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NIST Special Publication 759

High-Temperature Superconductivity:

Abstracts of NIST Publications,

1987-1988

Mary E. DeWeese, Robert A. Kamper, and Ronald M. Powell, Editors

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NIST Special Publication 759

High-Temperature Superconductivity:

Abstracts of NIST Publications,

1987-1988

Edited by

Mary E. DeWeese

Robert A. Kamper

and

Ronald M. Powell

Electromagnetic Technology Division

Center for Electronic and Electrical Engineering

National Institute of Standards and Technology

Boulder, CO 80303

NOTE: As of 23 August 1988, the National Bureau of Standards (NBS) became the National Institute of Standards and Technology (NIST) when President Reagan signed into law die Omnibus Trade and

Competitiveness Act

November 1988

U.S. Department of Commerce

C. William Verity, Secretary

National Institute of Standards and Technology ^>

(formerly National Bureau of Standards) <Ss^\©0^ c$$*

Ernest Ambler, Director <^\^#^

^
Library of Congress U.S. Government Printing Office

Catalog Card Number: 88-600602 Washington: 1988

National Institute of Standards

and Technology

Special Publication 759

Natl. Inst. Stand. Technol.

Spec. Publ. 759

35 pages (Nov. 1988)

CODEN: NSPUE2

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
CONTENTS

Page

1. INTRODUCTION 1

2. COMPOSITIONAL ANALYSIS 2

3. CRITICAL CURRENT 4

4. CRYSTAL STRUCTURE . 9

5. ELASTIC CONSTANTS AND PHONON SPECTRA 13

6. ELECTRICAL CONTACTS 16

7. ELECTRONIC STRUCTURE 18

8. ENERGY GAP AND TUNNELING SPECTRA 19

9. JOSEPHSON EFFECT AND DEVICES 22

10. MAGNETIC MEASUREMENTS 23

11. PHASE EQUILIBRIUM 25

12. PROCESSING- -BULK MATERIAL 26

13. PROCESSING- -THIN FILMS 28

AUTHOR INDEX 29

iii
ABBREVIATIONS

CAC = Center for Analytical Chemistry,

National Measurement Laboratory

CBS = Center for Basic Standards,

National Measurement Laboratory

CCP = Center for Chemical Physics,

National Measurement Laboratory

CEEE = Center for Electronics and Electrical Engineering,

National Engineering Laboratory

IMSE = Institute for Materials Science and Engineering

iv
HIGH -TEMPERATURE SUPERCONDUCTIVITY

:

ABSTRACTS OF NIST PUBLICATIONS, 1987-1988

Mary E. DeWeese, Robert A. Kamper, and Ronald M. Powell, Editors

We have collected abstracts from 61 papers published

between March 1987 and May 1988 covering various aspects

of superconductivity research. The work of nine divisions

of the National Institute of Standards and Technology

(formerly the National Bureau of Standards) in both

Boulder, Colorado, and Gaithersburg , Maryland, is

represented.

Key words: bismuth superconductors; critical current;

critical field; critical temperature; high Tc ; hightemperature superconductors; low- temperature superconductors; superconducting ceramics; superconductors;

thallium superconductors; YBCO ; yttrium superconductors

1 . INTRODUCTION

Robert A. Kamper

The National Institute of Standards and Technology (formerly the National

Bureau of Standards) has been involved with research in superconductivity since soon after Kammeriingh Onnes's original discovery in 1911. Our first contribution to the literature was the Silsbee hypothesis to account for the critical

current of a type -I superconductor, published by Francis Silsbee in 1916. The

discoveries of the high- field superconductors in the late fifties and the

Josephson effect in the early sixties introduced the possibility of serious

engineering with superconductors and were followed by significant expansion of

the program at NBS . A formal program to promote the application of superconductivity to physical standards and measurement techniques was established in 1969

and has been an active contributor to the development of superconducting electronics ever since. In the early seventies a program to define and establish

techniques to measure the engineering characteristics of practical high- field

superconductors was established. This has contributed to the development of

reliable conductors and to the success of many large-magnet projects.

The discovery of the high- temperature superconductors introduced the need

to extend research from metals to ceramics, for which NBS was also well prepared,

with an established program in ceramics and an extensive array of facilities to

analyze and map the composition, structure, and electronic energy spectrum of

materials in a wide range of scales.

This collection of abstracts represents the first results of applying all

these resources to high- temperature superconductors. Many of the papers have

authors from several institutions. The National Institute of Standards and

Technology encourages collaborative research projects to maximize the benefit

of Its resources to the U.S. research and development community. Anyone wishing

to discuss the possibility of a collaborative research project should feel free

to make contact with any of the NIST authors represented here.

i
2. COMPOSITIONAL ANALYSIS

STANDARD X-RAY DIFFRACTION POWDER PATTERNS OF FIFTEEN CERAMIC PHASES, Winnie

Wong-Ng, Howard F. McMurdie , Boris Paretzkin, Camden R. Hubbard, Alan L. Dragoo,

and James M. Stewart,* Powder Diffraction, Vol. 2, No. 2, pp. 106-117 (June

1987). See pp. 191-201 for accelerated publication with additional coauthors,

Yuming Zhang* and Katherine L. Davis. [ IMSE/Ceramics Division] (14a)

•^University of Maryland

See following abstract.

STANDARD X-RAY DIFFRACTION POWDER PATTERNS OF SIXTEEN CERAMIC PHASES, Winnie

Wong-Ng, Howard F. McMurdie, Boris Paretzkin, Yuming Zhang,* Katherine L. Davis,

Camden R. Hubbard, Alan L. Dragoo, and James M. Stewart,* Powder Diffraction,

Vol. 2, No. 3, pp. 191-201 (September 1987). [IMSE/Ceramics Division] (14b)

*University of Maryland

The following sixteen reference patterns of boride, silicide,

nitride, and oxide ceramics represent the second group of reference

patterns of ceramic phases measured at the National Bureau of Standards

under the project "High Quality Reference Patterns and Total Digital Powder

Patterns of Technologically Important Ceramic Phases." Included in the

sixteen reference patterns are reports for two high Tc superconducting

oxide phases (CuLa : 8 Sr0 20^ and Ba2 Cu3Y07 ) plus one related phase

(BaCuY205

) . In addition to these new phases, five other patterns represent

phases previously not contained in the PDF and eight represent major

corrections to data in the file. The general methods of producing these

X-ray powder diffraction reference patterns are described in this journal,

Vol. 1, No. 1, p. 40 (1986).

THERMAL ANALYSIS OF Ba2YCu3 07

_ x AT 700-1000°C IN AIR, L. P. Cook, C. K. Chiang,

W. Wong-Ng, and J. Blendell, Advanced Ceramic Materials: Ceramic

Superconductors, Vol. 2, No. 3B, pp. 656-661 (July 15, 1987). [IMSE/Ceramics

Division] (18)

Differential thermal analysis (DTA) and thermogravime trie analysis

(TGA) of Ba2YCu3 07 _ x using conventional platinum cells yields a series of

well-defined reversible thermal events between 850 and 1000°C. At least

one of these is interpreted as being due to the products of platinum

reaction. However, changes in slope are also observed on TGA curves taken

using non-reactive MgO cells. The other thermal events may therefore

represent minor phase changes in Ba2YCu3 07

_ x in response to variations in

temperature/stoichiometry

.

MEASUREMENT OF SUPERCONDUCTOR STOICHIOMETRY BY PROMPT AND DELAYED NEUTRON

ACTIVATION, Richard M. Lindstrom, Transactions, American Nuclear Society, Vol.

56, p. 231 (1988). [CAC/Inorganic Analytical Research Division] (49.3)

Prompt and delayed neutron activation analysis have been applied to

the nondestructive measurement of the major metals in small samples of

YBa2 Cu3 07 superconductors, starting materials, and intermediate products.

2
The small activation cross section and low gamma ray energies of Y-90 rn make the determination of yttrium imprecise in the presence of the large

quantities of Cu-64 radioactivity. The physical parameters for

neutron- capture prompt gamma- ray activation analysis (PGAA) are more attractive than delayed counting for stoichiometry measurements. The PGAA

spectra of Y, Ba, and Cu show three, four, and six analytically useful

gamma ray lines respectively, with nicely balanced intensities for the

composition range of interest. A 100 mg sample gives a Poisson precision

for yttrium (the weakest element) of 2% after only a 2 hour irradiation.

Since the concentrations and mole ratios Y/Cu and Ba/Cu may be calculated

immediately after the irradiation ends, the results can be returned within

a day

COMPOSITIONAL CONTROL OF THE MICROSTRUCTURE OF Ba2YCu3 06+x , John E. Blendell,

Carol A. Handwerker, Mark D. Vaudin, and Edwin R. Fuller, Jr. , Journal of Crystal

Growth, Vol. 89, pp. 93-100 (1988). [ IMSE/Ceramics Division] (74.3)

Although Ba2YCu3 06+x exhibits superconductivity at high temperatures,

the critical current density (J c

) in bulk polycrystalline materials,

prepared by sintering, is several orders of magnitude below useful values.

One possible source of low J c in polycrystalline materials is the presence

of resistive grain boundary junctions resulting from second phases or impurity segregation. Chemical composition plays a major role in determining the phases present and hence the grain boundary characteristics

Preliminary composition measurements of Ba2YCu3 06+x include the ratio of

major elements, impurity concentrations at the trace level, compositional

mapping of major elements by electron probe microanalyses, and observations

of sintered microstructures . Results on liquid phase formation and carbon

contamination introduced during powder processing and from exposure to

atmospheric C02 indicate that a great degree of compositional control is

required for any meaningful characterization of the Ba-Y-Cu-0 system.

INITIAL STAGES OF DEGRADATION OF SUPERCONDUCTOR SURFACES: 0 2 , H2 , C02

, and CO

CHEMISORPTION IN La2

. x SrxCuOA

, Richard L. Kurtz, Roger Stockbauer, Theodore E. Madey, Donald Mueller,* Arnold Shih,* and Louis Toth,* Physical Review B, Vol.

37, No. 13, pp. 7936-7939 (May 1, 1988). [CCP/Surface Science Division] (76.3)

*Naval Research Laboratory

The initial stages of degradation of high-Tc superconductor surfaces

by interaction with atmospheric gases have been studied using ultraviolet

photoelectron spectroscopy. 02 has little effect, while H20 adsorbs

dissociatively with an initial sticking coefficient S 0 - 1 , and forms a surface hydroxide. C02 adsorbs forming a carbonatelike surface species

with an initial sticking coefficient of -0.3. CO is observed to result in

similar adsorbed surface species with a much lower sticking coefficient of

~3 x 10~\
3. CRITICAL CURRENT

TRANSPORT CRITICAL- CURRENT CHARACTERISTICS OF Y1 Ba2 Cu3 0x , J. W. Ekin, A. J.

Panson,* A. I. Braginski,* M. A. Janocko,* M. Hong,f J. Kwo , | S. H. Liou.f D. W.

Capone , II, t and B. Flandermeyer

,

$ Materials Research Society, Vol. EA-11,

pp. 223-226 (June 1987). [ CEEE/Electromagnetic Technology Division] (6.3)

*West inghouse Research & Development Center

f AT&T Bell Laboratories, Murray Hill

JArgonne National Laboratory

Voltage versus current (V-I) characteristics were measured at

magnetic fields up to 24 T at a temperature of 77 K in several Y1 Ba2 Cu3 Ox samples fabricated at different laboratories. Critical temperatures, Tc , measured by resistivity were about 93 K. All samples showed linear V-I

characteristics at current levels much greater than the critical current,

I c

. However, the slope was significantly less than the normal resistance

at Tc. The slope increased with magnetic field and reached the normal

resistance value only at fields greater than 24 T. Values of the transport

critical-current density J c near zero magnetic field were generally low and

variable (about 1 to 200 A/cm2

) . The transport J c fell sharply when

magnetic field was applied, decreasing by about an order of magnitude

between 10~ 3 T and 1 T. This leads to an effective upper critical field

for transport critical currents in Y1 Ba2 Cu3 0x that is significantly less

than the upper critical field defined by the field at which the resistance

increases to the normal -state value. The transport J c appears to be

significantly less than J c calculated from magnetization data on similar

samples. These results are consistent with the transport critical-current

in the Y1 Ba2 Cu3 0x system being dominated by a "weak- link" region between

high-J c grains.

STUDIES OF NbTi STRANDS FROM CORELESS RUTHERFORD CABLES, L. F. Goodrich, E. S. Pittman, J. W. Ekin, and R. M. Scanlan, IEEE Transactions on Magnetics, Vol.

MAG-23, No. 2, pp. 1642-1645 (March 1987). [CEEE/Electromagnetic Technology

Division] (9a)

The electromechanical properties of NbTi strands extracted from

coreless Rutherford cables were studied to clarify the relative effects of

strand location and field angle on current degradation that occurs in

cables that have been compacted into a keystone shape. Detailed criticalcurrent measurements were made on two samples which were fabricated under

controlled conditions. These are prototype cables for high energy physics

applications. Specific factors that are addressed are the nature,

location, and amount of degradation. This information is intended to lead

to methods for reducing the amount of critical-current degradation in cable

manufacture

.

4
RELATIONSHIPS BETWEEN CRITICAL CURRENT AND STRESS IN NbTi, J. W. Ekin, IEEE

Transactions on Magnetics, Vol. MAG-23, No. 2, pp. 1634-1637 (March 1987).

[CEEE/Electromagnetic Technology Division] (9b)

The effects of various types of stress on the critical current of a multifilamentary NbTi superconductor are reported. Degradation of critical

current due to axial tension applied at 4 K, transverse compression applied

at 4 K, and hairpin bending strain applied at room temperature has been

measured. The degradation from axial tension is much greater than from

transverse compression in many practical cases because the soft copper

matrix limits the build-up of transverse compression. The degradation from

typical levels of transverse compression is only about 4% at 8 T, for

example. For axial tension, on the other hand, higher stresses can occur that will degrade the critical current by 24%, for example, at 7 T and 2.7%

strain. Both the axial- tensile and the transverse -compressive stress

effects are about 98% reversible; thus the degradation will be seen only

when the conductor is under operational stress. The results indicate that

a primary origin of the critical current degradation in NbTi is a stressinduced reversible decrease in the upper critical field.

EVIDENCE FOR WEAK LINK AND ANISOTROPY LIMITATIONS ON THE TRANSPORT CRITICAL

CURRENT IN BULK POLYCRYSTALLINE Y1 Ba2 Cu3 Ox , J. W. Ekin, A. I. Braginski,* A. J.

Panson,* M. A. Janocko,* D. W. Capone, II, f N. J. Zaluzec.f B. Flandermeyer ,

0. F. de Lima.f M. Hong,$ J. Kwo , £ and S. H. Liou,$ Journal of Applied Physics,

Vol. 62, No. 12, pp. 4821-4828 (December 15, 1987). [CEEE/Electromagnetic

Technology Division] (10.3)

*Westinghouse Research & Development Center

fArgonne National Laboratory

| AT&T Bell Laboratories, Murray Hill

Measurements of the transport critical-current density (J c ) , magnetization J c

, and magnetoresistance in a number of bulk sintered

samples of Y1 Ba2 Cu3 Ox from several different laboratories indicate that the

transport J c is limited by weak- link regions between high J c regions. The

weak- link J c has a Josephson character, decreasing by two orders of

magnitude as the magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain-boundary region in Y1 Ba2 Cu3 Ox shows no observable impurities or second phases to the scale of the [001] lattice

planes (-12 A) . The effect of intrinsic conduction anisotropy is

discussed. A current- transfer model is proposed in which weak conduction

along the c axis plays a role in limiting J c at grain boundaries.

Orienting the grains in the powder state during processing may result in

enhanced transport J c in bulk conductors

TRANSPORT CRITICAL CURRENT IN BULK SINTERED Y1 Ba2 Cu3 Ox AND POSSIBILITIES FOR ITS

ENHANCEMENT, J. W. Ekin, Advanced Ceramic Materials: Ceramic Superconductors,

Vol. 2, No. 3B, pp. 586-592 (July 1987). [CEEE/Electromagnetic Technology

Division] (12)

Several general processing methods for increasing the critical

current density, J c

, in bulk sintered Y1 Ba2 Cu3 0x superconductor are outlined. Data indicate that the transport J c in bulk polycrystalline

5
specimens is dominated by a weak- link region between high-J c grains and

that potentially much higher J c may be possible. Two possible causes of

such a weak- link phenomenon are considered: low-Tc phases or impurities

localized in the grain boundary region, and anisotropy of the

superconducting properties. Several methods for minimizing the weak- link

effects to increase the J c are discussed.

JOSEPHSON JUNCTION MODEL OF CRITICAL CURRENT IN GRANULAR Y1 Ba2 Cu3 07

_ 5 SUPERCONDUCTORS, Robert L. Peterson and Jack W. Ekin, Physical Review B, Vol.37,

No. 16, pp. 9848-9851, (June 1, 1988). [CEEE/Electromagnetic Technology Division]

(47)

We calculate the transport critical current density in a granular

superconductor in magnetic fields below about 5 x 10"3 T. The field

dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fit to

transport critical current bulk Y1 Ba2 Cu3 07

_

<5 ceramic superconductors , whose

average grain size somewhat exceeds 10 yum. The results yield an average

junction cross - sectional area (thickness x length) of 4-6 ^urn2

. If the

junctions are at the grain boundaries, a London penetration depth of about

150-300 nm is inferred, consistent with other estimates. We conclude that

Josephson junctions are limiting the transport critical current in these

samples and that they lie at the grain boundaries. The parameters of the

fit are not consistent with Josephson junctions at twinning boundaries.

IRREGULARITY IN Nb-Ti FILAMENT AREA AND ELECTRIC FIELD CURRENT CHARACTERISTICS,

J. W. Ekin, Cryogenics 1987, Vol. 27, pp. 603-608 (November 1987).

[CEEE/Electromagnetic Technology Division] (66.3)

There is a correlation between irregularity in filament area

( "sausaging" ) and the shape of a superconductor's electric field (E) versus current (I) relationship. The shape of the E-I characteristic is

quantified in terms of the resistive transition parameter, n, defined by

E a I

n

. Low values of n less than about 20 correlate with a wide filament

diameter distribution, while n values over 50 correspond to a distribution

more than 2.5 times smaller. It is proposed that the low- field (constant)

value of n be used as an index of filament quality in evaluating different

superconductors for practical applications. A model is also suggested to

explain this effect in terms of a locally depressed filament critical

current, which forces current to transfer across the normal matrix material

into neighboring filaments. The relationship between n and the statistical

distribution of filament diameters may be useful as an easy method of

estimating the extent of sausaging in practical multifilamentary Nb-Ti

superconductors from measurements of n.

TRANSVERSE STRESS EFFECT ON MULTI FILAMENTARY Nb3 Sn SUPERCONDUCTOR, J. W. Ekin,

Advances in Cryogenic Engineering Materials, Vol. 34, pp. 547-552 (1988).

[CEEE/Electromagnetic Technology Division] (67.3)

A large reversible degradation of the critical current of

multifilamentary Nb3 Sn superconductors has been observed under the

application of uniaxial compressive stress applied transverse to the

6
conductor axis at 4 K. In bronze -process multif ilamentary Nb 3 Sn, the onset

of significant degradation occurs at about 50 MPa. The intrinsic effect

of transverse stress on the upper critical field is about ten times greater

than for axial stress. Although transverse stress on Nb3 Sn filaments is

less than axial stress in most applications, it will need to be considered

in the internal stress design of large magnets because of the greater

sensitivity of Nb3 Sn to transverse stress. The effect scales with

conductor thickness and this will place limits on conductor dimensions

and the spacing between distributed reinforcement in large magnets.

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS, INTERIM REPORT, JANUARY 1986-

DECEMBER, 1987, L. F. Goodrich, ed. , National Bureau of Standards (U.S.), NBSIR

88-3088, 75 pages (February 1988). [CEEE/Electromagnetic Technology Division]

(68.3)

A cooperative program with the Department of Energy, the National

Bureau of Standards, other national laboratories, and private industry is

in progress to develop standard measurement practices for use in large

scale applications of superconductivity. This report describes research

for the period January 1986 through December 1987. It contains the results

of critical current studies on the effect of power- supply current ripple,

measurements on cable strands, an interlaboratory comparison (round robin) on a large NbTi monolithic conductor, and a Nb3 Srt round robin. Several

useful current supply circuits have been developed. The reduction in

coupling losses in multifilamentary NbTi conductors has been addressed by

a study of the magnetic properties of matrix material consisting of dilute

alloys of Mn in Cu. In addition, vibrating- sample magnetometry is shown

to be adaptable to the measurement of coupling losses, in addition to

hysteresis losses, in multifilamentary conductors.

CURRENT RIPPLE EFFECT ON SUPERCONDUCTIVE DC CRITICAL CURRENT MEASUREMENT, L. F. Goodrich, S. L. Bray, and A. F. Clark, Advances in Cryogenic Engineering

Materials, Vol. 34, pp. 1019-1026 (1988). [CEEE/Electromagnetic Technology

Division] (69.3)

The effect of sample -current power-supply ripple on the measurement

of dc critical current is reported. Measurements were made on multifilamentary NbTi superconductor. In general, ripple in a current

supply becomes more significant above 500 A because effective filtering

is difficult. The presence of current ripple reduces the measured dc

critical current. Ripple also causes noise at the input to the voltmeter

used for the measurements. The quantitative effect of current ripple was studied using a battery current supply modified to allow the creation of

ripple current with variable frequency and amplitude. Problems common to

all large -conductor critical-current measurements are discussed.

7
EFFECT OF TRANSVERSE COMPRESSIVE STRESS ON THE CRITICAL CURRENT AND UPPER

CRITICAL FIELD OF Nb3 Sn, J. W. Ekin, Journal of Applied Physics, Vol. 62, No. 12,

pp. 4829-4834 (December 15, 1987). [ CEEE/Electromagnetic Technology Division]

(70.3)

A large reversible degradation of the critical current of

multif ilamentary Nb 3 Sn superconductors has been observed when uniaxial

compressive stress is applied transverse to the conductor at 4 K. In

bronze -process multifilamentary Nb 3 Sn, the onset of significant degradation

occurs at about 50 MPa. In an applied field of 10 T, the magnitude of the

effect is about seven times larger for transverse stress than for stress

applied along the conductor axis. The transverse stress effect increases

with magnetic field and is associated with a reversible degradation of the

upper critical field. The intrinsic effect of transverse stress on the

upper critical field is about 10 times greater than for axial stress.

Although axial stresses on the Nb 3 Sn filaments are greater than transverse

stresses in most applications, the transverse stress effect will need to

be considered in the internal design of large magnets because of the

greater sensitivity of Nb3 Sn to transverse stress. It is shown that the

transverse stress from the Lorentz force on the conductor is proportional

to conductor thickness. This will place limits on conductor dimensions and

the spacing between distributed reinforcement in large magnets. The effect

may be particularly significant in cabled conductors where large transverse

stress concentrations can occur at strand crossover points.

8
4. CRYSTAL STRUCTURE

X-RAY STUDIES OF HELIUM -QUENCHED Ba2Y3 C 7

_ x , Winnie Wong-Ng and Lawrence P. Cook,

Advanced Ceramic Materials : Ceramic Superconductors, Vol. 2, No. 3B, pp. 624-631

(July 15, 1987). [ IMSE/Ceramics Division] (15)

A series of 11 samples of Ba2YCu3 07

_ x (x-0 to 1) was prepared from

single phase orthorhombic material by annealing at temperatures from 400

to 1000° C, followed by rapid quenching. All quenchings were performed

using a liquid nitrogen-cooled copper cold well through which helium gas was passed. Lattice parameters were obtained for these materials by X-ray

powder diffraction methods to obtain dependence of cell volumes and crystal

symmetry on annealing temperature.

X-RAY POWDER CHARACTERIZATION OF Ba2YCu3 07

. x , W. Wong-Ng, R. S. Roth, L. J. Swartzendruber , L. H. Bennett, C. K. Chiang, F. Beech, and C. R. Hubbard,

Advanced Ceramic Materials: Ceramic Superconductors, Vol. 2, No.3B, pp. 565-

576 (July 15, 1987). [IMSE/Ceramics Division] (17)

The X-ray powder diffraction technique was used in order to characterize the high Tc superconductor phase Ba2YCu3 07

_ x prepared under different

conditions. High quality reference powder patterns for three compositions

are presented. An attempt is made to correlate the differences of lattice

parameters with oxygen content and superconductivity.

CRYSTAL STRUCTURE OF THE HIGH -TEMPERATURE SUPERCONDUCTOR La^ggSro^CuO^ ABOVE AND

BELOW Tc

, R. J. Cava,* A. Santoro, D. W. Johnson, Jr.,* and W. W. Rhodes, * Physical Review B, Vol. 35, No. 13, pp. 6716-6720 (May 1, 1987). [ IMSE/Reactor

Radiation Division] (22)

-'AT&T Bell Laboratories, Murray Hill

The crystal structure of La1 . 85 Sr0 . 15 CuO4 has been determined at 300,

60, and 10 K by neutron- diffraction powder profile analysis. The structure

is of the tetragonal K2NiFA type at ambient temperature, but undergoes an orthorhombic distortion near 200 K which buckles the copper-oxygen planes.

The crystal structure of the orthorhombic phase is not significantly

different above and below Tc. The copper-oxygen coordination polyhedron

changes shape in a subtle manner at the tetragonal- to -orthorhombic phase

transition, but is unchanged at Tc.

NEUTRON STUDY OF THE CRYSTAL STRUCTURE AND VACANCY DISTRIBUTION IN THE

SUPERCONDUCTOR Ba2YCu3 0 9

_ 5

, F. Beech, S. Miraglia, A. Santoro, and R. S. Roth,

Physical Review B, Vol. 35, No. 16, pp. 8778-8781 (June 1, 1987). [IMSE/Reactor

Radiation Division] (23)

Two samples of the high- temperature superconductor Ba2YCu3 0g

_

(j with

5 = 2.0 and 2.2 have been studied at room temperature and at 10 K, with the

neutron powder diffraction method and profile analysis. The structure of

the compound is orthorhombic. The oxygen atoms are located on four sets

of sites and not on five sets as reported earlier in x- ray- diffraction

studies. Of the two sets of copper atoms, one is surrounded by four oxygen

9
atoms at distances 1.9299(4) and 1.9607(4) A. This arrangement forms a two-dimensional framework of Cu-0 atoms that extends indefinitely along

the a and b axis. A fifth oxygen atom is located at 2.295(3) A from the

copper atom, giving a pyramidal configuration. In the other set, the Cu

atoms are also surrounded by four oxygen atoms, with distances 1.9429(1)

and 1.846(2) A. In this case, however, the oxygen atoms form rectangles

connected by vertices, and resulting in chains along the b axis. In the

compound with 5=2.0 all oxygen sites are fully occupied. When 5=2.2

there are oxygen vacancies, but these are confined to one set of positions

only, specifically to the oxygen atoms of the chains, located on the b

axis. No detectable change of the structure has been observed between room temperature and low temperature.

THE STRUCTURE AND PROPERTIES OF Ba2YCu3 06

, A. Santoro, S. Miraglia, F. Beech,

S. A. Sunshine,* D. W. Murphy,* L. F. Schneemeyer , * and J. V. Waszczak,*

Materials Research Bulletin, Vol. 22, pp. 1007-1113 (1987). [ IMSE/Reactor

Radiation Division] (24)

*AT&T Bell Laboratories, Murray Hill

The structure of Ba2YCu3 06 has been determined by neutron diffraction

powder profile analysis. The cell is tetragonal P4/mmm with

a = 3.8570(1) A and c = 11.8194(3) A. The cations are in a perovskite type

arrangement, with Ba and Y ordered on the A site to give a cell tripled

along c. The oxygens occupy only 2/3 of the perovskite anion sites and are ordered such that 1/3 of the Cu is two- fold coordinated and 2/3 is fivefold coordinated. This configuration can be derived from that of the

superconductor Ba2YCu3 07 by removing the oxygen atoms along the b axis at

the (0,1/2,0) position. A pressed pellet exhibited semiconductivity with

a band gap of 0.21 eV from 125-300 K. The compound Is described as Ba2YCu1+ Cu|+06

.

X-RAY POWDER STUDY OF 2Ba0:Cu0, W. Wong-Ng, K L. Davis, and R. S. Roth, Journal

of the American Ceramic Society, Vol. 71, No. 2, pp. C-64-C-67 (1987).

[ IMSE/Ceramics Division] (34)

A compound of composition 2Ba0:Cu0 was synthesized during the phase

equilibria study of the Ba0-Y203 -Cu0x system. Phase characterization has

been carried out by using X-ray powder diffraction. The crystal symmetry

was found to be the same as that of Ca2Cu03 and Sr2Cu03

. It is orthorhombic

with space group Immm and lattice parameters a = 12.9655(14),

b = 4.1007(3), c = 3.9069(5) A, V = 207.72(3) A. The experimental pattern

shows good agreement, in general, with intensity values calculated by

assuming Ba2 Cu03 to have a structure similar to that of Sr2 Cu03 and Ca2 Cu03

. Intensity discrepancy for the hOO reflections might be due to preferred

orientation

.

10
NEUTRON POWDER DIFFRACTION STUDY OF ORTHORHOMBIC Ba2YCu3 06 . 5

, S. Miraglia, F. Beech, A. Santoro, D. Tran Qui, S. A. Sunshine,* and D. W. Murphy,* Materials

Research Bulletin, Vol. 22, pp. 1733-1740 (December 1987). [ IMSE/Reactor

Radiation Division] (38)

*AT&T Bell Laboratories, Murray Hill

The structure of orthorhombic Ba2YCu3 06 . 5 has been refined with

neutron diffraction profile analysis. The space group of the compound is

Pmmm, and the lattice parameters are a = 3.8468(1), b = 3.8747(1), and

c = 11.7466(5) A. The oxygen sites at (0,0,z) , 1/2, 0,z) , and (0,l/2,z) are fully occupied, while the sites at (0,1/2,0) are 50% full and the sites at

(1/2,0,0) are vacant. A comparison of bond distances in this sample and

in other compositions in the system Ba2YCu3 0(5 (6.0 < 8 < 7.0) shows that

the environment of the Ba atoms and of the Cu atoms at (0,0,0) and (0,0,z)

change significantly with the amount of oxygen in the unit cell.

THE X-RAY POWDER DIFFRACTION PROFILE STUDIES ON YBa2 Cu3 O7 . 0 AND YBa2 Cu3 06 . 8

, Y.

Zhang,* W. Wong-Ng, B. Morosin.f C. R. Hubbard, J. M. Stewart,* and S. W.

Freiman, PhysicaC, Vol. 152, pp. 130-132 (1988). [ IMSE/Ceramics Division] (53)

*University of Maryland

fSandia National Laboratories

Two barium yttrium cuprate compositions having different oxygen

stoichiome tries , YBa2 Cu3 07 0 and YBa2 Cu3 06 8

, were prepared from a single

phase material under different processing conditions. A comparison of the

X-ray powder diffraction (XRD) patterns of these two samples showed a significant difference in diffraction line breadth. X-ray diffraction line

profile analysis has been employed to study the profile differences.

POSSIBLE EVIDENCE FOR SUPERCONDUCTING LAYERS IN SINGLE CRYSTAL YBa2 Cu3 07. x BY

FIELD ION MICROSCOPY, A. J. Melmed, R. D. Shull, C. K. Chiang, and H. A. Fowler,

Science, Vol. 239, pp. 176-178 (January 8, 1988). [IMSE/Ceramics Division and

Metallurgy Division] (73.3)

The high- transition- temperature superconducting ceramic material

YBa2 Cu3 07

_ x (0 < x < 0.5) has been examined by field ion microscopy.

Specimens from nominally superconducting and nonsuperconducting samples

(determined by magnetic susceptibility measurements) were studied by field

ion microscopy and significant differences were found. Preferential

imaging of atomic or molecular layers, due to preferential field

evaporation, field ionization, or both, was found in the superconducting

phase below the transition temperature and is interpreted as possible

evidence for the occurrence of relatively highly conducting layers in the

YBa2 Cu3 07

_ x unit cell perpendicular to the orthorhombic c-axis. Similar

results were obtained for YbBa2 Cu3 07

_ x.

11
SIGNIFICANCE OF PLANE VERSUS CHAIN SITES IN HIGH -TEMPERATURE OXIDE

SUPERCONDUCTORS, Gang Xiao, M. Z. Cieplak, D. Musser, A. Gavrin, F. H. Streitz,

C. L. Chien, J. J. Rhyne , and J . A. Gotaas, Nature, Vol. 332, No. 6161, pp. 238-

240 (March 17, 1988). [ IMSE/Reactor Radiation Division] (75.3)

One of the outstanding questions concerning the high- temperature

superconductor YBa2 Cu3 07 is the relative importance of the Cu02 planes

(Cu(2) site) and the CuO chains (Cu(l) site) . Many theories have stressed

the importance of the 3d holes at the Cu sites, which provide an antiferromagnetic correlated background necessary for the coupling of the

superconducting electrons. We have substituted Zn(3d10 4s2

) and

Ga(3d10 4s24p1

) in the Cu(2) and Cu(l) sites, respectively, while maintaining

the oxygen stoichiometry at 7 . In the valence states of 2+ and 3+

respectively, the 3d bands of both elements are completely full, without

the complication of a magnetic moment. Here we discuss the structural

changes that accompany Zn and Ga substitution, as deduced from X-ray and

neutron diffraction, and correlate these changes with the superconducting

properties. We show that the integrity of the planes is much more important than that of the chains in sustaining high-Tc superconductivity.

12
5. ELASTIC CONSTANTS AND PHONON SPECTRA

THE BULK MODULUS AND YOUNG'S MODULUS OF THE SUPERCONDUCTOR Ba2 Cu3 Y07

, S. Block,

G. J. Piermarini, R. G. Munro , and W. Wong-Ng, Advanced Ceramic Materials:

Ceramic Superconductors, Vol.2, No. 3B, p. 601 (July 15, 1987). [ IMSE/Ceramics

Division] (19)

The isothermal equation of state of the high temperature

superconducting ceramic material Ba2YCu3 07 has been determined by

measurements in a diamond anvil pressure cell using an energy dispersive

x-ray diffraction method. The orthorhombic unit cell lattice parameters

(a = 3.8856 A, b = 11.6804 A, c = 3.8185 A) were found to have compressions

of (2.0%, 2.3%, 1.1%) respectively over the pressure range from one atmosphere to 10.6 GPa at room temperature. Subsequent equation of state

analysis of the approximately linear compression of the volume determined

that the isothermal bulk modulus was 196 ± 17 GPa. Young's modulus was estimated to be 235 ± 20 GPa assuming that the Poisson's ratio for

Ba2YCu3 07 was 0.3 which is typical of many ceramics.

PHONON DENSITY OF STATES OF SUPERCONDUCTING YBa2 Cu3 07 AND THE NONSUPERCONDUCTING

ANALOG YBa2Cu3 06 , J. J. Rhyne , D. A. Neumann, J. A. Gotaas , F. Beech, L. Toth,*

S. Lawrence,* S. Wolf,f M. Osofsky.f and D. U. Gubser.f Physical Review B,

Vol. 36, No. 4, pp. 2294-2297 (August 1, 1987). [ IMSE/Reactor Radiation

Division] (25.3)

Neutron scattering has been used to study the vibrational density

of states and the atomic structure of the high- temperature superconductor

YBa2 Cu3 07 and the analogous nonsuperconducting compound YBa2 Cu3 06

. The

density of states of the superconductor shows a strong double peak at about

20 meV and a second major maximum near 70 meV with additional less -intense

features present at intermediate energies. The YBa2 Cu3 06 material shows

a similar energy spectrum above 45 meV; however, below this energy there

are significant differences associated with the 0 vacancies in the linear

b-axis chain site.

ELASTIC CONSTANTS AND DEBYE TEMPERATURE OF POLYCRYSTALLINE Y1 Ba2 Cu3 07

. x , H. M.

Ledbetter, M. W. Austin, S. A. Kim, and Ming Lei, Journal of Materials Research,

Vol. 2, No. 6, pp. 786-789 (November-December 1987). [ IMSE/Fracture and

Deformation Division] (28.3)

Using ultrasonic methods, the quasi- isotropic elastic stiffness of

void-containing Y1 Ba2 Cu3 07

_ x were determined. By a composite-material

model, these were corrected to the void- free state. From these, the Debye

characteristic temperature was calculated. All the elastic stiffnesses

fall well below those of polycrystalline BaTi03, an approximate crystal

structural building block of Y1 Ba2 Cu3 07

_ x

. The low apparent stiffness may

result from oxygen vacancies , which soften interionic forces. Also, it may

result from microcracks , which reduce elastic stiffness without lowering

mass density.

13
SHEAR -MODULUS CHANGE BELOW Tc IN YBa2 Cu3 07

_ x , H. M. Ledbetter, M. W. Austin, S. A.

Kim, T. Datta,* C. E. Violet, f Journal of Materials Research, Vol. 2, No. 6,

pp. 790-792 (November-December, 1987). [ IMSE/Fracture and Deformation Division]

(29. 3)

^University of South Carolina

(Lawrence Livermore National Laboratory

The ultrasonic transverse -wave velocity in the high-Tc metal-oxide

superconductor YBa2 Cu3 07 _ x between 275 and 4 K was measured. Above Tc the

velocity shows normal behavior: during cooling, it displays a monotonic

increase with decreasing slope. During cooling below Tc the shear modulus

departs from normal behavior, increasing as AG/G = b[l - (T/Tc ) n

]

, where

n = 3. These results depart dramatically from those expected for a simple

second-order normal - superconducting phase transition.

REENTRANT SOFTENING IN PEROVSKITELIKE SUPERCONDUCTORS, T. Datta,* H. M. Ledbetter, C. E. Violet, f C. Almasan,* and J. Estrada,* Physical Review B, Vol.

37, No. 13, pp. 7502-7505 (May 1, 1988). [IMSE/Fracture and Deformation

Division] (39)

^University of South Carolina

fLawrence Livermore National Laboratory

We suggest a model- -reentrant elastic softening- - that achieves three

useful results. First, and principally, it reconciles existing soundvelocity-elastic-constant measurements with thermodynamics. Second, it

leads to Debye characteristic temperatures that agree with those from

specific-heat and phonon density- of - states determinations. Third, it links

elastic - cons tant - temperature behavior in Y-Ba-Cu-0 and La-Sr-Cu-0. The

model predicts a superconducting- state elastic stiffness lower than the

normal state.

ELASTIC PROPERTIES OF METAL-OXIDE SUPERCONDUCTORS, Hassel Ledbetter, Journal of

Metals, Vol. 40, No. 1, pp. 24-30 (January 1988). [IMSE/Fracture and Deformation

Division] (51.3)

This article reviews the elastic properties of the new perovskiterelated superconductors, especially Y1 Ba2 Cu3 07

_ x

. Considered are such

related matters as atomic size, crystal structure, microstructure , twins,

and microscopic mechanism. Also examined are related physical properties

such as Debye characteristic temperature, thermal expansivity, specific

heat, ultrasonic attenuation, and phonon density-of -states

.

REENTRANT SOFTENING IN COPPER-OXIDE SUPERCONDUCTORS, C. E. Violet,* T. Datta,

j

H. M. Ledbetter, C. Almasan, f and J. Estrada, f in High Temperature

Superconductors, Materials Research Society Symposium Series, Vol. 99, pp.

375-378. [IMSE/Fracture and Deformation Division] (55.3)

*Lawrence Livermore National Laboratory

fDept. of Physics and Astronomy, University of South Carolina

We suggest a model—reentrant elastic softening—that achieves three

useful results. First, and principally, it reconciles existing soundvelocity-elastic - constant measurements with thermodynamics. Second it

L4
leads to Debye characteristic temperatures that agree with those from

specific heat and phonon density-of -states . Third, it links elasticconstant-temperature behavior in Y-Ba-Cu-0 and La-Ba-Cu-O. The model

predicts a lower superconducting- state elastic stiffness.

ELASTIC CONSTANTS OF POLYCRYSTALLINE Y1 Ba2Cu3 07

_ x , H. M. Ledbetter, S. A, Kim, and

D. W. Capone,* in High-Temperature Superconductors II, Materials Research Society

Symposium Series, pp. 293-296 (1988). [ IMSE/Fracture and Deformation Division]

(61)

^Materials Science Division, Argonne National Laboratory

We report the elastic constants of a polycrystalline Y1 Ba2 Cu3 0 7_ x superconductor between 295 and 4 K. Roughly speaking, the elastic

constants behave regularly. However, small departures from regular

behavior provide much interest. We draw the following conclusions from the

measurements: (1) Elastic constants show irregularities below and above

Tc

. During cooling, all the elastic constants show an irregularity near

200 K. (2) Within measurement error (about 1 part in 1000), none of the

elastic stiffnesses shows an abrupt change at Tc

, 91 K. (3) Below Tc

, all

the elastic constants show regular behavior, except v (Poisson ratio) . (4)

During cooling, between 160 and 70 K, the material behaves like it

undergoes a sluggish phase transformation. (5) The large (4 percent)

decrease in Poisson ratio is unexpected. It indicates large interatomicforce changes. (6) The unusual flatness of u(T) near 295 K suggests

unusual material-property changes above ambient temperatures.

15
i <r - — v r r

6. ELECTRICAL CONTACTS

METHOD FOR MAKING LOW-RESISTIVITY CONTACTS TO HIGH Tc SUPERCONDUCTORS, J. W.

Ekin, A. J. Panson,* B. A. Blankenship , * Applied Physics Letters, Vol. 52, No. 4,

pp. 331-333 (January 25, 1988). [CEEE/Electromagnetic Technology Division]

(44.3)

*Wes tinghouse Research & Development Center

A method for making low- resistivity contacts to high Tc superconductors has been developed, which has achieved contact surface

resistivities less than 10 fj,Q cm2 at 76 K and does not require sample

heating above -150° C. This is an upper limit for the contact resistivity

obtained at high current densities up to 102 -103 A/cm2 across the contact

interface. At lower measuring current densities the contact resistivities

were lower and the voltage-current curve was nonlinear, having a superconducting transition character. On cooling from 295 to 76 K, the

contact resistivity decreased several times, in contrast to indium solder

contacts where the resistivity increased on cooling. The contacts showed

consistently low resistivity and little degradation when exposed to dry air over a four-month period and when repeatedly cycled between room temperature and 76 K. The contacts are formed by sputter depositing a layer of a noble metal—silver and gold were used—on a clean

superconductor surface to protect the surface and serve as a contact pad.

External connections to the contact pads have been made using both solder

and wire -bonding techniques.

HIGH-TC SUPERCONDUCTOR/NOBLE -METAL CONTACTS WITH SURFACE RESISTIVITIES IN THE

1CT 10 fi-cm RANGE, J. W. Ekin, T. M. Larson, N. F. Bergren, A. J. Nelson*, A. B. Swartz lander* , L. L. Kazmerski*, A. Pansonf , and B . Blankenshipj , Applied Physics

Letters, Vol. 52, No. 21, pp. 1819-1821 (May 1988). [CEEE/Electromagnetic

Technology Division] (63.1)

*Solar Energy Research Institute

fWestinghouse Research and Development Center

Contact surface resistivities (product of contact resistance and

area) in the 10" 10 fi-cm" 2 range have been obtained for both silver and gold

contacts to high-Tc superconductors. This is a reduction by about eight

orders of magnitude from the contact resistivity of indium- solder

connections. The contact resistivity is low enough to be considered for

both on-chip and package interconnect applications. The contacts were formed by sputter depositing either silver or gold at low temperatures

(<100°C) on a clean surface of Y1 Ba2 Cu3 07 _

(5 (YBCO) and later annealing the

contacts in oxygen. Annealing temperature characteristics show that for

bulk- sintered YBCO samples there is a sharp decrease in contact resistivity

after annealing silver/YBCO contacts in oxygen for 1 h at temperatures

above -500° C and gold/YBCO contacts for 1 h above -600° C. Oxygen annealing

for longer times (8 h) did not reduce the contact resistivity of silver

contacts as much as annealing for 1 h. Auger microprobe analysis shows

that indium/YBCO contacts contain a significant concentration of oxygen in

the indium layer adjacent to the YBCO interface. Silver and gold contacts,

on the other hand, contain almost no oxygen and have favorable interfacial

16
chemistry with low oxygen affinity. Silver also acts as a "switchable"

passivation buffer, allowing oxygen to penetrate to the YBCO interface at

elevated temperatures , but protecting the YBCO surface at room temperature

17
7. ELECTRONIC STRUCTURE

RESONANT PHOTOEMISSION STUDY OF SUPERCONDUCTING Y-Ba-Cu-O, Richard L. Kurtz,

Roger L. Stockbauer, Donald Mueller,* Arnold Shih,* Louis E. Toth,* Michael

Osofsky,* and Stuart A. Wolf,* Physical Review B Rapid Communications, Vol. 35,

No. 16, pp. 8818-8820 (June 1, 1987). [CCP/Surface Science Division] (21)

*Naval Research Laboratory

Ultraviolet photoelectron spectra of 93-K superconducting compound,

YBa2 Cu3 07

, have been obtained using photon energies ranging from 60 to

106 eV. Resonant photoemission is used to identify the chemical origin of

the features in the valence -band electronic structure.

SYNCHROTRON RADIATION STUDIES OF THE ELECTRONIC STRUCTURES OF HIGH-TC SUPERCONDUCTORS, Richard L. Kurtz, American Institute of Physics Conference

Proceedings No. 165, American Vacuum Society Series 3: Thin Film Processing and

Characterization of High-Tc Superconductors, J. M. E. Harper, R. J. Colton, and

L. C. Feldman, eds . , New York, pp. 222-234 (1988). [CBS/Surface Science

Division] (43.3)

Experimental measurements of the electronic structure of the highTc superconductors La2

_ x Srx Cu04 and YBa2 Cu3 07 _ x are reviewed and discussed

in the context of determining the strengths of the interactions within

the valence bands. It is concluded that the Cu 3d - 0 2p valence bands are highly hybridized and there are strong electron-electron correlations. It

is shown that the Cu3+ configuration (3d8

) is energetically unfavorable and

the Cu in these systems is best thought of as having an approximate valence

of +2. The influence of surface stoichiometry on the electronic structure

is discussed and preliminary molecular adsorption studies are reported.

18
8. ENERGY GAP AND TUNNELING SPECTRA

ELECTRON TUNNELING MEASUREMENT OF THE ENERGY GAP IN A La-Sr-Cu-0 SUPERCONDUCTOR,

J. Moreland, A. F. Clark, H. C. Ku,* and R. N. Shelton,* Cryogenics, Vol. 27,

pp. 227-228 (May 1987). [ CEEE/Electromagnetlc Technology Division] (1.3)

*Iowa State University

We have used the break junction technique to determine the energy gap

of lanthanum-strontium-copper-oxide, one of the new high critical temperature superconductors. The current-voltage characteristics demonstrated a variety of tunneling behaviors. The best characteristic indicating

quasiparticle tunneling between superconducting electrodes implied an energy gap of 7.0 ± 0.1 meV. Derivatives of other characteristics showed

weak structure indicating possible energy gaps up to 9 meV.

TUNNELING SPECTROSCOPY OF A La-Sr-Cu-0 BREAK JUNCTION: EVIDENCE FOR STRONGCOUPLING SUPERCONDUCTIVITY, John Moreland, A. F. Clark, and L. F. Goodrich, H. C. Ku,* and R. N. Shelton,* Physical Review B, Vol. 35, No. 16, pp. 8711-8713 (June

1, 1987). [CEEE/Electromagnetic Technology Division] (2.3)

*Iowa State University

Detailed structure in the quasiparticle tunneling has been observed

in La-Sr-Cu-0 superconductive tunneling junctions using the break- junction

technique. Variability in the energy gap and associated structure in the

current-voltage curves are observed indicating significant inhomogeneity

in the superconducting properties. Large energy gaps (7.0 meV) and deep

structure In the conductance derivative are evidence for a strong-coupling

mechanism

BREAK -JUNCTION TUNNELING MEASUREMENTS OF THE HIGH-TC SUPERCONDUCTOR

Y1 Ba2 Cu3 Og

_ (5

, J . Moreland, J. W. Ekin, L. F. Goodrich, T. E. Capobianco, A. F. Clark, J. Kwo,* M. Hong,* and S. H. Liou,* Physical Review B, Vol. 35, No. 16,

pp. 8856-8857 (June 1, 1987). [CEEE/Electromagnetic Technology Division] (3.3)

*AT&T Bell Laboratories, Murray Hill

Current-voltage tunneling characteristics in a high- criticaltemperature superconducting material containing predominantly Y1 Ba2 Cu3 09

_

(5 have been measured using the break- junction technique. Sharp gap structure

was observed, with the largest superconductive energy gap measured to be

A = 19.5 ± 1 meV, assuming a superconductor -insulator -superconductor

junction. This energy gap corresponds to 2A/kBTc = 4.8 at T = 4 K, for a critical temperature of 93 K (midpoint of the resistive transition)

ELECTRON TUNNELING MEASUREMENTS IN LaSrCuO AND YBaCuO, John Moreland, J. W. Ekin,

L. F. Goodrich, T. E. Capobianco, and A. F. Clark, Materials Research Society,

Vol. EA-11, pp. 73-75 (June 1987). [CEEE/Electromagnetic Technology Division]

(4.3)

The break junction technique whereby vacuum tunneling occurs within

the fracture of a bulk sample is used to study the LaSrCuO and YBaCuO

perovskite superconductors. Structure in the current versus voltage

19
characteristics is reminiscent of previous quasiparticle curves obtained

for BCS superconducting materials. Some curves have anomalous qualities,

including large dips in the junction conductance with increasing voltage

just above a well defined tunneling gap edge, linearly increasing junction

conductance with applied bias, along with features occurring near voltage

intervals with the 1, 3, 5 pattern.

ELECTRON TUNNELING MEASUREMENTS OF HIGH Tc COMPOUNDS USING BREAK JUNCTIONS, John

Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F. Clark,

Proceedings of the 18th International Conference on Low Temperature Physics,

Kyoto, Japan, August 20-26, 1987, Japanese Journal of Applied Physics, Vol.26,

Supplement No. 26-3, pp. 999-1000 (1987). [ CEEE/Electromagnetic Technology

Division] (8.3)

We report on the break junction technique and its application to the

high Tc superconductors LaSrCuO and YBaCuO. In this technique, bulk

samples are fractured and the freshly fractured surfaces adjusted to form

a tunneling junction with vacuum or liquid helium as the insulating

barrier. Precise mechanical adjustment permits the study of electron

tunneling phenomena between pieces of a bulk superconductor. The current

voltage characteristics of these break junctions are variable indicating

sample inhomogeneity . However, some junction settings result in the more familiar quasiparticle signatures in the current voltage characteristics.

Low leakage junctions indicate the presence of a sharp superconductive

energy gap as well as large variations in junction conductance above the

gap edge in both materials.

ANOMALOUS BEHAVIOR OF TUNNELING CONTACTS IN SUPERCONDUCTING PEROVSKITE

STRUCTURES, J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F. Clark, Advanced Cryogenic Engineering, Vol. 34, pp. 625-632 (1988).

[CEEE/Electromagnetic Technology Division] (11)

Our break junction results for electron tunneling spectroscopy of the

perovskite superconductors La-Sr-Cu-0 and Y-Ba-Cu-0 are similar to those

obtained using thin film, scanning tunneling microscopy, and point contact

methods. Energy gap structures are sometimes observed in the measured

current voltage characteristics. More often, however, the characteristics

are anomalous when compared to previous tunneling studies of BCS

superconductors. The anomalies include linearly increasing conductance

with voltage, large deviations in junction conductance above the gap edge,

and junction diode action. We discuss some possible explanations for these

observations

.

RECENT TUNNELING MEASUREMENTS OF 90 K SUPERCONDUCTORS AT NBS , John Moreland,

J. A. Beall, R. H. Ono , and A. F. Clark, Materials Research Society, Vol. EA14

,

pp. 351-353 (December 1987) . [CEEE/Electromagnetic Technology Division] (60.3)

Several tunneling measurements on oxide superconductors have been

made at NBS in the last year. These include break junction tunneling

measurements of the energy gap, break junction superconducting point

contacts, and the operation of a break junction point contact rf SQUID

above 77 K. Until recently, these tunneling experiments have been limited

20
to bulk samples cut from sintered pellets and a few small single crystals.

We present here further results on thin films of YBa2 Cu3 Ox (YBCO) using

squeezable electron tunneling (SET) junctions. In contrast to the break

junction tunneling experiments on bulk samples, where quite often tunneling

spectra are without energy gap features, the spectra for thin- film SET

junctions are rich with structure.

SINGLE CRYSTAL HoBa2 Cu3 Ox BREAK JUNCTIONS, John Moreland, A. F. Clark, M. A.

Damento , * and K. A. Gschneidner, Jr., International Conference on High

Temperature Superconductors—Materials and Mechanisms of Superconductivity,

Interlaken, Switzerland, pp. 1383-1384 (1988). [CEEE/Electromagne tic Technology

Division] (64.3)

*Iowa State University

Tunneling spectra of HoBa2 Cu3 Ox single crystals using the break

junction method show energy gap features. These features are variable

from junction to junction possibly due to an anisotropic gap function. The

I-V curves show the peculiar square law dependence of the current on voltage seen in many tunneling measurements of polycrystalline samples of

90 K superconductors. This may be an indication of an inherent

"granularity" built into the superconducting matrix of a single crystal.

21
9. JOSEPHSON EFFECT AND DEVICES

JOSEPHSON EFFECT ABOVE 77 K IN A YBaCuO BREAK JUNCTION , John Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, A. F. Clark, A. I. Braginski,* and A. J.

Panson,* Applied Physics Letters, Vol. 51, No. 7, pp. 540-541 (August 17, 1987).

[ CEEE/Electromagnetic Technology Division] (7.3)

*Wes tinghouse Research & Development Center

We have observed the Josephson effect in a YBaCuO break junction.

Critical currents as high as 10 mA were measured at 4 K for break junctions

with a point contact within the fracture of a sample. The junction was susceptible to microwave radiation showing Shapiro steps with the ratio of

V/f of 2.04 ± 0.05 /iV/GHz compared to the pair tunneling value of

h/2e = 2.068 yLiV/GHz. These steps were clearly visible in the currentvoltage characteristics at temperatures up to 85 ± 5 K.

A JOSEPHSON ARRAY VOLTAGE STANDARD AT 10 V, Frances L. Lloyd, C. A. Hamilton,

J. A. Beall, D. Go, R. H. Ono , and R. E. Harris, IEEE Electron Device Letters,

Vol. EDL-8, No. 10, pp. 449-450 (October 1987). [CEEE/Electromagnetic Technology

Division] (32.3)

The technology of Josephson voltage standards has been extended to an array of 14 184 junctions which is capable of generating over 150 000

quantized voltage levels spanning the range from -12 to +12 V. This makes

possible the direct calibration of 10-V Zener reference standards without

the use of a voltage divider.

OPERATION OF A Y-Ba-Cu-Ox RF SQUID AT 81K, J. E. Zimmerman, J. A. Beall, M. W.

Cromar, and R. H. Ono, Applied Physics Letters, Vol. 51, No. 8, pp. 617-618

(August 24, 1987). [CEEE/Electromagnetic Technology Division] (33.3)

An rf superconducting quantum interference device (SQUID) has been

made from bulk Y-Ba-Cu-0 . The device displays quantum interference effects

and operates with useful signal levels up to 81 K. The SQUID is formed

from a ring of Y-Ba-Cu-0 which is broken in the cryogenic environment and

then recontacted. Estimates of the SQUID noise performance are given.

EQUIVALENT FLUX NOISE IN A Yx Ba2Cu3 0x RF SQUID, J. E. Zimmerman, J. A. Beall, M.

W. Cromar, and R. H. Ono, Proceedings of the 18th International Conference on Low Temperature Physics, Kyoto, Japan, August 20-26, 1987, Japanese Journal of

Applied Physics, Vol. 26, Supplement 26-3, pp. 2125-2126 (1987).

[CEEE/Electromagnetic Technology Division] (42.3)

We have measured the noise in flux- locked rf SQUIDs made of bulk

YBa2 Cu3 0x both in a He cooled cryostat and in liquid nitrogen (LN2

) . Our

best results at 75 K show a spectral density of the equivalent flux noise

22
10. MAGNETIC MEASUREMENTS

AC SUSCEPTIBILITY MEASUREMENTS NEAR THE CRITICAL TEMPERATURE OF A Y-Ba-Cu-0

SUPERCONDUCTOR, R. B. Goldfarb, A. F. Clark, A. J. Panson,* and A. I. Braginski,*

Materials Research Society, Vol. EA-11, pp. 261-263 (June 1987).

[CEEE/Electromagnetic Technology Division] (5.3)

*Wes tinghouse Research & Development Center

The loss component of complex susceptibility of a Y-Ba-Cu-0

superconductor near its critical temperature is strongly dependent on ac field amplitude but virtually independent of frequency. This implies that

magnetic hysteresis is the major loss mechanism in these materials. The

temperature at which the loss first becomes positive upon warming

corresponds to an equivalence between the amplitude of the ac field and

the lower critical field of the superconductor.

EVIDENCE FOR TWO SUPERCONDUCTING COMPONENTS IN OXYGEN -ANNEALED SINGLE-PHASE

YBa-Cu-0, R. B. Goldfarb, A. F. Clark, A. I. Braginski,* and A. J. Panson,*

Cryogenics, Vol. 27, p. 475-480 (September 1987). [CEEE/Electromagnetic

Technology Division] (30.3)

*Wes tinghouse

The complex susceptibility of a sintered Y-Ba-Cu-0 superconductor is

strongly dependent on a.c. field amplitude, h. Very small values of h must

be used for the real part of susceptibility, x' > to reach a value

corresponding to bulk diamagnetism just below the critical temperature, Tc

. The imaginary part, x" > represents hysteresis loss in the sample. Thus,

X" versus temperature becomes positive when h exceeds the lower critical

field, Hcl , of the superconductor.

Annealing the material in oxygen gives rise to two distinct

components, a relatively high-Tc , high Hcl superconductor (denoted as "G" or "good") and a relatively low-Tc

, low Hcl superconductor (denoted as "B" or "bad"). Curves of susceptibility versus increasing temperature reflect

the dual nature of the annealed sample: x' has an inflection point at Tc of the B component and approaches zero at Tc of the G component, while x"

has a peak at each Tc. Both critical temperatures decrease linearly with

increasing h, though at very different rates. Hcl of the G component is

considerably greater than Hcl of the B component. The lower critical

fields are linearly decreasing functions of temperature.

Two models might explain the susceptibility data. In the grain

model, the G component consists of superconducting grains and the B

component is either intergranular material, unfavourably orientated

anisotropic grains, or oxygen- depleted grain boundaries. In the surface

model, the G component is in the interior of the sample and the B component

is at the sample's surface. This condition could arise if there was oxygen

depletion at the surface subsequent to total enrichment during annealing.

23
VI

MAGNETIC SUSCEPTIBILITY OF SINTERED AND POWDERED Y-Ba-Cu-O, D.-X. Chen,* R. B. Goldfarb, J. Nogues,* and K. V. Rao,* Journal of Applied Physics, Vol. 63, No. 3,

pp. 980-983 (February 1, 1988). [ CEEE/Electromagnetic Technology Division]

*Royal Institute of Technology, Sweden (40.3)

The real and imaginary parts of ac susceptibility of a sintered

Y1 Ba2 Cu3 07

_

(5 superconductor were measured before and after powdering. The

temperature - dependent susceptibility may be separated into two contributions, one sensitive and the other relatively insensitive to the magnitude

of the measuring field. The former is partially suppressed by coarsely

crushing the sample. It is completely suppressed after finely powdering,

whereupon the susceptibility curves become insensitive to the magnitude of

the measuring field. Several models apparently consistent with the results

are discussed.

24
11. PHASE EQUILIBRIUM

PHASE EQUILIBRIA AND CRYSTAL CHEMISTRY IN THE SYSTEM Ba-Y-Cu-O, R. S. Roth, K. L. Davis, and J. R. Dennis, Advanced Ceramic Materials: Ceramic Superconductors,

Vol. 2, No. 3B, pp. 303-312 (July 15, 1987). [ IMSE/Ceramics Division] (13.3)

Preliminary phase equilibria diagrams are constructed for the

"binary" systems BaO-hY203 , BaO-CuOx , ^Y203 -CuOx , and the "ternary" system

BaO-^Y2 03 -CuOx. Some melting data are used to outline an approximate

primary field for the phase responsible for high temperature

superconductivity in this system, Ba2YCu3 07_ x

. A discussion is given of the

crystal chemistry of the phase(s) near the composition Ba3YCu20x.

25
12. PROCESSING- -BULK MATERIAL

LOW TEMPERATURE THERMAL PROCESSING OF Ba2YCu3 07 _ x SUPERCONDUCTING CERAMICS, C. K.

Chiang, L. P. Cook, S. S. Chang,* J. E. Blendell and R. S. Roth, Advanced Ceramic

Materials: Ceramic Superconductors, Vol. 2, No . 3B , pp. 530-538 (July 15, 1987).

[IMSE/Ceramics Division] (16)

^Polymer Division

Thermal processing of superconducting ceramics Ba2YCu3 07

_ x , in the

temperature range between 50°C and 750°C was studied. The oxygen content

of the ceramic was reversible under thermal treatment. The temperature for

optimizing oxygen content was discussed.

PROCESSING - PROPERTY RELATIONS FOR Ba2YCu3 07_x HIGH Tc SUPERCONDUCTORS, J. E

.

Blendell, C. K. Chiang, D. C. Cranmer, S. W. Freiman, E. R. Fuller, Jr.,

E. Drescher-Krasicka , Ward L. Johnson, H. M. Ledbetter, L. H. Bennett, L. J.

Swartzendruber , R. B. Marinenko, R. L. Myklebust, D. S. Bright, and D. E. Newbury, Advanced Ceramic Materials: Ceramic Superconductors, Vol. 2, No. 3B,

pp. 512-529 (1987). [IMSE/Ceramics Division] (20)

Ceramic materials exhibit strong relationships between their physical

properties, their ceramic microstructures , and the processing sequence that

produced these microstructures and properties . This interrelation between

processing, microstructures, and properties is particularly true for the

new high Tc superconducting ceramic oxides of the type Ba2YCu3 07

_ x. To

characterize some of these relations, a number of properties have been

measured for these superconducting ceramics, and related to the

microstructures and processing sequence. The Ba2YCu3 07

_ x ceramics were prepared by powder processing techniques, followed by dry pressing and

sintering. Sintering was conducted in both air and flowing oxygen at

various temperatures, followed by annealing at various temperatures and

environments. Superconducting properties, such as the transition

temperature and the width of the transition, were measured by both

electrical conductivity and AC magnetic susceptibility; both showing a strong sensitivity to annealing temperature and annealing atmosphere. The

microstructure and density was also strongly dependent on processing

conditions. In this regard compositional mapping proved to be an important

technique for quantifying tho microstructural properties. Structural

properties, which will be important for the reliable use of the materials, were characterized through the fracture strength, the fracture toughness,

and the elastic moduli.

RELATIONSHIP OF ELECTRICAL, MAGNETIC, AND MECHANICAL PROPERTIES TO PROCESSING

IN HIGH Tc SUPERCONDUCTORS, J. E. Blendell, C. K. Chiang, D. C. Cranmer, S. W.

Freiman, E. R. Fuller, Jr. , E. Drescher-Krasicka, Ward L. Johnson, H. M. Ledbetter, L. H. Bennett, L. J. Schwartzendruber , R. B. Marinenko, R. L. Myklebust, D. S. Bright, and D. E. Newbury, American Chemical Society Symposium

Series 351, "Chemistry of High Temperature Superconductors," pp. 240-260

(September 1987). [IMSE/Ceramics Division, Fracture and Deformation Division,

and Metallurgy Division; and CAC/Gas and Particulate Science Division] (31.3)

26
The interrelation between processing, microstructure , and properties

is an important factor in understanding the behavior of ceramic materials.

This type of understanding will be particularly important in the

development of new high Tc superconducting' ceramic oxides of the type

Ba2YCu3 07

_ x

. As an initial effort in understanding these relations, a number of properties have been measured for these superconducting ceramics

and related to their microstructure and processing sequence. The

Ba2YCu3 07

_ x ceramics were prepared by powder processing techniques, followed

by dry pressing and sintering in both air and flowing oxygen at various

temperatures. The sintered bodies were annealed at various temperatures

and environments. Superconducting properties, such as the transition

temperature and the width of the transition, were measured by both

electrical conductivity and AC magnetic susceptibility; both of these

properties show a strong sensitivity to annealing temperature and

atmosphere. The microstructure and density were also strongly dependent

on processing conditions. In this regard, compositional mapping proved to

be an important technique for quantifying microstructural variations.

Mechanical properties, such as elastic modulus, hardness, and fracture

toughness, which will be important for the reliable use of these materials

in large scale structures, were also determined.

LOW TEMPERATURE APPROACHES TO SUPERCONDUCTIVE MATERIALS : A CHALLENGE IN CHEMICAL

SYNTHESIS, Joseph J. Ritter, Ceramics Powder Science II, American Ceramic

Society, Westerville, OH, pp. 79-84 (May 1988). [ IMSE/Ceramics Division] (48)

The synthesis of Ba-Y-Cu oxide ceramic powders is an essential factor

in the development of superconductive devices. While the major effort in

the superconductor research is supported with powders synthesized by

conventional high temperature techniques, the problem of generating

precursors to these materials by low temperature routes presents a significant challenge. Results from four different chemical approaches

ranging from the precipitation of Y-Ba-Cu hydroxycarbonates to reactions

between Y-Ba alkoxides with Cu(OH) 2 are given. All of these systems

progress through an intermediate triphasic mixture of BaC03

, CuO and Y2 03 before being converted to YBa2 Cu3 0

5+x between 800 and 950° . Superconductive

behavior is measured in each case after annealing the specimens in 02 at

-600° . The benefits of ideal and practical low temperature processing are discussed.

27
13. PROCESSING- -THIN FILMS

SUPERCONDUCTIVITY IN BULK AND THIN FILMS OF La^gsSro.^CuO^ AND Ba2YCu3 07

. 5 , K. Moorjani,* J. Bohandy,* F. J. Adrian,* B. F. Kim,* R. D. Shull, C. K. Chiang,

L. J. Swartzendruber , and L. H. Bennett, Physical Review B, Vol. 36, No. 7, pp.

4036-4038 (September 1, 1987). [ IMSE/Metallurgy Division and Ceramics Division]

*Johns Hopkins University (27.3)

A laser-ablation technique was used to deposit thin films from the

bulk oxides La-L 8 5Sr0 is^uO^ and Ba2YCu3 07

_ (5 , whose superconductivity

properties were investigated by dc resistivity, complex ac susceptibility,

and microwave response. The latter technique was employed to establish

that the thin films have superconducting regions with properties similar

to the bulk materials.

28
AUTHOR INDEX

Adrian, F. J. 28 Gotaas , J. A. 12 , 13

Almas an, C. 14 Gschneidner, Jr., K. A. 21

Austin, M. W. 13,14 Gubser, D. U. 13

Beall, J. A. 20, 22 Hamilton, C. A. 22

Beech, F. 9-11, 13 Handwerker, C. A. 3 Bennett, L. H. 9,26,28 Harris, R. E. 22

Bergren, N. F. 16 Hong, M. 4,5,19

Blankenship , B. A. 16 Hubbard, C. R. 2,9,11

Blendell, J. E. 2,3,26

Block, S. 13 Janocko, M. A. 4,5

Bohandy , J

28 Johnson, Jr ., D . W. 9 Braginski , A. I. 4,5,22,23 Johnson, W. L. 26

Bray, S. L. 7 Bright, D. S. 26 Kazmerski , L. L. 16

Kim, B. F. 28

Capobianco, T. E. 19,20,22 Kim, S. A. 13-15

Capone , D. W. 4,5,15 Ku, H. C. 19

Cava , R . J9 Kurtz, R. L. 3,18

Chang , S . S26 Kwo , J

4,5,19

Chen, D.-X. 24

Chiang, C. K. 2,9,11,26,28 Larson, T. M. 16

Chien, C. L. 12 Lawrence, S. 13

Cieplak, M. Z. 12 Ledbetter, H. M. 13-15,26

Clark, A. F. 7,19-23 Lei, M. 13

Cook, L. P. 2,9,26 Lindstrom, R. M. 2 Cranmer , D . C

26 Liou, S. H. 4,5,19

Cromar, M. W. 22 Lloyd, F. L. 22

Damento , M. A. 21 Madey , T . E

3 Datta, T. 14 Marinenko, R. B. 26

Davis, K. L. 2,10,25 McMurdie, H. F. 2 de Lima, 0. F. 5 Melmed, A. J. 11

Dennis, J. R. 25 Miraglia, S. 9-11

Dragoo, A. L. 2 Moorjani, K. 28

Drescher-Krasicka

E. 26 Moreland, J. 19-22

Morosin, B. 11

Ekin, J. W. 4-6 , 8 , 16 , 19 , 20, 22 Mueller, D. 3 , 18

Estrada , J

14 Munro , R . G

13

Murphy , D . W

10, 11

Flandermeyer , B. 4,5 Musser, D. 12

Fowler, H. A. 11 Myklebust, R. L. 26

Freiman, S. W. 11,26

Fuller, E. R. , Jr. 3,26 Nelson, A. J. 16

Neumann , D . A

13

Gavrin, A. 12 Newbury , D . E . 26

Go, D. 22 Nogues , J. 24

Goldfarb.R. B. 23,24

Goodrich, L. F. 4,7,19,20,22

29
Ono, R. H. 20,22

Osofsky, M. 13,18

Panson, A. J. 4,5,16,22,23

Paretzkin, B. 2 Peterson, R. L. 6 Piermarini, G. J. 13

Pittman, E. S. 4

Rao, K. V. 24

Rhodes, W. W. 9

Rhyne , J . J

.

12, 13

Ritter, J. J. 27

Roth, R. S. 9,10,25,26

Santoro , A. 9-11

Scanlan, R. M. 4

Schneemeyer , L. F. 10

Shelton, R. N. 19

Shih, A. 3,18

Shull, R. D. 11,28

Stewart, J. M. 2,11

Stockbauer, R. L. 3,18

Streitz, F. H. 12

Sunshine , S . A

.

10,11

Swartzendruber , L. J. 9,26,28

Swartzlander , A. B. 16

Toth, L. E. 3,13,18

Tran Qui, D. 11

Vaudin, M. D. 3 Violet, C. E. 14

Waszczak , J . V

.

10

Wolf, S. 13

Wolf, S. A. 18

Wong-Ng, W. 2,9-11,13

Xiao, G. 12

Zaluzec, N. J. 5 Zhang, Y. 2,11

Zimmerman, J. E. 22

30
NBS-114A IREV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

RFPORT NO

2. Performing Organ. Report No. 3. Publication Date November 1988

4. TITLE AND SUBTITLE

HIGH-TEMPERATURE SUPERCONDUCTIVITY

Abstracts of NIST Publications, 1987-1988

5. AUTHOR(S)

Mary E. DeWeese, Robert A. Kamper, Ronald M. Powell, Editors

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

Same as Item 6

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 88-600602

J

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information . If document includes a si gnificant

bibliography or literature survey, mention it here) We have collected abstracts from 61 papers published between March 1987 and May 1988

covering various aspects of superconductivity research. The work of nine divisions

of the National Institute of Standards and Technology (formerly the National Bureau

of Standards) in both Boulder, Colorado, and Gaithersburg , Maryland, is represented.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) bismuth superconductors; critical current; critical field; critical temperature; high

T ; high-temperature superconductors; low-temperature superconductors; superconducting

ceramics; superconductors; thallium superconductors; YBCO; yttrium superconductors

13. AVAILABILITY

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| | For Official Distribution. Do Not Release to NTIS

FX] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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PRINTED PAGES

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15. Price

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