Masashi Kawasaki

Preparation and nanoscale characterization of highly stable YBa2Cu3O7-δ thin films

High-quality c-axis-oriented YBa2Cu3O7-δ (YBCO) films were prepared using the laser ablation method. Films with a full width at half-maximum of the (005) X-ray diffraction peak as narrow as 0.1° and an χmin of Rutherford backscattering spectrometry as low as 3.5% were obtained by optimizing the experimental conditions. The atomic image and superconducting gap at 4.2 K were simultaneously observed for the first time on these high-quality YBCO films by scanning tunneling microscopy and scanning tunneling spectroscopy. Such observations, even after the film was stored in air for three weeks, indicates that YBCO film is sufficiently chemically and physically stable to prevent significant deterioration in superconductivity at the film surface.

Heteroepitaxial Growth of c-Axis-Oriented BaTiO3 Thin Films with an Atomically Smooth Surface

C-axis-oriented barium titanate (BaTiO3) epitaxial films with an atomically smooth surface have been fabricated on SrTiO3{100} substrates using the pulsed laser deposition technique. The crystal structure and epitaxial orientation of the films have been analyzed by means of 2θ/θ X-ray diffractometry and 4-circle X-ray diffractometry. The highly oriented and epitaxial crystallinity is also shown by means of Rutherford backscattering spectrometry which gave a χmin of 1.7%. The atomically smooth surface comparable to that of the SrTiO3 substrate is revealed by means of atomic force microscopy. The breakdown electric field of the BaTiO3 film is as high as 106 V/cm.

Cryogenic scanning tunneling microscopy/spectroscopy on the (110) surfaces of YBa2Cu3Oy epitaxial thin films

Abstract As-grown surfaces of (110) oriented laser-ablated YBa 2 Cu 3 O y films have been investigated by scanning tunneling microscopy and spectroscopy, for the first time, at room temperature and 4.2 K. We found semi-macro scale surface structures along the c -axis, which were composed of (110) top planes and (100)/(010) lateral planes. The orthorhombic atomic lattice image, corresponding to Cu atom arrangement of both CuO 2 double layers and CuO chain layers alternately, was observed on the (110) top plane at 4.2 K. Furthermore, broad swell-and-depression structures were observed as stripes perpendicular to the c -axis on the top plane. The STS results indicated superconducting nature on the swells and semiconducting nature on the depressions.

Evolution of Insulator–Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating

An interface between an oxide and an electrolyte gives rise to various processes as exemplified by electrostatic charge accumulation/depletion and electrochemical reactions such as intercalation/decalation under electric field. Here we directly compare typical device operations of those in electric double layer transistor geometry by adopting A-site vacant perovskite WO3 epitaxial thin films as a channel material and two different electrolytes as gating agent. In situ measurements of X-ray diffraction and channel resistance performed during the gating revealed that in both the cases WO3 thin film reaches a new metallic state through multiple phase transitions, accompanied by the change in out-of-plane lattice constant. Electrons are electrostatically accumulated from the interface side with an ionic liquid, while alkaline metal ions are more uniformly intercalated into the film with a polymer electrolyte. We systematically demonstrate this difference in the electrostatic and electrochemical processes, by comparing doped carrier density, lattice deformation behavior, and time constant of the phase transitions.

Crack formation and electrical properties of [100] and [110] oriented YBa2Cu3O7-δ thin films

We have grown [100] and [110] oriented YBa2Cu3O7−δ thin films on SrTiO3(100), (110) and LaSrGaO4(100) substrates using the PrBa2Cu3O7−δ template method. An array of cracks running perpendicular to the c-axis was observed on c-axis aligned YBa2Cu3O7−δ films apparently due to the uniaxial stress caused by the large thermal expansion mismatch along the c-axis. The [100] oriented YBa2Cu3O7−δ films deposited on SrTiO3(100) substrates contain two epitaxial domains with a different in-plane c-axis orientation by 90°. They show orthogonally crossing cracks developed due to the biaxial stress. The residual stress along the c-axis of YBa2Cu3 dramatically degrades the electrical properties.

Scanning tunneling microscopy/spectroscopy on the (110) surfaces of YBa2Cu3Oy thin films

Abstract As-grown surfaces of (110) oriented laser-ablated YBa 2 Cu 3 O y films have been investigated by scanning tunneling microscopy and spectroscopy. The orthorhombic atomic lattice image, corresponding to Cu atom arrangement of the (110) plane, was observed at 4.2 K. The STS results indicated that the surface electronic structure varied with the distance of a few to several nano-meters.