PHYSlCAELSEVIER

Physica C 263 (1996) 305-308

Influence of Cu site impurities on the thermal conductivity
of YBa2Cu307_
H. Fujishiro *, M. Ikebe, K. Nakasato, K. Noto
Department of Materials Science and Technology, Faculty of Engineering, lwate University, 4-3-5 Ueda, Morioka 020, Japan

Abstract
The effect of Zn substitution at in-plane Cu sites in 90 K and 60 K phase YBa2(CUl_xZnx)307 ~ sintered materials(0 < x < 0.04) was investigated from the viewpoint of thermal conduction. The characteristic enhancement of the thermal
conductivity K just below Tc in pure YBCO was rapidly suppressed in both phases by the Zn substitution. Based on thephonon heat conduction model, the disappearance of the K enhancement is attributed to the depressed phonon-electron
scattering caused by the Zn substitution.

1. Introduction
In the YBa2Cu307_ 8 (YBCO) system, two stablesuperconducting phases are known to exist, the 90 K
and the 60 K phase. Impurity Zn atom substitution at
Cu sites increases the electrical resistivity p in thenormal state and results in the reduction of thetransition temperature, Tc. The rapid reduction of Tc
has been attributed to a localization of Cu 2÷ d-holes.
[1] The thermal conductivity K is a valuable probefor the scattering processes of phonons in both normal and superconducting states. A characteristic feature of thermal conductivity K(T) in YBCO is a
rapid rise just below Tc and the existence of amaximum at about Tc/2 [2]. In this report, weinvestigate the effect of Zn impurity on the thermalconductivity K(T) for both the 90 K and 60 K phase* Corresponding author. Fax: +81 196 21 6373;
e-mail: fujishiro@ msv.cc.iwate-u.ac.jp.

YBCO samples. The experimental results are systematically analyzed on the basis of the phonon heat
conduction model.

2. Experimental procedure
90 K phase YBa2(CUl_xZnx)307_ 8 ( 0 < x <0.04) samples were prepared from stoichiometricmixtures of Y203, BaCO3, CuO and ZnO raw powders. The mixtures were calcined at 910°C for 24 hin air. They were pulverized, pressed into pellets andthen sintered at 955°C for 30 h in flowing oxygen.
60 K phase samples were fabricated by a quenchingprocess from 600°C down to liquid nitrogen temperature. The oxygen deficiency 8 of the 90 K phase and60 K phase samples was estimated to be ~ = 0.1 and= 0.3, respectively [3]. The density of these samples
was about 90% and independent of the oxygen deficiency and the Zn concentration. The thermal conductivity measurement was made by a continuous

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H. Fufishiro et a l . / Physica C 263 (1996)305-308

heat flow method between I0 and 150 K using anautomated measuring apparatus with Au(0.07at.%Fe)-chromel thermocouples as thermometers [4].

100 . . . . .
I ....
fYBa=(CUl.xZnx)30 r-6gii••••Oii80~- o.°%• °°o°°°,"OFig. 1 shows the temperature dependence of theelectrical resistivity p of 90 K and 60 K phase
YBa2(CUl_xZnx)3OT_, samples. The metallic behavior of p(T) was preserved for all 90 K phasesamples, though p increased with increasing Zn
concentration x. p(T) of the 60 K phase samples
showed a bump just above Tc and the height of thebump increased with increasing x. The inset in Fig.
1 shows the Zn concentration dependence of Tc ofthese samples. Tc decreased linearly with increasing
Zn concentration x and the Tc depression rate for x(ATc/Ax) was almost the same for 90 K and 60 K
phase samples.
Fig. 2 shows the thermal conductivity K as afunction of temperature T for the 90 K and 60 K
phase YBa2(Cu~_xZnx)307_ ~ samples. For 90 K
phase samples, the characteristic enhancement wassuppressed quite rapidly by the Zn substitution. The
enhancement was barely discernible even for thespecimen with x = 0.005, which showed a sharp
superconducting transition at Tc = 85 K. The ~(T)ii

*iI='~

iI,,~,. . . . . .

jI

I"d~'~""°'°1/ ,op-~.

I'-" 4°t "e~.6OK'phim.

iii-I I~.

aOK-+~x.,oI

Qx~.o'zI"x--o.o~x..o.o~.,:

0100

200

Temperature

(K)

i=i/

040-

oct**•.

*20d)

I--ii

0tiI50i==iIiii100TemperatureiIi

150iii200(K)Fig. 2. The temperature dependence of the thermal conductivity Kof the 90 K phase and 60 K phase YBa2(Cul_xZnx)307_~
samples with various Zn concentrations.

values decreased with increasing x in the normalstate. The r enhancement was also observed in the60 K phase YBCO below Tc(= 58 K) but it was lessobvious than that of the 90 K phase YBCO. The K
enhancement of the 60 K phase YBa2Cu307_ 8 is,
however, far clearer than that of the 90 K phaseYBaE(CU0.98Zno.02)307_8, in spite of almost thesame Tc values of both samples. In the 60 K phasethe enhancement was completely suppressed by theZn substitution, even for the specimen with x = 0.005(T~ = 40 K).

io~o0ix=o. l_ 1

6° f

3. Experimental r e s u l t s

6|9oK~ase, x-o I ~1x=o.oos I
×=o.ol I-4
x=o.o~ 1 460K-phase,X=0 | -I

300

Fig. 1. The temperature dependence of the electrical resistivity pof the 90 K and 60 K phase YBa2(CUz_xZnx)~O7_ ~ sinteredsamples. The inset shows the Zn concentration dependence of T~of these samples.

4. D i s c u s s i o n

The heat transport in conductors is due to bothelectrons ( r e) and phonons (Kph). In the normal state
the electron contribution ten can be estimated byusing the Wiedemann-Franz law. In the superconducting state, we assume that Kes follows the theory
proposed by Kadanoff and Martin [5], which predicts
the reduction of Ke below T~ because of the formation of Cooper pairs. Taking account of the phonon
scattering by various crystal defects and electrons,
the phonon contribution is given by Tewordt andWSlkhausen [6] in the following way:

3dnRT3v2 fOD/T x4e x
KPh-- M@3 "o( e ~ _ 1)2 ~'ph dx,(1)H. Fujishiro et aL / Physica C 263 (1996) 305-308

8060==: ~ ,..%:-':..."Z~ ~,.a ~20oo,90K'phase,, , , ,0,50
Temperaturezxot.j LJ

X=O
x=o.o05 I-I
X=O.01 I ], 13, X=O.02,, I]
100(K)

150

Fig. 3. The fitting of rph by the TW-BRT theory for the 90 K
phase samples with various Zn concentrations. The solid lines are
the calculated curves using parameter values in Table 1 (X =A(0)k/A(0)Bc s = 1). The dotted line for the x = 0.005 sample
presents the fitting in which X = 0.6 is used as the reduced gap.

where d is the mass density, M the mass of 1 mole,n(= 13) the number of atoms composing YBCO
compounds, R the gas constant, ~9o the Debye
temperature, o the average phonon velocity and x
the reduced phonon frequency. The phonon relaxation time Zph is given by1_ _ = ~.-~1+ sT2x 2 + pT4x 4 + ETxg( x, y).
rph(2)
Here, r b is the phonon relaxation time due to grainboundaries and s, p and E refer to the strength ofthe phonon scattering by sheet-like faults, point de-307

fects and electrons, respectively. The function
g(x, y ) = ~'phn/~'phs stands for the ratio of thephonon-electron relaxation time in the normal andsuperconducting states, which depends on the energy
gap through the parameter y - - A ( T ) / k s T as given
by Bardeen, Rickayzen and Tewordt [7].
Fig. 3 shows fitting curves of the phonon thermal
conductivity Xph(= K - Ke) for the 90 K phase sampies. The parameters used and determined in thefitting process are summarized in Table 1. As can beseen, the theoretical curves reproduced the measured
Kph(T) quite satisfactorily. The electron-phonon
coupling parameter A(= 2 a ( t ) E / ~ r u , where a and( t ) are the lattice spacing and the effective hopping
matrix element of electrons, respectively [6]) suddenly decreases by addition of Zn impurity. Thestrength of the phonon scattering by point defects, p,increases with increasing x because of the alloying
effects, but the increase is somewhat moderate. The
rapid suppression of the K(T) peak for Zn substitution might also be explained by a suppression of thesuperconducting gap, X = A(O)/A(O)ac s, because
then the phonon scattering by electrons survives atlower temperatures [8]. An example of the fitting fora reduced energy gap (X = 0,6) is shown in Fig. 3
for the 90 K phase sample with x = 0.005. However,the fitting curve did not reproduce the measured
Kph(T) in the low-temperature region. Thus, the disappearance of the K enhancement is mainly attributed to the depressed phonon-electron couplingstrength A. As can be seen in Fig. 1, p increases byaddition of Zn. As Pippard pointed out [9], theelectron-phonon interaction is diminished by shorter

Table 1
Characteristic parameters of the 90 K phase and 60 K phase YBa2(Cu I _xZlax)3OT_Sample

90 K phasex= 090 K phasex = 0.005

X
T~ (K)O D (K)d ( g / c m 3)
7~ I (s - I )p (K-4 s-*)s (K -2 s - *)E (K- J s - ~)A

191430
5.68
3 . 0 × l0 s
456
2.7 X 106
6.9 × 10 s
0.26

10.685430
5.47

3.1 × l0 s
1507
2.7 X 106
1.3 × 10s
0.05

2 . 9 × 108439
2.8 X 106
7.1 × 10 s
0.27

90 K phasex = 0.02

60 K phasex= 0157430
5.73
3 . 2 × 10 s
4497
2.9 X 106
2.9 X 1070.01

158380
5.53
3.2× 10 s
1402
4.5 X 106
3.3 × l0 s
0.13308

H. Fujishiro et aL / Physica C 263 (1996) 305-308

electron mean free paths. The rapid depression of A
might be related to the d-hole localization caused byZn impurities [1].
The electron-phonon coupling parameter A of the60 K phase YBaECU307_ 8 sample is larger than thatof the 90 K phase YBaE(CU0.98Zn0.02)307_ s sample
with almost the same T~ (see Table 1). This indicatesthat the small amount of Zn substitution at in-planeCu sites more directly and seriously suppresses thesuperconductivity than the oxygen reduction fromthe CuO chains, making the anomaly of the phonon
thermal transport below Tc disappear.
A central point in the recent dispute on the enhancement of K(T) is concerned with its origin;phonons or electrical carders. In this paper, weconsistently assumed the phonon origin model, andthe disappearance of the K peak caused by the Zn
substitution was attributed to the depression of electron-phonon interaction. If the electrical carrier isresponsible for the r enhancement, a very small
amount of Zn impurity should dramatically enhance
the quasi-particle scattering rate below Tc. Experi-ments to determine the quasi-particle scattering rate
in a Zn-doped crystal are highly desirable.

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