R. Di Leo

Niobium-titanium nitride thin films for superconducting rf accelerator cavities

The development of sputtering-magnetron techniques makes possible the manufacturing of thin-film coated rf cavities for high-energy accelerators of various superconducting alloys or compounds. In this context high-quality (Nb1−xTix)N films have been produced by dc magnetron sputtering and carefully characterized. These films, for x≤0.5, exhibit a critical temperature Tc as high as NbN (Tc=17 K) but show a markedly lower normal state resistivity. The calculated BCS surface impedance of the films is also lower than for NbN. The overall properties of the (Nb1−xTix)N films are compared with those of other superconductors, and the suitability of this material for the production of superconducting sputter-coated copper cavities for particle physics accelerators is demonstrated. The potential interest of (Nb1−xTix)N films for superconducting electronics is also briefly outlined.

Zero resistivity at 81 K in BSCCO films grown from liquid KCl solutions

Abstract Textured films of “BSCCO 2212”, with zero resistivity at 81 K, have been grown from KCl liquid solutions, onto (111)-oriented substrates of Gadolinium Gallium Garnet. This technique is particularly promising to grow large epitaxial films of both the “2212” and the “2223” phases of BSCCO.

An analysis of natural tunnel barriers in liquid-etched Y-Ba-Cu-O systems

Abstract We have analyzed the high-bias tunneling characteristics of chemically-etched 90 K, oxygen-deficient 60 K and tetragonal Fe-doped YBaCuO/Pb planar junctions with natural barriers. When a parabolic dependence is found, the barrier parameters can be deduced as in conventional junctions.

Electron tunneling into BiSrCaCuO

Abstract We have realized planar tunnel junctions on chemically etched BiSrCaCuO films and single crystals with natural barriers and Pb or Au films as counterelectrodes. The best data show a BiSrCaCuO gap-like structure at about ±22mV, a second feature at about ±40mV and high bias resistances decreasing with voltage. The majority of the junctions show smeared structures at intermediate voltages and flat resistances at high voltages. We speculate that differences could be due to the presence of different BiSrCaCuO phases with different Tc in the junction area.

Superconducting properties of YBaCuO/Pb “BULK” junctions by tunnel spectroscopy

Abstract The fabrication procedure of “bulk” junctions based on sinterized YBCO samples with a Ta 2 O 5 barrier and a Pb counterelectrode is described. The derivative dl/dV vs. V curves show in most samples a gap structure varying between 15–20 meV. Moreover narrow resistance peaks that vary from sample to sample and on thermal cycling are found. Probably these structures can be ascribed to the granular nature of the YBCO electrode.

Tunnelling anomalies in Bi2Sr2CaCu2O8-δ/Bi2Sr2YCu2O8-δ/Bi2Sr2CaCu2O8-δ planar junctions

Abstract We have prepared Bi2Sr2CaCu2O8−δ/Bi2Sr2YCu2O8−δ/Bi2Sr2CaCu2O8−δ (2 2 1 2 2 2 Y 2 2 2 1 2) planar heterojunctions (trilayer structures) using an in situ d.c.-sputtering process at high oxygen pressures onto (0 0 1) SrTiO3 substrates. Bi2Sr2YCu2O8−δ films (semiconducting-like) 10 and 12 nm thick were used as artificial barriers. Composition and structure of the barrier and the superconducting electrodes are entirely compatible which allows epitaxial growth of the entire heterostructure. Gap-like structures of about 30 mV have been determined from tunnelling characteristics of the junctions. Linear conductance backgrounds as well as Zero Bias Anomalies (ZBA) are reported and discussed in detail.

Preparation and characterization of sintered pellets and «Thick» films of Bi2Sr2YxCa1−xCu2O8

SummaryA systematic study of yttrium substitution for calcium in the high-temperature superconductor Bi2Sr2CaCu2O8 compound has been performed. The samples of Bi2Sr2YxCa1−xCu2O8 have been prepared by the sintering standard procedure using oxides and carbonates as precursors, weighted to obtain the nominal composition with 0≤x≤1. The real stoichiometric composition of the pellets and uniformity of the samples has been verified by EDS analysis. The grain dimension (≈1 μm) and the good pellet compactness have been established by SEM morphologic observation. By X-ray diffraction analysis the crystalline structure of orthorhombic type has been established. By increasing the yttrium concentration the resistivity dependence on temperature goes from metallic to insulating behaviour. For the superconductive samples Tc decreases for increasing doping percentage. «Thick» films on MgO substrates of Bi2Sr2CaCu2O8 have been obtained by melting-quenching-annealing procedure. These films showed critical temperatures higher than the pellets.

Etch pits and tunnel barrier in chemically etched YBa2Cu3O7−δ crystals by scanning tunneling microscopy

Abstract By imaging in air the (001) surfaces of YBa 2 Cu 3 O 7−δ single crystals etched in dilute CH 3 OH : Br 2 solution with scanning tunneling microscopy we have observed etch pits and followed their modification with time. The current-voltage characteristics of the junction are measured and described in terms of electron tunneling through a trapezoidal potential barrier of average thickness (≈ 25 A). Such a barrier results from the passivated layer formed on the superconductor surface exposed to room air after the etching. The superconductor surface is metallic. The normalized conductance G ( V ) measured at room temperature is that of a metal-insulator-metal junction except at small bias. Sharp peaks assigned to local density-of-states features within 1 eV from the Fermi level are observed.

Conductance characteristics of BiSrCaCuO-based trilayer structures

Abstract Tunneling spectroscopy has been performed on Bi2Sr2CaCu2O8−δ/Bi2Sr2YCu2O8−δ/Bi2Sr2CaCu2O8−δ trilayers deposited with a DC sputtering system in pure oxygen plasma. The conductance curves show either flat or linear increasing backgrounds, on which gap-like features are superimposed. The observation of the superconducting structures is consistent with the opening of the energy gap observed at the Bi2Sr2CaCu2O8−δ Tc in the zero-bias conductance measurements as a function of temperature. Zero-bias anomalies are also present in the tunneling spectra. These have been commented in the framework of the Appelbaum theory, as well as with respect to a possible d-wave symmetry of the order parameter.

Influence of Gd, Fe, Zn, and Al substitutions on the tunneling characteristics of the YBaCuO system.

Abstract We have investigation the effect of Gd, Fe, Zn and Al substitutions on the tunneling characteristics of planar tunnel junctions realized on single crystals of Yue5f8Baue5f8Cuue5f8O with different concentration doping. No relevant differences have been found in the conductance characteristics vs voltage of undoped 90 K and Gd junctions, so conforming that substitution of Y by other rare-earth does not affect the material superconducting properties. A different behavior has been found for substitutions on the Cu sites. Fe, Zn and Al doped junctions show broadened gap-like structures at about ±25 mV and dissapearance of the structures at ±5 mV, observed for the 90 K Yue5f8Baue5f8Cuue5f8O phase.