Session Q27 - Electronic Structure and Optical Properties of Metals.
ORAL session, Wednesday morning, March 14
Room 615, Washington State Convention Center

[Q27.001] Electronic band structure and Fermi surface of LaBiPt

Electronic structure and structural stability of LaBiPt are studied by using relativistic local-density FLAPW method. Total-energy calculations for three different atomic configurations within the MgAgAs-type crystal structure show that the Pt atom is definitely on the unique site. For the stable configuration, LaBiPt is semimetallic with hole and electron carrier concentrations of 10^-4e. Calculated hole Fermi surfaces have nearly cubic shape centered at \Gamma and their cross-sections perpendicular to different field angles are in good agreement with recently observed Shubnikov de Haas results. LaBiPt becomes a zero-gap semiconductor without the spin-orbit coupling, indicating LaBiPt being a spin-orbit induced semimetal. The semimetallic nature originates mostly in BiPt with the ZnS structure and may be inherent in wider material systems like RBiPt with rare-earth elements R.

[Q27.002] Oscillation of the Fermi Surface in Gold Induced by Coherent Acoustic Phonons

Using a two-color ultrafast optical pump-probe technique with an amplified Ti: sapphire femtosecond laser system, we study the heat-induced structural dynamics in gold by monitoring both the fundamental and third harmonic generation (THG) signals of the probe beam. For the first time, an oscillation is clearly observed in the THG signal with a period of tens of picoseconds. We show that the oscillation directly results from a modulation of the Fermi surface in gold driven by coherent acoustic phonons following laser heating.

[Q27.003] Model Calculations of Many Body Effects in EXAFS

Extrinsic and intrinsic excitations and their interference are calculated for an electron gas using a generalization of the GW approach and a plasmon pole dielectric function. These effects lead to an asymmetric quasiparticle peak and an energy dependent satellite in the spectral function. Interference between the extrinsic and intrinsic excitations leads to a reduction of the satellite and an enhancement of the main peak. These interference effects cause maximal cancellation of extrinsic and intrinsic losses at threshold and become negligible at high energies. The EXAFS is expressed as a convolution of the spectral function with the one-electron EXAFS expression. This yields a many-body amplitude reduction factor and a phase shift, in terms of a phasor summation, in addition to the usual mean free path. The theory yields results in good agreement with experiment for Cu K-edge EXAFS.

[Q27.004] Line Shape of the Pd M_4,5VV Auger Spectrum determined by Auger-Photoelectron Coincidence Spectroscopy (APECS)

Core valence valence (CVV) Auger spectra contain information about the valence band electronic structure of a solid at the site of the core hole. Measurements have been carried out of the M_5VV and M_4VV Auger spectra in coincidence with Pd 3d_5/2 and 3d_3/2 core photoelectrons, respectively, from a thick Pd film grown on the Ru(0001) surface. These experiments were carried out at station U16B of the VUV ring at the NSLS. The Pd overlayer grows in an epitaxial Pd(111) structure, so the spectra should be representative of the intrinsic lineshape of bulk Pd. Even in raw coincidence spectra, it is clear that the high kinetic energy edge of the M_4VV line differs from that of the M_5VV transition. This is a direct indication that these transitions cannot be described by a simple self-convolution of the valence band density of states (SCDOS), even with a Cini-Sawatzky distortion to account for electron correlation effects. As we found for the equivalent transitions of Ag, a full description of these lines requires consideration of the multiplet structure of the Pd d^7 configuration.

[Q27.005] Local field effects in x-ray absorption spectra

We develop a formalism based on the time dependent local density approximation (TDLDA) to account for local field effects on x-ray absorption spectra (XAS). These local fields are a polarization effect that screens the x-ray field in the dipole matrix elements, and is most important for soft x-rays (energies 1 keV or less). The formalism is incorporated into an extension of the self-consistent full multiple scattering code FEFF8 [1]. This yields a generalization of the approach of Zangwill and Sowen [2] to extended systems. Results are presented for XAS at the L_2,3 edge of selected 3d transition metals and for solid Xe.

[1] A.L. Ankudinov et al. Phys. Rev. B58, 7565 (1998). [2] A. Zangwill and P. Soven, Phys. Rev. A21, 1561 (1980).

[Q27.006] Charge Dynamics of the Half-Heusler TiCo1-xFexSb compounds

Half-Heusler phases are cubic XYZ ternary compounds, where X and Y are transition metals and Z an sp element. Some of these materials, often quoted as "half-metallic ferromagnets", exhibit rather large magneto-optical Kerr effects. We report on dc-transport and optical investigations of single crystals of the title compound with x=0, 0.015 and 0.05 below room temperature and covering the spectral range from 1 meV to 12 eV. For x=0 we found a metallic reflectivity with a Drude-component, gaining spectral weight with decreasing temperature, in agreement with the metallic-like feature of the dc-transport data. A strong phonon-type absorption at about 0.03 eV overlaps the Drude component. With decreasing temperature we note a considerable shift of spectral weight from higher energies into the phonon excitation, possibly indicating a strong electron-phonon coupling. The Fe-doped compounds are insulators without any temperature dependence of the optical spectra. Our data are in conflict with recent experimental results on polycrystalline samples and band structure calculations.

[Q27.007] O(N) Tight-binding CPA Approach to Chemical Order in Alloys on Massively Parallel Computer

The implementation of an O(N) tight-binding solution to the CPA (Coherent Potential Approximation) equations on IBM SMP massively parallel computer is discussed. The real-space approach is well suited for studying electronic structure and stability of complex multi-component alloys that exhibit both chemical and topological disorders. First, we briefly recall that the CPA equations can be solved with a recursion approach in the case of inhomogeneous systems. Second, combined with an orbital peeling technique, we show that this approach can be very efficiently applied to the determination of the effective interactions that build up the configurational part of the total energy within the embedded cluster method. Third the data distribution, communication routines and load-balancing scheme of the code are presented together with considerations on speed and scaling. Then performance results are presented for systems of various sizes on up to 1000 processors. The real-space solution to the problem, the inherent localization of the orbitals and the absence of global communications make the code efficiently scalable in terms of memory and speed on parallel computers. Preliminary results for amorphous Zr-Ni alloys are presented.

[Q27.008] AC conductivity in percolating Ag as measured by in situ ellipsometry.

Silver is a particularly good system for studying the effects of fractal geometry on electronic transport properties. It can be grown as thin, stable films very close to the percolation transition (\epsilon =(d-d_c )/d_c < 0.008, where d is the coverage of the film and d_c is the critical coverage for percolation) without any adverse, non-ohmic effects in the microscopic resistivity. Our experiments use simultaneous, in situ, DC-resistance and Ellipsometry measurements, during growth, to study the impact of microstructure on transport. The DC measurement shows behavior as a function of thickness and provides a yardstick to measure the microstructure of the film as a function of thickness. By correlating our ellipsometry signal with the resistance measurement we can see how the real and imaginary parts of the conductivity, measured at wavelengths between 2795 Åand 7629 Åvary through the percolation transition. Of particular interest is a strong electroreflectance signal, originating with the applied voltage of the DC measurement, which appears to change the effective connectivity of the infinite cluster.

[Q27.009] Microwave Size-Effect Resonances in Potassium

Using a microwave transmission technique at 80 GHz, size-effect resonances due to ballistic electrons have been studied in thin plates of high-purity potassium metal at a temperature of 1.5 K. With an applied magnetic field parallel to the surface of the plate, resonances occur whenever an integer number of extremal cyclotron diameters fit between the boundaries of the sample. The resonances consist of an oscillatory signal that extends from lower fields up to that of exact resonance, where the transmission is suddenly cut off. By comparing the experimental data with theoretical line shapes of D.S. Falk et al. [Phys. Rev. B 6, 377 (1972)], the signals are identified as arising from Type 2 electron trajectories, in which the centers of the orbits are located at the center of the specimen. A significant improvement in fitting the overall line shape is obtained by summing in the theory the contributions of all orbiting electrons, rather than considering only the contribution of the extremal belly orbit. While the overall fit is reasonably good, the experimental data show a phase anomaly at about 98

[Q27.010] STM light emission spectrum of oxygen atom on Cu(110)-(2x1) O surface

We have measured the scanning tunneling microscope (STM) light emission from the Cu(110)-(2x1)O surface with atomic spatial resolution. After cleaning in UHV the Cu(110) surface was exposed to 20L of oxygen. A clear LEED pattern was observed, indicating the formation of a reconstructed (2x1) surface. The STM light emission measurement was performed with the sample at 80K, using the exposure time of 100 s per spectrum. The sample was biased at +2.3V with respect to the tip, while the tunneling current was kept constant at 2nA. When the tip was located over the hollow site surrounded by four O atoms, a broad spectrum with a single peak was observed. This single peak split into a double peak, when the tip was moved over an oxygen atom. The spectrum from the hollow site can be understood as the emission from localized surface plasmons excited by the tunneling current. The double peak structure from the oxygen site can be explained by correlating the observed spectrum with the local density of states of the Cu(110)-(2x1) O surface.

[Q27.011] Effect of Interlayers on the Electronic Structures of Al/Organic Interfaces

We have investigated the electronic structures of interfaces between Al and tris-(8-hydroxyquinoline)aluminum (Alq_3), which is a prototypical organic electroluminescent (EL) material. It has been well known that the insertion of alkali metal fluoride, such as LiF, greatly enhances the EL performance and the fate of LiF at the interface has been controversial. We used X-ray and UV photoelectron spectroscopy (XPS and UPS) to probe the electronic structure change caused by the insertion of thin layer of LiF, MgF_2, and other fluoride materials.The presence of LiF layer enhanced gap-state formation, but even without the LiF layer, Al deposition significantly alters the electronic structures of Alq3. While the insertion of MgF_2 layer showed virtually no gap states, Al deposition alone formed weak, but clearly discernable gap states. For both fluoride insertion layers, the valence band electronic states moved to higher binding energy directions. We discuss the fates of LiF and MgF_2 at this interface and their implication in the performance of organic light-emitting devices.

[Q27.012] A high pressure study of phonons in oligophenyls using Raman scattering

We present a study of Raman phonons in three oligophenyls, namely, ter-, quarter- and hexaphenyl under hydrostatic pressure at low temperatures. The strongest modes observed are those due to the C-H bending mode at 1220 cm-1, the inter-ring C-C stretch Raman mode at 1280 cm-1, and the ring C-C stretch mode at 1600 cm-1. The intensity ratio of the first two modes is indicative of the degree of torsional motion between neighboring phenyl rings. Pressure brings the molecules closer to one another, thereby planarizing the molecule. In addition to the abrupt change in the intensity ratio caused by planarization, the frequencies of these modes increase discontinuously, and the linewidths change as well. The effect is clearly seen at pressures of 7.6, 7.8 and 13.6 kbar for the three materials, respectively.

[Q27.013] Dramatic electromagnetic enhancement in plasmon resonant nanowires

We investigate numerically the plasmon resonances of 10-50 (nm) nanowires with a non-elliptical section. Such wires have a much more complex behavior than elliptical wires and their resonances span a larger frequency range. The field distribution at the surface of these wires exhibits a dramatic enhancement, up to several hundred times the incident field amplitude. Each resonance has a particular topology that can be related to the polarization charge distribution. These strongly localized fields can provide an important mechanism for surface enhanced Raman scattering (SERS). They can also play a functional role in optical systems based on metallic nanostructures.

[Q27.014] Fermi-Liquid Theory for Anisotropic Superconductors

I present a Fermi-liquid theory for superconductors with anisotropic Fermi surfaces, Fermi-liquid interactions, and energy gaps. For d-wave superconductors, the Fermi-liquid interaction effects are found to be classifiable into strong and negligible renormalizaton effects. Furthermore, the leading clean-limit temperature-dependent correction to the superfluid density in a d-wave superconductor is found to be renormalized by a Fermi velocity (or mass) renormalization effect. The question is raised of whether or not the penetration depth in the high temperature superconductor YBa_2Cu_3O_6+xa can be accounted for with physically acceptable parameters within the framework of a quasiparticle model. Fermi-liquid corrections to the spin susceptibility and to the zero-energy magnetic-field-induced density of states are also evaluated.

Part Q of program listing