I. Takeuchi

Tunneling study of clean and oriented Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O surfaces

Abstract Cryogenically cleaved surfaces of epitaxially grown Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O films were studied using the film-edge junction technique. The junctions were analized based on a simple junction model. The junction qualities were degraded mainly due to the leaky tunneling barrier and the exposed normal material at the surface of the oxide superconductors. The gap energies were found to have the gap closing temperatures. The linearity between Tc (Δ=0) and Δ(0) was found to be better than that between Tc(R=0) and Δ(0). Such linearity was found in the Tc range between 40 K and 80 K. For Y-Ba-Ca-O and Bi-Sr-Ca-Cu-O samples, the normalized gap energy 2Δ(0)/kBTc that associated with the peak voltage value in the σ−V characteristics was about 6 (±0.2) in the direction of the Cu-O plane, in contrast to a smaller normalized gap energy found in the c-plane cleaved junction.

Energy gap of Tl-Ba-Ca-Cu-O compounds by tunneling

Abstract Tunneling spectroscopy measurements were carried out on surfaces of polycrystal Tl-Ba-Ca-Cu-O superconductors of two stoichiometric compositions (2212 and 2223). Broad gap-like structures were routinely observed on differential conductance curves, and taking their peak positions and resistive T c s, we obtain the estimated 2Δ k B T c of ∼ 6 for both compositions which is in good agreement with our earlier tunneling results on oriented film surfaces of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O compounds.

Observation of gap anisotropy in YBa2Cu3O7−δ by tunneling

Abstract Superconductive energy gap of YBa 2 Cu 3 O 7−δ are measured in anovel broken film edge junction. (001), (103) and (110) oriented films are broken in a cryogenic environment along the appropriate directions together with the SrTiO 3 substrate. Pb electrode is brought close in situ to the clean broken film edge. The normalized energy gap 2Δ(0)/k B T c measured in the direction along and perpendicular to the Cu-O plane are found to be 5.9±0.2 and 3.6±0.2 respectively. These values are independent of the variation in the values of T c within the examined range of 40K∽90K.

Characteristics of DC SQUIDs Patterned on Highly Granular Y–Ba–Cu–O Thin Films

Measurements have been made of the properties of thin granular film DC SQUIDs fabricated from the high Tc superconductor Y–Ba–Cu–O, paying special attention to peculiar effects arising from the unique granularity of the film. The output voltage modulation of the SQUIDs was found to be very much lower than expected from the theory, and we present here several possible explanations for it, based on such factors as the asymmetry of the junctions and high kinetic inductance of the film. Multiple periodicities and phase jumps on VΦ-characteristics were observed. Normal operations of the SQUIDs were observed till ~65 K. Estimating from the best data, the equivalent flux noise was below 3×10-4 Φ0/Hz1/2 in the frequency range above 600 Hz at around 50 K.

Electrical properties of SN contracts with cleaved oxide superconductor films

The authors have studied the feasibility of making sharp and clean electrical contacts with high-temperature superconductors by cleaving single-crystal Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O films in a stream of evaporated noble metals forming SN (superconductor/normal metal) contacts at the revealed cross-sectional edge surfaces of the films. With this technique, contact resistances on the order of 10/sup -9/ to 10/sup -8/ Omega cm/sup 2/ have been consistently obtained. For a structure with an oxide/Au/oxide configuration, the interface resistance was found to show a flat temperature dependence below T/sub c/. Material dependence and anisotropy were observed in the contact resistance. When

Comparison of the energy gaps in various high-Tc oxide supercinductors

Abstract Various superconductive oxides such as Tl2Ba2Ca2Cu3O10, Tl2Ba2Ca1Cu2O8, Tl2Ba2CuO6, Bi2Sr2CaCu2O8 and YBa2Cu3O7 as well as the electron doped Nd1.85Ce0.15CuO4 were studied by the tunneling technique. The energy gap structures were consistently observed for almost all the measurements. The peak voltages of such structures in the σ-V characteristics scaled linearly with the critical temperatures for all the compounds examined. But the details of such characteristics were not always reproducible to conjecture a particular density of states. The energy gap was inferred to be around 4∼6kBTc.

Epitaxial growth study of (Ba1−xRbx)BiO3−8 films by ion beam sputtering

Abstract Epitaxial growth of (Ba 1−x Rb x )BiO 3−8 films was studied. Oxygen deficient atmosphere was appropriate for the epitaxial growth, while the effective oxidation by atomic oxygen during cooling down process was required in order to obtain d the as-grown superconductivity. Typical Tc , zero of the film was about 10K. The films had very smooth surface and the in-plane epitaxial relationship was BRB// SrTiO 3 .

Single-Electron Tunneling Effects at the Surfaces of Oxide Superconductor Films

We have studied charging effects arising from tunnel junctions with ultrasmall capacitances using a cryogenic STM. Our samples are evaporated thin films of high temperature superconductors whose surface morphology is often rich in microstructures. All our measurements are made at 4.2K and above, and after approaching the surface, I-V characteristics are obtained. The I-V curves often displayed characteristics arising from the Coulomb blockade of ultrasmall tunneling junctions. In one measurement, we have observed clearly what appeared to be microwave induced current steps possibly due to the SET oscillations. Some of the results are compared with numerical calculations.

Cleaved Surfaces of High Tc Films for Making SNS Structures

In exploring the feasibility of fabricating high Tc SNS Josephson junctions with well-defined and clean SN interfaces, have been cleaved epitaxial thin films of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O of various crystal orientations in vacuum while the evaporation of a noble metal is taking place. The samples had structures which allowed in situ formation of SNS junctions when the films were cleaved together with substrate with the area of the SN interfaces defined by the revealed cross-sectional edge surfaces of the films. Electrical measurements of the resulting junctions show that oxide/metal interfaces possess contact resistance in the order of 10 to the 9th to 10 to the 8th ohm-sq cm which is at least two orders of magnitude larger than that of similarly made SN interfaces with metallic superconductors. The nature of this interface and the factors influencing and giving rise to the contact resistance are discussed.

Charging effects at the surfaces of HTSC films by CSTM

Abstract We have investigated the properties of ultrasmall capacitance junctions formed by the STM tip and conductive microstructures on the surfaces of evaporated YBaCuO films at 4.2K. We have found that most of the anomalous tunneling structures on the I–V curves that are in resemblance related to the superconductor and single junction Coulomb blockade behaviors can be explained by a voltage-biased double junction Coulomb blockade system. In one measurement, we have observed what appeared to be microwave induced current steps on the I–V curve.