493 research outputs found
Influence of the Third Dimension of Quasi-Two-Dimensional Cuprate Superconductors on Angle-Resolved Photoemission Spectra
Angle-resolved photoemission spectroscopy (ARPES) presents significant
simplications in analyzing strictly two-dimensional (2D) materials, but even
the most anisotropic physical systems display some residual
three-dimensionality. Here we demonstrate how this third dimension manifests
itself in ARPES spectra of quasi-2D materials by considering the example of the
cuprate BiSrCaCuO (Bi2212). The intercell, interlayer
hopping, which is responsible for -dispersion of the bands, is found to
induce an irreducible broadening to the ARPES lineshapes with a characteristic
dependence on the in-plane momentum . Our study suggests that
ARPES lineshapes can provide a direct spectroscopic window for establishing the
existence of coherent c-axis conductivity in a material via the detection of
this new broadening mechanism, and bears on the understanding of 2D to 3D
crossover and pseudogap and stripe physics in novel materials through ARPES
experiments.Comment: 5 pages, 4 figure
Low-energy electron diffraction study of potassium adsorbed on single-crystal graphite and highly oriented pyrolytic graphite
Potassium adsorption on graphite has been a model system for the understanding of the interaction of alkali
metals with surfaces. The geometries of the s232d structure of potassium on both single-crystal graphite
(SCG) and highly oriented pyrolytic graphite (HOPG) were investigated for various preparation conditions for
graphite temperatures between 55 and 140 K. In all cases, the geometry was found to consist of K atoms in the
hollow sites on top of the surface. The K-graphite average perpendicular spacing is 2.79±0.03 Ă
, corresponding
to an average C-K distance of 3.13±0.03 Ă
, and the spacing between graphite planes is consistent with the
bulk spacing of 3.35 Ă
. No evidence was observed for a sublayer of potassium. The results of dynamical LEED studies for the clean SCG and HOPG surfaces indicate that the surface structures of both are consistent with the truncated bulk structure of graphite
Spatially explicit assessment of roundwood and logging residues availability and costs for the EU28
Competition for woody biomass between material and energy uses is expected to further increase in the future, due to the limited availability of forest resources and increasing demand of wood for material and bioenergy. Currently, methodological approaches for modeling wood production and delivery costs from forest to industrial gates are missing. This study combines forest engineering, geographically explicit information, environmental constraints and economics in a bottom-up approach to assess costâsupply curves. The estimates are based on a multitude of wood supply systems that were assigned according to geographically explicit forestry characteristics. For each harvesting and transportation system, efficiencies were modeled according to harvesting sites and main delivery hubs. The costâsupply curves for roundwood and logging residues as estimates for current time and for the future (2030) show that there are large regional differences in the potential to increase extraction in the EU28. In most EU Member States, the costs of logging residues extraction increase exponentially already for low levels of mobilization, while extraction of roundwood can be increased to a larger extent within reasonable costs (30â40â
$/m3). The large differences between countries in their harvest potential highlight the importance of spatially explicit analyses
Optimization of Neutrino Rates from the EURISOL Beta-Beam Accelerator Complex
The beta beam concept for the production of intense (anti-)neutrino beams is now well established. A baseline design has recently been published for a beta-beam facility at CERN. It has the virtue of respecting the known limitations of the CERN PS and SPS synchrotrons, but falls short of delivering the requested annual rate of neutrinos. We report on a first analysis to increase the rate using the baseline ions of 6He and 18 Ne. A powerful method to understand the functional dependence of the many parameters that influence the figure of merit for a given facility is available with modern analytical calculation software. The method requires that a symbolic analytical description is produced of the full accelerator chain. Such a description has been made using Mathematica for the proposed beta beam facility at CERN. The direct access from Mathematica to an ORACLE database for reading basic design parameters and re-injecting derived parameters for completion of the parameter list is both convenient and efficient
Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars
There is no straightforward explanation for intrinsic X-ray emission from
intermediate-mass main-sequence stars. Therefore the observed emission is often
interpreted in terms of (hypothesized) late-type magnetically active companion
stars. We use Chandra imaging observations to spatially resolve in X-rays a
sample of main-sequence B-type stars with recently discovered companions at
arcsecond separation. We find that all spatially resolved companions are X-ray
emitters, but seven out of eleven intermediate-mass stars are also X-ray
sources. If this emission is interpreted in terms of additional sub-arcsecond
or spectroscopic companions, this implies a high multiplicity of B-type stars.
Firm results on B star multiplicity pending, the alternative, that B stars
produce intrinsic X-rays, can not be discarded. The appropriate scenario in
this vein is might be a magnetically confined wind, as suggested for the X-ray
emission of the magnetic Ap star IQ Aur. However, the only Ap star in the
Chandra sample is not detected in X-rays, and therefore does not support this
picture.Comment: 12 pages; accepted for publication in Astronomy & Astrophysic
Confirmation of Parity Violation in the Gamma Decay of
This paper reports measurements using the technique of On Line Nuclear
Orientation (OLNO) which reexamine the gamma decay of isomeric Hf and specifically the 501 keV 8 -- 6
transition. The irregular admixture of E2 to M2/E3 multipolarity in this
transition, deduced from the forward-backward asymmetry of its angular
distribution, has for decades stood as the prime evidence for parity mixing in
nuclear states. The experiment, based on ion implantation of the newly
developed mass-separated Hf beam at ISOLDE, CERN into an
iron foil maintained at millikelvin temperatures, produces higher degrees of
polarization than were achieved in previous studies of this system. The value
found for the E2/M2 mixing ratio, = -0.0324(16)(17), is in close
agreement with the previous published average value = - 0.030(2), in
full confirmation of the presence of the irregular E2 admixture in the 501 keV
transition. The temperature dependence of the forward-backward asymmetry has
been measured over a more extended range of nuclear polarization than
previously possible, giving further evidence for parity mixing of the 8 and 8 levels and the deduced E2/M2 mixing ratio.Comment: 28 pages, 9 figures, accepted for publication in Physical Review
Multiplicity of late-type B stars with HgMn peculiarity
Observations at various wavelengths of late B-type stars exhibiting strong
overabundances of the chemical elements Hg and Mn in their atmospheres indicate
that these stars are frequently found in binary and multiple systems.
We intend to study the multiplicity of this type of chemically peculiar
stars, looking for visual companions in the range of angular separation between
0.05" and 8".
We carried out a survey of 56 stars using diffraction-limited near-infrared
imaging with NAOS-CONICA at the VLT.
Thirty-three companion candidates in 24 binaries, three triples, and one
quadruple system were detected. Nine companion candidates were found for the
first time in this study. Five objects are likely chance projections. The
detected companion candidates have K magnitudes between 5.95m and 18.07m and
angular separations ranging from <0.05" to 7.8", corresponding to linear
projected separations of 13.5-1700 AU.
Our study clearly confirms that HgMn stars are frequently members of binary
and multiple systems. Taking into account companions found by other techniques,
the multiplicity fraction in our sample may be as high as 91%. The membership
in binary and multiple systems seems to be a key point to understanding the
abundance patterns in these stars.Comment: 12 pages, 5 figures, 3 tables, accepted for publication in A&
Topical Review on "Beta-beams"
Neutrino physics is traversing an exciting period, after the important
discovery that neutrinos are massive particles, that has implications from
high-energy physics to cosmology. A new method for the production of intense
and pure neutrino beams has been proposed recently: the ``beta-beam''. It
exploits boosted radioactive ions decaying through beta-decay. This novel
concept has been the starting point for a new possible future facility. Its
main goal is to address the crucial issue of the existence of CP violation in
the lepton sector. Here we review the status and the recent developments with
beta-beams. We discuss the original, the medium and high-energy scenarios as
well as mono-chromatic neutrino beams produced through ion electron-capture.
The issue of the degeneracies is mentioned. An overview of low energy
beta-beams is also presented. These beams can be used to perform experiments of
interest for nuclear structure, for the study of fundamental interactions and
for nuclear astrophysics.Comment: Topical Review for Journal of Physics G: Nuclear and Particle
Physics, published version, minor corrections, references adde
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