Influence of interface structure on electronic properties and Schottky barriers in Fe/GaAs magnetic junctions

The electronic and magnetic properties of Fe/GaAs(001) magnetic junctions are investigated using first-principles density-functional calculations. Abrupt and intermixed interfaces are considered, and the dependence of charge transfer, magnetization profiles, Schottky barrier heights, and spin polarization of densities of states on interface structure is studied. With As-termination, an abrupt interface with Fe is favored, while Ga-terminated GaAs favors the formation of an intermixed layer with Fe. The Schottky barrier heights are particularly sensitive to the abruptness of the interface. A significant density of states in the semiconducting gap arises from metal interface states. These spin-dependent interface states lead to a significant minority spin polarization of the density of states at the Fermi level that persists well into the semiconductor, providing a channel for the tunneling of minority spins through the Schottky barrier. These interface-induced gap states and their dependence on atomic structure at the interface are discussed in connection with potential spin-injection applications.Comment: 9 pages, 9 figures, to appear in PR

Optical transitions and nature of Stokes shift in spherical CdS quantum dots

We study the structure of the energy spectra along with the character of the states participating in optical transitions in colloidal CdS quantum dots (QDs) using the {\sl ab initio} accuracy charge patching method combined with the %pseudopotential based folded spectrum calculations of electronic structure of thousand-atom nanostructures. In particular, attention is paid to the nature of the large resonant Stokes shift observed in CdS quantum dots. We find that the top of the valence band state is bright, in contrast with the results of numerous {\bf k$\cdot$p} calculations, and determine the limits of applicability of the {\bf k$\cdot$p} approach. The calculated electron-hole exchange splitting suggests the spin-forbidden valence state may explain the nature of the ``dark exciton'' in CdS quantum dots.Comment: 5 pages, 4 figure

Impurity complexes and conductivity of Ga-doped ZnO

Using hybrid functional theory together with experimental measurements, we investigate the influence of gallium impurities and their complexes on electrical properties of ZnO. In contrast to the behavior of isolated Ga impurities and native defects, the calculated formation energies of Ga complexes are consistent with our experimental data. We show that for high levels of Ga doping the acceptor behavior of GaZn-VZn and GaZn-Oi complexes explains the conductivity measurements and compensation levels in ZnO. The computed binding energies of these complexes are also consistent with the binding energies obtained from the measurements of the temperature dependence of carrier mobility. Our results show that the formation of defect complexes, often overlooked by theory, can be indispensable in capturing the defect physics

Innovations of the Beginning of the Sixth Millennium BC in the Northern Pontic Steppe

This study focuses on the pottery-bearing ("Neolithic") sites of the northern Azov Sea region. The vessels ornamented with comb imprints appeared there in the sixth millennium BC. In the light of a recent re-dating of the Rakushechny Yar site sequence, the sites of the northern Azov region appeared to be the earliest evidence for this innovation. The innovation in the ceramic assemblage is accompanied by an innovative lithic tool set. The latter included macro-blades and fan-shaped end-scrapers, which were previously unknown in the studied region. Their reanalysis (including new field work at the single-layer site of Chapaevka) helped formulate a hypothesis of maritime transmission of comb-ornamented ceramics in the Black and Azov Sea. This hypothesis will stimulate further discussions regarding the ways of Neolithization in Eastern Europe. It underlines the connections between Balkan "classic"Neolithic and pottery-bearing sites of the Ukrainian Steppe. The impressed ware from Makri and other mainland Greek sites is treated as the closest analogy to the finds of the northern Azov Sea region

Effect of Particle-Hole Asymmetry on the Mott-Hubbard Metal-Insulator Transition

The Mott-Hubbard metal-insulator transition is one of the most important problems in correlated electron systems. In the past decade, much progress has been made on examining a particle-hole symmetric form of the transition in the Hubbard model with dynamical mean field theory where it was found that the electronic self energy develops a pole at the transition. We examine the particle-hole asymmetric metal-insulator transition in the Falicov-Kimball model, and find that a number of features change when the noninteracting density of states has a finite bandwidth. Since, generically particle-hole symmetry is broken in real materials, our results have an impact on understanding the metal-insulator transition in real materials.Comment: 5 pages, 3 figure

Verifying the chronology of Ukrainian Neolithic

This paper addresses the earliest Neolithic phases in Ukraine: aspects relating to the chronology, methods and time of Neolithisation. In general, the earliest Neolithisation relates to the times of the Linear Pottery culture. Nevertheless, Ukraine has numerous eco-zones so the process could have been different in other zones. The authors emphasize that the key to getting closer to solving this problem is the chronology-re-dating and validation program. Hitherto dates of one culture or period of culture have been treated against each other. This article presents a small series of AMS from well-defined contexts of four sites, previously dated by the Kyiv radiocarbon facility. The results are compared with existing dates in order to establish the validity of existing chronologies for the Neolithic of Ukraine. Obtained results allowed the authors to conclude that the Kyiv dates should not be obligatorily treated as wrong. Furthermore, the authors reopened the discussion to establish what was the role of LBK and Azov-Dnieper culture communities in spreading farming and herding in Ukraine during the second half of the VIth millennium BC

Electronic Structure of Transition-Metal Dicyanamides Me[N(CN)2_2]2_2 (Me = Mn, Fe, Co, Ni, Cu)

The electronic structure of Me[N(CN)$_2$]$_2$ (Me=Mn, Fe, Co, Ni, Cu) molecular magnets has been investigated using x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) as well as theoretical density-functional-based methods. Both theory and experiments show that the top of the valence band is dominated by Me 3d bands, while a strong hybridization between C 2p and N 2p states determines the valence band electronic structure away from the top. The 2p contributions from non-equivalent nitrogen sites have been identified using resonant inelastic x-ray scattering spectroscopy with the excitation energy tuned near the N 1s threshold. The binding energy of the Me 3d bands and the hybridization between N 2p and Me 3d states both increase in going across the row from Me = Mn to Me = Cu. Localization of the Cu 3d states also leads to weak screening of Cu 2p and 3s states, which accounts for shifts in the core 2p and 3s spectra of the transition metal atoms. Calculations indicate that the ground-state magnetic ordering, which varies across the series is largely dependent on the occupation of the metal 3d shell and that structural differences in the superexchange pathways for different compounds play a secondary role.Comment: 20 pages, 11 figures, 2 table

Optimizing thermal transport in the Falicov-Kimball model: binary-alloy picture

We analyze the thermal transport properties of the Falicov-Kimball model concentrating on locating regions of parameter space where the thermoelectric figure-of-merit ZT is large. We focus on high temperature for power generation applications and low temperature for cooling applications. We constrain the static particles (ions) to have a fixed concentration, and vary the conduction electron concentration as in the binary-alloy picture of the Falicov-Kimball model. We find a large region of parameter space with ZT>1 at high temperature and we find a small region of parameter space with ZT>1 at low temperature for correlated systems, but we believe inclusion of the lattice thermal conductivity will greatly reduce the low-temperature figure-of-merit.Comment: 13 pages, 14 figures, typeset with ReVTe

The COMPASS Experiment at CERN

The COMPASS experiment makes use of the CERN SPS high-intensitymuon and hadron beams for the investigation of the nucleon spin structure and the spectroscopy of hadrons. One or more outgoing particles are detected in coincidence with the incoming muon or hadron. A large polarized target inside a superconducting solenoid is used for the measurements with the muon beam. Outgoing particles are detected by a two-stage, large angle and large momentum range spectrometer. The setup is built using several types of tracking detectors, according to the expected incident rate, required space resolution and the solid angle to be covered. Particle identification is achieved using a RICH counter and both hadron and electromagnetic calorimeters. The setup has been successfully operated from 2002 onwards using a muon beam. Data with a hadron beam were also collected in 2004. This article describes the main features and performances of the spectrometer in 2004; a short summary of the 2006 upgrade is also given.Comment: 84 papes, 74 figure