15,966 research outputs found
On the determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy
We study the normal state electronic excitations probed by angle resolved
photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to
establish explicit criteria for determining the Fermi surface from ARPES data
on strongly interacting systems where sharply defined quasiparticles do not
exist and the dispersion is very weak in parts of the Brillouin zone.
Additional complications arise from strong matrix element variations within the
zone. We present detailed results as a function of incident photon energy, and
show simple experimental tests to distinguish between an intensity drop due to
matrix element effects and spectral weight loss due to a Fermi crossing. We
reiterate the use of polarization selection rules in disentangling the effect
of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite
all the complications, the Fermi surface can be determined unambiguously: it is
a single large hole barrel centered about (pi,pi) in both materials.Comment: Expanded discussion of symmetrization method in Section 5, figures
remain the sam
Renormalization of spectral lineshape and dispersion below Tc in Bi2Sr2CaCu2O8+d
Angle-resolved photoemission (ARPES) data in the superconducting state of
Bi2Sr2CaCu2O8+d show a kink in the dispersion along the zone diagonal, which is
related via a Kramers-Kronig analysis to a drop in the low-energy scattering
rate. As one moves towards (pi,0), this kink evolves into a spectral dip. The
occurrence of these anomalies in the dispersion and lineshape throughout the
zone indicate the presence of a new energy scale in the superconducting state.Comment: New Figure 3 with expanded discussio
Inconsistency in 9 mm bullets : correlation of jacket thickness to post-impact geometry measured with non-destructive X-ray computed tomography
Fundamental to any ballistic armour standard is the reference projectile to be defeated. Typically, for certification purposes, a consistent and symmetrical bullet geometry is assumed, however variations in bullet jacket dimensions can have far reaching consequences. Traditionally, characteristics and internal dimensions have been analysed by physically sectioning bullets – an approach which is of restricted scope and which precludes subsequent ballistic assessment. The use of a non-destructive X-ray computed tomography (CT) method has been demonstrated and validated Kumar et al., 2011); the authors now apply this technique to correlate bullet impact response with jacket thickness variations. A set of 20 bullets (9 mm DM11) were selected for comparison and an image-based analysis method was employed to map jacket thickness and determine the centre of gravity of each specimen. Both intra- and inter-bullet variations were investigated, with thickness variations of the order of 200 um commonly found along the length of all bullets and angular variations of up to 50 um in some. The bullets were subsequently impacted against a rigid flat plate under controlled conditions (observed on a high-speed video camera) and the resulting deformed projectiles were re-analysed. The results of the experiments demonstrate a marked difference in ballistic performance between bullets from different manufacturers and an asymmetric thinning of the jacket is observed in regions of pre-impact weakness. The conclusions are relevant for future soft armour standards and provide important quantitative data for numerical model correlation and development. The implications of the findings of the work on the reliability and repeatability of the industry standard V50 ballistic test are also discussed
HST Imaging in the Chandra Deep Field South: II. WFPC2 Observations of an X-Ray Flux-Limited Sample from the 1 Msec Chandra Catalog
We present HST/WFPC2 observations of a well-defined sample of 40 X-ray
sources with X-ray fluxes above the detection threshold of the full 1 Msec
Chandra Deep Field South (CDFS). The sensitivity and spatial resolution of our
HST observations are sufficient to detect the optical counterparts of 37 of the
X-ray sources, yielding information on their morphologies and environments. In
this paper we extend the results obtained in our previous study on the 300 ks
CDFS X-ray data (Schreier et al. 2001, Paper I). Specifically, we show that the
optical counterparts to the X-ray sources are divided into two distinct
populations: 1) an optically faint group with relatively blue colors, similar
to the faint blue field galaxy population, and 2) an optically brighter group,
including resolved galaxies with average colors significantly redder than the
corresponding bright field galaxy population. The brighter objects comprise a
wide range of types, including early and late type galaxies, starbursts, and
AGN. By contrast, we show that the faint blue X-ray population are most
consistent with being predominantly Type 2 AGN of low to moderate luminosity,
located at higher redshifts (z ~ 1 - 2). This conclusion is supported by
luminosity function models of the various classes of objects. Hence, the
combination of deep X-ray data with the high spatial resolution of HST are for
the first time allowing us to probe the faint end of the AGN luminosity
function at cosmologically interesting redshifts.Comment: AASTEX-Latex, 25 pages, 4 postscript figures, 9 jpg figures. Accepted
by the Astrophysical Journal. Full-size postscript images and figures,
included in the preprint, are available from:
http://www.stsci.edu/~koekemoe/papers/cdfs-hst
Collecting single molecules with conventional optical tweezers
The size of particles which can be trapped in optical tweezers ranges from
tens of nanometres to tens of micrometres. This size regime also includes large
single molecules. Here we present experiments demonstrating that optical
tweezers can be used to collect polyethylene oxide (PEO) molecules suspended in
water. The molecules that accumulate in the focal volume do not aggregate and
therefore represent a region of increased molecule concentration, which can be
controlled by the trapping potential. We also present a model which relates the
change in concentration to the trapping potential. Since many protein molecules
have molecular weights for which this method is applicable the effect may be
useful in assisting nucleation of protein crystals.Comment: 5 pages, 4 figure
Chandra X-ray Sources in the LALA Cetus Field
The 174 ks Chandra Advanced CCD Imaging Spectrometer exposure of the Large
Area Lyman Alpha Survey (LALA) Cetus field is the second of the two deep
Chandra images on LALA fields. In this paper we present the Chandra X-ray
sources detected in the Cetus field, along with an analysis of X-ray source
counts, stacked X-ray spectrum, and optical identifications. A total of 188
X-ray sources were detected: 174 in the 0.5-7.0 keV band, 154 in the 0.5-2.0
keV band, and 113 in the 2.0-7.0 keV band. The X-ray source counts were derived
and compared with LALA Bootes field (172 ks exposure). Interestingly, we find
consistent hard band X-ray source density, but 36+-12% higher soft band X-ray
source density in Cetus field. The weighted stacked spectrum of the detected
X-ray sources can be fitted by a powerlaw with photon index Gamma = 1.55. Based
on the weighted stacked spectrum, we find that the resolved fraction of the
X-ray background drops from 72+-1% at 0.5-1.0 keV to 63+-4% at 6.0-8.0 keV. The
unresolved spectrum can be fitted by a powerlaw over the range 0.5-7 keV, with
a photon index Gamma = 1.22. We also present optical counterparts for 154 of
the X-ray sources, down to a limiting magnitude of r' = 25.9 (Vega), using a
deep r' band image obtained with the MMT.Comment: 21 pages, including 6 figures, 1 table, ApJ accepte
Momentum anisotropy of the scattering rate in cuprate superconductors
We examine the momentum and energy dependence of the scattering rate of the
high temperature cuprate superconductors using angle resolved photoemission
spectroscopy. The scattering rate is of the form a + b*w. The inelastic
coefficient b is found to be isotropic. The elastic term, a, however, is found
to be highly anisotropic in the pseudogap phase of optimal doped samples, with
an anisotropy which correlates with that of the pseudogap. This can be
contrasted with heavily overdoped samples, which show an isotropic scattering
rate in the normal state
Probing the hydrogen melting line at high pressures by dynamic compression
We investigate the capabilities of dynamic compression by intense heavy ion beams to yield information about the high pressure phases of hydrogen. Employing ab initio simulations and experimental data, a new wide range equation of state for hydrogen is constructed. The results show that the melting line up to its maximum as well as the transition from molecular fluids to fully ionized plasmas can be tested with the beam parameters soon to be available. We demonstrate that x-ray scattering can distinguish between phases and dissociation states
Quasiparticles in the superconducting state of Bi2Sr2CaCu2O8
Recent improvements in momentum resolution by a factor of 32 lead to
qualitatively new ARPES results on the spectra of Bi2Sr2CaCu2O8 (Bi2212) along
the (pi,pi) direction, where there is a node in the superconducting gap. With
improved resolution, we now see the intrinsic lineshape, which indicates the
presence of true quasiparticles at the Fermi momentum in the superconducting
state, and lack thereof in the normal state. The region of momentum space
probed here is relevant for charge transport, motivating a comparison of our
results to conductivity measurements by infrared reflectivity.Comment: revised paper with new figure
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