6,172 research outputs found
Quantum oscillations and a non-trivial Berry phase in the noncentrosymmetric superconductor BiPd
We report the measurements of de Haas-van Alphen (dHvA) oscillations in the
noncentrosymmetric superconductor BiPd. Several pieces of a complex multi-sheet
Fermi surface are identified, including a small pocket (frequency 40 T) which
is three dimensional and anisotropic. From the temperature dependence of the
amplitude of the oscillations, the cyclotron effective mass is (
0.1) . Further analysis showed a non-trivial -Berry phase is
associated with the 40 T pocket, which strongly supports the presence of
topological states in bulk BiPd and may result in topological superconductivity
due to the proximity coupling to other bands.Comment: 5 pages, 3 figure
The three species monomer-monomer model in the reaction-controlled limit
We study the one dimensional three species monomer-monomer reaction model in
the reaction controlled limit using mean-field theory and dynamic Monte Carlo
simulations. The phase diagram consists of a reactive steady state bordered by
three equivalent adsorbing phases where the surface is saturated with one
monomer species. The transitions from the reactive phase are all continuous,
while the transitions between adsorbing phases are first-order. Bicritical
points occur where the reactive phase simultaneously meets two adsorbing
phases. The transitions from the reactive to an adsorbing phase show directed
percolation critical behaviour, while the universal behaviour at the bicritical
points is in the even branching annihilating random walk class. The results are
contrasted and compared to previous results for the adsorption-controlled limit
of the same model.Comment: 12 pages using RevTeX, plus 4 postscript figures. Uses psfig.sty.
accepted to Journal of Physics
The 5'-3' exoribonuclease pacman is required for epithelial sheet sealing in Drosophila and genetically interacts with the phosphatase puckered
Background information. Ribonucleases have been well studied in yeast and bacteria, but their biological significance to developmental processes in multicellular organisms is not well understood. However, there is increasing evidence that specific timed transcript degradation is critical for regulation of many cellular processes, including translational repression, nonsense-mediated decay and RNA interference. The Drosophila gene pacman is highly homologous to the major yeast exoribonuclease XRN1 and is the only known cytoplasmic 5′–3′ exoribonuclease in eukaryotes. To determine the effects of this exoribonuclease in development we have constructed a number of mutations in pacman by P-element excision and characterized the resulting phenotypes.
Results. Mutations in pacman resulted in flies with a number of specific phenotypes, such as low viability, dull wings, crooked legs, failure of correct dorsal/thorax closure and defects in wound healing. The epithelial sheet movement involved in dorsal/thorax closure is a conserved morphogenetic process which is similar to that of hind-brain closure in vertebrates and wound healing in humans. As the JNK (c-Jun N-terminal kinase) signalling pathway is known to be involved in dorsal/thorax closure and wound healing, we tested whether pacman affects JNK signalling. Our experiments demonstrate that pacman genetically interacts with puckered, a phosphatase that negatively regulates the JNK signalling pathway.
Conclusions. These results reveal that the 5′–3′ exoribonuclease pacman is required for a critical aspect of epithelial sheet sealing in Drosophila. Since these mutations result in specific phenotypes, our data suggest that the exoribonuclease Pacman targets a specific subset of mRNAs involved in this process. One of these targets could be a member of the JNK signalling pathway, although it is possible that a parallel pathway may instead be affected. The exoribonuclease pacman is highly conserved in all eukaryotes, therefore it is likely that it is involved in similar morphological processes, such as wound healing in human cells
A study of the Gribov copies in linear covariant gauges in Euclidean Yang-Mills theories
The Gribov copies and their consequences on the infrared behavior of the
gluon propagator are investigated in Euclidean Yang-Mills theories quantized in
linear covariant gauges. Considering small values of the gauge parameter, it
turns out that the transverse component of the gluon propagator is suppressed,
while its longitudinal part is left unchanged. A Green function, G_{tr}, which
displays infrared enhancement and which reduces to the ghost propagator in the
Landau gauge is identified. The inclusion of the dimension two gluon condensate
is also considered. In this case, the transverse component of the gluon
propagator and the Green function G_{tr} remain suppressed and enhanced,
respectively. Moreover, the longitudinal part of the gluon propagator becomes
suppressed. A comparison with the results obtained from the studies of the
Schwinger-Dyson equations and from lattice simulations is provided.Comment: 20 page
The Físchlár digital video system: a digital library of broadcast TV programmes
Físchlár is a system for recording, indexing, browsing and playback of broadcast TV programmes which has been operational on our University campus for almost 18 months. In this paper we give a brief overview of how the system operates, how TV programmes are organised for browse/playback and a short report on the system usage by over 900 users in our University
BeppoSAX observations of low power radio galaxies: possible detection of obscured nuclei
We present the first results of BeppoSAX observations of a small sample of
low brightness FRI radio galaxies. The flux of all the targets is consistent
with a thermal spectrum, as due to the presence of hot intracluster gas or
galactic corona. Moreover in three sources a non thermal absorbed spectrum can
be present in the MECS spectrum at energies larger than 7 keV, while for a
fourth object a high energy flux has been detected in the PDS instrument at
energies larger than 15 keV. This component could be related to the inner AGN
surrounded by an obscuring torus.Comment: 4 pages, LateX, 3 figures (included). Uses espcrc2.sty (included). To
appear in: "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE", Rome,
Italy, 21-24 October, 1997, Eds.: L. Scarsi, H. Bradt, P. Giommi and F. Fior
Fermi surface, possible unconventional fermions, and unusually robust resistive critical fields in the chiral-structured superconductor AuBe
The noncentrosymmetric superconductor (NCS) AuBe is investigated using a
variety of thermodynamic and resistive probes in magnetic fields of up to 65~T
and temperatures down to 0.3~K. Despite the polycrystalline nature of the
samples, the observation of a complex series of de Haas-van Alphen (dHvA)
oscillations has allowed the calculated bandstructure for AuBe to be validated.
This permits a variety of BCS parameters describing the superconductivity to be
estimated, despite the complexity of the measured Fermi surface. In addition,
AuBe displays a nonstandard field dependence of the phase of dHvA oscillations
associated with a band thought to host unconventional fermions in this chiral
lattice. This result demonstrates the power of the dHvA effect to establish the
properties of a single band despite the presence of other electronic bands with
a larger density of states, even in polycrystalline samples. In common with
several other NCSs, we find that the resistive upper critical field exceeds
that measured by heat capacity and magnetization by a considerable factor. We
suggest that our data exclude mechanisms for such an effect associated with
disorder, implying that topologically protected superconducting surface states
may be involved
Renormalization group aspects of the local composite operator method
We review the current status of the application of the local composite
operator technique to the condensation of dimension two operators in quantum
chromodynamics (QCD). We pay particular attention to the renormalization group
aspects of the formalism and the renormalization of QCD in various gauges.Comment: 13 latex pages, talk presented at RG0
Detection of water at z = 0.685 towards B0218+357
We report the detection of the H_2O molecule in absorption at a redshift z =
0.68466 in front of the gravitationally lensed quasar B0218+357. We detect the
fundamental transition of ortho-water at 556.93 GHz (redshifted to 330.59 GHz).
The line is highly optically thick and relatively wide (15 km/s FWHM), with a
profile that is similar to that of the previously detected CO(2--1) and
HCO^+(2--1) optically thick absorption lines toward this quasar. From the
measured level of the continuum at 330.59 GHz, which corresponds to the level
expected from the power-law spectrum already
observed at lower frequencies, we deduce that the filling factor of the H_2O
absorption is large. It was already known from the high optical thickness of
the CO, ^{13}CO and C^{18}O lines that the molecular clouds entirely cover one
of the two lensed images of the quasar (all its continuum is absorbed); our
present results indicate that the H_2O clouds are covering a comparable
surface. The H_2O molecules are therefore not confined to small cores with a
tiny filling factor, but are extended over parsec scales. The H_2O line has a
very large optical depth, and only isotopic lines could give us the water
abundance. We have also searched for the 183 GHz line in absorption, obtaining
only an upper limit; this yields constraints on the excitation temperature.Comment: 4 pages, 3 figures, accepted in ApJ Letter
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