ORAL SESSION 4D5: Superconducting Electronics

Thursday, Feb. 24, 10:00 a.m. – 12:15 p.m., Room 301F (GRB)

Chairs: H. Weinstock (AFOSR), D. Oates (MIT Lincoln Lab)

4D5.1 HTS SQUIDS: Status and Biological Applications

John Clarke, Department of Physics, University of California, Berkeley, CA 94720-7300 and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Presenting Author: J. Clarke

The current status of HTS dc SQUIDs is briefly reviewed. An HTS, first-derivative gradiometer is described, based on a planar, asymmetric flux transformer with a baseline of 48 mm. The outputs from two such gradiometers are subtracted to form a second-derivative gradiometer; in addition, the outputs from three orthogonal magnetometers are subtracted to reduce the response to uniform magnetic fields. The overall balance of the system, 20 ppm, is sufficient to obtain magnetocardiographs in an unshielded environment. The SQUID "microscope" consists of an HTS SQUID mounted just below a silicon nitride window. The microscope is used to study the binding of antigens to antibodies tagged with superparamagnetic particles. The antigen is attached to substrate. After a magnetic field has been applied briefly, the Néel relaxation of the magnetic particle is observed only if the antibody has been immobilized by binding to the antigen.

This work was performed in collaboration with M. Adamkiewicz, M. Alper, J. Borgmann, B.B. Buchanan, Y.R. Chemla, H-M. Cho, V. Gorgadze, H.L. Grossman, R.H. Koch, K.A. Kouznetsov, R. McDermott, Y. Poon and R. Stevens and supported by the U.S. Department of Energy under contract number DE-AC03-76SF00098.

4D5.2 Progress in Understanding of High Temperature SQUID

Alex I. Braginski*, Institut fuer Schicht- und Ionentechnik (ISI), Forschungszentrum Juelich GmbH (FZJ), D-52425 Juelich, Germany. *Present address: Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.

Presenting Author: A.I. Braginski

Conventional theories of dc and rf SQUIDs treat thermal fluctuations as a small perturbation. They are well verified experimentally for low-temperature (LTS) SQUIDs operating at or near liquid helium temperature. However, experimental data collected since early 1990s have been suggesting that conventional theories might not be sufficient to fully explain the properties of high-temperature (HTS) SQUIDs. I will mention earlier work pointing at these divergences between theory and experiment, or attempting to reconcile these, especially by Enpuku [1] for dc SQUIDs, and by Il'ichev et al [2] for rf SQUIDs. However, I wish to concentrate on overviewing the recent work performed at Juelich for both, dc and rf SQUIDs. The analytical theories of dc and rf SQUID by Chesca are based on analytical solutions of the Fokker-Planck equation and are applicable in the limit of high thermal fluctuations [3,4]. These theories could explain several experimentally observed features of SQUIDs behavior, such as the output voltage modulation dependence on temperature and loop inductance, the dependence of energy resolution on the noise parameter of dc SQUIDs, the ability of HTS rf SQUIDs to operate with high loop inductances, etc. In a restricted range of parameters, and with useful refinements by Greenberg for rf SQUIDs [5], it was possible to make a systematic comparison of theory with experiments, which were performed by Barthel et al. for dc SQUIDs [6], and by Zeng et al. for rf SQUIDs [7]. The agreement between experiment and theory is remarkably good for both types of SQUID, except for the thermal noise level in dc SQUIDs at low noise parameter values <1. In most, but not all, cases the measured noise is significantly higher than the theoretical prediction. The work has shown that at and near 77K the sensitivities of dc and rf SQUID are comparable. The optimum energy resolution of rf SQUID is obtained in highly dispersive (non-hysteretic) mode of operation, which extends to high values of the rf SQUID parameter exceeding 3, in contrast to LTS rf SQUID's optimum at the parameter value of 1.

[1] K. Enpuku et al., J. Appl. Phys. 78, 3498 (1995).

[2] E. Il'ichev et al., J. Low Temp. Phys. 106, 503 (1997).

[3] B. Chesca, J. Low Temp. Phys. 110, 963 (1998).

[4] B. Chesca, J. Low Temp. Phys. 112, 165 (1998).

[5] Ya. Greenberg, J. Low Temp. Phys. 114, 297 (1999).

[6] K. Barthel et al., Appl. Phys. Lett. 74, 2209 (1999) .

[7] X. Zeng et al., subm. to J. Appl. Phys.

4D5.4 Coherent Emission from Josephson-Junction Arrays*

Branimir Vasilic 1, Christopher J. Lobb 1, Sergey V. Shitov 2, and Paola Barbara 1. 1 Center for Superconductivity Research, Department of Physics, University of Maryland, College Park MD-20742-4111, USA. 2 IREE, Russia Academy of Sciences, Mokhovaya 11, 103907 Moscow, Russia.

Presenting Author: P. Barbara

We detected mm-wavelength radiation from two-dimensional arrays of underdamped Josephson junctions [1]. The emitted radiation shows novel features: coherent emission above a threshold number of activated junctions, synchronization up to array sizes bigger than the free space radiation wavelength, and DC-AC power conversion efficiency higher than 30%.

The experimental results suggest that Josephson junctions can synchronize in a manner analogous to atoms in a laser. A few theoretical works investigated this analogy in the early 70s’ and developed a formal comparison between radiative processes in Josephson junctions and two-level atoms. The experimental results can be described in terms of a model first introduced by Tilley [2], predicting that interconnected Josephson junctions can self-synchronize with a common radiation field, similarly to superradiant atoms in a resonant cavity.

[1] P. Barbara, A.B. Cawthorne, S.V. Shitov, and C.J. Lobb, Phys. Rev. Lett. 82, 1963 (1999).

[2] D.R. Tilley, Phys. Lett. 33A, 205 (1970).

*Project funded by the Air Force Office of Scientific Research under grant No. F49620-98-1-0072

4D5.5 High-T_c Superconducting rf Receiver Coils for Magnetic Resonance Imaging of Small Animals

Jaroslaw Wosik 1, Lei-Ming Xie 1, Mikhail Strikovski 1, Feinian Wang 1, Krzysztof Nesteruk 2, Mehmet Bilgen 3, and Ponnada A. Narayana 3. 1 Texas Center for Superconductivity at University of Houston and Electrical and Computer Engineering Department, University of Houston, Houston, TX 77204, USA. 2 Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/36, PL-02 668 Warszawa, Poland. 3 Radiology Department, University of Texas-Houston Medical School, 6431 Fannin, Houston, TX 77030, USA.

Presenting Author: J. Wosik

In the case of small-volume magnetic resonance imaging (MRI), the noise of the probe coil and/or preamplifier (not a body noise) set the system noise floor and hence the MRI performance. Thus, in such a case it is desirable to use high-temperature superconductors (HTS) to reduce the thermal coil noise to improve the image resolution and reduce the image acquisition time. Indeed, several demonstrations have shown already that for selected applications, HTS MRI receiver coils perform significantly better than comparable copper coils. We report on HTS rf receiver surface probe designs (twin horseshoe and butterfly), developed for two different applications: (1) 2 Tesla MRI imaging of spinal cord injuries in small animals and (2) low-field MRI of small biological samples. The 2 Tesla probe is used in lieu of an implanted copper coil being currently used in research on spinal cord injuries.

The influence on the SNR of both an examined body and rf receiver coil was analyzed numerically using complete Maxwell equations solution and the reciprocity principle for a coil of a rectangular shape facing a finite lossy dielectric cylinder. The effects of coil size, position, NMR signals source location, as well as the ultimate limit of the SNR were analyzed. The HTS probes were designed with a virtual ground plane, thus reducing the coil-to-ground losses and making its resonance frequency less sensitive to its proximity to the body. Each coil was fabricated using patterned double-sided YBa₂Cu₃O_x films deposited either on sapphire or LaAlO₃ substrates. Designing and cryo-packaging of HTS MRI probes will be discussed. Comparison of images obtained with superconducting and cooled copper probes also will be shown.

4D5.6 Application of Low R_s YBCO Films to Patch Antennas

Shigetoshi. Ohshima, Masashi Mukaida, Masanobu Kusunoki, Katufumi Ehata, Toshiyuki Chiba, Kazuaki Suzuki and Momoko Inadomaru, Faculty of Engineering, Yamagata University, Yonezawa 992-8510, Japan

Presenting Author: S. Ohshima

In order to reduce the surface resistance (R_s) of YBCO films we have examined in-plane alignment of YBCO films on MgO substrates prepared by an inductive coupled plasma sputtering system and an eximer laser deposition system. We could obtained perfect in-plane alignment of YBCO films on such substrates by using BaSnO₃ (BSO) and CeO₂ buffer layers for both systems. We measured R_s values of YBCO films by a dielectric resonator method. The R_s value of YBCO/BSO/MgO was approximately 0.3 mohm at 21 GHz, 30 K. We designed and fabricated the patch antenna using YBCO films. The gain of YBCO patch antenna was approximately 3 dB higher than that of copper patch antenna. The power handling capability was affected by the quality of YBCO films. The cooling system for the HTS patch antenna was also examined. The system was quite compact and hand-portable, and the gain and directivity of the YBCO antenna did not seem to be affected by the cryocooler system.

4D5.7 Josephson Junction Devices for quantum computing

Vadim. B. Geshkenbein 1, Gianni Blatter 1, Alban L. Fauchere 1, Lev B. Ioffe 2, and Mikhail V. Feigel'man 3. 1 Theoretische Physik, ETH-Hoenggerberg, 8093 Zuerich, Switzerland. 2 Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA. 3 L.D. Landau Institute for Theoretical Physics, 117940 Moscow, Russia.

Presenting Author: V.B. Geshkenbein

I review the concept for the solid state implementation of the "quiet" qubit, the building block of a quantum computer, that is effectively decoupled from the environment. This implementation is based on macroscopic quantum coherent states in a superconducting interference loop. These loops either include Josephson contacts between s-wave and d-wave superconductor or conventional Josephson junctions and frustrating elements, e.g. p -contact. These devices naturally form degenerate two level systems. Starting from a basic five-junction loop, we show how to construct degenerate two-level junctions and superconducting phase switches. These elements are then effectively engineered into a superconducting phase qubit which operates exclusively with switches, thus avoiding permanent contact with the environment through external biasing.