We confirm the observation of superconductivity at ambient pressure above 10 K in the organic metal (BEDT-TTF)2Cu(SCN)2 as reported recently by Urayama et al [12]. In addition we have measured ESR, ac-susceptibility and thermopower in crystals of (BEDT-TTF)2Cu(SCN)2 and have shown that in contrast to other organic superconductors here a relatively sharp superconducting transition even in the ac-susceptibility can be observed which saturates already around 8 K. The thermopower measurements indicate a clear metal-metal phase transition at 100 K and a possible second phase transition at around 50 K, while from the temperature dependence of the resistivity and susceptibility (ESR) these phase transitions cannot be observed

Gogu, Emil

Gärtner, Stephan

Heinen, Ilsabe

Keller, Heimo J.

Klutz, Thomas

Schweitzer, Dieter

Gärtner, S.

Gogu, E.

Heinen, I.

Keller, H.

Klutz, T.

Schweitzer, D.

MPG.PuRe

Superconductivity at 10 K and ambient pressure in the organic metal (BEDT - TTF)2Cu(SCN)2

Online Publikationen der Universität Stuttgart

Solid State Communications, Vol.65,No.12, pp.1531-1534, 1988. 
Printed in Great Britain. 
0038-1098/88 $3.00 + .00 
Pergamon Press plc 
SUPERCONDUCTIVITY AT 10 K AND AMBIENT PRESSURE IN THE ORGANIC METAL 
( BEDT - TTF )2 Cu ( SCN ) ~, 
S. G ~ t n e r ,  E. Gogu, I. Heinen,  H.J. Keller  ~, T. Klutz  and  D. Schwei tzer  # 
Max P l a n c k - l n s t i t u t  fu r  Medizinische Forschung,  AG MolekUlkrietal le ,  Heidelberg  and  
JAnorgan i sch -Chemisches  l n s t i t u t  de r  t ln ive rs i t~ t  Heidelberg,  Heidelberg,  FRG. 
(Received 12 December 1987 by Manuel Cardona) 
We conf i rm t he  obse rva t ion  of  superconduct iv i ty  a t  ambien t  p re s su re  above 10 K in t he  
organic  me ta l  ( B E D T - T T F } 2 C u ( S C N )  2 as r epo r t ed  recent ly  by Urayama e t  al [12].  In 
addi t ion  we have measu red  ESR, ac - suscep t ib i l l ty  and  t h e r m o p o w e r  in c rys t a l s  of  
(BEDT-TTF )2Cu (SCN)2 and have shown  t h a t  in c o n t r a s t  to  o t h e r  organic  supe rcon -  
duc to r s  here  a re la t ively  sha rp  supe rconduc t ing  t r ans i t i on  even in t he  ac - suscep t ib i l i -  
ty can  be obse rved  which s a t u r a t e s  already a round  8 K. The t h e r m o p o w e r  measu re -  
m e n t s  indicate  a c lea r  m e t a l - m e t a l  phase  t r ans i t i on  a t  100 K and  a poss ib le  second  
phase  t r a n s i t i o n  a t  a round  50 K, while f rom the  t e m p e r a t u r e  dependence  o f  the  
res is t iv i ty  and  suscept ib i l i ty  (ESR) t h e s e  phase  t r an s i t i ons  c a n n o t  be observed.  
Since the observation of superconductivity at tempe- 
ratures up to 100 K in the high T c ceramic Cu-oxides 
- first discovered by Bednorz and MUller [ I ] - super- 
conductivity has attracted again a lot of interest. 
Compared to the high T c anorganlc superconductors 
the transition temperatures into a superconducting 
state in organic materials is still rather low. Never- 
theless, the development of new superconductors in 
t h i s  c lass  of  mate r ia l s  is qui te  fast .  Since t he  f i r s t  
obse rva t ion  of  superconduc t iv i ty  in an  organic  me ta l  
- t h e  Bechgaa rd - sa l t s  -- in 1980 wi th  t r a n s i t i o n  
t e m p e r a t u r e s  a round  1 K [2,  3]  qui te  a n u m b e r  of  
d i f f e ren t  organic  radical  s a l t s  showed  supe rconduc -  
t ivi ty [ 4-11]. The h ighes t  observed  t r a n s i t i o n  t e m p e -  
r a tu re  in a s t ab l e  supe rconduc t ing  s t a t e  a t  ambien t  
p ressu re  in ~ t - ( B E D T - T T F ) 2 I  3 was found  a t  8 K 
[9,  10]. Ju s t  r ecen t ly  Hrayama e t  al [12, 13] r epo r t ed  
fo r  the  f i r s t  t ime  t he  syn thes i s  of  a new organic  
supe rconduc to r  (BEDT - T T F  )2 Cu (SCN) 2 wi th  a t r a n -  
s i t ion  t e m p e r a t u r e  above 10 K (Tcffil0.4 K). The supe r -  
conduct iv i ty  was e s t a b l i s h e d  by resis t ivi ty,  d c - s u s c e p -  
t ib i l i ty  and  cr i t ical  f ie ld  measu remen t s .  
In th i s  l e t t e r  we conf i rm  th i s  obse rva t ion  o f  super -  
conduct iv i ty  above 10 K in ( B E D T - T T F ) 2 C u ( S C N )  2 
o f  Urayama e t  a l [  12 ],  and  in addi t ion  we r e p o r t  ESR, 
t h e r m o p o w e r  and ac - suscep t ib i l i ty  measu remen t s .  
Crys ta l s  of  ( B E D T - T T F ) 2 C u ( S C N )  2 were g r o w n  by 
e lec t rochemica l  m e t h o d s  s imi lar  as desc r ibed  in [ 12 ]. 
#Reprint request to Prof. Dr. D. Schweltzer, MFI/AG 
Molakiilkristalle, Jahnetr. 29, 6900 Heidelberg, FRG. 
In contrast to [12] the crystals were grown under 
constant voltage conditions with a current density pf 
about I pAlcm 2. Distorted hexagon shaped crystals 
of a typical size of l.S x 0.5 x 0.05 mm 3 were ob- 
tained, and the largest crystals had the size of 3 x 
I x 0.05 mm 3. 
Dc- and ac-conductivity measurements were performed 
with the usual four point method. Fig. la shows the 
temperature dependence of the resistivity of (BEDT- 
TTF)2Cu{ SCN) 2 crystals. The resistivity is nornudized 
to the room temperature value (P3oo ~ 0.05 flcm }. 
Similar as in [ 12 ] the resistivity increases first until 
around I00 K, but in contrast to [ 12 ] the increase is 
weaker (only a factor of 2.S, and In two other cry- 
stals even only 1.5). Below 70 K the resistivity starts 
to decrease rapidly and the ratio PIOOK/PlSK~$O0. 
Fig I b shows the low temperature region of the re- 
sistivity of our crystals of (BEDT-TTF)2Cu(SCN) 2. 
The onset temperature for superconductivity lies at 
II.! K. The superconducting transition temperature 
as evaluated by the center of the resistive transition 
amounts to 10.2 K. At 7.8 K the superconducting tran- 
sition - as measured by resistivity - is complete and 
the resistivity zero within the experimental possibi- 
lities. 
Ac-susceptibility measurements at 3 MHz ( ~ 0.2 Gauss ) 
were done as described earlier [ 14 ]. Fig. 2 shows the 
increase in resonance frequency of the LC-clrcuit due 
to exclusion of the rf- field by diamagnetic shielding 
currents in the ( BEDT-TTF )2Cu(SCN) 2 crystals. There 
is a clear evidence of an onset of diamagnetic shlel- 
1531 
1532 
I 
m lff* 
SUPERCONDUCTIVITY AT 10K AND AMBIENT PRESSURE IN (BEDT-TTF)2 Cu(SCN)2 
+ + - O - ~ . ~  
/ 
! 
÷ 
. ,~+.#÷"** 
(BEDT_TTF)aCu(SCN) a 
. .÷J., 
T e m p l r l ~ u r e  [ K ]  
Vol. 65, No. 12 
ding around 10 K, somewhat lower than the observed 
onset in resistivity [12]. A similar lower transition 
temperature in the ac-susceptibility was already 
observed before for ~-(BEDT-TTF)213 [15] and 
~t-(BEDT-TTF)2]3 [10] crystals. In contrast to 
[3-(BFDT-TTF)213 and ~t-(BI=DT-TTF)213 the 
superconducting transition here in ( BFDT- TTF)2 Cu. 
(SCN)2 as observed in the ac-susceptibility is rather 
sharp and starts to saturate already at 8 K. Urayama 
et al [ 12 ] reported a diamagnetic signal measured by 
a Faraday susceptometer of about S ~ with respect 
to a perfect diamagnet. With the ac-susceptibility we 
3 0  
E 
> ,  
~J 
n~ 
/ 
+ 
+ 
"l" 
~d--d "+ td '++ , 
8 18 
/ "  
.+ .+~ 
.+ +-v, -~ 
'I" / 
÷ 
( B E n T - T T F ) 2 C u  ( S C N ) ; 3  
T e m p e r a t u e e  [ K ]  
20 
Fig. l a: Temperature dependence of  the  resistivity of  
( BEDT- TTF)2 Cu ( SCN)2 ( resistivity normalized to the 
room temperature value),  b: Low temperature region 
of  the resistivity (l inear scale)  o f  (BEDT-TTF)2Cu-  
(SEN) 2. 
10 
IO 
~ s 
N S 
~ 4 
~ 2 
m 
-2 
(BEDT-TTF)  2 
i ~ i ~ ~ ~ ~ d ~ i~ 1'1 l~ l~ 14 
T [ K e l v i n ]  
Fig. 2. Increase in resonance frequency of  a LC- circuit 
due to  exclus ion of  the  r f - f i e l d  by d iamagnet ic  shie l -  
ding cu r r en t s  in ( BEDT - TTF ) 2 Cu (SCN) 2 ( a c -  suscep -  
t ib i l i ty  ).  
L9 
U 
(- 
-(3 
B 
© 
(- 
25 
20 
15 
10 
0 
0 
J 
~** * 
j***** 
• . I** 
.I***** ***** 
.i 
(BEDT-TTF) Cu(SCN) 
2 2 
I I I I I 
50 100 150 200 250 
Temperature [K] 
3 0 0  
i.2 
r-] 
.8 
4J 
°~ 
_Q 
+~ . 4  
© 
o 
.P 
O3 
./ 
/ 
** J 
J 
* j 
I ~*** 
(BEDT-TTF)2 Cu (SCN) 2 
I I i I I 
0 50  100 150 2 0 0  2 5 0  3 0 0  
Temperature [ K ]  
Fig. 3 a: Tempera tu re  dependence  of  the  ESR- l ine -  
width  of  ( B E D T - T T F ) 2 C u ( S C N )  2 crys ta ls ,  b: Tem-  
pera ture  dependence  of  the  suscept ib i l i ty  - a s  obta ined  
by ESR - o f  ( B E D T - T T F ) 2 C u ( S C N ) 2 .  
Vol. 65, No. 12 
find a much stronger effect and see a diamagnetic 
shielding signal for our crystals of at least 70 Z with 
respect to a perfect superconductor indicating a strong 
volume effect {this strong effect can also be seen 
on a powdered sample). 
Figs. 3a,b show the temperature dependence of the 
ESR- linewldth as well as of the susceptibility. At room 
temperature the FSR-linewidth is about 26 Gauss 
while at I0 K a linewidth of only 14 Gauss is obser- 
ved. The l=$R -linewidth measurement can be used 
for the identification of the crystals [ 16]. The FSR- 
line is more or less symmetric over the whole tempe- 
rature range, and only below 20 K the typical Dyson- 
type lines as for metals with high conductivity are 
observed. As expected for a metal the susceptibility 
obtained by ESR is only weakly temperature depen- 
dent and looks very similar to the susceptibility 
measured with the Faraday susceptometer [ 12 ). 
Thermopower measurements were done on three 
crystals as described earlier [17]. All three crystals 
showed very similar results. An example is shown in 
Fig. 4. In contrast to the resistivity and susceptibility 
the thermopower measurement indicates clearly that 
between I00 and II0 K a phase transition exists. In 
addition at 50 K a second phase transition might 
SUPERCONDUCTIVITY AT 10K AND AMBIENT PRESSURE IN (BEDT-TTF)2Cu(SCN) 2 1533 
occur. Since above I00 K and between $0 and 80 K 
the thermopower depends linearly on the temperature~_
the phase transition at I00 K seems to be a metal- 
metal phase transition. At room temperature the 
anions are not linear but bent and positional disorder 
was observed [ 12) while at 100 K the disorder does 
not exist [ 13]. So, it seems that the ordering of the 
anions below 100 K results in a new electronic state 
as can be seen from the thermopower measurement. 
Fig. 5 shows the low temperature region (<50 K) of 
the thermopower of ( BEDT - TTF )2 Cu ( SCN )2 .Between 
18 und 50 K the typical behaviour of a metal is ob- 
served while below 12 K the thermopower is zero 
within the experimental error as expected for a super- 
conductor. 
In conclusion we have shown that (BEDT-TTF) 2 Cu. 
(SCN) 2 is a volume superconductor at ambient pres- 
sure above I0 K with even a relative sharp transition 
observed in the penetration depth measurement (ac- 
susceptibility). Thermopower measurements indicate 
a metal-metal phase transition at I00 K and a possible 
second phase transition around 50 K. 
We gratefully acknowledge financial support of this 
work by Deutsche Forschungsgemelnschaft. 
8 
:3. 
)_J 
L 
Io - I 0  
3 
0 
O. 
O 
E 
L. --28 I)  
C" 
I-" 
- 3 6  
(BFDT-TTF)~ Cu (SCN) 
/m ~ j  
/ N  
N N ~ .N  
m 
• se  l e e  l s e  2oe 2se  3ee  
Temper a%ure [K)  
Fig. 4. Temperature  dependence of  the the rmopower  
of  (BEDT-TTF)2Cu{SCN)  2. 
:3. 
L.J 
3 
0 
(1. 
0 
E 
f.. 
- 4  
- 8  
18 2e  3E 48 58  
T e m p e r a t u r e  [ K ]  
Fig. 5. Low tempera ture  region o f  the the rmopower  
of  {BEDT-TTF)2Cu(SCN)  2 (see t e x t ) .  
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Superconductivity at 10 K and ambient pressure in the organic metal (BEDT-TTF)2Cu(SCN)2

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Superconductivity at 10 K and ambient pressure in the organic metal (BEDT-TTF)2Cu(SCN)2

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