The Influence of Metal Substrates on the Electronic States of Metal Overlayers

The aim of this paper is to provide an introduction to the electronic structure of very thin film (one to ten monolayers) overlayers using examples from studies of Hg overlayers on Ag(100) and Fe thin film overlayers. Interracial states as a result of the interaction of the substrate and overlayer, as well as the new electronic states of the overlayer caused by a new crystallographic structure can modify the overlayer metal electronic structure. Additional changes in the overlayer electronic structure arise from the reduced dimensionality (2 dimensionality as opposed to 3 dimensionality) of the thin film. We discuss the photoemission techniques for determining the electronic structures of thin metal over layers. Layer by layer growth of the overlayer is important for such studies as is knowledge of the overlayer structure. We have summarized our current understanding of metal overlayers in tables and attempt to demonstrate that further progress in combining structural and photoemission studies is necessary for better fundamental understanding of metal overlayers

EPR identification of defects responsible for thermoluminescence in Cu-doped lithium tetraborate (Li2B4O7) crystals

Electron paramagnetic resonance (EPR) is used to identify the electron and hole traps responsible for thermoluminescence (TL) peaks occurring near 100 and 200 ◦C in copper-doped lithium tetraborate (Li2B4O7) crystals. As-grown crystals have Cu+ and Cu2+ ions substituting for lithium and have Cu+ ions at interstitial sites. All of the substitutional Cu2+ ions in the as-grown crystals have an adjacent lithium vacancy and give rise to a distinct EPR spectrum. Exposure to ionizing radiation at room temperature produces a second and different Cu2+ EPR spectrum when a hole is trapped by substitutional Cu+ ions that have no nearby defects. These two Cu2+ trapped-hole centers are referred to as Cu2+-VLi and Cu2+active, respectively. Also during the irradiation, two trapped-electron centers in the form of interstitial Cu0 atoms are produced when interstitial Cu+ ions trap electrons. They are observed with EPR and are labeled Cu0A and Cu0B. When an irradiated crystal is warmed from 25 to 150 ◦C, the Cu2+active centers have a partial decay step that correlates with the TL peak near 100 ◦C. The concentrations of Cu0A and Cu0B centers, however, increase as the crystal is heated through this range. As the crystal is futher warmed between 150 and 250 ◦C, the EPR signals from the Cu2+active hole centers and Cu0A and Cu0B electron centers decay simultaneously. This decay step correlates with the intense TL peak near 200 ◦C

Strain induced half-metal to semiconductor transition in GdN

We have investigated the electronic structure and magnetic properties of GdN as a function of unit cell volume. Based on the first-principles calculations of GdN, we observe that there is a transformation in conduction properties associated with the volume increase: first from halfmetallic to semi-metallic, then ultimately to semiconducting. We show that applying stress can alter the carrier concentration as well as mobility of the holes and electrons in the majority spin channel. In addition, we found that the exchange parameters depend strongly on lattice constant, thus the Curie temperature of this system can be enhanced by applying stress or doping impurities.Comment: 9 pages, 3 figure

Prediction of a surface state and a related surface insulator-metal transition for the (100) surface of stochiometric EuO

We calculate the temperature and layer-dependent electronic structure of a 20-layer EuO(100)-film using a combination of first-principles and model calculation based on the ferromagnetic Kondo-lattice model. The results suggest the existence of a EuO(100) surface state which can lead to a surface insulator-metal transition.Comment: 9 pages, 5 figures, Phys. Rev. Lett. (in press

Manganese surface segregation in NiMnSb

A quantitative analysis of the surface composition of the Heusler alloy NiMnSb has been undertaken using angle-resolved x-ray photoemission spectroscopy and the surface composition characterized. With sufficient annealing cycles, the stoichiometric surface evolves to a surface that is manganese rich. This indicates that the surface enthalpy is different from the bulk for the Heusler alloy NiMnSb. ©2000 American Institute of Physics

Experimental evidence for the dynamic Jahn-Teller effect in La\u3csub\u3e0.65\u3c/sub\u3eCa\u3csub\u3e0.35\u3c/sub\u3eMnO\u3csub\u3e3\u3c/sub\u3e

Recently, it has been argued that a strong electron-phonon interaction arising from the Jahn-Teller splitting of the outer Mn d level plays a crucial role in understanding the nonmetal-to-metal transition observed in the La1-xAxMnO3 (A=Ca, Sr, Ba) system. We show, by neutron powder diffraction, that La0.65Ca0.35MnO3 exhibits an anomalous volume and oxygen/manganese displacement change around Tc, in qualitative agreement with the theoretical prediction