Localization and superconductivity in (BEDT-TIF)2Cu[N(CN)2]Cl: Pressure effect

rhysica C 185-189 (1991) 2683-2684 North-Holland

LOC~_IZATION AND SUPERCON[XJCTIVITY IN (SEDT-TTF).-~-u[N(CN)=]CI : PRESSL~E EFFECT.

Yu. V. SUSHKO, V. A. ~

0

,

R. A. PETROSOV

I n s t i t u t e of Semiconductors, Ukrainian Academy of Sciences., Kiev, 252650, Ukraine, USSR and E. B. YAGUBSKII

N. D. ~

I n s t i t u t e of Chemical Physics, Academy of Sciences of the USSR, Chernogolovka, 142432,

At ambient pressure titled salt exhibits the insulating ground state. Superconductivity with T= near 12 K appears at the pressure of 95 bar and under the moderate pressures coexists with the high-temperature semiconducting regime. At 340 bar the metallic behavior of resistance is stabilized in whole temperature range and T= reaches the m a ~ i ~ n value (midpoint 12.8 K, onset 13.8 K). Further pressure increasing causes rapid T= decreasing.

l.

INTROOt~TION The

100

problem of

the

competition

between

superconducting and insulating ordering has a special

importance

superconductors. quite well compounds

in

S~Ich a

established of

field

the

of

competition,

(TMTSF)=×

(and P > 6 khan)

has

family

Ell.

-

O

firstly

85

75

95 (bar)

PRESSURE

The number of quasi

two-dimensional

superconducting

8EDT-TTF

rule,

state

a

n-

w~th T= near

exhibit,

as

V

been

in quasi-one-dimensional

discovered organic superconductors i K

o5 0 ~

organic

metallic

salts

in

wide

Pressure d e c a d e n c e

F IGb~E 1 rJf resistance at T=5 Ko

temperature range and their T= reaches I0.5-II~5 K at ambient pressure [2,3]. In 1990 the maximum

measurements

r= in organics was raised to 12.5 K by k-(SEDT-

illustrated

TTF)_-Cu[N(CN)=]CI

Figure

[4].

Interestingly,

ambient pressure its resistivity decreasing the

temperature,

localization

intrinsic state,

of

increases with

indicating, charge

under

therefore,

carriers

as

an

feature of this salt. Superconducting

according

to

[4],

appears

due

to

1

transition

temperature semiconducting k-(SEDT-TTF)=~u[N(CN)=]CI

near 12 K and

high-

regime both exist

in

at moderate pressures.

from

[he

results

of

electrical

resistance

0921-4534/9U$03.50 © 1991 - Elsevier Science Publishers B.V.

modest

the

transition starts

semicc~ducting

regime,

resistance

transition

near

#~

K~

instead only

a

of weak

near 32 K~ ~hich followed by r e ~ r k a b l e

Orop at 25 K. On further cooling the resistance sharply

increases,

transition,

from

RESKETSANODISCUSSION

at

is equal to 5 kQ~mo

bar

however

superconducting

2.

appears

fr~

sho~s,

P=-150

maxi~

superconducting

As is seen

At

work

shci~s that

are

where the s~mp!e resistance

pressure

Our

pressure

i, 2.

superconductivity

immediately

insulating

bar.

in Figures

gas

a pressure the superconducting

transition by the

300

he iium

pressure of 95 bar. Figure 2 shows that at such

suppressing of the insulating of

under

indicating at

12.8

transition

the semiconducting

the

starts

regime~

case of P=95 baro In principie~

All fights reserved.

K,

insulating complete

i~Tediately

similar in

to t ~

such a

case

Y~tK Sushko et al. / Localization and superconducti~ in (BEDT-TTF)2Cu[N(CN)2]Ch pressure effect

2684

15 ¸ 1 0 ~,.

I

2- 95

bor 3 - 1 5 0 bar

1\

10 " I ,-.

k,.

il

"10' o.%

1 bor

I-

j'~

4-250

\

bar

5-340 b=

oe

2, o

0 0

~I0

w10 = z 10 w

,

1

u~lO -' 10 -~ I 0 -'

FIGU~ 2 Temperature dependence of different pressures

resistance

should expect

Under

formstlcm

in

the preceding

order

to

met~llic state

provide

observed

direct

However we

ccmK~etitlon

localization and supezxx~ductivity,

of

the

concluding,

the

understood.

of

this

Similar

(TMTSF)mAsFe

ph~nc~ncn

behavior

crystals,

was

unstable

,

i

2O 30

highest

of

pressures

resistance

is

the

metallic

observed,

and

Tc

sharply decreases as it is seen from Figure 3.

6. CDNCV/JSIONS In summary, the helium gas pressAre effect on resistivity studied.

therefore, their puzzling prax~'ity. origin

l

ICD0 (bar) PRESSURE

necessary

reservoir of ccnductinE electrons.

The

l

,

at behavior

h~ve

,

FIGURE 8 Pressure dependence of superconducting transition temperature. Symbols o denote resistivity midpoint values we reported, * represent the dsts of [4].

1 TEMPER IA~ '=RE(K)

I0

we

I

)

of

k-(BEDT-TTF)sCu[N(CN)2]CI

Superocnductivity

started

w~s

inmm~iateiy

e~e

not

from the regime of charge localization has been

found

for

found at modest pressures

against

Spln

value

reaches

the ~mximum

P

>95 bar. (midpoint

The T= 12.8 K,

Density Waves (SDW) fozmKticm [i]. Assuming that

onset 13.8 K) at P=- 840 bar, where the metallic

SDW or CDW (Ch~ge Density Wave ) o~erlng occur

state is stabilized, and strongly decreases at

in k-(w~I]T-TTF)2Cu[N(C~)2]CI,

further pressure incre~slng.

attribute

the ancm~l~/s

it is tempting to

resistance

decreasing

This work ~ s

supported by the Scientific

near 25 K, observed ~t 150 bar Just before the

Council on the U.S.S.R. State Programme "High-To

insulating

Superconductivity".

phase

transition,

to

sliding

inc~m~nsurate SDW or CDW contribution into the conductivity.

Then

in ~a !~_t!rig

t_r~n__sitioD_ _

following this resistance drop may be attributed to some possible pinning process. As is seen from Figure 2 ,the semiconductorlike behavior is rapidl~ ~uppressed with further

REFERENCES 1. D. Jerome and (1982) 299.

H.J. Schultz,

2. H. Urayama

al.,

et

Chem.

3. A.M.Kini et ~i., iuorg. C ~ l .

Adv.Phys.

31

Lett. (1988) 55 29 (1990) 2555

pressure increasing, and at P=340 bar Tc reaches the maximum value of 12.8 K (onset at 13.8 K).

4. J.M. Williams et al., Inorg 3272.

Cham. 29 (1990)