I. Lindau

Beamline I311 at MAX-LAB: A VUV/soft X-ray undulator beamline for high resolution electron spectroscopy

We describe a beamline based on a plane-grating monochromator and an end-station designed for high resolution photoemission and photoabsorption spectroscopy on surfaces and interfaces using undulator radiation in the energy range 30 1500 eV. The general design of the beamline is based on a horizontally focusing pre-mirror, an SX-700 type of plane-grating monochromator and re-focusing optics of Kirkpatrick-Baez design.

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.

A photoemission spectroscopy and X-ray absorption study of Bi(2)Sr(2)CaCu(2)O(8) single crystal with adsorbed Cs: On the origin of states affected by electron doping and evidence for spatially resolved electron doping

The influence of electron doping, via deposition of small amounts of Cs, on the electronic structure of has been studied by high-resolution photoelectron spectroscopy (PES) and x-ray absorption spectroscopy (XAS), utilizing synchrotron radiation. The changes in the electronic structure were monitored by PES of the valence band and of the O 1s, Bi 4f, Bi 5d, Ca 2p and Sr 3d core levels, and by XAS at the O 1s, Cu 2p and Ca 2p edges. The experimental data suggest that the loss of the Fermi edge and the loss of spectral intensity down to about 2 eV below the Fermi level, and the substantial loss of spectral intensity of the pre-edge structure in the O 1s XAS spectrum are mainly due to annihilation of states with O 2p character in the Cu - O layer. It is evident from bulk- and surface-sensitive XAS spectra that the electron doping by Cs affects the electronic structure more strongly close to the surface. This implies that the doping occurs locally and that the charge transfer between the different layers in the unit cell is not uniform. Thus it seems possible to alter the electronic properties of locally through spatially resolved electron doping. When larger amounts of Cs are deposited, a chemical reaction occurs which causes a disruption of the Bi - O layer. This deposition regime is characterized by the presence of caesium oxide and reduced (metallic) Bi. (Less)

Studying the effect of oxygen content on the electron structure of Nd1.85Ce0.15CuO4 by means of photoelectron spectromicroscopy

Changes in the electron structure of the surface layer of Nd1.85Ce0.15CuO4 (NCCO) epitaxial films, which were caused by variation of the oxygen content and modification of the crystal structure of samples as a result of Ar+ ion etching and annealing, have been studied by means of photoelectron spectromicroscopy. A method is proposed for the cleaning the surface of oxygen-containing superconductors, which includes sequential stages of deep ion etching, high-temperature annealing in an oxygen-containing atmosphere (for the structural recovery and saturation with oxygen), a short-term ion etching (for the removal of an adsorbed layer of the oxidizer), and the final vacuum annealing of radiation-induced effects. The application of this procedure to NCCO films allowed an electron structure to be obtained, which was identical to that inherent in the surface of single crystals cleaved in situ in the measurement chamber.

Surface degradation of InxGa1−xN thin films by sputter-anneal processing: A scanning photoemission microscope study

The effects of nitrogen ion sputtering and thermal anneal processing on the surface electronic structure of the ternary III–V semiconductor In0.12Ga0.88N have been studied using scanning photoemission microscopy. No phase separation of the material is observed for annealing temperatures up to 650 °C. However, samples annealed at 700 °C for 5 h show clear evidence of phase separation. Furthermore, annealing at these temperatures with the sample surface directly exposed to ultrahigh vacuum produces a surface greatly deficient in In and with considerable surface roughness. This can be circumvented by using a sacrificial sample in physical contact with the film to artificially increase the local vapor pressure of Ga, In, and N during annealing.

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 .

Angle resolved photoemission on NiO: on the nature of the valence band

Angle resolved photoelectron spectroscopy has been performed on the valence band of NiO at the National Swedish Laboratory for Synchrotron Radiation. Normal emission spectra have been recorded for photon energies between 17 and 140 eV in order to study the electronic structure of the valence band. The experimentally determined band structure has been compared with a local density augmented plane wave band structure calculation. Signs of the influence of antiferromagnetic ordering are found and the general agreement between experimental and theoretical oxygen derived bands indicates strong hybridization between Ni 3d and O 2p orbitals.

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.