9,631 research outputs found
Electron-magnon scattering in elementary ferromagnets from first principles: lifetime broadening and band anomalies
We study the electron-magnon scattering in bulk Fe, Co, and Ni within the
framework of many-body perturbation theory implemented in the full-potential
linearized augmented-plane-wave method. To this end, a -dependent
self-energy ( self-energy) describing the scattering of electrons and
magnons is constructed from the solution of a Bethe-Salpeter equation for the
two-particle (electron-hole) Green function, in which single-particle Stoner
and collective spin-wave excitations (magnons) are treated on the same footing.
Partial self-consistency is achieved by the alignment of the chemical
potentials. The resulting renormalized electronic band structures exhibit
strong spin-dependent lifetime effects close to the Fermi energy, which are
strongest in Fe. The renormalization can give rise to a loss of quasiparticle
character close to the Fermi energy, which we attribute to electron scattering
with spatially extended spin waves. This scattering is also responsible for
dispersion anomalies in conduction bands of iron and for the formation of
satellite bands in nickel. Furthermore, we find a band anomaly at a binding
energy of 1.5~eV in iron, which results from a coupling of the quasihole with
single-particle excitations that form a peak in the Stoner continuum. This band
anomaly was recently observed in photoemission experiments. On the theory side,
we show that the contribution of the Goldstone mode to the self-energy is
expected to (nearly) vanish in the long-wavelength limit. We also present an
in-depth discussion about the possible violation of causality when an
incomplete subset of self-energy diagrams is chosen
Novel approaches in professional education to foster innovation in the chemical industry
Fostering innovation in the chemical industry demands a new approach to the competencies required by employees. To succeed in innovation processes, competencies are needed that transcend the mere scientific knowledge in chemistry: skills are needed to work in teams in a transdisciplinary manner in order to generate new ideas. Moreover, these skills will enable employees to exploit innovations that exist at the interface with other disciplines and industries. However, how can such competencies be trained and fostered? This article addresses competencies that are crucial to cross-industry and interdisciplinary innovation processes. Furthermore, it outlines principles for developing competencies through professional educational activities and provides both practical examples and domains for further researc
Field dependent collision frequency of the two-dimensional driven random Lorentz gas
In the field-driven, thermostatted Lorentz gas the collision frequency
increases with the magnitude of the applied field due to long-time
correlations. We study this effect with computer simulations and confirm the
presence of non-analytic terms in the field dependence of the collision
frequency as predicted by kinetic theory.Comment: 6 pages, 2 figures. Submitted to Phys. Rev.
Implications of Hyperon Pairing for Cooling of Neutron Stars
The implications of hyperon pairing for the thermal evolution of neutron
stars containing hyperons are investigated. The outcome of cooling simulations
are compared for neutron star models composed only of nucleons and leptons,
models including hyperons, and models including pairing of hyperons. We show
that lambda and neutron pairing suppresses all possible fast neutrino emission
processes in not too massive neutron stars. The inclusion of lambda pairing
yields better agreement with X-ray observations of pulsars. Particularly, the
surface temperatures deduced from X-ray observations within the hydrogen
atmosphere model are more consistent with the thermal history of neutron stars
containing hyperons, if the critical temperature for the onset of lambda and
nucleon pairing is not too small.Comment: 7 pages, 3 figures. To be published in ApJL. The postscript and
additional tables can be found at
http://www.physik.uni-muenchen.de/sektion/suessmann/astro/cool/schaab.089
Local Thermal Equilibrium in Quantum Field Theory on Flat and Curved Spacetimes
The existence of local thermal equilibrium (LTE) states for quantum field
theory in the sense of Buchholz, Ojima and Roos is discussed in a
model-independent setting. It is shown that for spaces of finitely many
independent thermal observables there always exist states which are in LTE in
any compact region of Minkowski spacetime. Furthermore, LTE states in curved
spacetime are discussed and it is observed that the original definition of LTE
on curved backgrounds given by Buchholz and Schlemmer needs to be modified.
Under an assumption related to certain unboundedness properties of the
pointlike thermal observables, existence of states which are in LTE at a given
point in curved spacetime is established. The assumption is discussed for the
sets of thermal observables for the free scalar field considered by Schlemmer
and Verch.Comment: 16 pages, some minor changes and clarifications; section 4 has been
shortened as some unnecessary constructions have been remove
Effects of P300-based BCI use on reported presence in a virtual environment
Brain-computer interfaces (BCIs) are becoming more and more popular as an input device for virtual worlds and computer games. Depending on their function, a major drawback is the mental workload associated with their use and there is significant effort and training required to effectively control them. In this paper, we present two studies assessing how mental workload of a P300-based BCI affects participants" reported sense of presence in a virtual environment (VE). In the first study, we employ a BCI exploiting the P300 event-related potential (ERP) that allows control of over 200 items in a virtual apartment. In the second study, the BCI is replaced by a gaze-based selection method coupled with wand navigation. In both studies, overall performance is measured and individual presence scores are assessed by means of a short questionnaire. The results suggest that there is no immediate benefit for visualizing events in the VE triggered by the BCI and that no learning about the layout of the virtual space takes place. In order to alleviate this, we propose that future P300-based BCIs in VR are set up so as require users to make some inference about the virtual space so that they become aware of it,which is likely to lead to higher reported presence
Flexible generation of correlated photon pairs in different frequency ranges
The feasibility to generate correlated photon pairs at variable frequencies
is investigated. For this purpose, we consider the interaction of an
off-resonant laser field with a two-level system possessing broken inversion
symmetry. We show that the system generates non-classical photon pairs
exhibiting strong intensity-intensity correlations. The intensity of the
applied laser tunes the degree of correlation while the detuning controls the
frequency of one of the photons which can be in the THz-domain. Furthermore, we
observe the violation of a Cauchy-Schwarz inequality characterizing these
photons.Comment: 5 pages, 4 figure
A Problematic Set of Two-Loop Self-Energy Corrections
We investigate a specific set of two-loop self-energy corrections involving
squared decay rates and point out that their interpretation is highly
problematic. The corrections cannot be interpreted as radiative energy shifts
in the usual sense. Some of the problematic corrections find a natural
interpretation as radiative nonresonant corrections to the natural line shape.
They cannot uniquely be associated with one and only one atomic level. While
the problematic corrections are rather tiny when expressed in units of
frequency (a few Hertz for hydrogenic P levels) and do not affect the
reliability of quantum electrodynamics at the current level of experimental
accuracy, they may be of importance for future experiments. The problems are
connected with the limitations of the so-called asymptotic-state approximation
which means that atomic in- and out-states in the S-matrix are assumed to have
an infinite lifetime.Comment: 12 pages, 3 figures (New J. Phys., in press, submitted 28th May
Spin dynamics in the Kapitza-Dirac effect
Electron spin dynamics in Kapitza-Dirac scattering from a standing laser wave
of high frequency and high intensity is studied. We develop a fully
relativistic quantum theory of the electron motion based on the time-dependent
Dirac equation. Distinct spin dynamics, with Rabi oscillations and complete
spin-flip transitions, is demonstrated for Kapitza-Dirac scattering involving
three photons in a parameter regime accessible to future high-power X-ray laser
sources. The Rabi frequency and, thus, the diffraction pattern is shown to
depend crucially on the spin degree of freedom
Know the Star, Know the Planet. III. Discovery of Late-Type Companions to Two Exoplanet Host Stars
We discuss two multiple star systems that host known exoplanets: HD 2638 and
30 Ari B. Adaptive optics imagery revealed an additional stellar companion to
both stars. We collected multi-epoch images of the systems with Robo-AO and the
PALM-3000 adaptive optics systems at Palomar Observatory and provide relative
photometry and astrometry. The astrometry indicates that the companions share
common proper motion with their respective primaries. Both of the new
companions have projected separations less than 30 AU from the exoplanet host
star. Using the projected separations to compute orbital periods of the new
stellar companions, HD 2638 has a period of 130 yrs and 30 Ari B has a period
of 80 years. Previous studies have shown that the true period is most likely
within a factor of three of these estimated values. The additional component to
the 30 Ari makes it the second confirmed quadruple system known to host an
exoplanet. HD 2638 hosts a hot Jupiter and the discovery of a new companion
strengthens the connection between hot Jupiters and binary stars. We place the
systems on a color-magnitude diagram and derive masses for the companions which
turn out to be roughly 0.5 solar mass stars.Comment: Accepted to Astronomical Journal, 16 pages, 5 Figure
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