Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.

/pdf/probing-bulk-electronic-structure-with-hard-x-ray-angle-nxuhbhy6u1.pdf

Probing Bulk Electronic Structure with Hard X-ray Angle-Resolved Photoemission

Electronic substrate-mediated interactions

In this paper we examine T-violating triple-product correlations (TP's) in B -> V1 V2 decays. TP's are excellent probes of physics beyond the standard model (SM) for two reasons: (i) within the SM, most TP's are expected to be tiny, and (ii) unlike direct CP asymmetries, TP's are not suppressed by the small strong phases which are expected in B decays. TP's are obtained via the angular analysis of B -> V1 V2. In a general analysis based on factorization, we demonstrate that the most promising decays for measuring TP's in the SM involve excited final-state vector mesons, and we provide estimates of such TP's. We find that there are only a handful of decays in which large TP's are possible, and the size of these TP's depends strongly on the size of nonfactorizable effects. We show that TP's which vanish in the SM can be very large in models with new physics. The measurement of a nonzero TP asymmetry in a decay where none is expected would specifically point to new physics involving large couplings to the right-handed b-quark.

Triple-Product Correlations in B -> V1 V2$ Decays and New Physics

http://www-old.mpi-halle.mpg.de/mpi/publi/pdf/9056_10.pdf

On the preparation of clean tungsten single crystals

We have re-investigated the well-studied hydrogen covered Si(100) surface using high resolution electron energy loss spectroscopy (HREELS) with a spectral resolution down to 13 cm1, representing an improvement by a factor of 3–4 compared to earlier work. For the first time in HREELS, it has been possible to clearly distinguish between the stretching vibrations of H-Si-Si-H units on monohydride surfaces with a 2×1 low energy electron diffraction pattern, on the one hand, and SiH2 units on dihydride surfaces with 1×1 symmetry on the other hand, the two being shifted by 5 cm-1 with respect to each other, in good agreement with results from infrared spectroscopy. Furthermore, we find trihydride units even for relatively low exposures beyond monohydride saturation coverage from their distinct stretching frequencies, and this points towards early etching stages. The question of the scattering mechanism applicable in our experiments is discussed. Since there are no spectral limitations in HREELS, we can analyze the bending and scissor vibrations with similar accuracy as the stretching vibration. Again, we observe fine structure in our loss peaks, the umbrella mode, for example, giving once more evidence for trihydride species at the surface. The complicated line shape of all vibrational modes can thus be used to deduce detailed structural information about the surface atomic structure, opening up entirely new possibilities by employing HREELS for the structural characterization of (silicon) surfaces with atomic hydrogen as a local probe.

Ultimate resolution electron energy loss spectroscopy at H/Si(100) surfaces

Normal-emission photoelectron spectra were recorded from W(110) using photons in the energy-range $11\mathrm{}\mathrm{eV}l~\mathrm{\ensuremath{\Elzxh}}\mathrm{\ensuremath{\omega}}l~30\mathrm{}\mathrm{eV}.$ The results are discussed in comparison to available band-structure calculations and experimental data obtained in earlier studies. We find reasonable overall agreement and summarize a set of ``best'' critical-point energies along the $\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Sigma}\ensuremath{-}N$ direction of the bulk Brillouin zone, both below and above the Fermi energy.

Photoemission from bulk bands along the surface normal of W(110)

Photoemission studies of the W(110)/Ag interface

Normal-emission photoelectron studies of the W(110)(1 × 1)O surface

High-resolution electron-energy-loss spectroscopy was applied to analyze in detail the spatial distribution of dopants in \ensuremath{\delta}-doped (Si) GaAs(100) samples, molecular-beam epitaxially grown at 550 \ifmmode^\circ\else\textdegree\fi{}C with different doping levels and depths. The fitting of the measured energy-loss spectra, based on a smooth self-consistent electron-density profile, reveals that at this growth temperature the silicon atoms are spread symmetrically around the initial doping plane for the doping level of ${\mathit{N}}_{\mathrm{Si}}$=1.3\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. However, the asymmetry in the dopant spreading becomes appreciable for the higher doped sample with ${\mathit{N}}_{\mathrm{Si}}$=6\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. In this case different diffusion coefficients of the dopant atoms were assumed in the growth direction and backward from the initial doping plane in order to obtain better fits to the measured energy-loss spectra. It was found that the damping of the plasmon excitations \ensuremath{\Gamma} changes versus a variation of the primary electron-beam energy ${\mathit{E}}_{0}$ with a significant decrease for the lowest primary energy applied. The observed dependence of \ensuremath{\Gamma} on ${\mathit{E}}_{0}$ is connected with the dominant optical phonon scattering, mostly contributing to the plasmon damping at room temperature. \textcopyright{} 1996 The American Physical Society.

Silicon spreading in delta -doped GaAs(100): A high-resolution electron-energy-loss-spectroscopy study.

We present a detailed investigation of the submonolayer coadsorption of Cl and O on Ag(110) at room temperature. Our main method is high-resolution electron energy loss spectroscopy (HREELS), supplemented by LEED, XPS and angle-resolved UPS. Preadsorbed chlorine at coverages below about 0.15 monolayer and subsequent exposure to causes the development of separate domains covered with either Cl or the O added row reconstruction. Above 0.2 monolayers of Cl precoverage, oxygen adsorption is inhibited completely. By contrast, oxygen preadsorbed in the O structure is almost completely removed by subsequent exposure to chlorine, and the added rows are dissolved.

Submonolayer coadsorption of chlorine and oxygen on Ag(110) studied by electron energy loss vibrational spectroscopy

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