59,146 research outputs found
The exclusive (e,ep) reaction at high missing momenta
The reduced (e,ep) cross section is calculated for kinematics that probe
high missing momenta. The final-state interaction is handled within a
non-relativistic many-body framework. One- and two-body nuclear currents are
included. Electron distortion effects are treated in an exact distorted wave
calculation. It is shown that at high missing momenta the calculated (e,ep)
cross sections exhibit a pronounced sensitivity to ground-state correlations of
the RPA type and two-body currents. The role of these mechanisms is found to be
relatively small at low missing momenta.Comment: 15 pages in REVtex with embedded psfigure
First Detection of Molecular Gas in the Shells of CenA
Shells are faint arc-like stellar structures, which have been observed around
early type galaxies and are thought to be the result of an interaction. HI gas
has recently been detected in shells, a surprising result in view of the
theoretical predictions that most of the gas should decouple from stars and
fall into the nucleus in such interactions. Here we report the first detection
of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128
(CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The
ratio between CO and HI emission in the shells is the same as that found in the
central regions, which is unexpected given the metallicity gradient usually
observed in galaxies. We propose that the dynamics of the gas can be understood
within the standard picture of shell formation if one takes into account that
the interstellar medium is clumpy and hence not highly dissipative. The
observed metal enrichment could be due to star formation induced by the AGN jet
in the shells. Furthermore our observations provide evidence that molecular gas
in mergers may be spread out far from the nuclear regions.Comment: Accepted for publication in Astronomy & Astrophysics Letters, (Vol.
356), 4 pages + 1 color figur
Hamiltonian mechanics on discrete manifolds
The mathematical/geometric structure of discrete models of systems, whether these models are obtained after discretization of a smooth system or as a direct result of modeling at the discrete level, have not been studied much. Mostly one is concerned regarding the nature of the solutions, but not much has been done regarding the structure of these discrete models. In this paper we provide a framework for the study of discrete models, speci?cally we present a Hamiltonian point of view. To this end we introduce the concept of a discrete calculus
Carbon cage-like materials as potential low work function metallic compounds: Case of clathrates
We present an ab-initio calculation of the electronic affinity of the
hypothetical C-46 clathrate by studying its bare and hydrogenated (100)
surfaces. We show that such a system shares with the diamond phase a small
electronic affinity. Further, contrary to the diamond phase, the possibility of
doping endohedrally these cage-like systems allows to significantly raise the
position of the Fermi level, resulting in a true metal with a small work
function. This is illustrated in the case of the Li8@C-46 doped compound. Such
a class of materials might be of much interest for the design of
electron-emitting devices.Comment: 4 pages, 3 figures, RevTe
Nearly Antiferromagnetic Fermi Liquids: A Progress Report
I describe recent theoretical and experimental progress in understanding the
physical properties of the two dimensional nearly antiferromagnetic Fermi
liquids (NAFL's) found in the normal state of the cuprate superconductors. In
such NAFL's, the magnetic interaction between planar quasiparticles is strong
and peaked at or near the commensurate wave vector, . For
the optimally doped and underdoped systems, the resulting strong
antiferromagnetic correlations produce three distinct magnetic phases in the
normal state: mean field above , pseudoscaling between and
, and pseudogap below . I present arguments which suggest that the
physical origin of the pseudogap found in the quasiparticle spectrum below
is the formation of a precursor to a spin-density-wave-state, describe
the calculations based on this scenario of the dynamical spin susceptibility,
Fermi surface evolution, transport, and Hall effect, and summarize the
experimental evidence in its support.Comment: LATEX + PS figures. To appear in the proceedings of the
Euroconference on "Correlations in Unconventional Quantum Liquids," Evora,
Portugal, October 199
X-ray photoemission characterization of La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} films
The Curie temperature and x-ray photoemission spectra of thin films of
La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} (LCSMO) have been studied as a function
of the Ca/Sr ratio. The films were grown by off-axis cosputtering from
individual targets of La_{0.67}Ca_{0.33}MnO_{3} (LCMO) and
La_{0.67}Sr_{0.33}MnO_{3} (LSMO) onto (100) oriented NdGaO_{3} substrates. The
films grow with a (100) orientation, with no other orientations observed by
x-ray diffraction. For the alloy mixtures, the Curie temperature, T_C, varies
slowly as the Ca/Sr is decreased, remaining 300 K, while for the LCMO
and LSMO films T_C is 260 and 330 K, respectively. The Mn-O valence structure
is composed of two dominant peaks, whose positions undergo a change as the Ca
fraction is decreased. The core lines behave as linear combinations of lines
from pure LCMO and LSMO.Comment: 3 pages, 5 eps figures. To be published in Journal of Applied Physics
(Proceedings of MMM'98
Encapsulation of DNA by cationic diblock copolymer vesicles
Encapsulation of dsDNA fragments (contour length 54 nm) by the cationic
diblock copolymer poly(butadiene-b-N-methyl 4-vinyl pyridinium) [PBd-b-P4VPQ]
has been studied with phase contrast, polarized light, and fluorescence
microscopy, as well as scanning electron microscopy. Encapsulation was achieved
with a single emulsion technique. For this purpose, an aqueous DNA solution is
emulsified in an organic solvent (toluene) and stabilized by the amphiphilic
diblock copolymer. The PBd block forms an interfacial brush, whereas the
cationic P4VPQ block complexes with DNA. A subsequent change of the quality of
the organic solvent results in a collapse of the PBd brush and the formation of
a capsule. Inside the capsules, the DNA is compacted as shown by the appearance
of birefringent textures under crossed polarizers and the increase in
fluorescence intensity of labeled DNA. The capsules can also be dispersed in
aqueous medium to form vesicles, provided they are stabilized with an osmotic
agent (polyethylene glycol) in the external phase. It is shown that the DNA is
released from the vesicles once the osmotic pressure drops below 105 N/m2 or if
the ionic strength of the supporting medium exceeds 0.1 M. The method has also
proven to be efficient to encapsulate pUC18 plasmid in sub-micron sized
vesicles and the general applicability of the method has been demonstrated by
the preparation of the charge inverse system: cationic poly(ethylene imine)
encapsulated by the anionic diblock poly(styrene-b-acrylic acid).Comment: 35 pages, 11 figures, accepted for publication in Langmui
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