层状磁电复合材料谐振频率下的巨磁电容效应

对三明治复合结构Tb_xDy_1-xFe_2-y/Pb(Zr, Ti)O₃/Tb_xDy_1-xFe_2-y的电容与频率及磁场的函数关系进行了实验和理论研究. 实验发现,该复合材料样品的电容随频率的增加而出现多个谐振峰,并且其谐振点随磁场的增加而发生频移. 在谐振点附近,观察到样品的阻抗随磁场的增加由容抗性转变为感抗性,从而同时观察到巨大的正磁电容效应和负磁电容效应. 由复合材料的弹性力学本构方程出发,对该类样品的电容随频率及磁场的变化进行了理论模拟. 结果显示,模拟曲线与实验结果符合得很好. 理论表明该磁致伸缩/压电复合材料的磁电容效应源于磁场诱变的铁磁相柔顺系数. 关键词： 层状复合材料 界面弹性耦合 磁电容效应     

Negative refraction at optical frequencies in nonmagnetic two-component molecular media

There is significant motivation to develop media with negative refractive indices at optical frequencies, but efforts in this direction are hampered by the weakness of the magnetic response at such frequencies. We show theoretically that a nonmagnetic medium with two atomic or molecular constituents can exhibit a negative refractive index. A negative index is possible even when the real parts of both the permittivity and permeability are positive. This surprising result provides a route to isotropic negative-index media at optical frequencies.     

Ultimately short optical pulses in carbon nanotubes in dispersive nonmagnetic dielectric media

We consider Maxwell’s equations for an electromagnetic field propagating in carbon nanotubes placed in dispersive nonmagnetic dielectric media. An effective equation having the form of an analog of the classical sine-Gordon equation was obtained and analyzed numerically. The dependence of the pulse on the type of carbon nanotubes, initial pulse amplitude, and dispersion constants of the medium was revealed.     

Magnetocapacitance: probe of spin-dependent potentials

The magnetic field dependence of the capacitance of Pd-AlOx-Al thin-film structures has been measured. The observed quadratic dependence of capacitance on the magnetic field is consistent with a theoretical model that includes the effect of a spin-dependent electrochemical potential on electron screening in the paramagnetic Pd. This spin-dependent electrochemical potential is related to the Zeeman splitting of the narrow d bands in Pd. The quantitative details depend on the electronic band structure at the surface of Pd.     

Effect of Microwave Radiation on Quantum Magnetocapacitance Oscillations

JETP Letters - The effect of microwave radiation on the amplitudes of the quantum magnetocapacitance oscillations in field-effect transistors has been studied. It has been found that the...     

Persistence in random walk in composite media

We consider a class of inhomogeneous media known as composite media that is often encountered in experimental sciences, and investigate the persistence probability of a random walker in such a system. Analytical and numerical results for the crossover time scales are obtained for a composite system with two homogeneous components and three homogeneous components respectively.     

Stability of solitons in nonlinear composite media

An analysis is made of the dynamic stability of soliton solutions of the Hamilton equations describing plane waves in nonlinear elastic composite media in the presence and absence of anisotropy. In the anisotropiccase two two-parameter soliton families, fast and slow, are obtained in analytic form; in the absence of anisotropy there is a single three-parameter soliton family. It is shown that solitons from the slow family in an anisotropic composite and solitons in an isotropic composite are dynamically stable if their velocities lie in a certain range known as the range of stability. The analysis of stability is based on the spectral properties of the “linearized Hamiltonian” ?. It is shown that the operator ? is positively semidefinite on some linear subspace of the main solution space from which stability follows. Problems of instability of the fast soliton family in the anisotropic case and representatives of soliton families whose velocities lie outside the range of stability in the presence and absence of anisotropy are discussed.     

Magnetocapacitance spectroscopy of multilayer electron systems in wide parabolic wells

We present a technique to detect quantum capacitance in the presence of strong density profile effects. We demonstrate our technique by performing capacitance-voltage subband spectroscopy on a system of coupled electron layers in a heterostructure which is the superposition of a wide parabolic well and multiple square wells. Events inducing Fermi level variations are detected as features in the derivative profile dC/dV_g as the electron gas width is reduced by an applied gate voltage. Quantum features are differentiated from density profile effects by considering differences in derivative profiles dC/dV_g for increasing magnetic field perpendicular to the electron layers. Results are compared with theory.     

Multiscale models of hard-soft composite media

The status of models of composite media for ultra-high density recording is reviewed. It is shown that the usual micromagnetic formalism may break down when dealing with interfaces where rapid spatial changes in magnetisation may occur. A multiscale approach is outlined, in which atomistic models are coupled with micromagnetic concepts to provide an improved physical model. The theory is applied to FePt/FeRh bilayers, which have potential as media for heat-assisted magnetic recording.     

Method of simulating magnetodielectric composite media

The paper introduces a method of simulating magnetodielectric composite media by analogy with Leinders's method (J. Phys. I (France) 5 (1995) 555), which starts from the study of the behavior of a magnetodielectric binary mixture inside an electromagnetic field. The work hypothesis is that the dimensions of the magnetic particles are much smaller than the wavelength in vacuum of the electromagnetic wave we used.     

Stability of solitary waves in nonlinear composite media

The system of equations for planar waves in elastic composite media in the presence of anisotropy is considered. In anisotropic case two two-parametric families of solitary waves are found in an explicit form. In case of the absence of anisotropy these two families coalesce into the unique three parametric family. The solitary wave solutions are found to be orbitally stable in a certain range of their phase speeds (range of stability) both in an anisotropic as well as in an isotropic materials. It is also shown that the initial value problem for the governing equations is locally well posed which is needed to prove the stability result. The local well-posedness of the initial value problem along with stability of solitary waves implies global existence result provided the initial data lie in a neighbourhood of a stable solitary wave. This complements the previous results of blow-up for this type of equations.     

Thermal conductivity of wet composite porous media

A model for the thermal conductivity of two-phase composite porous media, such as adsorbing desiccant bed with inert material, is presented. The unit cell model is extended to account for the inert materials and moisture sorbed in pores; and the effect of porosity, water content and presence of inert material is determined. The prediction of the effective thermal conductivity from this proposed model is compared with that given by other available models and experimental data and the proposed model is used to investigate the effect of porosity, water content and presence of inert material.     

Effect of the intrinsic nonlinearity on the propagation of ultrashort optical pulses in carbon nanotubes in dispersive nonmagnetic dielectric media

The Maxwell equations for the electromagnetic field that propagates in carbon nanotubes (CNTs) placed in dispersive nonmagnetic dielectric media are analyzed with allowance for the intrinsic nonlinearity of the medium. The dependences of the pulse on the initial pulse amplitude, dispersion constants, and nonlinearity are revealed.     

Poisson's ratio in composite elastic media with rigid rods

We calculate both the micromechanical response and bulk elastic constants of composites of rods embedded in elastic media. We find two fixed points for Poisson's ratio with respect to rod density: there is an unstable fixed point for Poisson's ratio =1/2 (an incompressible system) and a stable fixed point for Poisson's ratio =1/4 (a compressible system). We also derive approximate expressions for the elastic constants for arbitrary rod density, which agree with exact results for both low and high density. These results may help to explain recent experiments [Phys. Rev. Lett. 102, 188303 (2009)10.1103/PhysRevLett.102.188303] that reported compressibility for composites of microtubules in filamentous actin networks.     

颗粒复合介质在高温下的光学双稳特性

利用Maxwell-Garnett近似，结合谱表示方法，理论研究了金属/绝缘颗粒复合介质的光学双稳特性随环境温度的依赖关系.数值结果表明，体系的光学双稳区域随环境温度的升高而逐渐变窄，甚至消失.还进一步研究了复合体系的折射系数和消光系数随温度的变化关系. 关键词： 颗粒复合介质 光学双稳     

Giant negative magnetoresistance in a nonmagnetic semiconductor

Studies of a classical III–V semiconductor (InSb) doped with 3d magnetic ions (Mn²⁺, having a localized spin S=55/2) reveal some unexpected transport properties. It is found that the transition from the metallic to the low-temperature insulator phase occurs at an impurity concentration N _Mn∼N _cr=2× 10¹⁷ cm⁻³ and a temperature T<T _cr∼1 K. Under these conditions a giant negative magnetoresistance arises. The experimental results can be explained in terms of the onset of a hard Mott-Hubbard gap Δ in the impurity band formed by the shallow manganese acceptor in InSb at N _Mn∼N _cr. A model describing the gap formation is proposed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 358–362 (10 March 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.