5,692 research outputs found
The Micro Slit Gas Detector
We describe the first tests with a new proportional gas detector. Its
geometry consists in slits opened in a copper metallized kapton foil with 30
micron anode strips suspended in these openings. In this way the multiplication
process is similar to a standard MSGC. The fundamental difference is the
absence of an insulating substrate around the anode. Also the material budget
is significantly reduced, and the problems related to charging-up or
polarization are removed. Ageing properties of this detector are under study.Comment: 13 pages tex file, 10 figures ep
Absence of a structural transition up to 40 Gpa in MgB2 and the relevance of magnesium non-stoichiometry
We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a
monotonous decrease of the lattice parameters and of the c/a ratio, but no
structural transition down to parameters smaller than those of AlB2. The
transition superconducting temperature also decreases with temperature in a
sample dependent way. The results are explained by an increase of the filling
of the 2D pxy bands with pressure, the Mg stoichiometry determining the
starting position of the Fermi level. Our measurements indicate that these hole
bands are the relevant ones for superconductivity.Comment: submitted March 9th 2001, PRB accepte
Meson mass and confinement force driven by dilaton
Meson spectra given as fluctuations of a D7 brane are studied under the
background driven by the dilaton. This leads to a dual gauge theory with quark
confinement due to the gauge condensate. We find that the effect of the gauge
condensate on the meson spectrum is essential in order to make a realistic
hadron spectrum in the non-supersymmetric case. In the supersymmetric case,
however, only the spectra of the scalars are affected, but they are changed in
an opposite way compared to the non-supersymmetric case.Comment: 11 pages, 2 figure
Antimicrobial resistance characteristics and fitness of Gram-negative fecal bacteria from volunteers treated with minocycline or amoxicillin.
A yearlong study was performed to examine the effect of antibiotic administration on the bacterial gut flora. Gram-negative facultative anaerobic bacteria were recovered from the feces of healthy adult volunteers administered amoxicillin, minocycline or placebo, and changes determined in antimicrobial resistance (AMR) gene carriage. Seventy percent of the 1039 facultative anaerobic isolates recovered were identified by MALDI-TOF as Escherichia coli. A microarray used to determine virulence and resistance gene carriage demonstrated that AMR genes were widespread in all administration groups, with the most common resistance genes being bla TEM, dfr, strB, tet(A), and tet(B). Following amoxicillin administration, an increase in the proportion of amoxicillin resistant E. coli and a three-fold increase in the levels of bla TEM gene carriage was observed, an effect not observed in the other two treatment groups. Detection of virulence genes, including stx1A, indicated not all E. coli were innocuous commensals. Approximately 150 E. coli collected from 6 participants were selected for pulse field gel electrophoresis (PFGE), and a subset used for characterisation of plasmids and Phenotypic Microarrays (PM). PFGE indicated some E. coli clones had persisted in volunteers for up to 1 year, while others were transient. Although there were no unique characteristics associated with plasmids from persistent or transient isolates, PM assays showed transient isolates had greater adaptability to a range of antiseptic biocides and tetracycline; characteristics which were lost in some, but not all persistent isolates. This study indicates healthy individuals carry bacteria harboring resistance to a variety of antibiotics and biocides in their intestinal tract. Antibiotic administration can have a temporary effect of selecting bacteria, showing co-resistance to multiple antibiotics, some of which can persist within the gut for up to 1 year
INTEGRAL-RXTE observations of Cygnus X-1
We present first results from contemporaneous observations of Cygnus X-1 with
INTEGRAL and RXTE, made during INTEGRAL's performance verification phase in
2002 November and December. Consistent with earlier results, the 3-250 keV data
are well described by Comptonization spectra from a Compton corona with a
temperature of kT~50-90 keV and an optical depth of tau~1.0-1.3 plus reflection
from a cold or mildly ionized slab with a covering factor of Omega/2pi~0.2-0.3.
A soft excess below 10 keV, interpreted as emission from the accretion disk, is
seen to decrease during the 1.5 months spanned by our observations. Our results
indicate a remarkable consistency among the independently calibrated detectors,
with the remaining issues being mainly related to the flux calibration of
INTEGRAL.Comment: 6 pages, 3 figures. Figs. 2 and 3 are best viewed in color. Accepted
for publication in the INTEGRAL special edition of A&A
Friction force on slow charges moving over supported graphene
We provide a theoretical model that describes the dielectric coupling of a 2D
layer of graphene, represented by a polarization function in the Random Phase
Approximation, and a semi-infinite 3D substrate, represented by a surface
response function in a non-local formulation. We concentrate on the role of the
dynamic response of the substrate for low-frequency excitations of the combined
graphene-substrate system, which give rise to the stopping force on slowly
moving charges above graphene. A comparison of the dielectric loss function
with experimental HREELS data for graphene on a SiC substrate is used to
estimate the damping rate in graphene and to reveal the importance of phonon
excitations in an insulating substrate. A signature of the hybridization
between graphene's pi plasmon and the substrate's phonon is found in the
stopping force. A friction coefficient that is calculated for slow charges
moving above graphene on a metallic substrate shows an interplay between the
low-energy single-particle excitations in both systems.Comment: 13 pages, 5 figures, submitted to Nanotechnology for a special issue
related to the NGC 2009 conference (http://asdn.net/ngc2009/index.shtml
One-dimensional hydrogen atom with minimal length uncertainty and maximal momentum
We present exact energy eigenvalues and eigenfunctions of the one-dimensional
hydrogen atom in the framework of the Generalized (Gravitational) Uncertainty
Principle (GUP). This form of GUP is consistent with various theories of
quantum gravity such as string theory, loop quantum gravity, black-hole
physics, and doubly special relativity and implies a minimal length uncertainty
and a maximal momentum. We show that the quantized energy spectrum exactly
agrees with the semiclassical results.Comment: 10 pages, 1 figur
Predictive Power of Strong Coupling in Theories with Large Distance Modified Gravity
We consider theories that modify gravity at cosmological distances, and show
that any such theory must exhibit a strong coupling phenomenon, or else it is
either inconsistent or is already ruled out by the solar system observations.
We show that all the ghost-free theories that modify dynamics of spin-2
graviton on asymptotically flat backgrounds, automatically have this property.
Due to the strong coupling effect, modification of the gravitational force is
source-dependent, and for lighter sources sets in at shorter distances. This
universal feature makes modified gravity theories predictive and potentially
testable not only by cosmological observations, but also by precision
gravitational measurements at scales much shorter than the current cosmological
horizon. We give a simple parametrization of consistent large distance modified
gravity theories and their predicted deviations from the Einsteinian metric
near the gravitating sources.Comment: 12 pages, Latex, to be published in New Journal of Physic
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