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*******NOTE CHANGED E-MAIL ADDRESS INFORMATION JUST BELOW.

HIGH-Tc UPDATE E-MAIL VERSION, VOL. 6, NO. 3, FEBRUARY 1, 1992.
Published for the Division of Materials Sciences, Office of Basic Energy
Sciences, USDOE, under Contract W-7405-eng-82 with the Ames Laboratory,
Iowa State University.  Funded by DMS/BES/USDOE, DARPA/DOD, ONR/DOD,
EPRI, NASA, and other agencies, organizations, and individuals.
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PLEASE READ:  The electronic-mail version of High-Tc Update is generated
from a Macintosh Microsoft Word file and turned into a text file that
can be transferred electronically.  Formatting commands, Greek symbols,
diacritical marks, etc. are lost in this transformation.  In order to
improve the readability of the e-mail version, the newsletter staff add
explanatory marks as needed to the text file.  For example, a carat
(10^5) indicates a superscript (ten to the fifth).  A carat followed by
a bracket (cm^[-2]) indicates everything within the brackets is
superscripted (centimeter to the minus 2).  A bracket followed by a
carat ([18]^O) indicates everything before the carat is superscripted.
An underline (M_i) indicates a subscript (M subscript i).  Most Greek
letters are spelled out (Delta, mu, tau, pi, Omega), although delta is
left as "d."  In most instances, easily recognizable formulas or units
are left as they appear:  Tc, Jc, YBa2Cu3O7, O2.  Mu-m is changed to
micrometers.  Diacritical marks (accents, tildes, carats, etc.) are
removed, but the German umlaut (e.g., a, o, or u with two dots over it)
is changed into a, o, or u followed by e.  If needed for clarity,
hyphens are occasionally inserted between spelled-out Greek letters or
symbols (Omega-cm, sin-theta).
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NOTA BENE:  In the Jan. 1, 1992, High-Tc Update, we mentioned a report
that near-free-standing platelets of YBa2Cu3O7-d with 1-4 mm edges have
been reproducibly obtained using a self-decanting BaO:CuO flux method.
Unfortunately, we forgot to include the names of the authors:  T. A.
Vanderah (China Lake) et al.  The preprint is relisted in this issue.

Tl Copper Oxides

Remarkably high critical-current densities have been observed by R. S.
Liu (Cambridge) et al. in (Tl0.5Pb0.5)Sr2Ca2Cu3O9.  Based on magnetic
hysteresis measurements, the Jc of this single-Tl-O layer material was
found to be 1.24 x 10^5 A/cm^2 when measured at a temperature of 77 K
and a field of 1 T.  The authors note that this value of Jc is
comparable to that measured for YBa2Cu3O7-d (1.04 x 10^5 A/cm^2), which
has single layers of CuO chains, but is considerably greater than the Jc
of the double-Tl-O layer compound Tl2Ba2Ca2Cu3O10, which exhibited no
hysteresis under the same conditions.

The authors point out that, since the Tl/Pb-O sheets are insulating,
there is stronger Josephson coupling between adjacent CuO2 layers in the
single-Tl-O layer material than in the double-Tl-O layer material.  They
also note that, as suggested by D. H. Kim et al. [Physica C 177, 431
(1991)], the decoupling between planes in the double-Tl-O layer material
causes the vortex lattice to break into two-dimensional pancake
vortices, which are easily thermally activated.  The low Jc is then a
result of crossing the irreversibility line rather than a lack of
pinning centers.  The latter are known to be present, as shown by the
very strong pinning observed at 4.2 K.

The authors also measured the specific heat and found that the
(Tl0.5Pb0.5)Sr2Ca2Cu3O9 compound has a three-dimensional character.
They stress that the three-dimensional-like material
(Tl0.5Pb0.5)Sr2Ca2Cu3O9 may be of great promise for technological
applications because the material has a Jc comparable to YBa2Cu3O7 (Tc =
91 K) but has a higher Tc value (115 K).

Infinite-Layer Copper Oxides

Measurements of the superconducting transition temperature versus
pressure in the recently discovered infinite-layer electron-doped
copper-oxide compound Sr0.84Nd0.16CuO2 have been carried out by C. L.
Wooten et al. (Texas-Austin).  Nearly hydrostatic pressure in the range
0-20 kbar was used to examine the resistively determined Tc. The authors
observed, in contrast to other electron-doped copper oxides, a positive
and appreciable change in Tc with pressure:  dTc/dP = +0.06+-0.02
K/kbar.  While one might have expected Tc to decrease because of
pressure-enhanced intralayer magnetic correlations, the authors propose
that in this material Tc increases with pressure because of enhanced
interlayer coupling.

Bi Copper Oxides

Flux pinning has been studied by W. Gerhauser (Siemens and Erlangen) et
al. in Bi-2212 crystals containing well-defined defect structures
produced by 0.5 GeV iodine-ion irradiation.  These ions produce columnar
damage tracks 5 to 10 nm in diameter.  Even when flux lines are aligned
parallel to these tracks, the measured activation energies at low
temperatures and low fields do not exceed 70 meV, and the critical
currents decline rapidly with increasing temperature.  The small pinning
energies are explained quantitatively by pinning of two-dimensional
pancake vortices instead of complete flux lines.

A preprint by M. P. Maley (Los Alamos) et al. reports measurements of
the magnetic-field dependence of the transport critical-current density
of a high-Jc BSCCO/Ag tape sample.  At low temperature, Jc > 10^4
A/cm^2, which extends to high fields, as previously reported by several
groups.  For T > 20 K and with the applied field parallel to the c axis,
however, Jc declines precipitously with increasing field and
temperature, reflecting the properties of intragranular pinning.  The
variation of Jc with the angle between B and the tape normal is
consistent with a two-dimensional model of the vortex lattice.  The Jc
determined by magnetic hysteresis measurements shows rough agreement
with the transport Jc at low fields but falls below it by a factor that
increases with increasing field and temperature.  The differences can be
accounted for by a model for thermally activated flux motion.

The critical-current density Jc values of variously processed Ag-clad
Bi2Sr2CaCu2O8 tapes have been related to their microstructures as
observed by TEM, SEM, and x-ray pole-figure analysis, according to a
preprint by Y. Feng et al. (Wisconsin).  The microstructure-Jc
correlations confirm the general importance of a highly c-axis-aligned
microstructure for high critical-current density.  However, TEM
experiments revealed a "colony" microstructure in all samples, in which
groups of several grains with nearly perfect c-axis alignment (colonies)
are formed, but where the c axes of adjacent colonies are not parallel.
SEM imaging of etched cross sections showed that important features of
this colony misalignment are not accurately reflected by stand-alone x-
ray pole-figure analysis or TEM imaging.  Near-perfect alignment occurs
within a few microns of the Ag cladding for all processing methods.
However, the processing details control the macroscopic grain alignment
within the bulk of the superconductor.  In particular, second-phase
particles disrupt the alignment of macroscopic colonies.  These
misalignments may control the transport critical current, yet they are
not clearly evidenced by either macroscopic pole-figure analysis or by
very local TEM examination.

Oxygen Diffusion

Tracer diffusion of [18]^O has been investigated by M. Runde (Argonne)
et al. in dense polycrystals and in the c direction in single crystals
of Bi2Sr2CaCu2Ox between 300^oC and 700^oC under one atmosphere of
oxygen.  Penetration profiles were determined by secondary ion mass
spectrometry.  The diffusional anisotropy, as measured by the ratio of
the diffusion coefficient in the polycrystals (D = 1.75 x 10^[-5] exp(-
0.93 eV/k_BT) cm^2/s) to the diffusion coefficient parallel to the c
axis of the single crystal (D = 0.6 exp(-2.20 eV/k_BT) cm^2/s), is large
and temperature dependent.  The results suggest that diffusion in the ab
plane and in the c direction takes place by different mechanisms.

YBa2Cu3O7-d

A new process has been developed by M. R. Lees et al. (CNRS, Grenoble),
in which bulk textured YBa2Cu3O7-d ceramics have been prepared by
solidification in a magnetic field.  The method produces strongly
textured samples, the degree of induced alignment depending on both the
presence of a magnetic annealing field in the appropriate temperature
regime and the strength of the magnetic field.  The authors have
confirmed that it is the magnetic field and not, for example, the
presence of an unexpected temperature gradient within the furnace that
is responsible for the induced orientation.  At 77 K in a magnetic field
of 6.9 T, the transport current density exceeded 1.5 x 10^4 A/cm^2 for
currents flowing along the ab planes with the field applied parallel to
both the ab planes and the direction of current flow.

Thin Films

A subnanosecond photoresponse of a structured superconducting YBa2Cu3O7-
d thin film to infrared and visible radiation is reported by A. D.
Semenov (Moscow State) et al.  The voltage response to radiation pulses
was measured on a current-biased film (thickness 700 Angstroms) in the
resistive state.  From the results the authors conclude a response time
of about 90 ps and a responsivity of about 4 x 10^[10] Omega/J.  They
attribute the response to Cooper pair breaking and suppression of the
superconducting energy gap induced by nonequilibrium quasiparticles.

As reported by T. Sugimoto et al. (ISTEC), a-axis-oriented Bi-Sr-Ca-Cu-O
thin films have been prepared at substrate temperatures of 720-850^oC on
(100) MgO substrates by metalorganic chemical vapor deposition (MOCVD).
The a-axis films were found to grow preferentially at lower substrate
temperatures and faster deposition rates.  The formation of a-axis (c-
axis) domains was found to be preferred at ratios of Cu/(Sr+Ca) and
Bi/(Sr+Ca) lower (higher) than the stoichiometric composition of the
2212 and 2223 phases.  From scanning electron microscope (SEM)
observations, the surface morphology was found to have characteristic
triangular layered grains, and these grains were aligned with their long
dimensions along the a or b axis of the MgO substrate.  Four-probe dc
resistivity measure- ments revealed that the a-axis-oriented films had
an onset Tc around 80 K, but zero resistivity was not obtained above 15
K.  The authors note that this is evidently because the a-axis-oriented
grains do not form a continuous network, as shown in the SEM
observations.

Anomalous magnetoresistance has been observed by T. Wang et al.
(Minnesota) in ultrathin films (~35 Angstroms) of DyBa2Cu3O7-x whose
sheet resistances span the zero-field superconductor-insulator
transition.  For a film just on the superconducting side of the
transition, three magnetic-field-induced transitions were found, in
which the superconducting state was successively exited, re-entered, and
finally exited with increasing field.  The authors suggest that this
unusual behavior may result from the modification of the field-driven
superconductor-insulator transition by an interaction of the carriers
with antiferromagnetically ordered Cu^[2+] ions on the CuO2 sheets.

Tunnel Junctions

As reported by G. F. Virshup et al. (Varian), hysteretic Josephson
junctions have been fabricated by growing c-axis-oriented tri-layer
Bi2Sr2CaCu2O8/Bi2Sr2(Ca,Bi,Sr)7Cu8O20/Bi2Sr2CaCu2O8 single-crystal thin
films and patterning them into devices.  The authors note that, to their
knowledge, these are the first cuprate superconductor junctions to
exhibit current-voltage characteristics with substantial hysteresis.
Current flow in the devices is in the vertical (c-axis) direction,
through an ultra-thin (~30 Angstroms) barrier layer.  The current-
voltage response shows large, sharp Shapiro steps under microwave
illumination.

Moreland's break-junction technique has been extended by C. J. Muller et
al. (Leiden) to produce a clean, stable, mechanically adjustable
junction.  The coupling of two electrodes in vacuum was varied by
applying an external force.  The authors describe a junction with Nb
electrodes whose normal resistance could be varied from 1 Omega to 10^9
Omega.  From measurements of the tunneling characteristics over this
resistance range the authors found a transition from a weak-link regime
to a tunnel regime.  The current-voltage (I-V) curves of the
mechanically controllable break (MCB) junction were reproducible upon
adjustment changes over the whole resistance range.  The authors suggest
that this technique could be used to produce vacuum-barrier tunnel
junctions in high-Tc materials and heavy-fermion systems.

Theory

A preprint by J. C. Phillips (AT&T Bell Labs) proposes a conventional
explanation for the anomalous sharpening [D. S. Dessau et al., Phys.
Rev. Lett. 66, 2160 (1991)] observed in the angle-resolved photoemission
spectra of Bi2Sr2CaCu2O8+d.  The author explains the narrow peak at
0.010 eV = 80 cm^[-1] as being due to a surface phonon, strongly coupled
to a surface defect (probably an oxygen vacancy).

A simple model of anharmonic phonons has been applied by V. H. Crespi
and M. L. Cohen (UC-Berkeley and LBL) to study the superconducting
isotope effect.  Linear and quadratic electron-phonon coupling are
considered for various model potentials.  The results of the model
calculations are compared with experimental results for the high-
temperature superconductors La2-xSrxCuO4, Y1-xPrxBa2Cu3O7-d, and
YBa2Cu3-xMxO7-d with M = Zn, Ni, or Fe.  The authors note that strongly
anharmonic multiple-well phonons provide a means to account for
anomalous values of the isotope effect exponent alpha both above 0.5 and
below 0.0.

The microscopic mean-field equations for a BCS superconductor in a
strong magnetic field have been solved by M. R. Norman (Argonne) et al.
Calculations were performed for fields both above and below the
semiclassical upper critical field.  In the low-field regime the authors
find small quantum corrections to the results of semiclassical theory,
whereas in the reentrant strong-field regime they find that the vortex-
lattice state changes qualitatively.

C60

There are several papers listed in this issue on C60, none reporting
superconductivity.  Important industrial applications, however, may well
come out of the findings of M. Nunez Regueiro et al. (CNRS), reported in
a paper appearing this month in Nature.  The authors found that rapid
non-hydrostatic compression of C60 to pressures of 20+-5 GPa transforms
it instantly into bulk polycrystalline diamond at room temperature.  The
high efficiency and fast kinetics at room temperature suggest the
possibility of using this transformation for fabrication of industrial
diamonds.


Overviews
The fabrication and characterization of single crystals of high-
temperature superconductors are reviewed by S. E. Stupp and D. M.
Ginsberg (Illinois, Urbana-Champaign).  The authors concentrate on La2-
xSrxCuO4+y, YBa2Cu3O7-d, Bi2Sr2CaCu2O8+d, and Tl2Ba2Can-1CunO2n+3 (n =
2,3).  They note that, while high-quality single crystals are available
for several systems, some of the physical properties still vary,
depending upon how the crystals are made.  Sample composition, defect
structure, and defect density can be influenced by the processing
techniques.  The choice of crucible material also can be important;
several widely used types of crucibles (platinum for the La2-xSrxCuO4+y
system and gold for the YBa2Cu3O7-d system) may have important effects
on some of the physical properties.  Several techniques, such as the
traveling-solvent floating-zone method, are attractive because samples
can be grown without the use of a crucible. The authors also stress that
existing techniques of sample characterization are inadequate and that
future research on these systems should address the need for more
standardized practices (224 references).

A review chapter on electron microscopy investigations of irradiation
defect structures in the high-Tc superconductor YBa2Cu3O7-d has been
prepared by M. A. Kirk (Argonne) and H. W. Weber (Vienna).  Headings
include experimental considerations, atomic recoil parameters, the
isolated defect cascade, and cascade overlap.  There also are detailed
sections on the defect microstructure and properties produced upon
irradiation by light ions, protons, electrons, and heavy ions with GeV
energies (115 references).

A brief review of the open theoretical questions about high-Tc
superconductors has been prepared by Y. Lu (ICTP-Trieste and Beijing).
The author summarizes the basic experimental facts and notes that, while
the superconducting properties are not very different from those of the
ordinary superconductors, the normal-state properties cannot be
described by the standard Fermi-liquid (FL) theory.  The current
understanding of strongly correlated models is summarized.  In one
dimension, such systems behave like a Luttinger liquid, as opposed to a
Fermi liquid.  On the other hand, in two dimensions, the question of FL
vs. non-FL behavior is still open.  The author considers both the
single-hole problem in a quantum ferromagnet and the finite-doping
regime (104 references).

Chemical modification of high-temperature-superconductor (HTS) surfaces
and characterization of those surfaces with x-ray photoelectron
spectroscopy (XPS) are reviewed by R. P. Vasquez (JPL).  The XPS data
obtained from chemically etched HTS surfaces demonstrate that it is
possible to use chemical etching to obtain high-quality surfaces that
are comparable, and in some cases superior, to surfaces exposed in
vacuum (e.g., by cleaving single crystals).   Clear Fermi edges in the
valence-band regions, the most stringent test for high-quality surfaces,
are observed on chemically etched HTS surfaces.  Spectral features
intrinsic to the bulk of HTS materials are identified, including
features due to atoms occupying inequivalent lattice sites.  Spectral
features associated with the surface are identified with variable-angle
measurements.  Chemical treatments for the purpose of surface
passivation are also reviewed (103 references).

A review article by J. V. Yakhmi and R. M. Iyer (Bhabha, Trombay)
discusses the architecture of high-Tc superconducting layered cuprates
with the general formula (AO)_m(ACuO_[3-x])_n, implying a stacking of
rock-salt layers and perovskite units.  The article discusses Ba-free
123 phases with full Sr replacement, Sr-free Bi-2212, Sr substitutions
in Tl-1212, the Tl-1201 phase, Tl-2223, Pb-based cuprates, fluorite-
block insertions, and T*-phase materials (90 refs.).

Solidification from the melt is an effective way to produce high-
temperature superconductors that have textured microstructures and
critical-current densities in excess of 10^5 A/cm^2.  Paul J. McGinn
(Notre Dame) has prepared a book chapter that reviews the various means
of generating a textured microstructure, discusses the nucleation and
growth of YBa2Cu3O6+x, and considers the effects of additions on the
microstructure and properties.  The author notes that, for widespread
applicability, a means to speed up the texturing process will have to be
developed (89 refs.).

An overview by D. J. Bishop et al. (AT&T Bell Labs) of experiments on
the static and dynamic properties of vortices and flux-line lattices in
the mixed state of the copper-oxide superconductors has just been
published [Science 255, 165 (1992)].  Two types of experiments are
discussed:  (a) those that image the flux patterns via magnetic
decoration or neutrons, thereby giving information about static
structures, and (b) those that explore vortex dynamics either through
the resistivity or other electrodynamic responses of the material. The
authors stress that the experiments support a vortex-glass phase-
transition model in which there is a true phase transition from a low-
temperature superconducting vortex-glass phase into a disordered high-
temperature vortex-fluid phase (55 references).

A brief description of the present status of research on fullerene and
alkali-metal-doped fulleride films has been prepared by A. F. Hebard
(AT&T Bell Labs).  Deposition, metal doping, and characterization
techniques are discussed.  Measurement results including structure,
stoichiometry, infrared and visible absorption, electrical transport,
dielectric properties of the fullerenes, and superconducting properties
of the fullerides are reviewed (36 references).

Processing-induced transitions in the crystalline and electronic
structure of Bi2Sr2CaCu2O8 (BSCCO) thin films have been reviewed by J.-
J. Yeh (AT&T Bell Labs).  Thin-film growth, film characterization,
transitions at different annealing temperatures, and effects of
annealing period and composition are discussed.  The author notes that
superconducting BSCCO films with high Tc and Jc normally have smooth
surfaces, while films with rough surfaces normally have broad
superconducting transitions and low critical currents.  To grow high-
quality BSCCO films with oxygen post-annealing, the annealing
temperature and period must be adjusted to enhance the reaction rate in
forming the superconducting 2212 phase but at the same time minimize re-
evaporation of Bi from the film (30 refs.).

A review by C. H. Chen (AT&T Bell Labs) reviews the intricate
microstructure associated with the phase transitions in La2-
x(Sr,Ba)xCuO4 from a high-temperature tetragonal (HTT) phase to a low-
temperature orthorhombic (LTO) phase, and then to a new tetragonal (LTT)
phase at even lower temperatures.  In the HTT/LTO transition, it is
found that antiphase boundaries play a role in the microstructure in
addition to the twin/microtwin boundaries.  The author shows that the
detailed microstructure can be fully understood by considering the
configuration rules between the twin and antiphase boundaries (23
references).

An overview of models of the current density in Bi-based high-
temperature superconducting tapes has been prepared by A. P. Malozemoff
(American Superconductor).  The article focuses on the discovery that
the low-temperature critical-current density persists out to high
fields, violating early expectations of weak-link behavior in
polycrystalline YBa2Cu3O7 and Bi-based superconductors.  The author
assesses explanations in terms of non-weak-linked portions of grain
boundaries or the brick-wall model (22 references).

Ph.D. Thesis

Magnetic Raman scattering in rare-earth cuprates is the subject of L. J.
Marville's Ph.D. thesis (MIT).  Included are spin-pair Raman-spectra data

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