Impurity band for screened impurities in many-valley semiconductors

For a random distribution of screened donor impurities in many-valley semiconductors we study the impurity band as a function of carrier concentration. The results show that the often used assumption of an isolated screened impurity is unjustified, particularly for the critical concentration for which the binding energy becomes zero.     

Possibility of experimental determination of band structure parameters in many-valley semiconductors

We show possibility in principle to determine some parameters (density-of-states effective mass of different valleys, energy distances between valleys, energies of ionization of impurity levels split from different valleys) of the band structure in many-valley semiconductors, based on the experimental curve of temperature dependence of overall concentration of charge carriers.     

Effect of impurities scattering potential on NMR relaxation rate in impure d-wave superconductors

The purpose of our research is to study the nuclear spin lattice relaxation rate of impure d-wave superconductors. We use the Green’s function method to derive the approximation equation of density of states including the impurity scattering potential. We can get the analytic equation of the nuclear spin lattice relaxation rate that contained the impurity scattering potential in case of weak scattering potential and strong scattering potential in the simple form as the power series of Δ(T) and T. The numerical calculations show that there is coherence peak in the weak impurity scattering potential but there is no peak in the strong impurity scattering potential.     

On Suhl's approach to the Kondo problem

A model for the Kondo problem is studied in which the impurities are envisaged as a gas of infinitely heavy particles embedded in the gas of conduction electrons. Fors-wave interactions and low impurity concentrations the electron self-energy is expressed by the impurity-electron scattering matrix which is also shown to determine the thermodynamic potantial. Using standard Goldstone diagrams Suhl's equation is derived by the summation of leading singular graphs. For a special density of states, vanishing magnetic field, and no potential scattering the dispersion equation is solved exactly.     

Model calculations of anisotropic electron lifetimes due to dilute substitutional impurities in molybdenum

The de Haas-van Alphen technique forms a sensitive probe of the electronic structure of dilute alloys because of its dependence on the seattering rate in the forward direction. We report the results of a first principle model calculation of the scattering rate of dilute impurities in molybdenum which focuses on the anisotropy of the host wave function on the Fermi surface. This simplified model treats the impurity potential as an atomic potential screened by a Thomas-Fermi function. Substitutional impurities in bcc molybdenum have been studied using the Greens function (KKR) wave functions. The band structure calculated via the KKR method was fitted to a Fourier series representation in order to accurately determine a sufficiently large number of states on the Fermi surface. The KKR wavefunctions were used to calculate scattering rates for the substitutional impurity since the impurity potential can be best described in an angular momentum representation which is inherent to the Greens function method. Our detailed results, to be presented, suggest additional experiment to be done.     

Phonon growth in a many valley model semiconductor at low lattice temperatures

It is shown how the traditional method of neglecting the energy of acoustic phonons and approximating their distribution by the equipartition law leads to significant errors in the phonon growth rate in a many-valley model semiconductor when the lattice temperature is low.     

The van Hemmen spin glass revisited

We simulated the van Hemmen spin glass model by multicanonical algorithm. The exact results for this mean-field model are reproduced. Physical quantities such as energy density, specific heat, susceptibility and order parameters are evaluated at all temperatures. We also studied an alternate model with short range interactions, which displays the many-valley picture in 2D for random variables having values ±1.On leave of the absence from Department of Physics Engineering, Hacettepe University, Ankara, Turkey.     

Local impurity phase pinning and pinning force in charge density waves

Starting from the static Fukuyama-Lee-Rice equation for a three-dimensional incommensurate charge density wave (CDW) in quasi one-dimensional conductors a solvable model for local phase pinning by impurities is defined and studied. We find that average CDW energy and average pinning force show critical behaviour with respect to the pinning parameter h. Specifically the pinning force exhibits a threshold at h=1 with exponent . Our model exemplifies a general concept of local impurity pinning in which the force exerted by the impurity on the periodic CDW structure becomes multivalued and metastable states appear beyond a threshold. It is found that local impurity pinning becomes less effective at low temperatures and may eventually cease completely. These results are independent of spatial dimensionality as expected for local impurity pinning. Comparison with Larkin's model is also made. Received 8 July 1998     

Scattering from impurities in a crystal

A time independent scattering theory for a particle in a crystal with impurity is given. It is shown that the scattered wave is the solution of a Lippman Schwinger equation, and that the existence of bound states or narrow resonances is related more to the band structure than to the form of the impurity potential.     

Magnetic Impurity Band and Photoinduced Magnetic Phase Transition in Diluted Magnetic Semiconductors

Applying the dynamical coherent potential approximation (dynamical CPA) to a model of diluted magnetic semiconductors (DMSs), in which both random impurity distribution and thermal fluctuation of localized spins are taken into account, the spin-polarized band and the carrier spin polarization are calculated for various magnetizations. In order to clarify the role of impurity depth on the occurrence of ferromagnetism, three typical cases are investigated: (a) II-VI DMS, (b) deep impurity level, and (c) strong exchange interaction. The present study reveals that the impurity depth of magnetic ions strongly enhances the carrier spin polarization (CSP) and accordingly, leads to a high Curie temperature. This means that photoinduced ferromagnetism with high Curie temperature can be expected in a DMS with a deep impurity depth and strong exchange interaction.     

Residual electrical resistivity of dilute nickel alloys

A quantitative calculation of the residual electrical resistivity of dilute ferromagnetic nickel-based alloys has been performed in the framework of the four-current conduction model using the kinetic equation and ab initio approaches for and ab initio approaches for the determination of the scattering potential. The contributions to the residual electrical resistivity from scattering by inhomogeneities of the impurity Coulomb potential and the exchange interaction have been separated by comparing the calculated and experimental data.     

A new type of galvanomagnetic effect for hot electrons in many-valley semiconductors

An electric field transverse to the current density, but parallel and linear with respect to the magnetic field can arise for hot electrons in many-valley semiconductors. This new type of galvanomagnetic effect is investigated for n-Si at 77 K.     

杂质离子对有机共轭聚合物中极化子动力学性质的影响

基于一维紧束缚Su-Schrieffer-Heeger模型, 采用分子动力学方法, 讨论了杂质势的强度和杂质之间的距离对电子和空穴极化子动力学性质的影响. 研究结果表明: 1)当杂质势强度保持不变时, 两杂质离子之间的距离(d)在2-16个晶格常数变化时, 电子极化子的平均速度大于空穴极化子的平均速度, 这是由于电子、空穴极化子与杂质势的库仑作用不同而产生的差异, 同时极化子的平均速度随d的增加而增大; 若继续增加杂质离子之间的距离, 电子和空穴极化子的平均速度几乎保持不变, 仅有一些微小的振荡, 这是由于不同距离的杂质离子对电子和空穴极化子产生的势垒或势阱的叠加效果不同而引起的; 2)保持两杂质离子之间的距离不变时, 随着杂质势强度的增大, 电子和空穴极化子的平均速度均减小, 且空穴极化子的平均速度减小趋势更明显.     

Interaction of Solute Impurity with Grain Boundary: The Impurity Drag Effect

An equation of grain boundary motion in a binary polycrystal is derived. The derivation is based on minimization of free energy of the total systems. The equation takes into account an impurity segregation at the grain boundary, grain boundary curvature and energy.As an example, we apply this equation to the analysis of the impurity drag effect problem. It is shown, that the sign of the impurity effect on grain boundary velocity (delay or acceleration) does not depend on kinetic coefficients. The sign of the effect is determined by a thermodynamic function which combines the grain boundary segregation coefficient, the derivative of grain boundary energy with respect to absorbed impurity concentration, and the derivative of bulk free energy with respect to bulk impurity concentration.     

The electron wind force in electromigration

The problem of the electron wind force acting on an impurity atom in a jellium metal, arising from the polarization of the electron density created by the flow of the electric current under a constant electric field, has been investigated. The impurity concentration is assumed to be small so that the impurity-impurity interaction can be neglected. We have used the scattering theory to treat the screening of the impurity by the electron gas in a nonlinear manner, and the pertubation due to the electric field is considered within the framework of the linearized Boltzmann equation. It is found that the driving force on the impurity can be expressed in terms of its residual resistivity, and this relationship is independent of the strength of the impurity potential. In the weak scattering limit the results of Bosvieux and Friedel are recovered. A close examination of the electronic polarization induced by the current, in the weak scattering limit, shows that there are terms in the electron density of second order in the impurity potential, which can be identified as the residual resistivity dipoles. However, their contribution to the driving force is found to be exactly zero.     

An exactly solvable infinite-ranged model for sliding charge-density-waves

An infinite-ranged model for the sliding charge-density-wave (CDW) with periodic harmonic impurity potentials is shown to be solvable exactly in a whole range of the electric field strength. The solution yields some distinct features of the sliding CDW which were not reported in the same infinite-ranged model but with a sinusoidal impurity potential. The results on the nonlinear conductivity are discussed in comparison with the experiments on NbSe₃.     

Electromagnetically induced transparency in coupled quantum dot-ring structure under external electric field and donor impurity

The present study seeks to scrutinize the electromagnetically induced transparency of a coupled quantum dot-ring (CQDR). The effects of hydrogenic donor impurity and external electric field on real and imaginary parts of susceptibility are investigated. The Schrödinger equation with parabolic potential is solved by diagonalization method, and the real and imaginary parts of susceptibility in terms of photon energy are calculated using expanding of the density matrix operator. The results show that the susceptibility of coupled quantum dot-ring are affected by electric field, donor impurity and the depth of confinement potential.     

Impurity bound states in a compensated quantum well

A detailed study of the bound-state properties of an impurity in a compensated semiconductor quantum well is presented using the screened potential of a minority impurity ion in a compensated semiconductor due to Schechter (1981). Accurate eigenenergies for the first 15 states are obtained for this potential as a function of the screening parameter λ by numerical integration of the two-dimensional (2D) Schrödinger equation. The energies are found to decrease with increasing values of the screening parameter λ in all cases. The variation of splitting between adjacent levels for the same value of n with the screening parameter is also studied.     

The migration of impurities in the surface layer of a solid

The migration of an impurity atom on the surface of a solid initiated by the diffusion of vacancies was studied theoretically. With multiple collisions of a separate impurity with vacancies taken into account, impurity movements became Brownian, and the mean-square displacement at long times became proportional to time. An analytic equation for the diffusion coefficient was obtained. The equation only differed by a factor from the product of the impurity diffusion coefficient by impurity concentration