Divin Yu. Ya.

J. Mygind

N. F. Pedersen

P. Chaudhari

Soerensen O. H.

Yu. Ya. Divin

The ac Josephson effect was studied in YBa2Cu3O7−x grain-boundary junctions (GBJ) in the temperature range from 4 to 90 K. The temperature dependence of the linewidth of millimeter-wave Josephson oscillations was measured and it is shown that the derived effective noise temperatures may be as low as the physical temperature in the temperature range investigated. In the millimeter-wave range, linewidths as low as 380 MHz were found at liquid-nitrogen temperatures. Applied Physics Letters is copyrighted by The American Institute of Physics

Yu, Ya. Divin

Mygind, Jesper

Pedersen, Niels Falsig

Chaudhari, P.

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Divin; Mygind, Jesper; Pedersen, Niels Falsig; Chaudhari, P.Published in:Applied Physics LettersLink to article, DOI:10.1063/1.108007Publication date:1992Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Yu, Y. D., Mygind, J., Pedersen, N. F., & Chaudhari, P. (1992). Josephson oscillations and noise temperaturesin YBa2Cu3O7-x grain-boundary junctions. Applied Physics Letters, 61(25), 3053-3055. DOI: 10.1063/1.108007Josephson oscillations and noise temperatures in VB~&U~O~-~ grain-boundary junctions Yu. Ya. Divin,a) J. Mygind, and N. F. Pedersen Physics Laboratory A The Technical University of Denmark, DK-2800 Lyngby, Denmark P. Chaudhari IBM T. J. Watson Research Center, Yorktown Heights, New York 10598 (Received 28 July 1992; accepted for publication 26 October 1992) The ac Josephson effect was studied in YBa@tsO,-, grain-boundary junctions (GBJ) in the temperature range from 4 to 90’ K: The temperature dependence of the linewidth of millimeter-wave Josephson oscillations was measured and it is shown that the derived effective noise temperatures may be as low as the physical temperature in the temperature range investigated. In the millimeter-wave range, linewidths as low as 380 MHz were found at liquid-nitrogen temperatures. Recently, many different types of high T, supercon- ducting weak links have been studied and some of them have shown Josephson behavior.’ One of the most prom- ising types is the grain-boundary junction (GBJ) fabri- cated by the epitaxial growth of YBazCu@-, thin film on a bicrystal substrate.2 Such a GBJ is stable against thermal cycling and its parameters can be rather reproducibly con- trolled by changing the misorientation angle of the bicrys- tal substrate.’ Motivated by the promising applications of GBJ in dc SQUIDS, noise studies have mainly been con- fined to low frequencies and l/f noise. For high-frequency applications, detailed investigations of the ac Josephson effect and wideband fluctuations in GBJ are required. This can be carried out by studying the linewidth of the Joseph- son radiation, as was done with weak links fabricated from conventional low T, superconductors.3’4 To estimate the power and the spectral width of the Josephson radiation in a high T, GBJ, we consider the RSJ model to be applicable. The main parameters are the crit- ical current 1, and the normal-state resistance R,. The total power P of the Josephson radiation supplied to an external circuit with impedance R&R, at the fundamental fre- quency f= (2e/h) V is equal to5 P=2V2[(V2+V3-V]2/V~R,, (1) where V,=IJ,, is the characteristic voltage of the Joseph- son junction. The maximum power P= Vz/R, is reached at I/> V,. For an YBCO GBJ at 77 K, Vc is of the order 0.1 mV and with a typical R,- 100 a, we get P- lo-*’ W. Two main sources of voltage fluctuations may be con- sidered in GBJs, broadband thermal noise and low- frequency l/f noise. The latter may result both from fluc- tuations in the critical current and from fluctuations of the normal-state resistance.6 For broadband thermal fluctua- tions, which are the dominant here, the linewidth is5 Sf=4rr(2i/h)*kT(R2JR,) [ 1+(&‘21’)] , (2) where R, is the dynamic resistance. The linewidth in Eq. (2) is always larger than the minimum value 6f=4n(2e/ “Permanent address: Institute of Radio Engineering and Electronics of the eussian Academy of Sciences, Moscow 103907, Russia. h)2kTR, which is reached for INI, and will be used for estimates. At liquid-nitrogen temperatures and for a GBJ with R,= 1 a, we get Sf-3 GHz. In the present study of the Josephson linewidth, we use the same indirect technique based on the self-detected dc response of the Josephson junction to low-intensity micro- wave radiation, as was used in a study of low T, microcon- tacts.4 The technique is based on the analytical properties of the voltage dependence of the dc response AV( V), which is the voltage difference between the IV curves with and without applied microwave radiation. AV( V) shows an odd-symmetric resonance at the voltages V- (h/2e)f and the difference S V between voltages V+ and V._. corre- sponding to the positions. of the maximum and the mini- mum of the response A V in this region gives us a linewidth 6f=(2e/h)SV. Thin-film YBa,Cu,O,-, bicrystals were prepared by depositing c-axis oriented epitaxial YBazCu307-, films on SrTiO, and Y-ZrO, bicrystals using laser ablation deposi- tion. The thin-film bicrystal was then patterned by another laser into a number of bridges, each crossing a grain boundary and having widths ranging from 5 to 300 pm. The parameters are shown in Table I. The sample holder had a loosely coupled 60-90 GHz waveguide connection to the sample and -70 GHz radia- tion from a Gunn oscillator could be applied. A lock-in technique was used to measure the derivative of the dc IV characteristics and the dc response S V( V) . The response measurements were obtained by an on-off modulation of the microwave power; the modulation frequency was 531 Hz, determining the minimum frequency of noise observa- tions. The sample holder was placed inside a vacuum can immersed in a glass helium Dewar surrounded by a double p-metal shield. All measurements were made in an rf shielded room. The main results are based on the assumption that the RSJ model5 applies. Hence, we first discuss the validity of this assumption. For the GBJ used in this study, the dc IV characteristics agreed with the hyperbolic dependence pre- dicted by the RSJ model for R,-0.1 to 1 fl. For narrow and high-ohmic GBJs with R, > 1 R, hysteresis was ob- served in the IV curve at low temperatures with no stable 3053 Appl. Phys. Lett. 61 (25), 21 December 1992 0003-6951/92/503053-03$06.00 @ 1992 American Institute of Physics 3053 Downloaded 23 Oct 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspTABLE I. Parameters of grain-broundary junctions used for linewidth measurements. June. Misorient. Width R, 1J4.5 K) l’,(4.5 K) No. Substrate angle (pm) IfI) OW (mW GBJll SrTiO, 25’ 5 4.6 0.4 1.8 GBJ12 SrTiO, 25 16 0.57 2.0 1.1 GBJ13 SrTiO, 25’ 88 0.07 7.0 0.5 GBJ21 Y-zrOz 45” 34 0.67 0.11 0.07 GBJ22 Y-Zrol 45 93 0.22 0.35 0.08 bias point at voltages from zero to several hundreds pV, while for low-ohmic GBJ, there was no hysteresis in the IV curves. These observations suggest the hysteresis may be due to the shunting of the Josephson oscillations by the intrinsic stray capacitance,’ which we estimate to be around 1 pF for a 5-pm-wide GBJ on SrTiO,. The IV characteristics of the wider GBJ with resistance R, < 0.1 fi are also different from that of the RSJ model. In Fig. 1, the IV curve and mm-wave response A V versus voltage V are shown for a GBJ with R,=0.07 CL An excess current not consistent with the RSJ model is clearly seen on the IV curve at 1~1,. This excess current is due to tlux motion along the GBJ.’ The deviation from the RSJ model, how- ever, does not prevent us from measuring the response curve as in the hysteretic case. The corresponding data are shown in Fig. 1 (curve 3). The response AV is propor- tional to the differential resistance R, (curve 2) at low voltages; at voltages close to (h/2e) f, the response shows a resonance structure due to the interaction between the ex- ternally applied 72 GHz signal and the Josephson oscilla- tions. The width S V of this resonance structure, calculated as a difference in the voltages V+ and V- corresponding to -0.2 I-- ~~ ~-A----- I -_ 1 0 100 200 300 VOLTAGE, I-IV FIG. 1. (1) dc IV-characteristic, (2) voltage dependence of the differen- tial resistance, RD( V), and (3) response AV( I’) to 73 GHz radiation for YBa,Cu,O,-, grain-boundary junction GBJ13. The temperature is ‘l-=78 K. 600 2 z ? 400 3 Loo L k I i- 5 t 0 u’ -200 . ~~~~i~l~~ 400 ___ -- ~- -~.---_~ -. VOLTAGE PV J 0.6 FIG. 2. (1) dc IV characteristic, (2) differential resistance, and (3) response to 72 GHz radiation for YBa$u,O,-, grain-boundary junction GBJ12 at 7’=78 K. The inset shows the normalized response h(V) = (A V/RD) I. V, calculated and experimental. the minimum and the maximum response, was equal to (0.8 +=O. 1) PV at 78 K corresponding to a Josephson line- width Sf = (2e/h) S V= (380 f 50) MHz at liquid-nitrogen temperature. The amplitudes of the resonance response at V- (h/2e)f of low-ohmic GBJ were always less than the response at low voltage (see curve 3 in Fig. 1); this-means that the ac Josephson effect in.low-ohmic GBJ is compar- atively depressed. Figure 2 shows the same data for GB12. Here, the IV-curve is close to being hyperbolic and the differential resistance R,(V) has a maximum at low voltages and de- creases with increasing voltage approaching a constant value. The response AV is proportional to the differential resistance at low voltages, and at voltages around (h/2e) f, it shows an odd-symmetric resonance. The absence of res- onances at voltages n* (h/2e)f shows that the response is measured at sufficiently low intensity and the absence of (l/n) (h/2e)f resonances demonstrates a pure sin 9 current-phase relationship. In order to get quantitative results, we calculated the normalized response h ( V) = (A V/R )IV in the RSJ model with thermal fluctuations5” and compared it to the nor- malized response calculated from experimental data shown in Fig. 2. The comparison is shown in the inset. Two fitting parameters were used-the amplitude of the rf current in- duced in the GBJ by external radiation and the linewidth Sf of the Josephson radiation. As it can be seen from Fig. 2, the fit is good within an accuracy of a few percent. This confirms that the response of the GBJ can be described in terms of the RSJ model with thermal fluctuation. Other results of the RSJ model can therefore be considered ap- plicable to GBJ and we will use Eq. (3) to determine the effective temperature TN from the experimental values of linewidth b;J: 3054 Appl. Phys. Lett., Vol. 61, No. 25, 21 December 1992 Divin et a/. 3054 Downloaded 23 Oct 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspq n un IF/ , TEMPERATURE, K FIG. 3. Noise temperature vs physical temperature of the YBa,CusG7-X grain-boundary junctions derived from the experimental values of the millimeter-wave Josephson linewidth. Filled trianglesGBJ21; filled squares-GBJ22; open squares-GBJ12. In Fig. 3 we present the noise temperatures TN for three GBJs as a function of the physical temperature Tin the range from 4.5 to 90 K. The general trend is that, in this wide temperature range, the noise temperature follows the physical temperature. The best agreement between TN and T is obtained for GBJs having small characteristic voltages V,(T) ( (h/2e)f (filled squares and triangles). A larger difference between TN and T was observed for GBJ12 with V,(T) > (h/2e)f (unfilled squares) at inter- mediate temperatures. For some junctions of the micro- bridge type, a fit was not possible, indicating nonthermal noise sources dominating. The data presented in Fig. 3 shows that wideband ther- mal noise is the dominant cause of the observed Josephson linewidth: This is especially so for Josephson oscillations at voltages (h/2e)fs V,. In this case, the bias current I is much larger than critical current 1, so that low-frequency fluctuations of the critical current will not contribute to voltage fluctuations.” The l/f noise from resistance fluctu- ations can also give such a contribution6 but as it is seen from the data, this is not the case here. The largest difference between T and TN was observed for (h/2e)f< V, and in the range of intermediate temper- atures. We suspect l/f noise due to critical current fluctu- ations to be responsible for this extra contribution to the linewidth because, in this case, current bias is closer to c. The abrupt changes in the TN( 7) dependence in the inter- mediate temperature range correspond to. the spontaneous changes in I,(T) and this also indicates a contribution from the critical current fluctuations. The GBJ had a highly inhomogeneous spatial distribution of the critical current density, as was seen from the I,(H) dependence, and hence, the GBJ may be considered more as a multi- junction interferometer. Magnetic flux can come and go in the loops of this interferometer giving riseto changes in the critical current of the GBJ and to noise fluctuations. Like other interferometers, the GBJ should have a maximum responsivity SIJ6B when LIc-Qo, and this may be the cause of the extra noise as observed at some temperatures. In conclusion, we have shown that the effective noise temperature of high-quality YBa,Cu,O,-, grain-boundary junctions may be as low as their physical temperature in the range from 4.5 to 90 K. This allows us to use thermal fluctuations to get the limiting performance of high- frequency high T, devices utilizing the ac Josephson effect. The lowest value of the linewidth of 72 GHz Josephson oscillations at 78 K was equal to 380 MHz, which shows the applicability of the GBJ particularly in the field of radiation spectroscopy,’ even at liquid-nitrogen tempera- =_ tures. The authors are grateful to E. A. Stepantsov for sup- plying the bicrystal Y-ZrOz substrates, to P. M. Shadrin for the laser probe measurements of the inhomogeneity of the samples, and to J. Lacey and A. Gupta for help with the substrate and film fabrication. This work was partly funded under the Danish Technical Research Council (STVF) HTSC program. ’ R. Gross and P. Chaudhari, in Principles and Applications of Supercon- ducting Quantum Interference Devices, edited by A. Barone (World Scientific, Singapore, 199 1) . ‘D. Dimos, P. Chaudhari, J. Mannhart, and F. K. LeGoues, Phys. Rev. Lett. 61, 219 (1988). -‘O. H. Soerensen, J. Mygind,‘N. F. Pedersen, V. N. Gubankov, M. T. Levinsen, and P. E. Lindelof, J. 4ppl. Phys. 48, 5372 ( 1977). 4Yu. Ya. Divin and N. A. Mordovets, Sov. Tech. Phys. Lett: 9, 245 (1983). ‘K. K. Likharev, Dynamics of Josephson Junctions and Circuits (Gordon and Breach, New York, 1986): “M. Kawasaki, P. Chaudhari, and A. Gupta, Phys. Rev. Lett. 68, 1065 (1992).. ‘Yu. Ya. Divin, d; Yu. Polyanskii, and A.’ Ya. Shul’man, IEEE Trans. Magn. MAG-19, 613 (1983). 3055 Appl. Phys. Lett., Vol. 61, No. 25, 21 December 1992 Divin ef a/. 3055 Downloaded 23 Oct 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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