Surface structure of MgO(001) surface studied by LEED

MgO(001) surfaces prepared by three different heat treatments were studied by LEED. The first was the surface just after the vacuum cleavage; the second was that after the annealing of the first one at 300°C in UHV. The third was prepared by cleaving in air and heating in oxygen until the carbon Auger peak disappear. LEED I-V curves of the diffraction spots, (10), (11) and (20), were recorded and compared carefully. No remarkable difference corresponding to the preparing methods was found. Therefore, the atomic structures of the surfaces seem to be similar to each other. These experimental I-V curves were compared with the theoretical curves obtained by dynamical calculations for several grades of surface relaxation. The theoretical curves with no relaxation or with the smallest one in the calculation (2.5% expand) fitted best with the experimental curves. The experimental curves were also compared with the theoretical ones calculated for the rumpled surface. But no effect induced by the rumpling was found in the experimental curves.     

Surface charge-density wave phase transitions on Mo(001) and W(001)

The recently reported phase transitions of Mo(001) and W(001) are discussed in terms of surface charge-density waves. Band structure, surface lattice distortion and possible accompanying surface antiferromagnetism are considered. The expected two-dimensional critical behaviour is also investigated.     

Surface segregation of Ge at SiGe(001) by concerted exchange pathways

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.     

Surface composition analysis by low-energy ion scattering

Low-energy ion scattering (LEIS) is an analytical tool that provides information on the atomic composition of the outer surface, when noble gas ions are used as projectiles. In fact, quantitative composition analysis is currently done on a huge variety of materials, including catalysts and organic materials. The information on the surface composition is contained in the signal of backscattered ions (typically 1–3 keV He⁺,Ne⁺). In order to translate the LEIS signal to an elemental surface concentration all factors determining the LEIS signal must be known. These are in particular the scattering cross section and the ion fraction of the backscattered particles. The scattering cross section, which is due to the screened electrostatic potential between target atom and projectile, is well-known for the prevailing conditions of LEIS. It is an intriguing fact that, despite the large quantity of successful applications, the charge exchange processes in LEIS are not yet fully understood. It is e.g. not known why in LEIS for a given atomic species on the surface the signal usually does not depend on which other species are present (absence of matrix effects). Significant progress has recently been made in the understanding of the underlying charge exchange processes.Therefore, the aim of this review is twofold: on the one hand, to summarize the present understanding of the factors that determine the ion fraction of the scattered projectiles in LEIS, i.e. charge exchange processes. On the other hand, to summarize how quantitative surface composition analysis can be accomplished.In addition, we critically review publications that deal with surface composition analysis by LEIS, and analyze in which cases and by what means this was achieved and where and why it was successful or failed. After reading this review the reader will be able to deal with the pitfalls encountered in LEIS and to choose preferred experimental conditions for quantitative surface composition analysis.     

Hydrogen behavior in Pd0.75Ag0.25H x studied by PAC and PAS

Hydrogen behavior in Pd_0.75Ag_0.25H _x has been investigated by both perturbed angular correlation (PAC) and positron annihilation lifetime spectroscopy (PAS) techniques. The PAC measurements with the ¹¹¹Ag/¹¹¹Cd probe nuclei were conducted at room temperature as a function of the hydrogen concentration x up to 0.35. At x = 0 the probe nuclei experienced no quadrupole perturbation, while a distinct perturbation was observed after hydrogen charging. The quadrupole interaction frequency ω ₁ and its distribution width σ ₁ and relative fraction f₁ all decrease with increasing hydrogen concentration, indicating size growth and concentration reduction of the hydrogen induced vacancy clusters. The measured frequency distribution width σ ₁ reveals hydrogen diffusion. The PAS measurements were also performed at room temperature. The obtained lifetimes and their variation with hydrogen concentration strongly support the PAC results. The experimental results can be explained by the appearance of the β-phase in the Pd_0.75Ag_0.25H _x samples. The α-phase is the major phase in the Pd_0.75Ag_0.25H _x samples. Charging of the hydrogen leads to the formation of the β-phase and the fraction of the β-phase increases with increasing hydrogen concentration. The present PAC and PAS results show that as the fraction of the β-phase increases, hydrogen induced vacancy clusters are formed and become larger and larger, while the cluster density reduces due to the combination of the vacancy clusters into larger size vacancy clusters.     

Azimuthal anisotropy in low-energy ion scattering from SmB6 (001)

A remarkable azimuthal anisotropy has been observed in the scattering of He⁺ ions of an incident energy of 1 KeV from the SmB₆(001) surface. The anisotropy can be interpreted on the basis of “shadowing” effects and clearly reflects the structure of the surface.     

Pressure dependence of interconfiguration fluctuations in Eu0.25La0.75Rh2.00 studied by Mossbauer spectroscopy

The effect of pressure on the 21.6 keV Mössbauer resonance of ¹⁵¹Eu in Eu_0.25La_0.75Rh_2.00 was studied at various temperatures. The strong pressure induced shift of the resonance lines can be described by an increase of the energy separation between the Eu²⁺ and Eu³⁺ valence states.     

Selected topics in low-energy ion scattering: Surface segregation in Cu/Ni alloys and ion neutralization

The usefulness of ion scattering for studies of surface segregation in alloys has been demonstrated for the Cu/Ni system. A strong surface enrichment in Cu is found. As a step towards understanding the mechanism of neutralization of ions at a surface, the scattering of Ne⁺ from Au has been studied. The importance and origin of oscillations in the energy dependence of the ion fraction is discussed.     

Superconductive tunneling into Nb3(Al0.75Ge0.25)

Current and conductance characteristics have been obtained for Nb₃(Al_0.75Ge_0.25)-barrier-Pb_0.7Bi_0.3 superconducting tunneling junctions prepared on Nb₃(Al_0.75Ge_0.25) ribbon substrates. Thermal oxide, anodized, and amorphous carbon tunneling barriers were used. These junctions are relatively fragile: they deteriorate under temperature recycling or sustained bias. Typical junction resistances are of the order of 50Ω, and 2Δ(0) of Nb₃(Al_0.75Ge_0.25) is ～1.3 meV.     

The MgO(001) surface studied by helium diffraction

The diffraction of He atoms at thermal energy (~64 meV) from the (001) cleavage plane of MgO was studied. A hard-wall intensity analysis of in-plane spectra obtained at near normal incident angles and in the main azimuthal planes yields the corrugation function, which is simply sinusoidal in both unit-cell directions with a total corrugation amplitude of 0.47 ± 0.02 Å. This value is larger than the one previously reported by K.H. Rieder. A more detailed study of the surface is in progress.     

Measurement of the spin-depairing interaction in Sn0.75Eu0.25Mo6S8

Mössbauer effect measurements of the ¹⁵¹ Eu resonance in the Chevrel phase superconductor Sn_0.75Eu_0.25Mo₆S₈ have been used to obtain the temperature dependence of the Eu paramagnetic relaxation rate. This consists of a temperature independent part arising from spin-spin interactions and a part linear in T due to the Korringa process. From the slope we obtain $\text{I}$ = 0.0033/Eu atom-spin, where $\text{I}$ is the exchange coupling between the rare-earth spin and the conduction electron spin, and N(E_F) is the local density of states. This value is roughly one order of magnetude lower than that measured in binary superconductors, and accounts for the very weak dependence of the transition temperature on magnetic impurity concentration.     

Surface layer relaxation on Mo(111) measured by low energy alkali ion scattering

The surface of clean Mo(111) has been studied using Li⁺ ion scattering at 1000 eV. The dependence of single scattering intensity was measured as a function of incident polar angle in the [12̄1] and the [21̄1̄] azimuths for various total scattering angles. Very pronounced intensity cut-offs are observed and are readily assignable to shadowing and blocking effects in scattering from first, second or third layer atoms. After taking certain precautions to avoid interference from deeper layers, the measured positions of the features yield the first-second layer spacing which is found to be strongly contracted by (18 ± 2)% compared to the bulk spacing. The method also indicates that the second-third layer spacing is possibly expanded (4 ± 4)%, but this result is uncertain due to possible contributions from deeper layers which make this value an upper limit of the layer spacing. The physical implications of these results and the uncertainties in the technique are discussed.     

Surface segregation in liquid Ga-In alloys studied by bombardment-induced photon emission

The surface composition in liquid Ga-In alloys bombarded by Ar⁺ ions is mostly determined by thermal segregation effects. Photon emission from sputtered atoms was successfully used to study the surface segregation. The coordination numbers characterizing the surface layer of liquid Ga-In alloys were found.     

Bromine adsorption on Ge(001) surface: comparative study for coverages of 0.25, 0.5, 0.75 and 1 monolayer

We present theoretical analysis of bromine adsorption on the Ge(001)-p(1×2) surface. Bromine adsorbs dissociatively at the Ge(001) surface and the adlayer of bromine develops ordered structures of adatoms bound to substrate dimers. We analyse atomic configurations of the Ge(001) surface covered by 0.25, 0.5, 0.75 and 1 monolayer of bromine. Various stable surface structures, found in calculations, are presented and their energies are compared. Simulated STM images are also discussed for the considered configurations.