Magnetoinductance of Josephson junction array with frozen vortex diffusion

The dependence of sheet impedance of a Josephson junction array on the applied magnetic field is investigated in the regime when vortex diffusion between array plaquettes is effectively frozen due to low enough temperature. The field dependent contribution to sheet inductance is found to be proportional to f*ln(1/f), where f<<1 is the magnitude of the field expressed in terms of flux quanta per plaquette.Comment: 5 pages, no figure

Temperature and Frequency Dependence of Complex Conductance of Ultrathin YBa2Cu3O7-x Films: A Study of Vortex-Antivortex Pair Unbinding

We have studied the temperature dependencies of the complex sheet conductance of 1-3 unit cell (UC) thick YBa2Cu3O7-x films sandwiched between semiconducting Pr0.6Y0.4Ba2Cu3O7-x layers at high frequencies. Experiments have been carried out in a frequency range between: 2 - 30 MHz with one-spiral coil technique, 100 MHz - 1 GHz frequency range with a new technique using the spiral coil cavity and at 30 GHz by aid of a resonant cavity technique. The real and imaginary parts of the mutual-inductance between a coil and a film were measured and converted to complex conductivity by aid of the inversion procedure. We have found a quadratic temperature dependence of the kinetic inductance, L_k^-1(T), at low temperatures independent of frequency, with a break in slope at T^dc_BKT, the maximum of real part of conductance and a large shift of the break temperature and the maximum position to higher temperatures with increasing frequency. We obtain from these data the universal ratio T^dc_BKT/L_k^-1(T^dc_BKT) = 25, 25, and 17 nHK for 1-, 2- and 3UC films, respectively in close agreement with theoretical prediction of 12 nHK for vortex-antivortex unbinding transition. The activated temperature dependence of the vortex diffusion constant was observed and discussed in the framework of vortex-antivortex pair pinning. PACS numbers: 74.80.Dm, 74.25.Nf, 74.72.Bk, 74.76.BzComment: PDF file, 10 pages, 6 figures, to be published in J. Low Temp. Phys.; Proc. of NATO ARW: VORTEX 200

Dynamic Scaling and Two-Dimensional High-Tc Superconductors

There has been ongoing debate over the critical behavior of two-dimensional superconductors; in particular for high Tc superconductors. The conventional view is that a Kosterlitz-Thouless-Berezinskii transition occurs as long as finite size effects do not obscure the transition. However, there have been recent suggestions that a different transition actually occurs which incorporates aspects of both the dynamic scaling theory of Fisher, Fisher, and Huse and the Kosterlitz-Thouless-Berezinskii transition. Of general interest is that this modified transition apparently has a universal dynamic critical exponent. Some have countered that this apparent universal behavior is rooted in a newly proposed finite-size scaling theory; one that also incorporates scaling and conventional two-dimensional theory. To investigate these issues we study DC voltage versus current data of a 12 angstrom thick YBCO film. We find that the newly proposed scaling theories have intrinsic flexibility that is relevant to the analysis of the experiments. In particular, the data scale according to the modified transition for arbitrarily defined critical temperatures between 0 K and 19.5 K, and the temperature range of a successful scaling collapse is related directly to the sensitivity of the measurement. This implies that the apparent universal exponent is due to the intrinsic flexibility rather than some real physical property. To address this intrinsic flexibility, we propose a criterion which would give conclusive evidence for phase transitions in two-dimensional superconductors. We conclude by reviewing results to see if our criterion is satisfied.Comment: 14 page

Cell elongation in the grass pulvinus in response to geotropic stimulation and auxin application

Horizontally-placed segments of Avena sativa L. shoots show a negative geotropic response after a period of 30 min. This response is based on cell elongation on the lower side of the leaf-sheath base (pulvinus). Triticum aestivum L., Hordeum vulgare L. and Secale cereale L. also show geotropic responses that are similar to those in Avena shoots. The pulvinus is a highly specialized organ with radial symmetry and is made up of epidermal, vascular, parenchymatous and collenchymatous tissues. Statoliths, which are confined to parenchyma cells around the vascular bundles, sediment towards the gravitational field within 10–15 min of geotropic stimulation. Collenchymatous cells occur as prominent bundle caps, and in Avena , they occupy about 30% of the volume of the pulvinus. Geotropic stimulation causes a 3- to 5-fold increase in the length of the cells on the side nearest to the center of the gravitational field. Growth can also be initiated in vertically-held pulvini by the application of indole-3-acetic acid, 1-naphthaleneacetic acid or 2.4-dichlorophenoxyacetic acid. 2.3.5.-triiodobenzoic acid interferes with growth response produced by geotropic stimulation as well as with the response caused by auxin application. Gibberellic acid and kinetin have no visible effect on the growth of the pulvinus. Polarization microscopy shows a unique, non-uniform stretching of the elongating collenchymatous cells. Nonelongated collenchymatous cells appear uniformally anisotropic. After geotropic stimulation or auxin application, they appear alternately anisotropic and almost isotropic. Such a pattern of cell elongation is also observed in collenchyma cells of geotropically-stimulated shoots of Rumex acetosa L., a dicotyledon.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47463/1/425_2004_Article_BF00385422.pd

THE rf RESISTIVITY OF A PERFORATED SUPERCONDUCTING FILM IN A MAGNETIC FIELD

Des mesures de résistivité rf complexe à 75 MHz ont été effectuées en fonction de la température et du champ magnétique sur une couche d'aluminium dopée à l'oxygène et perforée suivant un réseau triangulaire de trous. La réponse rf pour des champs magnétiques commensurables est interprétée comme résultant de la superposition cohérente du mouvement des vortex dans un potentiel parabolique. Une dissipation anormalement faible a été trouvée pour les quanta piégés, en raison de l'absence d'excitations dans le coeur. La constante de rappel du potentiel de piégeage a été déduite et exprimée en fonction de la température.We report measurements of the complex rf resistivity at 75 MHz as a function of temperature and magnetic field on an oxygen-doped aluminum film, perforated with a triangular lattice of holes. The rf response at commensurate magnetic fields is modeled in terms of coherent superimposition of vortex motion in a parabolic potential. Anomalously small dissipation is found for trapped quanta, owing to the absence of core excitations. The force constant of the pinning potential is found as a function of temperature

Electrical resistivity and stoichiometry of KxC60, RbxC60, and CsxC60 films

Electrical resistance measurements as a function of stoichiometry have been carried out on KxC60, RbxC60 and CsxC60 thin films for 0<x<6. The annealed films show global resistance minima at K3C60, ρmin=4×10-3 Ω cm (60°C), Rb3C60, ρmin=4×10-3 Ω cm, and Cs1C60, ρmin = 7×10-2 Ω cm. All of the annealed films show additional features in the vicinity of x=4, but the manifestation of the A4C60 phase (A=K, Rb, Cs), in transport studies is dependent on the metal and the annealing conditions. The A6C60 phase is apparent for all of the metals studied and shows a relatively high resistivity. The activation energies of the conduction process show well defined stationary points at K3C60 (not activated), Rb3C60 (not activated). Cs1C60 (minimum), Cs4C60 (maximum), with less distinct features between 4<x<6 in all cases.