Screening of coulomb impurities in graphene

We calculate exactly the vacuum polarization charge density in the field of a subcritical Coulomb impurity, Z|e|/r, in graphene. Our analysis is based on the exact electron Green's function, obtained by using the operator method, and leads to results that are exact in the parameter Zalpha, where alpha is the "fine-structure constant" of graphene. Taking into account also electron-electron interactions in the Hartree approximation, we solve the problem self-consistently in the subcritical regime, where the impurity has an effective charge Z(eff), determined by the localized induced charge. We find that an impurity with bare charge Z=1 remains subcritical, Z(eff)alpha<1/2, for any alpha, while impurities with Z=2, 3 and higher can become supercritical at certain values of alpha.     

Contribution electronique au gradient de champ electrique sur une impurete de transition dans un environnement metallique anisotrope

The experimental data related to the electric field gradient at transition impurities either in hexagonal metals, or in cubic metals where the isotropy is perturbed by a next impurity, can be explained neither by the lattice contribution nor by the electronic contribution from the conduction band. A model is proposed here to investigate the electronic contribution arising from virtual bound 3d states on the impurity, by studying the local crystal field influence in a Friedel-Anderson model. It appears that at the 0°K limit, the localized electronic contribution to the EFG can be linearly related to the density n_d(?_F) of 3d states at the Fermi level. As a first approximation, this law is valid even at temperature different from 0°K so establishing a linear correlation between the EFG, the impurity resistivity and the amplitude of the charge perturbation around the impurity.     

Coulomb screening in graphene with topological defects

We analyze the screening of an external Coulomb charge in gapless graphene cone, which is taken as a prototype of a topological defect. In the subcritical regime, the induced charge is calculated using both the Green’s function and the Friedel sum rule. The dependence of the polarization charge on the Coulomb strength obtained from the Green’s function clearly shows the effect of the conical defect and indicates that the critical charge itself depends on the sample topology. Similar analysis using the Friedel sum rule indicates that the two results agree for low values of the Coulomb charge but differ for the higher strengths, especially in the presence of the conical defect. For a given subcritical charge, the transport cross-section has a higher value in the presence of the conical defect. In the supercritical regime we show that the coefficient of the power law tail of polarization charge density can be expressed as a summation of functions which vary log periodically with the distance from the Coulomb impurity. The period of variation depends on the conical defect. In the presence of the conical defect, the Fano resonances begin to appear in the transport cross-section for a lower value of the Coulomb charge. For both sub and supercritical regime we derive the dependence of LDOS on the conical defect. The effects of generalized boundary condition on the physical observables are also discussed.     

On the local quasiparticle density of states within the asymmetric Anderson model

We calculate the local density of states for the asymmetric Anderson model by applying a self-consistent perturbation approximation for the impurity electron's self-energy. The three-resonance structure obtained in the symmetric case at low temperatures, which features a narrow central peak at the Fermi level, is found to transform into a broad single resonance with increasing asymmetry. For any asymmetry the spectral density function approaches in the high temperature limit a broad two-resonance structure.     

Impurity pairs and excitation relaxation in doped silicon

The kinetics of photoconductivity is studied in silicon doped with B, Al, Ga, In, P, As, and Sb with concentrations of 10₁₆–10₁₈cm_?3 at 4.2 and 10.5 K placed in an 8-mm microwave electric field under pulsed impurity excitation. It is found that infrared absorption by impurity pairs and a slow component of photoresponse relaxation arise at close impurity concentrations. It is shown that this component is due to an increase in the polarization hopping conductivity in the presence of the optical charge exchange of impurity states—isolated impurities and impurity pairs and dipoles (pairs of the major and compensating impurities). The hopping transfer processes of ion charges in the course of relaxation are analyzed. It is shown that the main contribution to polarization photoconductivity comes from hopping transitions in impurity pairs at relatively small concentrations and from hopping with the participation of isolated ions at increased concentrations.     

Spin and charge polarization around magnetic impurities in metals

Our previously developed trial function scheme has been generalized to investigate the spin and charge polarization around the nearest neighbors of the impurity, and the influence of the polarization on the formation of local moments. While the reduction of impurity moment is mostly due to the charge polarization effect, both spin and charge polarizations together yield a polarized moment which compensates up to 60% of the impurity moment. Special case of cluster with completely compensated moment is also considered.     

Local density of states of one-dimensional Mott insulators and charge-density wave states with a boundary

We determine the local density of states of one-dimensional incommensurate charge-density wave states in the presence of a strong impurity potential, which is modeled by a boundary. We find that the charge-density wave gets pinned at the impurity, which results in a singularity in the Fourier transform of the local density of states at momentum 2k_{F}. At energies above the spin gap we observe dispersing features associated with the spin and charge degrees of freedom, respectively. In the presence of an impurity magnetic field we observe the formation of a bound state localized at the impurity. All of our results carry over to the case of 1D Mott insulators by exchanging the roles of spin and charge degrees of freedom. We discuss the implications of our result for scanning tunneling microscopy experiments on spin-gap systems such as two-leg ladder cuprates.     

To the theory of phase transitions in relaxors

It is shown that the process of charge carrier localization on impurity centers is of importance. Localized carriers produce local electric fields, thereby stimulating the appearance of an induced polarization near the phase transition point. The direction of this polarization is dictated by the spatial distribution of the centers occupied by charge carriers. The temperature dependence of the dielectric constant is determined by the dynamics of the local-center occupation with decreasing temperature. The dispersion of the dielectric constant is determined by the vibrational characteristics of the local states forming near a localized charge. The phase transitions in relaxors are considered in the framework of the thermodynamic approach.     

The effect of bulk impurities on localized surface states

The effect of the occurrence of bulk impurities in a crystal on the existence and localization of surface states is examined by investigating a simple one-dimensional model. It is found that the bulk impurity has a small effect on the existence conditions for localized states associated with the surface, but the degree of localization of the corresponding charge density can be altered appreciably by the presence of such impurities. The penetration depth associated with localized surface levels can be much greater that than estimated for a pure crystal. The charge density associated with the electrons which occupy localized surface states may vary in a real crystal over a broader region near the surface than would be deduced from models of pure crystals. This will affect physical quantities, such as surface impedance, which are dependent on this charge distribution.     

Screening of Local Magnetic Moment by Electrons of Disordered Graphene

Based on the Anderson impurity model and self-consistent approach, we investigate the condition for the screening of a local magnetic moment by electrons in graphene and the influence of the moment on electronic properties of the system. The results of numerical calculations carried out on a finite sheet of graphene show that when the Fermi energy is above the single occupancy energy and below the double occupancy energy of the local impurity, a magnetic state is possible. A phase diagram in a parameter space spanned by the Coulomb energy U and the Fermi energy is obtained to distinguish the parameter regions for the magnetic and nonmagnetic states of the impurity. We find that the combined effect of the impurity and finite size effect results in a large charge density near the edges of the finite graphene sheet. The density of states exhibits a peak at the Dirac point which is caused by the appearance of the edge states localized at the zigzag edges of the sheet.     

非对称Anderson模型的重整微扰展开

利用Yamada微扰论结合重整微扰方法来计算非对称Anderson模型,得到了局域电子占据数、重整化因子、重整化的局域能级以及重整化参数关于裸参数的展开式.计算了局域电子态密度和低温杂质电导,还计算了磁场对它们的影响,这些结果适用于从弱耦合到强耦合的整个耦合强度区域.由于在哈密顿量中已经将局域能级进行了初步重整,采用的重整微扰方法比Hewson的重整微扰方法更适合于研究非对称Anderson模型. 关键词： 非对称Anderson模型 重整化 磁场     

Double layers of H2 adsorption on an AlN sheet induced by electric field

In this paper, we propose an original functional method that makes it easy to determine the effect of any deviation in the shape of a nano-object from the well-studied shape (e.g., spherical) on the quantum characteristics of charge localized inside the nano-object. The maximum dimension of the object is determined by the magnitude of influence of quantum-size effects on quantum states of charge, and is limited by 100?nm. This method is ideologically similar to the perturbation theory, but the perturbation of the surface shape, rather than the potential, is used. Unlike the well-known variational methods of theoretical physics, this method is based on the assumption that the physical quantity is a functional of surface shape. Using the method developed, we present the quantum-size state of charges for two different complex shapes of nano-objects. The results from analyzing the quantum-size states of charge in the nano-objects with a deformed spherical shape indicated that the shape perturbations have a larger effect on the probability density of locating a particle inside the nano-object than on the surface energy spectrum and quantum density of the states.     

杂质P对α-Fe中刃型位错上扭折电子结构的影响

利用离散变分方法和DMol方法,研究了P对bcc Fe中［100］(010)刃型位错上扭折电子结构的影响,计算了杂质偏聚能、原子间相互作用能、电荷密度及态密度.计算结果表明：微量P引入体系后,电荷发生了重新分布,P原子得到电子,其周围Fe原子失去电子,由于P原子的3p轨道与近邻Fe原子的3d4s4p轨道之间杂化,使P原子与近邻Fe原子间有较强的相互作用,不利于扭折的迁移,使位错运动受阻,有利于材料强度的提高.同时,杂质P原子与基体原子间的成键主要是d,p轨道起作用,使得它们之间的成键有较强的方向性,有可能 关键词： 电子结构 刃型位错 扭折 杂质元素     

Spin-dependent current in resonant tunneling diode with ferromagnetic GaMnN layers

The spin-polarized tunneling current through a double barrier resonant tunneling diode (RTD) with ferromagnetic GaMnN emitter/collector is investigated theoretically. Two distinct spin splitting peaks can be observed at current-voltage (I-V) characteristics at low temperature. The spin polarization decreases with the temperature due to the thermal effect of electron density of states. When charge polarization effect is considered at the heterostructure, the spin polarization is enhanced significantly. A highly spin-polarized current can be obtained depending on the polarization charge density.     

Atomic collapse and quasi-Rydberg states in graphene

Charge impurities in graphene can host an infinite family of Rydberg-like resonance states of massless Dirac particles. These states, appearing for supercritical charge, are described by Bohr-Sommerfeld quantization of collapsing classical trajectories that descend on point charge, in analogy to the hydrogenic Rydberg states relation with planetary orbits. Strong tunnel coupling of these states to the Dirac continuum leads to resonance features in scattering on the impurities that manifest themselves in transport properties and in the local density of states.     

Effect of Ba in KDP crystal on the wavelength dependence of laser-induced damage

Ba impurity in potassium dihydrogen phosphate(KDP) is studied with the first-principle simulation method. The relaxed configurations and density of the states of KDP crystal with Ba impurity are calculated. We find that Ba can generate a K vacancy and an interstitial O-H unit for charge compensation. The band gap of KDP crystal narrowed down to about 3.9 eV,which is consistent with the experimental data from previously reported studies and indicates that Ba may be a source of low-damage threshold.     

Theory of the fano resonance in the STM tunneling density of states due to a single kondo impurity

The conduction electron density of states nearby single magnetic impurities, as measured recently by scanning tunneling microscopy (STM), is calculated, taking into account tunneling into conduction electron states only. The Kondo effect induces a narrow Fano resonance in the conduction electron density of states. The line shape varies with the distance between STM tip and impurity, in qualitative agreement with experiments, but is very sensitive to details of the band structure. For a Co impurity the experimentally observed width and shift of the Kondo resonance are in accordance with those obtained from a combination of band structure and strongly correlated calculations.     

Spectral and time resolved optical emission of a nonlinear dimer model

The relations between the different stationary and nonstationary states of molecular excitations described by a nonlinear dimer model with cubic site polarization terms and the corresponding optical emission spectra are considered in detail. For the stationary states emission is explicitly calculated for both the symmetric case with equal optical coupling constants of both molecules constituting the dimer as well as the asymmetric case where one of the molecules is optically active only. The time-resolved emission from the nonstationary states is treated by using a quasistationary approximation based on slow transfer dynamics compared to the polarization. Of particular interest in the nonstationary case is the chaotic transfer regime induced by a harmonic perturbation. For the latter case the evolution of the spectrum from an ensemble of dimers spreading within the stochastic layer on the Bloch sphere is presented.     

Interpretation of isomer shifts of substitutional119Sn and129I in group IV semiconductors

The measurements of the isomer shift in Mössbauer effect of substitutionally implanted¹¹⁹Sn and¹²⁹I atoms in group IV semiconductors show that the contact density of 5s electrons at the impurity ńucleus decreases when going from Ge to Si and diamond. A satisfactory interpretation of this dependence can be given based on a simple perturbation approach: Interaction of the host crystal with the bonding valence electrons of the impurity atom causes a charge redistribution of these electrons and results in decrease of occupancy of the 5s level at the impurity atom. Some general aspects of this problem with substitutional isoelectronic and donor impurities are discussed.     

Density of states in the impurity d band and inelastic electron scattering by a system of mixed-valence iron ions in HgSe: Fe crystals

The resistivity and the Hall coefficient of HgSe: Fe crystals with various iron content are experimentally studied in the temperature range 1.3≤T≤300 K and in magnetic fields up to 60 kOe. The temperature dependences of the density and mobility of conduction electrons in these crystals are determined. The influence of spatial charge ordering in the system of mixed-valence iron ions on impurity states in HgSe: Fe crystals is considered. The density of states in the impurity d band is theoretically analyzed, and inelastic electron scattering in which bi-and trivalent iron ions are recharged is discussed. It is shown that the experimentally detected features in the dependence of the density and mobility of electrons on temperature and iron impurity content can be explained by the influence of Coulomb correlations in the mixed-valence iron ion system on the structure of the impurity d band.