Physical interpretation of gauge invariant perturbations of spherically symmetric space-times

By calculating the Newman-Penrose Weyl tensor components of a perturbed spherically symmetric space-time with respect to invariantly defined classes of null tetrads, we give a physical interpretation, in terms of gravitational radiation, of odd parity gauge invariant metric perturbations. We point out how these gauge invariants may be used in setting boundary and/or initial conditions in perturbation theory.Comment: 6 pages. To appear in PR

The rotational modes of relativistic stars: Numerical results

We study the inertial modes of slowly rotating, fully relativistic compact stars. The equations that govern perturbations of both barotropic and non-barotropic models are discussed, but we present numerical results only for the barotropic case. For barotropic stars all inertial modes are a hybrid mixture of axial and polar perturbations. We use a spectral method to solve for such modes of various polytropic models. Our main attention is on modes that can be driven unstable by the emission of gravitational waves. Hence, we calculate the gravitational-wave growth timescale for these unstable modes and compare the results to previous estimates obtained in Newtonian gravity (i.e. using post-Newtonian radiation formulas). We find that the inertial modes are slightly stabilized by relativistic effects, but that previous conclusions concerning eg. the unstable r-modes remain essentially unaltered when the problem is studied in full general relativity.Comment: RevTeX, 29 pages, 31 eps figure

Quantum Andreev Oscillations in normal-superconducting-normal nanostructures

We show that the voltage drop of specially prepared normal-superconducting-normal nanostructures show quantum Andreev oscillations as a function of magnetic field or input current. These oscillations are due to the interference of the electron wave function between the normal parts of the structure that act as reflective interfaces, i.e. our devices behave as a Fabry-Perot interferometer for conduction electrons. The observed oscillations and field periods are well explained by theory.Comment: 5 pages and 4 figure

Calibration of the length of a chain of single gold atoms

Using a scanning tunneling microscope or mechanically controllable break junctions it has been shown that it is possible to control the formation of a wire made of single gold atoms. In these experiments an interatomic distance between atoms in the chain of ~3.6 Angstrom was reported which is not consistent with recent theoretical calculations. Here, using precise calibration procedures for both techniques, we measure length of the atomic chains. Based on the distance between the peaks observed in the chain length histogram we find the mean value of the inter-atomic distance before chain rupture to be 2.6 +/- 0.2 A . This value agrees with the theoretical calculations for the bond length. The discrepancy with the previous experimental measurements was due to the presence of He gas, that was used to promote the thermal contact, and which affects the value of the work function that is commonly used to calibrate distances in scanning tunnelling microscopy and mechanically controllable break junctions at low temperatures.Comment: 6 pages, 6 figure

A 350 to 700 GHz open structure SIS receiver for submm. radioastronomy

An open structure SIS receiver, used already for astronomy at 345 GHz was tested at 470 and 690 GHz. Receiver noise temperatures considerably better than those of standard systems based on Schottky diode mixers were accomplished. A specific merrit of this system is the option to change the range of input frequencies by simply mounting a specific mixer element in a standard mixer block and by exchange of the output horn at the solid state local ocillator

High temperature superconductivity in SmBa2Cu3O 7-x : transport properties and effect of pressure

Sintered powders of SmBa2Cu3O7-x have been prepared and characterized. The physical properties, superconducting temperature transition, up to 92 K, magnetic susceptibility, electrical resistivity and thermopower have been studied as a function of the preparation conditions and therefore of oxygen stoichiometry. The upper critical field has been estimated from resistivity measurements in fields up to 8 T and the slope (dBc2/dT)Tc found to be close to 4 TK-1 for the highest Tc samples. Studies of the electrical resistivity under hydrostatic pressures up to 18 kbar show an increase of Tc under pressure with dTc/dp ∼ 0.5 K kbar-1. Finally measurements of voltage-current characteristics indicate that either Josephson junctions or superconductor-insulator-normal metal junctions can be found inside the samples, as expected in inhomogeneous materials.Les propriétés physiques, température de transition supraconductrice, susceptibilité magnétique, résistivité électrique et pouvoir thermoélectrique de poudres frittées de SmBa2Cu3O7-x, ont été étudiées en fonction des conditions de préparation et donc de la stoéchiométrie en oxygène. Le champ critique Bc2 a été estimé à partir de mesures de résistivité en champ maximum de 8 T et la pente (dBc2/dT) Tc a été trouvée voisine de 4 T K-1 dans les échantillons à plus haut Tc. Une étude de la résistivité sous pression hydrostatique (0-18 kbar) montre que Tc augmente avec la pression (dTc/dp = 0,5 K kbar-1). Enfin des mesures de caractéristique courant-tension indiquent que des jonctions de type Josephson ou de type S-I-N (Supraconducteur-isolant-métal normal) existent à l'intérieur des échantillons, par suite de l'inhomogénéité de composition