SESSION R4: CRITICAL CURRENTS, PINNING AND GRAIN BOUNDARIES8:30 AM *R4.1
Chairs: Paul Grant and Yasuhiro Iijima
Wednesday Morning, April 2, 1997
Salon 15
cuprates has a
predominant 
-symmetry component. In the presentation,
the consequences of the -wave symmetry on the transport
properties of grain boundaries in high T superconductors will be
analyzed. It will be shown that a boundary region with a depressed
order parameter is 
at grain boundaries
in high T superconductors, so that for fundamental reasons the
boundary is a weak link or a Josephson junction with properties
controlled by the misalignment angle. 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 (
1 GeV) heavy ions
(e.g., Au) or by fission of heavy nuclei (such as Bi) within a
superconductor by energetic (0.8 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 (10
) 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. 
Cu
0
, 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. 
2 
if dislocation line is
parallel to vortex and dislocation density equals or exceeds matching
field. Indeed, the highest j (
A/cm
at 77 K) among any
high-T superconducting materials is observed in the highly
biaxially-oriented YBaCuO (YBCO) thin films,
where dense edge dislocation structure (up to 2
10
cm
) revealed by TEM study. TEM of YBCO films (2 
15 nm
thick, prepared by magnetron sputtering or PLD on MGO and SrTiO
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
15) being recalculated to real misorientation angle (
) 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 j 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 j on the misorientation angle of
the adjacent domains in 
-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. , BIAXIALLY ALIGNED YBaCuO 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.
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 
m. X-ray
diffraction shows typical in-plane textures of 7 10 FWHM,
with 
-perpendicular alignment to 1 FWHM. The
resulting films typically have T(R = 0) 
88 K, with
zero-field J values in the range 5-7 x 10
A/cm at 77 K.
In magnetic fields, J(H
c) 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.
, biaxially
textured YBCO thick films by means of the
Ion-Beam-Assisted-Deposition technology. Critical current densities,
Js, in excess of 1.0 MA/cm at 75 K can be achieved in film
thicknesses in excess of 2.0 microns and remain above 0.1 MA/cm
in magnetic fields above 4.0 T. The dependence of J 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 J 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.CuO
(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.
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 J
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 YBa
bicrystal thin
film grown on the CeO 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. 
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
5 may be approximated as follows: 
1/sin
(/2).
SrCaCuO
(2212) and
BiSrCuO
(2201) frequently occur. An atomic
model of the 13.5 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 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. |
| Society Information | Meetings | Membership | Publications | Marketing Opportunities | Materials Research Society 506 Keystone Drive, Warrendale, PA 15086-7573 USA Phone: 724 779-3003, Fax: 724 779-8313, webmaster@mrs.org |