Anisotropic resistivity and thermopower of the organic
superconductor (ET)2Cu[ N(CN)2] Br
L. Buravov, N. Kushch, V. Merzhanov, M. Osherov, A. Khomenko, E.
Yagubskii
To cite this version:
L. Buravov, N. Kushch, V. Merzhanov, M. Osherov, A. Khomenko, et al.. Anisotropic resistivity
and thermopower of the organic superconductor (ET)2Cu[ N(CN)2] Br. Journal de Physique
I, EDP Sciences, 1992, 2 (7), pp.1257-1262. <10.1051/jp1:1992207>. <jpa-00246618>
HAL Id: jpa-00246618
https://hal.archives-ouvertes.fr/jpa-00246618
Submitted on 1 Jan 1992
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Phys. I
J.
France
(1992)
2
JULY1992,
1257-1262
PAGE1257
Classification
Physics
Abstracts
74.70K
71.30
Short
Communication
Anisotropic resist1vlty and thermopower
superconductor (E~j)2Cu [N(CN)2] Br
Buravov,
L-Iand
Institute
of
Osherov,
M-V-
We
Br in
K-
so
Chemogolovka,
in
April 1992, accepted
23
has
Academy
Russian
the
maxima
two
10~
at
the
at
and
75
ambient
derivative
temperature
dependence of the resistivity of (ET)2Cu
thermopower in the a and c directions.
anisotropy is rather
95 K. The value of the resistivity
and
increases
with lowering
temperature
temperature
of resistivity
exhibits
maximum
50 Kstrong
at
a
and
the
for
the
c
transition
direction.
near
50 K
We
have
similarly
(ET)2Cu(NCS)2.
Introduction.
1.
ent
Chernogolovka
temperature
directions
Thermopower is positive for the a direction, while it is negative
concluded, that (ET)2Cu [N(CN)2] Br undergoes
insulator-metal
At
Sciences,
of
May 1992)
crystallographic
about
The
12
measured
have
three
The resistivity
large: pb/Pa is
to
Khomenko
A-G-
Russia
Abstract.
[N(CN)2]
Physics
Chemical
142432
(Received
below
Merzhanov,
VA.
Yagubskii
E-B-
MD,
Kushch,
N-D-
organic
of the
present,
transition
another
organic compound (ET)2Cu [N(CN)2] Br ii] has
ET-based organic superconductors.
Besides
the
among
pressure
temperature,
distinct
(ET)2Cu [N(CN)2]
feature
(ET)2Cu[N(CN)2]Br,
the
Br
retains
resistivity
anomalous
from
its
behaviour
the
its
highest Tc(rs 12 K) at ambiprominent superconducting
predece8sor, (ET)2Cu(NCS)2,
[2].
(ET)2Cu[N(CN)2]
belongs
by twodimensional
conducting sheets of ET molecules, located in the ac plane and composed of ET
dimers, alternating with polymer anion chains extending along the a direction [I].
describe the way of synthesis of high quality single crystals of (ET)2Cu
In th18 paper
we
[N(CN)2] Br and the results of temperature investigations of resistivity and thermopower in
difserent
crystallographic
directions.
to
the
family
of
ET
salts
as
well
with
as
a
its
relative
so-called
compound
crystal
~-type
structure
[4].
It is
Cl [3],
formed
JOURNAL
1258
PHYSIQUE
DE
I
N°7
Experimental.
2.
(ET)2Cu [N(CN)2]
electro-chemical
Br crysta18 were prepared by
ox10~~
the
under
galvanostatic
regime
constant
(2
at the
M)
x
on
temperature (20 °C). The electrolyte used was the mixture of CuBr (5 x 10~~ M) NaN(CN)2
(5 x 10~~ M) and 18- crown-6 ether (5 x 10~~ M) or CuBr and Ph4P [N(CN)2]. The solvents
I,1,2-trichloroethane (TCE), TCE with an addition of10ili of ethanol, and
nitrobenzene.
were
The magnitude of the current, applied to different cells, was
between 0.2 and 0.6 PA.
The quality of single crystals, determining their superconducting
transition
temperature Tc
width 6Tc, depends significantly on the synthesis
and
transition
conditions:
the magnitude
and the kind of the
solvent
dicyanarnide salt used.
of the
and
The
perfect
current,
most
crystals were produced in the pure TCE without absolute ethanol, by using NaN(CN)2, at the
electrocrystallization
of 0.5 PA (the anode
density of IA
I-S pA/cm~). At
current
current
these
conditions
the crystals grew on the
anode in 12-14 day8 and
quite
uniform.
The
were
crystals had the shape of plates with typical dimensions I x I x 0.05 mm~.
SYNTHESIS.
2, I
idation
Pt
The
electrodes
of ET
,
itESISTIVITY
2.2
AND
by
dependence
temperature
The
ANISOTROPY.
of the
electrical
resis-
techniques. Contacts
was
a-csample
by
graphite
the
made
using
diameter
platinum
10 pm
wires.
The values
to
paste
were
of pb/pa and pb/pc were large enough to perform direct
assuming
of pb,
the unimeasurements
form
distribution
of current in the sample, when voltage is applied normally to the conducting
plane. The resistivity anisotropy was measured also using an improved Montgomery method
tivity
measured
the
probe
four
method
both
with
d-c-
and
[7].
resistivity in the ac plane varies in the range
synthesis conditions. Usually crystals with lower
p(295 K) had higher Tc and lower 6Tc. For the best crystals, having perfect superconducting
transition, the typical resistivity values at the ambient
4 x 10~~ Qcm,
temperature
are:
pa
10~~
Qcm,
Qcm.
A
typical
dependence
of
6
the
resistivity of
50
temperature
x
pc
pb
At
the
the
temperature
ambient
Qcm for
0.02-1
difserent
batches
of the
value
depending
on
=
"
=
(ET)2Cu [N(CN)2]
the diminishing of
Br in all
the
directions
temperature,
is
shown
having
a
in the
figure
I.
At
first
broad
minimum
of
around
it
slightly
lowers
250 K,
then
with
grow8
approximately 100 K. Then the resistivity passes through the two maxima
at 95 K and
These two maxima, not equally pronounced in
75 K, the former being higher than the latter.
difserent
directions, are well reproduced in difserent samples. After this, the resistivity drops
until the beginning of the
superconducting
transition.
The
about 50 times
insert of figure I
(the midpoint
transition.
It
shows the region of the superconducting
ii.7
K
2~
at
occur8
determined
being
less
of the transition) with a width,
than
0A K. The
6Tc
To.9
To
as
i
down
=
=
temperature
derivative
of pa,
pb
and pc
exhibits
a
strong
maximum
at
50 K
(Fig. 2).
The
qualitatively similar.
anisotropy pb/pa and pb/pc for the Same
change significantly in the region above
decreases slower, than other
components,
pb/pa
noticeably.
of the resistivity anisotropy by the
both
and
pb/pc
increase
Measurements
so
dependence, but the
improved Montgomery method [7] gave essentially the same
temperature
values of pb/pa * 3000 in the range 70-295 K were
typical.
more
resistivity in all directions is
Temperature dependences of calculated resistivity
crystal are shown in figure 3. Both values do not
further lowering the
50-60 K. With
temperature pb
overall
2.3
behaviour
THERMOPOWER.
method
the
of the
same
[8].
The
crystal
by standard for an organic crystals
Thermopower was
measured
dependence of thermopower measured in directions a and c on
shown in the figure 4. The thermopower has difserent signs for difserent
temperature
is
RESISTIVITY
N°7
THERMOPOWER
AND
OF
(ET)2Cu [N(CN)2]
Br
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l-O
~
b£
LT~
C~
0.04
0.8
j~
p~
~~~
~
0.6
~
0.02
CL
~'~
0.01
0.00
0 2
o-o
Temperature,
Fig.
The
1.
directions.
has
a
of
the
of
about
of the
a
direction
+ 20
temperature
linearly.
almost
zero
dependence
superconducting
For
value
lowering
temperature
the
shows
Insert
the
normalized
thermopower
the
pV/K
at
the
for
the
c
on
broad
a
pV/K at the ambient
and
temperature
pronounced peak at 70 K, followed by a
all
measured
very
close
3.
Discussion.
to
samples.
Below
temperature,
maximum
direction
-18
well
resistivity
in
the
positive in the whole
is
ambient
then, after
and
Similarly,
of
b
a,
and
c
directions.
transition.
has
thermopower
an
extremum
10 K
at
50
both
at
slightly
range. It
with the
130 K,
decreases
towards
is
120
K, this
temperature
increases
around
the
minimum
approximately
then
negative with the
K. Remarkably, it
feature
components
of
value
has
a
being reproduced
thermopower are
zero.
Superconducting transition, occuring at Tc
ii]. Small observed 6Tc reflects high quality
K, is in agreements with previous results
crystals. The value of the resistivity at
our
ambient
is
somewhat
higher than it was reported earlier ii]. It should be also
temperature
ratio in
noted, that in spite of very sharp superconducting
transition
the
residual
resistance
(ET)2Cu [N(CN)2] Br is relatively low (p(75 K)/p(12 K) rs 50).
As it was mentioned, the
derivative of the resistivity
exhibits a strong peak at
temperature
As in the case of (ET)2Cu(NCS)2 [6], it can be an argument for the
50 K for all directions.
of a phase
in (ET)2Cu [N(CN)2] Br at this
transition
existence
transition
This
temperature.
insulator-metal
should be of an
anomalous
metallic
is
type, since the low-temperature
state
and
superconducting, and the high-temperature one is semiconducting. The
for
such
reasons
transition, as in the case of (ET)2Cu(NCS)2 [6], is presumably the change of the band
structure
ii.?
=
of
JOURNAL
1260
DE
PHYSIQUE
I
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0.06
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0.00
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:
0
,
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-0.02
20
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RESISTIVITY
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«
THERMOPOWER
AND
(ET)2Cu [N(CN)2]
OF
Br
4000
CL
j
CL
/~bll~a
p~/pc
Temperature,
Fig.
3.
The
temperature
dependence
of the
anisotropy.
resistivity
So
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o
o
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°
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1261
JOURNAL
1262
derivative
a
and
c
metal
rs
95 K
PHYSIQUE
N°7
I
resistivity occuring
of the
directions.
transition
may
DE
50 K
near
be
different signs of the thermopower in the
at 50 K, and
concluded, that (ET)2Cu [N(CN)2] Br undergoes an insulator to
similarly to (ET)2Cu(NCS)2. The additional anomaly observed at
with
another phase
transition.
have
We
concerned
Acknowledgements.
work
supported by the Scientific Council on the U-S-S-R- State Program "lligh-Tc
was
Superconductivity".
The
authors wish to thank I.F. Shchegolev and V-N- Laukhin for valuable
discussions, and A-V- Zvarykina for help in
measurements.
The
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