Page 415 - Electrical Properties of Materials

High-T c superconductors 397

critical temperature? The record, reigning for a number of years was 125 K,
achieved by an oxide with the chemical formula Tl 2 Ba 2 Ca 2 Cu 3 O 10 , known
as TBCCO. The latest figure is about 10 K higher. The compound is mercury
barium calcium copper oxide (HBCCO). By now there are quite a number of
high-T c superconductors. For a selection, see Table 14.3.
What is the basic structure of these superconductors? The first one dis-
covered, La 2–x Ba x CuO 4 , contains single CuO 2 planes separated by layers
which provide a charge reservoir, and the same is true for the two materials
mentioned so far, YBCO and TBCCO.
How do they work? Copper oxide is an insulator, so that is not much good
for the purpose. It needs dopants for creating carriers which will then flow
along the CuO 2 planes. The carriers may, for example, be provided by Ba
for holes and Ce for electrons. Note also that many of the properties of these
compounds are highly anisotropic, which may be measured on a single crystal
specimen. The electrical resistivity perpendicular to the CuO 2 layers may be
5
10 times as large as along the in-plane layers. The temperature dependence of
resistivity is also different: in the perpendicular plane resistivity increases with
temperature as in a metal, but in-plane resistivity decreases with temperature
as in a semiconductor. There are also different phases of these materials, which
depend on the doping level.
A generic phase diagram of cuprate superconductors is shown in
Fig. 14.22. As many as five different phases may be seen, starting with an
antiferromagnetic insulator. In a certain range of doping (roughly between 0.1
and 0.2 holes per copper oxide) and below a certain temperature they are
superconductors, above that temperature they are metals having rather odd
properties. In fact, theoreticians believe that it would be easy to work out
the physics of the transition to superconductivity once the properties of the
metallic phase are understood. And that is not the case as yet.
∗ We should perhaps add here a new
There has been no proper theory developed either for cuprates or for the
class of superconductors, whose dis-
other main type of oxide superconductors based on BaBiO 3 compounds. ∗ covery has made it even more dif-
However, a consensus exists concerning some aspects of the theory. There is no ficult to devise a theory. They are
based on the fullerene C 60 mentioned in
Chapter 5. Some of their representatives
are K 3 C 60 ,Rb 3 C 60 ,and Rb 2.7 Tl 2.3 C 60 ,
Table 14.3 Approximate critical temperat- with critical temperatures of 19 K, 33 K,
ures (K) of a selection of high-T c supercon- and 42 K, respectively.
ductors

YBa 2 Cu 3 O 7 93
Y 2 Ba 4 Cu 7 O 15 95
Bi 2 CaSr 2 Cu 2 O 8 92
Bi 2 Ca 2 Sr 2 Cu 3 O 10 110
Tl 2 CaBa 2 Cu 2 O 8 119
Tl 2 Ca 2 Ba 2 Cu 3 O 10 128
TlCa 2 Ba 2 Cu 3 O 8 110
EuBaSrCu 3 O 7 88
GdBaSrCu 3 O 7 86
HgBa 2 CuO 4 94
HgBa 2 Ca 2 Cu 3 O 8 133
HgBa 2 Ca 3 Cu 4 O 10 126