4PO9-50 Operation of a High T_c Single Flux Quantum Shift Register at 75K

Young-Hwan Kim 1, Jong Hyuk Park 1, Joonhee Kang 2, Taek Sang Hahn 1, Chang Hoon Kim 1, Jae Min Lee 1, and Sang Sam Choi 1. 1 Korea Institute of Science and Technology, Seoul 130-650, Korea. 2 University of Inchon, Inchon 402-749, Korea.

Presenting Author: Y.H. Kim

A 4-bit single flux quantum(SFQ) shift register circuit using YBa₂Cu₃O_x (YBCO) bicrystal junctions has been designed, fabricated and tested. The circuit consists of 4 shift register stages and a read SQUID placed next to each side of the stages to monitor the data shifts. Each SQUID was inductively coupled to the nearby shift register stage, respectively. The circuit was tested with a computer-controlled digital measurement set-up and operated correctly. A data sequence "1010" as well as a simpler data "0000" or "1000" was successfully shifted through the shift register stage by controlled current pulses used as clock pulses. Data shift from one stage to the next was controlled by a current pulse injected to the junction bias lines. Our new homemade probe equipped with high speed coax lines were used in this experiment to reduce the interference of the two read SQUIDs. The inductance of the data storage cell was measured upon the size of cell to investigate the optimum size for the correct operation of the shift register. To control the inductance of the data storage cell in an easier fashion and to build more complicated circuits, we also fabricated shift register circuits using HTS ramp edge junctions.

4PO9-51 Whole-head SQUID system in a superconducting magnetic shield

H. Ohta 1,2, M. Aono 2, T. Matsui 1, Y. Uchikawa 3, K. Kobayashi 3, K. Tanabe 3, S. Takeuchi 3, K. Narasaki 4, S. Tsunematsu 4, Y. Koyabu 4, Y. Kamekawa 4, K. Nakayama 5, T. Shimizu 5, K. Koike 6 K. Hoshino 6, H. Kotaka 6, E. Sudoh 6, H. Takahara 6, Y. Yoshida 8, K. Shinada 7, M. Takahata 7, and Y. Yamada 7. 1 Communications Research Laboratory, Ministry of Posts and Telecommunication, Nukuikitamachi 4-2-1, Koganei, Tokyo 184, Japan. 2The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama 351-01, Japan. 3 Tokyo Denki University, Oaza-Ishizaka, Hatoyama-cho, Hiki-gun, Saitama 350-03, Japan. 4 Sumitomo Heavy Industry, Soukai-cho 5-2, Niihama-shi, Ehime 792-8588, Japan. 5 Nihonkeiki Corporation, Goshogaoka 4-3-5, Moriyacho, Kitasoma-gun, Ibaragi 302-0119, Japan. 6 Mitsui Mining & Smelting Co, Ltd., Corporate R&D Center, Haraichi 1333-2, Ageo, Saitama 362, Japan. 7 Shimadzu Corporation, Keihanna Research Laboratory, Inuidani-Sunagatani, Seika-cho, Soraku-gun, Kyoto 619-02, Japan. 8 National Research Institute of Metals, Tsukuba, Ibaragi 305-0047, Japan.

Presenting Author: H.O. Ohta

A superconducting magnetic shield of high-T_c superconductor Bi(Pb)-Sr-Ca-Cu-O_x has been constructed whose diameter is 65 cm and length is ¹⁶0 cm respectively. We have successfully observed magnetic fields coming out from somatosensory-evoked human brains in the superconducting magnetic shield stimulating the median nerves of patients by current pulses. We made a 64-channel whole-head SQUID magnetometer of SNS (Superconductor/ Normal metal / Superconductor) junctions which does not low-frequency telegraph noise. The sensitivities of the dc-SQUID of mesoscopic SNS junctions are around 5 even in rather unfavorable surroundings. The magnetic shield can reduce a magnetic field to around - 80 dB or 10^-4 even at as low a frequency as 0.05 Hz. a The combination of a SQUID of SNS junctions and a superconducting magnetic could be a promising candidate of a mobile clinic for mental care.

4PO9-52 YBa₂Cu₃O₇ step-edge dc SQUID magnetometers prepared on sapphire substrates

In-Seon Kim 1, Hae-Ryong Lim 1,2, D.H. Kim 2, and Yong Ki Park 1. 1 Korea Research Institute of Standards and Science, Yusong PO Box 102, Taejon 305-600, Republic of Korea. 2 Department of Physics, Yeungnam university, Kyungsan, Republic of Korea.

Presenting Author: D.H. Kim

YBa₂Cu₃O₇ step-edge Josephson junctions and dc SQUID magnetometers on sapphire substrates have been fabricated. CeO₂ buffer layer and YBa₂Cu₃O₇ films were deposited in situ on the low angle (~30degree) steps formed on the sapphire substrates. Large I_cR_n product of 0.25 mV with junction resistance Rn ~ 5 ohm and critical current I_c ~ 0.05 mA at 77 K could be obtained reproducibly with 5-micron microbridges. Direct coupled SQUID magnetometers utilising the step-edge junctions exhibit voltage modulation depth of 0.016 mV and magnetic field noise of 170 fT/Hz0.5 at 100 Hz for the 70 pH-SQUID.

4PO9-53 Simulation of the Subharmonic SIS Mixer

W.L. Shan 1, S.C. Shi 2, Y.J. Feng 1, and P.H. Wu 2. 1 Department of Electronic Science and Engineering, Nanjing University, Nanjing, 210093, China. 2 Purple Mountain Observatory, Nanjing, 210008, China.

Presenting Author: Y.J. Feng

The performance of the subharmonic Superconductor-Insulator-Superconductor (SIS) mixer has been simulated thoroughly using a full five-port model and a quasi five-port model in which a sinusoidal local oscillation (LO) is assumed. It is shown that the accuracy of the quasi five-port model depends not only on the value of RNC, but also on the value of the embedding admittance. The results obtained from both models agree with each other when RNC is large enough to overcome the imaginary part of the embedding admittance of the RF sideband. By using these two models, the performance of a subharmonic mixer at fundamental frequency of 250 GHz has been studied successfully. The best performance has been obtained by optimizing eight parameters including embedding admittance, LO power and bias voltage. If the coupling efficiency is not considered, the optimized embedding admittance will lead to a very low noise temperature, while poor input and output coupling efficiencies are also obtained, which are disadvantageous for astronomy receivers. To get a more practical performance, a target function is designed to consider low noise, high gain and good coupling efficiency synthetically. The numerical results indicate that the SIS subharmonic mixer could be a very hopeful candidate for low-noise mixer at millimeter wave band.

4PO9-54 Optimizing the Slew Rate of SQUID System

F.X. Xie, D.F. He, T. Yang, P. Ma, R.J. Nie, L.Y. Liu, S.G. Wang, S.Z. Wang, and Y.D. Dai, Department of Physics, Peking University, Beijing 100871

Presenting Author: F.X. Xie

We study both theoretically and experimentally the optimization of the slew rate of SQUID (Superconducting QUantum Interference Device) system. Considering the total equivalent noise flux input in the SQUID, both for SQUID electronics with one pole and for electronics with two pole and one zero, we get new formulas about slew rate and find out the optimum bandwidth for maximum slew rate. For SQUID electronics with one pole, the optimum bandwidth and the corresponding maximum slew rate are both proportional to SF ^-1. While for system with two poles and one zero, the optimum bandwidth and the corresponding maximum slew rate are proportional to SF ^-1 and SF ^-2 respectively. SQUID sensor and multi flux quanta electronic simulator are used in the experiment and the results are in good agreement with the analysis.

4PO9-55 A Scalable, Tunable Phase Qubit, Based on a Clean Josephson Junction between Cuprate Superconductors

Alexandre M. Zagoskin, D-Wave Systems Inc., 119-1600 W. 6th Ave., Vancouver, B.C., V6J 1R3, Canada, and Physics and Astronomy Department, The University of British Columbia, 6224 Agricultural Rd., Vancouver, B.C., V6T 1Z1, Canada

Presenting Author: A.M. Zagoskin

Clean junctions between superconducting cuprates have unique properties, doubly degenerate ground state with tunable potential barrier between the "up" and "down" states and non-quantized spontaneous magnetic flux, which follow from d-wave superconducting pairing symmetry in these compounds. Here I show that they can be used as a basis for a scalable solid state qubit - the building block of a quantum computer. The role of quantum "pseudospin" variable is played by the sign of equilibrium superconducting phase difference on the junction, which is revealed in the direction of spontaneous supercurrent flow in equilibrium and can be read out, e.g., by existing magnetic force microscopes.

A working prototype of a register containing several dozen of such qubits can be built using existing experimental techniques and will be operational at temperatures of order few Kelvin. This lets us hope that current estimates of a quarter century lying between now and working quantum computer are way too pessimistic.

4PO9-56 Quantum Transport Characteristics of Quantum Dot Encountered by a Superconductor Point Contact

Attia A. Awad Alla 2, A.M. Hegazy 1, Adel H. Phillips 3 and Raafat K. Wassef 2. 1 Faculty of Science, Cairo University, Beni-Suef Branch, Egypt. 2 Faculty of Science, Cairo University, Giza, Egypt. 3 Faculty of Engineering, Ain-Shams University, Abassia, Cairo, Egypt.

Presenting Author: A.M. Hegazy

In the present paper Quantum transport characteristics of a mesoscopic system under the effect of Coulomb blockade and magnetic field is studied. This system is modeled as semiconductor quantum dot which is coupled to a superconducting leads via a quantum point contact. The Josephson current has been obtained in terms of the Andreev reflection amplitude. This Andreev reflection amplitude was deduced by solving the Bogoliubov-de Gennes (BdG) equation, describing the electron transport through the junction. Numerical calculation of the Josephson current has been performed for different values of the phase angle, temperatures, the magnetic field and the distance between the leads.

The obtained results show that the electron transport through the device has a coherent property. An important result was obtained for the variation of the magnetic length with the applied magnetic field. This result shows that the correlated electrons affected by an interplay between the magnetic length and the size of the present device.

The current dependence on the dimensions of the device shows an oscillatory behavior for multiple interacting channels, while for a single channel, this dependence is a decreasing function. These results are explained according to the clean limit theory of the mesoscopic system and found concordant with those in the literature.

4PO9-57 Resonance Tunneling Through Quantum Dot Coupled with Two Superconducting Reservoirs

Attia A. Awad Alla 1, A.M. Hegazy 1, Adel H. Phillips 3, and Raafat K. Wassef 2. 1 Faculty of Science, Cairo University, Beni-Suef Branch, Egypt. 2 Faculty of Science, Cairo University, Egypt. 3 Faculty of Engineering, Ain-Shams University, Abassia, Cairo, Egypt.

Presenting Author: R.K. Wassef

In this paper we consider a semiconductor quantum dot(Sm) is coupled via tunnel barriers to two superconducting (S) reservoirs. Electron transport through such junction is studied under the effect of magnetic field and Coulomb blockade. Under such conditions the state of the quantum dot is characterized by a set of occupation numbers, one for each energy "Landau" levels. Accordingly, the Josephson current is calculated in terms of the probability to find a number of electrons in e quantum dot in equilibrium with the superconductor reservoirs. This probability includes explicitly the free-energy of the dot. The transmission probability is derived by solving the Bogoliubov-de Gennes (BdG) equation. It is found that the current-phase angle (I-f ) of Cooper pairs relation exhibits a peak at f » 0.50 at low temperature. However, this peak shifts to a lower value as the temperature increases. Also, this relation (I-f ) does not change as the magnetic field varies. The relation (I-B) (B-magnetic field) has the same peak position for different values of f and temperatures. These results show a qualitative agreement with those results available in the literature. These results are explained according to the resonance tunneling regime of electrons through such junction.

4PO9-58 Microwave Characterization of HgBa₂CaCu₂O₆ Thin Films

Roberto S. Aga Jr., Yiyuan Xie, Judy Wu, and Siyuan Han, Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045

Presenting Author: R.S. Aga

We assessed the potential of Hg-based high-T_c superconducting thin films for microwave applications by measuring its microwave surface resistance R_s and power handling capability. Epitaxial thin films HgBa₂CaCu₂O₆ were fabricated using Cation-Exchange process. The films are c-axis oreinted with thickness of about 0.3 um. Their T_c’s are typically in the range of 112 K to 124 K. On unpatterned films, we obtained an R_s of 0.3 milli-Ohm at 10 GHz and 120 K using Nb cavity perturbation technique. Then we patterned the films into 5 mm long, 150 um wide microstip lines and put into package with SMA connectors to measure power handling. At 1 GHz and 77K, Insertion Loss (IL) was below 0.5 dB and RF J_c was comparable to that of YBCO and Tl-HTS. When temperature was increased to 100 K, IL just increased by 0.2 dB and critical input power dropped only by about 10 dBm, indicating that these Hg-based HTS thin films are very good for microwave device application.

4PO9-59 Remanent Magnetization of High-Temperature Josephson Junction Arrays

Wagner de A.C. Passos, Paulo N. Lisboa Filho, and Wilson A. Ortiz, Grupo de Supercondutividade e Magnetismo, Departamento de Física, Universidade Federal de São Carlos, Caixa Postal 676 - 13565-905 São Carlos, SP, Brazil.

Presenting Author: W.A. Ortiz

An explanation for the Wohlleben effect (WE) was given by Araujo-Moreira and coworkers [1], who studied the ac-susceptibility (c _AC) of Josephson junction arrays (JJA) varying the temperature and excitation field. Their single-plaquette model simulates the magnetic behavior of a JJA [1] and anticipates that the array, upon excitation by a field, will exhibit a remanent moment, in a temperature interval which depends on the critical current of the junctions.

Recently we have confirmed that WE is an inherent property of granularity [2] and demonstrated that tridimensional disordered JJAs (3D-DJJA) can be produced in a controlled manner [3] from granular superconductors. These arrays exhibit WE and, as predicted, the remanent moment in a window of temperatures [4].

Here we present a study of the remanent magnetization displayed by our newly produced 3D-DJJAs of YBa₂Cu₃O_7-d . The remanence appears in a limited range of temperatures and is a prospective tool for detection of anisotropy of the array and of the critical current dispersion of its constituent junctions.

[1] F.M. Araújo-Moreira et al., PRL 78 (1997) 4625.

[2] W.A. Ortiz et al., submitted to PRL (1999).

[3] W.A.C. Passos et al., to be submitted.

[4] W.A.C. Passos et al., submitted to J. Appl. Phys.