Yutaka Onodera

Transition temperatures and crystal structures of single‐crystal and polycrystalline NbNx films

Single‐crystal cubic NbNx films with thicknesses of a few thousand angstroms were epitaxially grown on cleaved (100) planes of single‐crystal MgO plates by the vapor phase growth technique. By heat treatment of some of these films in NH3+H2 or in H2 alone, films having the superlattice structures of Nb4N3, NbN, or Nb4N5 were prepared. The maximum Tc was observed in cubic NbNx. Both Nb4N3 and Nb4N5, the tetragonal phases with long‐range‐ordered arrangement of vacancies, exhibited superconductivity. NbN1.0 and Nb5N6, both with hexagonal structure, did not exhibit superconductivity down to 1.77 K. As the composition of cubic NbNx became close to stoichiometric, its Tc increased. However, the maximum Tc of cubic NbNx with nearly stoichiometric composition was limited by the lattice instability of cubic NbNx and by the resulting cubic NbNx to hexagonal NbN transformation. It might be expected that a higher Tc for compounds of the NbN family could be obtained if a NaCl‐type crystal with less vacancies could exi...

Numerical analysis of vortex motion on Josephson structures

The one‐dimensional and two‐dimensional sine‐Gordon equations with dimensionless loss factors and unitless normalized bias are numerically calculated by computer. The results are presented for accelerations, velocities, collisions, coupled states, and two‐dimensional propagation of solitons.

Logic design of Josephson network

New logic circuits are designed which employ as information bits the flux‐quantum vortices occurring in Josephson junctions of extended dimensions. It is shown that Josephson lines can be interconnected in certain direct ways so that complete logic capability can be achieved with networks of Josephson lines alone.

Superconducting Transition Temperatures of R. F. Sputtered NbN Films

NbN films are examined with respect to their electrical and superconducting properties. The films are deposited by r.f. reactive sputtering technique under argon gas atmosphere added with nitrogen. The films obtained have a wide range of resistivities and transition temperatures depending upon the partial nitrogen pressure in the argon atmosphere and the substrate temperature at which the films were deposited. The maximum transition temperature obtained is 17.3°K, higher by 0.9°K than the highest one reported so far.

Mechanical analogue of active Josephson transmission line

The mechanical analogue of the sine‐Gordon equation, which includes the effects of loss and the effect of a distributed bias source is constructed. The mechanical analogue itself is a active line transmitting kinks which are a kind of the solitons given by the sine‐Gordon equation. The various vortex interactions on an active Josephson transmission line can be demonstrated vividly by using the mechanical line. Our distributed bias source which consists of a series of air nozzles blowing against each of aluminum disks is able to supply energy to kinks on the line. The analogue gives a quick and new physical insight into the highly nonlinear behavior.

Temperature and angular dependences of upper critical fields for the layer structure superconductor 2H-NbSe2

The temperature and angular dependences of upper critical fieldsHc2have been measured for several 2H-NbSe2 single crystals by use of an electrical conduction method in magnetic fields up to 150 kOe. As the temperature approaches the transition temperatureTc, the value ofHc2‖(parallel to the layer planes) decreases with a positive curvature, while the value ofHc2⊥(perpendicular to the layer planes) decreases almost linearly. The ratio ofHc2‖toHc2⊥increases monotonically from 2.4 nearTcwith decreasing temperature and reaches the constant value of 3.2 at the lowest temperature. It becomes clear that the simple effective mass model based on the anisotropic Ginzburg-Landau theory does not explain our experimental results. The anisotropic behavior ofHc2can be accounted for by the Takanaka theory, which includes anisotropies of both the Fermi velocity and the energy gap and the effect of nonlocality. Agreement between experimental results and the theoretical prediction is obtained by the use of values of 0.16≲ε12≲0.25 and −0.6≲ε2≲−0.3, where ε1 is the mass anisotropy parameter and ε2 the gap anisotropy parameter. The coupling strength between layers is too strong to be explained by the Josephson phase coupling model proposed for quasi-two-dimensional layer superconductors.

Phase transformations in nearly stoichiometric NbNx

Processes of the phase transformation in nearly stoichiometric niobium nitride, from cubic δ‐NbN to hexagonal e‐NbN, are studied by selected‐area electron diffraction, by using NbNx films of the transition state prepared by annealing δ‐NbNx films at about 1000 °C in an atmosphere of H2 gas. The diffraction patterns taken from the annealed NbNx films suggest that the phase transformations (vacancy‐poor‐) δ‐NbNx (cub.) → (N‐vacancy‐) ordered δ‐NbN (tet.)  → ordered ζ‐NbN (hex.)  → ordered γ′‐NbN (mon.)  → δ′‐NbN (hex.)  → e‐NbN (hex.) occur. Ordered δ‐NbN, ordered ζ‐NbN, ordered γ′‐NbN, and δ′‐NbN are transient phases which occur during the transformation δ‐NbNx → e‐NbN, and the superlattice structures of the three ordered phases are described by the ordering of N vacancies with random Nb vacancies in their fundamental lattices. Mechanisms of the transformations from δ‐NbNx to e‐NbN at about 1000 °C are discussed on the basis of the transient phases observed during the phase transformations.

Nonequilibrium stationary coupling of solitons

By computer simulation, interaction of two solitons with the same screw sense of the sine‐Gordon equation, which retain their shapes and velocities upon collision with other solitons even in the presence of bias and loss terms, are examined. It is confirmed that they can couple when the bias is greater than a critical value. The conditions and mechanism of coupling are examined in detail. They are explained in terms of the energy of interaction between the ripple structures trailed by energy dissipative moving solitons. The distance D between coupled solitons can be expressed as D= (n−1/2−δ) λ (n is an integer), where λ is a wavelength of the ripple structure and δ≪1.

Magnetic‐Field Dependence of Josephson Current Influenced by Self‐Field

The magnetic‐field dependence of dc Josephson current IJ was measured using cross‐type square junctions made of Sn thin films. The results and analysis indicate that owing to the self‐field of IJ, the periodic pattern of IJ on magnetic field is modified and the value of the maximum IJ becomes smaller than theoretically expected value obtained Ambegaokar and Baratoff.

Kinetic momentum quantum effect in a superconducting closed loop composed of Josephson junctions

Superconducting closed loops composed of N series Josephson junctions are discussed. The conditions to achieve the n quantum state of a fluxoid in the loop are analyzed. It is found that the n quantum state exists in Josephson loops having more than 4n pieces of junctions even if it is assumed that the inductance L in the loop is negligibly small. This n quantum state is achieved by an asymmetric current feed to the Josephson loop. The properties of the loop are easily explained by using a pendulum analog.