Akira Fujimaki

Nagoya University, Nagoya, Aichi, Japan

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Publications (259)322.63 Total impact

  • Masamitsu Tanaka · Ryo Sato · Yuki Hatanaka · Akira Fujimaki
    No preview · Article · Jan 2016 · IEEE Transactions on Applied Superconductivity
  • [Show abstract] [Hide abstract] ABSTRACT: We demonstrate neutron detection using a solid state superconducting current biased kinetic inductance detector (CB-KID), which consists of a superconducting Nb meander line of 1 μm width and 40 nm thickness. 10B-enriched neutron absorber layer of 150 nm thickness is placed on top of the CB-KID. Our neutron detectors are able to operate in a wide superconducting region in the bias current-temperature diagram. This is in sharp contrast with our preceding current-biased transition edge detector, which can operate only in a narrow range just below the superconducting critical temperature. The full width at half maximum of the signals remains of the order of a few tens of ns, which confirms the high speed operation of our detectors.
    No preview · Article · Dec 2015 · Applied Physics Letters
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    Dataset: nmat4276
    T Wakamura · H Akaike · Y Omori · Y Niimi · S Takahashi · A Fujimaki · S Maekawa · Y Otani
    Full-text · Dataset · Oct 2015
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    Preview · Conference Paper · Sep 2015
  • No preview · Conference Paper · Sep 2015
  • Source
    T Wakamura · H Akaike · Y Omori · Y Niimi · S Takahashi · A Fujimaki · S Maekawa · Y Otani
    Full-text · Dataset · Jul 2015
  • [Show abstract] [Hide abstract] ABSTRACT: A superconducting nanowire detector with a B-10 conversion layer is useful for sensing a single neutron at a high operation rate. We use Li-7 ion and He-4 ion emitted as two independent heat sources in opposite direction, which are associated with nuclear reaction B-10(n, He-4)Li-7 to identify the XY position of neutron arrival. We probe a change rate in the kinetic inductance L-k originating from kinetic inertia of the Cooper pairs. Our detector is different from a well-known microwave kinetic inductance detector and, hence, is named as a current-biased kinetic inductance detector. We use two sets of meanderlines made of Nb nanowires with superconducting readout taps to monitor local signals. In between the X meander and the Y meander, we sandwiched a B-10 layer acting as a neutron conversion layer to two charged particles. We succeeded in observing signal induced by nuclear reaction. We plan to build a megapixel neutron imager by coupling 10-bit linear position-sensitive arrays with single-flux quantum circuits to read (X, Y) position of neutron arrival in the future.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
  • [Show abstract] [Hide abstract] ABSTRACT: We designed and fabricated rapid single-flux-quantum (RSFQ) circuits using a new 20-kA/cm2 process based on conventional Nb/AlOx/Nb Josephson junction (JJ) technology. Circular JJs were fabricated to improve their uniformity. They had sufficiently low spread of the critical currents for large-scale RSFQ circuits. We selected the circuit design parameters carefully, including the bias voltage and McCumber-Stewart parameter, balancing the energy consumption, switching speeds, margins, and integration density. We demonstrated several logic gates and shift registers at a clock frequency of 154 GHz with the designed bias voltage of 2.5 mV, and demonstrated energy-efficient shift registers using the low-voltage RSFQ (LV-RSFQ) technique at 83 GHz.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
  • [Show abstract] [Hide abstract] ABSTRACT: A current-biased kinetic inductance detector (CB-KID) is a new type of superconducting detector and senses a change in kinetic inductance in the superconducting nanowire biased by weak dc current Ib. Kinetic inductance depends on the density of Cooper pairs. Therefore, when Cooper pairs are broken by local energy dissipation, a change in kinetic inductance ΔLk can be obtained by monitoring a voltage V across the CB-KID sensor. The CB-KID has a wide operating temperature regime in the superconducting state whereas a current-biased transition edge detector senses a change in resistance at the superconductive transition edge and operates only at near vicinity of Tc. The confirmation for the validity of the CB-KID idea was preceded by using an MgB2 CB-KID meanderline detector. We extended this CB-KID method to a conventional Nb nanowire. We consider that our Nb-based CB-KID has versatile potentials in the various future applications. It is for the first time that the Nb-based CBKID operates at 4 K and has a capability of submicrometer position resolution.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
  • T Wakamura · H Akaike · Y Omori · Y Niimi · S Takahashi · A Fujimaki · S Maekawa · Y Otani
    [Show abstract] [Hide abstract] ABSTRACT: In some materials the competition between superconductivity and magnetism brings about a variety of unique phenomena such as the coexistence of superconductivity and magnetism in heavy-fermion superconductors or spin-triplet supercurrent in ferromagnetic Josephson junctions. Recent observations of spin-charge separation in a lateral spin valve with a superconductor evidence that these remarkable properties are applicable to spintronics, although there are still few works exploring this possibility. Here, we report the experimental observation of the quasiparticle-mediated spin Hall effect in a superconductor, NbN. This compound exhibits the inverse spin Hall (ISH) effect even below the superconducting transition temperature. Surprisingly, the ISH signal increases by more than 2,000 times compared with that in the normal state with a decrease of the injected spin current. The effect disappears when the distance between the voltage probes becomes larger than the charge imbalance length, corroborating that the huge ISH signals measured are mediated by quasiparticles.
    No preview · Article · May 2015 · Nature Materials
  • [Show abstract] [Hide abstract] ABSTRACT: The magnetic field generated by cryogenic PdNi ferromagnetic patterns was controlled at 4.2 K to fabricate superconducting phase-shift elements (PSEs) that would lead to production of energy-efficient devices. Cryogenic ferromagnetic patterns of Pd0.89Ni0.11 were magnetized by field cooling (FC) involving the application of external magnetic fields during sample cooling. The magnetic field from the PdNi patterns was evaluated by examining rectangular Josephson junctions in the vicinity of the patterns. Our experimental results show that the strength of the field induced by the PdNi patterns was nearly proportional to the applied field strength during FC. The field strength also depended on the end temperature of FC.
    No preview · Article · Apr 2015 · Japanese Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: A 4-bit bit-slice arithmetic logic unit (ALU) for 32-bit rapid single-flux-quantum microprocessors was demonstrated. The proposed ALU covers all of the ALU operations for the MIPS32 instruction set. It processes bit-sliced 32-bit data that are divided into eight slices of 4 bits. The bit-slice approach simplifies the circuit structure and reduces the hardware cost. The ALU uses synchronous concurrent-flow clocking and consists of eight pipeline stages. It was implemented using the 1.0- $mumbox{m} mbox{Nb}/mbox{AlO}_{x}/mbox{Nb}$ nine-layer advanced process 2 (ADP2) with a critical current density of 10 kA/cm2, and fabricated by National Institute of Advanced Industrial Science and Technology (AIST). It consists of 3481 Josephson junctions with an area of $3.09 times 1.66 mbox{mm}^2$. It achieved the target frequency of 50 GHz and a latency of 524 ps for a 32-bit operation, at the designed DC bias voltage of 2.5 mV, via precise control of interconnect delays and clock distribution. Furthermore, it achieved a throughput of $6.25 times 10^9$ 32-bit operations per second. All the correct ALU operations with measured DC bias voltage margins of around 10% at 50 GHz were successfully obtained. The proposed ALU can be used for any $4n$-bit processing.
    No preview · Article · Jan 2015 · IEEE Transactions on Applied Superconductivity
  • [Show abstract] [Hide abstract] ABSTRACT: We present not the results but the idea of a superconducting nanowire detector with ^{10} B conversion layer for sensing a single neutron. We use 7 Li ion and 4 He ion emitted as two independent heat sources, which appear in opposite direction associated with nuclear reaction ^{10} B(n,4 He)7 Li. We probe a change in the kinetic inductance L_k coming from inertia of the Cooper pairs. Our detector is different from a conventional kinetic inductance detector (KID), but is named as a current-biased KID. We use two sets of Nb nanowires with superconducting readout taps to monitor the local signal. In between the X meander and the Y meander, we inserted a ^{10} B layer acting as a conversion layer from neutrons to charged particles. We plan to fabricate a mega-pixel neutron imager by coupling 10 bit linear position-sensitive arrays along the X and Y directions with the single flux quantum readout circuits.
    No preview · Article · Mar 2014 · Journal of Low Temperature Physics
  • [Show abstract] [Hide abstract] ABSTRACT: We evaluated the effects of Fe3O4 magnetic nanoparticle (NP) films on the electrical characteristics of superconducting quantum interference devices (SQUIDs) at 4.2 K to enhance the performance of superconducting circuits. The NP films were formed directly on the SQUIDs. For SQUIDs with 5-nm-NP films, the SQUID inductance increased almost linearly with the NP film thickness, reaching 19.7% at an NP film thickness of 990 nm. An increase in NP size from 5 to 20 nm reduced that in the SQUID inductance. On the other hand, no clear effects on the current–voltage characteristics of SQUIDs were observed for 5- and 10-nm-NP films, while the critical currents of some SQUIDs with 20-nm-NP films were reduced.
    No preview · Article · Mar 2014 · Japanese Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: We describe the recent progress on a Nb nine-layer fabrication process for large-scale single flux quantum (SFQ) circuits. A device fabricated in this process is composed of an active layer including Josephson junctions (JJ) at the top, passive transmission line (PTL) layers in the middle, and a DC power layer at the bottom. We describe the process conditions and the fabrication equipment. We use both diagnostic chips and shift register (SR) chips to improve the fabrication process. The diagnostic chip was designed to evaluate the characteristics of basic elements such as junctions, contacts, resisters, and wiring, in addition to their defect evaluations. The SR chip was designed to evaluate defects depending on the size of the SFQ circuits. The results of a long-term evaluation of the diagnostic and SR chips showed that there was fairly good correlation between the defects of the diagnostic chips and yields of the SRs. We could obtain a yield of 100% for SRs including 70,000 JJs. These results show that considerable progress has been made in reducing the number of defects and improving reliability. Copyright © 2014 The Institute of Electronics, Information and Communication Engineers.
    No preview · Article · Mar 2014 · IEICE Transactions on Electronics
  • [Show abstract] [Hide abstract] ABSTRACT: Recently, we proposed a new data-path architecture, named a large-scale reconfigurable data-path (LSRDP), based on singleflux-quantum (SFQ) circuits, to establish a fundamental technology for future high-end computers. In this architecture, a large number of SFQ floating-point units (FPUs) are used as core components, and their high performance and low power consumption are essential. In this research, we implemented an SFQ half-precision bit-serial floating-point multiplier (FPM) with a target clock frequency of 50 GHz, using the AIST 10 kA/cm2 Nb process. The FPM was designed, based on a systolic-array architecture. It contains 11,066 Josephson junctions, including on-chip high-speed test circuits. The size and power consumption of the FPM are 6.66mm × 1.92mm and 2.83mW, respectively. Its correct operation was confirmed at a maximum frequency of 93.4 GHz for the exponent part and of 72.0 GHz for the significand part by on-chip high-speed tests. Copyright © 2014 The Institute of Electronics, Information and Communication Engineers.
    No preview · Article · Mar 2014 · IEICE Transactions on Electronics
  • [Show abstract] [Hide abstract] ABSTRACT: We report the successful operation of a low-power arithmetic logic unit (ALU) based on a low-voltage rapid single-flux-quantum (LV-RSFQ) logic circuit, whereby a dc bias current is fed to circuits from lowered constant-voltage sources through small resistors. Both the static and dynamic energy consumptions are reduced because of the reduction in the amplitudes of voltage pulses across the Josephson junctions, with a trade-off of slightly slower switching speeds. The designed bias voltage was set to 0.25mV, which is one-tenth that of our standard RSFQ circuit design. We investigated several issues related to such low-voltage operation, including margins and timing design. To achieve successful operation, we tuned the circuit parameters in the logic gate design and carefully controlled the timing by considering the interference of pulse signals. We show test results for the low-voltage ALU in on-chip high-speed testing. The circuit was fabricated using the AIST Nb/AlOx/Nb Advanced Process with a critical current density of 10kA/cm2. We verified that arithmetic and logical operations were correctly implemented and obtained dc bias margins of 18% at a target clock frequency of 20GHz and achieved a maximum clock frequency of 28GHz with a power consumption of 28µW. These experimental results indicate energy efficiency of 3.6 times that of the standard RSFQ circuit design.
    No preview · Article · Mar 2014 · IEICE Transactions on Electronics
  • [Show abstract] [Hide abstract] ABSTRACT: The single flux quantum (SFQ) is expected to be a nextgeneration high-speed and low-power technology in the field of logic circuits. CMOS as the dominant technology for conventional processors cannot be replaced with SFQ technology due to the difficulty of implementing feedback loops and conditional branches using SFQ circuits. This paper investigates the applicability of a reconfigurable data-path (RDP) accelerator based on SFQ circuits. The authors introduce detailed specifications of the SFQ-RDP architecture and compare its performance and power/performance ratio with those of a graphics-processing unit (GPU). The results show at most 1600 times higher efficiency in terms of Flops/W (floating-point operations per second/Watt) for some high-performance computing application programs. Copyright © 2014 The Institute of Electronics, Information and Communication Engineers.
    No preview · Article · Mar 2014 · IEICE Transactions on Electronics
  • [Show abstract] [Hide abstract] ABSTRACT: We describe a large-scale integrated circuit (LSI) design of rapid single-flux-quantum (RSFQ) circuits and demonstrate several reconfigurable data-path (RDP) processor prototypes based on the ISTEC Advanced Process (ADP2). The ADP2 LSIs are made up of nine Nb layers and Nb/AlOx/Nb Josephson junctions with a critical current density of 10 kA/cm2, allowing higher operating frequencies and integration. To realize truly large-scale RSFQ circuits, careful design is necessary, with several compromises in the device structure, logic gates, and interconnects, balancing the competing demands of integration density, design flexibility, and fabrication yield. We summarize numerical and experimental results related to the development of a cell-based design in the ADP2, which features a unit cell size reduced to 30-μm square and up to four strip line tracks in the unit cell underneath the logic gates. The ADP LSIs can achieve ~10 times the device density and double the operating frequency with the same power consumption per junction as conventional LSIs fabricated using the Nb four-layer process. We report the design and test results of RDP processor prototypes using the ADP2 cell library. The RDP processors are composed of many arrays of floating-point units (FPUs) and switch networks, and serve as accelerators in a high-performance computing system. The prototypes are composed of two-dimensional arrays of several arithmetic logic units instead of FPUs. The experimental results include a successful demonstration of full operation and reconfiguration in a 2×2 RDP prototype made up of 11.5k junctions at 45 GHz after precise timing design. Partial operation of a 4×4 RDP prototype made up of 28.5k-junctions is also demonstrated, indicating the scalability of our timing design. Copyright © 2014 The Institute of Electronics, Information and Communication Engineers.
    No preview · Article · Mar 2014 · IEICE Transactions on Electronics
  • [Show abstract] [Hide abstract] ABSTRACT: We recently proposed the idea of a novel sort of superconducting detector, i.e., a current-biased kinetic inductance detector (CB-KID). This detector is different from a current-biased transition edge detector studied previously, and is able to sense a change in kinetic inductance L_k given by Lk = Λ kl/S = msl/ns{qs}2S (Λ k ; kinetic inductivity, m_s ; mass of Cooper pair, n_s ; density of Cooper pairs, q_s ; charge of Cooper pair, l ; length of device, S ; cross sectional area) under a constant dc bias current I_b . In the present work, we first extend this idea to construct a multi-channel CB-KIDs array made of 200-nm-thick MgB_2 thin-film meanderline with 3-\upmu m thin wire. We succeeded in observing clear signals for imaging from the four-channel CB-KIDs at 4 K by irradiating focused pulsed laser. A scanning laser spot can be achieved by an XYZ piezo-driven stage and an optical fiber with an aspheric focused lens. We can see typical signals from all 4 channels at 4 K, and obtain the positional dependence of the signal as the contour in XY plane. Our CB-KIDs can be used as neutron detectors by utilizing energy released from a nuclear reaction between ^{10} B and cold neutron.
    No preview · Article · Jan 2014 · Journal of Low Temperature Physics