绝缘体转变同时伴随着铁磁相变,分析表明在低温下Co离子与Mn离子之间存在着铁磁交换作用,因此微量的Co掺杂会在反铁磁区域内形成部分铁磁小团簇,当铁磁团簇连通后,体系由反铁磁电荷有序态(AFMCO)向铁磁金属态(FMM)转变.研究结果表明了样品在电荷有序相变温度TCO与金属绝缘体转变温度TMI之间(TMI相似文献   

扩展的t-J模型中的非公度反铁磁自旋关联

在t-J模型和fermion-spin理论框架下,研究了电子次近邻跃迁t′对欠掺杂四方晶格铜氧化物材料的反铁磁非公度自旋关联的影响.结果表明,虽然电子次近邻跃迁项使得自旋结构因子的非公度峰强度增加,但对物理上合理的、较小的t′值,t-t′-J模型的非公度自旋关联与t-J模型定性一致.     

Y_(0.9)Ba_(0.1)CuO_3超导体的低温比热

测量了陶瓷超导体Y_(0.9)Ba_(0.1)CuO的低温比热,在Tc=90K附近未观察到比热跃变,而发现在12K附近存在典型的λ-相变,反映在样品中同时含有一个低Tc的超导相。从12K以上的低温比热数据计算出该超导体的德拜温度为400K左右,比热中线性项的系数较大,其中含有原子振动自由度的贡献。     

Magnetic phase diagram of an extended Hubbard model at half filling: Possible application for strongly correlated iron pnictides

We study the magnetic phase diagram within an extended half-filled Hubbard model, focusing on the roles of the next-nearest-neighbor (NNN) and the next-next-nearest-neighbor (3rd NN) hoppings in the magnetic configurations. We find that due to the spin frustration from the long range hopping and the competition between long-range hopping and Coulomb correlation, the striped antiferromagnetic (AFM) phase is stable when the NNN hopping is dominant, while the bicollinear AFM phase is robust when the 3rd NN hopping is considerably large. The triple points are found in various magnetic phase diagrams. Possible applications of the present theory on intermediately correlated LaFeAsO and strongly correlated FeTe are discussed.     

Phase diagram of the fcc(1 1 0) surfaces with adsorbate-induced missing-row reconstructions

The phase diagram of the fcc(1 1 0) surfaces with missing-row reconstructions induced by adatoms, is calculated by use of the Blume–Emmery–Griffiths model. In the model, we introduce adatom–adatom interactions to determine surface structures and dipole–dipole interactions to describe the effect of zigzag adsorption. The interactions between nearest-neighbor (NN) and next-nearest-neighbor (NNN) rows are considered. The calculation of the temperature versus adatom chemical potential phase diagram is performed using mean-field approximation. It is indicated that if NN and NNN interactions are competitive, there appear either dipole or coverage modulated (incommensurate) phases at high temperatures for a wide range of the interactions.     

Role of next-nearest-neighbour hopping in the internal structure of the ground state and finite temperature quantities of 2D t-J model

An exact diagonalization calculation for a small cluster in the two-dimensional t-J model has been studied to calculate two-hole correlation. Calculations reveal dominant hole-hole correlation for holes sitting on next-nearest-neighbour (NNN) sites and critical coupling occurs at J/t = 0.8. With the increase in negative-type NNN hopping, correlation decreases at NNN sites whereas it increases at other sites. The thermodynamic properties such as entropy and specific heat are studied as functions of temperature with various NNN hopping strength. Results show that with the inclusion of negative NNN hopping, the system becomes more ordered. A qualitative transition temperature region has been estimated. It is shown that with the increase in NNN hopping strength, T _c increases. Specific heat results show non-Fermi liquid-type behaviour of the system. All our calculations establish the importance of negative-type NNN hopping.     

Effective Hamiltonians and Phase Diagrams for Tight-Binding Models

We present rigorous results for several variants of the Hubbard model in the strong-coupling regime. We establish a mathematically controlled perturbation expansion which shows how previously proposed effective interactions are, in fact, leading-order terms of well-defined (volume-independent) unitarily equivalent interactions. In addition, in the very asymmetric (Falicov–Kimball) regime, we are able to apply recently developed phase-diagram technology (quantum Pirogov–Sinai theory) to conclude that the zero-temperature phase diagrams obtained for the leading classical part remain valid, except for thin excluded regions and small deformations, for the full-fledged quantum interaction at zero or low temperature. Moreover, the phase diagram is stable against addition of arbitrary, but sufficiently small further quantum terms that do not break the ground-state symmetries. This generalizes and unifies a number of previous results on the subject; in particular, published results on the zero-temperature phase diagram of the Falikov–Kimball model (with and without magnetic flux) are extended to small temperatures and/or small ionic hopping. We give explicit expressions for the first few orders, in the hopping amplitude, of equivalent interactions, and we describe the resulting phase diagram. Our approach yields algorithms to compute equivalent interactions to arbitrarily high order in the hopping amplitude.     

Topological Phase Transition with Larger Winding Number in the Non-Hermitian Dimerized Lattice

The topological phase transitions among normal insulator phase, two kinds of topological insulator phases, and topological semimetal phase are shown based on the non-Hermitian dimerized Su–Schrieffer–Heeger (SSH) model with the nonreciprocal intercell and long-range hopping. In contrast to the previous work, it is found that the topological insulator phase in the present SSH model can hold the larger non-Bloch winding number accompanied by exceptional winding of the generalized Brillouin zone around the gap-closing points. Compared with the usual topological insulator phase in non-Hermitian SSH model, the topological insulator with the larger winding number owns two pairs of zero energy modes with a distinct form of edge localization in the gap. The physical mechanism of the distinct edge localization for zero energy modes via a equivalent Hermitian version of the non-Hermitian SSH model is revealed. Additionally, the process of the phase transition is visualized among normal insulator phase, topological insulator phases, and topological semimetal phase in detail via the evolution of the gap-closing points on the plane of generalized Brillouin zone. This work further verifies the non-Bloch theory and enrich the investigation about the topologically nontrivial phase with the larger topological invariant in the non-Hermitian SSH model.     

Mott-Hubbard transition in the mass-imbalanced Hubbard model

The mass-imbalanced Hubbard model represents a continuous evolution from the Hubbard to the Falicov-Kimball model. We employ dynamical mean field theory and study the paramagnetic metal-insulator transition, which has a very different nature for the two limiting models. Our results indicate that the metal-insulator transition rather resembles that of the Hubbard model as soon as a tiny hopping between the more localized fermions is switched on. At low temperatures we observe a first-order metal-insulator transition and a three peak structure. The width of the central peak is the same for the more and less mobile fermions when approaching the phase transition, which agrees with our expectation of a common Kondo temperature and phase transition for the two species.     

Extended State to Localization in Random Aperiodic Chains

The electronic states in Thus-Morse chain （TMC） and generalized Fibonacci chain （GFC） are studied by solving eigenequation and using transfer matrix method. Two model Hamiltonians are studied. One contains the nearest neighbor （n.n.） hopping terms only and the other has additionally next nearest neighbor （n.n.n.） hopping terms. Based on the transfer matrix method, a criterion of transition from the extended to the localized states is suggested for CFC and TMC. The numerical calculation shows the existence of both extended and localized states in pure aperiodic system. A random potential is introduced to the diagonal term of the Hamiltonian and then the extended states are always changed to be localized. The exponents related to the localization length as a function of randomness are calculated. For different kinds of aperiodic chain, the critical value of randomness for the transition from extended to the localized states are found to be zero, consistent with the case of ordinary one-dimensional systems.     

Integrable correlated electron model with next-nearest-neighbour interactions

The exactly solvable model of supersymmetric t - J chains (STJC) of correlated electrons with next-nearest-neighbour (NNN) interactions is proposed and studied. The model with interactions between nearest neighbours and NNN interactions in one chain can also be considered as a two-chain model with zigzag-like coupling between the chains. The NNN interaction (coupling between chains) causes the onset of additional Dirac seas for low-lying charge and/or spin excitations. These Dirac seas change the low-energy (conformal) behavior of the model. The filling of those seas depends on the values of the NNN coupling (interactions between chains), external magnetic field and applied voltage. We identify the new ground state phases which appear due to the NNN as incommensurate ones. The NNN coupling in the incommensurate phases induces spontaneous magnetization and/or spontaneous filling of the Dirac sea for charge excitations (“spontaneous charge ordering”). The onset of this order implies a first order quantum phase transition driven by the field with hysteresis phenomena. Received 13 September 2000