Statistical description of the motion of dislocation kinks in a random field of impurities adsorbed by a dislocation

A model has been proposed for describing the influence of impurities adsorbed by dislocation cores on the mobility of dislocation kinks in materials with a high crystalline relief (Peierls barriers). The delay time spectrum of kinks at statistical fluctuations of the impurity density has been calculated for a sufficiently high energy of interaction between impurities and dislocations when the migration potential is not reduced to a random Gaussian potential. It has been shown that fluctuations in the impurity distribution substantially change the character of the migration of dislocation kinks due to the slow decrease in the probability of long delay times. The dependences of the position of the boundary of the dynamic phase transition to a sublinear drift of kinks x ∝ t^δ (δ σ 1) and the characteristics of the anomalous mobility on the physical parameters (stress, impurity concentration, experimental temperature, etc.) have been calculated.     

Kinetics of impurity segregation at grain boundaries in polycrystals. II. Concentrated solution

The isothermal impurity segregation from a finite-size grain into an intergrain boundary region or at an external free surface is investigated when the impurity concentration in the boundary region is not small. Simple algebraic equations are obtained that describe the impurity concentration for the case of several competing and interacting impurities. The process of segregation of two impurities is discussed in detail, and it is shown that the concentration of one of them can have a maximum as a function of time. Fiz. Tverd. Tela (St. Petersburg) 40, 251–253 (February 1998)     

A Bijection Theorem for Domino Tilings with Diagonal Impurities

We consider the dimer problem on a planar non-bipartite graph G, where there are two types of dimers one of which we regard as impurities. Computer simulations reveal a reminiscence of the Cheerios effect, that is, impurities are attracted to the boundary, which is the motivation to study this particular graph. Our main theorem is a variant of the Temperley bijection: a bijection between the set of dimer coverings and the set of spanning forests with certain conditions. We further discuss some implications of this theorem: (1) the local move connectedness yielding an ergodic Markov chain on the set of all possible dimer coverings, and (2) a rough bound for the number of dimer coverings and that for the probability of finding an impurity at a given edge, which is an extension of a result in (Nakano and Sadahiro in ).     

Grain Boundary Migration in Metals: Recent Developments

Current research on grain boundary migration in metals is reviewed. For individual grain boundaries the dependence of grain boundary migration on misorientation and impurity content are addressed. Impurity drag theory, extended to include the interaction of adsorbed impurities in the boundary, reasonably accounts quantitatively for the observed concentration dependence of grain boundary mobility. For the first time an experimental study of triple junction motion is presented. The kinetics are quantitatively discussed in terms of a triple junction mobility. Their impact on the kinetics of microstructure evolution during grain growth is outlined.     

Mobility of electrons upon scattering by a multicomponent correlated system of impurity centers

A system that contains two sorts of impurity centers spatially distributed in a random way is considered. Not all impurities of the first sort are ionized, and all the impurities of the second sort are ionized. Spatial correlations in the system of impurity ions of the first sort are investigated under conditions when the correlation radius of an impurity ion is limited from above due to a deficit of neutral impurities. The influence of randomly spatially arranged small-sized donors (impurities of the second sort) on correlations in the system of impurity ions is analyzed. The equations for describing the effect of small-sized donors on correlations in the system of impurity ions are obtained. The electron mobility at zero temperature is calculated by the example of HgSe: Fe (the correlated system of impurity centers consists of iron atoms and small-sized donors whose concentration is higher than the Mott concentration).     

Tuning the effective fine structure constant in graphene: opposing effects of dielectric screening on short- and long-range potential scattering

We reduce the dimensionless interaction strength alpha in graphene by adding a water overlayer in ultrahigh vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30%, due to the background dielectric constant enhancement leading to a reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short-range impurities is decreased by almost 40%, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron-hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.     

Calculation of the anisotropic mobility in (110) AlAs quantum wells at zero temperature

We calculate the mobility of the two-dimensional electron gas as realized in (110) AlAs quantum wells at zero temperature. In this structure the mass is strongly anisotropic which gives rise to an anisotropic mobility. By using a theoretical approach developed by Tokura [Phys. Rev. B 58, 7151 (1998)] we numerically calculate the anisotropic mobility. We study impurity scattering in quantum wells having an ellipsoidal Fermi surface. We find that increasing the electron density and/or the well width results in reduction of the anisotropy of the mobility while the anisotropy in the scattering time is increased. A strong dependence of the mobility anisotropy on the impurity position is predicted. Excellent agreement with a recently published experimental result is found under the assumption that impurities are located at the edge of the quantum well.     

Effect of magnetic impurities in chiral <Emphasis Type="Italic">p</Emphasis>-wave superconducting nanoloops

The effect of many magnetic impurities in symmetric chiral p-wave superconducting nanoloops is investigated by numerically solving the BdG equations self-consistently. Two magnetic impurities can lead to the appearance of two impurity bound levels close to the Fermi level. The arising bound states can cross the Fermi level at the same impurity strength for the case of two independent midway impurities, while multiple zero-energy states can be obtained at two separated values of impurity strength when two independent edge impurities are present. Moreover, the multiple zero modes can only show up for appropriate relative positions between two edge impurities due to the quantum interference effect. Particularly, for some appropriate strength of two independent midway impurities, the impurity bound levels cross the Fermi level twice with increasing threaded flux, while the multiple zero modes can not emerge in the flux evolution.     

Theory of Triple Junctions Mobility in Crystals with Impurities

We consider the steady state migration of the triple junction in the tricrystal with impurities which segregate strongly at the grain boundaries. If the mobility of impurities inside grain boundaries is much higher than the rate of impurity atoms jumps from the grain boundary into the bulk, the triple junction migration causes the divergence of the impurity content at the triple point. We show that this divergence can be relaxed either by the non-equilibrium segregation at the growing grain boundary or by the formation of the inclusion of the impurity-rich phase at the triple point. In the former case the dihedral angle at the triple point differs considerably from its equilibrium value and is strongly temperature-dependent. However, the triple junction cannot be described as an individual object with its own mobility. In the latter case of the cavity formation at the triple point the triple junction can be characterized by its own mobility. It is shown that the dependence of the triple junction migration rate on the driving force is approximately linear at the low migration rates and highly nonlinear at high migration rates. Moreover, there is the maximal allowable steady-state migration rate of the system triple junction-inclusion. For the higher migration rates the jerky motion of the triple junction occurs. Both models are in a good agreement with the experimental data.     

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.     

Conductance of metallic nanoribbons with defects

The conductances of two typical metallic graphene nanoribbons with one and two defects are studied using the tight binding model with the surface Green’s function method. The weak scattering impurities, U ～ 1 eV, induce a dip in the conductance near the Fermi energy for the narrow zigzag graphene nanoribbons. As the impurity scattering strength increases, the conductance behavior at the Fermi energy becomes more complicated and depends on the impurity location, the AA and AB sites. The impurity effect then becomes weak and vanishes with the increase in the width of the zigzag graphene nanoribbons (150 nm). For the narrow armchair graphene nanoribbons, the conductance at the Fermi energy is suppressed by the impurities and becomes zero with the increase in impurity scattering strength, U > 100 eV, for two impurities at the AA sites, but becomes constant for the two impurities at the AB sites. As the width of the graphene nanoribbons increases, the impurity effect on the conductance at the Fermi energy depends sensitively on the vacancy location at the AA or AB sites.     

Classical impurities and boundary Majorana zero modes in quantum chains

We study the response of classical impurities in quantum Ising chains. The Z₂${\mathbb{Z}}_2$ degeneracy they entail renders the existence of two decoupled Majorana modes at zero energy, an exact property of a finite system at arbitrary values of its bulk parameters. We trace the evolution of these modes across the transition from the disordered phase to the ordered one and analyze the concomitant qualitative changes of local magnetic properties of an isolated impurity. In the disordered phase, the two ground states differ only close to the impurity, and they are related by the action of an explicitly constructed quasi-local operator. In this phase the local transverse spin susceptibility follows a Curie law. The critical response of a boundary impurity is logarithmically divergent and maps to the two-channel Kondo problem, while it saturates for critical bulk impurities, as well as in the ordered phase. The results for the Ising chain translate to the related problem of a resonant level coupled to a 1d p-wave superconductor or a Peierls chain, whereby the magnetic order is mapped to topological order. We find that the topological phase always exhibits a continuous impurity response to local fields as a result of the level repulsion of local levels from the boundary Majorana zero mode. In contrast, the disordered phase generically features a discontinuous magnetization or charging response. This difference constitutes a general thermodynamic fingerprint of topological order in phases with a bulk gap.     

Optically enhanced hall mobility in GaAs

Photo-Hall effect measurements on n-type LPE GaAs: Si between 15 and 60 K indicate an enhancement of the Hall mobility in the ionized impurity scattering conduction regime. The mobility is enhanced by a factor of 2 upon illumination with band gap light of intensity 7 × 10¹⁴ photons/cm² sec. Quantitatively, this enhancement is partially accounted for in terms of a screening of impurities by photo-generated carriers. We discuss several other mechanisms which could also contribute to an enhancement, and conclude that an increase in carrier temperature and/or a decrease in the number of scattering centers through photo-neutralization are primarily responsible for the remainder of the enhancement.     

Cluster mobility in a system of light interstitial impurities

The contribution of cluster mobility to the low-temperature diffusion coefficient in a system of light interstitial impurities has been studied by computer simulation. It is shown that the magnitude of this contribution is dominated by the parameters of the impurity interaction potential, and the nonmonotonic character of its temperature behavior is related to a variation of cluster shape or freezing-out of one of several mobility mechanisms.     

Two magnetic impurities with arbitrary spins in open boundary t-J model

From the open boundary t-J model an impurity model is constructed in which magnetic impurities of arbitrary spins are coupled to the edges of the strongly correlated electron system. The boundary R matrices are given explicitly. The interaction parameters between magnetic impurities and electrons are related to the potentials of the impurities to preserve the integrability of the system. The Hamiltonian of the impurity model is diagonalized exactly. The integral equations of the ground state are derived and the ground state properties are discussed in detail. We discuss also the string solutions of the Bethe ansatz equations, which describe the bound states of the charges and spins. By minimizing the thermodynamic potential we get the thermodynamic Bethe ansatz equations. The finite size correction of the free energy contributed by the magnetic impurities is obtained explicitly. The properties of the system at some special limits are discussed and the boundary bound states are obtained.     

Impurity effects on adhesive energies

Some simple arguments are made about the effect of impurities on adhesive interactions at solid junctions. A universal adhesive force relation is derived for brittle adhesion. The adhesive binding energy, ΔE, is an important parameter in brittle adhesion forces. ΔE has also been shown by others to be important when there is plastic flow. We found that impurity effects on ΔE are determined by the segregation energies to the junction and to the free surfaces. At low temperatures, if it is energetically more favorable for impurities to segregate to the surfaces than to the junction, then the impurities will decrease ΔE. The converse is also true. For example, for self junctions which are in registry, ΔE is decreased if surface segregation is exothermic and increased if it is endothermic. These segregation energy relationships are consistent with the results of a number of experiments on the effects of impurities on adhesion forces and grain boundary embrittlement.     

Noninteracting electrons and the metal-insulator transition in two dimensions with correlated impurities

While standard scaling arguments show that a system of noninteracting electrons in two dimensions and in the presence of uncorrelated disorder is insulating, in this paper we discuss the case where interimpurity correlations are included. We find that for pointlike impurities and an infinite interimpurity correlation length, a mobility edge exists in 2D even if the individual impurity potentials are random. In the uncorrelated system we recover the scaling results, while in the intermediate regime for length scales comparable to the correlation length, the system behaves like a metal but with increasing fluctuations, before strong localization eventually takes over for length scales much larger than the correlation length. In the intermediate regime, the relevant length scale is given by the interimpurity correlation length, with important consequences for high mobility systems.     

硅中钛杂质对激光电场分布影响的数值模拟

采用T-matrix数值方法模拟光学系统常用的光学材料硅中金属钛杂质对激光电场分布的影响。分析了不同球形杂质数目、不同杂质球半径、不同球形杂质间距、不同球形杂质排列以及不同相对介电常数杂质对激光电场分布的影响,通过数值模拟了解到:电场幅值的最大值总是出现在沿入射波极化方向上,电场幅值的增大与相邻两球的耦合有关,且两球间距越小电场幅值的最大值越大;相对介电常数实部越小、虚部越大的杂质会导致较大的电场幅值最大值。     

Influence of interstitials and non magnetic impurities on the Kondo effect

The influence of interstitials and non magnetic impurities on the anomalous resistivity, thermopower and Kondo temperature of dilute magnetic alloys was investigated generalizing a model proposed by Bohnen and Fischer. Numerical results are given as a function of the distance between the interstitial (or non magnetic impurity) and the magnetic impurity using their scattering phase shifts as parameters. The Kondo anomalies are altered considerably, if the magnetic impurity is very close to the non magnetic scattering potential, e.g. if it is part of an interstitial dumbbell.This work is part of a doctoral thesis of G.Wehr at the Technische Universität München     

Grain boundary migration initiated by diffusion

A review is presented of work related to two new processes which arise in crystalline materials when impurities diffuse along grain boundaries from the surface of the material: diffusion-initiated grain boundary migration and recrystallization. We analyze the conditions under which DIGM occurs, the kinetics of the process, its driving forces, and also the changes in the grain fine structure and the near-grain regions, as well as the nucleation of new grains on the migrating boundaries, caused by an uncompensated impurity atom flux. We consider the mechanisms for DIGM. It is shown that not one of the mechanisms proposed to describe DIGM is capable of explaining all the experimentally observed properties of grain boundary migration under DIGM conditions. We note that changes in the grain boundary structure caused by the impurity atoms diffusing along it are due to diverse grain-boundary processes, which have important technological implications.V. D. Kuznetsov Physicotechnical Institute, Tomsk State University, Siberia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 34–57, May, 1992.