PhysicaC185-189 (1991) 2659-2660 North-HoUand PHYSICA

SUPERCONDUCTING TRANSITION IN K-(BEDT-TTF)2Cu[N(CN)2]Br UNDER MAGNETIC FIELD

Hiroshi ITO, Masashi WATANABE, Yoshio NOGAMI, Takebiko ISHIGURO, Tokutaro KOMATSU*, Gunzi SAITO*, Nobuyoshi HOSOITO t

~ hysics Department, Kyoto Univ., Kyoto 606-01 ; * Chemistry Department, Kyoto Univ., Kyoto 606-01; Inst. Chem. Research, Kyoto Univ. , Uji 611

~ nter layer and i~ t ra laye r coherence lengths of ~-(BEDT-TTF)2Cu[N(CN)2]Br are derived as ~b(0)=6±2 , ~ac(0)=24±5 A, respect ive ly , by magnetic measurement. The renormalized theory of the order

parameter f l u c t ua t i on near the superconducting t r a n s i t i o n region was employed for the analysis. The broad r e s i s t i v e t rans i t ions under magnetic f i e l d are discussed on the same foot ing,

Organic superconductors to date exh ib i t

notable broadening in the r e s i s t i v e t r ans i t i on

under magnetic f i e l d . In such a case one cannot

ru le out ambiguity in determining Hc2 and hence

Ginzburg-Landau (GL) coherence length ~°

Dominant cause of the broadening is not sample

inhomogeneity f o r the case of

~-(BEDT-TTF)2Su[N(CN)2]Br, since the t r ans i t i on

in the absence uY the magnetic f i e l d is rather

sharp. Simi lar broaae~ings are found in the

oxide superconductors with two-dimensional i ty .

In order to describe the t r ans i t i on

charac te r i s t i cs fo r the layered superconductors,

a renormalized theory of order parameter

f l uc tua t ion has been developed base~! on

Ginzburg-Landau formalism. 1,2 The e f fec t s of

the magnetic f i e l d perpendicular to the two-

dimensional plane have been ca lcu lated fo r the

i n te r l aye r conduc t i v i t y I and the dc

magnetization 2.

In th is paper we report the GL coherence

length at 0 K, ~(0), for organic superconductor

K-(BEDT-TTF)2Cu[N(CN)2]Br through the magnetic

and the r e s i s t i v e measurements, Sample crysta ls

grown by the electrochemical method exhib i ted

sharp res i s t i ve t r a n s i t i o n at 11.6±0o5 K in zeru

magnetic f i e l d ,

The e lec t r i ca l resistance under the magnetic

f i e l d up to 1 T applied normal to the

two-dimensional plane was measured by

four- terminal method. The dc magnetization was

measured by a SQUID susceptometer in the

f i e ld -coo led condi t ion,

Figures 1 and 2 show the temperature

dependence of the in te r layer e l ec t r i ca l

r e s i s t i v i t y and the dc magnetizaiton under

magnetic f i e l d perpendicular to the plane. In

order to deduce ~(0), we f i t these data near the

t r a n s i t i o n region by using the renormalized

f l uc tud t ion theory. 1,2 As for the spec i f i c heat

gap AC at the superconducting t r ans i t i on , we

~ 0.8 0

~ 0.4

0 v

0.687• f 4 ,e o

..~ .~" O. 18T

8 i n L2 A u

Tempevglure (g)

]4

FIGURE 1 In te r l aye r r e s i s t i v i t y under magnetic f i e l d perpendicular to the layer° The experimental data are f i t t e d wi th C=2300, ~b(O)=4,0 A, ~ac(O)=20 ~. The broken l ine is for an estimated normal r e s i s t i v i t y °

0921-4534191/$03.50 © 1991 - Elsevier Science Publishers B.V. All figh~s rese~wed.

2660 H. lto et at / Superconducting transition in r-(BEDT-TTF)zCu[N(CN)z]Br under magnetic field

0

-2

-3

4-}

c~ -4 bo (d

-56

. " 0 . 3 7 T " ~ / • O.18T j O,05T

t • • •

i I i | i I i

8 I0 12 14

Temperature (Z)

FIGURE 2 dc magnetization under magnetid f i e ld perpendicular to the layer. ~he experimental data are f i t t e d with Cb(0)=6.2 A, ~ac(O)=21 A.

employed 600±150 mJ/K-mol estimated from the AC

value for K-(BEDT-TTF)2Cu(NCS)2 .3,4

F i r s t , we take up magnetization data, since

they can be f i t t ed well, as shown in Fig. 2,

with eq. (2.6) of ref . 2. Then, together with

the value of Tc=I0.9±0.3 K, the in ter layer ~(0)

and in t ra layer one are derived as Cb(0):6±2

and Cac(0)=24±5 ~, respect ively. These are

rather close to the values, ~b(O)=4 ~ and

~ac(0)=37 ~, by Kwok et a l . 5 Based on these

values, which is shorter than the in ter layer

distance, they argued the dimensional cross-over

near t rans i t i on region. We note that the theory

adopted here is based on the Lawrence-Doniach

model for inherently two-dimensional super-

conductor, and hence the apparent cross-over

phenomenon is taken into account automatically.

For the res is t ive t rans i t i on , the to ta l

, e s , ~ , v , ~ y is w, , ~ n as

P = [I/Dn + o/C] - I ,

where Pn is the normal r e s i s t i v i t y and o is the

calculated f luctuat ion conduct iv i ty , given by

eq. (3.16) of ref. I . When we t r y to f i t with

the C(O) close to the magnetically determined

value, a parameter C with order of 103 is

required, although a proper value seems to be I0

or less. As for the reason we point out

indefini teness due to size and shape of used

as-grown samples. Further we have to admit

unadequacy of Pn deduced by smooth extrapolat ion

from the temperature region above T c, since the

nature of the normal conduction may not be so

simple. On the other hand, to f i t with smaller

values of C in the order of I0, shorter ~b(O)

I~2 ~ is deduced.

We found that the f i t t i n g for the current

configurat ion normal to the magnetic f ie ld was

poorer than that for paral le l case. As a cause

of the not ic iab le discrepancy, we remined of the

contr ibut ion from the vortex motion to the

magneto-conductivity: for th is sa l t f lux melting

has been claimed. 6 The deta i ls Of the f i t t i n g

of the res i s t i ve t rans i t ion data w i l l be

presented elsewhere.

We thank Dr. R. Ikeda for handing refs. 1 and

2 pr io r to publ icat ion and useful discussion.

This work was par t ly supported by the

Grant- in-Aid for Sc ient i f i c Research from

Minist ry of Education, Science and Culture,

Japan.

REFERENCES I . R. Ikeda et a l . , J. Phys. Soc. Jpn. 60 (1991)

1051.

2. R. Ikeda and T. Tsuneto, J. Phys. Soc. Jpn. 60 (1991) 1337.

3. B. Andraka et a l . , Phys. Rev. B40 (1989) 11345.

4. J.E. Graebner et a l . , Physo Rev. B41 (1990) 4808.

5. W.K. Kwok et a i . , Phys. Rev. B42 (i990) 8686.

6o T. Takahashi et al°, Syntho Metals 27 (1988) A319.