Yoichi Okabe

Selective minimum-norm solution of the biomagnetic inverse problem

A new multidipole estimation method which gives a sparse solution of the biomagnetic inverse problem is proposed. This solution is extracted from the basic feasible solutions of linearly independent data equations. These feasible solutions are obtained by selecting exactly as many dipole-moments as the number of magnetic sensors. By changing the selection, the authors search for the minimum-norm vector of selected moments. As a result, a practically sparse solution is obtained; computer-simulated solutions for L/sub p/-norm (p=2, 1, 0.5, 0.2) have a small number of significant moments around the real source-dipoles. In particular, the solution for L/sub 1/-norm is equivalent to the minimum-L/sub 1/-norm solution of the original inverse problem. This solution can be uniquely computed by using linear programming.<<ETX>>

Ion-sensitive field-effect transistors with inorganic gate oxide for pH sensing

Ion-sensitive field-effect transistors (ISFETs) have been fabricated by using silicon films on sapphire substrates (SOS). Using this structure SiO<inf>2</inf>, ZrO<inf>2</inf>, and Ta<inf>2</inf>O<inf>5</inf>films are examined as hydrogenion-sensitive materials, and Ta<inf>2</inf>O<inf>5</inf>film has been found to have the highest pH sensitivity (56 mV/pH) among them. The measured pH sensitivity of this SOS-ISFETs is compared with the theoretical sensitivity based on the site-binding model of proton dissociation reaction on the metal oxide film and good agreement between them is obtained.

A robust reconstruction of sparse biomagnetic sources

Inequality constraints are introduced to a normalized minimum-L/sub 1/-norm estimator, which gives a sparse solution of the biomagnetic inverse problem. The constraints have a numeric tolerance to take into account the measurement ambiguity caused by noise. Computer simulation and phantom-data analysis show how the solution is improved by the constraints with a moderate tolerance; the improvement is examined in noisy conditions such that signal-to-noise ratios (SNRs) are lower than 10 dB.

Scattering of Electrons by Potential Clusters in Ternary Alloy Semiconductor

By assuming potential clusters as one of the scattering centers for electrons in ternary alloy semiconductors, the drift mobility of electrons was calculated and compared with experimental results in (In1-xGax)As, (In1-xGax)Sb and In(As1-xPx). An explanation of the experimental results using the present model is given.

Y–Ba–Cu–O/Nb Tunnel Type Josephson Junctions

Y-Ba-Cu-O/Au/AlOx/Nb tunnel type Josephson junctions were fabricated and their current-voltage relation exhibited typical characteristics of Josephson tunnel junctions such as supercurrent, hysteresis and rf-induced voltage steps at 4.2 K.

High frequency properties of all-NbN nanobridges with gap structure in I-V curves

All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness of c R N products of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.

Josephson single‐flux‐quantum logic circuits using niobium weak links

Josephson logic circuits using fluxoid quantization in a one‐junction interferometer to represent logical states have been fabricated from thin films of Nb by means of photolithography, electron beam lithography, and reactive ion etching. A variable thickness bridge is used as a Josephson junction in the one‐junction interferometer. The logic circuits have shown AND and OR logic functions.

Inductance calculation of 3D superconducting structures with ground plane

An inductance calculation method, which is based on calculating the current distribution of a fluxoid-trapped superconducting loop by using the expression of momentum and the Maxwell equations, is reconstructed to enable calculation of arbitrary 3D structures which have a ground plane (GP). Calculation of the mutual inductances of the superconductor system is also incorporated into the algorithm. The method of images is used to save computational resources, and the mirror plane is demonstrated to be just at the effective penetration depth below the upper boundary of the GP. The algorithm offers accurate results with reasonable calculation time.

Y-Ba-Cu-O/AlOx/Nb Josephson Tunnel Junctions

We have fabricated Y-Ba-Cu-O/AlOx/Nb Josephson tunnel junctions using electron-beam evaporation of Al and Nb films and natural oxidation. Superconducting Josephson current, hysteresis, and rf-induced voltage steps at a voltage greater than 0.4 mV have been clearly observed at 4.2 K. Moreover, rf-induced subharmonic steps have appeared. Magnetic field dependence of the superconducting Josephson current has also been studied.

Stochastic resonance in a pulse neural network with a propagational time delay

Stochastic resonance in a coupled FitzHugh-Nagumo equation with a propagational time delay is investigated. With an appropriate set of parameter values. i.e. the frequency of the periodic input, the propagational time delay, and the coupling strength, a deterministic firing induced by additive noise is observed, and its dependence on the number of neurons is examined. It is also found that a network composed of two assemblies shows a competitive behavior under control of the noise intensity.