Charge manipulation and imaging of the Mn acceptor state in GaAs by Cross-sectional Scanning Tunneling Microscopy

An individual Mn acceptor in GaAs is mapped by Cross-sectional Scanning Tunneling Microscopy (X-STM) at room temperature and a strongly anisotropic shape of the acceptor state is observed. An acceptor state manifests itself as a cross-like feature which we attribute to a valence hole weakly bound to the Mn ion forming the (Mn$^{2+}3d^5+hole$) complex. We propose that the observed anisotropy of the Mn acceptor wave-function is due to the d-wave present in the acceptor ground state.Comment: Proceedings of the SIMD-4 conference. Hawaii, USA (December 1-5, 2003

The Effect of Gravity on the Nervous System

Gravity affects the nervous system of living organisms. This book chapter reviews historical and recent findings on how changes in gravity affect cellular and subcellular parameters of human and animal cells as well as the timing and shaping of complex sensorimotor responses. With an emphasize on weightlessness, partial, and hypergravity conditions, the gravity dependencies of living organisms have been manifested on different levels of organization, ranging from changes in biophysical properties of single cells to the intact nervous system. An effort has been made to integrate the various findings into a consistent model for a better understanding of how the components of the nervous system interact as a response to acute and long-term gravitational variation. Especially with planned long-term manned missions to Mars and beyond, knowledge about the impact of increased and decreased gravity on the nervous system is essential for the physical and cognitive preparation to assure the success of space missions and human survival in space

Influence of the Characteristics of the STM-tip on the Electroluminescence Spectra

We analyze the influence of the characteristics of the STM-tip (applied voltage, tip radius) on the electroluminescence spectra from an STM-tip-induced quantum dot taking into account the many-body effects. We find that positions of electroluminescence peaks, attributed to the electron-hole recombination in the quantum dot, are very sensitive to the shape and size of the confinement potential as determined by the tip radius and the applied voltage. A critical value of the tip radius is found, at which the luminescence peak positions as a function of the tip radius manifest a transition from decreasing behavior for smaller radii to increasing behavior for larger radii. We find that this critical value of the tip radius is related to the confinement in the lateral and normal direction.Comment: 15 pages, 5 figure

Image resonance in the many-body density of states at a metal surface

The electronic properties of a semi-infinite metal surface without a bulk gap are studied by a formalism that is able to account for the continuous spectrum of the system. The density of states at the surface is calculated within the GW approximation of many-body perturbation theory. We demonstrate the presence of an unoccupied surface resonance peaked at the position of the first image state. The resonance encompasses the whole Rydberg series of image states and cannot be resolved into individual peaks. Its origin is the shift in spectral weight when many-body correlation effects are taken into account

Exchange-correlation vector potentials and vorticity-dependent exchange-correlation energy densities in two-dimensional systems

We present a new approach how to calculate the scalar exchange-correlation potentials and the vector exchange-correlation potentials from current-carrying ground states of two-dimensional quantum dots. From these exchange-correlation potentials we derive exchange-correlation energy densities and examine their vorticity (or current) dependence. Compared with parameterizations of current-induced effects in literature we find an increased significance of corrections due to paramagnetic current densities.Comment: 5 figures, submitted to PR

Theory of band gap bowing of disordered substitutional II-VI and III-V semiconductor alloys

For a wide class of technologically relevant compound III-V and II-VI semiconductor materials AC and BC mixed crystals (alloys) of the type A(x)B(1-x)C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A(x)B(1-x)C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight binding models for the pure AC and BC systems. The electronic properties of the A(x)B(1-x)C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1-x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA). We apply this treatment to the II-VI system Cd(x)Zn(1-x)Se, to the III-V system In(x)Ga(1-x)As and to the III-nitride system Ga(x)Al(1-x)N.Comment: 14 pages, 10 figure

Accurate evaluation of the interstitial KKR-Green function

It is shown that the Brillouin zone integral for the interstitial KKR-Green function can be evaluated accurately by taking proper care of the free-electron singularities in the integrand. The proposed method combines two recently developed methods, a supermatrix method and a subtraction method. This combination appears to provide a major improvement compared with an earlier proposal based on the subtraction method only. By this the barrier preventing the study of important interstitial-like defects, such as an electromigrating atom halfway along its jump path, can be considered as being razed.Comment: 23 pages, RevTe

The role of occupied d states in the relaxation of hot electrons in Au

We present first-principles calculations of electron-electron scattering rates of low-energy electrons in Au. Our full band-structure calculations indicate that a major contribution from occupied d states participating in the screening of electron-electron interactions yields lifetimes of electrons in Au with energies of $1.0-3.0 {\rm eV}$ above the Fermi level that are larger than those of electrons in a free-electron gas by a factor of $\sim 4.5$. This prediction is in agreement with a recent experimental study of ultrafast electron dynamics in Au(111) films (J. Cao {\it et al}, Phys. Rev. B {\bf 58}, 10948 (1998)), where electron transport has been shown to play a minor role in the measured lifetimes of hot electrons in this material.Comment: 4 pages, 2 figures, to appear in Phys. Rev.

Basis Functions for Linear-Scaling First-Principles Calculations

In the framework of a recently reported linear-scaling method for density-functional-pseudopotential calculations, we investigate the use of localized basis functions for such work. We propose a basis set in which each local orbital is represented in terms of an array of `blip functions'' on the points of a grid. We analyze the relation between blip-function basis sets and the plane-wave basis used in standard pseudopotential methods, derive criteria for the approximate equivalence of the two, and describe practical tests of these criteria. Techniques are presented for using blip-function basis sets in linear-scaling calculations, and numerical tests of these techniques are reported for Si crystal using both local and non-local pseudopotentials. We find rapid convergence of the total energy to the values given by standard plane-wave calculations as the radius of the linear-scaling localized orbitals is increased.Comment: revtex file, with two encapsulated postscript figures, uses epsf.sty, submitted to Phys. Rev.

Improved Slater approximation to SIC-OEP

We propose a simplification of the Optimized Effective Potential (OEP) applied to the Self Interaction Correction (SIC) scheme of Density Functional Theory (DFT). The new scheme fulfills several key formal properties and turns out to be both simple and accurate. We show examples of applications on model molecules in terms of observables known to be especially sensitive to details of the SIC-OEP approach.Comment: 3 figure