Akihiro Odagawa

Structural, normal-state, and superconducting properties of (La1−xBax−ySry)2CuO4

Abstract We report on the interplay among the structural transitions, transport anomalies and superconductivity in (La 1− x Ba x − y ) 2 CuO 4 . For (La 1− x Ba x ) 2 CuO 4 y =0, it is now well established that high- T c superconductivity is suppressed in a narrow range of x around 0.0625. In this study, the amount of doping is held at x =0.0625 and the structural transition temperatures are controlled by a new parameter y . The appearance of the low-temperature tetragonal (TLT) phase and the depression of superconductivity correlate well, but unlike in (La 1− x Ba x ) 2 Cuo 4 the transport anomalies can be decoupled from structural transition, implying the presence of a new instability in the electronic state, which precedes the structural transition to the TLT phase.

Suppression of Inhomogeneous Segregation in Graphene Growth on Epitaxial Metal Films

Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films.

Control of Epitaxial Growth Orientation and Anisotropic Thermoelectric Properties of Misfit-Type Ca3Co4O9 Thin Films

We have investigated the control of crystal orientation in misfit-type layered cobaltite Ca3Co4O9 thin films by rf-planar magnetron sputtering and succeeded in growing epitaxial films with c-axis, a-axis and b-axis orientations normal to the substrate. Anisotropic transport properties (parallel to the CoO2 layers and perpendicular to the CoO2 layers) were measured using the b-axis-oriented epitaxial films with a layered structure perpendicular to the substrate surface. The resistivity parallel to the CoO2 layers (ρa) is about 8 mΩ cm at room temperature while that perpendicular to the CoO2 layers (ρc) is about 300 mΩ cm; the anisotropy is estimated to be about 40. Thermoelectric anisotropy is not considerably pronounced; the parallel Seebeck coefficient Sa is measured to be 110 µV/K and the perpendicular Sc is 40 µV/K at room temperature.

Characteristics of Intrinsic Josephson Junctions in a Thin Stack on Bi-2223 Thin Films

We have successfully fabricated a small thin stack with a small number of intrinsic junctions on (Bi, Pb)2Sr2Ca2Cu3O10+x thin films. Mesa structures with the junctions are fabricated on the surface of high-quality films prepared by rf-sputtering and subsequent heat treatment. Current-voltage (I–V) responses along the c-axis show large distinct hysteresis, a clear multiple branching structure with a periodic voltage jump and the edge structure, which represents the superconductive gap. On reducing the number of the intrinsic stacked junctions, the estimated superconductive gap value increases. The thinnest stack exhibits the I–V characteristic of a single junction. The value of the superconductive gap estimated from the single junction sample is about 75 meV at 4.2 K. The obtained I–V curve is explained quite well by assuming dx2-y2 symmetry for the superconductive order parameter without the term of gap suppression.

Observation of Intrinsic Josephson Junction Properties on (Bi,Pb)SrCaCuO Thin Films

We have fabricated intrinsic Josephson junctions for (Bi,Pb)2Sr2Ca2Cu3Ox thin films and investigated their electronic characteristics. Mesa structures with the junctions are fabricated on the surface of high-quality films prepared by rf-sputtering and subsequent heat treatment. The junctions show distinct hysteresis and a multiple branching structure with a periodic voltage jump at a current-voltage response. These results demonstrate that the fabricated mesas consist of stacked series SIS junctions. From this periodic structure, a voltage jump of 26–28 mV is obtained for the 2223 phase at 4.2 K.

Collective switching and heat diffusion of stacked intrinsic Josephson junctions

Abstract We have investigated a mechanism for the switching of stacked intrinsic Josephson junctions in current–voltage characteristics. In a current-biased measurement, collective switching has been observed, while in a voltage-biased measurement, clear branching structure has been observed. In the voltage-biased measurement, the interaction between different intrinsic junctions has clearly appeared in decrease of values of Josephson critical current ( I c ) with increasing the number of junctions in the resistive state. The I c decrease leading to the collective behavior has been explained by taking effective heat diffusion into account.

Preparation and characteristics of 90° rotated biepitaxial Fe3O4 thin films

In-plane 90° rotated biepitaxial Fe 3 O 4 thin films have been successfully prepared onto MgO (110) substrates using a CeO 2 seed layer and their microstructure, electric, and magnetic properties were investigated. From the x-ray φ-scan measurements, the in-plane epitaxial relations were determined as 〈110〉Fe 3 O 4 //〈110〉MgO and 〈001〉Fe 3 O 4 //〈001〉MgO for the no-seeded Fe 3 O 4 layer, and 〈001〉Fe 3 O 4 //〈110〉MgO and 〈110〉Fe 3 O 4 //〈001〉MgO for the CeO 2 (110) seeded Fe 3 O 4 layer. The CeO 2 seed layer was found to rotate the upper Fe 3 O 4 lattice at 90° upon normal axis to the layer against the no-seeded Fe 3 O 4 . The transmission electron microscopy and electron diffraction analyses revealed that the transition region of the biepitaxial Fe 3 O 4 boundary between CeO 2 -seeded and no-seeded portions consisted of columnarlike polycrystalline grains. The Fe 3 O 4 films exhibited single-crystallinelike electric and magnetic properties, however, substantial spin-dependent-tunneling magnetoresistance across the 90° grain boundary was not observed even in the antiparallel situation for each Fe 3 O 4 portion.

Structural transitions and superconductivity in (La1-xBax-ySry)2CuO4

We report on the interplay among the structural transitions, transport anomalies and superconductivity in (La1-xBax-ySry)2CuO4. Here the amount of doping is held at x = 0.0625 and the structural transition temperatures are controlled by a parameter y. The appearance of the low-temperature tetragonal phase and the depression of superconductivity correlate well, but unlike in (La1-xBax)2CuO4 the transport anomalies can be decoupled from the structural transition, implying the presence of a new instability in the electronic state.

Investigation of I-V characteristics of intrinsic Josephson junctions in Bi-2223 thin films

Abstract We have investigated current-voltage (I-V) characteristics of intrinsic Josephson junctions in c -axis-oriented (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+ x (Bi-2223) superconductive thin films. Clear branching structures peculiar to stacked junctions are observed. We also have evaluated the quasi-particle current of a single intrinsic junction by adopting the modified mesa structure. The obtained quasi-particle I-V curve is quite well explained theoretically by assuming d x 2 − y 2 symmetry for the superconductivity order parameter.

Observation of microwave induced steps for a single intrinsic tunnel Junction

We report the successful fabrication of a single intrinsic Josephson junction on a (Bi,Pb)-Sr-Ca-Cu-O thin film and investigate the current-voltage characteristics. We have observed voltage steps in the current-voltage characteristic induced by microwave irradiation. The voltage step shifts to higher voltages with increasing irradiation power while the height of the step remains practically unchanged. The shift of the step voltage and the independence of the step current on irradiated power are different from the Shapiro-step beahavior. It is proposed that this behavior is caused by fluxon motion.