747 research outputs found
Evolution of Mid-gap States and Residual 3-Dimensionality in LaSrCuO
We have carried out extensive first principles doping-dependent computations
of angle-resolved photoemission (ARPES) intensities in LaSrCuO
(LSCO) over a wide range of binding energies. Intercell hopping and the
associated 3-dimensionality, which is usually neglected in discussing cuprate
physics, is shown to play a key role in shaping the ARPES spectra. Despite the
obvious importance of strong coupling effects (e.g. the presence of a lower
Hubbard band coexisting with mid-gap states in the doped insulator), we show
that a number of salient features of the experimental ARPES spectra are
captured to a surprisingly large extent when effects of -dispersion are
properly included in the analysis.Comment: 5 pages, 4 figure
Reply to Comment on Circular Dichroism in the Angle-Resolved Photoemission Spectrum of the High-Temperature Bi2Sr2CaCu2O8 Superconductor http://arxiv.org/abs/1004.1648
We conclude that arguments of Norman et al. in their Comment do not provide a
significant basis for their claim that the geometric mechanism for explaining
the observations reported by them is not viable. More generally, our study
highlights the importance of assessing structural issues before invoking exotic
mechanisms for explaining unusual spectroscopic observations, especially in
complex materia
Beam Dynamics Studies for the HIE-ISOLDE Linac at CERN
The upgrade of the normal conducting (NC) Radioactive Ion Beam EXperiment
(REX)-ISOLDE heavy ion accelerator at CERN, under the High Intensity and Energy
(HIE)-ISOLDE framework, proposes the use of superconducting (SC) quarter-wave
resonators (QWRs) to increase the energy capability of the facility from 3
MeV/u to beyond 10 MeV/u. A beam dynamics study of a lattice design comprising
SC QWRs and SC solenoids has confirmed the design's ability to accelerate ions,
with a mass-to-charge ratio in the range 2.5 < A/q < 4.5, to the target energy
with a minimal emittance increase. We report on the development of this study
to include the implementation of realistic fields within the QWRs and
solenoids. A preliminary error study is presented in order to constrain
tolerances on the manufacturing and alignment of the linac.Comment: 3 pages, 8 figures, 1 table, submitted to the Particle Accelerator
Conference (PAC) 2009 in Vancouver. Page formatting changed to US letter siz
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
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
Molecular tendrils feeding star formation in the Eye of the Medusa - The Medusa merger in high resolution 12CO 2-1 maps
Studying molecular gas properties in merging galaxies gives us important
clues to the onset and evolution of interaction-triggered starbursts. NGC4194
is particularly interesting to study since its FIR-to-CO luminosity ratio
rivals that of ULIRGs,despite its lower luminosity compared to ULIRGs, which
indicates a high star formation efficiency that is relative to even most
spirals and ULIRGs.We study the molecular medium at an angular resolution of
0.65"x .52" through our observations of CO2-1 emission using the SMA. We
compare our CO2-1 maps with optical HST and high angular resolution radio
continuum images to study the relationship between molecular gas and other
components of the starburst region. The molecular gas is tracing the
complicated dust lane structure of NGC4194 with the brightest emission being
located in an off-nuclear ring-like structure with ~320pc radius, the Eye of
the Medusa. The bulk CO emission of the ring is found south of the kinematical
center of NGC4194. The northern tip of the ring is associated with the galaxy
nucleus, where the radio continuum has its peak. A prominent, secondary
emission maximum in the radio continuum is located inside the molecular ring.
This suggests that the morphology of the ring is partially influenced by
massive supernova explosions. From the combined evidence, we propose that the
Eye of the Medusa contains a shell of swept up material where we identify a
number of giant molecular associations. We propose that the Eye may be the site
of an efficient starburst of 5-7M_sun/yr, but it would still constitute only a
fraction of the 30-50M_sun/yr SFR of NGC4194. Furthermore, we find that ~50% of
the molecular mass of NGC4194 is found in extended filamentary-like structures
tracing the minor and major axis dust lanes. We suggest that molecular gas is
transported along these lanes providing the central starburst region with fuel.Comment: accepted for publication in A&A, 12 pages, 9 figure
How different Fermi surface maps emerge in photoemission from Bi2212
We report angle-resolved photoemission spectra (ARPES) from the Fermi energy
() over a large area of the () plane using 21.2 eV and 32 eV
photons in two distinct polarizations from an optimally doped single crystal of
BiSrCaCuO (Bi2212), together with extensive
first-principles simulations of the ARPES intensities. The results display a
wide-ranging level of accord between theory and experiment and clarify how
myriad Fermi surface (FS) maps emerge in ARPES under various experimental
conditions. The energy and polarization dependences of the ARPES matrix element
help disentangle primary contributions to the spectrum due to the pristine
lattice from those arising from modulations of the underlying tetragonal
symmetry and provide a route for separating closely placed FS sheets in low
dimensional materials.Comment: submitted to PR
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Thorium Energy Futures
The potential for thorium as an alternative or supplement to uranium in fission power generation has long been recognised, and several reactors, of various types, have already operated using thorium-based fuels. Accelerator Driven Subcritical (ADS) systems have benefits and drawbacks when compared to conventional critical thorium reactors, for both solid and molten salt fuels. None of the four options â liquid or solid, with or without an accelerator â can yet be rated as better or worse than the other three, given today's knowledge. We outline the research that will be necessary to lead to an informed choice
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