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:ONDUCTIVITY 



:ts 



ERCONDUCTIVITY 



The New Superconductors 



)NDUCTIVITY 



Frank J. Owens 



Army Armament Research Engineering and Development Center 
Picatinny, New Jersey 

and Hunter College of the City University of New York 
New York, New York 



and 

Charles P. Poole, Jr. 

Institute of Superconductivity 
University of South Carolina 
Columbia, South Carolina 



on order will bring delivery of 
d only upon actual shipment. 



Plenum Press • New York and London 



On file 



Library of Congress Cataloging-in-Publication Data 



ISBN 0-306-45453-X 



© 1 996 Plenum Press, New York 

A Division of Plenum Publishing Corporation 

233 Spring Street, New York, N. Y. 10013 

1098765432 1 

All rights reserved 

No part of this book may be reproduced, stored in a retrieval system, or transmitted ii 
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Printed in the United States of America 



98 



CHAPTERS 



HE W HIGH-TEMPERATUI 



Table 8.1. Progress in Raising the Superconducting Transition Temperature T c 
Since the Discovery of Cuprates in 1986 



Material 


T C (K) 


Year 


Ba x La 5 _ x Cu 5 0 9 


30-35 


1986 


(Lao^Bao ,)2Cu 4 04_ x (at 1-GPa pressure) 0 


52 


1986 


YBa 2 Cu 3 0 7 . x 


95 


1987 


Bi 2 Sr2Ca2Cu3O 10 


no 


1988 


Tl 2 Ba 2 Ca 2 Cu3O 10 


125 


1988 


Tl 2 Ba 2 Ca 2 Cu 3 Oio (at 7-GPa pressure) 


131 


1993 


HgBa 2 Ca 2 Cu 3 0 8+x 


133 


1993 


HgBa 2 Ca 2 Cu 3 O 10 (at 30-GPa pressure) 


147 


1994 



°A pressure of 1 GPa is about 10,000 atm. 



While this increase in T c itself is an amazing result, a high-transition tempera- 
ture is not the only property required to make new compounds useful for applica- 
tions. For example if materials are to be used as wires in magnets, they must be 
malleable and ductile rather than brittle; in addition they must have high critical 
currents in large magnetic fields. Critical currents as high as those in niobium-tin 
have not yet been achieved in forms of the new materials that can easily be made 
into wires, although there are reports of comparable values in thin films on various 
substrates. 

The Holy Grail that is being sought is a transition temperature much above 
room temperature. We say much above because devices must operate significantly 
below the transition T c so that the critical current J c and critical magnetic field B c 
are sufficiently high. Very close to the transition temperature, the critical magnetic 
field is usually quite small, but we see from Figs. 3.4 and 3.5 that B c and i c 
continuously increase as the temperature is lowered below T c . We need an operating 
temperature far below the critical surface in Fig. 3.15 so that both B c and J c are 
sufficiently large for the desired application. 

8.3. LAYERED STRUCTURE OF THE CUPRATES 

All cuprate superconductors have the layered structure shown in Fig. 8.1: The 
flow of supercurrent takes place in conduction layers, and binding layers support 
and hold together the conduction layers. Conduction layers contain copper-oxide 
(Cu0 2 ) planes of the type shown in Fig. 8.2; each copper ion (Cu 2+ ) is surrounded 
by four oxygen ions (O 2- ). These planes are held together in the structure by calcium 
(Ca 2+ ) ions located between them, as indicated in Fig. 8.3. An exception to this is 
the yttrium compound in which the intervening ions are the element yttrium (Y 3 *) 
instead of calcium. These Cu0 2 planes are very close to being flat. In the normal 
state above T c , conduction electrons released by copper atoms move about on these 



Ci 



c 



o 



figure 8.1. Layering schem 
layers for different sequence 
for several cuprates. 



^gure 8.2. Arrangement ol 
in a Cu0 2 plane of the condi 



CHAPTERS 

Transition Temperature 7 C 
n 1986 

Year 

1986 
1986 
1987 
1988 
1988 
1993 
1993 
1994 



t, a high-transition tempera- 
npounds useful for applica- 
s in magnets, they must be 
ley must have high critical 
igh as those in niobium-tin 
als that can easily be made 
ues in thin films on various 

n temperature much above 
; must operate significantly 
d critical magnetic field B c 
ature, the critical magnetic 
•A and 3.5 that B c and 7 C 
w T c . We need an operating 
so that both B c and J c are 



ES 

ure shown in Fig. 8.1: The 
ind binding layers support 
yers contain copper-oxide 
;r ion (Cu 2+ ) is surrounded 
in the structure by calcium 
.3. An exception to this is 
the element yttrium (Y 3 *) 
> being flat. In the normal 
toms move about on these 



MEW HIGH-TEMPERATURE SUPERCONDUCTORS 99 



BINDING LAYERS 




CONDUCTION LAYERS WITH Cu0 2 



BINDING LAYERS 



CONDUCTION LAYERS with Cu0 2 



BINDING LAYERS 



CONDUCTION LAYERS WITH Cu0 2 



BINDING LAYERS 



Figure 8. 7 . Layering scheme of the cuprate superconductors. Figure 8.3 shows details of the conduction 
layers for different sequences of copper oxide planes, and Fig. 8.4 presents details of the binding layers 
for several cuprates. 



OXYGEN COPPER 

i / 

• o • o • o • 

o o o o 

• o • o • o • 



f igure 8.2. Arrangement of copper and oxygen atoms O O O O 

in a Cu0 2 plane of the conduction layer. • O • O • O * 



100 



CHAPTERS 



Cu0 2 



Conduction layer with one copper oxide plane 



Cu0 2 
Ca 
Cu0 2 



Conduction layer with two copper oxide planes 



Cu0 2 
Y 

CuO, 



Conduction layer of yttrium compound with two copper oxide planes 



La: 



Neodymii 



Yt1 



Cu0 2 
Ca 

Cu0 2 
Ca 

CuO, 



Conduction layer with three copper oxide planes 

Figure 8.3. Conduction layers of the various cuprate superconductors showing sequences of O1O2 and 
Ca (or Y) planes in the conduction layers of Fig. 8. 1 . 



Cu0 2 planes carrying electric current. In the superconducting state below T c , these 
same electrons form the Cooper pairs that carry the supercurrent in the planes. 

Each particular cuprate compound has its own specific binding layer consisting 
mainly of sublayers of metal oxides MO, where M is a metal atom; Fig. 8.4 gives 
the sequences of these sublayers for the principal cuprate compounds. These 
binding layers are sometimes called charge reservoir layers because they contain 



Bismutr 



Thalliu 



Mercur 

Figure 8.4, Sequences c 
metal ions. The parenthe: 



CHAPTERS 



r oxide plane 



r oxide planes 



■ two copper oxide planes 



>er oxide planes 

:tors showing sequences of Cu0 2 and 



•nducting state below T c1 these 
supercurrent in the planes, 
►ecific binding layer consisting 
is a metal atom; Fig. 8.4 gives 
J cuprate compounds. These 
ir layers because they contain 



LaO 

LaO 

Lanthanum Superconductor La 2 Cu0 4 



NdO 
NdO 



Neodymium (electron) Superconductor Nd 2 Cu0 4 



BaO 
CuO 
BaO 



yttrium Superconductor YBa 2 Cu 3 0 7 



Sro 
BiO 
BiO 
SrO 



Bismuth Superconductor Bi 2 Sr 2 Ca n _ 1 Cu n 0 2n+4 



BaO 
TIO 
TIO 
BaO 



Thallium Superconductor Tl 2 Ba 2 Ca n _ 1 Cu n 0 2n+4 



BaO _ 
_Hg(0). 
BaO _ 



Mercury Superconductor HgBa 2 Ca n . 1 Cu n 0 2n+2 

Figure 8.4. Sequences of MO sublayers in the binding layers of Fig. 8.1, where M stands for various 
metal ions. The parentheses around the oxygen atom O in the lowest panel indicates partial occupancy. 



CHAPTERS 

of randomly oriented grains. I n 
he current flow capability 0 f 



La,_ x ,Sr x ) 2 Cu0 4 are hole-type 
^rium-copper oxide, (Nd Ux 
trons rather than holes. The 
have trivalent positive ions: 

(8.6) 
(8.7) 

itium (Sr 24 ) and cerium (Ce 4 *), 



,Cu0 4 ) 



l 2 Cu0 4 ) 



(8.8) 



(8.9) 



one extra electron to form an 
-ontium subtracts one electron, 
iperconductor is hole-like. Any 
int both of these examples of 
lar, but not identical structures; 
;ause most experiments are not 



UCTURES 

ferred to as ceramics, they are 
-rovskite refers to the particular 
iral perovskite, calcium titanate 
;) parts of the lanthanum com- 
perovskite, with Cu present in 
lot shown in Fig. 8.9) positions, 
Similarities between these two 
;all La 2 Cu0 4 a perovskite-type 



HEW high-temperature superconductors 



109 



TITANIUM " 



OXYGEN- 




Figure 8.9- Sketch of the cubic unit cell of the mineral Perovskite, CaTi0 3 , showing titanium at the 
vertices and oxygen in the middle of the edges. Calcium, not shown, is in the center of the cube. 



In contrast the ceramic designation is not based on structural grounds but on 
the similarity of the cuprate-superconducting compound and ceramic manufactur- 
ing process. For example La-Sr-Cu-0 is made by heating mixtures of lanthanum 
oxide, strontium carbonate, and copper oxide in air at 900-1000 °C for 20 hours. 
Proportions of atoms in the initial mixture should be the same as in the end product, 
and for the compound (Lao 9 Sr 0 ,) 2 Cu0 4 the ratio La:Sr:Cu is 1.8:0.2:1. Materials 
are usually ground to a fine mixture before heating; after heating in air, they are 
cooled, pressed into pellets, and reheated from 900- 1 000 °C for several more hours. 

We see in Fig. 8.10 that the superconductor (La t _ x Sr x ) 2 Cu0 4 has only one 
copper oxide plane in its conduction layer and each copper ion is surrounded by 



conduction layer 

binding layer 
conduction layer 
binding layer 
conduction layer 




pervoskite 
like 



Figure 8.10. Atom positions in the tetragonal unit cell of the La 2 Cu0 4 compound. When strontium is 
substituted for lanthanum in the superconducting compound (Lai_ JC Sr jr ) 2 Cu0 4 it replaces lanthanum in 
some of the La sites. 



uo 



CHAPTERS 



six neighboring oxygen ions; these form an 8-sided figure called an octahedron, as 
shown. The Cu0 6 complex of one copper and six oxygens is present in all cuprate 
superconductors that have a single Cu0 2 plane in their conduction layer. Figure 
8.1 1 shows atom arrangements in the mercury compound HgBa 2 Ca 2 Cu 3 O I0 , which 
has three such planes in its conduction layer. In the upper and lower planes, copper 
ions have five neighboring oxygens forming a CuO s group with the shape of a 
pyramid, as shown. The middle copper ions have only four nearby oxygens, forming 
what is called a square planar group Cu0 4 . If we consider removing the central 
copper oxide plane and one calcium layer from Fig. 8. 1 1 , we generate the two-plane 
structure in which all copper ions form Cu0 5 pyramids. These structural details 
may somehow constitute important factors in determining why cuprates are such 
good superconductors. 



r 

BINDING 
LAYER 



r 



CONDUCTION 
LAYER 



r 



BINDING 
LAYER 




jS/fW HIGH-TEMF 

8.8. YTTRIU 

The discov 
the initial report 
Miiller (see Fij 
compound YBa : 
nitrogen, as sho 1 
between the rest 
Wu of the Univt 



6 
o 

a 
a 



O.CK 



O.Oi 



0.04 



0.01 



0.0* 



Figure 8.11. Atom positions in four unit cells of the superconducting compound HgBa 2 Ca 2 Cu 3 0 8w 
which has T c = 133 K. The copper ions of the upper Cu0 2 plane are hidden by the pyramids, and some 
partially occupied oxygen sites in the mercury Hg plane are not shown. 



Hgure 8.1 2. First] 
Bednorz and K. A. I 



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