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Contents:

Topics: What’s New in the World of Superconductivity

Feature Article: Cryogenic Digital Devices-The Effects of Magnetic Thin Films Employed in Superconducting Digital Circuits-Reversible Computing Using Adiabatic Superconductor Logic to Breakthrough the
Minimum Energy Limitation Required for Logic Operations-Superconducting Detector and Hybrid Device Using SFQ Circuits

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This work was subsidized by JKA using promotion funds fromKEIRIN RACE
http://ringring-keirin.jp

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What’s New in the World of Superconductivity초전도 뉴스 –세계의 동향超导新闻-世界的动向-chāo dǎo xīnwén

- shìjiè de dòngxiàng-

Yutaka Yamada, Principal Research Fellow
Superconductivity Research Laboratory, ISTEC★News sources and related areas in this issue

►Wire

선 재료

缐材料 [xiàn cáiliào]

Mechanical Test Facilities for Customer
SuperPower (26 Jan, 2015)
SuperPower Inc. has announced the start of its own dedicated facilities for the testing of
mechanical/electromechanical properties of its 2G HTS wires. The superconducting characteristics of such
HTS wires are affected by the applied stresses/strains, and thus a better understanding is very important in
the design, fabrication and operation of HTS devices. It is therefore anticipated that these facilities will
ensure a continuous and robust improvement in the performance characteristics of its 2G HTS products.
SuperPower is also closely working with the IEC/TC90 to ensure the testing methods employed conform to
the applicable standards. The data from these tests will be provided to its customers to support theirapplications.

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Specifically, the completion of a recent project has enabled Superpower to have the capabilities toundertake mechanical/electromechanical tests in liquid nitrogen and also measure critical currents undereither longitudinal or transverse tensile stress conditions. Additional to anvil tensile tests, pin-pull and peel
tests are also being employed to investigate the mechanical behaviors of the wires under a transverse
tensile stress. Currently, SuperPower is working on setting up for compressive testing and twist testing, andthus their commitment to mechanical/electromechanical testing capabilities continues.
Source:”Superpower has established comprehensive testing capabilities for the assurance of themechanical/electromechanical performance of its 2G HTS wires”(26 Jan, 2015) News Release
http://www.superpower-inc.com/content/superpower-establishes-comprehensive-testing-capabilities-assura
nce-mechanicalelectromechani
Contact: Mickey Lavicska mlavicska@superpower-inc.com►Electronics

엘렉트로닉스

电子应用 [diànzǐyè yìngyòng]Nano-memory for Superconducting Computer
NIST (23 Jan, 2015)
The growing demands of cloud computing in recent years have prompted the creation of centralized
computing facilities at sites around the world. Such facilities operate 24 hours per day and utilize
semiconductor-based electronics, which are typically power hungry and generate substantial amounts ofheat that also needs energy to remove.
A promising replacement technology is superconducting (SC) computers, employing instead, Josephson
junctions (JJs) that dissipate small amounts of energy and can operate at gigahertz frequencies, compared
to a few gigahertz for semiconductor computers. Recent research work has led The Intelligence Advanced
Research Projects Activity (IARPA) to ascertain that major breakthroughs are forthcoming and haslaunched a multi-year program to investigate the practical viability of SC computing.
For the IARPA program, NIST scientists have developed and demonstrated a new nanoscale memory
technology for superconducting computers that functions at cryogenic temperatures, utilizing the fastswitching speeds of JJs thus offering the potential of low-energy computing. The NIST team’s module is thefirst to employ spintronic effects, which are acutely difficult to characterize on the nanoscale. Their memory
module, described in a 2014 publication, is a modified JJ, 100 nanometers thick comprising a multi-layer
barrier consisting of two different magnetic materials separated by a non-magnetic metal. The behaviors ofdifferent barrier configurations and materials for use in memory are now being investigated.
“The combination of low-loss superconducting logic and nonvolatile, hybrid magnetic memory could
revolutionize mainframe computation and data storage within a decade,” says Ron Goldfarb, leader of the
NIST Magnetics Group and a supporting member of the NIST team.

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Source:” Hybrid memory device for superconducting computing”(23 Jan, 2015) News/Multimedia
http://www.nist.gov/pml/electromagnetics/magnetics/hybrid-memory-device-for-superconducting-computing.cfm
Contact: Burm Baek burm.baek@nist.gov►Basics기초 基础[jīchǔ]Superconducting Nano-wire
University of Copenhagen (12 Jan, 2015)
Researchers at the University of Copenhagen are behind the breakthrough of a new type of ‘nanowire’
crystal based on aluminium, which can fuse semiconducting and metallic materials on the atomic scale.
Their work appears in Nature Materials, demonstrating a perfect contact and also showing that they canmake a chip with billions of identical semiconductor-metal nanowire hybrids.
This new material comprises of both a semiconductor and metal, having superconducting characteristics at
cryogenic temperatures. This hybrid material has great potential according to Associate Professor Thomas
Sand Jespersen, who has worked in the field for more than 10 years, ever since research into nanowire
crystals was launched at the Nano-Science Center at the Niels Bohr Institute. The challenges withnanowires have been in creating good electrical contacts to the outside world. Groups around the world
have fabricated nanowires and contacts separately, however with the new approach, both the quality andthe reproducibility of the contacts have improved significantly. Assistant Professor Peter Krogstrup explainsthat this opens many opportunities to make new types of electronic components on the nanoscale, leading
to intensified studies about the electrical properties with much greater precision than ever before.
Source:” New superconducting hybrid crystals developed at the University of Copenhagen”(12 Jan, 2015) News
http://www.nbi.ku.dk/english/news/news15/new-superconducting-hybrid-crystals-developed-at-the-universit
y-of-copenhagen/Contact: Peter Krogstrup krogstrup@nbi.ku.dk

HTS Fingerprint
Cornell University (6 Jan, 2015)
Scientists at Cornell have isolated the “fingerprint” which is believed pinpoints the specific fluctuations ofelectrons that force them into pairs, producing resistance-free superconductivity in “unconventional”
superconductors. The group has proved that spin fluctuations are the cause of the electrons to form pairs.
In conventional superconductors, it is well understood that superconductivity occurs because electron
pairing is driven by an exchange of lattice vibrations, which become strong enough to overcome Coulomb
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repulsion, but only at very low temperatures. However, the mechanism of superconductivity in“unconventional” superconductors has been a mystery. In such superconductors, the electrons tend to form
antiferromagnets, and this tendency, prior to onset of the antiferromagnetic ordering, produces the spin
fluctuations.
Associate professor of physics Eun-Ah Kim, explained that, “the fact that these spin fluctuations could
provide electron pairing has been conjectured many times over, but proving it has been a big challenge andthe methods that had worked for simple metals like aluminum don’t quite work for higher temperature
superconductors.” A new class of iron-based unconventional superconductors, the so-called multiband
systems, means that electrons at a given energy can have several different momentum states, each withdifferent velocities. By employing an energy-momentum measurement technique called quasiparticle
interference imaging, they successfully differentiated between the fingerprint due to spin fluctuations andlattice vibrations. Their findings are published online Dec.22 in Nature Physics, in an article entitled,“Identifying the ‘Fingerprint’ of Antiferromagnetic Spin Fluctuations in Iron Pnictide Superconductors”.
Source:” High-temperature superconductor 'fingerprint' found”(6 Jan, 2015) Cornell Chronicle
http://news.cornell.edu/stories/2015/01/high-temperature-superconductor-fingerprint-foundContact: Anne Ju amj8@cornell.edu

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Feature Article: Cryogenic Digital Devices-The Effects of Magnetic Thin Films Employed in Superconducting
Digital Circuits
Hiroyuki Akaike, Associate Professor
Graduate School of Engineering, Nagoya University
A particular characteristic of superconducting digital circuits including single flux quantum (SFQ) circuits istheir low power operation. In recent years, this advantageous characteristic has gathered attention leading
to buoyant R&D activities aimed at further low power operations. One of the targeted aims ofsuperconducting digital circuits is the prospect of realizing energy-efficient high-performance computing
systems. Therefore, they possibly offer the key in addressing the future issues associated with the vast
amounts of power consumed by supercomputers and data center facilities. In fact, the development of an
ultra-low power circuit and their systematization has been actively ongoing under the Cryogenic Computing
Complexity (C3) program 1) in the USA, and under the JST-ALCA project 2) and Grant-in-Aid in Japan. In
recent years, the introduction of magnetic materials into circuits or Josephson junctions in circuits has beenstudied in order to realize greater circuit performance and functionality. This article briefly highlights theadvantages of magnetic thin films utilized in superconducting digital circuits including the trials undertakenby the author and his research team.
One of the most notable characteristics of magnetic materials is their magnetization. The switching of theirmagnetization used in for example, Magneto-resistive Random Access Memory (MRAM), is applicable torealization of high-capacity superconducting memory devices for practical use, because the utilization of themagnetization would lead to elimination of the conventional use of superconducting loops for storing flux
quanta as a storage cell. The above-mentioned C3 program places the development of high-density
cryogenic memory as an important factor that is driving R&D activities aimed at magnetic Josephson
Junctions and superconducting MRAMs. The author’s research team has commenced studies on magnetic
Josephson Junctions, employing magnetic PdNi alloys as a junction barrier. Measured tolerance
characteristics for potential integration have been proved as having a 0.7 % device-to-device variation(1standard deviation). Other advantageous characteristics afforded by magnetic materials used forcircuits include the ability to bias their magnetic flux. In other words, a micro magnetic pattern incorporated
into the circuit allows a magnetic flux to be induced locally.
The magnetic flux biasing is one of the methods that enhance the functionality of superconducting circuitsthat are constructed by parallel connection of SQUIDs, or superconducting loops containing Josephson
Junctions. A superconducting loop within a circuit is now highlighted. The phase change in the microscopic
wavefunction along a closed superconducting loop is equal to an integer multiple of 2 π, based upon thequantization condition. With the application of an external magnetic flux to the loop, an additional phasedifference is produced dependent upon the magnitude of the flux applied eventually shifting the phasemodulation. Elements that induce additional phase difference are called superconducting phase shifters,
and a variety of such elements have been proposed (Table 1).

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Table 1 A comparison between each type of superconducting phase shifter

Phase shifting element

Degree ofshift

Characteristics

Issue/disadvantage

References

Element size, Flux trap
operation is required3)

Flux-trap loopπSimple fabrication

YBCO corner-SQUID

πd -wave superconducting gap
HTS process, layout
symmetry

4)

Ferromagnet-based Josephson πjunctionπProcess applicability, Small
sized element

5)

Magnetic-patterned

Discretional

Control of element
characteristics

Simple fabrication, Extremely Magnetization control,
small-sized element
Flux leakage

NagoyaUniversity

Junction Critical Current Ic [A]

Significant advantages attained by utilizing phase shifters include reduced power consumption of circuits4)
and reliable circuit operation 6). When a phase shifter is implemented in a SQUID loop, a circulating current
naturally forms in the loop because of phase change of the superconducting macroscopic wavefunction.
On the other hand, a circulating current in a conventional circuit is generated by applying a magnetic flux via
mutual inductance or by supplying bias currents asymmetrically to the loop storing SFQ. Therefore,
implementing a phase shifter into the loop in circuits can reduce the numbers of bias lines thereby reducing
the total amount of bias current, further reducing power dissipation. Additionally, with improved operating
margin and bit error rate further operational reliability can be achieved.
A phase shifter utilizing magnetic materials
Field cooling
Zero field coolingwhere the author’s research team has
been studying, offers advantageous
characteristics
toallow
simplefabrication/down-scaling of elements, inaddition exhibiting a great deal of flexibility
in being able to specify phase shift
Josephson External Field H
Junctionchanges.
Magnetization
controlisinevitable for employing such phaseshifters. The magnetic effects provided by
Magnetic-pattern
the phase shifter have been studied up to
now by using a single Josephson Junction
or SQUID with magnetic patterns. An
External Field
example is shown in Figure 1. When an
external magnetic field is applied during the
Fig. 1 Critical current of a Josephson Junction with acooling-down process of the sample (fieldmagnetic pattern as a function of the external field. A shift incooling), the effect of a magnetic flux
the characteristics due to field cooling can be observed.
biasing can be observed. The degree offlux bias is understood to be controllable by the strength of the magnetic field applied during field cooling.
Currently, the advantages offered by a phase shifter implemented in a circuit (Figure 2) are being
progressively investigated.
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Magnetic-patternFig. 2 Circuit with a Magnetic pattern

It is anticipated that the utilization of magnetic materials for applications ranging from junctions to circuits willallow further performance enhancements of superconducting digital circuits and offer significant progress inrealizing a practical circuit. In order to accomplish such research and development it is desirable to include
not only researchers specialized in the field of superconducting digital circuits, but also trans-disciplinary
research groups, including those researchers specializing in the field of magnetic materials. The formationof such research taskforces has led to advancements as seen in typical examples of the C3 program. The
launch of such a full-scale research project in Japan is awaited.
Reference:
1) IRPA C3 program, http://www.iarpa.gov/index.php/research-programs/c3
2) JST-ALCA program, http://www.jst.go.jp/alca/kadai.html#K02
3) J.B. Majer et al., Appl. Phys. Lett. 80, 3638 (2002).
4) Ortlepp et al., IEEE Tans. Appl. Supercond., 17, 659 (2007).
5) M. I. Khabipov, et al., Supercond. Sci. Technol. 23 (2010) 045032.
6) O. Wetzstein et al., IEEE Tans. Appl. Supercond., 21, 814 (2011).

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Feature Article: Cryogenic Digital Devices-Reversible Computing Using Adiabatic Superconductor Logic to
Breakthrough the Minimum Energy Limitation Required for Logic
Operations
Naoki Takeuchi, Senior Research Fellow
Nano ICT Laboratory, Advanced ICT Research InstituteNational Institute of Information and Communications Technology
In order to realize the next generation high-end computers exceeding ExaFLOPS, an innovative logic circuitwith much lower power dissipation than CMOS circuits is required. A common theme emerging in the
pursuit of greater efficiencies in computation has long since involved the relationship between computerand energy. Amongst the arguments involved Landauer’s principle 1) is well known. He predicted, “There isno minimum energy limitation for computing if there is no reduction in entropy”. Some recent experimental
demonstrations reported by Bérut and his research team has confirmed the validity 2). Under these
conditions, it is therefore anticipated to produce reversible logic operations 3), which can perform
computations with unlimited minimum energy dissipation accompanied with no changes in entropy.
However, research examples demonstrating reversible computing achieved at a device level have yet to bereported.
Here, the author and his research team have joined in their efforts to realize practical reversible computing
by employing Adiabatic quantum-flux-parametron (AQFP) 4),5), a low-power superconducting logic gate.
AQFP gates can ultimately limit switching energies by adiabatically changing the circuit potential energy. Up
to now, experimental demonstrations of 10 zJ bit-energy operations 6) as well as demonstrations of sub-kBT
bit-energy operations using numerical analysis 7) have been successfully undertaken. Additionally, it has
been proved that the energy-time product was engineered as having approximately the small dimensionsof the quantum-mechanical limit (ħ/2) 8). The author considers these research findings to show that AQFP
logic is a good candidate for use as a low-power building block composed of actual reversible computing.
A physically reversible device is desired in circuit design in order to perform reversible logic operations, 9).
Figure 1a shows a reversible quantum-flux-parametron (RQFP) gate 10), which the author’s research teamhas proposed as reversible logic gate. The RQFP gate comprises of three Splitter (SPL) gates and three
Majority (MAJ) gates. Since SPLs and MAJs have the same circuit topography as an AQFP logic circuit, an
RQFP gate is symmetrical or proven to be physically reversible. Figure 1b shows simulation data of theenergy dissipation for a logic operation using a RQFP gate. For all input data combinations the energy
dissipation decreases almost linearly with an increase in the rise/fall time for AC bias currents,
demonstrating that there is no minimum energy dissipation for reversible logic operations. The reason
behind this is because the potential energy of all AQFPs comprising RQFP gates varies adiabatically as aresult of the physical reversibility characteristics. The above-mentioned research findings successfully
demonstrated the first practical reversible computing using RQFP gates with unlimited minimum energy
dissipation. RQFP logic operations were also demonstrated successfully utilizing Nb integrated circuit
fabrication process technology developed by the Advanced Industrial Science and Technology.
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In non-adiabatic circuits that include CMOS, large amounts of power exceeding kBT for computation are
required since the minimum energy dissipation is thermodynamically determined. Contrary to this,reversible computing utilizing RQFP gates proposed in this article can perform low-power logic operations
with smaller amounts of energy than kBT, which is anticipated to be greater than six-order superiority from ahigh-energy efficiency viewpoint. Future plans include further research studies towards the realization ofpractical ultra low-power reversible computation using RQFP gates.

Fig. 1 RQFP gate (a) Block diagram – MAJ (a, b, c) = ab+bc+ca, (b) Simulated energy dissipation. Horizontal
axis shows a rise/fall time of AC-bias currents. Simulated energy dissipation including input/output buffer energy
consumption

Reference：
1) R. Landauer, “Irreversibility and Heat Generation in the Computing Process,” IBM J. Res. Dev., vol. 5, no.
3, pp. 183–191, Jul. 1961.
2) A. Bérut, A. Arakelyan, A. Petrosyan, S. Ciliberto, R. Dillenschneider, and E. Lutz, “Experimental
verification of Landauer’s principle linking information and thermodynamics.,” Nature, vol. 483, no. 7388,
pp. 187–9, Mar. 2012.
3) E. Fredkin and T. Toffoli, “Conservative logic,” Int. J. Theor. Phys., vol. 21, no. 3–4, pp. 219–253, Apr.
1982.
4) N. Takeuchi, D. Ozawa, Y. Yamanashi, and N. Yoshikawa, “An adiabatic quantum flux parametron as anultra-low-power logic device,” Supercond. Sci. Technol., vol. 26, no. 3, p. 035010, Mar. 2013.
5) N. Takeuchi, K. Ehara, K. Inoue, Y. Yamanashi, and N. Yoshikawa, “Margin and Energy Dissipation of
Adiabatic Quantum-Flux-Parametron Logic at Finite Temperature,” IEEE Trans. Appl. Supercond., vol.
23, no. 3, p. 1700304, Jun. 2013.
6) N. Takeuchi, Y. Yamanashi, and N. Yoshikawa, “Measurement of 10 zJ energy dissipation of adiabatic
quantum-flux-parametron logic using a superconducting resonator,” Appl. Phys. Lett., vol. 102, no. 5, p.
052602, 2013.
7) N. Takeuchi, Y. Yamanashi, and N. Yoshikawa, “Simulation of sub-kBT bit-energy operation of adiabatic
quantum-flux-parametron logic with low bit-error-rate,” Appl. Phys. Lett., vol. 103, no. 6, p. 062602, 2013.
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8) N. Takeuchi, Y. Yamanashi, and N. Yoshikawa, “Energy efficiency of adiabatic superconductor logic,”
Supercond. Sci. Technol., in press.
9) K. K. Likharev, “Classical and quantum limitations on energy consumption in computation,” Int. J. Theor.
Phys., vol. 21, no. 3–4, pp. 311–326, Apr. 1982.
10) N. Takeuchi, Y. Yamanashi, and N. Yoshikawa, “Reversible logic gate using adiabatic superconducting
devices,” Sci. Rep., vol. 4, p. 6354, Sep. 2014.

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Feature Article: Cryogenic Digital Devices-Superconducting Detector and Hybrid Device Using SFQ Circuits
Takekazu Ishida, Professor
Graduate School of Engineering, Osaka Prefecture University
Osaka Prefecture University (Ishida) andNagoya University (Fujimaki) have proposed abasic principle to detect changes in kinetic
inductance in the time domain triggered bydifferent
external
sourcesalongacurrent-biased superconducting nanowire,
proving the transient voltage pulse. The
detector is termed a current-biased kinetic
inductance detector (CB-KID), and the diagramin Figure 1 shows its principle of operation. The
detector can be applicable in a variety ofFig. 1. Principle of current-biased kinetic inductance detector
external-signal
detection
applications.
High-quality as-grown Nb thin films were
patterned in a compact meander geometry producing a Nb nanowire. The central part of themeander-patterned Nb nanowire is enriched with a 10B (200 nm) utilizing Molecular Beam Epitaxy (MBE).
Here, the Nb nanowire remains superconductive when current flows. The research team based at Osaka
Prefecture University have demonstrated that nuclear reactions between neutrons and the boron nuclei inthe multilayer boron film breaks some Cooper pairs in the superconducting Nb nanowires since the nuclear
reactions yields 4He-7Li nuclei releasing significant thermal energies of 2.3 MeV in the opposite direction.
Thus, the demonstrations have verified that the breakage of some Cooper pairs caused a change in kinetic
inductance, leading to the capability of neutron detection by measuring the transient voltage signals.
Whilst conventional neutron detectors requirelarge-scale high voltage power supplies, thisnew-type compact/lightweight neutron detector
proposed by Osaka Prefecture University has
been successful in detecting neutrons, offering
advantageous operational characteristics atonly several volts. The detector is not based onrare nuclear reactions of 3He, and can be
operated at 4K to detect changes in kinetic
inductance. These advantages allow theassembly of single flux quantum (SFQ) circuits
operating at 4K to be placed in series withmultiplexing readout systems. As shown in
Figure 2, the author’s research teamstandardized an 80-pin package into 22 mm x
March, 2015

Red: bias
current port (+)
Blue: bias
Current port (-)

Fig. 2. The Nb nanowire detector mounted at the centerof a 22mmx22mm Si-substrate, surrounded by 80-units
of SFQ circuit electrode pads.

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22 mm size. By having X directional and Y directional sensor arrays, it is possible to assemble thesuperconducting detector (CB-KID), the neutron reactive layer (10B) and single flux quantum (SFQ)
read-out circuit all together onto a single chip. This could potentially lead to a superconducting chip similar tosemiconductor-based CMOS. Scaled–down 0.6μm-wide Nb nanowaire with sub-micron spatial resolution
could revolutionize technology able to realize one million pixels imaging in the near future. Specifically, thedetector is of interest as a performance enhancement for SFQ read-out circuit.
At the Japan Proton Accelerator Research Complex (J-PARC), demonstrations of neutron detection using
pulsed neutrons have concluded. Figure 3 shows a picture of the demonstration system with the element
mount that was utilized. Practical realization of this new imaging system will pave the way for neutron
detectors having ultra-high speeds, greater resolutions and ultra-high spatial resolution capabilities, as thepotential tool applicable towards innovating existing technologies, neutron intensity measurements at
nuclear reactor sites, non-destructive testing, new materials development and local-field distribution such as
spintronics. Ultimately, further contribution towards progress in material science and magneto-science isanticipated.

Fig.3. 4K refrigerator cryostat and 76-units of semi-rigid cables having the CB-KID installed in the middle

This research was undertaken in collaborations with Osaka Prefecture University (Takekazu Ishida,
Shigeyuki Miyajima, Hiroaki Shishido, Yoshito Narukami, Naoto Yoshioka, Hirotaka Nakayama, andHiroyuki Yamaguchi), Nagoya University (Akira Fujimaki), AIST (Mutsuo Hidaka), J-PARC (Kenichi Oikawa,
Masahide Harada, Takayuki Oku, and Masatoshi Arai). The research is also supported by the fundamental
research (S) entitled “All solid state superconducting system for neutron radiography with one million pixels
and submicron resolution” (Takekazu Ishida et al) Grant Number:23226019, and Materials and Life Science
Experimental Facility based at the J-PARC, 2013P0800 entitled “Development and the application ofneutron optical devices and detector system” (Takayuki Oku et al).

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