Hiroshi Takashima

TUNNELING SPECTROSCOPY AND PAIRING SYMMETRY OF THE HIGH-TC SUPERCONDUCTORS

Abstract Influences of d-wave pairing symmetry on superconducting electronic states is studied by tunneling spectroscopy. Detailed experiments on spatial and orientational dependences of the surface electronic states of YBa2Cu3O7−δ thin films and preliminary results of the proximity effect observation in Au layer deposition on YBCO are presented. By comparing these experimental results with recent theoretical works, the existence of anomalous interference effects are verified in the electronic states of the high-Tc superconductor, due to the d-wave symmetry of the pair potential.

Single crystal growth of superconducting La2-xBaxCuO4 by TSFZ method

Abstract Single crystals of La2-xCuO4 with 5 mm diameter and 25 mm length have been grown by the traveling solvent floating zone (TSFZ) method. The composition of the single crystal containing maximum Ba content and showing a superconducting transition at 30 K was determined to be La1.935Ba0.065CuO4. Electrical resistivities along the c-axis and in the ab-plane reveal significant anisotropy. Magnetic susceptibilities have been measured under a field of 50 Oe from 5 to 50 K. Under fields H⊥ ab-plane and H∥ab-plane, the single crystal demonstrates 3.6% and 0.7% Meissner fractions, respectively, at 5 K.

Control of Step Arrays on Normal and Vicinal SrTiO3(100) Substrates

The surface morphologies of SrTiO3 substrates have been investigated by atomic force microscopy. Periodic step arrays could be developed on both normal and vicinal SrTiO3 (100) substrates by a new method employed both buffered NH4F-HF (BHF) solution and O2 annealing. An explanation is given for the surface morphologies observed which involves the surface stoichiometrical change upon BHF solution and surface diffusion upon oxygen annealing.

Adsorption of anionic nanosheets from their dilute colloidal suspensions onto gas-liquid interfaces with and without a Langmuir film of cationic surfactant.

The adsorption of [(Ca(2)Nb(3)O(10))(-)](n) (CNO) polyanionic nanosheets from their dilute colloidal suspensions onto gas-liquid interfaces with and without a cationic [N(CH(3))(2)(C(18)H(37))(2)](+) (DOA) surfactant film has been experimentally investigated. Our concern has been focused particularly on their dynamical aspects, which can be observed owing to the long specific time of the adsorption. The theoretical framework of the Langmuir adsorption model has enabled a quantitative analysis of the observed data, and that analysis has indicated that the presence of a positively charged Langmuir film enhances the ratio of the adsorption and desorption rate constants by approximately 30 times. Furthermore, the experimental results have shown that a balanced hybrid Langmuir film, in which both organic and inorganic constituents are densely packed, can be prepared.

Orientation dependence of tunneling spectra in YBCO and NCCO

Abstract We performed a crystal orientation dependent study of the tunneling spectra in high-temperature superconductors. The comparison of the tunneling spectra for different orientations and materials as measured by Low Temperature Scanning Tunneling Microscopy/Spectroscopy (LTSTM/S) gives information about the relation between tunneling spectra and order parameter symmetry. Our data — obtained on well characterized, high quality (100), (110), and (001) epitaxial YBa 2 Cu 3 O −δ thin films and Nd 1.85 Ce 0.15 CuO 4− y single crystals — can be interpreted by assuming zero-energy surface states resulting from the sign change of the pair potential in d -wave superconductors. The observed spatially continuous and bias voltage independent zero bias conductance peak (ZBCP) on (110) YBa 2 Cu 3 O 7−δ surfaces is then due to the d χ 2 − y 2 wave symmetry. Compared with that, Nd 1.85 Ce 0.15 CuO 4− y never exhibited any ZBCP as expected because of its s -wave superconductivity.

Structure and Dielectric Behavior of Epitaxially Grown SrTiO3 Film between YBa2Cu3O7-δ Electrodes

To obtain a high insulating SrTiO3 layer, the parallel capacitors of epitaxially grown YBa2Cu3O7-δ / SrTiO3 / YBa2Cu3O7-δ have been fabricated by a Chemical Mechanical Planarization method. The dielectric constant and tan δ of the SrTiO3 film at 2.2 K was 27000 and 0.027 at 10 kHz, respectively. The temperature variation of the dielectric constant could be well fitted by the formulae of Curie-Weiss and Barrett in the temperature ranges above 100 K and 30<T<100 K, respectively. The best-fit parameters were coincident with those for SrTiO3 single crystals. In order to examine the influence of degraded SrTiO3 layers at the YBa2Cu3O7-δ/SrTiO3 interfaces on the dielectric constant, a multilayer model involving two kind of SrTiO3 layers with different dielectric characteristics was proposed. The results revealed that the degraded SrTiO3 layer is very thin and the dielectric constant of the SrTiO3 film was dominated by that of single-crystal-like SrTiO3 layer.

Development of intrinsic surfaces, and their electronic structures and stability of non-c-axis YBCO epitaxial films

Surface-cleaning using 100% ozone beams or atomic oxygen beams has been applied for the fabrication of high quality [110]- and (100)-axis oriented YBCO epitaxial films. The treatment recovers metallic surfaces in spite of the semiconducting properties of the as-received surfaces. High quality [110]- and (100)-oriented films with high T/sub c/ and high orientation were deposited by temperature-gradient self-template method. On both kinds of film clear photoelectron Fermi edges, evidence of a metallic feature, were observed at room temperature. For the [110] films, well-defined photoelectron Fermi edges are observed for the first time, For the (100) surface, the rather high electron density of states Fermi edge is as high as about 15% of Cu 3d signal in valence band, On the other hand, the edge-height of the [110] surface was less than 5% of the Cu 3d and it was reduced as a function of time. Detailed electronic structures and the origin of the difference of the surface-stabilities is discussed.

Non-destructive Detection of Defects in Carbon Fiber-Reinforced Carbon Matrix Composites Using SQUID

Carbon fiber composites are increasingly used as structural materials because of their unique and advantageous characteristics. Carbon fiber reinforced carbon matrix composite (C/C) has the characteristics of high fatigue resistance, fracture toughness and heat resistance up to 3000 K, and is an important component of refractory tiles and nozzles in space shuttles. Useful nondestructive testing methods for C/C are now required. We have developed a SQUID-NDT system based on a nonmagnetic coaxial pulse tube cryocooler (PTC), a HTS-SQUID gradiometer and a field generator with ferrite cores that induces high currents in specimens with low electric conductivity. The cryostat with the PTC is compact, at 50 mm in diameter and 400 mm in height. It weighs a total of 4 kg. The system noise is 80 μΦ 0 /Hz 1/2 corresponding to 1.3 nT/m/Hz 1/2 at 100 Hz. We used the system to investigate the usefulness of the SQUID-NDT in detecting flaws in C/C composites. Hidden cracks in C/C multi-layered specimens were detected up to depth of 15 mm. Hidden cracks in C/C-Al stacked sample was also clearly detected. In addition, we magnetically detected the mechanical breaking process of a C/C specimen under tensile load using the current injection method. For this study, a technique for visualizing current detouring defects was developed for detection of deteriorating areas in the specimen. The deteriorating area, identified from the current map, expands during breaking process and agrees with the results obtained by the microscopic observation of the breaking process. The interrupted current I int , estimated by summing the detour current, clearly changed depending on the stage of the breaking process, suggesting that I int may be applicable as good index for distinguishing each stage in the breaking process. It is concluded that a SQUID-NDT is applicable to C/C composites and advanced complex materials with low electric conductivity in addition to metallic materials.

Regulated Epitaxy of YBa2Cu3O7-δ by Atomic Control of Step Arrays on Vicinal SrTiO3(100) Substrates

We have investigated the surface morphologies of vicinal SrTiO3 substrates and YBa2Cu3O7-δ (YBCO) thin films by atomic force microscopy (AFM). Extremely regular step arrays had been successfully developed vicinal SrTiO3 with 5.7° misorientation. AFM reveals a great improvement of the surface microstructures of YBCO thin films grown on such terraced vicinal substrates, exhibiting a regular step-flow growth mode and almost precipitate-free surfaces of practical device sizes. By a high growth rate and low growth temperature, the step bunching and therefore the total roughness have been substantially suppressed.

Fabrication of High-Quality YBa2Cu3O7-δ Multilayer Structure Using Chemical Mechanical Planarization for Superconducting Quantum Interference Device Gradiometer

We have developed a technology to fabricate a YBa2Cu3O7-δ (YBCO) multilayer structure of high quality using chemical mechanical planarization (CMP) for the end of fabricating a high-performance superconducting quantum interference devices (SQUIDs) gradiometer. An annealing process in oxygen gas atmosphere was introduced to achieve epitaxial growth of upper films. A double-cross structure on five layers, YBCO/SrTiO3 (STO)/YBCO/STO/YBCO, was fabricated to show the performance. The insulation resistivity between layers at 77 K, Tc and Jc of each layer at 77 K were more than 1.5×107 Ωcm, over 88 K and more than 106 A/cm2, respectively. The possibility of fabricating grain boundary Josephson junctions on the top layer with a bicrystal substrate is investigated using STO (100) bicrystal substrates with 24 and 30 degrees misorientation angles on a YBCO trilayer structure. The IcRn products of both junctions were the same as that of a single YBCO layer.