4PO9-60 Bicrystal grain boundary Josephson junctions with epitaxial Tl₂Ba₂CaCu₂O_x thin film

J. Wang 1, B. Han 1, F.Z. Xu 1, G.H. Chen 1, Q.S. Yang 1, R.T. Lu 2, M. He 2, and S.L. Yan 2. 1 Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China. 2 Department of Electronics, Nankai University, Tianjin 300071, P.R. China.

Presenting Author: G.H. Chen

Epitaxial Tl₂Ba₂CaCu₂O_x thin film grain boundary Josephson junctions have been fabricated on 24° STO bicrystal substrates by magnetron sputtering and post-annealing process. The junction characteristics have RSJ-like behavior with V_c = I_cR of 40 m V at 95 K.The critical current of the junctions near T_c (105K) has a temperature dependence similar to that of SNS type ones. The junctions have been used to form a dc-SQUID, which can operate at temperature up to 99K.

4PO9-61 Monte Carlo Simulation of Ion Beam Damage Profile in YBCO Thin Film under Various Mask Structures

Nianhua Peng 1, Ivan Chakarov 2, Chris Jeynes 1, and Roger Webb 1. 1 Surrey Centre for Research in Ion Beam Applications, School of Electronic Engineering, Information Technology and Mathematics, University of Surrey, Guildford GU2 5XH, United Kingdom. 2 SILVACO International, Santa Clara, CA 95054, USA.

Presenting Author: N. Peng

Energetic ion beam irradiation damage profiles in masked superconducting YBCO thin film have been studied by Monte Carlo simulation with various high aspect ratio mask structures. Both proton and oxygen beams with energies between 20 keV and 200 keV were used for the simulations. The simulation results reveal that controlled damage area with dimension down to nanometer scale is possible through various nanometer scale mask structures, though side-wall interaction plays an important role in the damage profile distribution inside YBCO thin film under the nanometer scale masks. The optimisation of appropriate mask materials and mask structures has been discussed. These results are useful for superconducting electronics device fabrication using nanometer scale superconducting and non-superconducting interfaces, such as Josephson junction based devices.

4PO9-62 Characteristics of YBa₂Cu₃O_7-y Josephson junctions prepared with smooth ramp-edge surfaces

L.M. Wang 1, H.H. Sung 1, H.C. Yang 2, and H.E. Horng 3. 1 Department of Electrical Engineering, Da-Yeh University, Chang-Hwa 515, Taiwan, ROC. 2 Department of Physics, National Taiwan University, Taipei 106, Taiwan, ROC. 3 Department of Physics, National Taiwan Normal University, Taipei 117, Taiwan, ROC.

Presenting Author: L.M. Wang

YBa₂Cu₃O_7-y(YBCO) ramp-type Josephson junctions with PrBa₂Cu₃O_7-y(PBCO) barriers have been fabricated on SrTiO₃(001) substrates. The morphologies of ramp-edge surfaces were examined using the atomic force microscopy (AFM). The photolithography process, the ion beam incident angle, and the post-annealing process were optimized to improve the roughness of the ramp-edge surfaces. Typically, smooth ramp-edge surface with an angle of about 25° and an average roughness of about 2 nm was achieved. The I-V curves, microwave induced Shapiro steps, and field modulation pattern were used to characterize the junctions. The junctions showed a RSJ-like behavior with I_cR_N ~ 400 m V and a uniform Josephson current distribution in the junction area. Results indicate the junction properties are improved with a smooth ramp-edge surface. The details are discussed.

4PO9-63 A general expression for Josephson penetration depth in junction arrays

D.-X. Chen, J.J. Moreno, and A. Hernando, Instituto de Magnetismo Aplicado, RENFE-UCM-CSIC, 28230 Las Rozas, Madrid, Spain

Presenting Author: D.X. Chen

We have calculated analytically and numerically the screening field profiles in a slablike Josephson-junction array of lattice constant a with normalized line density of maximum Josephson-junction current i_max. It is shown that the Josephson penetration depth, l _J = a/ln[1 + p i_max + square root of (1 + 1_max)²-1], whose low and high-i_max limits are a/square root of 2p i_max and a/ln(2 + 2p i_max) respectively. The first limit is consistent with that for uniform long junctions and continuous superconductors and the second limit should be used for the intergranular properties of granular superconductors. The presence of both limits indicates a transition resulting from the enhancement of the structural discreteness.

4PO9-64 Demagnetizing effects on the critical state in Josephson-junction arrays

D.-X. Chen 1, J.J. Moreno 1, A. Hernando 1, and A. Sanchez 2. 1 Instituto de Magnetismo Aplicado, RENFE-UCM-CSIC, 28230 Las Rozas, Madrid, Spain. 2 Grup Electromagnetisme, Dept. Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Catalonia, Spain.

Presenting Author: D.-X. Chen

It is well known that an almost perfect critical state in infinitely long Josephson-junction (JJ) arrays occurs at a sufficiently large maximum JJ current density imax and the corresponding critical current density i_c is a step function of i_max. It is shown by model calculations that a critical state can also occur in planar JJ arrays with strong demagnetizing effects if the maximum JJ current i_max is sufficiently large. However, the critical state in the planar arrays is never perfect, with field and current profiles always containing an oscillating component; and with increasing demagnetizing effects, the critical current i_c becomes a linear function of i_max.

4PO9-65 Pseudo-hysteretic behavior in superconductor-semiconductor junction

Roberto Nicolsky, Instituto de Fisica, Universidade Federal do Rio de Janeiro, C.P. 68528, Rio de Janeiro 21945-970, Brazil, and Youri A. Gorelov, General Atomics, San Diego, CA, USA

Presenting Author: R. Nicolsky

A recent work by Th. Schäpers et al. [Appl. Phys. Lett. 71 (1997) 3575] showed the coupling to a superconductor of the two-dimensional electron gas in a doped semiconductor heterostructure. The presented I-V characteristic exhibits a hysteresis at the low-voltage range, similarly to I-V curves of other works for analogous structures. In this work we simulate that kind of junction using the Pereira-Nicolsky [Physica C 282-287 (1997) 2411] simplified model for superconductor-normal metal-superconductor systems applied to the dimensional parameters of that junction and the appropriate Andreev density of states calculated for that doped semiconductor. We show that, at sub-gap voltages, pseudo-hysteresis is due to the current-bias experimental set-up used in the measurements and actually it hiddens a negative-differential-resistance behavior. The negative derivative in the I-V characteristic is due to the presence of multiple Andreev scattering mechanisms at the superconductor-semiconductor interfaces as a consequence of the density of states within the semiconductor gap. It follows from the competition between the increasing voltage and the decreasing of the allowed number of reflections. Finally, we suggest the kind of experiment should be done for avoiding that pseudo-hysteresis in those junctions.

*4PO9-66 Interpretation of a microwave induced current step in a single intrinsic Josephson junction on Bi-2223 thin films

Ch. Helm 1, A. Odagawa 2, M. Sakai 2, H. Adachi 2, K. Setsune 2, and R. Kleiner 3. 1 Los Alamos National Laboratory, T-11, B-262, Los Alamos NM 87545, USA. 2 Central Research Laboratory, Matsushita Electric Industrial Co. Ltd., Seika, Soraku, Kyoto, 619-0237, Japan. 3 Physical Institute III, University of Erlangen-Nurnberg, D-91058 Erlangen, Germany.

Presenting Author: Ch. Helm

The microwave response of stacks of intrinsic Josephson junctions on (Bi,Pb)-Sr-Ca-Cu-O thin films is reported. The thinnest stack is formed by a single junction und exhibits a single resistive branch and a clear superconducting gap structure in the I-V-Characteristic. In external microwave fields a current step in the I-V-curve is observed, which shifts to higher voltages with increasing radiation power, while the height of the step remains practically unchanged. This dependence of the microwave power is different from the behaviour of Shapiro-steps. With the help of a numerical simulation based on coupled Sine-Gordon-Equations including an ac-magnetic field parallel to the layers the experimentally observed dependences can be interpreted by fluxon motion, which is driven by the ac field.

4PO9-67 Responses of BSCCO-2212 intrinsic Josephson junctions to microwave irradiation at short millimeter waveband

H.B. Wang 1, Y. Aruga 2, T. Tachiki 2, Y. Mizugaki 2, J. Chen 2, K. Nakajima 2, T. Yamashita 3, and P.H. Wu 4. 1 CREST, Japan Science & Technology Corporation (JST), Japan. 2 Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan. 3 New Industry Creation Hatchery Center, Tohoku University, Sendai, Japan. 4 Department of Electronic Science & Engineering, University of Nanjing, Nanjing 210093, China.

Presenting Author: H.B. Wang

Intrinsic Josephson junctions (IJJs) with a-b plane sizes from 1 to 100 squared microns were fabricated on argon-annealed Bi-Sr-Ca-Cu-O (2212) single crystals, adopting an overlap structure to improve the coupling. With characteristic voltage up to a few milli-volts, high frequency responses were carefully investigated from a few to 200 GHz. The IJJs were very sensitive to the irradiation at all frequencies, with typical suppression of critical currents and unchanged last resistive branch. The I-V curves were chaotic under moderate microwave power, especially at low frequencies. At higher power levels, instead of distinct Shapiro steps, current upturns were typically observed even at 100 GHz. Possible explanation based on the envelop of high-order Shapiro steps was suggested, and it was supported by harmonic mixing experiments.

4PO9-68 Decrease of the HTS mm-wave Josephson Mixer Noise in Small dc Magnetic Field

J. Konopka 1,2, S.J. Lewandowski 1, M. Darula 3, O. Harnack 3, A. Konopka 1, and I. Wolff 2. 1 Institute of Physics PAN, Al. Lotników 32/46, 02-668 Warszawa, Poland. 2 IMST, Karl Friedrich Gauss Str. 2, Kamp Lintfort, Germany. 3 KFA Jülich , D-52425 Jülich, Germany.

Presenting Author: J. Konopka

Properties of YBaCuO grain boundary junction (GBJ) Josephson mixer incorporated into a log-spiral antenna in quasi-optical configuration were investigated at frequencies 105 and 140 GHz in the temperature range 12 to 55K. Noise temperature and conversion efficiency were measured using semi-automatic cold-hot load technique. In particular, the influence of small (< 1mT) dc magnetic field of different orientation on noise temperature of the mixer was investigated.

The best performance of the mixer (T_N ~ 950 K) was obtained in the presence of a small dc magnetic field applied at an angle of about 50° to the HTS film surface and parallel to the GBJ with the driving local oscillator level appropriately reduced. The intensity of optimizing magnetic field was close to 0.45 mT at 15 K. The decrease of the noise temperature was up to 20 % with respect to the T_N value measured at zero field.

We explain our result by the fact, that in HTS Josephson junctions, where I_C behavior in function of applied dc magnetic field significantly deviates from the Fraunhofer pattern, some regions of the grain boundary are driven out of superconductivity and total fluctuations of both normal resistance R_N and critical current I_C are markedly reduced.

4PO9-69 Josephson Effects in YBa₂Cu₃O_7-ƒÂ SIS Tunnel Junctions with Double Layer Barrier

You-song Jiang, Tadayuki Kobayashi, and Toshinari Gotom, The University of Electro-Communications, Chofu-Shi, Tokyo, 182-8585, Japan

Presenting Author: Y.S. Jiang

For proposes of electronic applications and the research of fundamental physics fabrication of SIS tunnel junction is very important. In this study, we have fabricated vertical SIS tunnel junctions with a-axis oriented YBa₂Cu₃O_7-ƒÂ(YBCO) electrodes. In order to prevent the pinhole in the barrier layer, double layer consists of PrGaO₃(PGO) and CeO₂ was used. The standard lithography technology and ion milling were employed to define the junctions with area of 3ƒÊm8~15ƒÊm. The junctions with barriers 50 ¶ thick PGO and 25 ¶ thick CeO2 showed supercurrents and RSJ like voltage-current(I8½ V) characteristics. Fraunhofer like magnetic field dependence of Ic were observed, and the results proved the existence of Josephson effects in these junctions. The typical I_c, R_n and I_cR_n were 0.41mA, 0.172ƒ¶ and 0.07mV at 4.2K, respectively.