228 research outputs found
Effective calculation of LEED intensities using symmetry-adapted functions
The calculation of LEED intensities in a spherical-wave representation can be substantially simplified by symmetry relations. The wave field around each atom is expanded in symmetry-adapted functions where the local point symmetry of the atomic site applies. For overlayer systems with more than one atom per unit cell symmetry-adapted functions can be used when the division of the crystal into monoatomic subplanes is replaced by division into subplanes containing all symmetrically equivalent atomic positions
Spin Motion in Electron Transmission through Ultrathin Ferromagnetic Films Accessed by Photoelectron Spectroscopy
Ab initio and model calculations demonstrate that the spin motion of
electrons transmitted through ferromagnetic films can be analyzed in detail by
means of angle- and spin-resolved core-level photoelectron spectroscopy. The
spin motion appears as precession of the photoelectron spin polarization around
and as relaxation towards the magnetization direction. In a systematic study
for ultrathin Fe films on Pd(001) we elucidate its dependence on the Fe film
thickness and on the Fe electronic structure. In addition to elastic and
inelastic scattering, the effect of band gaps on the spin motion is addressed
in particular.Comment: 4 pages, 5 figure
Manifestations of Broken Symmetry: The Surface Phases of Ca(2-x)Sr(x)RuO4
The surface structural phases of Ca(2-x)Sr(x)RuO(4) are investigated using
quantitative Low Energy Electron Diffraction. The broken symmetry at the
surface enhances the structural instability against the RuO6 rotational
distortion while diminishing the instability against the RuO6 tilt distortion
occurring within the bulk crystal. As a result, suppressed structural and
electronic surface phase transition temperatures are observed, including the
appearance of an inherent Mott metal-to-insulator transition for x = 0.1 and
possible modifications of the surface quantum critical point near xc ~ 0.5.Comment: 4 pages, 4 figure
Dynamic image potential at an Al(111) surface
We evaluate the electronic self-energy Sigma(E) at an Al(111) surface using the GW space-time method. This self-energy automatically includes the image potential V-im not present in any local-density approximation for exchange and correlation. We solve the energy-dependent quasiparticle equations and calculate the effective local potential experienced by electrons in the near-surface region. The relative contribution of exchange proves to be very different for states above the Fermi level. The image-plane position for interacting electrons is closer to the surface than for the purely electrostatic effects felt by test charges, and, like its classical counterpart, is drawn inwards by the effects of atomic structure
Bias-voltage dependence of the magneto-resistance in ballistic vacuum tunneling: Theory and application to planar Co(0001) junctions
Motivated by first-principles results for jellium and by surface-barrier
shapes that are typically used in electron spectroscopies, the bias voltage in
ballistic vacuum tunneling is treated in a heuristic manner. The presented
approach leads in particular to a parameterization of the tunnel-barrier shape,
while retaining a first-principles description of the electrodes. The proposed
tunnel barriers are applied to Co(0001) planar tunnel junctions. Besides
discussing main aspects of the present scheme, we focus in particular on the
absence of the zero-bias anomaly in vacuum tunneling.Comment: 19 pages with 8 figure
Origin of the photoemission final-state effects in Bi2Sr2CaCu2O8 by very-low-energy electron diffraction
Very-low-energy electron diffraction with a support of full-potential band
calculations is used to achieve the energy positions, K// dispersions,
lifetimes and Fourier compositions of the photoemission final states in
Bi2Sr2CaCu2O8 at low excitation energies. Highly structured final states
explain the dramatic matrix element effects in photoemission. Intense c(2x2)
diffraction reveals a significant extrinsic contribution to the shadow Fermi
surface. The final-state diffraction effects can be utilized to tune the
photoemission experiment on specific valence states or Fermi surface replicas.Comment: 4 pages, 3 Postscript figures, submitted to Phys. Rev. Lett; major
revision
Steam reforming on transition-metal carbides from density-functional theory
A screening study of the steam reforming reaction (CH_4 + H_2O -> CO + 3H_2)
on early transition-metal carbides (TMC's) is performed by means of
density-functional theory calculations. The set of considered surfaces includes
the alpha-Mo_2C(100) surfaces, the low-index (111) and (100) surfaces of TiC,
VC, and delta-MoC, and the oxygenated alpha-Mo_2C(100) and TMC(111) surfaces.
It is found that carbides provide a wide spectrum of reactivities towards the
steam reforming reaction, from too reactive via suitable to too inert. The
reactivity is discussed in terms of the electronic structure of the clean
surfaces. Two surfaces, the delta-MoC(100) and the oxygen passivated
alpha-Mo_2C(100) surfaces, are identified as promising steam reforming
catalysts. These findings suggest that carbides provide a playground for
reactivity tuning, comparable to the one for pure metals.Comment: 6 pages, 4 figure
Science and society in education
This booklet is for teachers who want to expand their
teaching approaches to include socio-scientific issues
which enrich and give meaning to core scientific principles.
It is meant to enhance young people’s curiosity about the
social and scientific world and raise important questions
about issues which affect their lives. We call this approach
Socio-Scientific Inquiry-Based Learning, or ‘SSIBL’ for
short. Chapters 1 and 2 present an introduction to the
theoretical background of SSIBL. In chapter 3, SSIBL will
be approached from a classroom perspective, providing a
simplified version of the framework and showing teaching
examples
Quantitative versus standard pupillary light reflex for early prognostication in comatose cardiac arrest patients: an international prospective multicenter double-blinded study.
To assess the ability of quantitative pupillometry [using the Neurological Pupil index (NPi)] to predict an unfavorable neurological outcome after cardiac arrest (CA).
We performed a prospective international multicenter study (10 centers) in adult comatose CA patients. Quantitative NPi and standard manual pupillary light reflex (sPLR)-blinded to clinicians and outcome assessors-were recorded in parallel from day 1 to 3 after CA. Primary study endpoint was to compare the value of NPi versus sPLR to predict 3-month Cerebral Performance Category (CPC), dichotomized as favorable (CPC 1-2: full recovery or moderate disability) versus unfavorable outcome (CPC 3-5: severe disability, vegetative state, or death).
At any time between day 1 and 3, an NPi ≤ 2 (n = 456 patients) had a 51% (95% CI 49-53) negative predictive value and a 100% positive predictive value [PPV; 0% (0-2) false-positive rate], with a 100% (98-100) specificity and 32% (27-38) sensitivity for the prediction of unfavorable outcome. Compared with NPi, sPLR had significantly lower PPV and significantly lower specificity (p < 0.001 at day 1 and 2; p = 0.06 at day 3). The combination of NPi ≤ 2 with bilaterally absent somatosensory evoked potentials (SSEP; n = 188 patients) provided higher sensitivity [58% (49-67) vs. 48% (39-57) for SSEP alone], with comparable specificity [100% (94-100)].
Quantitative NPi had excellent ability to predict an unfavorable outcome from day 1 after CA, with no false positives, and significantly higher specificity than standard manual pupillary examination. The addition of NPi to SSEP increased sensitivity of outcome prediction, while maintaining 100% specificity
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