Certain Novel Aspects of Thallium and Bismuth Cuprate Superconductors
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CERTAIN NOVEL ASPECTS OF THALLIUM AND BISMUTH CUPRATE SUPERCONDUCTORS C. N. R. RAO Solid State and Structural Chemistry Unit Indian Institute of Science, Bangalore-560012,
India.
ABSTRACT Novel superconducting thallium cuprates of the type TlCa 1xLn xSr 2Cu2 06+ (Ln
=
Y or
described.
rare
earth),
TlSr n+_xLnx Cuny
and
Tl -xPb xSr n+Cun0y
These cuprates as well as Bi 2Ca 1xLn xSr 2Cu208+
are
and TlCaIx Yx
Ba 2Cu206+6 show maximum Tc around a specific composition or oxygen content. They
also show
thermcpower slope in
of
these
interesting
with the
the
in
systems
can
contributions.
the
sign
Specially
thermopower-temperature
two-band
quasiparticle
changes
canposition.
plots.
The
be understood
It
appears
and
magnitude
noteworthy
in
that
is
the
thermopower terms
of
of
the
negative behaviour
entropic
Tl 1xPb xCaSr 2Cu206+ 6
and is
a genuine high T c electron superconductor. Intr oducti on A variety in
the recent
of cuprate superconductors literature
[1-3].
These
have been described adequately
are:
La2x MxCuO4 0
(Ln = Y or rare earth), LnBa 2 Cu 4 0 8 , Bi 2 (Ca, CunO2n+4' T nCanBa2Cun 2 2n+3 and Pb 2 Sr 2 (Ca, seem to be hole-superconductors. of
the
TCaSr2Cu 206+6
Sr) n+Cun Ln)Cu3 0
and
LnBa 2Cu3 07
2 n+ 4 '
Tl 2 CanBa2 Most of these
In this report, we shall discuss variants
system
which
show
interesting
superconducting
properties depending on the substitution of Tl and Ca by Pb and Ln respectively;
the
be prepared
parent in
compound
pure form.
(without
conductors not containing Ca or Ba. to
be two-band
systems
or
Tl
Ca
substitution)
We shall also describe new Tl cuprate
and have
the thermopower behaviour.
not
super-
The Tl cuprates discussed here appear therefore
enabled
the manipulation
the nature of charge carriers depending on the composition, in
can
Accordingly,
of
as reflected
TlCa 1xLn xSr2 Cu2y and TlCa 1x
Ln xBa2 Cu20y show maximum Tc around a specific composition; this behaviour is also shown by Bi2Ca 1 xLn xSr 2Cu2 0 8+6. More power data indicates electron-like behaviour We
discuss
the
thermopower
behaviour
in
interestingly, the thermoin many of these systems.
terms
particle contributions.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
of
entropic
and
quasi-
112
Bi2Ca1_LnSr2Cu20+ and TiCa1_LnxSr2Cu20+ Bi2 Ca1-x Inx Sr2 CU2 08+6 an la1-x nx Sr2 Cu2 06+6 The Bi2 Cax n x Sr2CU208+6 family of cuprates possessing the orthorhambic
structure
superconductivity In
Fig.
1, we
similar with
to
all
the
rare
parent earth
compound
(x
substitutions
show the compositional dependence
= up
0.0) to
exhibit x-
0.75.
of the superconducting
110 100-
0
+
0 0
90-
+
0 a
0
Fig.
80
s u pe r -
1. Variation of the
conducting transition x 70
temperature with x in Bi 2 Ca
60
50
30r
xLnxSr2 Cu208+6'
eNd
+DYb Dy 3Y
40-
1
OY 0 Pr B
I 0.2
I
0.1
i 0.4
0.3
I 05
x
transition
temperatures of
Tc attains
a maximum
Bi 2 Ca 1 x LnxSr
value
around
a
2 Cu 2 0
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