Yoshio Ohashi

Fluctuation effects in the niobium oxynitride (Nb0.87Si0.09□0.04)(N0.87O0.13) superconductor

Resistive broadening observed for the Si-doped niobium oxynitride superconductor (Nb0.87Si0.090.04)(N0.87O0.13) has been analyzed in terms of fluctuation effects. The zero field superconducting transition of this system is 17.32 K, where the resistance becomes zero. We have analyzed the broadening of the superconducting transition in the zero field in terms of Aslamazov–Larkin predictions, while the broadening under strong applied magnetic fields of up to 14 T has been analyzed using the thermally activated flux flow (TAFF) model. The data is further analyzed to understand the applicability of lowest Landau level (LLL) scaling in order to deduce the dimensionality of the system. Zero field fluctuation conductivity, just above the mean field transition, shows a cross-over from 2D to 3D scaling and is in line with the analysis of LLL scaling. The evidence clearly establishes the 3D nature of fluctuations in this superconductor. Other relevant superconducting parameters like Ginzburg numbers, coherence length, penetration depth and upper critical fields are estimated and discussed.

TAFF and vortex glass state in a niobium oxy-nitride Nb(N0.98O0.02)

Resistivity broadening of niobium oxy-nitride [Nb(N0.98O0.02)] is analyzed in terms of fluctuation conductivity, thermally activated flux flow (TAFF) and vortex glass study in magnetic fields up to 14 T. Zero field fluctuation analysis of excess conductivity using the Aslamzov-Larkin theory shows an indication of a 3D character that is further supported by the lowest Landau level (LLL) scaling. We could obtain a clear vortex glass region from an extended analysis of the resistivity data. Different regions of interest are estimated that include vortex glass and vortex liquids of the phase diagram. The TAFF has been analyzed using conventional Arrhenius relation, as well as its modified version that includes a temperature- and field-dependent pre-factor. While the conventional TAFF scaling is well suited for the present system, the modified version yields deeper insight into the material parameters of interest. Ginzburg number, coherence length, critical exponents related to the vortex glass transition, etc, are estimated and discussed.

Structural behaviour of niobium oxynitride under high pressure

High pressure investigation of niobium oxynitrides (NbN0.98O0.02) employing synchrotron based angle dispersive x-ray diffraction experiments was carried out in very fine pressure steps using membrane driven diamond anvil cell. Ambient cubic phase was found to be stable up to ∼18 GPa. At further high pressure cubic phase showed rhombohedral distortion.

Local structure around the flux pinning centers in superconducting niobium silicon oxynitride (Nb{sub 0.87}Si{sub 0.09}□{sub 0.04})(N{sub 0.87}O{sub 0.13})

The superconducting transition temperature of niobium silicon oxynitride (Nb0.87Si0.09□0.04)(N0.87O0.13) exhibits a gradual reduction from 16.8 K to around 11 K under an increasing applied magnetic field of up to 14 T. This relatively small Tc reduction under an applied magnetic field suggests a robustness of its superconducting behavior in comparison to that in the parent niobium oxynitride. It was similar to the flux pinning effect observed in the large magnetic hysteresis of the niobium-silicon oxynitrides in our previous study. Both Si K-edge XANES and 29Si MAS-NMR indicated that the local structure of pinning centers around the silicon atoms close to cationic vacancies was similar to that of Si in amorphous SiO2 in the rock-salt structure of niobium oxynitride.

Structural behaviour of Mg, Al and Si doped niobium oxynitrides under high pressures

We have investigated the high pressure behaviour of Mg, Al and Si doped niobium oxynitrides employing synchrotron based angle dispersive x-ray diffraction experiments. All the oxynitrides are found to be stable up to the highest pressure studied. Equation of state and bulk modulii have been determined and the latter are found to be linearly related to the ambient volumes for these compounds.