13,830 research outputs found
Superconducting nanobridges under magnetic fields
We report on the study of superconducting nanotips and nanobridges of lead
with a Scanning Tunnelling Microscope in tunnel and point contact regimes. We
deal with three different structures. A nanotip that remains superconducting
under a field of 2 T. For this case we present model calculations of the order
parameter, which are in good agreement with the experiments. An asymmetric
nanobridge of lead showing a two steps loss of the Andreev excess current due
to different heating and dissipation phenomena in each side of the structure. A
study of the effect of the thermal fluctuations on the Josephson coupling
between the two sides of a superconducting nanobridge submitted to magnetic
fields. The different experiments were made under magnetic fields up to twenty
five times the volume critical field of lead, and in a temperature range
between 0.6 K and 7.2 K.Comment: 17 pages, 7 figure
Sagittarius: The Nearest Dwarf Galaxy
We have discovered a new Galactic satellite galaxy in the constellation of
Sagittarius. The Sagittarius dwarf galaxy is the nearest galaxy known, subtends
an angle of degrees on the sky, lies at a distance of 24 \kpc from the
Sun, \sim 16 \kpc from the centre of the Milky Way. Itis comparable in size
and luminosity to the largest dwarf spheroidal, has a well populated red
horizontal branch with a blue HB extension; a substantial carbon star
population; and a strong intermediate age stellar component with evidence of a
metallicity spread. Isodensity maps show it to be markedly elongated along a
direction pointing towards the Galactic centre and suggest that it has been
tidally distorted. The close proximity to the Galactic centre, the
morphological appearance and the radial velocity of 140 km/s indicate that this
system must have undergone at most very few close orbital encounters with the
Milky Way. It is currently undergoing strong tidal disruption prior to being
integrated into the Galaxy. Probably all of the four globular clusters, M54,
Arp 2, Ter 7 and Ter 8, are associated with the Sagittarius dwarf galaxy, and
will probably share the fate of their progenitor.Comment: MNRAS in press, 22pp uuencoded PS file, 26 printed figures available
on request from [email protected]
Effective axial-vector coupling of gluon as an explanation to the top quark asymmetry
We explore the possibility that the large forward-backward
asymmetry measured by the CDF detector at Tevatron could be due to a universal
effective axial-vector coupling of gluon. Using an effective field theory
approach we show model independently how such a log-enhanced coupling occurs at
1-loop level. The interference with QCD gluon vector coupling naturally induces
the observed positive forward-backward asymmetry that grows with
invariant mass and is consistent with the cross section measurements.
This scenario does not involve new flavor changing couplings nor operators that
interfere with QCD, and, therefore, is not constrained by the LHC searches for
4-quark contact interactions. We predict top quark polarization effects that
grow with energy and allow to test this scenario at the LHC. Our proposal
offers a viable alternative to new physics scenarios that explain the
forward-backward asymmetry anomaly with the interference between QCD and tree
level new physics amplitudes.Comment: Few small changes in the text, same as published version, 5 pages, 2
figure
Radiative return at NLO and the measurement of the hadronic cross-section in electron-positron annihilation
Electron-positron annihilation into hadrons plus an energetic photon from
initial state radiation allows the hadronic cross-section to be measured over a
wide range of energies. The full next-to-leading order QED corrections for the
cross-section for e^+ e^- annihilation into a real tagged photon and a virtual
photon converting into hadrons are calculated where the tagged photon is
radiated off the initial electron or positron. This includes virtual and soft
photon corrections to the process e^+ e^- \to \gamma +\gamma^* and the emission
of two real hard photons: e^+ e^- \to \gamma + \gamma + \gamma^*. A Monte Carlo
generator has been constructed, which incorporates these corrections and
simulates the production of two charged pions or muons plus one or two photons.
Predictions are presented for centre-of-mass energies between 1 and 10 GeV,
corresponding to the energies of DAPHNE, CLEO-C and B-meson factories.Comment: 13 pages, 15 figure
Perspectives for the radiative return at meson factories
The measurement of the pion form factor and, more generally, of the cross
section for electron-positron annihilation into hadrons through the radiative
return has become an important task for high luminosity colliders such as the
Phi- or B-meson factories. This quantity is crucial for predictions of the
hadronic contributions to the anomalous magnetic moment of the muon, and to the
running of the electromagnetic coupling. But the radiative return opens the
possibility of many other physical applications. The physics potential of this
method at high luminosity meson factories is discussed, the last upgraded
version of the event generator PHOKHARA is presented, and future developments
are highlighted.Comment: Presented at SIGHAD03: Worskhop on Hadronic Cross Section at Low
Energy, Pisa,Italy, October 8th-10th, 200
Evolution of the local superconducting density of states in ErRhB close to the ferromagnetic transition
We present local tunneling spectroscopy experiments in the superconducting
and ferromagnetic phases of the reentrant superconductor ErRhB. The
tunneling conductance curves jump from showing normal to superconducting
features within a few mK close to the ferromagnetic transition temperature,
with a clear hysteretic behavior. Within the ferromagnetic phase, we do not
detect any superconducting correlations. Within the superconducting phase we
find a peculiar V-shaped density of states at low energies, which is produced
by the magnetically modulated phase that coexists with superconductivity just
before ferromagnetism sets in.Comment: 5 pages, 4 figure
Topological superconductivity in lead nanowires
Superconductors with an odd number of bands crossing the Fermi energy have
topologically protected Andreev states at interfaces, including Majorana states
in one dimensional geometries. Superconductivity, a low number of 1D channels,
large spin orbit coupling, and a sizeable Zeeman energy, are present in lead
nanowires produced by nanoindentation of a Pb tip on a Pb substrate, in
magnetic fields higher than the Pb bulk critical field. A number of such
devices have been analyzed. In some of them, the dependence of the critical
current on magnetic field, and the Multiple Andreev Reflections observed at
finite voltages, are compatible with the existence of topological
superconductivity
Transient lateral photovoltaic effect in patterned metal-oxide-semiconductor films
The time dependent transient lateral photovoltaic effect has been studied
with us time resolution and with chopping frequencies in the kHz range, in
lithographically patterned 21 nm thick, 5, 10 and 20 um wide and 1500 um long
Co lines grown over naturally passivated p-type Si (100). We have observed a
nearly linear dependence of the transitorial response with the laser spot
position. A transitorial response with a sign change in the laser-off stage has
been corroborated by numerical simulations. A qualitative explanation suggests
a modification of the drift-diffusion model by including the in uence of a
local inductance. Our findings indicate that the microstructuring of position
sensitive detectors could improve their space-time resolution.Comment: 4 pages, 4 figure
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