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 $> 10$ 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 $t\bar t$ 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 $t\bar t$ asymmetry that grows with $t\bar t$ 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 $t\bar t$ 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 ErRh4_4B4_{4} close to the ferromagnetic transition

We present local tunneling spectroscopy experiments in the superconducting and ferromagnetic phases of the reentrant superconductor ErRh$_4$B$_{4}$. 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