The differential specific heat study on the superconductivity for the “214” phase and the doped superconductors

Physica C 185-189 (1991) 1361-1362 North-Holland

THE D I F F I ~ E ~ I A L SPECIFIC HEAT STUDY ON THE ~ D ~ O ~ 0 ~ T I V I T Y

Chen

Zhaojia,

Yar~

Zhan XiaononE

HonEshun,

and Wang

FOR THE "214" PHASE AND THE DOP~

Keqin,

I)epertment of Physics, University of Science and TechnoloEy of China, Hefei, Anhui 230026, P.R. China The differential specific heats of the L ~ .~lzCuO4_ ~ (M=Sr and Ba) and I ~ 1.$sSro.,. IsCUl-~Co ~.044 polyc~Istalline • . _ samples are measured. F o r L~.lSrzCuO{. ~ samples, the superconduct~nE trans~txon temperature Tc and the specific heat anomalies AC(T~) all present maximma at x=0.15, while they disppear nest x:0.14 for the L~.,B~CuO~.~ samples. The magnitude of the specific heat anomaly is reduced quickly when the concentration x deviate from the optimal value, which are 0.15 for iSO0 and 0.10 and 0.22 for iBOO. The Te is lowered and the specific heat anomaly is smeared out with foreign ion doping at Cu site.

IIl. R~SULTS AND DISCUSSION

I. INTRO~V]TION to

The specific heat anomalies were measured with

investigate the physical properties of various

high resolution differential c a l o r i m e t e r Ill . The

materials.

reference

Specific

heat

is a

Much

obtained

physical

with

mes~urement.

useful

information

differential

Because

bulk probe

of

can

specific

the

be heat

difficulty

in

sampls

condition.

non-supe~tir~

to

The difference of specific heats is

prepsrinE the samples and in measurement, there were few reports about the

superconduct i n g

study on

the

LaSrCtf) which is made with different sinterinE due

systematic

are

the

electron and

contributions the

of

the

non-supe~uctinE

the specific heat of [~.~CuO4_~ (M=Sr and Ba) and

samples for the lattice specific heats of two

the Cu site doping samples. In this paper, we give

samples ~re supposed equal, ie. AC(T):CeB-Ce~. The

the differential specific heat results of those

experiment data was modified with entropy balance. The

samples.

maxim%~

excess

specific

heat

at

supeI~onductinE transition temperature AC(T,) of II. SAMPLE PREPARATION AND CHARAC[£RIZATION The remction

samples were method.

prepared by

The

starting

solid state

materials

are

I~.rSrzCAK)68

samples

sample with

Sr content x=0°16 [~]. %~en the Sr

is

corresponding

to

the

content d i ~ t e s 0.16, the ~C(Tc) and Tc become

powders o f L~%(3N-purity),SrC~, Ba~(3N), CoO and

CuO(3N).

The

mixture

with

correct

Fig.l

The Sr

stoichiometry is heated at 90(~C for 24 hours in

Content depen-

air atmosphere,

dence

repeated

then Er0und. This procedure is

two times.

superconducting

pellets and sintered at I025°C for 48 hours in

transition tem-

air. ~ t e r annealed at 525°C for 24 hours in air,

perature TC(*) &

the

the magnitude

samples

were

it is

obtained.

measurements

The

were

powder X-ray

carried

out

by

Rigaku D/M~x 7A with ~=1.540562A CuK~ radiation indicatinE

that there is no

these samples.

!

of the

pressed into

diffraction

Then

To(K) AC{Tc)(md~r~|) SO0

40

20 ran~

@omo-

Ca~

of the specific

is0

heat anomaly AC( + )

$r c o ~ n t

impurity phase in

X

smeared out or lop, red ~.~i!e the fo~--'-mer%~Jries

0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

C Zhaojia et at / Superconductivityfor the "214"phase

1362

much faster. The behavior of Tc obtained by

0.12, 0.20 and 0.24 respectively. The specific

differential specific heet measurement is the same

heat jumps at transition temperature AC(Tc) are

as that of resistivity measurement result[I]• The

50.412.5(x=0.10), 13.1,0.7(x=0.12), 11.1a0.6 (x=0.20) and 26.1,1.3(x=0.24) mJ/K'mol. The

ohange o f t h e Sommerfeld oonstantA¥(O) is o b t a i n e d

the

llneam term of excess specific heat at

disappeering of the specific heat anomaly around

low t e m p e r a t u r e far below Tc. The a¥(O) velue a l s o

x=0.14 may be related to the existence of the low

presante

temperature tetregonel phase[3].

from

maximum

at

x:O. 16.

The

bulk

superconductin~ l~ume diagram is drawn in Fig. 1.

Fig.4 illustrates

the mee.sured specific

heat

We think the specific heat jump is smeared by inhomogeneity.

jumps o f Lal.8sSro.l$CUl.iCoTO44.It can be s e e n t h a t the specific heat jump is smeared out rapidly and

For the L~.~CU04. ~, the temperature dependence of the differen(ial specific heat for

the superconducting transition temperature becomes

each sample is shown in Fig.2. From this figure,

y=0.005, the specific heat jtmrp is reduced to

bC(mJIK.mol) 4o o 40 0

Fig.2

dependence

of

differentia/

the

about one seventh

bO(md/K.mol)

x,0.078

Fig.4 The temperature

x,O.090

dependence Of

4~

L~.~Cu04. | pol~rystalline samples

heat for

X'0.120

300l

the

[

.,,

0 ' X-0.140

~0

y,,O.O000~ Y'0"0016/j~l

"°'°°'°/EII

differe.tial - ifio 150 heat for L~.8sSr0.lsCUl. ~ CoIO~.~ polycrystalline 0

4O

specific

of its initial undoped value.

x,O.O7O

40 0

The temperature

lower when the Co doping content increases. For

,pO,OO60/'k/IyU

samples

0 ' X,0.160 40 0 x-O.200

2O T(K)

0

40

40

Our experiment results which will be published

4O

0

x-O.300 0

soon also indicated that if the foreign ion in the 30

15

T(K)

Cu site at Cu-O plane is magnetic or not, it will cause the same effect: the To is lowed and the

we can see that the specific heat anom~ly at

specific heat anomaly is smeared out with doping.

superconducting transition temperature can only be

The

doping effect

gives

us a

clue

to

get

conclusion that the perfectnessof the O/-0 plane in Fig.3 The super-

40

laSrCuO system is significant to forming optimal

conducting phase diagram of L ~ . x

so

Ba~CuO4-a°btained by differential ~

20

specific heat~, DC susceptibility(e) and

resistivity(~

measurements

!

I0

Laz.zBaxCu04.6

superconducting phase.

i7 ","

REFERENCES

| i

i

%

A i

iU

(1989)361-363.

iI %

O 0.000

oaoo

o.ioo

I. Chen Zhsojia, Wang Keqin et al, Progress in

%.aoo

Ba content: X observed for four samples corresponding to x = 0 . 1 0 j

2. Yang Hongshun, Zhang Xiaonongj Zhu Changfei, Wang Keqin, Cao Liezhao and Chen Zhaojia, Physics C172(1990)71-74. 3. Tsksshi SUZUKI and Toshizo FUJITA, Physica C159(1989) 111-116.