J. Guimpel

Photoinduced enhancement of superconductivity

We show clear and conclusive experimental evidence for the enhancement of superconductivity in GdBa2Cu3Ox and YBa2Cu3Ox thin films by photoexcitation. Upon laser illumination the critical temperature increases while the resistivity of the material decreases. The relaxation back to the original state is very slow, of the order of days at room temperature. The existence of this effect opens the possibility of fabricating weak‐link devices with in situ tunable superconducting characteristics.

High Tc thin films with roughness smaller than one unit cell

We have developed a method for the growth of epitaxial high Tc superconducting thin films with roughness smaller than one unit cell using conventional magnetron sputtering. In this method the substrate is positioned above one edge of the target (off axis) to avoid resputtering, and oscillated back and forth between the two symmetrical edges of the target to improve film thickness homogeneity. Finite size peaks in the x‐ray diffraction spectra of thin GdBa2Cu3O7−δ films and satellite peaks on a GdBa2Cu3O7−δ/YBa2Cu3O7−δ superlattice show the excellent thickness control and smoothness obtained with this technique.

Anomalous proximity effect in underdoped YBa2Cu3O6+x josephson junctions

Josephson junctions were photogenerated in underdoped thin films of the YBa2Cu3O6+x family using a near-field scanning optical microscope. The observation of the Josephson effect for separations as large as 100 nm between two wires indicates the existence of an anomalously large proximity effect and shows that the underdoped insulating material in the gap of the junction is readily perturbed into the superconducting state. The critical current of the junctions was found to be consistent with the conventional Josephson relationship. This result appears to constrain the applicability of SO(5) theory to explain the phase diagram of high critical temperature superconductors.

Synthesis and properties of a‐axis and b‐axis oriented GdBa2Cu3O7−δ high Tc thin films

We report the growth and properties of a‐axis oriented GdBa2Cu3O7−δ high Tc thin films on (100) SrTiO3 substrates by dc magnetron sputtering. It is found that GdBa2Cu3O7−δ films on (100) SrTiO3 exhibit a‐oriented growth at higher substrate temperatures compared with YBa2Cu3O7−δ films. By utilizing low‐temperature‐grown a‐axis GdBa2Cu3O7−δ films (200 A in thickness) as a self‐template, pure a‐axis films can be grown at elevated temperatures. The growth of b‐axis film on vicinal (100) SrTiO3 under similar growth conditions is also reported.

The granular nature of bulk superconductivity at 40K in La1.8Sr0.2CuO4

Abstract We have measured the electrical resistivity, magnetic flux expulsion and critical fields of the new high temperature superconductor La 1.8 Sr 0.2 CuO 4 after different heat treatments. Our data show that the complete flux expulsion in these samples is consistent with the presence of shielding currents induced in multiple connected structures. The magnetization measurements indicate that the lowest field where flux penetrates into the sample should not be taken to be the bulk Hcl. It is shown that the absolute value of the electrical resistivity and its temperature dependence are not correlated with the superconducting critical field and temperature. We discuss the behaviour of the superconducting parameters and their relation with the electrical resistivity.

Thickness dependence of the superconducting transition temperature of YBCO

Abstract We have performed detailed structural and superconducting studies of YBCO single layers, YBCO/PrBCO/YBCO tri-layers, and YBCO/PrBCO superlattices. The results for single layers indicate that a nominally one unit cell thick YBCO film sandwiched between PrBCO layers is not superconducting. However, a superlattice with nominally one unit cell thick YBCO layers has a measurable superconducting temperature onset ( T c o ). These apparently contradictory results are reconciled by experiments on YBCO/PrBCO/YBCO tri-layers which, due to coupling across the PrBCO insulating layer, exhibit superconductivity for PrBCO thicknesses below 200 A. Structural studies show that the length scale of the superconductive coupling is much larger than the length scale of the step disorder and interdiffusion present in the film.

Scaling of critical currents in high-temperature superconducting superlattices and thin films

We report the measurement of transport critical current densities in a wide variety of high‐temperature superconducting (high Tc) superlattices and thin films. We find that the temperature dependence of the critical current density Jc for all samples may be collapsed into a single curve through the scaling relation Jc(t)=Jc(0)(1−t)1.6, where t=T/Tc(H,Θ). This scaling relation remains valid in fields of up to 5 T and in all field directions Θ with respect to the c axis of the sample with the field kept perpendicular to the current.

Electrical resistivity of amorphous Zr70Cu30 and the Kondo like model

Abstract Electrical resistivity measurements of amorphous Zr70Cu30 as a function of the concentrations of two level systems show that a Kondo like theory cannot explain the observed temperature dependence.

New buffer layer for high‐temperature superconducting ceramics on sapphire: LaBa2Cu3Oy/Ag bilayers

We have grown highly oriented LaBa2Cu3Oy (110)/Ag (111) bilayers on sapphire (1120). This structure constitutes a potentially excellent buffer layer for the growth of 1‐2‐3 ceramic oxides on sapphire substrates. This bilayer does not show superconductivity in resistive measurements. A DyBa2Cu3Oy film grown on this bilayer also exhibited (110) orientation.

Size dependence of the superconducting critical temperature and fields of Nb/Al multilayers

The critical temperatureTcof Nb/Al multilayers decreases as the total sample thicknessdTis decreased while the thickness of each Nb and Al layer is kept constant. To understand this behavior, models based on the proximity effect and on weak two-dimensional (2D) localization are employed. The latter uses a characteristic length, the thermal diffusion length, in relation todTto obtain 2D behavior and leads to a reasonable explanation ofTc(dT). It is also found that the slope atTc(dT) of the critical magnetic field perpendicular to the layers is independent ofdTwhen the Nb and Al layer thicknesses are kept constant. The angular dependence of the critical field is also measured.