Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations

Using a single coil mutual inductance technique, we measure the low temperature dependence of the magnetic penetration depth in superconducting NbN films prepared with similar critical temperatures around 16 K but with different microstructures. Only (100) epitaxial and weakly granular (100) textured films display the characteristic exponential dependence of conventional BCS s-wave superconductors. More granular (111) textured films exhibit a linear dependence, indicating a gapless state in spite of the s-wave gap. This result is quantitatively explained by a model of thermal phase fluctuations favored by the granular structure.Comment: 10 pages, 4 figures, to appear in Phys. Rev.

Structural transition of ammonium aluminium alums studied by e.p.r.

EPR spectroscopy results for Cr3+ in two alums A1NH4 and A1ND4 are reported. These alums undergo a phase transition at respectively 60 K and 56 K. The results for T > Tc are qualitatively explained as resulting from fluctuations of distortions associated with the structural change. As A1NH4 looks internally fractured after being cooled below Tc, only the low temperature phase of A1ND4 has been studied. The EPR spectra below T c depend on the rate of cooling. After a slow cooling, the crystal contains three types of domains, each belonging to the orthorhombic system. After a fast cooling, complicated spectra are observed and partially analysed. The analysis suggests they are due either to some form of disorder or to the existence of many domains with large walls.Les résultats de spectroscopie R.P.E. de deux aluns AINH4 et A1ND4 dopés avec des ions Cr3+ sont donnés. Ces aluns subissent un changement de phase à 60 et 56 K. Les résultats obtenus pour T > Tc sont qualitativement interprétés comme résultant de distorsions fluctuantes dues au changement de structure. L'alun A1NH 4 présentant des fractures internes après refroidissement, seule la phase basse temperature de l'alun A1ND 4 a été étudiée. Elle dépend de la vitesse de refroidissement. La phase obtenue après un refroidissement lent appartient au système orthorhombique et trois types de domaines sont présents dans le cristal. Par contre, après un refroidissement rapide, les spectres compliqués obtenus n'ont été analysés que partiellement. Cette analyse suggère qu'ils sont dus soit à un certain désordre dans le cristal, soit à l'existence de nombreux domaines séparés par des parois épaisses

Anomalous resistive states created by light pulses in superconducting films

Experiments on the behaviour of aluminium superconducting films when submitted to light or short electric pulses are reported. They show that permanent resistive states can be created at temperatures below the transition temperature. The value of the resistance of these states, lower than the normal resistance, can be varied continuously by changing the experimental conditions.Des expériences sur le comportement de films d'aluminium superconducteur soumis à des impulsions lumineuses ou électriques sont rapportées. Elles montrent que des états résistifs permanents peuvent être créés à des températures inférieures à la température de transition. La valeur de la résistance de ces états, inférieure à celle dans l'état normal, peut être modifiée de façon continue en changeant les conditions expérimentales

ANALYSIS OF PERMANENT RESISTIVE STATES CREATED BY LIGHT PULSES IN SUPERCONDUCTING FILMS

Les états résistifs permanents créés dans des films supraconducteurs, soit par une impulsion lumineuse, soit par réduction rapide du courant dans le film, sont analysés expérimentalement. Leur comportement pour une faible variation du courant est décrit et la preuve est donnée qu'ils ne résultent pas simplement d'effets thermiques.Permanent resistive states in superconducting films created either by a light pulse, or by fst decrease of the electrical current in the film are analysed experimentally. Their behaviour under small variation of the current is described and the proof is given that they are not due to purely heating effects

Processing of DC SQUIDs for radiofrequency amplification

The goal of this studies is to use dc Squids as low noise radiofrequency amplifier (Tn ≈1K) in the frequency range 1-150 Mhz and to integrate them in NMR probe. DC Squids have been built using planar technology and with two types of superconducting materials : low Tc(Nb) and High Tc(YBa2Cu3O7). Contrary to the traditional set up used in magnetometry (flux locked loop) the dc Squid is here fluxed biased near Φz=(2n+1)Φo/4 and no flux locked loop is connected. Small variations of flux (Φs<<Φo) introduced from signal source via the input coil are amplified following the transfer characteristic VΦ=dV/dΦ. The low Tc devices are set up to now the best, concerning the reproducibility and the qualities of the Josephson Junctions obtained with a three layer process Nb/Al-Al2O3/Nb. Such junctions have been realized following the so called SNOP process, without anodization. The main characteristic obtained at 4.2K are RJ/RN=10, 2Δ≈2.8 meV and a Josephson current density J≈103 A/cm2. The high Tc devices have been built with bi-epitaxial YBa2Cu3O7 junctions and with a 30x30µm2 area loop. I(V) and V(Φ) characteristics curves obtained are promizing for applications as RF detection in the temperature range : 4-77K. Modelization studies are currently in progress for the optimization of those devices (low Tc and high Tc) as RF amplifiers (minimization of parasitic capacitance between the input coil and the Squid loop) in order to increase their upper frequency limit. * This work is partially supported by Ultimatech (CNRS), by the "région Rhône-Alpes" and E.E.C. "HTSC-GBJ" programs n°7100

Tunneling spectroscopy in NbN based Josephson junctions

SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman