David W. Lynch

Superconducting Gap in Bi-Sr-Ca-Cu-O by High-Resolution Angle-Resolved Photoelectron Spectroscopy

Detailed studies indicate a superconducting gap in the high-temperature superconductor Bi2Sr2CaCu2O8. Photoemission measurements with high energy and angle resolution isolate the behavior of a single band as it crosses the Fermi level in both the normal and superconducting states, giving support to the Fermi liquid picture. The magnitude of the gap is 24 millielectron volts.

Comments on the Optical Constants of Metals and an Introduction to the Data for Several Metals

Publisher Summary This chapter presents tables and plots of the optical constants N = n - ik for the metals Ag, Au, Cu, Ir, Mo, Ni, Os, Pt, Rh, W, and Al as a function of wavelength from the soft x-ray region to the near infrared. Preceding each data set is a short critique that outlines the procedures used to obtain the constants and the conditions under which they are measured. It is observed that each critique has its own independent set of references. Of all the metals, aluminum appears to have the most widespread spectral application, being used in optical components from the infrared to the soft x-ray region. The chapter also briefly illustrates the methods used to obtain the optical constants. There are four general methods for obtaining experimental data from which the optical constants can be deduced. The first three methods are based on the Fresnel relations for the reflection and transmission of radiation at a flat, smooth interface between two media. The fourth method uses electron-loss spectrum.

A photoemission study of benzotriazole on clean copper and cuprous oxide

Abstract Photoemission spectra of benzotriazole (BTA) chemisorbed on clean Cu and on cuprous oxide were compared with the spectra of condensed- and gas-phase BTA. Chemisorbed BTA bonds to both Cu and Cu 2 O via lone-pair orbitais on the nitrogen ring. The lack of a chemical shift for the π- orbitais indicates that BTA does not lie flat on the surface. We propose a model for the geometry and bonding of chemisorbed BTA which accounts for its corrosion inhibition on Cu, and for the corrosion inhibition, or lack of inhibition, by molecules similar to BTA.

Optical properties of metals.

Attention is drawn to a forthcoming publication of optical data on all the metallic elements in the photon energy range 0.1--500 eV. (AIP)

Optical transitions, XPS, electronic states in NiPS3

Abstract We have measured the optical absorption below the fundamental threshold, the normal-incidence reflectivity between 1.5 and 30 eV and the X-ray photoemission spectra of NiPS3. Shake-up satellites present at the Ni 2p and 3p core levels are strong evidence for the ionicity of the NiS bonds. We have also derived a qualitative molecular orbital model of NiPS3 in which the trigonal crystal field splits the P and S 3pxpy-3pa states, and strong covalent hybridization between P and S pxpy orbitals leads to covalent electronic bonding. Ni is envisaged as a divalent ion which plays little role in the electronic bonding and its 3d levels are localized, lying near the top both of the valence states. This model accounts well for both the valence band XPS data and the low energy optical transitions. Our model should represent, at the center of the Brillouin zone but not at the boundaries, the energy level sequence in NiPS3 and other related MPX3 layer-type compounds where M Co2+, Mn2+, Fe2+, Zn2+ and X is sulfur or selenium. The XPS spectra and optical properties of NiPS3 have been obtained and interpreted on a qualitative molecular orbital model in which the Ni is a divalent positive ion which plays little role in the bonding. Evidence for such ionicity appears in the optical properties and XPS satellite structures, as well as in the magnetic properties. The model should represent qualitatively the band structure at the center of the Brillouin zone, but not at the boundaries. It should also be valid for other compounds similar to NiPS3, i.e. those with other metals in place of Ni and those with Se in place of S.

X-ray photoemission and optical spectra of NiPS3, FePS3 and ZnPS3

Abstract X-ray-induced photoemission spectra were obtained for the valence bands and core levels of ZnPS3 and FePS3 for comparison with earlier XPS spectra of NiPS3. The valence band spectra are interpreted on the basis of a two-dimensional model in which the transition metal is ionized, with the two electrons appearing in covalent bonds in the SP3 groups. The absorption edge for each crystal was obtained by optical transmission measurements and the dielectric function for energies above the edge was obtained from a Kramers-Kronig analysis of the reflectance up to an energy of 30 eV. The optical properties are consistent with the view that the transition metals exist primarily as ions, with localized 3d levels. There is evidence, however, that transition metal 4s and 4p states mix with ligand states.

Infrared absorption in magnesium silicide and magnesium germanide

Abstract Infrared absorption spectra of several n-type Mg2Si and Mg2Ge single crystals were obtained in the wavelength regions from 1–13 μ and 1–7 μ, respectively, over the temperature range from 85–370°K. Free carrier absorption occurred in Mg2Si at the longer wavelengths. The absorption edges appeared to be due to indirect transitions and the shift of the optical energy gap with temperature is estimated to be −5 × 10−4 eV (°K)−1 for Mg2Si and −6.5 × 10−4 eV (°K)−1 for Mg2Ge. No values for the gaps were determined from the present data but they can be seen to be approximately equal to the gaps determined by electrical measurements. An absorption peak at 0.40 eV in n-type Mg2Si was observed and is discussed.

No well-defined remnant Fermi surface in Sr2CuO2Cl2

In angle-resolved photoelectron spectra of the antiferromagnetic insulators Ca2CuO2Cl2 and Sr2CuO2Cl2 a sharp drop of the spectral intensity of the lowest-lying band is observed along a line in k space equivalent to the Fermi surface of the optimally doped high-temperature superconductors. This was interpreted as a signature of the existence of a remnant Fermi surface in the insulating phase of the high-temperature superconductors. In this paper it is shown that the drop of the spectral intensity is not related to the spectral function but is a consequence of the electron-photon matrix element

On van der Waals dipole-dipole coefficients in alkali halides

Abstract The van der Waals dipole-dipole coefficients have been calculated for KC1, KBr, KI and AgCl from recent optical data using a method introduced by Mayer. A unique separation of the absorption into anion and cation contributions cannot be made, but by making a reasonable division into such components, ionic polarizabilities and van der Waals dipole-dipole coefficients can be calculated. These depend strongly on the local field correction made in calculating the electric dipole matrix element from the absorption coefficient. Using both no correction and the Lorentz-Lorenz correction, van der Waals coefficients considerably different from those of Mayer are found. For AgCl the concept of interionic dipole-dipole forces is vague, so that the van der Waals coefficients and ionic polarizabilities obtained from optical data are not meaningful.

Infrared and visible optical properties of single crystal Ni at 4K

Abstract The absorptivity of single-crystal Ni and Ni + 1% Cu has been measured from 0.08 to 3.0 eV at 4K, where the free carrier background is suppressed. The data have been Kramers-Kronig analyzed. Maxima in the interband conductivity occur at 0.28 and 1.6 eV with minima at 0.4 and 1.0 eV. Critical points may not be significant in their interpretation. Structure previously reported between 2.0 and 2.5 eV and attributed to X 5 - X ′ 4 transitions has not been found, neither in these data nor in thermomodulation measurements at 78 K.