Specific heat of the high Tc organic superconductor $\kappa - ({\rm ET})_2{\rm Cu[N(CN)}_2]{\rm Br}$

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Submitted on 1 Jan 1993

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Specific heat of the high Tc organic superconductor κ − (ET)_2Cu[N(CN)_2]Br

V. Kopylov, A. Palnichenko

To cite this version:

V. Kopylov, A. Palnichenko. Specific heat of the high Tc organic superconductor κ − (ET)_2Cu[N(CN)_2]Br. Journal de Physique I, EDP Sciences, 1993, 3 (3), pp.693-695.

�10.1051/jp1:1993156�. �jpa-00246750�

J. Phys. I France 3

(1993)

693-695 _MARCH 1993, _PAGE 693

Classification Physics Abstracts 74.70K 65.40

Short Communication

Specific heat of the high _Tc organic superconductor

~c

(ET)~Cu[N(CN)~]Br

V-N-

Kopylov

and A-V- PaInichenko

Institute of Solid State Physics Chemogolovka, Moscow District, 142432 Russia

(received

16 October 1992, accepted 23 November

1992)

~c-

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

where ET is the cation-radical donor molecule

bis-(ethylenedithio) tetrathiafulvalene,

has the

highest ambient-pressure

transition temperature Tc for _an

organic superconductor

to date. The resistive onset transition temperature is 12.5 K and the inductive onset is 11.6 K

[Ii.

In this work _we present ^the

precise

measurements of the

specific

heat

capacity

of the _com-

pound

at _zero

magnetic

field.

The measurements have been

performed by

means of _a

laboratory-made

relaxation calorime-

ter.

Specific

heat values for _a fixed temperature,

T,

have been determined

by averaging

_{over a} temperature interval

equal

to 1.5 Sl of T. The _accuracy of _our measurements has been tested

by measuring

a

sample

of

high purity

Cu. The

discrepancy

between the

published

_[2] and measured

specific

heat of Cu _was less than 2 Sl _at all

temperatures.

^{A few}

single crystals

with

a total

weight

of 20.3 mg were chosen for measurements. Addenda contribution to the total heat

capacity

_was smaller than 20 $i.

The

dependence

of the

specific

heat divided

by

temperature

C/T

_vs.

^T~

_over the temperature range 4.2 to 19 K is

presented

in

figure

I. One mole is defined

as 6.02 _x 10~~ formula units

(rather

than unit

cells).

The dotted vertical lines

correspond

to the temperature _range from 9 to II K where 90 $i of the transition takes

place,

measured

inductively

_[3].

Figure

2 shows

the

dependence

of the

C/T

derivative with respect to T~

vs. T~.

The diRerence between the _zero field and 5 T field

specific

heat divided

by

temperature has been

reported

to be 45

+10mJ/K~

mol at Tc cs 11.5 K which

corresponds

to

AC/C

_cs 0.02 [4]. ^In our measurements the relative

scattering

of the measured values

AC/C

in the

vicinity

of Tc _was less thant 2 _x

10~~

_but

no clear

anomaly

_was observed at Tc in both

plots.

On the _one

hand,

the

specific

heat

anomaly

in _our

experiments

_may be smeared out

owing

to the broadness of the

transition,

_cs 2 K. On the other

hand, however,

^the onset of _a broad heat

capacity anomaly

in [4] was at 13 K. This is about 0.5 K and 1.5 K above the

superconductivity

onset in the electrical

resistivity

and the

diamagnetic susceptibility, respectively. Therefore,

the 13 K

anomaly

in the

specific

heat _may be _a result of _some field induced structural _or

694 JOURNAL DE PHYSIQUE I N°3

3

m

b~

~

-n ,_,

,

- ,

,

E- 1 ,,

, ,

~ _{, ,}

, ,

CJ _,

, ,

, ,

, ,

, ,

, ,

, ,

, ,

, ,

, ,

0

0 loo 200 300 400

T~( K~)

Fig.

I. Specific heat divided by temperature vs. temperature _square for _~

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

The dotted vertical fines show the temperature range from 9 to ii K where 90 il of superconducting transition takes place. Bold points correspond to values of heat capacity calculated from the Debye model.

magnetic

transition

slightly

above Tc [4]. ^{The unusual feature in the} upper critical

magnetic

field of this

superconductor

_{[5] may} also suggest ^{the existence of} a field-induced transition.

Thus,

the diRerence between the _zero field and 5 T field

specific

heat

might

_represent the eRect of such transition rather than of

superconducting

_one.

The

plateau,

shown

by

^the dotted horizontal line in

figure 2, corresponds

to the cubic

dependence

of _a

phonon

_part of

specific

heat _on the temperature

Cph

"

flT~.

The deviation

pf

the C _vs. T

dependence

from _the cubic

dependence

is manifested at T _> 7.7 K which is

an indication of the

relatively

low

(50

-100

K)

8D. ^The value of

fl

₌ 15.8 +

0.2mJ/K~ mol,

calculated from

d(C/T)dT~

_vs. T~

dependence

is in _a

good

agreement ^with [4].

For

calculating

the

Debye

temperature, 8D, the

Debye

function has been used:

~~

~~~

~~

~~

~~~~

(ex ~~~-

_e-x

_I'

where R ₌

8.31J/K

mol and _N is the number of eRective oscillators _per molecule. Each of the oscillators is formed

by

_a set of atoms

rigidly

bound to _one another within _a molecule. N is not

necessarily equal

to the number of atoms in the molecule

owing

to the diRerence between

interatomic bounds and is not known _a

priori.

Due to such _an uncertainty in the value of N it is

generally impossible

to _{use a}

simple

low temperature

approximation

to the

Debye

function:

Cv(J/K mol)

₌

1944N(T/8D)~

for _an evaluation of 8D for

complex crystals.

We have determined the 8D and N

by fitting

of data calculated from the

Debye

function C _vs. T

dependence

to the

corresponding experimental

data. The best fit _was achieved with 8D ₌ 81.5 K and N

= 4.I and is shown in

figure

I. The

discrepancy

between the

experimental

and calculated

points

is less than 3$i. Such _an

agreement

^{is rather}

unexpected, taking

into

N°3 SPECIFIC HEAT OF

~

(ET)~Cu[N(CN)~]Br

695

20 O

~

NE

~

⁸

7z

~

~

~

~

'Cl 0 0

T~(

Fig.

vs.