L. Antognazza

Study of interface resistances in epitaxial YBa2Cu3O7−x/barrier/YBa2Cu3O7−x junctions

We have measured interface resistances in YBa2Cu3O7−x/barrier/YBa2Cu3O7−x junctions with different barrier materials in an edge junction geometry. CaRuO3, La0.5Sr0.5CoO3, Y0.7Ca0.3Ba2Cu3O7−x, YBa2Cu2.79Co0.21O7−x, and La1.4Sr0.6CuO4 have been employed as the epitaxial barrier materials. We observe interface resistances of the order of 1×10−8 Ω cm2 when we use CaRuO3 and La0.5Sr0.5CoO3 barriers. These two barrier materials are cubic perovskites. However, in the case of the layered barrier materials, the measured interface resistances are smaller than 1×10−10 Ω cm2. Our study suggests that the oxygen disorder at the YBa2Cu3O7−x surface, due to stress created by the thermal expansion mismatch between YBa2Cu3O7−x and the barrier, may be the origin of the interface resistances, and that the magnitude of this stress can change the resistance by orders of magnitude.

Josephson coupling of YBa2Cu3O7−x through a ferromagnetic barrier SrRuO3

Epitaxial barriers of ferromagnetic SrRuO3 have been used to fabricate high Tc superconductor‐normal‐superconductor Josephson junctions in the edge junction geometry. At small barrier thicknesses the SrRuO3 junctions follow the behavior of nonferromagnetic but otherwise closely related CaRuO3 junctions. A rapid disappearance of critical current is observed when the barrier thickness is increased to 250 A. Possible origins of such a large critical thickness are discussed.

Properties of YBa2Cu3O7−x/YBa2Cu2.79Co0.21O7−x/YBa2 Cu3O7−x edge junctions

We report the properties of YBa2Cu3O7−x/YBa2Cu2.79Co0.21O7−x/YBa2 Cu3O7−x junctions in an edge junction geometry as a function of temperature and barrier thickness. The barrier material used in this work, YBa2Cu2.79Co0.21O7−x, is an underdoped version of YBa2Cu3O7−x, which has a lower carrier density, a lower Tc, and a higher anisotropy than YBa2Cu3O7−x. The resistances of the junctions are proportional to the thicknesses of their barriers, suggesting the presence of very little interface resistance. The temperature dependences of critical currents and junction resistances show behavior consistent with that predicted by the conventional proximity effect.

Ti and Ca substitution in SrRuO3 thin films by sequential deposition process

We report on the growth and properties of (Sr1−xCax)RuO3 and Sr (Ru1−xTix)O3 thin films obtained by sequential deposition of submonolayers from end members of each compound. Magnetization measurements as well as transport properties exhibit a very different behavior for each type of substitution. A simple model, which assumes a random distribution of the substituted sites, allows us to account quantitatively for the reduction of the magnetization with increased doping and further confirms the homogeneity of our samples. A strong distortion due to the presence of the Ti on the Ru site could explain the different behavior observed between both types of doping.

High-Tc edge junctions with Y0.8Pr0.2Ba2Cu2.7Co0.3O7−δ barrier layers near the metal–insulator transition

We report the use of an epitaxial barrier of Y{sub 0.8}Pr{sub 0.2}Ba{sub 2}Cu{sub 2.7}Co{sub 0.3}O{sub 7{minus}{delta}} in order to study the effect of Pr in the barrier layer. The bulk behavior of this Pr and Co-doped YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) material shows it is just on the insulating side of the metal{endash}insulator transition. We present the temperature dependence of the critical current I{sub c} and the resistance R{sub n} for junctions with 150-, 300-, and 450-{Angstrom}-thick barriers of Y{sub 0.8}Pr{sub 0.2}Ba{sub 2}Cu{sub 2.7}Co{sub 0.3}O{sub 7{minus}{delta}}. The temperature dependence of the critical currents was analyzed within the framework of the proximity effect of de Gennes. The fittings show that the junctions are in the dirty limit with a diffusion constant of D=2.3cm{sup 2}/s and a pair-breaking scattering time of 8.6{times}10{sup {minus}14}s. The value of the diffusion constant is three times lower than the value we have found in the pure 14{percent} Co-doped YBCO barrier, showing that the Pr doping decreases the mean free path in the junction barrier. The presence of the inelastic pair-breaking process was identified as a key issue in the barrier near the metal{endash}insulator transition. {copyright} {ital 1997 American Institute of Physics.}We report the use of an epitaxial barrier of Y0.8Pr0.2Ba2Cu2.7Co0.3O7−δ in order to study the effect of Pr in the barrier layer. The bulk behavior of this Pr and Co-doped YBa2Cu3O7−δ (YBCO) material shows it is just on the insulating side of the metal–insulator transition. We present the temperature dependence of the critical current Ic and the resistance Rn for junctions with 150-, 300-, and 450-A-thick barriers of Y0.8Pr0.2Ba2Cu2.7Co0.3O7−δ. The temperature dependence of the critical currents was analyzed within the framework of the proximity effect of de Gennes. The fittings show that the junctions are in the dirty limit with a diffusion constant of D=2.3 cm2/s and a pair-breaking scattering time of 8.6×10−14 s. The value of the diffusion constant is three times lower than the value we have found in the pure 14% Co-doped YBCO barrier, showing that the Pr doping decreases the mean free path in the junction barrier. The presence of the inelastic pair-breaking process was identified as a key issue in the ...

CONDUCTANCE-VOLTAGE CHARACTERISTICS OF INTERFACES BETWEEN YBA2CU3O7 AND METALLIC OXIDES

We present the conductance–voltage characteristics of interfaces between YBa2Cu3O7 and three conductive metallic oxides; CaRuO3, SrRuO3, and La0.5Sr0.5CoO3. In the case of all the YBa2Cu3O7/(Ca,Sr)RuO3 interfaces, we observe the zero bias conductance peaks with the increasing conductance background at high bias. The zero bias conductance peaks decrease with increasing temperature and disappear at temperatures between 30 and 40 K. However, we did not observe zero bias anomalies in the YBCO/La0.5Sr0.5CoO3 contacts, in spite of an interface resistance of similar magnitude. This work suggests that the origin of the zero bias conductance peak lies in the defects on the YBCO surface and the exact defect structure depends on the adjacent epitaxial material.

Transport and magnetic properties of Sr(Ru1−xTix)O3 thin films

We report on the growth and the properties of Sr(Ru1−xTix)O3 thin films deposited by laser ablation. By substituting Ti for Ru in SrRuO3, we observe a continuous change of the magnetic and electrical properties of the perovskite. The Curie temperature is reduced from 150 K (x=0) to zero with increasing amount of Ti. The resistivity as a function of temperature shows a crossover from metallic to semiconducting behavior. A comparison with Ca substitution for Sr is presented.