M. Qvarford

Alkali metal adsorption on Al(111)

The submonolayer adsorption of Na, K, Rb, and Cs on the Al(111) surface at 100 K and at room temperature is investigated by high resolution core level spectroscopy and low energy electron diffraction. It is found that the first alkali atoms on the surface adsorb at surface defects. At higher coverages, up to approximately one third of the maximum submonolayer coverage, alkali atoms adsorbed at defects coexist with a dispersed phase. At higher coverages island formation is found to occur for the majority of the systems. It is argued that all of the ordered structures formed at room temperature involve a disruption of the Al(111) surface in contrast to the situation at 100 K where the alkali atoms adsorb as adatoms.

Layer dependent core level binding energy shifts: Na, K, Rb and Cs on Al(111)

Layer resolved core level spectra are presented for Na, K, Rb and Cs films on Al(111). From the development of the spectra, it is possible to distinguish emission from alkali atoms at the interface, in the bulk, and at the surface of the adsorbed layers. The core level binding energy shifts are discussed in terms of adhesion and interface segregation energies. It is found experimentally that the Al induced core level binding energy shifts of the alkalis are increasing with increasing atomic number of the alkali metal. This trend is qualitatively reproduced by semi-empirical calculations.

ELECTRONIC-STRUCTURE OF NIO(1 0 0) WITH ADSORBED NA

Adsorption of Na on single crystals of NiO(1 0 0) at room temperature has been investigated via angle-resolved photoemission spectroscopy and measurements of work-function changes. The drastic changes in the valence band spectra upon Na deposition are assigned to a chemical reaction between Na and O where metallic Ni is left as by-product. At short deposition times, Na mainly acts as an inert electron donor, and at these coverages no dispersion is observed in the angle-resolved photoemission spectra which indicates that the electronic states are localized. The measurements were performed both for a polished and an in-situ cleaved NiO crystal.

Polarisation-dependent X-ray absorption in high- and low-Tc Bi2Sr2Can-1CunO4+2n superconductors

Abstract Taking advantage of the linear polarisation of synchrotron radiation, the polarisation dependence of the O K and Cu L 3 X-ray absorption edges has been compared for in situ cleaved Bi 2 Sr 2 CaCu 2 O 8 and Bi 2 Sr 2 CuO 6 single crystals. The X-ray absorption spectra were measured by means of total electron yield detection. The spectra show for both crystals that the lowest unoccupied Cu 3d and O 2p orbitals are dominantly oriented parallel to the a−b plane, but also the presence of a small amount of unoccupied orbitals oriented perpendicular to the a−b plane can be deduced from both the O K and Cu L3 absorption spectra. A shift of the order of 0.5 eV between absorption into the in-plane and the out-of-plane Cu 3d orbitals was measured for the Bi 2 Sr 2 CaCu 2 O 8 crystal, but no such shift was found for the Bi 2 Sr 2 CuO 6 crystal.

X-ray absorption study of oxygen in the high-Tc superconductor Bi2Sr2CaCu2O8 near the interfaces to Cu, Ag and Au

Abstract The influence on O 2p holes in single crystalline Bi 2 Sr 2 CaCu 2 O 8 upon the interface formation to Cu, Ag and Au has been studied by O K edge X-ray absorption measurements. It was found that Cu reduces the amount of doping induced O 2p holes significantly in the vicinity of the interface, whereas Ag and Au gave a much smaller reduction of these states. Photoemission spectra confirmed previous findings that Cu causes a strong chemical reaction at the BiO surface of Bi 2 Sr 2 CaCu 2 O 8 , in contrast to Ag and Au which induced only a minimal reaction. The results support the opinion that the BiO layers are essential for the doping of the CuO 2 layers in Bi 2 Sr 2 CaCu 2 O 8 .

Alkali core level binding energy shifts from buried interfaces between alkali films and metallic substrates with different surface index

Core level binding energy shifts from approximately three layers of different alkali metals deposited on Pd(111), Pd(100), Rh (111), and Rh (110) are presented in order to demonstrate how the packing of the substrate surface affects the layer-resolved alkali core level binding energy shifts. It is found that a more open surface induces a larger alkali core level binding energy shift. It is shown that this behaviour can be explained as a consequence of the higher coordination of the alkali metal atom to the substrate on a more open surface than on a more closed packed surface.

Surface electronic structure of CeCo2, CeRh2 and CeRh3 probed by valence band resonant photoemission spectroscopy

Abstract The 4f surface versus bulk electronic structure of three strongly hybridized Ce intermetallic compounds (CeCo 2 , CeRh 2 and CeRh 3 ) is investigated by exploiting the 4f photoemission resonant enhancement at the Ce 3d–4f and Ce 4d–4f thresholds. The much higher surface sensitivity at the shallow 4d threshold allows to extract the surface and bulk 4f electronic structure. Our results show a decreasing of the hybridization strength at the surface as compared to the bulk. The analysis of the whole set of results highlights the effect on the 4f spectral function obtained by varying the Ce metallic partner in the same crystal structure (CeCo 2 versus CeRh 2 ) and by changing the stoichiometry in the CeRh phase diagram (CeRh 2 versus CeRh 3 ).

Photoemission study of the Bi2CaSr2Cu2O8 superconductor with Cu, Ag and Au overlayers

In this paper, we present a photoemission study of the interaction of Cu, Ag and Au with clean single-crystal Bi2CaSr2Cu2O8 superconductor surfaces. Both the valence-band and the Bi 5d, O 1s and Sr 3d core levels were monitored for all overlayers. Cu, Ag and Au were deposited as consecutively thicker layers starting with a third of a monolayer and progressing in steps up to a deposition in the range of eight monolayers. Comparing the results for the different overlayers reveals the Ag overlayers to be less reactive than Au which causes the formation of metallic Bi on cleaved Bi2CaSr2Cu2O8 surface. Cu is shown to be the most reactive of the three metals. The Au and Ag overlayers display an island-growth mode, while Cu grows in a layer-by-layer fashion.

Electronic and structural properties of the Cu : Bi2CaSr2Cu2O8 interface

Abstract The formation of the Cu-Bi 2 CaSr 2 Cu 2 O 8 interface has been studied by photoelectron spectroscopy using synchrotron radiation. Photon energies in the range 20–1000 eV were utilized in order to probe both the valence band and to monitor chemical changes upon Cu deposition, as revealed by the core-level shifts. A strong chemical reaction between Bi 2 CaSr 2 Cu 2 O 8 and Cu is manifested by the formation of metallic Bi. From theb density variations as a function of electron emission angle it is shown that the metallic Bi segregates to the top surface layer.

Valence band resonant photoemission spectroscopy on Ce compounds at Ce 3d-4f and 4d-4f thresholds: surface vs. bulk electronic structure

Abstract Electron correlation in 4f orbitals strongly influences the low energy properties of Ce and its inter-metallic compounds. By exploiting the different surface sensitivity of valence band resonant photoemission (VBRP) at Ce 3d-4f and Ce 4d-4f thresholds, a lowering of the 4f hybridization has been recently observed on going from the bulk to the surface layers. We report an extensive VBRP work at both the thresholds (3d and 4d) on three Ce compounds (Ce7Rh3, CeRh2, and CeCo2). In addition to surface effects, we found different resonance behaviours for f0 and f1 configurations. These differences should not be disregarded when extracting the 4f bulk and surface contributions from the photoemission spectra.