CW Operation of a Submillimeter Wave Gyrotron (Gyrotron Fu IV) for High Stability of the Output Frequency

The first cw operation of our submillimeter wave gyrotron (Gyrotron FU IV) using a 12 T superconducting magnet has been successfully carried out. Output power is more than 20 W at a frequency of 301 GHz in the TE₀₃₁ resonant cavity mode. Time-resolved frequency measurement s shows that the frequency fluctuation of the gyrotron output is smaller than 2 MHz. This frequency fluctuation is mainly due to the fluctuation in the output voltage of the power supply.     

A delay-encoding-logic array processor for dynamic-programming matching of data sequences

Computationally very expensive dynamic-programming matching of data sequences has been directly implemented in a fully-parallel-architecture VLSI chip. The circuit operates as digital logic in the signal domain, while analog processing is carried out in the time domain based on the delay-encoding-logic scheme. As a result, a high-speed low-power best-match-sequence search has been established with a small chip area. The typical matching time of 80 ns with the power dissipation of 2 mW has been demonstrated with fabricated prototype chips.     

Bioinspired Magnetite Crystallization Directed by Random Copolypeptides

Control over magnetite (Fe₃O₄) formation is difficult to achieve in synthetic systems without using non‐aqueous media and high temperatures. In contrast, Nature employs often intrinsically disordered proteins to tightly tailor the size, shape, purity, and organization of the nanocrystals to optimize their magnetic properties. Inspired by such “flexible polyelectrolytes,” here random copolypeptides having different amino acid compositions are used as control agents in the bioinspired coprecipitation of magnetite through a ferrihydrite precursor, following a recently developed mineralization protocol. Importantly, the copolypeptide library is designed such that the amino acid composition can be optimized to simultaneously direct the size of the nanoparticles as well as their dispersibility in aqueous media in a one‐pot manner. Acidic amino acids are demonstrated to regulate the crystal size by delaying nucleation and reducing growth. Their relative content thus can be balanced to tune between the superparamagnetic and ferrimagnetic regimes, and high contents of negatively charged amino acids result in colloidal stabilization of superparamagnetic nanoparticles at high pH. Conversely, with positively charged lysine‐rich copolypeptides ferrimagnetic crystals are obtained which are stabilized at neutral pH and self‐organize in chains, as visualized by cryo‐transmission electron microscopy. Altogether, the presented findings give important insights for the future development of additive‐mediated nanomaterial syntheses.     

Queuing Scheme for Improved Downlink Throughput on WLANs

A transmission queuing scheme is described that increases downlink throughput on wireless local area networks (WLANs) while also increasing the total throughput. When the amount of uplink traffic increases on a WLAN, the carrier sense multiple access with collision avoidance (CSMA/CA) protocol, which is the prescribed scheme for IEEE 802.11 WLAN channel access, may substantially reduce the rate of downlink data frame transmission. This results in severe throughput degradation for mobile stations with downlink traffic. The proposed scheme comprises a transmission control function based on consecutive transmission, as described in the IEEE 802.11e standard, and a dynamic queue prioritization algorithm. Simulation results demonstrate that the proposed scheme increases the maximum total throughput for uplink and downlink traffic by 17% compared with the conventional distributed coordination function (DCF) scheme and that it reduces the difference between uplink and downlink throughput. In an environment where transmission errors occur, the difference in throughput is reduced by about 50% compared with the conventional schemes.     

DNA Computing Boosted by a Cationic Copolymer

The huge information storage capability of DNA and its ability to self‐assemble can be harnessed to enable massively parallel computing in a small space. DNA‐based logic gates are designed that rely on DNA strand displacement reactions; however, computation is slow due to time‐consuming DNA reassembly processes and prone to failure as DNA is susceptible to degradation by nucleases and under certain solution conditions. Here, it is shown that the presence of a cationic copolymer boosts the speed of DNA logic gate operations that involve multiple and parallel strand displacement reactions. Two kinds of DNA molecular operations, one based on a translator gate and one on a seesaw gate, are successfully enhanced by the copolymer without tuning of computing conditions or DNA sequences. The copolymer markedly reduces operation times from hours to minutes. Moreover, the copolymer enhances nuclease resistance.     

Hydrogen plasma passivation of bulk GaAs and Al0.3Ga0.7As/GaAs multiple-quantum-well structures on Si substrates

Hydrogen (H) plasma passivation effects on GaAs grown on Si substrates (GaAs on Si) are investigated in detail. H plasma exposure effectively passivates both the shallow and deep defects in GaAs on Si, which improves both the electrical and optical properties. It was found that the minority carrier lifetime is increased and the deep level concentration is decreased by the H plasma exposure. In addition, after H plasma exposure, room temperature photoluminescence (PL) for Al_0.3Ga_0.7As/GaAs multiple-quantum-well (MQW) on Si is enhanced with a decrease in the spectral width.     

Strong Surface‐Termination Effect on Electroresistance in Ferroelectric Tunnel Junctions

Tunnel electroresistance in ferroelectric tunnel junctions (FTJs) has attracted considerable interest, because of a promising application to nonvolatile memories. Development of ferroelectric thin‐film devices requires atomic‐scale band‐structure engineering based on depolarization‐field effects at interfaces. By using FTJs consisting of ultrathin layers of the prototypical ferroelectric BaTiO₃, it is demonstrated that the surface termination of the ferroelectric in contact with a simple‐metal electrode critically affects properties of electroresistance. BaTiO₃ barrier‐layers with TiO₂ or BaO terminations show opposing relationships between the polarization direction and the resistance state. The resistance‐switching ratio in the junctions can be remarkably enhanced up to 10⁵% at room temperature, by artificially controlling the fraction of BaO termination. These results are explained in terms of the termination dependence of the depolarization field that is generated by a dead layer and imperfect charge screening. The findings on the mechanism of tunnel electroresistance should lead to performance improvements in the devices based on nanoscale ferroelectrics.     

Development of 170 GHz/500 kW gyrotron

A development of 170GHz/500kW level gyrotron was carried out as R&D work of ITER. The oscillation mode is TE_31,8. In a short pulse experiment, the maximum power of 750kW was achieved at 85kV/40A. The efficiency was 22%. In the depressed collector operation, 500kW/36%/50ms was obtained. The maximum efficiency of 40% was obtained at P_RF=470kW whereas the power decrease by the electron trapping was observed. Pulse extension was done up to 10s at P_RF=170kW with the depressed collector operation. The power was limited by the temperature increase of the output window.     

30-GHz bandwidth 1.55-/spl mu/m strain-compensated InGaAlAs-InGaAsP MQW laser

High-speed 1.55 /spl mu/m laser diodes with a 3-dB modulation bandwidths of 30 GHz were fabricated by using short-cavity mushroom structures with undoped, strain-compensated InGaAlAs-InGaAsP twenty-quantum-well active regions. The bandwidths were achieved at low bias current of 100 mA. The laser exhibited a high differential gain of 1.54/spl times/10/sup -15/ cm/sup 2/ and a small K factor of 0.135 ns. These results were achieved by using an In/sub 0.386/Ga/sub 0.465/AlAs barrier with 0.83% tensile strain to reduce the thermal emission time of holes from wells and hence the hole transport time.