Hirohiko Izumi

Partial oxygen pressure effects on the morphology of Y‐Ba‐Cu‐O thin films in laser deposition process

The characterization of Y‐Ba‐Cu‐O thin films prepared by 193‐nm‐ArF laser deposition at partial oxygen pressures ranging wide from 0.01 mTorr to 5 Torr has been investigated. These films have been grown on (100) SrTiO3 substrates held at 700 °C. Surface morphology, superconducting transition temperature and crystalline orientation strongly depend on a partial oxygen pressure during deposition. The smooth and highly crystal‐oriented films with the superconducting transition temperature above 80 K and transport critical current densities of 1.0×105 A/cm2 (without magnetic field at 77 K) have been obtained at 10 to 50 mTorr of oxygen.

Superconductivity and crystallinity of Ba2Y1Cu3O7−δ thin films prepared by pulsed laser deposition with substrate bias voltage

Substrate bias voltages were found to be significantly effective in preparing high‐quality laser‐deposited superconducting Ba2Y1Cu3O7−δ films at reduced substrate temperatures. The zero‐resistance temperature of the biased films, positive bias being more effective than negative, decreased very slightly when the substrate temperature was lowered, whereas that of the unbiased films decreased considerably. In addition, the surface morphology and c‐axis orientation have been improved by applying substrate bias voltages. Bias voltages within ±500 V hardly affect the composition of the resulting films so that stoichiometric films have been obtained from a stoichiometric target. A time‐resolved optical observation revealed that a short time emission, probably being oxygen plasma, occurred in a few μs after the laser pulse impingement. The improvement in crystallinity of the resulting films is attributed to this emission. The velocity of emissive species in the plume was determined to be 6×105 cm/s. Under positiv...

Effect of an Oxygen Pressure on Crystal Growth of a-Axis Oriented YBa2Cu3O7-x Films Prepared by Pulsed Laser Deposition

Abstract The effect of an oxygen partial pressure on the growth of a-axis oriented YBa2Cu3O7−x films has been investigated by cross-section transmission electron microscopy and scanning electron microscopy. The samples were deposited on SrTiO3 (100) substrates at 700°C and oxygen partial pressure of 20, 50 and 100 mTorr. Under 20 mTorr, a-axis and c-axis oriented domains equally grow over the whole substrate, resulting in the alternate structure of both with a separation of about 150 A in TEM images. The alternate structure continues from the substrate to the film surface. At 50 and 100 mTorr, the a-axis oriented domain becomes dominant above a thickness of 40 and 100 A, respectively. A disordered layer is observed between the a-axis oriented domain and substrate for 50 mTorr and the c-axis oriented domain grows on the substrate for 100 mTorr. The surface morphology is improved with decreasing the c-axis oriented domain.

Deposition Pressure Effects on the Laser Plume of YBa2Cu3O7-δ

The control of the preferential orientation in oxide superconducting films, which is generally done by deposition temperatures, was achieved by choosing the oxygen pressures during the deposition. Time-resolved optical observations of the laser plume were carried out at the same time with the film deposition. Upon increasing the oxygen pressure, the velocities of the emissive species ablated from the target were decreased. The kinetic energy effects of the ablated species on the film orientation are discussed, and the orientation change can be qualitatively understood by a surface migration model as well as the case of the deposition temperature dependence.

Epitaxial growth of a-axis oriented laser deposited YBa2Cu3O7−x thin films

Abstract YBa 2 Cu 3 O 7− x superconducting thin films with zero resistance temperature of over 80 K with crystallographic orientation of the a -axis perpendicular to the surface have been epitaxially grown on SrTiO 3 (100) single crystalline substrates at 700°C by pulsed laser deposition. The a -axis oriented film deposited at 50 mTorr of an oxygen partial pressure the closet to the thermodynamic stability condition of tetragonal YBa 2 Cu 3 O 6.0 phase exhibited very smooth and grain boundary-free surface. Cross-sectional high-resolution transmission microscopic observation has revealed microstructure of a -axis oriented thin films showing almost defect-free a -axis oriented region, and defective c-axis oriented or disordered regions in the interface between the film and the substrate.

Formation of Ca-doped YBa2Cu4O8 phase in thin films

Abstract Ca-doped YBa2Cu4O8 (124) thin films are prepared on (100) SrTiO3 substrates by annealing the amorphous films deposited using a pulsed laser deposition technique. The X-ray diffraction measurements show that the Ca-doped YBa2Cu4O8 phase is formed by annealing below 800°C at a oxygen pressure of 1 atm. The 124 films have c-axis orientation normal to the substrates. As the Ca content increases, the proportion of the 123 impurity phase in the samples increases. The onset temperature of superconductivity of the Y(Ca)Ba2Cu4O8 films increases from 79 K to 88 K with an increase Ca-substitution for 5 to 10% of Y.

Comparison of I–V characteristics in a-axis- and c-axis-oriented YBa2Cu3O7−x thin films

Abstract Current-voltage characteristics of a -axis- and c -axis-oriented YBa 2 Cu 3 O 7− x thin films at temperatures near the superconducting transition have been investigated, using the Kosterlitz-Thouless transition model. The voltage exhibits a power-law dependence on the current in both films. The exponent α( T ) of the c -axis oriented films shows a “universal jump” at the Kosterlitz-Thouless transition temperature, while that of the a -axis-oriented films shows a linear dependence down to α ( T )=1 with increase in temperature.

Magnetic circular dichroism in YBa2Cu3Ox thin films

Abstract Magnetic circular dichroism (MCD) in superconducting and insulating YBa 2 Cu 3 O x thin films is studied. The MCD spectra in superconducting samples exhibit a linearly field dependent structure at 4–5 eV. The field and temperature dependences, the latter seeming to obey the Curie law, suggest that this structure is associated with the intraionic 3d-4p transitions of localized Cu ions at the Cu-O chains. These transitions are thought to be the origin of the peak at 4.1 eV observed in insulating ones. A comparison of the spectra of insulating YBa 2 Cu 3 O x dielectric function studies and this work is also given.

YBa2Cu3O7−δ - PrBa2Cu2TaO8 heterostructures made by pulsed laser deposition

Abstract The YBa 2 Cu 3 O 7−δ (YBCO)/PrBa 2 Cu 2 TaO 8 (PBCT) heterostructure was formed by pulsed laser deposition. The great possibility of PBCT for the barrier of junctions using YBCO has been confirmed. The lattice flexibility of PBCT is poorer than that of YBCO and YBCO can be used as a buffer layer between PBCT and substrates being lattice mismatch.

Magnetization Study of High-Tc Oxide Superconductors by Magneto-Optical Observations

Magnetization and the spatial variation of magnetic flux in thin film samples of YBa2Cu3Ox were studied by using a high contrast magneto-optical observation method(HICOM). The demagnetizing effect in the mixed state was also investigated in comparison with ferromagnetic materials. The magnetic flux distribution conformed to the critical state model in a YBa2Cu3Ox thin film made by laser deposition. The trapped flux in the sample with high Jc showed the nature of permanent magnet. In particular, a thin film magnet with perpendicular magnetization could be realized because the flux density in the center of the sample was not reduced by the demagnetizing field. However, the flux gradients in the edges of the sample were changed following the critical state determined by the demagnetizing field.