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Josephson properties of basal-plane-faced tilt boundaries in YBa2Cu3O72d thin films B. H. Moeckly and R. A. Buhrman School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853-2501 ~Received 29 August 1994; accepted for publication 7 October 1994! The Josephson properties of 90° basal-plane-faced tilt boundaries formed between c-axis and a-axis normal grains in YBa2Cu3O72d thin films are reported. These boundaries have a low conductance which results in underdamped junction behavior. The junction capacitance and kinetic inductance both scale directly with junction critical current and conductance. The results emphasize the inhomogeneous and filamentary nature of the superconducting properties of cuprate grain boundaries. © 1994 American Institute of Physics. Considerable effort has been directed toward understanding the weak link properties of high-angle grain boundaries in the cuprate superconductors, and, in the thin film case, toward the utilization of such weak links as Josephsonlike elements in quantum device applications. This latter effort has been largely concerned with the study of YBa2Cu3O72d ~YBCO! tilt boundaries ~TBs! created by the union of two crystalline grains whose in-plane a – b orientations differ by a large ~.15°! amount.1 Such TBs are generally found by electron microscopy to be clean, abrupt, and free of secondary phases.2 The current–voltage ~I – V! characteristics of these high-angle TBs have more or less the character of a Josephson element nonideally shunted by a linear resistor. Notable features of TBs are the much lower than ideal value of the characteristic voltage I c R n and the widely observed scaling behavior of I c R n ;J ac , with reported3,4 values of a ranging from 0.5 to 0.7 depending on the type or method of formation of the TB. ~Here, I c is the critical current, J c is the average critical current density, and R n is the normal-state resistance of the TB.! The response of in-plane TBs to microwave radiation and magnetic field H has indicated that they are filamentary with respect to their superconducting properties.5– 8 The boundaries can be modeled6 as possessing small areas of material on each side which have a high degree of basalplane oxygen order and low vacancy density, thereby providing strong superconducting contact. These contact areas are adjoined by material with lower oxygen content and/or higher disorder, giving rise to both normal conducting paths, most likely via localized states,9 and capacitive connections across the TB. The areas of oxygen disorder are created by the distressing ease by which the basal-plane oxygen ion can diffuse through YBCO and by the concomitant ability of very weak atomic forces to create vacancy aggregates.6,10 Here, we report on studies of an alternative type of TB Josephson junction consisting of a 90° basal-plane-faced boundary which is formed by growing one YBCO grain so that its c axis lies in the plane of the substrate, while its neighboring grains ~compromising the bulk of the film! grow with their c axes normal to the substrate. The behavior of these ‘‘a – c’’ TBs is quantitatively different from that of our naturally occurring in-plane TBs: The scaling behavior of I c R n with J c is substantially weaker, and the TB resistance is considerably higher at a given I c . The resultant under3126 Appl. Phys. Lett. 65 (24), 12 December 1994 damped Josephson junction behavior provides new insights into the electrodynamics of cuprate TBs. To produce such TBs we adopted the simple expedient of lowering the temperature of an MgO substrate in a laser ablation YBCO growth process to the point at which a fraction ~;1%–5%! of the grains nucleating on the substrate have an a- or b-axis-normal orientation within the predominantly c-axis-normal film. Since these a-axis grains are, typically, .2 mm long in the in-plane a- ~or b-! axis direction, the film can be easily patterned so that an ‘‘a grain’’ completely bisects a 1- or 2-mm-wide microbridge. Perhaps because no epitaxial seed layers are necessary in this bruteforce approach, we find by electron microscopy that the resultant a – c 90° TBs are abrupt and cleanly bounded on one side by a basal plane of the a-axis-normal grain.2 The bulk of the transition of microbridges containing a-grain inclusions typically occurs near 88 K, but there are often two steps in the transition to zero resistance which occurs at lower T, <75 K. We attribute these to the separated transitions of the two TBs formed by the presence of the a-grain inclusion. The transition of the bulk of the a grain is generally not distinctly evident. We conclude that the a grains contain a sufficient density of stacking faults and other extended defects such that the conductivity of the bulk of the grain is considerably higher than is observed in less-defected single crystals.11,12 As shown in Fig. 1, the I – V characteristics of these samples exhibit Josephson behavior in the underdamped or nearly underdamped regime13,14 which is attributable to the weaker of the two a – c TBs. The Josephson character of the TB is supported by a strong Shapiro step response to microwave radiation. Unlike in-plane TBs, the specific resistance of these a – c TBs tends to be quite high, ranging from 531028 to 1025 V cm2 for the devices examined. Despite these high resistances and the low values of J c , which range from 102 to 105 A/cm2, the 4.2-K values of I c R n of the contacts are fairly high, of order 1 mV. While the I c R n of these a – c TBs does scale roughly with J c , the mean variation as illustrated by Fig. 2~a! is quite weak, a'0.3. The hysteresis that is generally seen in the lowtemperature I – V characteristics can be used to establish the specific shunt capacitance C/A of the junction.13,14 ~A is the junction area.! C/A is highly variable for our devices and is correlated with the variation in the junction R n A and/or J c as 0003-6951/94/65(24)/3126/3/$6.00 © 1994 American Institute of Physics Downloaded¬21¬Apr¬2011¬to¬128.84.158.108.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/about/rights_and_permissions FIG. 1. Current–voltage characteristics of two YBCO microbridges, each of which is bisected by an a-axis-normal grain. In ~a! I c R n 5712 mV; in ~b! I c R n 5620 mV. The inset in ~a! illustrates the current path, and the inset in ~b! shows the response to 25-GHz microwave radiation. shown in Fig. 2~b!. This variation in C/A by more than an order of magnitude, while J c charges by less than a factor of 100, clearly indicates that the a – c contact is not a typical tunnel barrier, while the high R n A and underdamped behavior also rules out the possibility of a uniform superconductornormal-superconductor junction. Additional information can be obtained from the very distinct structure that is often seen in their I – V characteristics, examples of which are shown in Fig. 3. This structure has all the hallmarks of the ‘‘beating mode’’ behavior15–18 of an underdamped two-junction SQUID in the regime bL 5L s I c /F 0 >1. ~Here, L s is the SQUID inductance, and F0 is the superconducting flux quantum.! In this situation an additional solution of the SQUID equations exists which describes a behavior distinctly different from that given by the standard zero-voltage supercurrent solution and the solution for bias currents above the maximum critical current of the device where the phases of the two Josephson elements increase or ‘‘free-run’’ in phase. In the intermediate, beatingmode ~BM! regime, the phase of one junction is approximately stationary, while that of the other junction precesses by some multiple of 2p. The junctions periodically reverse their roles in a process that allows one or more flux quanta nF 0 to enter the SQUID loop through one junction and subsequently exit through the other. An observable consequence of this BM state is an additional branch in the I – V characteristic between the two usual SQUID states. For symmetrically fed SQUIDs with two identical junctions, a bias H is required to access the BM regime. For an asymmetrically fed Appl. Phys. Lett., Vol. 65, No. 24, 12 December 1994 FIG. 2. ~a! The variation of a – c boundary I c R n with critical current density J c . ~b! The variation of junction capacitance ~open circles! as determined from the I – V hysteresis at 4.2 K with R n A, and the variation of the minimum junction inductance ~filled circles! necessary to produce the observed SQUID-like beating-mode behavior. device, or for one with junctions having substantially different critical currents, this solution can be obtained without a field bias, although the behavior is strongly varied by a small H, sufficient to change the enclosed flux by F0/2. This BM behavior is often very closely mimicked by the behavior of our a – c TBs as can be seen by comparing Fig. 3 with the predictions of Refs. 15–17. In some instances we require a small H in order to observe the apparent beating mode; in other cases the resonance occurs at zero H but can be readily removed by application of a small field. These results strongly suggest that the a – c TBs, which are only 1–2 mm wide, can often be effectively modeled as consisting of two ~or more! very small, localized underdamped Josephson junctions connected by a substantial inductance to effectively form a SQUID-like structure. The calculations15–17 indicate that the BM behavior requires that bL *3/2 and 1&bc &pbL . This condition puts a lower bound on the effective inductance of the a – c contacts that ranges from 10211 to 1028 H, depending upon the TB J c , or R n A, as also shown in Fig. 2~b!. We note that the positions of the BM branches on the a-grain I – V curves do roughly correspond to the LC resonance value predicted from our estimated values of capacitance and inductance. If the areas of superconducting contact at the TB can be modeled as filaments of dimensions less than lL , then the kinetic inductance ~L k ! of these filaments can easily supply the necessary L s for the beating-mode behavior. The onedimensional Ginzburg–Landau model of a uniform narrow B. H. Moeckly and R. A. Buhrman 3127 Downloaded¬21¬Apr¬2011¬to¬128.84.158.108.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/about/rights_and_permissions FIG. 3. Examples of a – c TB I – Vs exhibiting SQUID-like BM behavior which is seen as an intermediate branch of the I – V located between the zero-voltage and high-voltage behavior. superconducting filament of length l gives a kinetic inductance19 L k5 S D 2 F0 ~ l/ j ! 3 11 , 2 p I c 3 A3 ~ l/ j ! 2 23 p 2 where j is the superconducting coherence length. Given the extremely short values of j for YBCO in either the a – b or c-axis directions, a mean l of less than 5 nm at each contact is sufficient to provide the requisite L k . If the actual tilt boundary contact is a high transparency tunnel barrier, then I c of the filaments leading up to the barrier will be greater than that of the contact, and a somewhat longer, but still quite short, effective length would be required. The fact that the BM behavior is observed over a range of TB I c s varying by more than a factor of 100 strongly supports an explanation in which the effective SQUID inductance scales at least roughly with the TB I c . We additionally observe ‘‘subharmonic’’ ~half-integral! Shapiro steps on our a-axis-grain TB I – Vs in the presence of a magnetic field, consistent with high-inductance contacts at the grain boundary.7 We have previously shown6,10 that weak electromigration ~EM! forces can readily displace basal-plane oxygen ions and thereby affect the superconducting properties of both uniform YBCO thin films and in-plane TBs. The a – c TBs are also extremely easily affected by room-temperature EM, and increases in I c by over a factor of 20 have been accomplished. EM can also lead to decreases in R n by up to eight times, and overall increases in I c are easily produced. The persistence of BM behavior in the I – Vs after such 3128 Appl. Phys. Lett., Vol. 65, No. 24, 12 December 1994 changes provides substantial evidence for the picture of the EM process as one in which the biased oxygen diffusion acts to add or subtract filamentary contacts. There is little in the a – c electromagnetic behavior that distinguishes them from in-plane TBs. We have also observed BM behavior in the latter structures, albeit not always so clearly or so commonly. The 1/f noise character of the a – c TBs is nearly identical, when properly scaled, to that of in-plane TBs.20 We conclude that the transport properties of both types of TBs are established largely by the inhomogeneous oxygen disorder in the material immediately adjacent to the boundaries. The higher R n A and generally lower J c of the a – c boundaries also suggest that the transport across the boundary occurs only at sparsely distributed locations, possibly at places where extended defects at the basal plane face of the a-grain provide normal and superconducting pathways that effectively short the ideal a – c contact. This is simply an extension of the argument that the low resistivity of the a-axis grain itself indicates that the intragrain transport in the c-axis direction proceeds predominately via extended defects in the thin film grain and not by true c-axis conduction. This research was supported by the Office of Naval Research ~N0014-89-J-1692! and the Naval Research Laboratory ~N0014-93-K-2001!. Additional support was provided by the National Science Foundation through use of the National Nanofabrication Facility. 1 For a recent review see: R. Gross in Interfaces in High-Tc Superconducting Systems, edited by S. L. Shindé and D. A. Rudman ~Springer, New York 1994!, pp. 176 –209. 2 D. H. Shin, J. Silcox, S. E. Russek, D. K. Lathrop, B. Moeckly, and R. A. Buhrman, Appl. Phys. Lett. 57, 508 ~1990!; Science and Technology of Thin Film Superconductors 2, edited by R. D. McConnell and R. Noufi ~Plenum, New York, 1990!, pp. 3191. 3 S. E. Russek, D. K. Lathrop, B. H. Moeckly, R. A. Buhrman, D. H. Shin, and J. Silcox, Appl. Phys. Lett. 57, 1155 ~1990!. 4 R. Gross, P. Chaudhari, M. Kawasaki, and A. Gupta, Phys. Rev. B 42, 10 735 ~1990!. 5 D. K. Lathrop, B. H. Moeckly, S. E. Russek, and R. A. Buhrman, Appl. Phys. Lett. 58, 1095 ~1991!; D. K. 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