S. C. Gausepohl

Strain stabilized metal–insulator transition in epitaxial thin films of metallic oxide CaRuO3

We report the observation of both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO3 deposited under identical conditions. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single-crystal (100) SrTiO3 substrates, metallic films with lattice parameters close to the bulk material grew on (100) LaAlO3 substrates and poor crystalline quality SrTiO3 substrates. It is believed that a strain induced substitution of the small Ru4+ cations by the larger Ca2+ cations occurs, breaking the conduction pathway within the three-dimensional network of the RuO6 octahedra and leading to a metal–insulator transition. This unique phenomenon, which is not observed in bulk material, can be significant in technologically important epitaxial perovskite oxide heterostructures.

Magnetoresistance probe of spatial current variations in high‐Tc YBa2Cu3O7–SrRuO3–YBa2Cu3O7 Josephson junctions

The low frequency current–voltage characteristics of YBa2Cu3O7–SrRuO3–YBa2Cu3O7 Josephson junctions show an increasing resistance and phase shift in fields of 0 to 8 T. A critical current is also evident up to 8 T. Data are analyzed in terms of field‐induced perturbations of the current paths through the junction. Results indicate a highly inhomogeneous interface where the mean width of spatial variations in both normal and supercurrent is of order 50 to 100 A.

Influence of SrTiO3 bicrystal microstructural defects on YBa2Cu3O7 grain-boundary Josephson junctions

Using near-field scanning optical microscopy (NSOM), we observe an inhomogeneous distribution of submicron-sized structural defects at the fusion boundary of polished SrTiO3 bicrystal substrates. Both NSOM and scanning force microscopy show that these substrate defects cause the grain boundary of a YBa2Cu3O7 thin film grown on the bicrystal to wander up to a micron in the film. These structural defects are shown to correlate qualitatively with the electrical characteristics of grain-boundary Josephson junctions patterned on the YBa2Cu3O7 film.

Contact resistance improvement by dielectric breakdown in semiconductor-dielectric-metal contact

We propose an approach for reduction of the contact resistance by inducing dielectric breakdown in a Si-dielectric-metal contact stack. We observe a 36% reduction in the contact resistance as well as an improvement in the uniformity in the distribution after dielectric breakdown. The results open up interesting device applications in complementary metal oxide semiconductor technology.

Supercurrent distributions in YBa2Cu3O7–YBa2Co0.21Cu2.79O7–YBa2Cu3O7 edge Josephson junctions

We report the measurement and analysis of critical current (Ic) modulations in magnetic field (H) for YBa2Cu3O7–YBa2Co0.21Cu2.97O7–YBa2Cu3O7 edge Josephson junctions operating at 77 K. The Ic(H) patterns are clean and reproducible enough to allow a quantitative, self‐consistent Fourier transform analysis that reveals the major features of the spatial supercurrent density distribution. Use of YBa2Co0.21Cu2.97O7 as the weak link is seen to result in significantly more homogeneous characteristics than in most other high‐Tc junctions reported to date.

Microstructural defects in bicrystal substrates and their influence on yttrium barium copper oxide grain boundary Josephson junctions

Using a near-field scanning optical microscope (NSOM), we found non-uniformly distributed microstructural defects near the fusion boundary of bicrystal substrates. These defects depict themselves as circular dark spots in the optical transmission images. We attribute these optical features to near-surface voids at the boundary previously found in transmission electron microscopy studies. Our results show a direct link between the presence of these defects and the superconducting properties of the YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) grain boundary Josephson junctions (GBJJs). We find that junctions that are fabricated over highly defected regions of the bicrystal substrates have lower critical temperatures and currents. The positions of defects across a junction also play a role in determine the device characteristics. Strain fields associated with these defects are most likely responsible for affecting YBCO film growth locally and junction performance. We employ an NSOM to survey these microstructural defects in bicrystals of various tilt angles and of different materials that are commonly used for fabrication of YBCO GBJJs. We find that these defects and their strain fields are the result of the bicrystal fusion process and are not intrinsic to the materials.

Supercurrent distributions and flux penetration in YBa2Cu3O7–YBa2C o0.15Cu2.85O7–YBa2Cu3O7 edge junctions

We report the measurement and analysis of critical current (Ic) modulation and microwave response in magnetic field for a set of YBa2Cu3O7-YBa2Co0.15Cu2.85O7-YBa2Cu 3O7 ramp-edge Josephson junctions. These junctions have been fabricated with various widths w to span the transition from the moderately small (w 2λJ) junction limit at 77 K. As w increases, both the Ic modulation and microwave absorption behavior independently suggest a growing contribution from edge supercurrents. The addition of a superconducting ground plane was shown to reduce the edge supercurrent effects.

Effect of Lattice Strain on Growth Mechanisms and Electrical Transport Behavior of Epitaxial CaRuO 3 Thin Films

We have observed both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO 3 deposited under identical conditions on (100) SrTiO 3 substrates of varying crystalline quality. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single crystal SrTiO 3 substrates, metallic films with lattice parameters close to the bulk material grew on poor crystalline quality SrTiO 3 substrates. The films deposited on (100) LaAlO 3 substrates consistently showed metallic behavior. Atomic force microscope images suggest that the semiconducting films had a coherent two dimensional nucleation. In contrast, three dimensional island-like incoherent growth was seen in the metallic films. It is believed that in the coherent films a strain induced substitution of the small Ru 4+ cations by the larger Ca 2+ cations occurs, breaking the conduction pathway within the three dimensional network of the RUO 6 octahedra and leading to a metal-insulator transition. This unique phenomenon - which is not observed in bulk material - can be significant in technologically important epitaxial perovskite oxide heterostructures.

Supercurrent distribution and flux penetration in high-T c edge Josephson junctions

The spatial distribution of supercurrent in high-Tc Josephson junction devices has been studied extensively using field modulation measurements of the critical current and microwave absorption. The devices are edge junctions composed of YBa2Cu3O7-YBa2Co0.21Cu2.79O7-YBa2Cu3O7. The lc(H) patterns allow a quantitative Fourier transform analysis to obtain a self-consistent spatial supercurrent density distribution, Js(x). These junctions are found to be more homogeneous than in most other high-Tc Josephson junctions reported to date.

Absence of proximity coupling in ultra-thin c-axis oriented YBa2Cu3O7 - Au bilayers

Abstract We have measured the mutual inductance signal of the superconducting transition in ultra-thin YBa 2 Cu 3 O 7 - Au bilayers. The samples consist of d S = 50 A of c -axis oriented YBa 2 Cu 3 O 7 , capped with initially d N = 50 A of Au. As d N is increased on each sample, no decrease in T c is observed, to within our experimental resolution. These results set an upper limit of ≤ 15 A on the proximity coupling range of YBa 2 Cu 3 O 7 , through Au.