Junpei Kasai

Low-temperature Fabrication of Transparent Conducting Anatase Nb-doped TiO2 Films by Sputtering

We present a low-temperature (~300 °C) process for preparing transparent conducting anatase Nb-doped TiO2 (TNO) polycrystalline films by sputtering. We first deposited amorphous films composed of an oxygen-rich bottom layer and oxygen-deficient top layer at room temperature. These films were then crystallized in a reducing atmosphere. The oxygen-rich bottom layer behaved as a seed layer during crystallization of the top layer, resulting in significant improvement of crystallinity and reduction of crystallization temperature. We obtained TNO polycrystalline films showing a resistivity of 6.4×10-4 Ω cm and absorption below 10% in the visible region by post-deposition annealing at 400 °C. The developed low-temperature process was applied to fabricating TNO films on plastics and glass with low glass-transition temperature.

Heteroepitaxial Growth of Rutile TiO2 on GaN(0001) by Pulsed Laser Deposition

Rutile TiO2(100) thin films have been grown on GaN(0001) surfaces by using the pulsed laser deposition method. Reflection high-energy electron diffraction (RHEED) measurements during the deposition clearly revealed the layer-by-layer growth of TiO2 at a substrate temperature of 400°C under an oxygen pressure of 1×10-5 Torr. X-ray diffraction and atomic force microscopy confirmed that the obtained films have high crystallinity with atomically flat surfaces. Pole figure measurements revealed the epitaxial relationship between TiO2 and GaN, namely that the in-plane TiO2 axis aligns parallel to the GaN .

Direct observations of vortices in Bi2212 single crystals by scanning SQUID microscopy

In order to investigate the behaviour of vortices in high-Tc superconductors under low magnetic fields, we performed scanning SQUID microscopy (SSM) on the ab surfaces of Bi2.1Sr1.8Ca(Cu0.98Co0.02)2Oy (Bi2212) single crystals at different temperatures and under different magnetic fields up to 0.8 G. The observed magnetic images clearly demonstrated vortex assembly structures for Bi2212. Vortices trapped inside Bi2212 single crystals tended to be arranged in a one-dimensional manner.

Local Magnetic Properties of Underdoped La2-xSrxCuO4 Single Crystal Probed by Scanning Superconducting Quantum Interference Device Microscopy.

In order to investigate the local magnetic properties of high-Tc superconductors, we have performed scanning superconducting quantum interference device (SQUID) microscopy (SSM) on the ab-surfaces of an underdoped La1.9Sr0.1CuO4 (LSCO) single crystal (Tc=26 K) at various temperatures. The obtained magnetic images below Tc clearly demonstrated diamagnetic behavior together with trapped vortices in units of Φ0 (= hc/2e). Above Tc up to 97 K, on the other hand, we did not observe any magnetic structures within an experimental error of ≤0.2 µT, contrary to the recent SSM results of LSCO thin films exhibiting diamagnetic regions of over-micron scales. From the present SSM observations, it can be safely said that local diamagnetism of such scales is unique to thin film specimens.

Heteroepitaxial growth of ferromagnetic rutile CoxTi1−xO2−δ on GaN (0001)

A rutile CoxTi1−xO2−δ (100) film was heteroepitaxially grown on GaN (0001) by pulsed laser deposition. Magneto-optical Kerr effect and cross-sectional transmission electron microscope (TEM) measurements revealed that Co0.03Ti0.97O2−δ films prepared at an oxygen partial pressure of 10−6–10−5Torr with a carrier density ne⩾3×1018cm−3 exhibit room-temperature ferromagnetism without any precipitates or secondary phase. High-resolution TEM observations confirmed that the interface between CoxTi1−xO2−δ and GaN is atomically smooth without intermixing. These results lead us to conclude that CoxTi1−xO2−δ is promising as a spin injector in GaN-based spin-electronic devices.

Observation of Compositional Fluctuation by Scanning Superconducting Quantum Interference Device (SQUID) Microscope in Superconducting La1.8Sr0.2CuO4

In order to visualize spatial distributions of vortices strongly pinned in La1.8Sr0.2CuO4, we have performed scanning superconducting quantum interference device (SQUID) microscopic (SSM) measurements near Tc. The obtained SSM images have revealed that stripe-shaped regions with higher Tc run in an arbitrary direction with respect to crystallographic axes. Vortices tend to align in a one-dimensional fashion, negotiating around the higher Tc regions. High-resolution electron probe micro-analysis confirmed that the spatial variation in Tc corresponds well to the modulation of Sr/La composition. We also found that small-angle grain boundaries with tilt angles of 1–3 degrees are less effective in trapping vortices, indicating that superconductivity is not degraded very much.

Direct observation of interlayer Josephson vortices in heavily Pb-doped Bi2Sr2CaCu2Oy by scanning superconducting quantum interference device microscopy

Josephson vortices trapped in cross-sectional edge surfaces of Pb0.6Bi1.4Sr2CaCu2Oy has been directly observed by using a scanning superconducting quantum interference device (SQUID) microscope. The magnetic field distribution Bz around each vortex is substantially anisotropic, compared with the usual vortex in the ab-plane, and is extended over 100 µm toward the in-plane direction. By fitting a theoretical Bz function to experimental ones, c-axis penetration depth λc was estimated to be 11.2 ±0.7 µm, which is in good agreement with the literature value, 12.6 µm, obtained from the Josephson plasma edge frequency.

Local magnetic properties of high-Tc superconductors probed by scanning SQUID microscopy

Abstract We have performed scanning SQUID (superconducting quantum interference device) microscopy (SSM) on the ac -surfaces of heavily Pb-doped Bi2212, Bi 1.6 Pb 0.6 Sr 1.8 CaCu 2 O y , single crystals at low temperatures. The observed images demonstrated interlayer Josephson vortices below T c . The value of c -axis penetration depth, λ c , evaluated from the SSM images was 11.2±0.7 μm, which is approximately one order of magnitude smaller than that of pure Bi2212 in the optimal doping regime. This clearly indicates that Pb doping substantially reduces the anisotropy of Bi2212.

Cryogenic STM/STS observation of NdBa2Cu3O7−δ single crystals

Abstract To investigate the spatial distribution of electronic structure, responsible for the flux pinning, surfaces of a high Jc material NdBa 2 Cu 3 O 7−δ ( Nd 123) were probed by STM/STS at cryogenic temperatures below Tc. The obtained STM images clearly demonstrated island structures buried in a matrix with a dimension up to ∼50 nm . From the detailed STS analysis, it was concluded that the island possesses more ‘insulating’ tunneling behavior than the matrix. This result suggests that the boundaries between two regions may act as flux pinning centers.