耦合电磁场对石墨烯量子磁振荡的影响

我们研究了包含自旋轨道耦合与杂质散射在内的石墨烯量子磁振荡对外加电磁场的响应.我们发现,石墨烯中自旋轨道耦合、电磁场以及边界共同修正了朗道能谱,且当电场与磁场比值超过某一临界值时,量子磁振荡会突然消失,这与非相对论二维电子气的情况显著不同.这种现象可以通过朗道量子化轨道由封闭转化为开放的半经典理论来解释.此外,我们还发现杂质散射和温度的共同作用会使得磁振荡振幅衰减.我们的结果可用于分析石墨烯及其类似结构(硅烯、锗烯、锡烯等)的费米能级与朗道能谱的相互作用,进而探测自旋轨道耦合引起的能隙.     

Susceptibility of the Kagomé lattice Ising model

We explicitly calculate the zero-field magnetic susceptibility of the anisotropic Kagomé lattice Ising model on two different varieties of the parameter space. One of them is the limitH=0 of the solubility condition, obtained in a previous paper by Giacomini, for the model with magnetic field. The other one is the disorder variety of the model, for which a dimensional reduction occurs. These varieties do not contain any nontrivial critical behavior of the model. A functional relation is also established, which relates the zero-field susceptibility for ferromagnetic and competing interactions.     

Magnetic susceptibility and order parameter of the spin-glass-like phase of the double-exchange model

The magnetic susceptibility and Edwards-Anderson order parameter q of the spin-glass-like (SGL) phase of the double-exchange model are evaluated in the weak-coupling or RKKY limit. Dynamical mean-field theory is used to show that q = M(T/T(SGL))2, where M is the classical Brillouin function and T(SGL) is the SGL transition temperature. The correlation length of the SGL phase is determined by a correlation parameter Q that maximizes T(SGL) and minimizes the free energy. The magnetic susceptibility has a cusp at T(SGL) and reaches a nonzero value as T --> 0.     

Spatial confinement of ferromagnetic resonances in honeycomb antidot lattices

We report on a theoretical investigation of the magnetic static and dynamic properties of a thin ferromagnetic film with honeycomb lattice of circular antidots using micromagnetic simulations and analytical calculations. The theoretical model is based on the Landau–Lifshitz equations and directly accounts for the effects of the magnetic state nonuniformity. A direct calculation of local dynamic susceptibility tensor yields that the resonance spectra consist of four different quasi-uniform modes of the magnetization precession related to the confinement of magnetic domains by the hole mesh. Three of four resonant modes follow a two-fold variation with respect to the in-plane orientation of the applied magnetic field. The easy axes of these modes are mutually rotated by 60° and combine to yield the apparent six-fold configurational anisotropy. Additionally, a mode with intrinsic six-fold symmetry behavior exists, as well. Micromagnetic calculations of the local dynamic susceptibility tensor allow identifying the magnetic unit cell areas/domains responsible for each resonance mode.     

磁记忆检测的力磁耦合型磁偶极子理论及解析解

磁偶极子理论在缺陷漏磁场解释中被成功广泛使用.由于磁荷密度等参数不易定量,磁偶极子理论在应用中常常进行归一化处理,被认为不适用于对应力相关的磁记忆信号做量化分析.本文通过建立力磁耦合型磁偶极子理论模型,以适用于分析磁记忆检测中应力对磁信号的影响.基于铁磁学理论确定应力和磁场联合作用下的等效场强度,基于弱磁化状态的一阶近似,获得了各向同性铁磁材料微弱环境磁场下的应力磁化解析解.结合磁信号二维问题中矩形和V形磁荷分布假定,建立了光滑与破坏试件表面磁信号、矩形和V形表面缺陷所诱导磁信号的力磁耦合型磁偶极子理论分析模型,并获得其解析解.基于力磁耦合型磁偶极子理论的解析解,对拉伸实验中试件破坏前后的信号差异、矩形和V形表面缺陷诱导磁信号,以及磁信号的影响因素和规律等进行了详细分析.理论研究表明,基于本文理论模型的解析解可实现对磁记忆检测中的一些基本实验现象和规律的解释.     

基于拓扑结构的碱金属化合物摩尔磁化率的支持向量回归研究

基于经典电动力学导出的表征简单离子磁化率的磁性点价g_i所构建的分子磁性连接性指数?^mF及45种碱金属化合物的摩尔磁化率χ_m的实测数据集,利用粒子群寻优的支持向量回归（SVR）方法,建立了基于⁰F和¹F的碱金属化合物χ_m的预测模型,并与基于多元线性回归（MLR）模型的计算结果进行了比较.结果显示,基于9次交叉验证的SVR模型预测的平均绝对误差、平均相对误差绝对值以及均方根误差均比MLR模型小,表明SVR模型的回归预测能力优于MLR.研究表明,磁性连接性指数^mF是一种合适的分子描述符,SVR是一种预测碱金属化合物χ_m的有效方法. 关键词： 碱金属化合物 摩尔磁化率 支持向量回归 预测     

一维扩展Hubbard模型热力学性质(Ⅱ)——U<0

本文计算了一维负U扩展Hubbard模型的熵、内能、比热和磁化率,并得到了强耦合极限下磁化率的解析形式。结果表明系统的热力学性质可分为两温区来讨论,高温区对应着双占据态破裂,低温区对应着电荷的各种分布。相互作用W对热力学性质有较大影响,而自旋分布的影响不很明显。 关键词：     

Staggered potential and spin polarization effects on RKKY interaction in armchair graphene nanoribbon

Indirect exchange interaction between two magnetic external atoms, named by Ruderman–Kittle–Kasuya–Yosida (RKKY) interaction, has been presented in the staggered armchair graphene nanoribbon. We have studied RKKY interaction as a function of distance between localized moments. It has been shown that a magnetic ordering along the z-axis mediates an anisotropic interaction which corresponds to a XXZ model interaction between two magnetic moments. The static spin susceptibility components of armchair graphene nanoribbon have been calculated to find exchange interaction between arbitrary components of magnetic moments. We have exploited Green’s function approach in order to calculate spin susceptibility components of electronic gas in nanoribbon structure in the context of tight binding model Hamiltonian. The effects of parameter and ribbon width on the dependence of exchange interaction on distance between moments are investigated. Our results show the spin polarization along perpendicular to the plane leads to anisotropic behavior for exchange interaction between the two magnetic moments. In other words the spatial behavior of RKKY interaction between longitudinal components of magnetic moments is different from that of transverse components.     

Dynamic magnetic susceptibility of a superconductor with a relaxing magnetic moment

Within the framework of the diffusion approximation for the dynamic magnetic flux, a model is constructed for the dynamic magnetic susceptibility of a superconductor with a nonuniform and time-dependent magnetic field distribution in the sample. The possibility of appearance of peaks in the temperature (or frequency) dependence of χ″ in a structurally homogeneous superconducting sample in the presence of a magnetic-field-induced nonuniformity of the diffusion parameter is demonstrated. The character of the temporal evolution of the magnetic susceptibility during relaxation of the magnetic moment of the sample is predicted and its properties are investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 811–815 (May 1997)     

Effect of Longitudinal Magnetic Field on Vibration Characteristics of Single-Walled Carbon Nanotubes in a Viscoelastic Medium

The effect of longitudinal magnetic field on vibration response of a sing-walled carbon nanotube (SWCNT) embedded in viscoelastic medium is investigated. Based on nonlocal Euler-Bernoulli beam theory, Maxwell’s relations, and Kelvin viscoelastic foundation model, the governing equations of motion for vibration analysis are established. The complex natural frequencies and corresponding mode shapes in closed form for the embedded SWCNT with arbitrary boundary conditions are obtained using transfer function method (TFM). The new analytical expressions for the complex natural frequencies are also derived for certain typical boundary conditions and Kelvin-Voigt model. Numerical results from the model are presented to show the effects of nonlocal parameter, viscoelastic parameter, boundary conditions, aspect ratio, and strength of the magnetic field on vibration characteristics for the embedded SWCNT in longitudinal magnetic field. The results demonstrate the efficiency of the proposed methods for vibration analysis of embedded SWCNTs under magnetic field.     

Scattering of low-energy electrons by positive ions in a magnetic field. II. Resonances,transition probabilities,and transport frequencies

We analyze quantum-mechanically electron-ion collisions in a magnetic field at a low temperature, for which the electron's thermal energy is less than the energy gap between two Landau levels and the electron's Larmor radius is less than the characteristic impact parameter of close collisions without the magnetic field. To calculate transition probabilities, we use the analytical procedure proposed in the first part of our paper. We calculate the energy and lifetime of the resonant (autoionization) states of an electron embedded in the Coulomb electric field of an ion and in a uniform magnetic field. The obtained values coincide in order of magnitude with the known exact numerical values. We find that the electron backward scattering probability irregularly (chaotically) depends on the particle energy and the magnetic field. We propose analytical approximations for the collision transport frequencies, one of which describes the electron braking along the magnetic field and another, equalizing of the temperatures corresponding to the electron motion along and across the magnetic field. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 8, pp. 682–699, August 2008.     

Blume–Emery–Griffiths纳米管的热力学与相变性质

利用有效场理论研究了圆柱形纳米管上Blume-Emery-Griffiths系统的热力学与相变性质,得到了系统的磁化强度、磁化率、比热和相图.讨论了四次交换作用与二次交换作用的比值 与晶格场对系统热力学量和相图的影响.研究发现:系统存在三临界点,且三临界点由参数 和晶格场共同决定,即若确定了参数 ,则三临界点所对应的晶格场也能确定.随着参数 的增加,系统出现三临界点时所对应的温度和晶格场也相应增大.     

Magneto-elasto-dynamic analysis of an elastically confined conducting nanowire due to an axial magnetic shock

Vibration of a conducting nanowire embedded in an elastic matrix due to an axial magnetic shock is of concern. Based on Maxwell?s and Cauchy?s equations, a model is proposed to study the problem in the context of nonlocal continuum theory. For solving the equations of motion of the nanowire, an analytical approach and a semi-analytical technique are proposed for low and high levels of small-scale parameter, respectively. The effects of small-scale parameter, stiffness of the surrounding matrix, and duration of the applied magnetic shock on the maximum dynamic elastic fields are examined.     

Particle motion and electromagnetic fields of rotating compact gravitating objects with gravitomagnetic charge

The exact solution for the electromagnetic field occuring when the Kerr–Taub–NUT compact object is immersed (i) in an originally uniform magnetic field aligned along the axis of axial symmetry (ii) in dipolar magnetic field generated by current loop has been investigated. Effective potential of motion of charged test particle around Kerr–Taub–NUT gravitational source immersed in magnetic field with different values of external magnetic field and NUT parameter has been also investigated. In both cases presence of NUT parameter and magnetic field shifts stable circular orbits in the direction of the central gravitating object. Finally we find analytical solutions of Maxwell equations in the external background spacetime of a slowly rotating magnetized NUT star. The star is considered isolated and in vacuum, with monopolar configuration model for the stellar magnetic field.     

A generalized Ginzburg-Landau approach to second harmonic generation

We develop a generalized Ginzburg-Landau theory for second harmonic generation (SHG) in magnets by expanding the free energy in terms of the order parameter in the magnetic phase and the susceptibility tensor in the corresponding high-temperature phase. The non-zero components of the SHG susceptibility in the ordered phase are derived from the symmetries of the susceptibility tensor in the high-temperature phase and the symmetry of the order parameter. In this derivation, the dependence of the SHG susceptibility on the order parameter follows naturally, and therefore its nonreciprocal optical properties. We examine this phenomenology for the magnetoelectric compound Cr₂O₃ as well as for the ferroelectromagnet YMnO₃. Received 27 August 1999     

One-dimensional ising model with next-nearest-neighbour interaction in magnetic field

The exact solution for the one-dimensional Ising model with nearest-neighbour and next-nearest-neighbour interactions in an external magnetic field is obtained within the framework of the Kramers-Wannier transfer matrix. The explicit analytical expressions for thermodynamic quantities such as magnetization, magnetic susceptibility and specific heat are derived and analyzed at length as functions of temperature, magnetic field, and the signs and values of the interaction parameters.     

A new method for calculation of the magnetic properties in one- and two-dimensional spin systems with anisotropic Heisenberg exchange

A new approximation method is proposed for the calculation of the magnetic susceptibility of one-dimensional assembly of spins and the critical temperature of two-dimensional one both with the anisotropic Heisenberg exchange. In a linear chain system, every spins are grouped into pairs of adjacent spins (pair-approximation) or clusters of adjacent three spins ((q+1)-approximation), and the partition function of the total spin system is approximated as a sum of products of the partitions functions for the pairs or the clusters. Then the partition function of the anisotropic Heisenberg spin system is shown to reduce into a form of the Ising spin system with modified coupling constants. The exact result for the Ising chain system enables us to obtain an analytical expression for the magnetic susceptibility of anisotropic Heisenberg chain system. The same approximations are also applied to two-dimensional lattices, and the critical temperatures of the square, triangular, and honeycomb lattices with anisotropic Heisenberg exchange are calculated as a function of anisotropy parameter. The results are compared with those of the existing theories and shown to be quite excellent.     

Magnetic properties of a one-dimensional integrable electron model with correlated hopping

We investigate ground-state properties of a one-dimensional correlated hopping electron model in the presence of an external magnetic field which is solvable by Bethe ansatz. We present a general method of calculating magnetization, susceptibility, and chemical potential in exactly solvable spin-1/2 fermion models by deriving a parametric representation of these functions and the magnetic field in terms of the charge and spin distributions, the dressed charge matrix, and the dressed energy at the Fermi points in parameter space. For the correlated hopping model, we numerically calculate the dressed properties-which are given by sets of coupled integral equations-for general values of the field, the band filling, and the interaction parameter. In the special limits of magnetic saturation or large interaction parameter the results are presented in analytic form.     

Indirect RKKY interaction in doped armchair nanotube due to electron phonon interaction effects

We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in doped armchair nanotube in the presence of gap parameter. The effects of both next nearest neighbor hopping parameter and electron-Holstein phonon on RKKY interaction have been addressed. RKKY interaction as a function of distance between localized moments have been analyzed. In order to calculate the exchange interaction along arbitrary direction between two magnetic moments, we should obtain the transverse static spin susceptibility of armchair graphene nanoribbon in the presence of electron-phonon coupling and gap parameter. The spin susceptibility components are calculated using Green’s function approach for Holstein model Hamiltonian. The effects of electron doping on dependence of exchange interaction on distance between moments are investigated via calculating correlation function of spin density operators. Our results show the influences of next nearest neighbor hopping parameter on the spatial behavior of RKKY interactions are different in the presence of electron phonon coupling.     

Simulation of magnetic phase transitions in amorphous alloys of the Re-Gd system

Using the Monte Carlo method within the Heisenberg model, we studied the magnetic properties of amorphous alloys of the Re-Gd system and of pure amorphous gadolinium. The temperature dependences of spontaneous magnetization, magnetic susceptibility, and the Edwards-Anderson order parameter were determined. The possibility of the system transitioning to the spin-glass state was examined.