Search

Lensless X-ray imaging techniques such as coherent diffraction imaging and ptychography, and Fourier transform holography can provide time-resolved, diffraction-limited images. Nearly all examples of these techniques have focused on transmission geometry, restricting the samples and reciprocal spaces that can be investigated. We report a lensless X-ray technique developed for imaging in Bragg and small-angle scattering geometries, which may also find application in transmission geometries. We demonstrate this by imaging a nanofabricated pseudorandom binary structure in small-angle reflection geometry. The technique can be used with extended objects, places no restriction on sample size, and requires no additional sample masking. The realization of X-ray lensless imaging in reflection geometry opens up the possibility of single-shot imaging of surfaces in thin films, buried interfaces in magnetic multilayers, organic photovoltaic and field-effect transistor devices, or Bragg planes in a single crystal.

Valence-band spectra of Cr and Fe metal were measured with photon energies around their respective $2p$ energies. An Auger signal is found to be superimposed on the valence-band photoemission signal for photon energies at and above the $2p$ absorption energy, but also for excitation energies down to $\ensuremath{\sim}4$ eV below the $2p$ absorption energy. This is the radiationless resonance Raman (resonant Raman Auger) regime and gives rise to a signal that is equivalent, in terms of the final state, to the 6 eV satellite in Ni with energies at 3.5 eV below ${E}_{\mathrm{F}}$ in Cr and 3.2 eV below ${E}_{\mathrm{F}}$ in Fe.

Self-assembled indium linear chains on the Si(111) surface are found to exhibit instability of the metallic phase and 1D charge density wave (CDW). The room-temperature metallic phase of these chains undergoes a temperature-induced, reversible transition into a semiconducting phase. The 1D CDW along the chains is observed directly in real space by scanning tunneling microscopy at low temperature. The Fermi contours of the metallic phase measured by angle-resolved photoemission exhibit a perfect nesting predicting precisely the CDW periodicity.

Two-dimensional Fermi contours of clean and hydrogen-induced surface electronic states have been systematically measured vs coverage of H on W(110) using angle-resolved photoemission. We observe a new pair of spin-orbit-interaction-split surface states, whose splitting grows systematically with hydrogen coverage and whose Fermi contours are strongly nested due to their opposing curvatures. Momentum-dependent electronic transitions between these states drive two previously observed anomalous behaviors: The phonon anomalies along {ovr {Gamma}H} and {ovr {Gamma}S} observed with inelastic helium scattering and electron energy loss spectroscopy, and the shifted-upper-layer reconstruction observed by low-energy electron diffraction for coverages greater than 0.5 monolayer. {copyright} {ital 1998} {ital The American Physical Society}

We report a detailed experimental and theoretical study of the electronic structure of \(Mo_{1-x}Re_x\) random alloys. We have measured electronic band dispersions for clean and hydrogen-covered \(Mo_{1-x}Re_x\) ( 110) with x = 0-0.25 using angle-resolved photoemission spectroscopy. Our results suggest that the bulk and most surface electronic bands shift relative to the Fermi level systematically and approximately rigidly with Re concentration. We distinguish and quantify two contributions to these shifts: a raise of the Fermi energy and an increase of the overall bandwidth. Alloy bands calculated using the first-principles Korringa-Kohn-Rostoker coherent-potential-approximation method accurately predict both of these effects. As derived from the rigid band model, the Fermi energy shift is inversely related to the bulk density of states in this energy region. Using our results, we also characterize an electronic topological transition of the bulk Fermi surface and relate this to bulk transport properties. Finally, we distinguish effects beyond the rigid band approximation: a highly surface-localized state and a composition-dependent impact of the spin-orbit interaction.

The kinetics of NH3 and ND3 desorption from Cu(001) surface have been measured by time‐resolved electron energy loss spectroscopy. The desorption precess is observed to be characterized by pseudo‐first‐order kinetics. The desorption energy and the prefactor have been determined for both NH3 and ND3. The prefactors are unusually low as compared to those observed for simple diatomic molecules. Moreover, we observed an unusual behavior consistent with an isotope kinetic compensation effect. Both of these observations can be explained within the context of transition state theory by the interplay between isotopic masses and the shifts of frustrated vibrational modes of adsorbed molecules. Our results imply that the conventional Arrhenius form would not be precisely applicable to molecular desorption, if a large enough temperature range could be measured, particularly for hydrogen‐containing molecules.

Angle-resolved photoemission measurements of the surface localized electronic states of the clean Pd(001) surface are presented. Several surface localized levels were observed, in qualitative agreement with an existing surface band-structure calculation. In particular, a state was observed which crosses the Fermi level (${\mathit{E}}_{\mathit{F}}$) near the X\ifmmode\bar\else\textasciimacron\fi{} point of the surface Brillouin zone. The segment of the Fermi contour derived from this crossing lies largely within a gap in the projected bulk Fermi surface and is parallel to the Brillouin-zone edge. The nesting properties of this segment of the Fermi contour suggest the possibility of a fairly strong electronic coupling to phonon excitations.

We present measurements of the Fermi surface and underlying band structure of a single layer of indium on Si(111) with $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ periodicity. Electrons from both indium valence electrons and silicon dangling bonds contribute to a nearly free, two-dimensional metal on a pseudo-4-fold lattice, which is almost completely decoupled at the Fermi level from the underlying hexagonal silicon lattice. The mean free path inferred from our data is quite long, suggesting the system might be a suitable model for studying the ground state of two-dimensional metals.

The electron bands of the Peierls compound NbSe 3 are mapped with angle-resolved photoemission. Data of the Fermi level crossings show the nesting condition responsible for the charge density wave along the one-dimensional axis. The instability with periodicity q =0.44 A −1 induces a remnant backfolding of the electron bands in the nominally metallic state high above the critical temperature.

Angle-resolved photoemission (ARP) of valence bands is a mature technique that has achieved spectacular success in band-mapping metals, semiconductors, and insulators. The purpose of the present study was the development of experimental and analytical techniques in ARP which take advantage of third generation light sources. Here the authors studied the relatively simple Cu surface in preparation for other metals. Copper and related metals themselves are of current interest, especially due to its role as an interlayer in spin valves and other magnetic heterostructures. A major goal of this study was the development of a systematic technique to quickly (i.e. in a few hours of synchrotron beamtime) measure the FS and separate it into bulk and surface FS`s. Often, one needs to avoid bulk features altogether, which one can achieve by carefully mapping their locations in k-space. The authors will also show how they systematically map Fermi surfaces throughout large volumes of k-space, and, by processing the resulting volume data sets, provide intuitive pictures of FS`s, both bulk and surface.

The 3-year activity report covers surface structure and phonon anomalies (surface reconstruction on W(001) and Mo(001), adsorbate lateral ordering, surface Fermi contours and phonon anomalies on Pt(111) and Pd(001)), adsorbate vibrational damping, charge transfer in momentum space: W(011)-K, surface states and resonances (relativistic effects {ampersand} computations, surface resonances).

Using angle-resolved photoemission, we have mapped the dispersion relations and Fermi contours for surface-localized electron states onto clean and hydrogen-covered Cr(110) surfaces. In particular, we have probed the relationship between hydrogen adsorption and the evolution of the spin density wave (SDW) periodicity in chromium thin films observed previously. We find qualitatively similar surface band dispersion relations to those on W(110) and Mo(110), although with a narrower bandwidth, broader spectral features, and a smaller impact from the spin–orbit interaction. We compare our results to existing first-principles calculations and find a significant disagreement for a surface band that produces a prominent surface Fermi contour. Upon hydrogen adsorption, the Fermi contour for a particular surface band becomes well nested at a wave vector that stabilizes a commensurate SDW. We suggest that a competition between commensurate two-dimensional (2D) and incommensurate 3D Fermi surface nesting plays an important role in the SDW energetics in thin Cr(110) films.

This paper discusses the following topics: surface states and resonances; surface fermi surfaces; and non-adiabatic adsorbate vibrational damping. (LSP)

Chromium films offer an excellent system to study the impact of dimensional confinement on physical properties associated with the spin-density-wave (SDW) ground state observed in bulk materials. These properties are also of some technological importance since chromium is a common component of thin film magnetic structures. We prepared chromium (1?1?0) films of high crystalline quality on a W(1?1?0) substrate with a wedge-shaped thickness profile so that the impact of confinement can be systematically studied. We have characterized these films using a combination of low-energy electron diffraction and microscopy as well as high-resolution angle-resolved photoemission spectroscopy. We have probed the Fermi surface and the nesting vectors therein that are relevant to the SDW ground state. We find these to predict accurately the observed bulk SDW periodicity. We have also characterized the SDW periodicity in the film directly by measuring the splitting between backfolded bands, and we find that this periodicity deviates markedly from the bulk periodicity for thinner films at higher temperatures. We have systematically mapped the SDW incommensurability and phase diagram as a function of both film thickness and temperature. We find commensurate and incommensurate phases that are separated by nearly continuous transitions. Our results suggest a simple model to explain the delicate interplay between commensurate and incommensurate phases that involves a balance between SDW stabilization energy and surface and interface energetics.

Angle-resolved photoemission measurements of the Fermi surfaces of several surface localized states of clean and hydrogen-covered W(110) are reported. Three hole orbits and one electron orbit have been characterized. The hole-orbit states are rapidly attenuated by hydrogen, while the electron-orbit states are shifted to higher binding energy resulting, initially, in an expansion of the Fermi surface, and ultimately, in its conversion to two hole orbits. These data represent the most detailed study of the behavior of the two-dimensional Fermi surface to date.