Anisotropic resistivity and thermopower of the organic superconductor (ET)2Cu[ N(CN)2] Br

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Anisotropic resistivity and thermopower of the organic superconductor (ET)2Cu[ N(CN)2] Br

L. Buravov, N. Kushch, V. Merzhanov, M. Osherov, A. Khomenko, E.

Yagubskii

To cite this version:

L. Buravov, N. Kushch, V. Merzhanov, M. Osherov, A. Khomenko, et al.. Anisotropic resistivity

and thermopower of the organic superconductor (ET)2Cu[ N(CN)2] Br. Journal de Physique I, EDP

Sciences, 1992, 2 (7), pp.1257-1262. �10.1051/jp1:1992207�. �jpa-00246618�

Classification Physics Abstracts

74.70K 71.30

Short Communication

Anisotropic resist1vlty and thermopower of the organic superconductor (E~j)2Cu [N(CN)2] Br

L-I-

Buravov,

N-D-

Kushch,

VA.

Merzhanov,

M-V-

Osherov,

A-G- Khomenko and E-B-

Yagubskii

Institute of Chemical Physics in Chemogolovka, Russian Academy of Sciences, Chernogolovka MD, 142432 Russia

(Received

23 April 1992, accepted 12 May

1992)

Abstract. We have measured the temperature dependence of the resistivity of

(ET)2Cu [N(CN)2]

Br in three crystallographic directions and the thermopower in the _a and _c directions.

The resistivity has two maxima at 75 and 95 K. The value of the resistivity anisotropy is rather

large:

pb/Pa

is about 10~ _{at the} ambient temperature and increases with lowering temperature below so K- The temperature ^derivative of resistivity exhibits _a strong maximum at 50 K-

Thermopower is positive for the _a direction, while it is negative for the _c direction. We have concluded, that

(ET)2Cu [N(CN)2]

Br undergoes insulator-metal transition

near 50 K similarly

to

(ET)2Cu(NCS)2.

1. Introduction.

At present, the

organic compound (ET)2Cu [N(CN)2]

Br

ii]

has the

highest Tc(rs

12

K)

at ambi- ent pressure among ET-based

organic superconductors.

Besides its

prominent superconducting

transition temperature,

(ET)2Cu _[N(CN)2]

Br retains from its

predece8sor, (ET)2Cu(NCS)2,

another distinct feature the anomalous

resistivity

behaviour [2].

(ET)2Cu[N(CN)2]Br,

_as well _as its relative

compound (ET)2Cu[N(CN)2]

Cl [3],

belongs

to the

family

of ET salts with _a so-called ~-type

crystal

structure [4]. It is formed

by

two- dimensional

conducting

sheets of ET

molecules,

located in the _ac

plane

and

composed

of ET

dimers, alternating

with

polymer

anion chains

extending along

the _a direction

[I].

In th18 _{paper we} describe the _way of

synthesis

of

high quality single crystals

of

(ET)2Cu [N(CN)2]

Br and the results of temperature

investigations

of

resistivity

and

thermopower

in

difserent

crystallographic

directions.

1258 JOURNAL DE PHYSIQUE I N°7

2.

Experimental.

2, I SYNTHESIS. The

(ET)2Cu _[N(CN)2]

Br

crysta18

_were

prepared by

electro-chemical _ox- idation _on Pt electrodes of ET

(2

x

10~~ M)

^{under the}

galvanostatic regime

at the constant

temperature

(20 °C).

The

electrolyte

used _was the mixture of CuBr

(5

x 10~~

M)

,

NaN(CN)2 (5

x

10~~ M)

^{and 18- crown-6 ether}

(5

x

10~~ M)

or CuBr and

Ph4P [N(CN)2].

^{The solvents}

were

I,1,2-trichloroethane (TCE),

TCE with _an addition of10ili of

ethanol,

and nitrobenzene.

The

magnitude

of the current,

applied

to different

cells,

_was between 0.2 and 0.6

PA.

The

quality

of

single crystals, determining

their

superconducting

transition temperature Tc and transition width

6Tc, depends significantly

on the

synthesis

conditions: the

magnitude

of the current, and the kind of the solvent and

dicyanarnide

salt used. The most

perfect crystals

_were

produced

in the _pure TCE without absolute

ethanol, by using NaN(CN)2,

at the

electrocrystallization

current of 0.5

PA (the

anode current

density

of IA I-S

pA/cm~).

At these conditions the

crystals

_{grew on} the anode in 12-14

day8

and _were

quite

uniform. The

crystals

had the

shape

of

plates

with

typical

dimensions I _x I _x 0.05 mm~.

2.2 itESISTIVITY _AND ANISOTROPY. The temperature

dependence

of the electrical resis-

tivity

_was measured

by

the four

probe

method with both d-c- and _a-c-

techniques.

Contacts to the

sample

_were made

by graphite

paste

using

10 pm diameter

platinum

wires. The values of

pb/pa

and

pb/pc

_were

large enough

to

perform

direct measurements of pb,

assuming

the uni- form distribution of current in the

sample,

when

voltage

is

applied normally

to the

conducting plane.

The

resistivity anisotropy

_was measured also

using

_an

improved Montgomery

method [7].

At the ambient temperature ^{the value of the}

resistivity

in the _ac

plane

varies in the _range 0.02-1 Qcm for difserent batches

depending

on

synthesis

conditions.

Usually crystals

with lower

p(295 K)

had

higher

Tc and lower 6Tc. For the best

crystals, having perfect superconducting transition,

the

typical resistivity

values at the ambient temperature are: pa = 4 _x 10~~

Qcm,

pb " 50

Qcm,

_{pc =} 6 _x

10~~

_{Qcm. A}

typical

temperature

dependence

of the

resistivity

of

(ET)2Cu [N(CN)2]

Br in all directions is shown in the

figure

I. At first it lowers

slightly

with the

diminishing

of the

temperature, having

_a broad minimum of around 250

K,

then grow8 down

approximately

100 K. Then the

resistivity

_passes

through

the two maxima _at 95 K and 75

K,

the former

being higher

than the latter. These two

maxima,

not

equally pronounced

in

difserent

directions,

_are well

reproduced

in difserent

samples.

After

this,

the

resistivity drops

about 50 times until the

beginning

of the

superconducting

transition. The insert of

figure

I shows the

region

of the

superconducting

transition. It _occur8 at 2~ ₌ ii.7 K

(the midpoint

of the

transition)

with _a

width,

determined _as 6Tc ₌ To.9 To

i

being

less than 0A K. The temperature ^{derivative of} _{pa, pb} ^and pc exhibits _a

strong

^maximum at 50 K

(Fig. 2).

The overall behaviour of the

resistivity

in all directions is

qualitatively

similar.

Temperature dependences

of calculated

resistivity anisotropy pb/pa

and

pb/pc

for the _Same

crystal

_are shown in

figure

3. Both values do not

change significantly

in the

region

above 50-60 K. With further

lowering

^the temperature _pb ^decreases

slower,

than other components,

so both

pb/pa

and

pb/pc

increase

noticeably.

Measurements of the

resistivity anisotropy by

the

improved Montgomery

method [7] gave

essentially

the _same temperature

dependence,

but the values of

pb/pa

_* 3000 in the _range 70-295 K _{were more}

typical.

2.3 THERMOPOWER.

Thermopower

_was measured

by

standard for _an

organic crystals

method [8]. ^The temperature

dependence

of

thermopower

measured in directions _a and _{c on} the _same

crystal

is shown in the

figure

4. The

thermopower

has difserent

signs

for difserent

.2

/~b

~

l-O b£

LT~

C~ 0.8 0.04

j~ _p~

~ 0.6

~~~

~ 0.02

CL

~'~ 0.01

0 2 0.00

o-o

Temperature,

Fig. 1. The temperature dependence of the normalized resistivity in the _a, b and _c directions.

Insert shows the superconducting transition.

directions. For the _a direction the

thermopower

is

positive

in the whole temperature range. It has _a value of about + 20

pV/K

_at the ambient temperature, then increases

slightly

with the

lowering

of the temperature and

then,

after _a broad maximum around 130 K, decreases towards

zero almost

linearly. Similarly,

for the _c direction the

thermopower

is

negative

with the value of -18

pV/K

at the ambient temperature and has _an extremum at 120 K.

Remarkably,

it has _a well

pronounced peak

at 70

K,

followed

by

_a minimum _at 50

K,

this feature

being reproduced

on all measured

samples.

Below

approximately

10 K both components ^of

thermopower

_are

very close to _zero.

3. Discussion.

Superconducting transition, occuring

at Tc ₌ ii.?

K,

is in

agreements

^with

previous

results

ii].

Small observed 6Tc reflects

high quality

of _our

crystals.

The value of the

resistivity

at ambient temperature ^{is somewhat}

higher

than it _was

reported

earlier

ii].

It should be also

noted,

that in

spite

of _very

sharp superconducting

transition the residual resistance ratio in

(ET)2Cu [N(CN)2]

Br is

relatively

low

(p(75 K)/p(12 K)

rs

50).

As it _was

mentioned,

the temperature ^derivative of the

resistivity

exhibits _a strong

peak

at 50 K for all directions. As in the _case of

(ET)2Cu(NCS)2

_[6], it _can be _an

argument

^{for the}

existence of _a

phase

transition in

(ET)2Cu [N(CN)2]

Br _at this temperature. ^{This transition} should be of _an anomalous insulator-metal type, since the

low-temperature

state is metallic and

superconducting,

and the

high-temperature

_one is

semiconducting.

The _reasons for such

transition,

_as in the _case of

(ET)2Cu(NCS)2

_{[6], is}

presumably

the

change

of the band structure

1260 JOURNAL DE PHYSIQUE I N°7

0.08

0.06 fi..

~ O '

~ ^. ,

7J $

~

~g ^{# .}

, ..,

~/ _,

' _:., _,

0.00 ^~-~

-0.02

0

20 1

K

« 4000

CL

j

CL

/~bll~a

p~/pc

Temperature,

Fig. 3. The temperature dependence of the resistivity anisotropy.

So

~ ^o o o o

o o

o o o o a o o oo

~~ ^o ° ° ~ ~

~oo ao

b£

~

~ap°

/

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1262 JOURNAL DE PHYSIQUE I N°7

derivative of the

resistivity occuring

at 50

K,

and different

signs

of the

thermopower

^{in the}

a and _c directions. We have

concluded,

that

(ET)2Cu [N(CN)2]

Br

undergoes

_an insulator to metal transition _near 50 K

similarly

to

(ET)2Cu(NCS)2.

The additional

anomaly

observed at

rs 95 K may be concerned with another

phase

transition.

Acknowledgements.

The work _was

supported by

^the Scientific Council _on the U-S-S-R- State

Program "lligh-Tc Superconductivity".

The authors wish to thank I.F.

Shchegolev

and V-N- Laukhin for valuable

discussions,

and A-V-

Zvarykina

for

help

in measurements.

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