高温超导磁通跳跃过程中的磁致伸缩效应

文中基于超导磁通动力学理论,考虑电磁力与热激活对磁通运动的影响,基本模型包括由等效电阻率随超导体温度和磁场变化的磁通扩散方程,以及比热随超导体温度变化的热传导方程组成.在此基础上,用数值方法求解了这组非线性磁热耦合方程,主要研究了有磁通跳跃状发生状态时环境温度和外磁场速度对于高温超导磁致伸缩的影响.结果表明:磁通进入超...     

基于集成最小二乘支持向量机方法的近红外光谱分析

针对近红外（Near Infrared,NIR）光谱测量中的小样本问题。本文提出了一种集成最小二乘支持向量机（Ensemble Least Squares Support Vector Machine,ELS-SVM）新算法。首先使用随机子空间算法（Random Subspace Method,RSM）原始高维变量空间划分为若干个低维度的子空间,然后分别在各个子空间建立最小二乘支持向量机（LS-SVM）模型,最后构造一个集成结果来进行预测。针对一批柴油样本的实验结果表明,本法对柴油十六烷值的预测精度优于传统的LS-SVM方法。     

Multilayer transition in a spin-1 Blume–Capel model with RKKY interaction and quantum transverse anisotropy

Using mean-field theory, we have studied the effect of quantum transverse anisotropies with RKKY interaction on the multi-layer transition and magnetic properties of the spin-1 Blume--Capel model of a system formed by two magnetic multi-layer materials, of different thicknesses, separated by a non-magnetic spacer of thickness M. It is found that the multilayer magnetic order--disorder transition temperature depends strongly on the value of the transverse anisotropy. The multilayer transition temperature decreases when increasing the transverse anisotropy. Furthermore, there exists a critical quantum transverse anisotropy Δ_xL beyond which the separate transitions occur in the two magnetic layers. The critical transverse anisotropy Δ_xL decreases (increases) on increasing the non-magnetic spacer of thickness M (on increasing the crystal field), and Δ_xL undergoes oscillations as a function of the Fermi level.     

Propagation and interaction of ion-acoustic solitary waves in a quantum electron-positron-ion plasma

This paper discusses the existence of ion-acoustic solitary waves and their interaction in a dense quantum electron-positron-ion plasma by using the quantum hydrodynamic equations.The extended Poincar’e-Lighthill-Kuo perturbation method is used to derive the Korteweg-de Vries equations for quantum ion-acoustic solitary waves in this plasma.The effects of the ratio of positrons to ions unperturbation number density p and the quantum diffraction parameter H e (H p) on the newly formed wave during interaction,and the phase shift of the colliding solitary waves are studied.It is found that the interaction between two solitary waves fits linear superposition principle and these plasma parameters have significantly influence on the newly formed wave and phase shift of the colliding solitary waves.The investigations should be useful for understanding the propagation and interaction of ion-acoustic solitary waves in dense astrophysical plasmas (such as white dwarfs) as well as in intense laser-solid matter interaction experiments.     

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho_2−xY_xTi₂O₇ (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1 −1 0) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6 K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30 mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi₂O₇ and compare our results with pure spin ice Ho₂Ti₂O₇. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.     

Encapsulation of methane molecules into carbon nanotubes

Methane gas (CH₄) is a chemical compound comprising a carbon atom surrounded by four hydrogen atoms, and carbon nanotubes have been proposed as possible molecular containers for the storage of such gases. In this paper, we investigate the interaction energy between a CH₄ molecule and a carbon nanotube using two different models for the CH₄ molecule, the first discrete and the second continuous. In the first model, we consider the total interaction as the sum of the individual interactions between each atom of the molecule and the nanotube. We first determine the interaction energy by assuming that the carbon atom and one of the hydrogen atoms lie on the axis of the tube with the other three hydrogen atoms offset from the axis. Symmetry is assumed with regard to the arrangement of the three hydrogen atoms surrounding the carbon atom on the axis. We then rotate the atomic position into 100 discrete orientations and determine the average interaction energy from all orientations. In the second model, we approximate the CH₄ molecule by assuming that the four hydrogen atoms are smeared over a spherical surface of a certain radius with the carbon atom located at the center of the sphere. The total interaction energy between the CH₄ molecule and the carbon nanotube for this model is calculated as the sum of the individual interaction energies between both the carbon atom and the spherical surface and the carbon nanotube. These models are analyzed to determine the dimensions of the particular nanotubes which will readily suck-up CH₄ molecules. Our results determine the minimum and maximum interaction energies required for CH₄ encapsulation in different tube sizes, and establish the second model of the CH₄ molecule as a simple and elegant model which might be exploited for other problems.     

Electron-TO-phonon interaction in polar crystals

In the present work, the interaction between electrons and long-wavelength transverse optical (TO) vibrations in a polar insulator is considered. The crystal model employed for the theoretical analysis includes classical potentials and electronic polarizabilities. A significant enhancement of the strength of the electron-TO-phonon interaction in ferroelectrics has been found. A microscopic justification of this effect is given. A bridge that relates the interaction of electrons with the polar long-wavelength TO modes of lattice vibrations to the long-range dipole-dipole interaction is established. As an application of our analysis, a novel method for quantitative predictions of the electron-TO-phonon interaction constants in polar crystals via the data of experimental studies is suggested.     

The effect of the interdot Coulomb interaction on Kondo resonances in series-coupled double quantum dots

We theoretically investigate the effect of the interdot Coulomb repulsion on Kondo resonances in the series-coupled double quantum dot coupled to two ferromagnetic leads. The Hamiltonian of our system is solved by means of the slave-boson mean-field approximation, and the variation of the density of states, the transmission probability, the occupation number, and the Kondo temperature with the interdot Coulomb repulsion are discussed in the Kondo regime. The density of states is calculated for various interdot Coulomb repulsions with both parallel and antiparallel lead-polarization alignments. Our results reveal that the interdot Coulomb repulsion greatly influences the physical property of this system, and relevant underlying physics of this system is discussed.     

Magnetic moment of phonons in graphene induced by a magnetic field

A magnetic field not only changes the electronic structure in graphene but also affects the phonon excitations via the electron-phonon interaction and even enables the phonons to generate magnetism. In this paper, we evaluate the magnetic moment of phonons in graphene using a generating-functional technique. The calculation results indicate that the phonon magnetic moment exists only in a weak magnetic field. The step-like change of the magnetic moment with the magnetic field reflects a macroscopic quantum effect.     

Critical behavior of XY spin chain with Dzyaloshinsky-Moriya interaction studied in terms of Loschmidt echo

In this paper, the critical behavior of the general XY spin chain with the Dzyaloshinsky-Moriya (DM) interaction is studied by means of a Loschmidt Echo (LE) calculation. LE presents a Gauss decay in the region of magnetic field intensity |λ|<1 and an exponential decay in the region of |λ|>1. There exists a critical spin chain size N_C. When spin chain size is larger than N_C, the value of λ corresponding to the minimum value of LE (λ_m) is independent of the spin chain size and keeps a stable value. In the region of λ<0, the stable value is same for different DM interactions. In the region of λ>0, the stable value varies with changing DM interaction.