Self-induced resonances in YBCO bicrystal grain boundary Josephson junctions

PHYSICA

PhysicaB 194-196 (1994) 1771-1772 North-Holland

Self-Induced Resonances in YBCO Bicrystal Grain Boundary Josephson Junctions D. Winkler a, Y.M. Zhang a, P.,~. Nilsson a, E. A. Stepantsov b, and T. Claeson a aDepartment of Physics, Chalmers University of Technology, S-412 96 G6teborg, Sweden blnstitute of Crystallography, Russian Academy of Sciences, 59 Leninskii pr., 117 333 Moscow, Russia We have investigated YBCO grain boundary junctions formed on YSZ bicrystal (0-32 °) substrates. We observed strong magnetic field dependent peaks in the Josephson current for certain voltage biases. We relate the structures to self-induced Fiske steps, where the junction electrodes along the grain boundary form a resonator of length L, and the ac Josephson current sets up a standing wave for frequencies fn = n" c / 2 L . For L = 20 ~tm, resonances occurred at fl = 280 GHz, f2 = 490 GHz, f3 = 640 GHz. A good agreement between the calculated Fiske-modes and the experimental dependence was obtained.

1. I N T R O D U C T I O N YBCO thin films grown on bicrystal substrates give reproducible and high quality Josephson weak links [1-4]. To determine the junction uniformity and properties, magnetic "fingerprint" measurements can be devised. We have made artificial grain boundary junctions on yttria stabilized zirconia (YSZ) bicrystals. 200 nm of c-axis oriented YBa2Cu307 was deposited at 760 ° C in 0.2 mbar of oxygen using laser ablation. The films generally have Tc - 90 K and Jc > 106 A/cm2 at 77 K. Using bicrystal substrates, Jc could be varied from 102 to 105 A/cm 2 at 77 K. Usually, RSJ-type I - V curves

were obtained, but at lower misorientations, the critical current was not much reduced from the film value, and the I-V characteristics diverged from the ideal RSJ-type. The 0-32 ° [001] tilt misorientation gave the best current uniformity along the boundary compared to several other orientations (e.g. 8-8 °, 1616 ° , and 0-45°). The magnetic field response was symmetric and nearly Fraunhofer-like for the first lobes and the I-V characteristic was of the RSJ type. Here, we pay special interest to the magnetic field response of a L=20 gm long 0-32 ° [001] tilt grain boundary junction on YSZ.

I [la.A]

Fig. 1.1-Vcurves for increasing magnetic field values from -0.6 G (lower right) to 0.6 G (upper left). T=I5 K. 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93) 1517-P

1772 2. RESULTS For zero magnetic field, the I-V curve resembled the ordinary RSJ behavior, and from the Jc dependence on the magnetic field, we see that the junction is in the "small" limit compared to the Josephson penetration depth, Zj ; L/Zj 2.3. To demonstrate the magnetic field dependence, in Fig. 1 we plot 100 I-V curves with an incremental offset of -10 btV, and +0.5gA with AH about 12 raG. The magnetic field is varied from -0.6 G (lower =

4O

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the plot, we have subtracted the normal current given by I n = V/R n, where Rn = 13.6 ,(-2is the normal state resistance. An increase of the effective flux area of about 10% from n=0 to n=3 is used in the fit. The theoretical O-values [5] (On = 1.8; 4.1 ; 7.0) can be compared to the fitted experimental ones (On = 1.5; 3.1; 5.3), which brings us to the conclusion, that the major loss contribution comes from R n.

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right curve) to +0.6 G (upper left). When a magnetic field of 0.1 G is applied (corresponding to about a flux quantum, g'o = h/2e, in the junction), a strong enhancement of the Josephson current is observed in the I-V curve at around 0.58 mV. When the field is increased further, a second and a third peak emerge at 1.01mV and 1.32mV. In Fig. 2, we show the comparison of the theoretical field dependence of the Fiske steps [5]:

ACKNOWLEDGMENT

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This work was supported by the Swedish Research Council for Engineering Sciences and the Swedish National Board for Industrial and Technical Development.

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Fig. 2. Magnetic field dependencies for the critical current, Io (n=0), and for the first three Fiske modes (n=l-3). The solid lines give the best theoretical fit to the experimental data (dots). T=15 K.

1. D. Dimos, P. Chaudhari, J. Mannhart and F. K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. 2. J. Mannhart, P. Chaudhary, D. Dimos, C. C. Tsuei and T. R. McGuire, Phys. Rev. Lett. 61 (1988) 2476. 3. Z. G. Ivanov, P.-A. Nilsson, D. Winkler, J, A. Alarco, T. Claeson, E. A. Stepantsov and A. Y. Tzalenchuk, Appl. Phys. Lett. 59, (1991) 3030. 4. J. Mannhart, R. Gross, H. Hipler, R. P. Huebener, C. C. Tsuei, D. Dimos and P. Chaudhari, Science 245 (1989) 839. 5. A. Barone and G. Paterno, Physics and Applications of the Josephson Effect, (John Wiley & Sons, New York, 1982).