LT 21

Proceedings of the 21st International Conference on Low Temperature Physics
Prague, August 8-14, 1996
Part $3 - Superconductivity 2: HTS - Transport, etc.

Effect of Y-site and Ba-site Substitution

on Thermal Propertiesof YBazCu30~. e

Hiroyuki Fujishiro, Kiminari Nakasato, Shigeo Takahashi and Manabu lkebe
Faculty of Engineering, lwate University, 4-3-5, Ueda, Morioka 020, JapanThe effect of partial replacement of Y site with Ca or Ba site with Sr on the thermal and electrical properties ofYBa2CU3OT.~ superconductors was investigated. For Y(Bai.xSrx).2CU3OT.b system, the characteristic enhancement ofthe thermal conductivity K(T) below the transition temperature T c was decceasedmore slowly by substitution than thatfor (~I.XCaX)I~I2C,'u307.6system. Electron-phonon interaction in these systems were discussed based on the phonon
heat conduction model under the weak coupling d-wave energy gap.
1. I N T R O D U C T I O N
A characteristic feature of thermal conductivity K(r)in YBa2Cu307.6 ~ )superconductors is a rapid rise
just below Tc with decreasing temperature T and existence of a maximum at about TJ2. The origin of thisbump is under controversy between the phonon and thecarrier model. We have investigated the effects ofoxygen deficiency 6 and Zn impurity of Cu site on K(T)and the experimental results have been systematicallyanalyzed on the basis of the phonon heat conduction
model [1,2]. It has been reported that the Y site
replacement with Ca has the effects of increasing theplanar hole concentration [3] and that the Ba site
replacement with Sr changes the oxygen nonstoichiometry and the copper valence [4]. in this paper,
the effect of partial replacement of Y site with Ca or Ba
site with Sr on the thermal properties of YBCO is investigated. The variation of phonon scattering isdiscussed for various Ca or Sr concentrations on thebasis of the additive relaxation rate approximation.
2. EXPERIMENTAL(Y1.xcax)B82CU3OT.6 (0<X~0.3)
and Y(Bal.
xSrx)2C'~307_6 (0~;X.~0.4) samples were prepared fromstoichiomelric mixtures of Y203, CaCO3, BaCO3,
SrCO3 and CaO raw powders. The mixtures were calcined at 910"(3 for 24h in air. They were pulverized,
pressed into pellets and then simelld at 955"C for 30h in
flowing oxygen. All samples were confirmed to be asingle (123) phase by the x-ray diffraction method. The
density of these samples was about 90% and was
independent of the Ca or Sr concentration. The thermal
conductivity (K) measurement was made by a continuous
heat flow method and the thermal diffusivity (a)measurement was performed by an arbitra~ heating
method quasi-simultaneously. An automated measuringapparatus was used with Au(0.07at.%Fe)-chmmd
thermocouples as thermometers between 10 and 200K [5].

Czechoslovak Journal of Physics, V01.46 (1996), Suppl. $3

3. RESULTS AND DISCUSSION
Fig. 1 shows the electrical resistivity p vs. T for
typical substituted samples. The inset in Fig. 1 shows
the transition temperature Tc vs. the concentration X ofCa and Sr. Tr of Y(Bal.xSrx)2Cu3OT~ system deceased
linearly with increasing Sr concentration X. in (Y~.
XCaX~IBa2Cu307.6 system, Tr decreased molt rapidJy forX<0.2 than that of Sr substitution and seemed tobecome almost constant at 3(--0.2--0.3. Similar results
have been rq~orted by other workers [6]. Fig. 2 shows
the temperature dependence of r. For Y(Bal.
xSrx)zCu30~+ system, the K enhancement was reduced
more moderately by substitution than for (Y].
xCax)Ba2Cu307.b system.
Generally the heat transport is due to both electrons(~:c) and phonons (~:vh)"In the superconducting state, we
assume that the electronic component re, follows ~etheory proposed by Kadanoff and Martin [7]. We also
assume the d-wave energy gap anisotropy A--Amaxcos20
with 2Anm/]~'l'c=7 (AmJABCS=2) in the weak coupling3|,/i,w|

i|

/ ~-t

00.1,,,, |I020.3

1 I-w,,it

9s~.x=o., II

1, .**~,"-/0.4+,,+"*

t09J I .........

-"~'--. ~/0!

9'=J . . . . . . .

...Jf.

,~

§I.

.

.

.

100
TemperatureI

200(K).

.

.

.

I

300

Fig.1 The electrical resistivity p vs. T and the transition
temperature Tc vs. the concentration X (in the inset).

1207

Table I. Summary of fitting parameters by TW theory for (Yt.xCax)Ba2Cu3OT.~ and Y(Ba~.xSrx)~Cu3OT.~ systems.
samplesTo(K) xBq(see-l)/B(ttm)p(K'4sec"1)

s(K'2sec"1)E(K-lsecq)

7.

YBa2Cu3OT.8(Yo.gCaej)Ba2Cu3OT.~(Yo.scao.=)Ba2Cu3OT.8(Yo.TCao.z)Ba.zCuzOT.eY(Bao.gSro.~).zCu30~.~
Y(Bao.sSro.2)2Cu3OT.~
Y(Bao.TSro.3)2Cu3OT.~

91
83
79
81
89
87
85

14.3
2.1
2.3
2.1
3.4
1.3
1.1

533
2387
3681
5635
1000
1972
1987

2.2x 106
2.1x106
2.8x 10~2.5x106
5.8x 106
4.1x 106
8.6x1OS

7.8x l0 s
2.5xlos
7.2x 107
7.0x1078.5x los
8.7x los
7.3x1OS

0.30
0.10
0.03
0.03
0.32
0.33
0.28

2" so

9'"

'I''''

2.3x los
1.6x109
1.4x 109
1.6x1099.7x 10s2.6x 1093.1x109

I''''

I

"''

4O'

9999I

999~

|

9'

''.

5,oo

30
m

~ ~ - -.,,,,~f~o~.

, ~^oOoOoooe~ovy.A ALloY

OA=~ ~ e*4,

@41,@------

9-9##-

9-'e O@

9_..88##$## *#**#'~.-~.*

I

Ioc=x=o.1*-Ioc.

9&ca: •9x.o

Sr:•

x=0.11:

I,

x=ol:

t=LO

"t--0I

0all|

50

I

P'

9J,.& ,t .t.t.t.=..--. ............

gSOlO-~@4MI,4HI'v------

II|

100
Temperature

=9.

l

150
(K)

.

.

200

==

107.t:

0I

0Ioz~

I'

9I

e.

a.

II50,,=

C=" X=0.11
C= x=o.2 I;

",I

100
Temperature (K)' -I

Sr: XfO'2 It150

Fig. 2. Temperature dependence of the thermal
conductivity K for various Ca and Sr concentrations.

Fig. 3. Examples of the fitting curves by TW theory.

limit [8]. In order to calculate Kph' in the superconducting state, Xph, is assumed to be given by [9],is more conspicuous in CYl.xC.ax)Ba2Cu307.s system inwhich the reduction 7. is also more salient. From theestimation of the specific heat C using the relation
C.=rffct, a tendency of slight reduction of the De,byetemperahne Ov was noted in both substituted systems.
The details of the behavior of C is to be reportedelsewhere.

Xph,-I ffi%q + sT2x2 + pT4~c4 + ETxg,(1)where Xb, S, p and E refer to the phonon scatteringstrength by grain boundary, sheet-like faults, point
defects and the charge carriers, respectively. The function
gm'gphn/'gphsis the ratio of the phonon-electron scattering
rate in the normal and superconducting state and was
calculated under the assumption of the weak coupling dwave gap. Table I shows the values of thesecharacteristic parameters determined in the analyses. Fig.
3 shows the examples of the fitting curves, which
reproduced the Kph. (=K-K=) data satisfactorily. It was
found that pointdefect scattering (p parameter) increased
remarkably with increasing substitution concentration

X. In (Yl.xcax)Ba,zCu3OT.a, electron-phonon coupling
parameter 7. (=E) dearessed rapidly with X, but not so
rapidly as YBa.z(CUl.xZnx)3OT.8 previously studied [2].
In marked contrast, 7. remained almost constant of X inY(Bal.xSrx)2Cu3OT. ~. It Is noteworthy that Tc reduction

1208REFERENCES[1] H. Fujishiro et at.: Physica C 235-240 (1994) 825.
[2] H. Fujishim et al.: to be published in Physica C(1996).
[3] B. Fisher et al.: Phys. Rev. B47 (1993) 6054.
[4] ]L Fueki etal.: Physicu C 166 (1990) 261.
[5] M. Ikebe etal.: J. Phys. Soc. Jpn. 63 (1994) 3107.
[6] R.G. Buckley etal.: Physica C 174 (1991) 383.
[7] L.P. Kadanoff and P.C. Martin: Phys. Rev. 124(1962) 670.
[8] M. lkebe etal.: submitted to J. Low Temp. Phys.
[9] L Tewordt and T. W61khausen: Solid State Com.
75, (1990) 515.

Czech. J. Phys. 46 (1996), Suppl. S3