Kazunori Miyahara

VORTEX FLOW CHARACTERISTICS OF HIGH-TC FLUX FLOW TRANSISTORS

High‐Tc superconducting flux flow transistors were fabricated from thin films of coevaporated YBaCuO. The vortex flow characteristics of the devices were experimentally investigated. The flow velocity of a vortex along a channel was measured from the applied magnetic field dependence of the I‐V characteristics. It is 3.6–6.9×105 m/s when the bias current is 3–6 mA. This value is of the same order as that of a Josephson vortex flowing along a barrier region in a super‐normal‐super junction, and more than three orders of magnitude larger than that in type‐II metal superconducting thin films.

Transresistance and current gain of high‐Tc flux flow transistors

The transresistance and current gain of high‐Tc flux flow transistors were investigated phenomenologically and experimentally in relation to the flux flow characteristics of the devices. It was shown that the transresistance was in proportion to the vortex flow velocity, the channel width, and the magnetic coupling coefficient between the channel and the control current line. The devices were fabricated from thin films of coevaporated NdCeCuO, and the control current dependence of their transresistance was measured. It was found that the transresistance peak appeared at a high control current level that corresponded to the saturation of the flow voltage. The peak value of the transresistance was 1.25 Ω. The flow voltage saturation mechanism is discussed in relation to the channel etching of the devices.

Abrikosov vortex memory

A nondestructive readout random access memory (RAM) cell based on the use of Abrikosov vortices in thin‐film type‐II superconductors is newly designed and experimentally tested. The cell occupies an area of 30×60 μm2 with a 5‐μm design rule. Proper memory cell operation is achieved in the so‐called 1, −1 mode using the shift saturation effect, which occurs on the shift value characteristics of the sense gate threshold curves as a function of the write currents. This is the first Abrikosov vortex RAM cell to operate properly. The write current level has been reduced one order of magnitude using a Pb/In/Au film co‐evaporated at 90 K for the vortex storage region.

Abrikosov vortex memory with improved sensitivity and reduced write current levels

Etudes dune cellule de memoire a acces direct basee sur lutilisation des vortex dAbrikosov dans les supraconducteurs de type II en couches minces

Millimeter Wave Mixing Properties of EuBa 2Cu 3O 7-δ Bicrystal Grain Boundary Junction Arrays

We report the millimeter wave mixing properties of high-T c superconductor Josephson junction arrays using a waveguide-to-microstrip converter in the 100 GHz frequency range. The junction arrays consist of 3 weak links fabricated from EuBa2Cu3O7-δ thin films grown on an MgO bicrystal substrate. In these junction arrays, Shapiro steps are observed in the presence of 100 GHz irradiation at intervals of 3 times the fundamental step voltage. Millimeter wave mixing with an input local oscillator frequency of 99.142 GHz and an input signal frequency of 99.880 GHz is detected as the IF output at 738 MHz. The IF output power peaks at voltages corresponding to those of the dynamic resistance dV/ dI. A mixing conversion efficiency of -24.1 dB is obtained for a 3-junction array.

Critical currents of ion‐beam sputtered amorphous beryllium thin films and their application to an Abrikosov vortex memory

Critical current and flux pinning were examined for ion‐beam sputtered (IBS) amorphous beryllium (a‐Be) thin films with Tc 6 K. Pinning forces in IBS a‐Be films were one or more than one order of magnitude smaller than those for crystalline superconductors, but were still rather large for amorphous superconductors. The viscosity coefficient for IBS a‐Be was very small, 1–2×10−9 Nu2009s/m2, which was only a few tenths as large as viscosity coefficients for other metallic superconductors. Write operation characteristics for Abrikosov vortex memory with an IBS a‐Be vortex storage region was tested. The write current level was reduced to about two thirds of that for fine‐grained PbInAu previously reported. IBS a‐Be films were proved to be a potential material for use in an Abrikosov vortex memory.

Coherent Effects in Array Junctions Using Eu 1 Ba 2 Cu 3 O 7-x Thin Films

Since a weak-link Josephson junction has low impedance, it is difficult to match a junction and an external circuit. We have therefore investigated an array junction using Eu1Ba2Cu3O7-x thin film with a view of increasing impedance. This array junction is designed as a 10µm wide superconducting meander line formed above the boundary of an MgO bicrystal substrate and their dc/rf properties are measured. The N-series junctions exhibit large Shapiro steps under microwave irradiation. The steps appear at voltages N times the first Shapiro step voltage V0. This result shows that each junction worked coherently and indicates the uniformity of the junction properties. Furthermore, good responsivity is obtained with this array junction in the 10 GHz range.

Temperature Dependence of the Vortex Flow Velocity in High-Tc Flux Flow Transistors

High-Tc superconducting flux flow transistors have been fabricated from thin films of YBaCuO and NdCeCuO. The temperature dependence of the vortex velocity flowing in the device was experimentally investigated. The vortex flow velocity was obtained from the slope of the flow voltage vs. control current characteristics. The measured vortex flow velocity was plotted against temperature. The flow velocity decreases in the higher temperature region and it appears to reach zero at the critical temperature of the channel. The measured vortex flow velocity is ~ 106 m/sec. The flow velocity of YBaCuO-device is nearly twice as large as that of NdCeCuO-device at the same reduced temperature.