Polaron effects in cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowires

By the fractal dimension method, the polaron properties in cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowire are explored. In this study, the polaron effects in GaAs/Al_xGa_(1-x)As core-shell nanowire at different values of shell width and aluminum concentration are discussed. The polaron binding energy, polaron mass shift and fractal dimension parameter are numerically worked out each as a function of core radius. The calculation results show that the binding energy and mass shift of the polaron first increase and then decrease as the core radius increases, forming their corresponding maximum values for different aluminum concentrations at a given shell width. Polaron problems in the cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowire are solved simply by using the fractal dimension method to avoid complex and lengthy calculations.     

Holstein极化子能带和有效质量的温度依赖性

使用正则变换方法,考察了一维Holstein极化子能带和有效质量的温度依赖性。结果表明,对于一定的电子声子耦合强度,Holstein极化子能带宽度随温度升高而变窄,有效质量随温度升高而增大。特别是当电子声子耦合强度足够大时,极化子能带宽度在很小的温度范围内会迅速地变为零,我们认为这种情况实际上是极化子从能带状态向自陷局域态的迅速转变,这与通常的相变现象有点相类似。当电子声子耦合常数越大时,极化子有效质量随温度的升高而增加得越快。很显然,研究电子声子相互作用,对理解固体的光学和输运等性质将有重要的意义。     

Small polaron mass with a long range density-displacement type interaction

In this work renormalization of the effective mass of an electron due to a small polaron formation is studied within the framework of the extended Holstein model. It is assumed that an electron moves along the one-dimensional chain of ions and interacts with ions vibrations of a neighboring chain via a long-range density-displacement type force. By means of the exact calculations a renormalized mass of a nonadiabatic small polaron is obtained at strong coupling limit. The obtained results compared with the mass of small polaron of ordinary Holstein model. The effect of ions vibrations polarization on the small polaron mass is addressed.     

Dynamics and transport of a large acoustic polaron in one dimension

We have studied the dynamics and transport relaxation of a large polaron in a one-dimensional (1-D) system with an acoustic- (Debye) type phonon spectrum and a deformation potential electron-phonon interaction. The dynamics is treated in a collective coordinate formalism which shows that such a polaron moves as a heavy quasi-particle that carries energy and (crystal) momentum. For thermal energies less than the polaron binding energy, its transport relaxation is dominated by collision processes wherein a thermal phonon is reflected off the polaron with a momentum transfer that is small compared to the thermal polaron momentum. The phonon reflectivity is estimated and found to exhibit a maximum (resonance) for phonon wavelengths that match the polaron size. Furthermore, the reflectivity is largely independent of the polaron momentum. We incorporate these results into a semi-classical (Boltzmann) kinetic theory and obtain a polaron mobility that is independent of the polaron effective mass and decreases monotonically with increasing temperature. These results are compared to and found to be substantially different from those obtained recently for the large polaron in the 1-D molecular cystal model wherein the underlying phonon spectrum has optical (Einstein) character.To study the effects of three-dimensional (3-D) coupling on the 1-D polaron, we propose a simple 3-D extension of the 1-D acoustic model which includes both electronic and elastic interchain interactions. We briefly discuss the question of polaron stability in the presence of 3-D coupling and the criteria of validity for the purely 1-D treatment.     

有限深势阱里量子盘中极化子的基态性质

采用平面波展开、幺正变换和变分相结合的方法推导出有限深势阱里量子盘中极化子的基态能量公式.采用极化子单位进行数值计算,结果表明极化子的基态能量随势垒高度和势垒宽度的增大而增大,原因是势垒愈高、愈宽,电子穿透势垒的可能性愈小,导致电子能量增大,进而导致极化子基态能量增大.数值计算结果还表明极化子的基态能量随量子盘有效受限长度和量子盘半径的增大而减小;声子效应导致极化子能量较电子能量低.     

外电场作用下纤锌矿氮化物抛物量子阱中极化子能级

The energy levels of a polaron in a wurtzite nitride finite parabolic quantum well (PQW)are studied by a modified Lee-Low-Pines variational method. The ground state of the polaron, the transition energy from first exited state to the ground state and the 关键词： 氮化物抛物量子阱 电子-声子相互作用 极化子     

Exciton and polaron properties in GaxIn1−xAs ternary mixed crystals

Based on the pseudopotential method under the virtual crystal approximation that takes into account the effect of compositional disorder, the electron and heavy-hole effective masses and the dielectric constants in Ga_xIn_1−xAs (0≤x≤1) have been calculated. The results are firstly used in the Wannier equation, which allowed the determination of the exciton reduced mass, binding energy and Bohr radius; then, the polaron properties have been investigated. In this respect, the Fröhlich coupling parameter, Debye temperature and polaron effective mass are calculated and their dependence on the Ga concentration is examined. For InAs and GaAs, our results are generally in reasonable agreement with the known data in the literature, while for compositions x in the range 0-1, our treatment represents the first theoretical predictions. It is found that the exciton and polaron properties for compositions 0<x<1 differ from those of the parent compounds suggesting thus more diverse opportunities to describe most exciton and polaron properties in ternary mixed crystals of interest.     

Pressure-dependent dynamical properties of Zn-based II–VI semiconductors

The effect of pressure on optical phonons and polaron properties in ZnS, ZnSe, and ZnTe II–VI compound semiconductors has been investigated. The calculations are performed in the framework of ab initio pseudopotential approach based on the density functional perturbation theory. At zero pressure, a reasonable degree of agreement is generally found between our results and data available in the literature. It is found that when pressure is increased the phonon modes at Г in the Brillouin zone are shifted towards high energies. The pressure dependence of features such as Fröhlich coupling parameter, the Debye temperature of the longitudinal optical phonon frequency and the polaron effective mass showed that the polaron properties are sensitive to the pressure effect.     

Mean-field properties of impurity in Bose gas with three-body forces

We exactly analyze, on the mean-field level, the low-momentum properties of a single impurity atom loaded in the dilute one-dimensional Bose gas with two- and three-body short-range interactions. Particularly the Bose polaron binding energy and the quasiparticle residue are calculated for the considered system in the broad region of parameters change. We also explore the generic mean-field formula for the polaron effective mass which was shown to depend on the density profile of bath particles with a motionless impurity immersed.     

The Properties of Surface Polaron in the Semi-Infinite Polar Crystal

The effective Hamiltonian and the renormalized mass of surface polaron, where the electron is strongly coupled with the surface optical (SO) phonons but weakly or intermediately coupled with the bulk longit udinal-optical (LO) phonons, were first reviewed. The self-energy and the renormalized mass of the polaron for the polar crystals of potassium iodide, silver chloride and silver bromide are evaluated as a function of the depth Gom the crystal surface. Results of our calculation indicate that the effect of the electron-SO phonon interaction is dominant only within a very small layer from the crystal surface. Its justification awaits further experimental results.     

Localized States of an Excess Electron in an Ionic Cluster

A theory of an electron affinity for an ionic cluster is proposed both in a quasiclassical approach and with quantization of a polarization electric field in a nanopartiele. A critical size of the cluster regarding in formation of an electron＇s autolocalized state, dependencies of energy and radius of a polaron on a cluster＇s size are obtained by a variational method. It has been found that binding energy of the electron in the cluster depends on a eluster＇s radius but a radius of electron＇s auto-localization does not depend on the cluster＇s radius and it equals to the polaron radius in a corresponding infinity crystal. A bound state of the electron in a cluster is possible only if the duster＇s radius is more than the polaron radius.     

Localized States of an Excess Electron in an Ionic Cluster

A theory of an electron affinity for an ionic cluster is proposed both in a quasiclassical approach and with quantization of a polarization electric field in a nanoparticle. A critical size of the cluster regarding in formation of an electron's autolocalized state, dependencies of energy and radius of a polaron on a cluster's size are obtained by a variational method. It has been found that binding energy of the electron in the cluster depends on a cluster's radius but a radius of electron's auto-localization does not depend on the cluster's radius and it equals to the polaron radius in a corresponding infinity crystal. A bound state of the electron in a cluster is possible only if the cluster's radius is more than the polaron radius.     

量子环中极化子的温度效应

采用求解能量本征方程和LLP幺正变换方法,研究了量子环中极化子的温度效应.数值计算表明:当温度较低时,温度对极化子的基态能量无影响,当温度较高时,极化子的基态能量随温度的升高而增大;还表明极化子的基态能量随电子-声子耦合强度的增大而减小,随电子受限程度的增强(即量子环内径增大或外径减小)而增大,说明其量子尺寸效应非常显著.     

Frequency dependent magnetoconductivity and conductivity study in Ni-dispersed silica nano-composite produced by sol-gel technique

The low frequency (20 Hz to 1 MHz) ac conductivity and magnetoconductivity behaviour of ceramic nanocomposite (Ni-SiO₂) at low temperature down to 77 K are reported. The frequency dependent conductivity followed the power law, σ(ω) ∝ ω ^s . The fractional exponent s is a function of temperature and was found to increase with increasing temperature. This type of variation may be attributed to small polaron hopping. A peak present in the loss tangent indicates the presence of a Debye relaxation process. The magnetoconductivity of the samples is positive, which strongly depends on frequency. A firm theoretical explanation of frequency dependent magnetoconductivity is still lacking.     

Fractal dimension study of polaron effects in cylindrical GaAs/AlxGa1−xAs core–shell nanowires

Polaron effects in cylindrical GaAs/Al_xGa_1−xAs core–shell nanowires are studied by applying the fractal dimension method. In this paper, the polaron properties of GaAs/AlxGa1-xAs core–shell nanowires with different core radii and aluminum concentrations are discussed. The polaron binding energy, polaron mass shift, and fractal dimension parameter are numerically determined as functions of shell width. The calculation results reveal that the binding energy and mass shift of the polaron first increase and then decrease as the shell width increases. A maximum value appears at a certain shell width for different aluminum concentrations and a given core radius. By using the fractal dimension method, polaron problems in cylindrical GaAs/Al_xGa_1-xAs core–shell nanowires are solved in a simple manner that avoids complex and lengthy calculations.     

束缚极化子有效质量的温度特性

采用改进的线性组合算符和幺正变换方法研究强、弱耦合束缚极化子的有效质量的温度特性,对RbCl晶体进行数值计算。结果表明,强耦合束缚极化子的振动频率和有效质量随温度的升高而增加,随库仑势的增加而增大。     

Polaron self-energy and effective mass in a freestanding cylindrical quantum wire

The polaron self-energy and correction to the electron effective mass in a freestanding quantum wire is investigated by the perturbation approach.The polaron effect of the electron-confined longitudinal optical (LO) phonon and surface optical (SO) phonon interactions are separately worked out. Numerical calculation on a GaAs quantum wire shows that the confined LO phonon contribution to the polaron self-energy is relatively small for a narrow wire and gradually approach that of the bulk material when the radius of the wire increases. While the contribution of the SO phonon modes is big for small wire radius and then decreases as the radius increases.     

Effect of the Aharonov-Bohm Potential on the Optical Polaron Soliton Motion in One-Dimensional Rings

The effect of the Aharonov-Bohm potential on the optical polaron solitons is considered by making use of vibration configuration resulting from the energy minimum. Two new polaron solutions with solitary wave form and their relevant properties have been obtained. The results show that the soliton's shape, location, self-trapping energy and effective mass depend on the Aharonov-Bohm dux. At the same time, the bell solitons appear at the polaron energy band, while the kink excitation at the top.     

极化子在正弦色散声子场中的性质

本文计及纵光学声子的色散,在正弦近似下,用微扰法了多原子晶体中极化子的基态能量,有效质量和自能。     

有限深抛物势量子盘中极化子的激发态性质

量子点作为一种重要的低维纳米结构, 近年来在单光子光源和新型量子点单光子探测器的研究引起了人们的广泛关注, 对各种势阱中量子点性质的研究已取得了重要成果. 但是大多理论研究都局限于无限深势阱, 而有限深势阱更具有实际意义. 利用平面波展开、幺正变换和变分相结合的方法研究了有限深势阱中极化子激发态能量及激发能随势阱形状和量子盘大小的变化规律. 数值计算结果表明: 极化子的激发态能量、激发能随势垒高度或宽度的增大而增大, 原因是势垒愈高、愈宽, 电子穿透势垒的可能性愈小, 电子在阱内运动的可能性愈大, 进而导致极化子的激发态能量和激发能均随势垒高度和宽度的增大而增大; 极化子的激发态能量和激发能随量子盘半径的增大而减小, 表明量子盘具有显著的量子尺寸效应; 极化子的激发态能量随有效受限长度的增加而减小, 原因是有效受限长度愈大, 有效受限强度愈小, 电子受到的束缚愈弱、振动愈慢、势能愈小, 进而导致基态能量、激发态能量减小; 同时由于激发态能量较基态能量减小慢, 使得激发能随之增加. 研究结果对量子点的应用具有一定的理论指导意义.