Superconducting-Ferromagnetic Transistor - IEEE Xplore

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 4, AUGUST 2014

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Superconducting-Ferromagnetic Transistor Ivan P. Nevirkovets, Oleksandr Chernyashevskyy, Georgy V. Prokopenko, Oleg A. Mukhanov, Fellow, IEEE, and John B. Ketterson

(Invited Paper)

Abstract—We report experimental results on the dc and ac characterization of multiterminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx ), and a ferromagnetic material (Ni), respectively), which display transistor-like properties. We investigated two types of such superconducting–ferromagnetic transistors (SFTs): ordinary devices with a single acceptor (SIS) junction, and devices with a double acceptor. The devices with the single SIS acceptor were investigated and demonstrated a modulation of the maximum Josephson current as a function of the SFIFS current injection level. For devices of the second type, by applying an ac signal (in the kilohertz range) with a constant dc bias current to the injector (SFIFS) junction, we observed a voltage gain of about 25 on the double acceptor with the operating point chosen in the subgap region of the acceptor current-voltage characteristic. We also observed an excellent input–output isolation in our SFIFSIS devices. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications. Index Terms—Ferromagnetic–superconducting hybrid structures, Josephson effect, proximity effect, quasiparticle injection, superconducting transistor, superconductivity.

I. I NTRODUCTION

S

UPERCONDUCTING electronics offers unprecedented speed, sensitivity, and low energy consumption, which no other technology can match [1], [2]. However, the basic device of current superconducting electronics—the Josephson junction—is intrinsically a two-terminal device, which significantly limits its functionality. Development of superconducting three-terminal devices with good input/output isolation, the capability to amplify signals, and working at 4.2 K is impor-

Manuscript received December 31, 2013; revised February 7, 2014; accepted March 11, 2014. Date of publication April 17, 2014; date of current version May 15, 2014. This work was supported in part by the Army Research Office under Contract W911NF-09-C-0036. This paper was recommended by Associate Editor A. Kleinsasser. I. P. Nevirkovets and O. Chernyashevskyy are with the Department of Physics and Astronomy, Northwestern University, Evanston, IL 602083112 USA (e-mail: [email protected]; o-chernyashevskyy@ northwestern.edu). G. V. Prokopenko and O. A. Mukhanov are with HYPRES, Inc., Elmsford, NY 10523 USA (e-mail: [email protected]; [email protected]). J. B. Ketterson is with the Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208-3112 USA, and also with the Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208-3118 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2014.2318317

tant for many superconducting applications. This has been an elusive dream for many years inspiring different implementation approaches [3]–[12]. However, such devices have not yet demonstrated practically useful performance. An early version of a superconducting transistor-like device was the quiteron [4]. The quiteron has a symmetric twojunction S1 IS2 IS3 structure (here S and I denote a superconductor and an insulator, respectively); one tunnel junction (e.g., S1 IS2 ) is called the acceptor, and the second tunnel junction (S2 IS3 ), stacked on top of the first one, is called the injector The injector current (at a voltage V ≥ (Δ2 + Δ3 )/e, where Δi is the superconducting energy gap in the respective superconductor Si ), produces a nonequilibrium quasiparticle population in the middle S2 layer, thereby suppressing its energy gap Δ2 and modifying the current-voltage characteristic (CVC) of the acceptor. Since the S2 layer is common to both the injector and the acceptor, biasing the acceptor junction at V ≈ (Δ1 + Δ2 )/e affects the CVC of the injector junction to nearly the same degree as the injector junction affects the CVC of the acceptor. This parasitic back action leads to a lack of isolation between the input and output and to latching logic operation, which was the main obstacle for implementing the devices into circuits. In recent years, a device called the quatratran was proposed [8], [9]. This is a three-terminal device with an N1 IS1 N2 IS2 structure (where N is a normal metal), composed of two stacked tunnel junctions, similarly to the device described above. The operating principle of this device is based on quasiparticle trapping in the N2 layer. A current through the N1 IS1 junction injects quasiparticles into the S1 layer. They are then trapped in the N2 layer where they lose energy and excite more quasiparticles. Due to the increased quasiparticle population, the current through the N2 IS2 junction increases, resulting in a current amplification. However, the current gain of this type of device was not confirmed in a later work [13]. In addition, since the operation of the device is based on quasiparticle trapping in the superconducting Al (known to be a “slow” material [14]), the operating speed may be too low, and therefore, this may be a limiting factor for some applications. Some of the more recent proposals for amplifying devices are hard to realize experimentally [10], [11], or they are very specialized [12]. Aiming to develop a superconducting transistor suitable for wide applications, we have explored hybrid superconductor– ferromagnet multiterminal devices [15]–[18]. Our goal is to

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design a device with the following basic properties: (1) voltage and current amplification; (2) good input/output isolation; and (3) compatibility with existing Nb/Al Josephson junction technology. In earlier work one of the co-authors [15]–[17] showed that introducing a thin ferromagnetic (F) layer between the insulating barrier and the superconducting middle electrode on injector side (i.e., making SIFSIS device instead of a “symmetric” SISIS one) significantly improves input/output isolation in the quasiparticle-injection type three-terminal devices. Further improvement of the isolation is possible in SFIFSIS devices [18] due to the fact that the CVC of the injector SFIFS junction is linear, and is not sensitive to perturbations such as temperature variations, quasiparticle injection, etc. Furthermore, introducing a ferromagnetic layer to the structure offers the possibility to exploit the additional spin degree of freedom in a more subtle way, e.g., by making π-junctions [19], [20] or structures with non-conventional pairing mechanisms [21], with the potential to significantly extend the functionality of the devices. The use of superconductor–ferromagnet structures for the implementation of multiterminal SFT devices complements the recent interest in superconductor–ferromagnet magnetic Josephson junctions (MJJs) for low-power, high-density cryogenic magnetic memories [22]–[28] for high-end computing applications [29], [30]. The compatibility of electrical properties and fabrication processes of SFT, MJJ, and JJ devices offers the opportunity to combine these devices in memory circuits. The high input/output isolation expected in SFT devices will be quite useful in random access memory (RAM) arrays for implementation of memory cell selectors. The SFT gain will be indispensable in RAM line drivers and sense circuits. It is expected that SFT devices should also be compatible with various energy-efficient single flux quantum (SFQ) digital circuits [31] and extend the SFT applicability to digital and mixed-signal applications. Recently, we have demonstrated that SFIFSIS SFT devices display an excellent input/output isolation and a voltage gain above unity [18]. Here, we show that SFT devices with improved junction quality are able to display a considerable voltage gain; in addition, the Josephson current of the acceptor SIS junction can be controlled by current injection from the SFIFS injector junction into the common S electrode. II. SFIFSIS D EVICE S TRUCTURE Both three- and four-terminal devices were fabricated from Nb/Ni/Al/AlOx /Ni/Nb/Al/AlOx /Nb multilayers deposited on sapphire substrates and tested at 4.2 K. Nominal thicknesses of the layers were as follows: 120 nm and 80 nm for the bottom and top Nb layers, respectively; 4 nm for Ni layers; and 9 nm for the initial thickness of unoxidized Al used to form the Al/AlOx barrier. In this paper, we consider devices of the two types. Devices of the first type are single-acceptor, three-terminal devices with the thickness of the middle Nb layer of 30 nm. Lateral dimensions are 5 μm × 7.5 μm for the injector (SFIFS) and 5 μm × 5 μm for the acceptor (SIS) junctions. Fourterminal devices of the second type have two acceptor junctions

Fig. 1. SFT device. (a) Schematic cross-sectional view and biasing for type 1 (single-acceptor) device. (b) Microphotograph of the fabricated device. Panels (c) and (d) show the same for type 2 (double-acceptor) SFT.

connected in series; the thickness of the middle Nb layer in these devices is 35 nm. Lateral dimensions are 10 μm × 12 μm for the injector and 4 μm × 8 μm for each of two acceptor junctions. Fig. 1 shows a schematic cross-sectional view and the actual top view (optical microphotographs) for both types of devices (panels (a), (b) are for the type 1 device; panels (c) and (d) are for the type 2 device). Anodization and deposition of SiO2 was used for proper insulation of the electrodes from each other. More details on the device fabrication procedure can be found in [15]–[17]. III. C HARACTERISTICS OF THE S INGLE -ACCEPTOR D EVICE We consider devices in which the injector junction has specific tunneling resistance RT = 3.1 × 10−7 Ω × cm2 , and the acceptor junction has RT = 3.3 × 10−7 Ω × cm2 . All devices were characterized at 4.2 K. Current-voltage characteristics of a type 1 device having a single acceptor [see Fig. 1(a) and (b)] are shown in Fig. 2. The acceptor junction has a tunneling SIS-type CVC (curve 1). The Josephson critical current density for the junction is 4.4 × 103 A/cm2 . The SFIFS injector junction has a linear CVC (curve 2), in spite of the fact that both junctions have a common (FS) electrode. The physical reason for this was considered in detail in our prior work [15]–[17]. Briefly, if the thickness of the F layer is larger than the superconducting coherence length in it (which is the case here), then the superconducting correlations within the layer are suppressed and the nonlinear features associated with the superconductivity are not manifested in the tunneling CVC. Other properties of the F layers (magnetization orientation, domain structure etc.) do

NEVIRKOVETS et al.: SFT

Fig. 2. Current–voltage characteristics of the type 1 device at 4.2 K. Curve 1 is for the SIS acceptor junction; curve 2 is for the SFIFS injector junction.

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Fig. 4. Dependence of the maximum Josephson current of the acceptor junction on the level of current injection from the injector junction [see biasing schematic in Fig. 1(a)].

IV. C HARACTERISTICS OF THE D OUBLE -ACCEPTOR D EVICE

Fig. 3. Maximum Josephson current versus magnetic field dependence for the acceptor junction of the same device (cf. Fig. 2).

not play any role in the devices considered here. The fact that linear CVC of the SFIFS junction does not depend on external perturbations is important to eliminate the back action from device output to input. The dependence of the maximum Josephson current, Ic , vs. magnetic field (applied parallel to the device plane) for the acceptor junction of the same device is shown in Fig. 3, indicating a good quality of the junction. For intended memory applications, in which SFT operates as a memory cell selector integrated with MJJ memory element, it is important to be able to alter Josephson current in a controllable manner. Our devices provide this capability, as can be seen from Fig. 4. The figure shows dependence of the maximum Josephson current of the acceptor junction on the level of current injection, Ii , from the injector junction, according to the layout depicted in Fig. 1(a). Increasing the injection current results in a reduction of the Josephson current. It is expected that more efficient control (higher “gain” δIc /δIi ) can be achieved in optimized devices.

In our previous work [18], we demonstrated that multiterminal SFIFSIS devices exhibited a voltage gain above unity, and a high input–output isolation; the devices were tested in MHz range. Since the quasiparticle CVC of the acceptor junction (that serves as an output) is nonlinear, we choose the operating point in the “flat” subgap region to ensure the most efficient voltage amplification. There are two straightforward ways to increase the voltage amplification in this regime: reduce subgap “leakage” current, and use double-acceptor configuration proposed in [4]. Here, we exploit both ways. Next we consider the characteristics of the double-acceptor device with the structure shown in Fig. 1(c) and (d). In this device, the specific tunneling resistance is 3.9 × 10−7 Ω × cm2 for the injector and 2.5 × 10−7 Ω × cm2 for each of the acceptor junction; the Josephson critical current density for the acceptor junctions is 3.8 × 103 A/cm2 . The CVCs of this device are shown in Fig. 5. The measurements were done at 4.2 K. Curve 1 is the unperturbed CVC of the double acceptor, and curve 2 is the CVC of the injector SFIFS junction. Curve 3 is the double acceptor CVC recorded in applied magnetic field of 250 G; curve 4 is the same but recorded when an injection current of about −2.5 mA (corresponding to a voltage of −0.8 mV) was fed through the injector junction (point A in curve 2). Here, we present results of the experiment in which the operating point in the acceptor CVC was chosen at −1.35 mV (shown by the red point B in Fig. 5), and the injection level corresponded to the bias current at −0.8 mV across the injector junction (point A in curve 2). Fragments of the CVCs 3 and 4 including the operation point B (enclosed by the red rectangle in the main panel) are shown on a magnified scale in the inset of Fig. 5. The role of the injection is to obtain the appropriate shape of the subgap portion of the CVC in order to maximize the voltage amplification. The dc injection current level (and respective

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Fig. 5. Unperturbed CVC of the double acceptor (curve 1) and injector junction (curve 2) at 4.2 K for a device shown in Fig. 1(c) and (d). Orange curve 3 and red curve 4 is the subgap part of the acceptor CVC in an applied magnetic field of 250 G at zero injection current and under influence of injection at the level denoted by point A in curve 2, respectively. Red point B in curve 4 is the acceptor operation point in the “forward” transmission experiment. The portions of CVCs 3 and 4 enclosed by the red rectangle are shown on a magnified scale in the inset. Points C and D (open symbols) are operation points in the injector and acceptor CVC, respectively, in the “reverse” transmission experiment.

power consumption) can be minimized by proper choice of the device parameters, in particular, the superconducting transition temperature, Tc , the thickness of the middle S layer, lateral dimensions, and the transparency of the injector barrier. In real circuits, pulsed injection will be used, resulting in further reduction of the power consumption. On top of the dc injection offset, we applied a small ac signal across the injector junction (i.e., device input) in the range from several hundreds Hz to several kHz, and observed the response at the device output by measuring the ac component of the acceptor voltage, Va , as shown in Fig. 1(c). An image of the oscilloscope screen where both input (channel 1) and output (channel 2) signals are displayed is shown in Fig. 6. The voltage scale is 20 μV per division for channel 1, and 500 μV per division for channel 2 (both signals were preamplified by a factor of 1000). One can infer from Fig. 6 that the voltage gain is about 25, so that the input signal with the peak-to-peak amplitude of about 20 μV is amplified to about 0.5 mV. In order to verify the isolation property of this device, we swapped the input and output. In a particular case shown in Fig. 7, the dc offset on the SIS junction (now serving as the input) was 2.2 mV (point D in curve 4; cf. Fig. 5), superimposed with an ac signal (displayed in channel 1). The response was observed on the SFIFS junction (detected in channel 2) biased at 0.3 mA (point C in curve 2; cf. Fig. 5). One can see that, within the noise level, no reverse transmission was detected, which implies very good input–output isolation in our device. One may raise a question as to what is the role of Joule heating in operation of these devices. We tried to minimize heating by using relatively thin Nb films and the sapphire substrates. Sapphire is known to have very good thermal conductivity and thus provides efficient heat removal. It is known that the thermal phonons (with the energies less than twice the superconduct-

Fig. 6. Input signal (channel 1) is fed through the SFIFS junction, and the respective output signal (channel 2) is detected on the SIS double acceptor. The voltage scale is 20 μV per division for channel 1 and 500 μV per division for channel 2.

Fig. 7. Reverse transmission experiment: Input signal (channel 1) is fed through the SIS double acceptor, and the response (channel 2) is observed on the SFIFS junction. The voltage scale is 500 μV per division for channel 1 and 5 μV per division for channel 2.

ing gap) have large mean free path, unlike the recombination phonons with the energies equal to or greater than twice the superconducting gap [32]. Since the injector (SFIFS) junction is the bottom junction in contact with the sapphire substrate, and the total thickness of the bottom electrode is about 130 nm, we suggest that most of the thermal phonons created in the injector junction escape into the substrate. On the other hand, the recombination phonons created when the injected quasiparticles recombine into Cooper pairs are short-lived, and they are easily trapped in the Nb electrodes, thus the energy gap is affected in both Nb electrodes of the acceptor junction. Reabsorption of the recombination phonons slows down the temporal response of the superconducting nonequilibrium devices. According to analysis performed by Iguchi [33], for Nb-based devices with reasonably thin films, one may expect operation in GHz range. Faster operation can be achieved for materials with higher Tc and lower electron density of states.

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Considering the observed properties and theoretical estimates [16], [33], we believe that the SFT device can potentially serve as a cryogenic preamplifier in superconducting electronics circuits. The high input/output isolation of SFT can be useful working as a memory cell selector device in superconducting magnetic memory arrays similar to an isolating transistor in MRAM memory cells. The SFT device can also be valuable as a readout device in certain types of cryogenic memories. V. C ONCLUSION We fabricated and studied dc and ac characteristics of multiterminal SFIFSIS (Nb/Ni/Al/AlOx /Ni/Nb/Al/AlOx /Nb) devices at 4.2 K. We observed modulation of the maximum Josephson current of the SIS acceptor junction under the influence of current through the SFIFS injector junction. A voltage gain about 25 is demonstrated in devices with the double acceptor when the acceptor junctions are biased in the subgap region of the current-voltage characteristic. In comparison with formerly studied SISIS- and SIFSIS-type devices, the SFIFSIS devices display considerably better input/output isolation. The main effect of introducing the ferromagnetic layers in the injector junction is to create a linear CVC that is insensitive to back action from the acceptor junction; at the same time, the thin ferromagnetic layers do not prevent tunneling injection of quasiparticles into the middle S layer. This combination of properties in nonequilibrium superconducting devices is hard to realize without ferromagnetic layers. It is worth noting that in the current version of SFT devices, magnetic layers role is different from some known for magnetic Josephson junctions [22]–[28]: in the latter case, the magnetic layers serve to switch Josephson current magnitude between different values, whereas in our device, the magnetic layers suppress Josephson current in the injection junction completely. Optimization of the device parameters would result in further improvement of its characteristics. We suggest that SFIFSIS devices are promising new devices with a significant potential for various applications in superconducting electronics. ACKNOWLEDGMENT The authors would like to thank I. Vernik for the useful discussions and M. Manheimer and S. Holmes for their encouragement. R EFERENCES [1] T. Van Duzer, “Superconductor digital electronics past, present, future,” IEICE Trans. Electron., vol. E91-C, no. 3, pp. 260–271, Mar. 2008. [2] K. Likharev and V. Semenov, “RSFQ logic/memory family: A new Josephson-junction technology for sub-terahertz clock-frequency digital systems,” IEEE Trans. Appl. Supercond., vol. 1, no. 1, pp. 3–28, Mar. 1991. [3] E. Gray, “Superconducting transistor,” U.S. Patent 4 157 555, Jun. 5, 1979. [4] S. M. Faris, S. I. Raider, W. J. Gallagher, and R. E. Drake, “Quiteron,” IEEE Trans. Magn., vol. MAG-19, no. 3, pp. 1293–1295, May 1983. [5] I. Iguchi and A. Nishiura, “Transient response of a nonequilibrium superconductor to pulsed injection of quasiparticles through a tunnel barrier,” J. Low Temp. Phys., vol. 52, no. 3/4, pp. 271–287, Mar. 1983. [6] W. J. Gallagher, “Three-terminal superconducting devices,” IEEE Trans. Magn., vol. MAG-21, no. 2, pp. 709–716, Mar. 1985.

[7] B. D. Hunt, R. P. Robertazzi, and R. A. Buhrman, “Gap suppression devices,” IEEE Trans. Magn., vol. MAG-21, no. 2, pp. 717–720, Mar. 1985. [8] N. Booth, P. Fisher, M. Nahum, and J. Ullom, “A superconducting transistor based on quasiparticle trapping,” Supercond. Sci Technol., vol. 12, no. 8, pp. 538–554, Aug. 1999. [9] G. P. Pepe et al., “Superconducting device with transistor-like properties including large current amplification,” Appl. Phys. Lett., vol. 77, no. 3, pp. 447–449, Jul. 2000. [10] F. Giazotto and J. P. Pekola, “Josephson tunnel junction controlled by quasiparticle injection,” J. Appl. Phys., vol. 97, no. 2, pp. 023908-1– 023908-4, Jan. 2005. [11] F. Giazotto, F. Taddei, R. Fazio, and F. Beltram, “Huge nonequilibrium magnetoresistance in hybrid superconducting spin valves,” Appl. Phys. Lett., vol. 89, no. 2, pp. 022505-1–02250531, Jul. 2006. [12] S. Pagano et al., “Nano-strip three-terminal superconducting device for cryogenic detector readout,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 717–720, Jun. 2011. [13] H. Myoren, A. Shigihara, T. Taino, and S. Takada, “Quasiparticles injection effects for Nb/Al/AlOx /Al/Nb tunnel junctions at 0.4 K using stacked Josephson junctions,” J. Phys., Conf. Ser., vol. 43, pp. 1326–1329, 2006. [14] S. B. Kaplan et al., “Quasiparticle and phonon lifetimes in superconductors,” Phys. Rev. B, Condens. Matter, vol. 14, no. 11, pp. 4854–4873, Dec. 1976. [15] I. P. Nevirkovets, “A superconducting transistorlike device having good input-output isolation,” Appl. Phys. Lett., vol. 95, no. 5, pp. 052505-1– 052505-3, Aug. 2009. [16] I. P. Nevirkovets, “A superconducting transistor with improved isolation between the input and output terminals,” Supercond. Sci. Technol., vol. 22, no. 10, p. 105 009, Oct. 2009. [17] I. P. Nevirkovets and M. A. Belogolovskii, “Hybrid superconductorferromagnet transistor-like device,” Supercond. Sci. Technol., vol. 24, no. 2, p. 024009, Feb. 2011. [18] G. V. Prokopenko, O. A. Mukhanov, I. P. Nevirkovets, O. Chernyashevskyy, and J. B. Ketterson, “DC and RF measurements of superconducting-ferromagnetic multi-terminal devices,” in Proc. IEEE 14th ISEC, Cambridge, MA, USA, Jul. 7–11, 2013, pp. 1–4, PF12. [19] V. V. Ryazanov et al., “Coupling of two superconductors through a ferromagnet: Evidence for a π junction,” Phys. Rev. Lett., vol. 86, no. 11, pp. 2427–2430, Mar. 2001. [20] T. Yamashita, S. Takahashi, and S. Maekawa, “Controllable π junction with magnetic nanostructures,” Phys. Rev. B, Condens. Matter, vol. 73, no. 14, pp. 144517-1–144517-4, Apr. 2006. [21] A. F. Volkov, F. S. Bergeret, and K. B. Efetov, “Odd triplet superconductivity in superconductor-ferromagnet multilayered structures,” Phys. Rev. Lett., vol. 90, no. 11, pp. 117006-1–117006-1, Mar. 2003. [22] V. V. Ryazanov et al., “Magnetic Josephson junction technology for digital and memory applications,” Phys. Proc., vol. 36, pp. 35–41, 2012. [23] T. I. Larkin et al., “Ferromagnetic Josephson switching device with high characteristic voltage,” Appl. Phys. Lett., vol. 100, no. 22, pp. 222601-1– 222601-5, May 2012. [24] I. V. Vernik et al., “Magnetic Josephson junctions with superconducting interlayer for cryogenic memory,” IEEE Trans. Appl. Supercond., vol. 23, no. 3, p. 1701208, Jun. 2013. [25] S. V. Bakurskiy et al., “Theoretical model of superconducting spintronic SIsFS devices,” Appl. Phys. Lett., vol. 102, no. 19, pp. 192603-1– 192603-4, May 2013. [26] E. Goldobin et al., “Memory cell based on a ϕ Josephson junction,” Appl. Phys. Lett., vol. 102, no. 24, pp. 242602-1–242602-4, Jun. 2013. [27] B. Baek, W. H. Rippard, S. P. Benz, S. E. Russek, and P. D. Dresselhaus, Hybrid Superconducting-Magnetic Memory Device Using Competing Order Parameters. [Online]. Available: http://lanl.arxiv.org/abs/1310.2201 [28] M. Qader et al., “Switching at small magnetic fields in Josephson junctions fabricated with ferromagnetic barrier layers,” Appl. Phys. Lett., vol. 104, no. 2, pp. 022062-1–022062-5, Jan. 2014. [29] D. S. Holmes, A. L. Ripple, and M. A. Manheimer, “Energy-efficient superconducting computing—Power budgets and requirements,” IEEE Trans. Appl. Supercond., vol. 23, no. 3, p. 1701610, Jun. 2013. [30] S. Nishijima et al., “Superconductivity and the environment: A roadmap,” Supercond. Sci. Technol., vol. 26, no. 11, p. 113 001, Sep. 2013. [31] O. A. Mukhanov, “Energy-efficient single flux quantum technology,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 760–769, Jun. 2011. [32] M. R. Hauser, R. Gaitskell, and J. P. Wolfe, “Imaging phonons in a superconductor,” Phys. Rev. B, Condens. Matter, vol. 60, no. 5, pp. 3072– 3075, Aug. 1999. [33] I. Iguchi, “Evaluation of the performance of superconducting nonequilibrium devices,” J. Appl. Phys., vol. 59, no. 2, pp. 533–539, Jan. 1986.

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Ivan P. Nevirkovets received the M.S. degree in physics from the National T. Shevchenko University of Kyiv, Kyiv, Ukraine, in 1977 and the Ph.D. degree in solid-state physics and the Dr.Sc. degree in superconductivity from G. V. Kurdyumov Institute for Metal Physics of the National Academy of Sciences of Ukraine, Kyiv, in 1985 and 2000, respectively. In 1997, he joined the Department of Physics and Astronomy, Northwestern University, Evanston, IL, USA, as a Visiting Scholar, and became a Research Assistant Professor in 2002. His research interests include tunneling in superconductors, hybrid superconductor–ferromagnet structures, and electric transport in low-dimensional nanodevices.

Oleksandr Chernyashevskyy received the M.S. degree in physics from the National T. Shevchenko University of Kyiv, Kyiv, Ukraine, in 1977 and the Ph.D. degree in solid-state physics from G. V. Kurdyumov Institute for Metal Physics of the National Academy of Sciences of the Ukraine, Kyiv, in 1990. In 2002, he joined Department of Physics and Astronomy, Northwestern University, Evanston, IL, USA, as a Visiting Scholar. He is currently a Senior Research Associate. His current research interests include electric transport in superconducting multiterminal devices.

Georgy V. Prokopenko received M.S. degree in physics from Moscow Institute of Physics and Technology, Moscow, Russia, in 1982 and the Ph.D. degree in physics from the Institute of Radio Engineering and Electronics, Russian Academy of Science, Moscow, Russia, in 2003. In 2009, he joined HYPRES Inc., Elmsford, NY, USA, as member of technical staff to work on a SQUID array for antenna and low-noise amplifier applications. Prior to HYPRES, he was a Visiting Researcher with NIST (USA), AIST (Japan), Denmark Technical University (Denmark), Forschungszentrum Jülich, (Germany), Technical University of L’Aquila (Italy), and Chalmers University of Technology (Sweden). He also worked as Staff Scientist with the Institute of Radio Engineering and Electronics, Russian Academy of Science. His research interest include broadband low-noise SQUID amplifiers, SQUID and SQIF arrays, metamaterials, SIS mixers, and new superconducting devices for memory and RF applications.

Oleg A. Mukhanov (M’94–SM’01–F’12) received the M.S. degree in electrical engineering from Moscow Engineering Physics Institute, Moscow, Russia, in 1983 and the Ph.D. degree in physics from Moscow State University, Moscow, in 1987. In April 1991, He joined HYPRES as a superconductive integrated circuit designer to initiate the development of digital circuits based on rapid single flux quantum logic family. He is currently the Chief Technology Officer and Senior Executive Vice President with HYPRES Inc., Elmsford, NY, USA. He has initiated and been involved in multiplicity of HYPRES projects, including development of energy-efficient digital and memory circuits, high-performance A/D, D/A, and T/D converters, fast Fourier transform hardware, SQUID arrays, and high-speed digital signal processors for a variety of applications including computing, communications, radar, EW, and instrumentation. Prior to joining HYPRES, he was a Staff Scientist with the Cryoelectronics Laboratory, Moscow State University, where he worked on the single-flux-quantum digital devices since 1984. His current research interest include superconducting energy-efficient digital and memory circuits, superconducting–ferromagnetic devices, SQUID arrays, metamaterials, and quantum computing readout and control circuits.

John B. Ketterson received the Ph.D. degree in physics from the University of Chicago, Chicago, IL, USA, in 1962. He is currently a Professor with the Department of Physics and Astronomy and with the Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL, USA. He is an Experimental Condensed Matter Physicist with wide ranging interests, including superconductors, superfluids, magnetic nanoarrays, and plasmonic–photonic structures. In particular, he has written or edited several books on these topics.