Kazunuki Yamamoto

Single-Crystal YBa2Cu3O7-x Thin Films by Activated Reactive Evaporation

By means of the activated reactive evaporation, YBa2Cu3O7-x single-crystal thin films with the c axis perpendicular to the substrate plane have been directly and epitaxially grown on the (100) surface of SrTiO3. The substrate temperature was kept below 600°C and the oxidation treatment was performed at below 500°C. The ac-resistive measurement for a film with 2000 A thickness gave a sharp superconducting transition, where the endpoint was 90.2 K and ΔTc(10–90%)=1.7 K. From the complex susceptibility measurement we confirmed the Meissner effect as well as the structural uniformity of the specimen.

In situ reflection high energy electron diffraction observation during growth of YBa2Cu3O7−x thin films by activated reactive evaporation

Epitaxial growth of (001) oriented YBa2Cu3O7-x (YBCO) on SrTiO3(100) and MgO(100) by activated reactive evaporation was investigated by in situ reflection high energy electron diffraction (RHEED). RHEED observation of the YBCO films on both the substrates demonstrated the formation of the perovskite structure even at initial deposit. A sharp streak in every deposition stage revealed the layer-by-layer growth. The initial deposit on the substrate of MgO with the large mismatch of 9% kept the same in-plane lattice spacing as MgO. The lattice spacing converted from the bulk value of MgO to that of YBCO when the layer became thicker than 12 A.

Alignment Controls and Switching Characteristics in a Ferroelectric Liquid Crystal with the Phase Sequence of N*-SC*

The switching characteristics in a SC* ferroelectric liquid crystal with the N*-SC* phase sequence, were studied. Two kinds of alignment methods were tried. The rubbing method gave a better alignment than the improved temperature gradient method. Although rubbing confines the molecular direction, the two possible layer directions could be altered by applying a DC electric field. The molecular switching between the up and down polarization states in the rubbing cell showed two characteristic asymmetric behaviors due to the strong monostable surface condition as follows. The response time from the stable to the unstable state was slower by a factor of several tens than that of the reverse process. The slow process was accompanied by the nucleation and growth of an unstable domain, while only a color change was observed in the fast process without any domain nucleation. It was also found that during this process surface molecules stayed unswitched.

Observation of gap anisotropy in YBa2Cu3O7−δ by tunneling

Abstract Superconductive energy gap of YBa 2 Cu 3 O 7−δ are measured in anovel broken film edge junction. (001), (103) and (110) oriented films are broken in a cryogenic environment along the appropriate directions together with the SrTiO 3 substrate. Pb electrode is brought close in situ to the clean broken film edge. The normalized energy gap 2Δ(0)/k B T c measured in the direction along and perpendicular to the Cu-O plane are found to be 5.9±0.2 and 3.6±0.2 respectively. These values are independent of the variation in the values of T c within the examined range of 40K∽90K.

Is There a Magnetic-Field-Induced Breakdown in the Universality of Conductance Fluctuations?

Over the past few years, there have been many reports of the breakdown of universality in conductance fluctuations in mesoscopic systems when a high magnetic field is applied, particularly in the case of quantum wires. Normally, conductance fluctuations are described by a single scaling parameter-the coherence length l Φ . The increase in the magnetic coherence length B c is generally attributed to a decrease in l Φ which is not observed in the amplitude of the fluctuations. Worse, in high mobility material, the fluctuations seem to increase in amplitude, which is also inconsistent with a decrease in l Φ . Here, we argue that (1) the wire behavior is governed by surface scattering, (2) there is a formation of edge states in the high magnetic field, and (3) a breakdown of diffusive transport and transition to quasi-ballistic transport in the edge states in high-mobility material. The transition to high field behavior occurs when the cyclotron orbit at the Fermi surface becomes smaller than the width, so that proper treatment of the amplitude of the fluctuations and of the magnetic correlation length remains in keeping with theoretically expected behavior without loss of universality.

Third-Harmonic Susceptibility in Single-Crystal YBa2Cu3O7-x Thin Films

Higher-harmonic susceptibility was investigated for single-crystal YBa2Cu3O7-x thin films located in the ac magnetic field perpendicular to the film plane. We found for the first time that the third-harmonic term of the thin films has nonzero values in the region of ac-susceptibility transition. The nonlinear response against ac magnetic field indicates that the Maxwell-Strongin model is not applicable and there must be some mechanism obstructing the vortex motion in the films.

Formation and properties of YBa2Cu3O7−x single-crystal thin films by activated reactive evaporation

Single-crystal YBa2Cu3O7−x thin films have been directly and epitaxially grown on (100) planes of SrTiO3 at relatively low temperatures ( ∽ 600°C) using three metal sources by activated reactive evaporation. Films by this method have a novel superiority in the structural and superconducting properties.

Formation and structures of NiO-ZnO artificial superlattices

Abstract Artificial superlattices, composed of ultrathin layers of NiO and ZnO, (NiO) m (ZnO) n × l , were epitaxially grown on the (0001) plane of α-Al 2 O 3 by activated reactive evaporation. The two oxides in nature have different crystal structures and a large misfit of the lattice. X-ray and reflection high energy electron diffraction measurements revealed that the lattice structures of the superlattices were classified into two types depending on m and n : (I) n n > 6, substantially NiO has the NaCl structure and ZnO has the wurtzite structure.

Structural Study on Nd1.85Ce0.15CuO4-δ Epitaxial Thin Film Grown by Activated Reactive Evaporation Method

New electron-superconductor Nd1.85Ce0.15CuO4-δ epitaxial thin film was successfully grown in situ by means of the activated reactive evaporation method. Temperature dependences of the lattice properties were investigated by means of X-ray diffraction in the temperature range of 10 K–300 K. Debye temperature was estimated to be 430 K using an anharmonic approximation. Evidence of structural change such as a tetragonal-orthorhombic phase tansition was not found, implying that the average structure is tetragonal at the superconducting state.

Single crystal YBa2Cu3O7−x thin films by activated reactive evaporation

By means of the activated reactive evaporation (ARE), single-crystal thin films of YBa 2 Cu 3 O 7−x with its c axis perpendicular to the substrate plane have been directly and epixtaxially grown on the (100) surface of SrTiO 3 . The substrate temperature was kept below 600°C and the oxidation treatment was performed below 500°C. The ac-resistive measurement for a film 1500A thick showed a sharp superconducting transition whose end point was 90.2K and AT (10–90%)=1.7K. The Meissner effect as well as the structural uniformity of the specimen were confirmed from the complex susceptibility measurements.