On the low-temperature states of highly correlated BETS conductors

ELSEVIER

Synthetic Metals 102 (1999) 1654-1657

On the low-temperature H. Kobayashi

a, H. Akutsu a, H. Tanaka b, A. Kobayashi alnstitute

b Department dLPMC,

states of highly

for

Molecular

of Chemistry,

School ’ Electrotechinical

SNCMP, eLCC,

INSA, CNRS,

route

Science,

Scientifique

BETS conductors

b, M. Tokumoto Okazaki

31077

Japan

of Tokyo, 305-0045,

de Rangueil,

de Narbonne,

c, L. Brossard d, P. Cassoux e

444-8585,

of Science, the University Laboratory, Tsukuba

Complexe 205

correlated

31077

Toulouse

Tokyo Japan

113-0033

Toulouse

Cedex,

Cedex,

Japan France

France

Abstract h-BETS2MX4 (M=Ga. Fe; X=CI, Br) afford an The x--type BETS conductors (BETS=bis(ethylenedithio)tetrathiafulvalene), extremely large variety of low-temperature properties by substitution of metal (M) and halogen (X) atoms. In this paper the and the coupling (or decoupling) of the antiferromagnetic and metal-insulator superconducting properties of h-BETS2GaBrxC14-, While h?\-BETS2GaBr.$1,, is an organic superconductor at x10.8. transitions of h-BE?lYS,FeBr,CI,, are reported. The maximum T, WIS BETS2GaBrl,5C12,5 is a semiconductor at ambient pressure and becomes a superconductor at high pressure. The anisotropic decrease of magnetic almost the same as that of so-called “10 K-class organic superconductor” K-ET&u(NCS)2. insulating state is Iocatednear superconducting susceptibility of h-BETS 2GaBr,CIhx ( x>l.O) suggested that non-antiferromagnetic At ~~0.2, the phase. The x-dependencies of resistivity and magnetic susceptibility of h-BETS2FeBrxClq_, were also examined. Around x=0.3, these two transitions tend to be metal-insulator and antiferromagnetic transitions take place cooperatively. separated. At x>O.6, the coupling between n and d electron systems becomes very weak and the Fe3’ spins system undergoes an When the rc-d coupling is strong (xO.6). of the antiferromagnetic structure is parallel to c. resemble to each other. The superconducting phase of hThe T-x phase diagrams of h-BETS,GaBr,CI,, and h-BIZ$,FeBr,Cl,+, BETS2GaBrxC1+, appears at the x-region where the rc-d coupling can be observed in h-BETS 2FeBrxClq,. Keywords:

Organic

superconductors,

Superconducting

phase transition,

Metal-insulator

phase transition,

Magnetic

phase transition

1. Introduction

2.Experimental

We have recently synthesized a series of organic conductors based on BETS molecules (BETS= bis(ethylenedithio)tetraselenafulvalene) h-BETS2MX4 (M=Ga, Fe; X=Cl, Br), whose physical properties can be controlled continuously by exchanging metal (M) and halogen (X) atoms. Besides the superconductivity of h-BEI’S2GaBrxC14-x [l], many novel phenomena have been found at low-temperature, that is, (1) colossal magnetoresistance of h-BEZIS2FeC14 [2, 31 (2) ferromagnetic organic metal phase of h-BETS2FeC14 above 10 T [2, 41 (3) antiferromagnetic metal phase of h-BET$FeCl, at high pressure [4] and (4) superconductor-to-insulator transition and superconductor-to-metal transition in h-BEIJ,Fe,Ga,,Br,Cl,, [4, 51. In this paper, we will report briefly the “chemical pressure effect” on the superconducting properties of h-BETS2GaBrxClq, and coupled decoupled) (or antiferromagnetic and metal-insulator transitions of hBETSZFeBrxClq,,

The crystals of BETS conductors were prepared electrochemically. For example, in the case of hBETS2FexGal-,Cl+ the crystals were grown from ethanol (lo%)-chlorobenzene solution containing BETS, The x-value was [(C+-&$lFeC~4 and [(C2H=J4NJGaC14 determined by EPMA (electron probe microanalyses). The xvalue was also estimated from the linear relation between x and V (unit cell volume). X-Ray experiments were made on Rigaku four circle diffractometer (RIGAKU AFC-5R and AFC-7R). Resistivities were measured by conventional four probe method. The high-pressure experiments were made by using clamp-type cell. Magnetic susceptibilities were measured by SQUID magnetometer (QUANTUM DESIGN MPMS7, MPMS-SS and MPMS2)

3. Results

0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00245-S

and discussion

H. Kobayashi

3.1. Superconductivity

of h-BETS,GaBrxC14~,

et al. I Synthetic

Metals

102 (1999)

[ 1. 71

The crystals of h-BETS2GaBrxC14, have not magnetic ions. Nevertheless, owing to the highly correlated state of the JC conduction electrons, there is a large possibility of the development of antiferromagnetic state at low temperatures. Figure 1 shows the resistivity of h-BETS,GaBr,Cl,,. hBEZS2Ga.C14 showed a broad resistivity maximum around 90 K which is asign of the strong correlation of JCelectron system. Below 90 K the resistivity decreases fairly rapidly with lowering temperature and exhibited a superconducting In the resistivity measurements of htransition at 6 K we have frequently encountered the crystals BETS,GaCl,, showing theonset of resistivity drop at fairly high temperature (=9 K). But we have recently noticed that for the systematic comparison of T, of a series of h-BETS,Fe,Ga,-,Br,Cl,, conductors, Te must be taken as the temperature where the The magnetization resistivity decreases sharply to zero. measurements of h-BETSZGaC14 indicated the superconducting state to be in almost perfect Meissner state with Tel of 12 Oe (H//c) and 8 Ce (Hlc). The high-temperature non-metallic state is stabilized tith increasing x and the temperature corresponding to the broad resistivity maximum is lowered at higherx. At the same time, T, is enhanced up to 7.5 K At x>O.8, the system takes non-metallic state down to low temperature. These resistivity behavior suggests the enhancement of the electron correlation in higher-x system. When x (Br-content) is increased, the unit cell is expanded (“negative chemical pressure”) to result in the narrowing of the bandwidth. Similar resistivity behavior can be reproduced by applying pressure to the semiconducting crystals of hBETS,GaBrt,,Cl,, (Fig. 2) which clearly shows the validity of the idea of the chemical pressure. T, was about 9.7 K (mid point) at 3 kbar, which is almost the same to the recently determinedT, value of the “10 K-class ET superconductor”, K-

I...~~....~.~~‘~~~..““.~....1 10-3d 50 100 1.50 200 250

5

Resistivities of h-BEIS2GaBrXC14-, and h-BETS2GaC13F.

10

20

50

100 200

TIK Fig. 2.

Resistivities

of hBEXSZGaBr,,SC12,5

at high pressure.

The magnetic susceptibility (x) of h-BETS,GaRr,Cl,, increases gradually with decreasing temperature down to 60 K Below it, x becomes x-dependent. Unexpectedly, x of h BETS2GaBrxC14-, ( x>l.O) decreases isotropically below 20-30 K indicating the insulating state to be not antiferromagnetic but rather non-magmetic. Considering the fourfold stacking structure of BETS moecules along the a axis, the non-magnetic properties seems to be natural. Owing to the strong correlation, the x electrons till tend to localize on BETS dimers in the higher-x system. Since the antiferromagnetic interaction can be expected between neighboring dimers, a spin-Peierls like antiferromagnetic state till be realized if the magnitudes of two independent interdimer interactions (B, C) along the a axis are sufficiently different to each other.

300

0

T/K Fig. 1.

1655

X6.54-1657

Fig. 3.

T-V(x)

0.5

1.0

phase diagram of

1.5

--

h-BETS2GaBrxC14,

H. Kobayashi

1656

et al. I Synthetic

Metals

102 (1999)

Recent systematic X-ray structure analyses and the calculation of the intermolecular transfer integrals indicates that the difference between B and C becomes small and the transverse intermolecular interaction between BETS stacks is enhanced with decreasing x. Therefore the system will approach to the The strong antiferromagnetic state when decreasing x. suppression of x observed in h-BETS2GaBr0~7C13,3 below 60 K role of the antiferromagnetic suggests the important fluctuation around x=0.7, where the highest T, was obtained (Fig. 3). 3.2. Coupled antiferromagnetic of h-BETS2FeBrxC1,, [3]

and metal-insulator

1654-1657

a 1.3kbar b 1Skbar c 2.Okbar 10’

.. . :. \

Qt; 2%

transition r

10C

We have examined the electrical and magnetic properties of a series of highly correlated organic JC conductors ions (Fe3+ with S=512), hmagnetic incorporating Except the low-temperature BElS2FeBr,Clq-x (~~0.8). region, general resembles that

feature of the resistivity behavuior closely A broad of h-BETS2GaBrxClq, (Fig. 4).

resistivity maximum was observed at 90 K (X m 0) - 50 K (x * 0.7), indicating the enhancement of the correlation of ZThe metal-insulator conduction electrons at higher-x region. transition temperature (TMI) was enhanced from 8.5 K (x=0) to Thecrystal with x=0.8 shows 18 K(x~0.7) with increasingx. a semiconductor-insulator transition around 20 K Similar to the case of h-BETSZGaBrxClq_,, resistivity

behaviors

of

reproduced by applying BETS2FeBr0,$J3,2 (Fig. that

MI

and

the systematic

change of the

h-BETS2FeBrxCl~x

could

be

pressure to the semiconducting ?Lshow 5). Magnetic susceptibilities

antiferromagnetic

transitions

took

place

Fig. 5. Resistivities cooperatively 6a.). Alarge

of hBETSZFeBro

8C13.2 at high pressure.

around 8.5 K (TN*&) at 0~~~0.2 (Figs. 4 and magnetization drop was observed at TMf for the

magnetic field parallel to the c axis (H//c), which indicates the appearance of localized n spins (S=1/2) and the x-d coupled antiferromagnetic spin systems (Fig. 6a). At 0.3a~
to AF ordering small magnetization

of the Fe3+ spins drop observedat

(TN).

The

TM1 suggests a

small coupling of x and d spin systems. It should be noted that contrary to the system with ~~0.35, the system with ~0.35 showed the characteristic susceptibility drop at TM1 when H

- ‘a

was perpendicular to c (Fig. 6b). magnetization drop at rMr disappeared

For ~0.6, which shows

the the

decoupling of JC and d electron systems. Then R electron system undergoes MI transition independently of the d spin systems. The disappearance of the suscepti bility anomaly at TM~ indicates that the x electron system transforms to nonmagnetic

insulating

state

below

TM1 (Fig.

6~).

The anisotropy of M obserced at low temperature shows the development of antiferromagnetic structure of Fe spins for every x. The T-x phase diagram is shown in Fig. 7, which resembles that of h-BETSZGaBrxClg, (Fig. 3). It should be noted that the superconducting realizedat thex-region in h-BETSzFeBr,Clq_~.

Fig. 4.

Resistivities

of h-B’El’S2FeBrxCl~,.

phase of h-BETSZGaBrxCf~x

where therz-d coupling

is

can be observed

This shows the important role of the spin fluctuation in the superconducting transition of L-type BETS superconductors. The systematic x-dependence of the

H. Kobayashi

0

5

10

15

0

5

et

10

al. I Synthetic

15

Metals

102 (1999)

20

1

T/K

T/K

(a) x =O.O Fig. 7.

2.OT LOT 0.6-I. 0.3T

‘1

10

20

0

10

20

30

1657

1654-1657

0

0.5 x (Br contents)

1

T-x phase diagram of h-BETSZFeBrXClq-,

susceptibility behavior indicates that thedirection of easy axis of the AF spin structure changes gradually from parallel to the c axis (~~0.2) to perpendicular to it (x>O.6) (see Fig. 6). In other words, the direction of easy axis is varied according to Magnetoresistance the magnitude of z-d coupling. measurements showed that TM~ of h-BEIS2FeBr0,7C13.3 was almost independent of H below 1 kbar. The field-restored metallic state similar to that of h-BETS2FeC14 was observed at high pressure. The Weiss temperature (8) estimated from the M-T curve at TMI
T/K

T/K

(b) x =0.4 References [l] H. Kobayashi, H. Akutsu, E. Arai, H. Tanaka, and A. Kobayashi. Phys. Rev. B56, R8526 (1997). [2] L. Brossard. R. Clerac, C. Coulon, D. K. Petrov, V. N. Laukhin,

1ST I nnc

F. Gaze, A. Kobayashi,

d l.OT

t

l3u.Phys.J. b 1OOG a SOG

Kobayashi, 10

20

0

10

20

30

T/K

T/K

(c) x =0.8 Fig 6.

Magnetic

susceptibilities

439

M. J. Naughton,

H. Kobayashi,

of h-BETS,FeBr,Cl,,.

T. Ziman,

A. Audouard.

and P. Cassoux,

(1998).

[ 31 H. Akutsu, K. Kato, E. Arai. H. Kobayashi, il

0

BI,

M. Tokumoto,

and P. Cassoux, Phys. Rev.

H. Tanaka, A. B.,

in press,

[4] H. Kobayashi, er al., to be published. [5] H. Kobayashi, A. Sato, E. Anti, H. Akutsu, A. Kobayashi, andP. Cassoux, J. Am. Chem. Sot. 119. 12392 (1997). [6] H. Akutsu. E. Arai, H. Kobayashi, H. Tanaka, A. Kobayashi, andP. Cassoux, J. Am. Chem. Sot. 119. 12681 (1997). [7] H. Tanaka et al., to be published.