Electronic structure of hydrogen and muonium in Al,Mg and Cu

The electronic structure of hydrogen and muonium in simple metals is investigated. The spherical solid model potential is used for the discrete lattice and the Blatt correction for lattice dilation. The proton and muon are kept at the octahedral sites in the fcc and hcp lattices and self-consistent non-linear screening calculations are carried out. The scattering phase shifts, electronic charge density, effective impurity potential, self-energy, charge transfer, residual resistivity and Knight shift are calculated. The spherical solid potential changes the scattering character of impurity. The phase shifts are found slowly converging. The scattering is more prominent in Al than in Mg and Cu. The virtual bound states of proton and muon are favoured in all the three metals. The calculated value of residual resistivity for CuH is in good agreement with the experimental value. The results for Knight shift forμ ⁺ in Cu and Mg are in reasonable agreement with the experimental values while those forμ ⁺ in Al are lower than the experimental value. The analytical expressions for effective impurity potential and electronic charge density are suggested.     

Infrared absorption of various isolated impurities in GaAs

We have calculated the infrared absorption at the local mode frequency for various isolated impurities such as B, Al, Mg, Si, C, and P in GaAs. To calculate the local mode frequency we take into account the effects from impurity mass defect and local force constant shifts and use the symmetry properties of the lattice. To calculate the absorption we consider contributions to the dipole moment from impurity and nearest neighbour charges and from charge migration effects. We suggest a physical model which accounts for impurity and nearest neighbour charges and polarizabilities.     

Effect of transition-metal substitution in iron-based superconductors

We study theoretically the effect of transition-metal (TM) substitution in iron-based superconductors through treating all of the TM ions as randomly distributed impurities. The extra electrons from TM elements are localized at the impurity sites. In the meantime the chemical potential shifts upon substitution. The phase diagram is mapped out and it seems that the TM elements can act as effective dopants. The local density of states (LDOS) is calculated and the bottom becomes V-shaped as the impurity concentration increases. The LDOS at the Fermi energy ρ(ω = 0) is finite and reaches the minimum at the optimal doping level. Our results are in good agreement with scanning tunneling microscopy experiments.     

Transition metal impurity effect on charge and spin density in iron: Ab initio calculations and comparison with Mössbauer data

Density functional theory was applied to study influence of the isolated impurity located on the regular site of the α-Fe crystal on the charge and spin density (hyperfine interactions) on the iron nucleus. Calculations were performed using both pseudopotential and the full potential methods. The scalar relativistic approximation was applied. Perturbations of the charge and spin density on iron were calculated for all d impurities soluble in iron and additionally for Ga impurity. It was found that impurities have measurable effect on the iron charge and spin density up to the second or third coordination shell depending on the impurity. Hyperfine parameters of iron adjacent to the impurity are affected by two intermixed physical mechanisms, i.e., the volume mismatch due to the impurity and electron band mixing caused by the electronic configuration of the impurity outer shells. Some correlations between ab initio calculations and Mössbauer experimental results are discussed. A table is provided with the parameters allowing calculate Mössbauer spectrum of the binary iron alloy with d impurity or Ga. On the other hand, provided parameters allow extraction from the Mössbauer data information about impurity concentration and eventual order.     

Vacuum polarization and screening of supercritical impurities in graphene

Screening of charge impurities in graphene is analyzed using the exact solution for vacuum polarization obtained from the massless Dirac-Kepler problem. For the impurity charge below a certain critical value, no density perturbation is found away from the impurity, in agreement with perturbation theory. For the supercritical charge, however, the polarization distribution is shown to have a power law profile, leading to screening of the excess charge at large distances. The Dirac-Kepler scattering states give rise to standing wave oscillations in the local density of states which are prominent in the supercritical regime.     

Distribution of electric field gradients around impurities in aluminum

The electric field gradients caused by a vacancy, Mg, Ga, In, Si, Ge and Sn impurities at several near-neighbor sites in Al hosts have been calculated. The theory takes into account the contribution from the conduction electron screening cloud and the lattice strain caused by the impurities. The perturbed core as well as conduction electron densities around the impurities are calculated selfconsistently using the density functional theory. Assuming that the charge distribution around the impurity is spherically symmetric, an exact expression, valid at all distances, is derived for the conduction electron contribution to the electric field gradient. While this result is substantially different from those using the conventional asymptotic or pre-asymptotic expressions, it is found to be entirely inadequate in explaining the observed asymmetry and magnitude of the electric field gradient distribution in cubic metal alloys. The contribution due to the lattice strain is calculated using the point-ion model and a new analytic form for the elastic strain tensor. The combined strain and charge screening effect provides a satisfactory agreement between calculated and experimental electric field gradients. The difficulties standing in the way of an overall quantitative understanding of the electric field gradient in cubic metal alloys are discussed. The subsequent stages of improvement in both theory and experiment that can result in a better understanding of the problem are pointed out.     

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.     

CPA density of states and conductivity in a double-exchange system containing impurities

We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated. Received 13 June 2000 and Received in final form 5 January 2001     

Visualization of nano-plasmons in graphene

We study localized plasmons at the nanoscale (nano-plasmons) in graphene. The collective excitations of induced charge density modulations in graphene are drastically changed in the vicinity of a single impurity compared to graphene's bulk behavior. The dispersion of nano-plasmons depends on the number of electrons and the sign, strength and size of the impurity potential. Due to this rich parameter space the calculated dispersions are intrinsically multidimensional requiring an advanced visualization tool for their efficient analysis, which can be achieved with parallel rendering. To overcome the problem of analyzing thousands of very complex spatial patterns of nano-plasmonic modes, we take a combined visual and quantitative approach to investigate the excitations on the two-dimensional graphene lattice. Our visual and quantitative analysis shows that impurities trigger the formation of localized plasmonic excitations of various symmetries. We visually identify dipolar, quadrupolar and radial modes, and quantify the spatial distributions of induced charges.     

Ab initio calculation of local vibrational modes by the Green’s function method. Application to GaAs:C and GaN:As

We present an ab initio technique for the calculation of vibrational modes at deep defects in semiconductors outside and inside the host-phonon bands. The dynamical matrix is calculated using density-functional theory in the local density approximation. In the results presented here all interatomic harmonic forces up to the eleventh nearest neighbour of a particular atom of the perturbed or unperturbed crystal are included. The Green's function method is used to obtain the difference of the density of phonon states between the perturbed and the perfect crystal. This technique is applied to calculate the split-off mode at the C impurity at As site in GaAs and its isotope shifts, which are in good agreement with Raman scattering experiments. It is demonstrated that the impurities generate resonances and localized modes inside the host-phonon bands. The resonances arise at specific energies of the density of phonon states of the perfect crystal which are practically independent of the chemical nature of the defect, whereas the localized modes show distinct impurity or ligand isotope shifts. Our calculations of GaAs and cubic GaN lead to the assignment of a number of low energy Raman-scattering peaks between 7.2 meV and 31.0 meV, observed at a layer of cubic GaN on a GaAs substrate, to resonances inside the phonon bands of GaAs and GaN. Received 5 March 1999     

Bondlength distortion of atomic substitutions in semiconductors

The distortion in bondlength is calculated for a wide range of impurities, both isovalent and heterovalent, in all I-VII, II-VI and HI-V compound semiconductors and in elemental semiconductors, silicon and germanium. Universal parameter tight binding (UPTB) theory of Harrison combined with the spring constant model is used. The results agree well with earlier theoretical estimates and also with the available extended X-ray absorption fine structure (EXAFS) data. A systematic behaviour of the bondlength distortion of impurities in semiconductors with respect to (i) the difference in covalent energy of the host-impurity bond, (ii) the difference in electronegativity between the host and that of the impurity atom and (iii) the change in covalency of the bond when the impurity replaces the host is observed.     

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     

Screening of light impurities in simple metals

The Kohn-Sham density functional method is used to calculate self-consistent electron densities around H, He and Li impurities in jellium host corresponding to Li, Al and Mg metals. The differences in the density profiles for interstitial and substitutional impurities are investigated. Residual resistivities, relaxation energies and charge transfer in dilute alloys are evaluated.     

Calculations on the three-nucleon system using a separable expansion of local potentials

A separable expansion for local potentials called the unitary pole expansion (UPE) has been applied to the singlet-S soft-core Reid potential. The separable potentials obtained are used in conjunction with a separable tensor potential to calculate the triton binding energy, the nd doublet scattering length and the nd phase shifts in the state above and below break up. The convergence of the UPE is good especially for repulsive terms. The one term approximation (UPA) gives a triton energy differing by 0.04 MeV and a doublet scattering length differing by 0.14 fm from the values found for the local potential. The calculated phase shifts are in good agreement with the phase-shift analysis of Van Oers and Seagrave.     

Tunneling density of states for the two-band model of superconductivity

In the presence of interband scattering of quasiparticles by impurities only a single gap in the excitation spectrum is found. The dependence of this gap and the two order parameters on interband coupling and impurity content is discussed. Graphical representations of the density of states for several sets of parameters are given. For certain ranges of parameters our calculated density of states is qualitatively similar to the experimental results.     

Local mode sideband IR absorption and second harmonic intensity calculation for various isolated impurities in GaAs

We have calculated the intensity and shape of the local mode sidebands and the second harmonic intensity for various isolated impurities in GaAs. To calculate the absorption we have considered the second order dipole moment in the harmonic approximation. The absorption depends on the values of the impurity charge and polarizability, on the corresponding quantities for the nearest neighbours of each impurity and on a charge migration factor. The latter accounts for charge variations during the motion. Although these quantities vary from impurity to impurity, the results are presented in a general form valid also for other impurities. The point group symmetry properties are used in order to give a physical meaning to the results. The effects on the dynamical properties from the mass defect and the local changes in the force constant are also taken into account.     

Multiple scattering theory of the density of states in semiconductors

A method for calculating the electronic density of states of a semiconductor in the presence of impurity scattering is given. The approach is based on a low density multiple scattering expansion. The calculation treats the density of states of the conduction band, shifts of the band edge, and the profiles of the impurity bands. It is performed for a simple model, using a spherical well potential for the individual impurity centers, but could be extended to any scattering potential. The first impurity band forms when the potential is sufficiently strong that one center can bind an electron. The impurity bands are of finite width, have a flat upper edge, and an asymmetrically broadened tail on the low energy side.     

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.     

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.     

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/AlxGa1-xN heterojunctions under pressure

The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/Al x Ga 1 x N heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation.The variations of Stark energy shift with electric field,impurity position,Al component and areal electron density are discussed.Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states.For a given impurity position,the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect.The weakening of the blue and red shifts,induced by the screening effect,strengthens gradually with the increase of electric field.Furthermore,the screening effect weakens the mixture crystal effect,thereby influencing the Stark effect.The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.