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PHYSICAL REVIEW B
VOLUME 58, NUMBER 6
1 AUGUST 1998-II
80 K anomaly and its effect on the superconducting and magnetic transition in deuterated
k -„BEDT-TTF…2Cu†N„CN…2‡Br
X. Su and F. Zuo
Department of Physics, University of Miami, Coral Gables, Florida 33124
J. A. Schlueter, A. M. Kini, and Jack M. Williams
Chemistry and Materials Science Divisions, Argonne National Laboratory, Argonne, Illinois 60439
~Received 16 April 1998!
In this paper, we report careful transport and magnetic measurements on single crystals of deuterated
k -~ET!2Cu@N~CN!2!#Br. By cooling the sample at different rates, it is found that cooling through 80 K has a
dramatic effect on the normal state metal-insulator transition and the superconducting transition temperatures.
In-plane resistivity depends strongly on the cooling rate for temperatures below 80 K, above which all resistivity curves cooled at different rates converge. By comparing it with the nondeuterated salt and the quasi-1D
Bechgaard salt, we speculate that 80 K corresponds to a structural phase transition in the anion chain. Fast
cooling through 80 K will freeze the high temperature magnetic phase to low temperatures and the presence of
local magnetic moments suppresses the superconducting T c . @S0163-1829~98!51330-6#
The low temperature ground state of the organic
k -(BEDT-TTF) 2 X @BEDT-TTF is bis~ethylenedithio!tetrathiafulvalene, abbreviated as ET# is known to depend on
the anion, X, and the pressure.1–4,6,7 At ambient pressure,
k -~ET!2Cu@N~CN!2!#Cl is an insulator with an antiferromagnetic ground state with a Néel temperature of 27 K. Under a
pressure of 0.3 kbar, it superconducts at the highest transition
temperature, 13 K, in the ET family. k -~ET!2Cu~NCS!2 and
k -~ET!2Cu@N~CN!2!#Br, on the other hand, have superconducting ground states with T c of 10 K and 11.6 K, respectively. T c decreases with increasing pressure at a typical rate
of 23 K/kbar. Deuterated k -~ET!2Cu@N~CN!2!#Br, however,
is presumably at the boundary between the magnetic and
superconducting phases, with both superconducting and
magnetic transitions.4,5
The anion and pressure dependence are very similar to the
quasi-one-dimensional Bechgaard salts (TMTSF) 2 X, ~where
TMTSF denotes tetramethyltetraselenafulvalenium; X
5ClO4, PF6! with a ground state bordering the superconducting state and magnetic spin-density-wave state.8–11 Furthermore, in ~TMTSF!2ClO4, the ground state is also found
to depend strongly on the cooling rate with a superconducting ground state when slowly cooled and an insulating magnetic state when cooled rapidly. X-ray diffraction and neutron scattering measurements reveal that the thermal history
dependence of the ground state arises from a structural phase
transition in the one-dimensional ~1D! molecular stacks of
anions.10,11 The presence of the polymeric anion chain in the
quasi-two-dimensional k -(ET) 2 X system, especially the
deuterated k -~ET!2Cu@N~CN!2!#Br, calls for a close examination of possible ordering of the chains.
In this paper, we report careful transport and magnetic
measurements on several deuterated k -~ET!2Cu@N~CN!2!#Br
samples. By cooling the sample at different rates, it is found
that cooling through 80 K has a dramatic effect on the normal state metal-insulator transition and the superconducting
transition temperature. In-plane resistivity depends strongly
0163-1829/98/58~6!/2944~4!/$15.00
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on the cooling rate for temperatures below 80 K, above
which all resistivity curves cooled at different rates converge. The cooling rate dependence is very analogous to that
of the Bechgaard salt. We speculate that 80 K corresponds to
a structural phase transition temperature in the anion chain.
Cooling through 80 K at different rates will freeze the high
temperature phase to low temperatures and the presence of
local moments suppresses the superconducting transition
temperature.
Single crystals of the deuterated k -~ET!2Cu@N~CN!2#Br
superconductor were synthesized at the Argonne National
Laboratory as described elsewhere.12 Several crystals were
used in this study. The interlayer resistance was measured
with use of the four-probe technique. Contact of the gold
wires to the sample was made with a Dupont conducting
paste. Typical contact resistance between the gold wire and
the sample was about 1–10 V. A current of 1–10 mA was
used to ensure linear I-V characteristics. The room temperature resistivity was (260.25) V cm. The sample was initially cooled slowly from room temperature to liquid helium
temperature over 3 h. Subsequent coolings were done by
warming up the sample slowly from below 10 K to about
140 K, and then cooled at different rates to below 10 K by
pumping helium vapor in the sample space. The data were
taken while the sample was slowly warmed up. Magnetic
measurements were done with a Quantum Design SQUID
magnetometer. Each measurement was done with the sample
cooled in zero field to 5 K and magnetizations were done in
an applied field of 1 Oe.
Figure 1 shows the resistivity as a function of temperature, r (T), for the sample cooled initially from a high temperature and subsequently raised to and cooled from several
low temperatures of T q 535 K, 55 K, and 67 K. The data
clearly overlap with one another in the same temperature
range, independent of T q and the cooling rate. Similar low
T q (,80 K) independence was observed when the sample
was initially cooled at different rates from high temperatures.
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© 1998 The American Physical Society
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80 K ANOMALY AND ITS EFFECT ON THE . . .
FIG. 1. Resistivity as a function of temperature for a sample
with T q ,80 K.
For T q above 80 K, r (T) is found to depend strongly on
the cooling rate. Shown in Fig. 2 is a plot of r (T) for the
sample cooled down from 140 K at five different cooling
rates. The five different curves marked 1 to 5 correspond to a
cooling rate at 1 K/min, 10 K/min, 15 K/min, 30 K/min, and
60 K/min, respectively. The cooling rate is defined as the
dT/dt at around 80 K since the temperature does not decrease linearly with time. It is clear from the data that at
T;80 K, r (T) curves branch out. The resistivity as well as
the peak position changes according to the cooling rate. For
T.80 K, r (T) curves overlap, regardless of the cooling
FIG. 2. Resistivity as a function of temperature for a sample
cooled at different rates. Curves 1 to 5 correspond to a cooling rate
of 1, 10, 15, 30, and 60 K/min, respectively. The inset is a plot of
the peak resistivity versus peak temperature.
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FIG. 3. Resistivity as a function of temperature near the transition for a sample cooled at different rates. The labels are the same
as in Fig. 2.
rate. The inset is a plot of the peak resistivity, r peak , versus
the peak temperature, T peak . A monotonic increase in r peak
with decreasing T peak is observed.
The cooling rate has dramatic effect on the low temperature r (T) and superconducting transition temperature T c .
Shown in Fig. 3 is an expanded view of the low temperature
part of the data. Several features are clear: ~a! The low T
resistivity increases with increasing dT/dt; ~b! the superconducting transition temperature shifts lower with increasing
dT/dt; ~c! the r (T) for the slowly cooled sample displays
stronger semiconducting behaviors near T c and resistive
jumps above T c .
If we define the T c as the temperature where r drops to
50%, 75%, and 90% of resistivity at 12 K, the T c is found to
decrease quasilinearly with increasing r~12 K!, as shown in
Fig. 4. The choice of resistivity at 12 K is somewhat arbitrary because of the temperature dependence above T c , especially for the slowly cooled sample. Nevertheless, it gives
a good qualitative description of the cooling rate dependence
of T c .
Similar cooling dependent magnetization measurements
have been performed on another deuterated single crystal
sample. Shown in Fig. 5 is an overlay of magnetization as a
function of temperature for the sample cooled at different
T q ’s. For T q <78 K, M (T) basically overlaps. For 80 K
<T q <100 K, M (T) decreases in magnitude with increasing
T q . For T q >100 K, M (T) curves of different T q are indistinguishable. It is clear that the M (T) for the slowly cooled
sample has the highest onset T c and the onset transition temperature gradually decreases once T q is greater than 80 K.
Similar to the resistivity data, if the sample was initially
cooled at different rates from high T, M (T) for the low
T q (,80 K) will overlap with the M (T) of the initial data.
The T q and cooling rate dependence of the resistivity and
magnetization for the deuterated sample is very similar to the
cooling rate dependence of resistivity in the ~TMTSF!2ClO4
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SU, ZUO, SCHLUETER, KINI, AND WILLIAMS
FIG. 4. Superconducting transition temperature vs resistivity at
12 K.
compound.9 When cooled rapidly through 24 K, the resistivity of the Bechgaard salt is found to depend strongly on the
cooling rate for T,24 K and merges for T.24 K. The origin for the cooling rate dependence is associated with a
structural phase transition of the anion chain at 24 K. Measurements of specific heat jump at the superconducting transition find that T c as well as the volume fraction of superconducting phase decreases with increasing cooling rate.
The similarity suggests that 80 K is related to a possible
structural phase transition. The nature of the transition is
unclear. Several related anomalies have been reported in the
nondeuterated compound.13,14 Thermal expansion and x-ray
studies show that the a-axis ~along the polymer chain! lattice
parameter has an anomalous peak at around 80 K and a corresponding jump in the thermal expansion coefficient dl/dT.
Qualitatively similar cooling rate dependence has been ob-
FIG. 5. Shielding magnetization as a function of temperature at
1 Oe for a sample cooled at different temperatures.
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served in the nondeuterated k -~ET!2Cu@N~CN!2!#Br, i.e., the
resistivity curves for different cooling rates separate out at
temperature below 80 K and merge above it.15 It was initially
suggested that 80 K anomaly might be due to an orderdisorder transition of the ethylene groups.14 Recent studies
including x-ray diffraction and thermal relaxation measurements suggest strongly that the order-disorder transition has
an onset temperature of about 140 K and the transition occurs continuously over a wide temperature range.16,17 The
same 80 K transition temperature, independent of the ethylene groups, excludes the possibility of an order-disorder
transition of ethylene groups for the anomaly. Rather it suggests strongly that the anomaly in both deuterated and nondeuterated compounds could be from a structural transition
of the same anion chains, similar to an anion disorder model
proposed for the k -~ET!2Cu~NCS!2. 18,19
Magnetic susceptibility, 13C NMR, and the effect of cooling rate have been studied on the deuterated sample.4 It was
reported that the deuterated system is situated in the critical
region of the transition between a superconducting phase and
an antiferromagnetic phase with a T N of 14–15 K. Each
sample contains a mixture of magnetic and superconducting
phases. The magnetic phase is very similar to the
k -~ET!2Cu@N~CN!2!#Cl with spin cantings. Rapid cooling
suppresses the spin canting as observed from magnetic susceptibility measurement. However, the NMR data suggest
the same Néel temperature and a decreased superconducting
fraction upon rapid cooling through 80 K. The reduced moment may be due to structural disorders in the magnetic state.
Our measured r (T) and the cooling rate dependence are
consistent with the picture of increased fraction of the antiferromagnetic phases with rapid cooling, in analogy to the
1D system. In this picture, the merge of the resistivity curves
above and the independence of T q (,80 K) suggest strongly
a phase separation at around 80 K. Rapid cooling quenches
the high temperature phase to low temperatures. The resistivity peak around 40 K may be indicative of a competition
of the insulating magnetic and the metallic phases. With increasing fraction of the magnetic phase, the peak temperature
decreases and the magnitude of the resistivity increases. In
comparison with the nondeuterated compound, the secondary
bump or a shoulderlike feature in r (T) at around 70 K for
the rapidly cooled nondeuterated sample can arise from the
same argument.15 However, it is noted that the 100 K peak in
the nondeuterated k -~ET!2Cu@N~CN!2!#Br is not observed in
the deuterated compound.
The low temperature resistivity jumps may be signatures
of antiferromagnetic transition. It is not clear how the resistivity near T c is related to the cooling rate or volume fractions of the superconducting and magnetic phases. At first
sight, it might be contradictory to the picture of an increasingly larger magnetic volumes since the upturn in r (T) is
less pronounced with increasing cooling rate, somewhat
similar to the reduced moment from the dc susceptibility
measurement.4 However, it may be plausible that disordered
magnetic state will contribute to the observed temperature
dependence. Another possiblility is that the measured resistivity in this temperature range is not intrinsic due to the
nature of the mixed phases. The presence of various current
paths might complicate the measured quantity.
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80 K ANOMALY AND ITS EFFECT ON THE . . .
Like its quasi-1D counterpart, rapid cooling through 80 K
not only reduces the superconducting fraction, it suppresses
the superconducting transition temperature as well. With increasing cooling rate or increased resistivity, the superconducting transition curves shift toward zero. The quasilinear
T c dependence on r~12 K! is qualitatively similar to the
T c ( r ) in the nondeuterated compound.15 Although it is noted
that the resistivity above T c is very flat for the nondeuterated
compound, r is rather temperature dependent for the deuterated system. Nevertheless, the similarity suggests that the
reduction in T c may be of the same origin.
The linear dependence on r implies that DT c }1/t ; here t
is the relaxation time of charge carriers. For nonmagnetic
impurities at small concentrations, Anderson’s theorem that
the T c is little affected by the amount of disorders works for
most metallic superconductors. For dirty superconductors in
the strong coupling limit, the T c is found to increase or decrease with an increasing residual resistivity r, dependent on
the T c0 , the transition temperature in the clean limit.20 For
various materials, d t/ r , where t5(T c 2T c0 )/T c0 , was found
to have a magnitude of (1 – 100) mV cm21. For the organic
superconductor studied here, a similar calculation would
yield d t/ r ;2231024 mV cm21. The very small d t/ r suggests against the dominant role of nonmagnetic disorders in
the organic materials. In the presence of a secondary antiferromagnetic ~AFM! phase, it is not clear how T c will be affected. One possibility is that the local moment due to canted
spins in the AFM state will introduce spin scatterings. An-
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other possibility is that the rapid cooling may induce localized moments such as the Cu21 ions in the superconducting
phase. Both cases will give rise to a reduced T c ~Ref. 21! and
DT c } r . However, if we assume the cooling introduces a
similar amout of the Cu21 ions, the fact that DT c ;2 K in the
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In summary, we have reported detailed transport and
magnetic measurements and their cooling rate dependence in
the deuterated k -~ET!2Cu@N~CN!2!#Br. Comparisons with
the nondeuterated and the quasi-1D system demonstrate unequivocally a phase transition near 80 K, most likely in the
anion chain. Rapid cooling through 80 K suppresses the superconducting phase and increases the magnetic phase. The
reduction in T c may arise from the spin scattering of the
canted moment in the antiferromagnetic phase. This calls for
a careful structural determination in the k -(ET) 2 X family
and the role of anion chains in the interplay of superconductivity and magnetism.
The work was supported in part by NSF Grant No. DMR9623306. Work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Basic
Energy Sciences, Division of Materials Sciences, under Contract No. W-31-109-ENG-38.
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