SESSION R4: CRITICAL CURRENTS, PINNING AND GRAIN BOUNDARIES
Chairs: Paul Grant and Yasuhiro Iijima
Wednesday Morning, April 2, 1997
Salon 15
8:30 AM *R4.1
IMPLICATIONS OF d-WAVE SYMMETRY FOR GRAIN BOUNDARY CRITICAL CURRENTS, Jochen Mannhart, H. Hilgenkamp, Univ of Augsburg, Dept of Experimentalphysik VI, Augsburg, GERMANY.

Various experiments have shown that the order parameter in most high Ttex2html_wrap_inline73 cuprates has a predominant tex2html_wrap_inline75-symmetry component. In the presentation, the consequences of the tex2html_wrap_inline75-wave symmetry on the transport properties of grain boundaries in high Ttex2html_wrap_inline73 superconductors will be analyzed. It will be shown that a boundary region with a depressed order parameter is tex2html_wrap_inline81 at grain boundaries in high Ttex2html_wrap_inline73 superconductors, so that for fundamental reasons the boundary is a weak link or a Josephson junction with properties controlled by the misalignment angle.

9:00 AM *R4.2
"HARD" AND "EASY" DIRECTIONS OF VORTEX MOTION THROUGH SPLAYED COLUMNAR DEFECTS, Lia Krusin-Elbaum, D. Lopez, A. D. Marwick, J. Z. Sun, H. Safar, V. M. Vinokur, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.

One of the principal factors limiting current conduction in high-Ttex2html_wrap_inline73 superconductors is easy motion of magnetic vortices in the bulk material. The most efficient way to pin vortices is with columnar defect structure, which can be installed either by irradiation with swift (tex2html_wrap_inline871 GeV) heavy ions (e.g., Au) or by fission of heavy nuclei (such as Bi) within a superconductor by energetic (tex2html_wrap_inline870.8 GeV) protons. The former results in a nearly aligned columnar tracks of amorphous damage, while the latter produces splayed tracks of similar diameter. We have shown that uniformly splayed fission tracks boost current conduction by orders of magnitude and enlarge the useful field-temperature range of highly anisotropic Bi- and Hg-based cuprates. In a less anisotropic YBCO, there is a splay angle (tex2html_wrap_inline8710tex2html_wrap_inline93) for which the performance is optimal. In this talk we discuss results of the experiment designed to probe the anisotropy of vortex motion through a forest of splayed columnar pins. C axis oriented epitaxial YBCO films, with two crisscrossing families of parallel columnar tracks, were patterned in a geometry allowing control of the relative orientation between the external current and the splay planes in the same sample. This permits us to establish ''hard'' and ''easy'' directions of vortex motion. Our results indicate that vortex motion is favored in the direction normal to the splay plane, in contradiction with present theoretical models. The dynamics of vortices in the presence of small-angle splay, tilt, and isotropic splay will be compared.
* In collaboration with-D. Lopez, A.D. Marwick, J. Sun. H. Safar, and V.M. Vinokur.

9:30 AM *R4.3
EDGE DISLOCATIONS AND CRITICAL CURRENT DENSITY IN EPITAXIAL YBatex2html_wrap_inline95Cutex2html_wrap_inline970tex2html_wrap_inline99, Vladimir M. Pan, V. L. Svetchnikov, Inst for Metal Physics, Kiev, UKRAINE; A. I. Kasatkin, Inst for Metal Physics, Superconductivity, Kiev, UKRAINE; H. C. Freyhardt, Univ of Goettingen, Inst fur Metallphysik, Goettingen, GERMANY.

The most effective pinning is ensured by linear defect like a nonsuperconducting core of edge dislocation with cross-size tex2html_wrap_inline101 2 tex2html_wrap_inline103 tex2html_wrap_inline105 if dislocation line is parallel to vortex and dislocation density equals or exceeds matching field. Indeed, the highest jtex2html_wrap_inline73 (tex2html_wrap_inline109 A/cmtex2html_wrap_inline111 at 77 K) among any high-Ttex2html_wrap_inline73 superconducting materials is observed in the highly biaxially-oriented YBatex2html_wrap_inline95Cutex2html_wrap_inline97Otex2html_wrap_inline99 (YBCO) thin films, where dense edge dislocation structure (up to 2tex2html_wrap_inline121 10tex2html_wrap_inline123 cmtex2html_wrap_inline125) revealed by TEM study. TEM of YBCO films (2 tex2html_wrap_inline127 15 nm thick, prepared by magnetron sputtering or PLD on MGO and SrTiOtex2html_wrap_inline97 substrates) is done on plane-view specimens and demonstrated very high-density of edge dislocations having their lines perpendicular to substrate. Due to a use of Moire pattern technique, the images of a single dislocation and edge dislocation arrays are clearly observed. The power spectrum (optical diffraction pattern) over the Moire fringes picture is exhibited two reflections due to regularity in the fringes sequence; the angular splitting of reflections (about 15tex2html_wrap_inline93) being recalculated to real misorientation angle (tex2html_wrap_inline133 ) confirms a presence of low-angle tilt boundaries (LAB) in YBCO films. Rows (''walls'') of edge dislocations arranged in domain sub-boundaries are responsible for the azimuthal ' 'mosaic''spread in YBCO film which is detectable by X-ray diffraction analysis. A consistent model of vortex depinning from linear defect and its motion along the equidistant array of parallel dislocation lines in the LAB is developed to evaluate jtex2html_wrap_inline73 and its dependence upon the misorientation of adjacent domains. It is taking into account (for the thin film case) the surface potential interaction with a depinned fraction of vortex which is deviated from a linear defect along the surface under the Lorentz force action. The critical current should be observed when this depinned vortex ''tongue'' aligned to a surface reaches the adjacent dislocation with a current increase. In the framework of this model it is easy to explain quantitatively the dependence of jtex2html_wrap_inline73 on the misorientation angle of the adjacent domains in tex2html_wrap_inline139-plane which is the intrinsic feature of vortex behavior in such a LAB, assuming nothing as to the misorientation angle effect on its quasiparticle transparency and Josephson properties.

10:30 AM *R4.4
SUPERCONDUCTING PROPERTIES OF HIGH-Jtex2html_wrap_inline73, BIAXIALLY ALIGNED YBatex2html_wrap_inline95Cutex2html_wrap_inline97Otex2html_wrap_inline99 THICK FILMS ON METALLIC TAPE SUBSTRATES, David K. Christen, David P. Norton, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Amit Goyal, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; John D. Budai, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Qing He, Univ of Tennessee, Knoxville, TN; C. E. Klabunde, D. M. Kroeger, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN ; Dominic F. Lee, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; F. A. List, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Mariappan Paranthaman, Oak Ridge National Laboratory, Dept of Chemical & Analytical Science, Oak Ridge, TN; B. Saffian, Univ of Tennessee, Knoxville, TN; E. D. Specht, Matthew F. Chisholm, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN.

Thick films of biaxially-aligned YBCO have been deposited on short segments of sharply textured nickel tapes produced by thermomechanical processing. The nickel tapes are coated with appropriate epitaxial oxide buffer layers using vapor deposition techniques, including evaporation, sputtering, and pulsed laser ablation. To date, bilayer buffers of CeOtex2html_wrap_inline95 and YSZ have served to provide both chemical isolation from the nickel and conferred biaxial texture in YBCO films grown to thicknesses of 1 to 3 tex2html_wrap_inline151m. X-ray diffraction shows typical in-plane textures of 7 10tex2html_wrap_inline93 FWHM, with tex2html_wrap_inline155-perpendicular alignment to 1tex2html_wrap_inline93 FWHM. The resulting films typically have Ttex2html_wrap_inline73(R = 0) tex2html_wrap_inline161 88 K, with zero-field Jtex2html_wrap_inline73 values in the range 5-7 x 10tex2html_wrap_inline165 A/cmtex2html_wrap_inline111 at 77 K. In magnetic fields, Jtex2html_wrap_inline73(Htex2html_wrap_inline171c) and irreversibility lines are comparable to, or exceed, those of epitaxial films on single crystal oxides. Comparative measurements confirm the strongly coupled nature of electrical transport, and reveal properties that are characteristic of strong pinning by extended defects. Atomic resolution, Z-contrast TEM shows that the YBCO low-angle grain boundaries are free of impurities, with periodicity in the disorder along the boundary.
*Managed by Lockheed Martin Energy Research Corp. under contract DE AC05-96OR22464 with the U.S. Department of Energy.

11:00 AM R4.5
CRITICAL CURRENTS AND FLUX PINNING IN BIAXIALLY TEXTURED YBCO THICK FILM CONDUCTORS, Jeffrey O. Willis, Martin P. Maley, Los Alamos National Laboratory, Los Alamos, NM; J. Yates Coulter, Paul N. Arendt, Stephen R. Foltyn, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM; Hugo F. Safar, Los Alamos National Laboratory, Superconductivity Technology Center, Los Alamos, NM.

Recently, we have produced high Jtex2html_wrap_inline73, biaxially textured YBCO thick films by means of the Ion-Beam-Assisted-Deposition technology. Critical current densities, Jtex2html_wrap_inline73s, in excess of 1.0 MA/cmtex2html_wrap_inline111 at 75 K can be achieved in film thicknesses in excess of 2.0 microns and remain above 0.1 MA/cmtex2html_wrap_inline111 in magnetic fields above 4.0 T. The dependence of Jtex2html_wrap_inline73 on magnetic field angle shows a prominent peak with the field aligned with the c-axis and other evidence for strong pinning defects extending through the film thickness along the film normal. Patterned bridge studies show strong in-plane anisotropy of Jtex2html_wrap_inline73 correlated with current flow along or diagonal to the prevailing twin-plane mosaic orientation, and indicating a substantial contribution from twin boundary pinning. We report results of critical current measurements as a function of temperature magnetic field and field orientation on films of various thickness and discuss pinning mechanisms.

11:15 AM R4.6
TEM MICROSTRUCTURAL ANALYSIS OF YBatex2html_wrap_inline95Cutex2html_wrap_inline97Otex2html_wrap_inline189 (001) FACETED ASYMMETRIC AND SYMMETRIC BOUNDARIES GROWN USING LASER ABLATION TECHNIQUE AND CORRELATION WITH ITS ELECTRICAL PROPERTIES, Jack W.H. Tsai, Columbia Univ, Dept of Materials Science, New York, NY; Steven C. Tidrow, U.S. Army Research Laboratory, Dept of Physical Sciences, Ft. Monmouth, NJ; Siu-Wai Chan, Columbia Univ, Dept of Materials Science, New York, NY.

The studies of the various grain boundary structures in the high Ttex2html_wrap_inline73 superconductor has long been an interest both from understanding the fundamental properties of superconductivity and electron devices application perspective. Given the short coherence length of its order parameter for the HTS materials, the grain boundary behaves as SNS junction in which the electrons tunnel across the weak links as quasiparticles. Recent findings attributing the depression of Jtex2html_wrap_inline73 across the boundary is due to combination boundary faceting and the wave symmetry of the pairing state [1]. It was also observed that a macroscopically straight boundary plane can contain many microscopic facets [2]. The faceted boundary plane can be thermodynamic stable if the energy sum of the individual facet planes is less than the energy of the straight boundary plane. Here, we study the faceting mechanism of [001] tilt boundaries of the YBatex2html_wrap_inline195 bicrystal thin film grown on the CeOtex2html_wrap_inline95 bicrystal templates using the laser ablation technique. We use the Transmission Electron Microscope to study the faceted plane and correlate the boundary structure to its electrical properties.
[1] J. Mannhart, H. Hilgenkamp, B. Mayer, Ch. Gerber, J.R. Kirley, K.A. Moler, and M. Sigrist, Phys. Rev. B, p 2782-2785, vol. 77, 1996.
[2] I-Fei Tsu, S.E. Babcock, and D.L. Kaiser, J. Mater. Res. p 1383-1397, Vol. 11, 1996.

11:30 AM R4.7
GENERALIZED RANDOM DISLOCATION THEORY OF INTERGRAIN BOUNDARY CRITICAL CURRENT, Evgenii Z. Meilikhov, R. M. Farzetdinova, RRC Kurchatov Inst, Moscow, RUSSIA.

Dislocation model of the weak link at the boundary between two superconducting grains is based on the assumption of order parameter suppression in the regions subjected to strong stresses. It was shown [1] that a critical current theory based on calculating the stresses for a system of periodical edge dislocations (have been considered up to not) is not consistent with experiments. The theory taking into account randomization of dislocations at the intergrain boundary (e.g., due to thermal fluctuations or incommensurability of dislocation period with the lattice constant) is developed. At small misorientation angles tex2html_wrap_inline199 produced stress field is weakly sensitive to random displacements of dislocations. With the rise of the angle, the mutual compensation of individual dislocations' stresses ceases that results in the fast lowering of the intergrain critical current ic. Corresponding angular dependence for tex2html_wrap_inline199tex2html_wrap_inline203 5tex2html_wrap_inline93 may be approximated as follows: tex2html_wrap_inline207 tex2html_wrap_inline73 tex2html_wrap_inline211 1/sintex2html_wrap_inline213 (tex2html_wrap_inline199/2).
We conclude that the adequate description of intergrain boundary superconductive features could only be obtained in the framework of the randomly distributed dislocations model.
[1] Meilikhov E.Z., Proc. 21st Int. Conf. on Low Temp. Phys., Czech J. Phys., v. 46 - Suppl., Part S2, 1015 (1996).

11:45 AM R4.8
HRTEM INVESTIGATIONS OF THE ATOMIC STRUCTURE OF INTERPHASE GRAIN BOUNDARIES IN Bi-Sr-Ca-Cu-O-TYPE CERAMICS, N. D. Zakharov, D. Hesse, Max-Planck-Inst, Dept of Microstructure Physics, Halle/Saale, GERMANY; H. Frank, R. Stollmann, G. Guntherodt, RWTH Aachen, Aachen, GERMANY.

The atomic structure of grain boundaries between different BSCCO-type phases in BSCCO ceramics is investigated by high-resolution transmission electron microscopy (HRTEM). Interphase boundaries between Bitex2html_wrap_inline95Srtex2html_wrap_inline95CaCutex2html_wrap_inline95Otex2html_wrap_inline223 (2212) and Bitex2html_wrap_inline95Srtex2html_wrap_inline95CuOtex2html_wrap_inline229 (2201) frequently occur. An atomic model of the 13.5tex2html_wrap_inline93 low-angle 2212/2201 tilt boundary is proposed. The interphase boundary consists of an array of edge dislocations which, as a rule, are separated by three or four planes of type a/2(100). Remarkably, the Bi layers cross the grain boundary almost without any disturbances, whereas each central Cu layer of the 2201 phase splits into two within the 2212 phase. Low-angle grain boundaries between 2212 and 1212 phases are also frequently observed. An atomic model of the 2212/1212 low-angle grain boundary is proposed. It is formed by inserting an extra Bi plane into the 1212 phase. This procedure is equivalent to the introduction of a partial dislocation at the end of each extra Bi plane with a Burgers vector IbI tex2html_wrap_inline101 0.27 nm and the formation of an extended stacking fault in the Ca plane between these partial dislocations. Single lamellae of the 2212 phase incorporated into the 1212 matrix also occur. They obviously form by the insertion of an extra Bi-O plane into the 1212 matrix.
This work was supported by BMBF No. 13N6451/1.



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