G. Koren

Flux creep characteristics in high‐temperature superconductors

We describe the voltage‐current characteristics of YBa2Cu3O7−δ epitaxial films within the flux creep model in a manner consistent with the resistive transition behavior. The magnitude of the activation energy, and its temperature and magnetic field dependences, are readily derived from the experimentally observed power law characteristics and show a (1−T/Tc)3/2 type of behavior near Tc. The activation energy is a nonlinear function of the current density and it enables the determination of the shape of the flux line potential well.

Epitaxial films of YBa2Cu3O7−δ on NdGaO3, LaGaO3, and SrTiO3 substrates deposited by laser ablation

Frequency‐doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm2 and 10 ns duration were used to deposit thin films of YBa2 Cu3 O7−δ by laser ablation on NdGaO3 , LaGaO3, and SrTiO3 substrates held at 725±5u2009°C in 0.2 Torr of O2 ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with Tc (R=0) at 92–93 K. Critical current densities in 0.3–0.6 μm thick, 200 μm long, and 10–30 μm wide strips were measured to be 106 A/cm2 at 60, 77, and 80 K in the films on LaGaO3, NdGaO3, and SrTiO3, respectively. X‐ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90° with respect to each other in the a‐b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8–2.1%) result in epitaxial growth of the films.Frequency-doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm/sup 2/ and 10 ns duration were used to deposit thin films of YBa/sub 2/ Cu/sub 3/ O/sub 7-//sub delta/ by laser ablation on NdGaO/sub 3/ , LaGaO/sub 3/, and SrTiO/sub 3/ substrates held at 725 +- 5 /sup 0/C in 0.2 Torr of O/sub 2/ ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with T/sub c/ (R = 0) at 92--93 K. Critical current densities in 0.3--0.6 ..mu..m thick, 200 ..mu..m long, and 10--30 ..mu..m wide strips were measured to be 10/sup 6/ A/cm/sup 2/ at 60, 77, and 80 K in the films on LaGaO/sub 3/, NdGaO/sub 3/, and SrTiO/sub 3/, respectively. X-ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90/sup 0/ with respect to each other in the a-b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8--2.1%) result in epitaxial growth of the films.

Laser wavelength dependent properties of YBa2Cu3O7−δ thin films deposited by laser ablation

YBa2Cu3O7−δ thin films were deposited onto (100)u2009SrTiO3 substrates using 1064, 532, 355, 248, and 193 nm laser ablation. Transport measurements show lower normal‐state resistivities and higher critical currents in films deposited by the shorter wavelength lasers. The surface morphology of the films was rough with large particulates when the 1064 nm laser was used whereas much smoother surfaces with fewer and smaller particulates were obtained with the UV lasers. It is suggested that the better film quality obtained when the UV lasers are used is due to a small absorption depth of the UV photons in the ceramic target and to higher absorption by the ablated fragments. This leads to smaller ablated species and further fragmentation in the hotter plume and, therefore, to smoother and denser films.

Epitaxial films of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ on NdGaO/sub 3/, LaGaO/sub 3/, and SrTiO/sub 3/ substrates deposited by laser ablation

Frequency‐doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm2 and 10 ns duration were used to deposit thin films of YBa2 Cu3 O7−δ by laser ablation on NdGaO3 , LaGaO3, and SrTiO3 substrates held at 725±5u2009°C in 0.2 Torr of O2 ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with Tc (R=0) at 92–93 K. Critical current densities in 0.3–0.6 μm thick, 200 μm long, and 10–30 μm wide strips were measured to be 106 A/cm2 at 60, 77, and 80 K in the films on LaGaO3, NdGaO3, and SrTiO3, respectively. X‐ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90° with respect to each other in the a‐b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8–2.1%) result in epitaxial growth of the films.Frequency-doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm/sup 2/ and 10 ns duration were used to deposit thin films of YBa/sub 2/ Cu/sub 3/ O/sub 7-//sub delta/ by laser ablation on NdGaO/sub 3/ , LaGaO/sub 3/, and SrTiO/sub 3/ substrates held at 725 +- 5 /sup 0/C in 0.2 Torr of O/sub 2/ ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with T/sub c/ (R = 0) at 92--93 K. Critical current densities in 0.3--0.6 ..mu..m thick, 200 ..mu..m long, and 10--30 ..mu..m wide strips were measured to be 10/sup 6/ A/cm/sup 2/ at 60, 77, and 80 K in the films on LaGaO/sub 3/, NdGaO/sub 3/, and SrTiO/sub 3/, respectively. X-ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90/sup 0/ with respect to each other in the a-b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8--2.1%) result in epitaxial growth of the films.

Role of atomic oxygen in the low‐temperature growth of YBa2Cu3O7−δ thin films by laser ablation deposition

Thin films of YBa2Cu3O7−δ were deposited on (100)u2009SrTiO3 substrates held at 600 and 700u2009°C in N2O and O2 ambients using 355 nm Nd‐YAG laser pulses for ablation of the target. The experiments were done either in the presence or absence of 193 nm excimer laser irradiation of the ambient gas between the target and the substrate. Results without the excimer irradiation show that in 0.2 Torr of both N2O and O2, at 700u2009°C substrate surface temperature, excellent smooth films with Tc (R=0) of 93 K and Jc (88 K) of 1.3×106 A/cm2 were obtained. At 600u2009°C, semiconducting films with no superconducting transition were obtained in O2 ambient, whereas in N2O, semiconducting normal state behavior with broad superconducting transition was found. With the 193 nm irradiation, no change was observed in the electrical properties of the films deposited in O2 at 600u2009°C, whereas in N2O reasonably good superconducting films with normal metallic behavior and Tc (R=0) of 84 K were found. Since the 193 nm photons hardly dissociate ...

All high Tc edge junctions and SQUIDs

We present the first observations of superconducting quantum interference in multilevel, all high Tc, lithographically patterned edge junction structures. The current‐voltage characteristics are nonhysteretic and have well‐defined critical currents. The dynamic resistance is independent of current above the critical current. These devices show periodic sensitivity to magnetic fields and low levels of magnetic hysteresis up to temperatures around 60 K.

Y1Ba2Cu3O7−δ thin films grown by a simple spray deposition technique

The preparation of high Tc superconducting thin films of Y1Ba2Cu3O7−δ on (100) single crystals of MgO, ZrO2 with 9% Y2O3 (yttria stabilized zirconia, or YSZ), and SrTiO3 using a simple spray deposition technique is described. Typical film growth procedure involves (a) the spraying of a stoichiometric solution of the nitrate precursors on the heated substrate (180u2009°C), (b) prebaking in air of the sprayed film (20 min at 500u2009°C), and (c) oven annealing of the film under flowing O2 (900–950u2009°C followed by slow cooling to 200u2009°C in about 3 h). X‐ray diffraction analysis of the films after each of the growing steps mentioned above shows primarily the presence of crystalline phases of the nitrates, the oxides, and the orthorhombic superconducting phase, respectively. Resistivity versus temperature measurements show that the onset and completion of the superconductive transition occur at 92 and 87 K, respectively, in films on YSZ substrate; at 95 and 80 K, respectively, in films on SrTiO3 substrate; and at 82 an...The preparation of high T/sub c/ superconducting thin films of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub delta/ on (100) single crystals of MgO, ZrO/sub 2/ with 9% Y/sub 2/O/sub 3/ (yttria stabilized zirconia, or YSZ), and SrTiO/sub 3/ using a simple spray deposition technique is described. Typical film growth procedure involves (a) the spraying of a stoichiometric solution of the nitrate precursors on the heated substrate (180 /sup 0/C), (b) prebaking in air of the sprayed film (20 min at 500 /sup 0/C), and (c) oven annealing of the film under flowing O/sub 2/ (900--950 /sup 0/C followed by slow cooling to 200 /sup 0/C in about 3 h). X-ray diffraction analysis of the films after each of the growing steps mentioned above shows primarily the presence of crystalline phases of the nitrates, the oxides, and the orthorhombic superconducting phase, respectively. Resistivity versus temperature measurements show that the onset and completion of the superconductive transition occur at 92 and 87 K, respectively, in films on YSZ substrate; at 95 and 80 K, respectively, in films on SrTiO/sub 3/ substrate; and at 82 and 77 K, respectively, in films on MgO substrate.

Emission spectra, surface quality, and mechanism of excimer laser etching of polyimide films

Emission spectra in the visible and ultraviolet range, and scanning electron microscopy of polyimide films exposed to 193, 248, and 351‐nm laser radiation were used to elucidate the mechanism of the laser etching process. It was found that rough laser etched surfaces, with nonuniformities larger than 0.5 μm, are accompanied by a continuum emission from the blow‐off materials, while smooth surfaces are characterized by strong C2 emission in the Au20093Πg→X′u20093Πμ bands. Smoothly etched surfaces were obtained for all laser wavelengths provided the energy absorbed per unit volume in the surface exceeds a threshold value of (5.0±0.5)×104 J/cm3. It is suggested that surface smoothness results from transient melting and that the laser etching mechanism is mostly a statistical thermodynamic process without complete energy randomization.

Particulates reduction in laser‐ablated YBa2Cu3O7−δ thin films by laser‐induced plume heating

Experimental demonstration of reduction in the number and size of particulates formed in the laser ablation deposition of YBa2Cu3O7−δ thin films is obtained by the use of a second laser which further heats and fragments the blowoff material in the plume formed by the first laser. This results in a smoother film with higher critical current density as compared to that obtained without the second laser irradiation of the plume.

Quadratic temperature dependence of the in-plane resistivity in superconducting Nd1.85CuO4 — Evidence for Fermi-liquid normal state

Abstract The in-plane normal-state resistivity, ϱ|;( T ), of the electron-doped superconductor Nd 1.85 Ce 0.15 CuO 4 has been measured and analyzed as a function of temperature in the range of T c to 300 K where T c ( R =0)=20 K. A dominant T 2 temperature dependence of ϱ|( T ) for T T 200 K, the data indicate a logarithmic correction to the quadratic temperature dependence. These findings are consistent with the Fermi-liquid model for electron-electron scattering, which is greatly enhanced in the CuO 2 -layered oxides. The experimental results of this work lend strong support to the validity of a Fermi-liquid description for the normal-state of electron-doped Cu-oxide superconductors.