Bunju Shinozaki

Weak Localization and Electron–Electron Interaction Effects in Indium Zinc Oxide Films

Electron weak localization (WL), electron–electron (el–el) interaction and electron–phonon (el–ph) scattering effects on the transport properties have been investigated for two to three-dimensional...

Relationship between variable range hopping transport and carrier density of amorphous In2O3–10 wt. % ZnO thin films

The electrical transport characteristics in amorphous Zn doped In2O3 films have been investigated in the range from 2 × 1017 cm−3 to 6 × 1020 cm−3 of the carrier concentration Ne. For films with Ne > 3 × 1020 cm−3, it is found that the Hall mobility μH is limited by ionized impurity scattering. However, for films with Ne   0 to insulating behavior with dρ/dT < 0 near Ne≈1 × 1020 cm−3 with decreasing Ne. The transport mechanism of carriers in the high-resistivity region is discussed by considering a model based on the Ioffe-Regel criterion. For the film with highest resistivity with Ne ≈ (5 − 6) × 1017 cm−3 among the present films, the ρ(T) show a change from Mott variable-range hopping (ρ ∝ exp T−1/4) to ρ ∝ expT−1/2 at approximately 10 K with decreasing temperature.

Scanning SQUID microscope study of vortex polygons and shells in weak-pinning disks of an amorphous superconducting film

Direct observation of vortices by the scanning superconducting quantum interference device microscopy was made on large mesoscopic disks of an amorphous MoGe thin film. Owing to the weak pinning nature of the amorphous film, vortices are able to form geometry induced, quasi-symmetric configurations of polygons, and concentric shells in the large disks. Systematic measurements made on selected disks allow us to trace not only how the vortex pattern evolves with magnetic field, but also how the vortex polygons change in size and rotate with respect to the disk center. The results are in good agreement with theoretical considerations for mesoscopic disks with sufficiently large diameter. A series of vortex images obtained in a disk with a pinning site reveals a unique line symmetry in vortex configurations, resulting in modifications of the shell filling rule and the magic number.

Localization and interaction effects in ultrathin epitaxial NbN superconducting films

For epitaxial NbN films with thickness d, 2.0 nm ≤ d ≤ 20.5 nm, we observed a sharp superconducting transition, for which the transition temperature T(c) monotonically decreased with increasing 1/d. Regarding the suppression of T(c), the sheet resistance R(sq) dependence of T(c) closely fitted the Finkelstein formula from localization theory, with a reasonable value of the electron mean free path comparable to atomic distance, which was used as a fitting parameter. On the other hand, the critical sheet resistance R(c), at which the superconducting-insulator transition was expected, was approximately one-third of the universal value R(q) = h/4e(2) suggested by the dirty boson model for self-duality. It is concluded that T(c) depression in the present NbN system is determined by localization theory but not the dirty boson model.

Observation of the Intermediate State in Superconducting Lead Films by Shadow Electron Microscopy

By employing an electron microscope with a liquid helium cooled stage, periodic shadow patterns which are formed by an array of normal and superconducting domains have been observed for lead films of thickness from 0.32 µm to 82 µm. These patterns have not been confirmed for films thinner than 0.19 µm. By using a generalized Landau laminar model, the surface energy parameter Δ ( T =0 K) in estimated from the observed period. Thickness dependence of the Δ (0) can hardly be found within the range from 0.32 µm to 82 µm; the avarage Δ (0)=380 A±30%. On the basis of the experimental results, the transition of the magnetic behavior between the type-I and they type-II is discussed.

Microstructural analysis and Transport Properties of MoO and MoC nanostructures prepared by focused electron beam-induced deposition

By electron-beam-induced deposition, we have succeeded in the direct fabrication of nanowires of molybdenum oxide (MoOx) and molybdenum carbide (MoC) on a SiO2 substrate set in a scanning electron microscope. In order to prepare MoOx specimens of high purity, a precursor gas of molybdenum hexacarbonyl [Mo(CO)6] is used, mixed with oxygen gas. On the other hand, MoC is grown by mixing H2O gas with the precursor gas. The electrical transport properties of the nanowires are investigated by the DC four-terminal method. A highly resistive MoOx nanowire prepared from an as-deposited specimen by annealing in air shows nonlinear current-voltage characteristics and a high photoconductivity. The resistivity ρ of an as-deposited amorphous MoC (a-MoC) nanowire takes its maximum at a temperature T ≈ 10 K and decreases to ≈ 0 with decreasing temperature. This behavior of ρ(T) indicates the possible occurrence of superconductivity in a-MoC nanowires. The characteristic of ρ(T) below the superconducting transition temperature Tc ≈ 4 K can be well explained by the quantum phase-slip model with a coherence length ξ(0) ≈ 8 nm at T = 0.

Direct Imaging of Vortex Polygons and Vortex Shells in Mesoscopic Squares of a Weak Pinning Superconducting Thin Film

We report on the direct visualization of vortices in mesoscopic squares of a weak pinning amorphous superconducting thin film using a scanning superconducting quantum interference device microscope. From systematic measurements made on selected dots, we trace how the vortex pattern evolves with an applied magnetic field. The observed images clearly illustrate the formation of symmetric polygons and concentric shells of vortices, from which the rule of shell filling with increasing vorticity is formulated. We analyze the size and shape of vortex polygons and obtain insight into the flow pattern of the shielding current governing vortex configurations.

Duality picture of Superconductor-insulator transitions on Superconducting nanowire

In this study, we investigated the electrical transport properties of niobium titanium nitride (NbTiN) nanowire with four-terminal geometries to clarify the superconducting phase slip phenomena and superconducting-insulator transitions (SIT) for one-dimensional superconductors. We fabricated various nanowires with different widths and lengths from epitaxial NbTiN films using the electron beam lithography method. The temperature dependence of resistance R(T) below the superconducting transition temperature Tc was analyzed using thermal activation phase slip (TAPS) and quantum phase slip (QPS) theories. Although the accuracy of experimental data at low temperatures can deviate when using the TAPS model, the QPS model thoroughly represents the R(T) characteristic with resistive tail at low temperatures. From the analyses of data on Tc, we found that NbTiN nanowires exhibit SIT because of the change in the ratio of kinetic inductance energy and QPS amplitude energy with respect to the flux-charge duality theory.

Fluctuation conductance and the Berezinskii-Kosterlitz-Thouless transition in two dimensional epitaxial NbTiN ultra-thin films

We study on the electric transport properties of epitaxial NbTiN ultrathin films in a range from 2 to 8nm. The films with 4 nm thick shows superconductivity of which mean-field superconducting transition temperature is TC0 = 9.43 K The excess conductance due to superconducting fluctuations was measured at temperatures above TC0. The paraconductivity shows a two-dimensional like behaviour at close to TC0. Experimental results are in good agreement with the sum of Aslamazov − Larkin and Maki − Thompson term for superconducting fluctuation theory. Decreasing temperature below TC0, the current-voltage characteristic shows a crossover from linear to nonlinear behaviour. The exponent α of current-voltage relation, V ~ Iα showed universal jump at TCBKT = 9.33 K As results, we find that there is a consistency between the parametrization of the2D characteristics of fluctuation paraconductivity above TC0 and Berezinskii-Kosterlitz-Thouless type behaviour below TC0.

Localization and pair breaking parameter in superconducting molybdenum nitride thin films

We have investigated the superconductor-insulator transition in molybdenum nitride films prepared by deposition onto MgO substrates. It is indicated that the T c depression from [Formula: see text] for thick films with increase of the normal state sheet resistance [Formula: see text] was well explained by the Finkelstein formula from the localization theory. Present analysis suggests that the superconducting-insulator transition occurs at a critical sheet resistance [Formula: see text]. It is found that the [Formula: see text] above [Formula: see text] shows different characteristics of [Formula: see text] and [Formula: see text] in the regions [Formula: see text] and [Formula: see text], respectively, where [Formula: see text] is the classical residual resistance and A is a constant. The excess conductance [Formula: see text] due to thermal fluctuation has been analyzed by the sum of the Aslamazov-Larkin and Maki-Thompson correction terms with use of the pair breaking parameter [Formula: see text] in the latter term. The sum agrees well with the data, although the experimental results of the [Formula: see text] dependence of [Formula: see text], that is, [Formula: see text] shows the disagreement with a linear relation [Formula: see text] derived from the localization theory.