Yoshinao Mizugaki

Implementation of new superconducting neural circuits using coupled SQUIDs

A novel superconducting neuron circuit and two types of variable synapses, which are based on superconducting quantum interferometer devices (SQUIDs), are presented. A neuron circuit with good input-output isolation and steep threshold characteristics is accomplished using a combination of a single-junction SQUID coupled to a double-junction SQUID. The quantum state of the single-junction SQUID represents the neuron state, and the output voltage of the double-junction SQUID, which is operated in a nonlatching mode with shunt resistors, is a sigmoid-shaped function of the input. Both variable synapse circuits are composed of multiple shunted double-junction SQUIDs. The first type changes its conductance value by using both superconducting and voltage states. The second variable synapse circuit changes its output current digitally by switching its bias currents. Besides numerical simulations of the circuit characteristics, we have fabricated superconducting neural chips in a Nb/AlO/sub x//Nb Josephson junction technology. The fundamental operation of each element and a 2-bit neural-based A/D converter have been successfully tested. A learning system with a variable synapse is also discussed.<<ETX>>

Microwave-induced current steps in intrinsic Josephson junctions patterned on Bi2Sr2CaCu2O8 single crystal

With a 10 μm×10 μm mesa patterned on Bi2Sr2CaCu2O8 single crystals, we measure the current–voltage (I–V) curves of a stack of intrinsic Josephson junctions. Current steps are observed at an equal voltage spacing of 4 mV when the sample is subjected to microwave radiation at around 7 GHz. With increase of the microwave power, more steps occur while the spacing between neighboring steps does not seem to change. The magnitude of each step depends on the microwave power in an oscillating way. Tuning the microwave frequency causes such steps to occur over separate frequency ranges, and each range is quite narrow. A temperature rise from 4.2 to 14.3 K completely quenches the step structures. Possible explanations for the step structures, based on resonances excited by microwave or geometric resonances in the junction cavity, are discussed.

Subgap structures in the current–voltage properties of La2−xSrxCuO4 intrinsic Josephson junctions

Abstract We investigated current–voltage ( I – V ) properties of intrinsic Josephson junctions in c -axis micro-bridges (∼20 μm 2 area for ab plane and ∼1 μm length along the c -axis) of La 2− x Sr x CuO 4 single crystals. The micro-bridges were fabricated by the focused-ion-beam etching technique. We found periodic conductance peaks below the gap voltage in the RCSJ-like I – V curves of the intrinsic Josephson junctions. We discuss possible explanations and show that the subgap structures can be explained by coupling between ac Josephson oscillation and Josephson plasma oscillation in La 2− x Sr x CuO 4 .

Intrinsic Josephson junctions in oxygen-deficient YBa2Cu3O7−δ film deposited on a substrate step

Using c-axis-oriented oxygen-deficient YBa2Cu3O7−δ film deposited across a low-angle step on a SrTiO3 substrate, we successfully demonstrated intrinsic Josephson effects. In addition to several voltage jumps of large amplitudes (a few millivolts) and remarkable hysteresis on the dc current–voltage curves, we observed upturns on the current–voltage curves under microwave irradiation which appeared at increasingly high voltages with increasing microwave power. We proposed to explain this observation in terms of high-order microwave-induced Shapiro steps.

Superconducting neural circuits using fluxon pulses

A superconducting neural circuit is fabricated for the first time by use of a niobium integrated‐circuit technology. Fluxon pulses on Josephson transmission lines (JTLs) are used as neural impulses. In this circuit a threshold element (a neuron) is composed of two JTL elements connected through a resistor. The conductance value of the resistor represent a synaptic strength. The fan‐in and the fan‐out are accomplished by the biased JTL branches. The operation of 2‐bit neural based A/D converter is successfully observed. These circuits do not require any hysteretic Josephson junctions, and hence, have a potential to be fabricated with the high‐Tc superconductors.

5-Bit Quasi-Sinusoidal Voltage Waveform Synthesized Using Single-Flux-Quantum Pulse-Frequency Modulation

Synthesis of sinusoidal voltage waveform is a unique application in single-flux-quantum (SFQ) digital electronics. In this paper, the authors present a waveform synthesizer based on SFQ pulse-frequency modulation. It comprises a variable SFQ pulse-number-multiplier (PNM) and a code generator (CG) integrated on the same chip. The output voltage is determined by the multiplication factor in the variable-PNM, which is controlled by the code from the CG instead of room-temperature electronics. The variable-PNM realizes 5-bit resolution with the multiplication factors between 33 and 64. The CG generates the 5-bit codes for 16-step quasi-sinusoidal waveform. The whole circuits including I/O elements are designed using an SFQ digital cell library, and fabricated using a niobium integration process. The modulation of the multiplication factor for the 16-step quasi-sinusoidal waveform is confirmed by counting SFQ pulses in the low-speed testing, whereas the voltage waveform of 11 μV peak-to-valley is demonstrated using a high-speed SFQ input of 156.25 MHz.

Intrinsic Josephson junctions in c-axis-oriented La1.85Sr0.15CuO4 thin films

We investigated current–voltage characteristics (IVCs) of intrinsic Josephson junctions (IJJs) for c-axis oriented La1.85Sr0.15CuO4 thin films. We fabricated IJJs with two kinds of structures. One was a mesa structure with a junction area of 22–194 μm2 fabricated by using conventional photolithography and Ar-ion milling, and the other was a microbridge structure with a junction area of 1.1–3.6 μm2 fabricated by using a focused-ion-beam technique. The mesa-type IJJs exhibited resistively-shunted-junction-like IVCs with no hysteresis. The temperature dependence of their critical current followed the Ambegaokar–Baratoff relation. IVCs of the microbridge-type IJJs, on the other hand, exhibited voltage jump and clear hysteresis. The different behaviors between the mesa-type and the microbridge-type IJJs were explained by the different numbers of grain boundaries involved in the IJJs.

Single-Electron Signal Modulator Designed for a Flash Analog-to-Digital Converter

We demonstrate a 4-bit flash analog-to-digital converter (FADC) composed of a single-electron signal modulator (SESM) with a capacitive voltage divider (CVD). In the CVD, we utilize the self-capacitance of floating metal islands, and hence, maximum input impedance can be realized with the CVD. We also analyze the characteristics of the CVD with stray capacitance. The signals from the CVD are detected by four single-electron transistors (SETs), and their outputs, processed at quantizers, represent gray-coded digital signals. We designed two kinds of layouts for the CVDs, and fabricated two samples for each layout by photolithography, electron-beam lithography, and the shadow evaporation of aluminum. Measurements were executed at approximately 30 mK. Four-bit FADC operation was successfully simulated using experimental results. We also discuss the possibility of higher resolution with our circuit configuration.

Harmonic frequency mixing in Bi2Sr2CaCu2O8+X intrinsic Josephson junctions

In a c-axis junction stack (10 μm×10 μm in a-b plane) patterned on a Bi2Sr2CaCu2O8+X single crystal, we performed harmonic mixings between a 100 GHz signal and up to the 98th harmonic of a local oscillator at about 1 GHz. The dependence of PIF on V, where PIF is the mixing output at the intermediate frequency and V is the dc voltage across the junction stack, was strongly affected by the local oscillator power levels. For low values of local oscillator power, the optimum operation point where PIF became maximum was around zero bias, and typical multi-branch structure of PIF vs V curves were observed. For high values of local oscillator power, the optimum operation point moved to the high-voltage region. In both cases, interesting oscillations occurred on the PIF vs V curves. Our experimental results led us to believe that intrinsic Josephson junctions can be good candidates for high frequency applications and the harmonic mixing may be a useful probe to investigate plasma phenomena in the layered supercon...

Superconducting implementation of neural networks using fluxon pulses

The authors fabricated neural-based superconducting integrated circuits by using Nb/AlO/sub x//Nb Josephson junctions, and demonstrated the operation of a 2-b neural-based A/D converter. Fluxon pulses were used as neural impulses, and a Josephson junction was used as a threshold element. The conductance values of resistors by which Josephson transmission lines are connected represent fixed synaptic strengths. The preliminary experimental result suggests that variable critical currents of DC-SQUIDs (superconducting quantum interference devices) may provide synapses with variable strength.<<ETX>>