Thermochemistry of the gadolinium-copper interface

We have characterized the thermochemistry of the gadolinium-copper interface using valence-band photoemission. We find that the copper substrate binds strongly with the gadolinium overlayer, based upon valence-band binding-energy shifts that occur with increasing gadolinium coverages on Cu(100). The net potential for copper at the interface is 3.82 eV/atom which is some 0.51 eV/atom greater than the heat of sublimation for copper. The strong bonds formed between copper and gadolinium provide a thermochemical driving force for copper gadolinium-alloy formation. We observed a surface-to-bulk core-level shift for gadolinium of 0.3–0.4 eV. We also observed a Cu 3d binding-energy increase of 0.73 eV with increasing coverage

Thermochemistry of the gadolinium-copper interface

We have characterized the thermochemistry of the gadolinium-copper interface using valence-band photoemission. We find that the copper substrate binds strongly with the gadolinium overlayer, based upon valence-band binding-energy shifts that occur with increasing gadolinium coverages on Cu(100). The net potential for copper at the interface is 3.82 eV/atom which is some 0.51 eV/atom greater than the heat of sublimation for copper. The strong bonds formed between copper and gadolinium provide a thermochemical driving force for copper gadolinium-alloy formation. We observed a surface-to-bulk core-level shift for gadolinium of 0.3–0.4 eV. We also observed a Cu 3d binding-energy increase of 0.73 eV with increasing coverage

The chemistry of the gadolinium-nickel interface

Gadolinium overlayers on Ni(111) have been studied by angle resolved photoemission, angle resolved AES, LEED, and RHEEB. We have observed pronounced interdiffusion of nickel with the gadolinium overlayer at temperatures as low as 150 K. This is in marked contrast with gadolinium overlayers on Cu(108) where substantial interdiffusion is not observed until 360 K, but is consistent with studies of ytterbium on nickel. [A. Nilsson, B Eriksson, N. Martenssom, J. N. Andersen, and J. Onsgaard, Phys. Rev. B 38,10357, ( 1988) and I. Chorkendorff, K. Onsgaard, J. Schmidt-May and R. Nyholm, Surf. Sci. 160, 587, (1985) .] There is a strong interfacial heat of interaction observed with gadolinium on both copper and nickel resulting in pronounced binding energy shifts observed in photoemission. An extremely small kinetic barrier to rare earth diffusion through nickel has been measured. The results are compared to transition metal overlayers on transition metal substrates

Temperature dependence of the superconducting gap anisotropy in Bi2_{2}Sr2_{2}Ca1_{1}Cu2_{2}O8+x_{8+x}

We present the first detailed data of the momentum-resolved, temperature dependence of the superconducting gap of $Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x}$, complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the $\Gamma-\bar{M}$ and $\Gamma-X$ directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors such as lead. Specifically, the size of the superconducting gap along the $\Gamma-X$ direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the $\Gamma-\bar{M}$ direction.Comment: APS_REVTEX. 19 pages, including 8 figures, available upon request. UW-Madison preprin

The chemistry of the gadolinium-nickel interface

Gadolinium overlayers on Ni(111) have been studied by angle resolved photoemission, angle resolved AES, LEED, and RHEEB. We have observed pronounced interdiffusion of nickel with the gadolinium overlayer at temperatures as low as 150 K. This is in marked contrast with gadolinium overlayers on Cu(108) where substantial interdiffusion is not observed until 360 K, but is consistent with studies of ytterbium on nickel. [A. Nilsson, B Eriksson, N. Martenssom, J. N. Andersen, and J. Onsgaard, Phys. Rev. B 38,10357, ( 1988) and I. Chorkendorff, K. Onsgaard, J. Schmidt-May and R. Nyholm, Surf. Sci. 160, 587, (1985) .] There is a strong interfacial heat of interaction observed with gadolinium on both copper and nickel resulting in pronounced binding energy shifts observed in photoemission. An extremely small kinetic barrier to rare earth diffusion through nickel has been measured. The results are compared to transition metal overlayers on transition metal substrates

Observation of a van Hove Singularity in Bi2Sr2Ca1Cu2O8+xBi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x} with Angle Resolved Photoemission

We have performed high energy resolution angle-resolved photoemission studies of the normal state band structure of oxygen overdoped $Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x}$. We find that there is an extended saddle point singularity in the density of states along $\Gamma-\bar{M}-Z$ direction. The data also indicate that there is an asymmetry in the Fermi surface for both the $\Gamma-\bar{M}-Z$ and perpendicular directions.Comment: APS_Revtex. 28 pages, including 16 figures, available upon request. UW-Madison preprint#

Localization Effects in Bi2Sr2Ca(Cu,Co)2O8+y High Temperature Superconductors

Doping Bi2Sr2Ca1Cu2O8+y with Co causes a superconductor-insulator transition. We study correlations between changes in the electrical resistivity RHOab(T) and the electronic bandstructure using identical single crystalline samples. For undoped samples the resistivity is linear in temperature and has a vanishing residual resistivity. In angle resolved photoemission these samples show dispersing band-like states. Co-doping decreases TC and causes and increase in the residual resistivity. Above a threshold Co-concentration the resistivity is metallic (drab/dT >0) at room temperature, turns insulating below a characteristic temperature Tmin and becomes super- conducting at even lower temperature. These changes in the resistivity correlate with the disappearance of the dispersing band-like states in angle resolved photoemission. We show that Anderson localization caused by the impurity potential of the doped Co-atoms provides a consistent explanation of all experimental features. Therefore the TC reduction in 3d-metal doped high- temperature superconductors is not caused by Abrikosov Gor'kov pair- breaking but by spatial localization of the carriers. The observed suppression of TC indicates that the system is in the homogenous limit of the superconductor-insulator transition. The coexistance of insulating (dRHOab/dT <0) normal state behavior and super- conductivity indicates that the superconducting ground state is formed out of spatially almost localized carriers.Comment: Postscript file 11 pages plus 4 figures available on reques

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