Host Band Structure Research Articles

Temperature dependence of Ce-emission kinetics in YAG:Ce optical ceramic

We have measured the scintillation decay kinetics of the Ce-emission from a YAG:Ce optical ceramic material over a temperature range between 110 and 450 K. Above room temperature the decay can be well described by the sum of two exponential terms, but below room temperature a third term is needed. This new component has a characteristic time shorter than the radiative lifetime of the Ce ion, varying from 33 ns at 110 K to 18 ns at 300 K. The presence of the short component can be explained in terms of the distortion of the YAG host band structure and the introduction of surface-boundary states into the otherwise forbidden energy region, which drains the emitting 5d levels of Ce ions located in the vicinity of the grain-boundaries.

Theory of incompletely isovalent -doped semiconductors

In order to calculate the dependence of the electronic structure of single -doped layers on the area concentration c of isovalent impurities, the averaged t-matrix approximation (ATA) is applied to the case of a few impurities within a layer, and generalized to the case of a few missing impurities in an otherwise completely filled layer. In this way, exact results for the limits and are obtained. An interpolation between these limits results in Green's functions which allow the calculation of c-dependent electronic properties. In the case of GaAs:In, the top of the heavy-hole subband is calculated for a short-range (-like) difference between the host atom (Ga) and the impurity (In) potentials on the basis of a parabolic host band structure as well as by applying a three-band Hamiltonian. The comparison with the results from a virtual-crystal approximation proves the latter to describe the subbands well, provided that single impurities do not bind charge carriers. The subband dispersion for heavy holes, calculated for AlAs:Ga on the basis of a realistic ten-band Hamiltonian, shows a camel-back structure caused by the warping of the host heavy-hole band. This structure is proved to result in a pronounced high-energy peak of the subband density of states, in contrast to its step-like behaviour in the case of parabolic bands.

Interpretation of the equilibrium composition of ammoniated transition metal disulfides

In order to study intercalates formed by the complete reaction of ammonia with TiS 2 and NbS 2, very long reaction times are required. These intercalates attain equilibrium ammonium concentrations of 25 mole% according to the formula (NH + 4) y' (NH 3) y' TS y'- 2, where T=Ti of Nb, only after reaction periods of months. A value of y'=0.25 also is consistent with the charge transfer of 0.25 e -/TaS 2 observed for ammoniated TaS 2. The above ammonium concentration is in good agreement with a proposed 2 a×2 a ammonium superstructure for the above stage I intercalates, suggesting that the extent of ammonium formation is not primarily governed by the host band structure, which is substantially different for the above hosts, but by Coulombic intercalant repulsions. This superstructure is consistent with the presence of planar ammonia in (ND + 4) 0.22(ND 3) 0.34TiS 0.22- 2, since the ammonia positions are always midway between nearest-neighbor ammonium positions, with the ammonia plane perpendicular to the corresponding ammonium-ammonium vector. The sensitivity of the ammonia structure to guest-guest interactions is evidenced by the observation of a non-planar ammonia structure for Li + 0.23(ND 3) 0.63TiS 0.23- 2, where ammonia cannot be symmetrically and linearly coordinated by Li +.

μ+knight shifts and trapping in diluteSbSn alloys

Giant μ+ Knight shifts Kμ have been studied previously in Sb andSbBi alloys. Here we report μ+SR measurements on Sb with dilute heterovalent Sn impurities. A dramatic concentration dependence is observed: Kμ is increased slightly (relative to the value in pure Sb) by Sn concentrations of 60K. The amplitude of the low Kμ signal grows with increasing T at the expense of the amplitude of the high-Kμ, low-T signal, suggesting that the μ+ migrates through the host lattice to trap sites. A simple trapping model correctly describes the observed T-dependence of the amplitudes, phases and relaxation rates of the two signals. We conclude that the low-T Knight shift is truly characteristic of the host band structure while the much lower Kμ value of the high-T site is characteristic of a specific trap site, presumably adjacent to a Sn impurity.

Optical absorption in binary semiconductors containing substitutional impurities

AbstractThe optical absorption spectrum of binary semiconductors with substitutional impurities is studied in terms of a one‐electron theory. A previous tight‐binding calculation of the imaginary part ε2(ω) of the dielectric constant in a two‐independent‐band insulator is extended to a general hybridized multiband binary semiconductor as well as to interatomic optical excitations. As in the previous simplified model it is shown that it is still possible to explore the critical points of the host band structure through wave‐vector non‐conserving transitions due to the presence of the impurity. In order to visualize that, a numerical calculation is performed in the case of a binary linear chain of atoms containing one substitutional impurity.

Effect of the Host Band Structure on Capture and Emission Processes at DX Centers in AlGaAs

Effect of the Host Band Structure on Capture and Emission Processes at DX Centers in AlGaAs

Determination of the bulk elastic properties of silicon via the Chadi method

One of the simplest and yet most successful methods for determining the bulk elastic properties and surface geometries of the covalent solids has been the Chadi total energy minimization scheme. As a result of recent work, however, there are now strong grounds for questioning the validity of one of the basic assumptions which have been employed in implementing this particular approach. The assumption in question is the so-called 1 d 2 approximation whereby the tight-binding matrix elements appearing in the model Hamiltonian description of the host bandstructure are assumed to scale simply as the inverse square of the appropriate interatomic distance. We have therefore endeavoured to test this 1 d 2 model, and the possible alternative of using data derived from pressure dependent bandstructure calculations, by employing them both within the Chadi scheme in a determination of some of the lattice dynamical properties of silicon. In an earlier publication we have reported the results of such calculations on the two elastic constants C 11- C 12 and C 44, and some of the Γ and X point phonon mode distortions. In the present paper we extend these silicon calculations to incorporate the various L-point phonon modes. This enables us to draw quite general conclusions concerning the relative merit of these two spatial models, and the overall reliability of the Chadi total energy minimization scheme.

On the solution of the Boltzmann equation for anisotropic electron-impurity scattering in metals

Sondheimer's exact solution of the Boltzmann equation with a degenerate kernel takes a particularly simple form when the scattering can be described by a small number of phase shifts. The authors discuss the practical implementation of this form, stressing its economy (which is comparable with commonly used non-trivial approximations) and its generality (independence of the formalism used to compute the phase shifts). Various approximations are compared with the exact resistivity as functions of the phase shifts, using aluminium as the host metal. This comparison helps distinguish several 'band-structure' effects as compared with free-electron predictions. The most dramatic band-structure effect (in the example of aluminium) is the vanishing of the resistivity as all phase shifts except for the s wave approach zero. They discuss the potential usefulness of these results in free-electron-based calculations as well as those which include host band structure from the beginning.

Energy Structure and Quantized Hall Effect of Two-Dimensional Holes

Combined magnetotransport and cyclotron-resonance experiments in a two-dimensional hole system at a modulation-doped GaAs-(AlGa)As heterojunction show that the Kramers degeneracy of the lowest subband is lifted for finite $k$ giving rise to two cyclotron masses ${{m}_{1}}^{*}=0.38{m}_{0}$ and ${{m}_{2}}^{*}=0.60{m}_{0}$ at ${E}_{\mathrm{F}}=2.4$ meV. The observation of plateaus in ${\ensuremath{\rho}}_{xy}$ shows that the quantized Hall effect is independent of the details of the host band structure.

Model for optical absorption in transition metals containing d impurities

AbstractThe effect of a ‘d’ impurity is investigated on the contribution to the optical absorption in a transition metal from electronic intra‐ and interband transitions. Within a tight‐binding approximation a previous calculation of the imaginary part ϵ2(ω) of the dielectric constant in simple models is extended to realistic multiband dilute alloys. The possibility is shown of exploring critical points in nickel host band structure, for example, by wave‐vector non‐conserving transitions induced by impurities, in relation with recent experimental data.

Charge transfer effects in X-ray core level photoemission from layered transition metal intercalates

The authors report X-ray photoemission measurements of core level shifts in TaS2, Fe13/TaS2, Mn14/TaS2 and Ni13/TaS2. The shifts are related to charge transfer from the intercalate 3d ions to the host band structure, and have implications concerning observations of charge density wave amplitudes.

Electronic structure of magnetic impurities calculated from first principles

The electronic structure of magnetic $3d$ impurities in Cu and Ag is calculated self-consistently from first principles. Using the density functional theory the exchange and correlation is treated in the local spin-density approximation of von Barth and Hedin. Our method is based on the Kohn-Korringa-Rostocker Green's-function method and the impurity is described by a single perturbed muffin-tin potential in an otherwise periodic lattice. We give results for the local density of states, the magnetic moments, and the phase shifts at the Fermi energy for the $3d$ impurities in Cu and Ag. Our results are in qualitative agreement with the Anderson model, however modifications due to the host band structure are important, especially for Mn and Fe in Cu. For all cases studied, the exchange integral $I$ lies between 0.65 and 0.8 eV.

Wave-vector-nonconserving optical transitions in a model dilute alloy

We explore the theory of the imaginary part ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ of the dielectric constant for a simple model alloy, with emphasis on the possibility of studying critical points in the host band structure through wave-vector-nonconserving transitions induced by impurities. We find Van Hove singularities in the host band structure produce discontinuities in slope of $\ensuremath{\partial}\frac{{\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})}{\ensuremath{\partial}\ensuremath{\omega}}$ at photon energies equal to the absolute magnitude of the difference between the Fermi energy and those of the critical points. The paper presents a study of ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ and its frequency derivative for a single impurity placed in a host with nondegenerate conduction band of tight-binding form.

Theoretical study of hyperfine fields of rare-earth impurities in transition hosts

We study theoretically the hyperfine field at the nucleus of a rare-earth impurity embedded in a transition host. This system is a limiting case of the Blandin-Campbell problem, namely, that of the hyperfine field at a nonmagnetic impurity site in a metallic matrix doped with a magnetic impurity at a distance ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{R}}}_{0}$ (Heusler alloys). In the case we study both the polarizing spin and the charge impurity potential collapse into the same site (i.e., ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{R}}}_{0}=0$). Furthermore, one must consider a more complicated band structure: an $s$- and $d$-character conduction band. We perform numerical computations for a number of model band structures taking into account the two-band nature of the transition metals and treating the exchange couplings of the impurity $f$ spin with the $s$- and $d$-electron gas to first order in perturbation. The $s$ and $d$ hyperfine contributions are calculated in terms of local magnetic responses ${\overline{\ensuremath{\chi}}}^{\ensuremath{\lambda}\ensuremath{\mu}}(0)$ ($\ensuremath{\lambda},\ensuremath{\mu}=s,d$) which incorporates the host band structure, charge potential and local correlations, and ratios between the hyperfine parameters ${A}_{\mathrm{cp}}$ and $A(Z)$. We find that the $s$ contribution to the hyperfine field may change sign with the impurity potential difference between rare-earth impurity and the matrix. However, the $d$ contribution does not; as it is the dominant one, the total hyperfine field remains always positive (negative) throughout the transition-metal series depending on whether the exchange coupling ${J}^{(d)}$ between the $d$-electron gas and the $f$ local spin is negative (positive). Our theory may be useful to extract information on the order parameter in certain spin-glass systems from hyperfine-field measurements. Our theoretical results are compared with some available experimental data and further systematic experimental studies are suggested, including spin-glass systems.

Multiple scattering theory for magnetoresistance of localized impurities in metals

AbstractIt is shown that starting from a multiple scattering formalism for localized impurities in metals the linearized Boltzmann equation can be solved by the degenerate kernel technique. In this case the Boltzmann equation reduces to an algebraic set of linear equations of finite rank. An expression for the magneto‐resistance, valid for arbitrary strength of external magnetic fields is derived. In the limit of a nearly‐free‐electron‐like host band structure the well‐known results for magnetoresistance and impurity resistivity are reproduced.

The electronic structure of dilute transitional impurities in simple metals

The pseudo-Green function formalism of Moriarty is extended to the case of dilute transitional and noble metal impurities in simple metals. When applied to the case of dilute AlCu alloys this theory predicts a virtual bound state arising from the s-d hybridization with a width in good agreement with experiment. The effects of changes in the host band structure upon this virtual bound state are also considered and shown to be small for AlCu.

Effect of localized perturbations on the spin polarization: Two-band model

The conduction-electron spin polarization, induced by a magnetic rare-earth impurity embedded in a transition-metal-like host, is calculated, using the Green's-function formalism. The $s$ and $d$ magnetizations are calculated to first order in the exchange parameters, in terms of partial static susceptibilities ${\overline{\ensuremath{\chi}}}^{\mathrm{dd}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$, ${\overline{\ensuremath{\chi}}}^{\mathrm{ds}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$, ${\overline{\ensuremath{\chi}}}^{\mathrm{ss}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$, and ${\overline{\ensuremath{\chi}}}^{\mathrm{sd}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$. These susceptibilities are expressed in terms of the host band structure, phase-shifts, and strength of matrix elements associated to the charge impurity potential and local Coulomb correlation parameter at the impurity site. These results are compared to previous ones and possible applications are discussed.

Modulation spectroscopy studies of defects

We examine some relationships between defect characteristics and the modulated optical properties of solids. In particular, the effects of an electric field ℰ on the N absorption in GaP are presented. The crystallographic-orientation dependence is used to determine symmetry properties of the impurity electron states and their interaction with the host band structure. These data also yield an impurity concentration which is in agreement (to within a factor of 2) with results obtained using other techniques. The Franz-Keldysh mechanism responsible for the electro-absorption suggests that in indirect gap semiconductors, an electric field should double the luminescence. This enhancement will be greatest for ℰ along the orbital axis of the luminescent center. Generalizations of this work to other chemical and structural defects are discussed.

Electronic structure of noble (or transition) metal based alloys: charge oscillations, Knight shifts and pair interaction energies

The electronic structure is calculated, of noble or transition metal based alloys with transitional impurities, using an hybridized tight binding and nearly free electron (TB-NFE) model for the host. In this Hartree-Fock model for which parameters appear only in the host band structure, the author studies selfconsistently the screening of one isolated impurity and of two nth neighbouring impurities (n=1,...,4) in copper. The results are compared with those deduced in the Friedel-Anderson model from the asymptotic approximation: the charge oscillations and the spin polarizations on the nearest neighbours around an impurity cannot be rigorously obtained by the previous asymptotic approximation which does not take into account the selection rules required by the crystal symmetry.

Electronic Density of States in Cu-Based Alloys

A detailed theoretical investigation has been made of the manner in which the electronic density of states of copper changes with the introduction of dilute nonmagnetic impurities. The formalism used is exact for a single impurity in a system of muffin-tin potentials, and takes into account the detailed character of the host band structure. The impurity potential is calculated through a self-consistent procedure utilizing a generalized Friedel sum rule. The results show a marked difference from the predictions of the rigid-band model. We find that the changes in the single-particle electronic density of states are not sufficient to account for the measured changes in the linear component of the specific heat. But due to the comprehensive nature of the calculations, it is possible to conclude that the discrepancy is due to other physical effects.