手性ABM态p波超导体上临界磁场的角依赖关系

利用格林函数方法,通过Klemm-Clem变换,计算具有正交晶格结构椭球形费米面的手性ABM态p波超导体上临界磁场的角依赖关系.超导序参量选取具有手性ABM对称性的等自旋配对单分量形式.当椭球形费米面满足一定条件时,上临界磁场随角度呈现非单调变化,表明除配对电子有效质量的各向异性外,手性ABM态p波超导体也具有超导序参量的各向异性.计算结果可用于判断重费米子超导体Sr2RuO4的配对电子空间结构.     

硅烯/dx2-y2+idxy混合波超导隧道结隧道输运性质研究

运用Blonder-Tinkham-Klapwijk(BTK)理论研究了硅烯/dx2-y2 +idxy 混合波超导隧道结的隧穿性质.研究发现:垂直施加的电场、超导配对势的方向角和两种混合波配对能隙的比值Δ1/Δ0 强烈地影响正常反射、Andreev反射和隧穿电导的值;当两种混合波的序参量比值较大时,隧道谱线在外加偏压E =Δ1 处出现谐振峰;系统的隧穿电导、正常反射幅和Andreev反射幅随超导方向角成周期性变化,变化周期为π/2;由于dxy-波的存在,通过改变外加电场可以对隧穿电流加以调控。     

硅烯/d_(x~2-y~2)+id_(xy)混合波超导隧道结隧道输运性质研究

运用Blonder-Tinkham-Klapwijk (BTK)理论研究了硅烯/d_(x~2-y~2)+id_(xy)混合波超导隧道结的隧穿性质.研究发现:垂直施加的电场、超导配对势的方向角和两种混合波配对能隙的比值Δ_1/Δ_0强烈地影响正常反射、Andreev反射和隧穿电导的值;当两种混合波的序参量比值较大时,隧道谱线在外加偏压E=Δ_1处出现谐振峰;系统的隧穿电导、正常反射幅和Andreev反射幅随超导方向角成周期性变化,变化周期为π/2;由于d_(xy)-波的存在,通过改变外加电场可以对隧穿电流加以调控.     

电子型超导材料的超流密度响应

超导电性是凝聚态物理学中研究的热点.超导材料基本构成分为电子型与空穴型.对于空穴型超导材料,其电子配对对称性普遍接受的是具有dx2-y2波对称性;而对于电子型超导材料,电子配对对称性是dx2-y2波对称性还是s波对称性,目前还存在着很大的争议.在线性近似下,dx2-y2波超导体超流密度是温度的线性函数,而s波超导体的超流密度是温度的指数函数.因此通过对超流密度的研究可以明确电子型超导材料的电子配对对称性.本文首先从微观t-t′-t″-J模型出发,通过隶波色子平均场近似,计及反铁磁关联序和超导配对项,得到两带超导哈密顿量,进而推导出两带超流密度.最后验证了电子掺杂型超导体电子配对对称性和空穴掺杂型一样,同样具有dx2-y2波对称性,和早期唯象的两带模型的研究相一致.明确超流密度随掺杂的变化,是两带准粒子随掺杂变化相互作用的结果.     

时空调制对可激发介质螺旋波波头动力学行为影响及控制研究

本文首先研究了时空调制对可激发介质中周期螺旋波波头动力学行为的影响. 随着时空调制的增大, 螺旋波经历了周期螺旋波、外滚螺旋波、旅行螺旋波和内滚螺旋波的显著变化. 通过定义序参量来定量的描述由时空调制引起的螺旋波在不同态之间非平衡跃迁的临界条件, 及漫游螺旋波波头圆滚圆半径随调制参数的变化情况. 当时空调制增大到某个临界值时, 螺旋波发生了破碎; 再增加时空调制, 螺旋波则发生了衰减, 系统最终演化为空间均匀静息态. 在文中给出了螺旋波发生破碎和衰减的机理和原因. 最后将时空调制方法运用于漫游螺旋波, 实现了将漫游螺旋波控制成周期螺旋波, 或将其控制为空间均匀静息态.     

铁磁超导态/绝缘层/自旋三重态p波超导体结的直流Josephson电流

由Bogoliubov-de Gennes方程得到铁磁超导共存态(FS)的自洽方程，利用推广的Furusaki-Tsukada的电流公式计算了铁磁超导态/绝缘层/自旋三重态p波超导体(FS/I/p)结的直流Josephson电流随结的温度、相位差以及FS中磁交换能、结界面的势垒散射强度的变化关系.研究表明：FS中磁交换能、结界面的势垒散射均抑制FS/I/p结的直流Josephson电流.当自旋三重态超导体具有p_x波配对势时，自旋三重态超导体结的直流Josephson电流随结两侧相位差的振荡周期是π. 关键词： 铁磁超导态 自旋三重态超导体 p波超导体 直流Josephson电流     

铁磁-p波超导结中的自旋极化隧道谱与散粒噪声

考虑到界面的势垒散射和铁磁层中的磁交换作用，依照Sr2RuO4超导体具有自旋三重配对态的p波对称结构，研究铁磁-p波超导结中的自旋极化隧道谱与散粒噪声，研究表明：1.对于幺正配对态，随着铁磁层中磁交换劈裂增强，隧道谱与散粒噪声都减少；2.对于非幺正配对态，隧道谱与散粒噪声都依赖于铁磁层的磁化轴方向，当磁化轴平行于非幺正配对态的自旋轴时，在低偏压下磁交换作用能使隧谱与散粒噪声增强，而当磁化轴反平行于自旋转轴时，其结果相反。     

超导体-正常导体界面的邻近效应

本文应用以前一项工作中所得到的超导-正常界面系统格林函数研究了S-N界面的邻近效应.界面两边的超导有序参量随空间变化,变化展布的范围为相平长度量级.此外,有两点值得一提:1)序参量是一和频率有关的复量,且呈现出振荡的行为.2)在界面的近邻可以出现无能隙区域.     

配对对称性与带间作用

研究d p模型的超导性质.可出现d p配对占主导的情形，配对对称性取决于CuO2面内空穴作用的各向异性，可以是纯d波配对，也可以是纯s波配对.CuO2面内各向排斥作用不能导致空穴配对.欠掺杂区域可以出现“预配对”.当库伯对是局域的则不能根据配对函数求超导临界温度.空穴的退局域以及配对参量对称性的演化也能得到理解. 关键词： d波对称性 d p配对 超导电性     

一维化学势调制的p波超导体中的拓扑量子相变

本文研究了一维公度势和非公度势调制下的p波超导量子线系统的拓扑相变.在公度势调制下,通过计算Z2拓扑不变量确定系统的相图,指出系统的拓扑相变强烈地依赖于调制参数α和相移δ.在非公度势调制下,以α=(√5-1)/2,δ=0为例,计算系统的低能激发谱、Z2拓扑不变量以及逆参与率等,发现p波配对强度△∈(0,0.33)时,系统存在拓扑非平庸超导相,拓扑平庸超导相和拓扑平庸局域相的转变.而当p波配对强度△>0.33时,系统存在拓扑非平庸超导相和拓扑平庸局域相的转变.     

Dynamic Behavior of a Spin- 1 Ising Model. I. Relaxation of Order Parameters and the “Flatness” Property of Metastable States

The dynamic behavior of a spin-1 Ising system with arbitrary bilinear and biquadratic pair interactions is studied by using the path probability method, and approaches of the system toward the stable or metastable equilibrium states according to the ratio of interaction parameters and rate constants are presented. In particular, we investigate the relaxation of the order parameters for temperatures less than, equal to, and greater than the second-order and first-order phase transitions. From this investigation, the “flatness” property of metastable states is seen explicitly. We also show how a system freezes in a metastable state as well as how it escapes from one metastable state to the other.     

Nonlinear bound-free pair creation in the strong electromagnetic fields of a heavy nucleus and an intense x-ray laser

We study e(-)e(+) pair creation in the collision of a heavy nucleus with an intense x-ray laser, where the produced e(-) is simultaneously captured into the K shell of the projectile nucleus. The parameters of the colliding system are chosen such that the absorption of at least two photons from the laser wave is required in order to surmount the energy threshold of the reaction. Considering this fundamental nonlinear strong-field process for the first time, we estimate the total production rate as well as the angular and energetic distributions of the emitted positrons. According to our results, the process of nonlinear bound-free pair creation could become observable by the aid of the upcoming x-ray laser facilities.     

Resonant activation of a synthetic antiferromagnet

The magnetic decay time of a synthetic antiferromagnet comprised of two closely spaced magnetic dipoles is measured in the presence of microwave excitation. The system is known to be highly stable with respect to switching between its two antiparallel ground states under quasistatic magnetic fields. We show that an order of magnitude lower field can switch the pair, provided the field is applied in resonance with the optical eigenmode of the collective spin dynamics in the system. We furthermore show that thermal agitation can play an essential role in spin-flop switching for resonant excitations of near- or subcritical amplitude.     

Investigation of multicomponent chemical short-range order in NiZr2

The local atomic configuration of multicomponent chemical short-range order (MCSRO) in NiZr2 has been investigated by means of molecular dynamics simulation (MD) in a wide temperature range. The potential functions for the system based on the embedded atom method are constructed and the parameters are obtained by fitting the structure and properties of NiZr2 crystal. The static structures such as pair distribution functions and the distribution of coordination number have been calculated. The local atomic configurations of the MCSI~Os in the melt were demonstrated as distorted coordination polyhedron of the compound structure and/or the structure similar to cubooctahedron analogues. It is indicated by the results of MD simulation that above the melting point the atomic packing of long-range order disappears, but the chemical interaction of coordinated atoms still exists, which leads to the formation of various MCSROs with atomic configurations similar to the stable or metastable unit cell of NiZr2 compound. When the system is just melted, many icosahedral polyhedron configurations appear, which decrease as the over-heating temperature increases.     

Optimal working conditions for an absorption heat transformer—analysis of the H2O/LiBr theoretical cycle

This article studies the optimal working conditions of an absorption heat transformer based on a theoretical cycle using the H₂O/LiBr working pair. The efficiency parameters of the system are discussed and a new parameter, namely the exergetic index, directly related to the exergetic efficiency but more significant for evaluating the performance of the system, is introduced. Distinction is made between energetic optimisation—which involves maximal thermodynamic efficiency—and economic optimisation—for which the knowledge of both the energy and equipment costs are necessary. The existence of two kinds of decision variables related to each type of optimisation are discussed. Four criteria for energetic optimisation are defined from which simple prediction equations are obtained. These equations enable us to predict for a given pair, a range of energetically optimal working conditions, valid for a large number of cold and waste energy source temperatures. Finally, practical applications of the proposed predictions are given.     

Dynamic stability of rotor-bearing systems subjected to random axial forces

This paper investigates the lateral vibration of a spinning disk-shaft system supported by a pair of ball bearings and subjected to a pair of random axial forces at both ends. The axial forces are assumed as the sum of a static force and a random process with a zero mean. Due to the random axial forces, the rotor-bearing system may experience parametric random instability under certain situations. In this work, the finite element method is applied to yield a set of discretized system equations first. The set of discretized system equations is partially uncoupled by the modal analysis procedure suitable for gyroscopic systems. The stochastic averaging method is then adopted to obtain Ito's equations for the response amplitudes of the system. Finally the first- and second-moment stability criteria are utilized to determine the stability boundaries of the system. Numerical results show that the rotor-bearing system is always stable in the sense of the first-moment stability, and the effects of the average axial compressive force and the disk mass, which will lower all frequencies of the system, tend to destabilize the second-moment stability of the system.     

Chargino-Pair Productions via Drell-Yan Process at the LHC

We present the analytical and numerical investigations of chargino-pair productions via Drell-Yan process a t proton-proton collider in the frame of minimal supersymmetric extension at tree level. For the χ_i⁺χ_j^-(i,j= 1,2) pair productions at the LHC, the cross section is in the order of 3～2.5 ×10^-3 pb. We find that the cross sections of all kinds of chargino-pair production processes are strongly related to the SUSY Lagrangian parameters M₂ and μ.     

Superexchange pairing and magnetic ordering in cuprate superconductors

The first analytic study of superexchange ion interaction in the cuprate layer of high-T _c superconductors is reported. It is shown that the superexchange nonadditive contributions are dominant in the onset of long-range magnetic order in a system, as well as are responsible for hole pairing to produce an energetically preferable, stable spin configuration. The Heisenberg parameter and the pair binding energy obtained ab initio are in a good quantitative agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 2127–2131 (December 1999)     

Harmonic analysis of time-limited signals

The problem of the estimation of the harmonic content of a signal is studied. The study is limited to the class of causal signals which are the response of linear stable systems to pulse inputs. Two cases are examined: (i) the output of a system whose model has one real pole; (ii) the output of a system whose model has a complex conjugate pole pair. An analytical expression for the error that arises when using the finite Fourier transform is obtained. It is shown that the method required to deal with these two cases are sufficient to deal also with systems having many zeros and poles. The principal dependence of the error on the observation interval length is discussed, and also its dependence on other parameters, such as the ratio of the input pulse length to the observation interval length, and the position of the poles of the system model in the s-plane.