Andreone, A

Aruta, C

Cassinese, A

Iavarone, M

Palomba, F

Vaglio, R

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Particle Accelerators, Vol. 60, pp. 161-169Reprints available directly from the publisherPhotocopying permitted by license only© 1998 OPA (Overseas Publishers Association) N.V.Published by license underthe Gordon and Breach SciencePublishers imprint.Printed in India.RF PROPERTIES OFLOW AND HIGH TEMPERATURESUPERCONDUCTING FILMSA. CASSINESE*, A. ANDREONE, c. ARUTA,M. IAVARONE, F. PALOMBA and R. VAGLIODipartimento di Scienze Fisiche, Universita' di Napoli "Federico II" andI.N.F.N., Piazzale Tecchio 80,1-80125 Napoli, Italy(Received in final form 15 January 1998)In this paper we report on r.f. measurements performed on Nb3Sn and YBa2Cu307-8(YBCO) films grown by sputtering in different configurations. The measurements areperformed using a microstrip resonator technique. The rJ. power dependence of thesurface impedance Zs is studied at different temperatures (4 K < T < Tc) and frequenciesin the range l-lOGHz. For Nb3Sn films a set of consistent data is reported in describ-ing the non-linear behavior observed. The nature of the breakdown field is investigatedconsidering the effects of a d.c. field externally applied. For YBCO films, differentloss mechanisms are observed, depending on the quality of the sample. The data arediscussed in the context of different models, in the attempt to underline similaritiesand differences between Low Temperature (LTS) and High Temperature Super-conductors (HTS).Keywords: Superconductivity; Radiofrequency; Cavities; Surface impedance;Microstrip resonatorsINTRODUCTIONConsiderable progress has been made worldwide in the depositionof cuprates as well as intermediate Tc Nb-compounds and A15materials for microwave applications. However, the r.f. performance* Corresponding author. Tel.: 003981 7682420. Fax: 0039812391821.E-mail: CASSINESE@AXPNA1.NA.INFN.IT. Present address: University ofWuppertal, FB PhysiK, GauBstras. 20, D-42097 Wuppertal, Germany.-161162 A. CASSINESE et al.of superconductors degrades drastically at high power. This repre-sents one of the main limiting factors in many applications, and adeep understanding of the high power response of both low and highTc superconductors is yet to be achieved.The losses are usually well characterized by a residual surface resis-tance Rres , a breakdown field H;, and the exponent n of the powerlaw experimentally observed in the field dependence of the surfaceimpedance: Zs(H) ex H n. Considering the rich spectrum of causes fornon-linearities, a good description of the loss mechanisms can beachieved only by investigation of the surface impedance dependenceon several parameters, including the effect of a d.c. external field. Inthe case of HTS films, the comprehension of the r.f. response is alsocomplicated by the difficulty in discriminating between extrinsiceffects, connected to the morphological and structural properties ofthe samples, and intrinsic behavior, related to the unconventionalmicroscopic mechanism of superconductivity in cuprates.In this paper, we are mainly focusing on the study of the lossmechanisms observed in HTS films in comparison with LTS films,since in this last case the intrinsic behavior is well described by theBCS theory. In particular, we report on microwave measurementsperformed on YBCO, which appears nowadays the most promisingmaterial to use in superconducting microwave devices,! and onNb3Sn, which has attracted a renewed interest for recent resultsobtained on coated cavities. 2EXPERIMENTAL DETAILSThe investigation of the r.f. properties is performed by a microstripresonator technique. All details are reported in Ref. [3] The advan-tages of this method are:- a good spatial uniformity in the electrodynamic response observed,due to the small dimensions of the resonator (10 x 10 mm2);- the achievement of high magnetic fields inside the microstrip with arelatively low input power;- the possibility of studying the dependence of Rs and Xs not only ontemperature and r.f. field but also on frequency, with the measure-ment of higher order modes.RF PROPERTIES OF LTS AND HTS FILMS 163One of the main disadvantages is the build-up of the current at theedges of the microstrip, where a degradation of the sample can bepresent due to the photolithographic process.The resonator is formed by a couple of films deposited under iden-tical conditions, separated by a thin spacer made of sapphire (thick-ness h == 125 Ilm) or teflon (h == 12 Ilm). The width w of the microstripcan be varied between 100 and 200 Ilm.The top film is patterned by standard wet photolithography in ameander line geometry (L == 30 mm) yielding, in the case of thesapphire spacer, a fundamental mode at 1.4 GHz.The measurements are performed using a network analyzer in thefrequency domain. The resonator is inserted in a liquid helium dewarand the temperature varied between 4.2 K and Te . The signal launch-ers are capacitively coupled to the device and adjusted in order tomeasure the resonator in the unloaded regime. The maximum inputpower is Pin == 33 dB m, using a commercial room temperature ampli-fier. A pair of Helmholtz coils can be externally applied with a vari-able orientation, to investigate the effect of a d.c. field (Hd .e . :::; 200 Oe)on the microwave response.The surface resistance Rs and the penetration depth A (propor-tional to the surface reactance: Xs == /-lowA) of the sample are relatedrespectively to the quality factor Q and to the resonance frequency fof the resonator by the following formulas:rQ = Rs(coth(tl>') + (tl>')1 sech2(tl>.))'~f -/-low~(Acoth(t/A))f 2r(1)where r is the geometrical factor of the resonator and t the thicknessof the films.The peak magnetic field Hr.f. (Oe) is evaluated considering a phe-nomenological relation between the current and the field inside amicrostrip:4Hr.f ~ 32.54e-O.61n wI, (2)here w is the width of the microstrip (em) and I the total current (A),evaluated as reported in Ref. [5].164 A. CASSINESE et al.RESULTS AND DISCUSSIONIn Figure 1 the r.f. field dependence of a Nb3Sn sample, measured atdifferent temperatures, is reported. The residual surface resistancevalue is Rres == 11lf2 at f == 1.4 GHz. The frequency scaling of the sur-face resistance at T == 4.2 K is almost quadratic, as expected by theBCS theory (Figure 2).The data in Figure 1 show a quick increase of the surface imped-ance at low fields, followed by a nearly linear dependence. At highfields (Hr.f. ~ 120-140 Oe) "breakdown" phenomena occur with arelevant distortion of the resonant peak. As described in Ref. [3], thedata are well described by the Halbritter-Portis model which assumeslosses related to Josephson coupling between grains.6,7 The quickincrease at low fields is related to the flux penetration in the grainboundaries above H c1j , the Josephson junction critical field.A quantitative agreement can be found between the values of H c1j ,Rres and A predicted by the model and the measured ones, taking into180o T=4.2 KT=4.5 KT=4.8 K• T=5.2 Kv T=6.1 KT=7.3 KT=8.2 KT=11.7 K150120906030oFIGURE 1 Rs vs HLf. at different telnperatures for the first resonant lTIode (I\ =1.4 GHz) of a Nb3Sn sample.RF PROPERTIES OF LTS AND HTS FILMS 165200 ...160r---'l 120 ~C:t~~tIl 8040o -0 2......'....••......I4 6f[GHz]I8 10FIGURE 2 Frequency dependence of the surface resistance Rs at T = 4.2 K forNb3Sn. The dotted line represents the best fit of the data using the expression:Rs = olD, with 8 = 1.9 ± 0.1.account the critical current density (fe == 5 x 105 A/cm2) and the aver-aged grain dimension (a == 20 nm) as determined by experiments. 3As reported in Figure 3, a good consistence is observed between the"breakdown" field H; as obtained increasing the input power feedingthe microstrip or externally applying a d.c. magnetic field parallel tothe film surface.We believe that the nature of this field is related to the lower crit-ical field H e1 , somewhat reduced by disorder effects due to the smalldimension of the grains.The temperature dependence of H; in Figure 4 is well described bya linear law: H;(T) == H;(O) x (1 - T/Tc ). A very similar behavior,likely due to the granularity of the films, is also observed in the crit-ical current density measurements feeT).(b) YBCOIn case of films grown by an Inverted Cylindrical Magnetron Sputter-ing (ICMS) technique,8 the dependence at low fields9 appears verysimilar to what observed for granular Nb3Sn superconductors(Figure 5).166 A. CASSINESE et al.8.0x1040 0 Hr.f. ; Hd.c=O7.0x104 0 0 Hd .c ; Hr.f. =200e6.0x1045.0x104 o 0 0 000Q DO 0 0 DOD DOo 0 ~) 0 Q4.0x104 eP 0 00 03.0x104 0 002.0x1040001.0x1040 30 60 90 120 150H[Oe]FIGURE 3 Q vs Hr.f. (circles) and Q vs H d .c . (squares) at T=4.2K for a Nb3Snsample. The measurements as a function of the d.c. field are performed with an inputpower Pin = -10 dB m, corresponding roughly to Hr.f. = 20 Oe.150!! ! !120 ~!90 ~Q)~(,) 60 I-~30 ~0 I I4 6 10T [K]FIGURE 4 The breakdown field H~ (T) at different temperatures evaluated by ther.f. field measurements.RF PROPERTIES OF LTS AND HTS FILMS 1675040302010• 7 KA 11 K* * *r******* T ~~ ~0000000 <>53K~DDD D77K<> <> <> <> <> <> <> <> <> <> * 88' K0000 .. '- ...~\ \.\.""'\ .. ..oFIGURE 5 Rs vs Hr.f. at different temperatures for the first resonant mode(11 = 2.2 GHz) for a YBCO film grown by the ICMS technique.9On the contrary, the grain model is unable to describe the behaviorobserved on YBCO films obtained by a high pressure oxygen sputter-ing technique and reported in Figure 6. In this case, no relevantincrease of the surface impedance was observed at low fields. The Rsvs r.f. field dependence appears flat up to Hr.f. == 400 Oe; at higherfields an almost linear increase is observed, but no breakdown effectsare seen up to a maximum field higher than 8000e.The residual surface resistance is lower than in case of films grownby ICMS technique (R res ~ 20 J.lO compared to Rres ~ 50 J.lO at fi ==2 GHz) but higher than the best films reported in literature. 10Loss mechanisms related to the uniform or global heating of thesample10 can also be excluded. A simple way to show this is to com-pare the Rs and Xs field dependence considering the r-factor,ll,12defined as:~Rs ~~(l/Q)rH == ~Xs == -2~f/f· (3)It can be easily shown that the r-factor is independent of the geometri-cal factor r and only sensitive to the properties of the superconductor.168 A. CASSINESE et al.1000800400 600HrJ. [Oe]200o10-3 r-------------------------,o T=4.3KT=32.2K'J T=33.7Ko T=36.2K~ T=42.2KA T=47.1K'W T=54.3 K.. T=58.4Km T=66.2KT=75.6KFIGURE 6 Rs vs Hr.f. at different temperatures for the first resonant mode(fl = 2.2 GHz) for a YBCO film grown by a high oxygen d.c. sputtering technique.In the case of uniform heating the rH value must be similar to whatexpected varying the temperature:fJQ-1/arrT == -a//2ar (4)while the experimental data show rH';:::j0.6, much larger thanrT';:::j 0.05.Local heating effects can also be excluded in this last case, in fact itshould be rH» 1,12 which in our measurements is observed only onpoor quality YBCO samples (Rs';:::j 1mn at / == 1.4 GHz).CONCLUSIONSR.f. measurements performed with a microstrip resonator techniqueon Nb3Sn and YBCO films grown by sputtering techniques have beenreported.The Josephson junction model seems to describe the behavior ofgranular superconductors very well, independently of the material.In high quality YBCO filn1s, a better understanding of the intrinsicRF PROPERTIES OF LTS AND HTS FILMS 169properties is still lacking. Losses related to the heating of the sampledo not seem to playa relevant role, and cannot explain the experi-mental data in this last case.AcknowledgmentsThe authors wish to thank J. Halbritter and M. Rein for critical dis-cussions. The technical assistance of A. Maggio and S. Marazzo isgratefully acknowledged.References[1] N-. Newman and G. Lyons, J. Supercond. 6, 119 (1993).[2] G. Miiller, P. Kneisel, D. Mansen, H. Piel, J. Pouryamout and R.W. Roeth,Proceedings of the 5th European Particle Accelerator. Conference EPAC'96, Sitges,Vol. 3, 1996, pp. 2085-2087.[3] A. Andreone, A. Cassinese, A. Di Chiara, M. Iavarone, F. Palomba, A. Ruosi andR. Vaglio, J. Appl. Phys. 82, 1736 (1997).[4] S.M. EI-Ghazaly, T. Hoh and R.B. Hammond, IEEE Microwave and Guided WaveLett. 1,255 (1991).[5] D.E. Oates, A.C. Anderson and P.M. Mankiewich, J. Supercond. 3, 251 (1990).[6] J. Halbritter, J. Appl. Phys. 68, 6315 (1990).[7] A.M. Portis, "Electrodynamics of High-Temperature Superconductors", WorldScientific Ed., Singapore, 1992.[8] A. Andreone, A. Di Chiara, F. Fontana, F. Miletto Granozio, G. Peluso, U. Scottidi Uccio and M. Valentino, Sov. J. Low. Temp. Phys. 18, 1113 (1992).[9] A. Andreone, C. Cantoni, A. Cassinese and A. Di Chiara, Proceedings of the 2ndEuropean Conference on Applied Superconductivity, lOP Publishing, Bristol, 1995,p.1115.[10] M. Hein, In "Studies of High Temperature Superconductors", Vol. 18, A. Narlikar(ed.), Nova Science Publ., New York, 1996; M. Hein, Proceedings of the 3rdEuropean Conference on Applied Superconductivity, 1997 (in press).[11] J. Halbritter, J. Supercond. 8, 691 (1995).[12] M.A. Golosovsky, H.J. Snortland and M.R. Beasley, Phys. Rev. B 51,6462 (1995).

RF properties of low and high temperature superconducting films

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