~ ) Solid State Communications, Vol. 85, No. 12, 1005-1009, 1993. PP. Printed in Great Britain.

0038-1098/93 $6.00+.00 Pergamon Press Ltd

ON SUPERCONDUCTIVITY OF THE ORGANIC CONDUCTOR

a-(BEDT-TTF)~ KHg(SCN)4

Hiroshi Ito, Hiroshi Kaneko and Takehiko Ishiguro Department of Physics, Kyoto University, Kyoto 606-01, Japan

Hidehiko Ishimoto and Kimitoshi Kono Institute for Solid State Physics, University of Tokyo, Tokyo 106, Japan

Sachio Horiuchi, Tokutaro Komatsu and Gunzi Saito Department of Chemistry, Kyoto University, Kyoto 606-01, Japan

(Received 7 January 1993 by H. Kamimura)

The resistivity decrease in a-(BEDT-TTF)~KHg(SCN)4 appearing in the tem- perature region below 300 mK was found to be suppressed by a magnetic fields of less than 0.1 T. We argue that the decrease in resistivity is due to the super- conductivity occurring in a fibril-like structure. The observed behavior is in part due to the inhomogeneous molecular arrangement, but is also related to the subtle interplay between magnetism and superconductivity in this system.

INTRODUCTION

THE QUASI two-dimensional organic conductors c~- (BEDT-TTF)~X (X=KHg(SCN)4, NH4Hg(SCN)4, RbHg(SCN)4, TIHg(SCN)4, hereafter each salt is indi- cated by the name of the anion) have a layered struc- ture in which conducting sheets consisting of zigzag aligned BEDT-TTF (bis(ethylenedithio)tetrathia- fulvalene) are sandwiched between triple sheets of thick anion layers [1 ,,, 3]. The coupling between the con- ducting layers is very weak and the anisotropy of the conductivity reaches 103 in the case of the X=KHg- (SCN)4 salt. The low temperature electronic and mag- netic phases of this material have attracted much at- tention recently in relation to the electronic states [4]. Among the salts, only the NH4Hg(SCN)4 salt exhibited superconductivity [2, 5] although they are isomorphous with respect to the crystal structure. As possible cause of this difference the effect of magnetic order has been argued, together with the difference in the density of states at the Fermi surface [2]. For the KHg(SCN)4 salt, the resistivity shows metallic behavior down to 0.5 K, with a step-like anomaly near 8 K [6]. Coin- cident with this anomaly, it has been found that the magnetic susceptibility decreases, reminiscent of the emergence of antiferromagnetic ordering [7]. Similar temperature dependences of the resistivity have been found for RbHg(SCN)4 and TIHg(SCN)4 salts [8, 9]. It is tempting to interpret these results in terms of sup- pression of the superconductivity by magnetic order. Meanwhile we reported a decrease of the resistivity in the KHg(SCN)4 salt below 200 mK for all crystalline directions [10]. Due to the gradual resistivity decrease and the remarkable residual resistance, superconduc-

1005

tivity was ruled out as an origin, but it was not conclu- sive. In this paper, we present the effect of a magnetic field on the observed decrease and argue the presence of superconductivity in KHg(SCN)4 salt.

EXPERIMENTAL

Single crystals of t~-(BEDT-TTF)2KHg(SCN)4 were grown by the electrochemical method. Four gold pads were evaporated onto the sample for use as elec- trodes on a line parallel to the diagonal of the diamond- shaped crystal for the in-plane resistivity measure- ments. X ray diffraction study showed that this direc- tion was parallel to the crystallographic a axis. Gold wires were attached onto the pads using gold paste. The contact resistance was less than 20 ohms. For in- terlayer resistivity measurements along the b axis, two gold wires were directly attached to each side of the crystal using gold paste. In this case the resistance of the sample itself was high enough in comparison with the contact resistance. We used standard ac-current lock-in techniques at a frequency of 13 Hz for the mea- surement. We checked that the measured resistivity was frequency-independent in the range of 10 to 10 a Hz. To eliminate any effects of self-heating in the sample, the measuring current was kept as low as possible in the low temperature measurements. The typical measuring ac-current was 1/zA (corresponding to 1 x 10 -3 Acm -2) for the intralayer case and 10 nA (4 x 10 -6 Acm -2) for the interlayer case and no evidence of the self-heating was observed within the present current level. To ob- tain reliable resistance values in the low-current-level measurements, we deduced the sample resistance from the slope of the voltage-current relation with several

1006

101 ........ • ....... . ........ i ....... ~ ......... " 103

100 / / 102

J~ g , , ~ 1 0 - ' Ob . . . . . . J / 10, ~_.~

" " " " ° ° ' • ~ 1 .1~

10 -2 . . . . . ' P a 1 1 0 °

] 11q-3 ....... ~ ........ ' ........ I . . . . . . . . e . . . . . . . . I , , , t 1N_ 1 ""0.001 0.01 0.1 1 10 100 ""

Temperature (K)

Fig. 1 Temperature dependence of the resistivities along the a axis (pa) and the b axis (Pb).

current levels in some eases. To see the influence of the magnetic field on the resistivity, measurements in high fields were carried out by varying the temperature un- der a fixed field in order to avoid eddy current heating in the samples. A magnetic field of up to 6 T was ap- plied to the sample to measure the magnetoresistance parallel and perpendicular to the conducting plane. We cooled the samples to 1 mK applying the combination of a dilution refrigerator and a nuclear adiabatic de- magnetization stage. The samples were immersed in liquid 3He to ensure good thermal contact.

RESULTS

The temperature dependences of the resistivity for the KHg(SCN)4 salt along the a and the b axes, Pa and Pb, respectively, from room temperature to 1 mK at zero magnetic field are shown in Fig. 1. In both cases the 8 K-anomaly was observed. The resis- tivity became almost temperature-independent below 3 K, until it showed a decrease below 300 mK. The resistivities in a magnetic field of up to 6 T parallel to the conducting plane along the a and b directions are shown in Fig. 2(a) and Fig. 2(b), respectively, in the temperature region from 10 mK to 1 K. The resistivity decreases below 200 mK were almost sup- pressed at 0.044 T in Pb and at 0.14 T in pa. Above 0.4 T, the resistivity increased with increasing magnetic field: the temperature dependence of the resistivity was monotonic and increased weakly with decreasing temperature. This behavior is understood as a smooth extension of the magnetoresistance effect observed be- low 8 K [6]. In the case of the magnetic field normal to the conducting plane, the resistivity decrease exhibited a similar recovery in relatively low magnetic fields as shown in Fig. 3. The field dependence of the resistiv- ity below 0.01 T is also shown in Fig. 4. In this case

SUPERCONDUCTIVITY OF THE ORGANIC CONDUCTOR Vol. 85, No. 12

(a) 0.05

0.04 u

"~" 0.03

.~ 0.02

0.01

%

2 g ~ ~ 8 * * 8 g ** A A Jt A A A A A A A

v V v • v • • • • • •

+ 4 + + ÷ + + .¢- •

• • 0 T t , I o l , ,

• 0.044T 0.14-I"

" 0.46T " 1T

3T ,:' 6T

. . . . . . . . t . . . . . . .

100 1000

Temperature (mK)

(b) 20

°1(

o

• ," 10 ;>

09

. . . . . . . . i . . . . . . . .

¢ O * * * * O ¢ , v 6 T ¢ _

3T D [ ] 0 o o 0 0 0 0

0.46T 1T

v v v v v v T v

. . . . " ~ "<0.14T"~''- 0T

0.044T

. . . . . . . . t . . . . . . .

100 1000

Temperature (mK)

Fig. 2 Temperature dependence of the resistivity below 1 K under magnetic field in the in-plane direction for Pa at typical current level of 1 #A (shown in (~)) and p~ (b) at ~ 10 nA.

the resistivity decrease started near 300 mK and the decrement reached 80 % at 25 inK. The extent of the decrease depended on the current level as indicated in Fig. 5.

DISCUSSION

The quick recovery of the resistivity decrease below 200 mK (Fig. 2) or 300 mK (Pig. 3) under relatively low magnetic field strongly suggests that the decrease is due to superconductivity, in spite of the broadness of the transition and the remarkable residual resistance even below 20 inK in any direction. At the same time the resistivity decrease along the a axis depended on the current level. These facts indicate that the super- conductivity in this salt appears not in bulk but in a fibril-like configuration. The broad transitions against

Vol. 85, No. 12

0.15

O.lO

0.05

SUPERCONDUCTIVITY OF TIIE ORGANIC CONDUCTOR

. . . . . . . . , . . . . . . . . 0.04 . . . . . . . . ,

1007

' i . . . .

_ . 6.25T

. 4.48T

3.08T

2.0T

1.0ST 0.5T - , ~ - - - . . . . . . . . . . . 0 . 2 T ~ "."

0.054T

1 1 ' , . . . . . . f . . . . . . . .

0 100 1000

Temperature (inK)

0.03

-~ 0.02 . , . . ~

0.01

/ ~ 0 . 3 1 4 / z A

i i t I i t t t [ i , i . , ,

10 100 1000

Temperature (mK)

Fig. 3 Temperature dependence of the resistivity along the a axis under magnetic field perpendic- ular to the conducting plane at current level of 0.314 pA.

temperature and magnetic field are reminiscent of quasi one-dimensional superconductors. We recall the case of (TMTSF)2FSOa with very broad transition [11]. Due to the broadness, it is difficult to determine the tran- sition temperature, but we may say that the onset lo- cates near 300 mK provided that the effect of fluctua- tion is neglected.

We have to comment on recent reports where the resistance decrease was not detected [12, 13] even below 200 mK. A possible reason for this discrepancy may be that we used very low current levels in our work. As shown in Fig. 5, the decrease in the resistivity de- pends on the current. This might be related with a

0.03

0.02-

> •

.~ 0.01 - .

G 0

I I I i r

T=I6 inK

I I I I I

2 4 6 8 10 12

Magnetic field (mT)

Fig. 4 Magnetic field dependence of the resistance at 16 InK for pa in low field region with measuring cur- rent of 0.314 pA. The field was applied perpen- dicular to the conducting plane.

Fig. 5 Temperature dependence of the resistivity at zero magnetic field along the a axis measured with currents of 0.314 #A and 6.28 pA.

low critical current density in a fibril-like structure of the superconducting portion. We notice also that the decrease was found irrespective to the difference in the temperature dependence above 100 K, which is thought to be caused in part by structural difference relevant to disorder [1] as well as by the anisotropy [9].

For the KHg(SCN)4 salt, the presence of antiferro- magnetic order below 8 K was proposed from the mag- netization measurements. The antiferromagnetic order may result from the nesting of the plane-like Fermi sur- face [14]. Incidentally the resistivity shows a step-like anomaly at this temperature [6]. The anomaly at 8 K may be interpreted as the resistance increase caused by the decrease in the effective number of the carriers due to an antiferromagnetic transition of the itiner- ant spins, as in the case of chromium [15]. However, searches for indications of the magnetic ordering by means of NMR and pSR were not successful [16, 17]: according to the NMR result, the amplitude of the an- tiferromagnetic order is very small, less than 0.01 PB

per molecule. In addition the in-plane magnetization measurement to find the easy axis of the antiferromag- netic ordering did not find any appreciable anisotropy [18]. These facts indicate that the magnetic phase can- not be described in terms of ordinary antiferromagnetic ordering. As a cause of the anomalous magnetic order- ing in (BEDT-TTF)2KHg(SCN)4, the smallness of the nesting wavevector (with 1/5 to 1/4 of the size of the Brillouin zone) and thereby the possible contribution of the internal degrees of freedom in the nesting con- figuration was pointed out by the present authors [10].

The interplay of superconductivity and magnetism is currently very intriguing subjects in the study of or- garlic superconductors. The relevance of the spin den-

1008 SUPERCONDUCTIVITY OF THE

sity wave state in (TMTSF)2X is one case. We would like to point out more recent results on K-(BEDT- TTF)2Cu[N(CN)2]C1, where weak ferromagnetism was reported as a counter part to the superconductivity [19]. More recently, a dramatic interplay with reentrant superconducting behavior has been discovered [20]. For the present salt it is possible that superconductivity is interplaying with magnetism whose microscopic feature is open for future study.

So far we have deferred the discussion of magne- toresistance in the higher field region (higher than O.1 T). The magnetoresistance in the KHg(SCN)4 salt has attracted attention not only due to the Shubnikov-de Haas oscillation [12, 13], but also due to an anoma- lously large magnetoresistance below 8 K [4, 21, 22]. The magnetoresistance for the field perpendicular to the conducting planes shown in Fig. 3 is positive up to 6 T and smoothly connected to the reported results. The magnetoresistance for the field parallel to the con- ducting plane is affected strongly by the angle between the plane and the field direction. The difference be-

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ORGANIC CONDUCTOR Vol. 85, No. 12

tween the field dependences shown in Figs. 2(a) and 2(b), where the magnetoresistance in the field above 0.46 T is small for Pa while it is pronounced for Pb, is ascribed to imperfect alignment of the field against the conducting plane.

In conclusion, in view of the recovery under rel- atively low magnetic fields, the resistivity decreases found in the organic conductor a-(BEDT-TTF)2KHg- (SCN)4 below 300 inK, in directions both parallel and perpendicular to the conducting planes, are attribut- able to a superconducting state with a critical field of less than 0.1 T. The configuration of the superconduct- ing portion is thought to possess a fibril-like structure. For this case, the subtle competition between the mag- netic order and the superconductivity may be involved with structural disorder in microscopic molecular ar- rangements.

This research was supported in part by Grant-in- Aid for Scientific Research (~ 3302017) from the Min- istry of Education, Science and Culture, Japan.

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Vol. 85, No. 12 SUPERCONDUCTIVITY OF THE ORGANIC CONDUCTOR

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