Physica B 263—264 (1999) 636—639

Enhanced phonon scattering due to 3d-transition metal
substitution for Cu in YBa Cu O  \BManabu Ikebe*, Hiroyuki Fujishiro
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, JapanAbstract
The characteristic enhancement below T in the thermal conductivity i of high-T oxides, YBa Cu O
(90 K phase)   and YBa Cu O (60 K phase), is promptly quenched by a small amount of 3d-transition metal (Fe, Co, Ni) substitution  for Cu. The quench of the i enhancement can be understood as being due to the enhanced phonon scattering below T by
residual unpaired electrons. The existence of a considerable amount of the residual electrons is consistent with the d-wave
symmetry of the superconducting energy gap under strong electron scattering.  1999 Elsevier Science B.V. All rights
reserved.
Keywords: Thermal conductivity; Oxide superconductors; Cu site substitution; Magnetic impurity

The thermal conductivity i of high-T cuprates
(HTSC) has been widely investigated because i canbe measured below the superconducting transitiontemperature T and can provide important in
formation concerning the superconductivity ofHTSC. YBa Cu O
is one of the representative  \BHTSC systems and show two typical transition
temperatures at T +90 K (90 K-YBCO: d+0.08)
and T +60 K (60 K-YBCO: d+0.3) depending onthe oxygen deficiency d. T of conventional super
conductors is drastically reduced by addition ofmagnetic impurities. In this paper, the effect of
3d-transition metal impurities M (M"Fe, Co, Ni)
substituted for Cu is investigated for both 90 Kand 60 K-YBCO and is compared with that of* Corresponding author. Fax:#81-19-621-6373; e-mail:ikebe@iwate-u.ac.jp.

non-magnetic Zn substitution. The main interest isplaced on the impurity effect on the thermal transport in the YBCO system.
YBa (Cu M ) O
polycrystals were pre\V V  \Bpared by a standard solid state reaction method.
The raw materials were powders of Y O , BaCO , CuO and transition metal oxides (NiO, CoO orFe O ). They were calcined at 910°C for 24 h in air, pulverized, pressed into pellets, sintered at 955°C
for 30 h in flowing oxygen and then slowly cooled
to the room temperature in about 24 h. Thus prepared samples correspond to the 90 K-YBCO(M )Vsamples. The 60 K-YBCO(M ) samples were obV
tained by quenching 90 K-YBCO(M ) into liquidVnitrogen from 600°C. The oxygen deficiency d of60 K-YBCO was estimated to be &0.3 from thec-axis lattice parameter and from relative weightloss after quenching process. The thermal conductivity was measured by a standard continuous heat

0921-4526/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 4 3 6 - 7

M. Ikebe, H. Fujishiro / Physica B 263—264 (1999) 636—639

Fig. 1. T as a function of impurity concentration x for (a)90 K-YBCO(M ) and for (b) 60 K-YBCO(M ).
VV

Fig. 2. Magnetic susceptibility as a function of temperature for(a) 90 K-YBCO(M ) and for (b) 60 K-YBCO(M).
  flow method [1]. The magnetic susceptibility s wasmeasured by a commercial SQUID magnetometer.
Fig. 1a and b show the superconducting transitiontemperature T as a function of Fe, Co, Ni and Zn
impurity concentration x for 90 K-YBCO(M ) andV60 K-YBCO(M ), respectively. In Fig. 1a and b, weV
notice that the T depression for increasing x is themost steep for the nonmagnetic Zn impurities andis the most gradual for Fe impurities. Fig. 2a andb show the temperature dependence of the magnetic susceptibility s of 90 K-YBCO(M ) and 60 K-YBCO(M ). In Fig. 2a and b, Fe impurities 637

Fig. 3. (a) The thermal conductivity i of 90 K-YBCO(M ) and (b) i of 60 K-YBCO(M ).
 

are the most magnetic followed by Co and Ni inthis order. In contrast, T depression is stronger in
rather the reverse order, i.e., the more weakly magnetic ions damages the superconductivity moreseriously. The results in Figs. 1 and 2 seem to indicatethat the magnetic scattering for conduction electrons[2] is not the main origin of the T depression in both
90 K-YBCO(M ) and 60 K-YBCO(M ).
VVFig. 3a and b show the temperature dependenceof the thermal conductivity i of 90 K-YBCO(M ) and 60 K-YBCO(M ). i of 90 K- and 60 K YBCO [3] is also presented for comparison. Thethermal conductivities of both 90 K-YBCO and60 K-YBCO show the characteristic enhancement
below T , taking the maximum around T &T /2.

In the present YBCO polycrystals, the upper limitof the electron thermal conductivity i is under
strong ceiling by relatively large electrical resistivity and i is estimated to hardly contribute to thei enhancement [4,5]. Then similarly to conventional alloy superconductors [6], the i enhancement is considered to come from the reducedphonon scattering into the Cooper pairs. We analyze the phonon thermal conductivity on the basisof the following formula [7]:  

3dn R1v2 ¹  p
i " d
2pM
H"xeV;
q (x) dx,(eV!1) H"

2


(1)

638

M. Ikebe, H. Fujishiro / Physica B 263—264 (1999) 636—639

where x is the reduced phonon frequency, n ("13!
d) the number of atoms composing YBa Cu O ,  \B
R the gas constant, d the mass density and M themolar weight. The total phonon scattering rate
1/q is assumed to be given by the sum of scattering rates by various centers [7],q\"q\#s¹x#p¹x#E¹xg(x, y)H#E¹x#º¹x exp ! " .
b¹





(2)

Here, parameters q\, s, p, Eg(x, y), E and U repres
ent the phonon scattering strength due to grainboundaries, sheet-like faults, point defects, electrons, residual normal electrons and other phonons(umklapp process). The ratio of the phonon scattering by electrons g(x, y)"q /q in the normal state to in the superconducting state was first given
by BRT [8,9]. We assumed the d-wave superconducting energy gap, D"D cos 2 [10]. The electron thermal conductivities i was subtracted from
observed i by use of the Wiedemann—Franz law for¹'¹ and by use of Kadanoff’s formula for
¹(¹ [11]. i is always much smaller than i .

Examples of the fitting curves for i are shown in
Fig. 4 for 90 K-YBCO(M). Table 1 summarizes 
the used fitting parameters. The most impressive
result is the absence of the i enhancement below
T caused by the impurities. As can be seen fromparameter values in Table 1, we attributed thedisappearance of the enhancement to the phonon
scattering by residual electrons (E term) which do
not condense into Cooper pairs even at 0 K.
Miyake calculated ¹ /¹ versus N /N relation  (T /T is the transition temperature in the presence of impurity scattering normalized by the pureFig. 4. Examples of the fitting curves based on the Eqs. (1) and (2)for 90 K-YBCO(M ).
 sample transition temperature T and N /N is the 
fraction of the residual electron density at 0 K) [12].
The calculation pointed out very large N /N in 
the case of the d-wave energy gap and in the limit ofstrong (unitarity limit) electron scattering by impurities. The results of our analyses of i are semiquantitatively consistent with the theoretical prediction of Miyake in the case of strong scattering.
As for Zn impurities, however, a considerable reduction of the phonon—electron scattering must be
postulated to explain the stronger reduction effectof Zn for the thermal conduction in the YBCOsystem.
The authors wish to thank Mr. K. Nakasato ofShibaura Engineering Works Co. Ltd., for his experimental contribution. This work is partially supported by a Grant-in-Aid for Scientific Research

Table 1
Summary of the parameters used and determined in the fitting processes in pure 90 K-YBCO and 90 K-YBCO(M ) (M"Fe, Co, Ni, Zn) samples
SampleT (K)q\ (s\)s(K\s\)p(K\s\)

E (K\s\)E (K\s\)

U(K\s\)YBCO
M"FeM"CoM"NiM"Zn

91
88
77
79
55

4.6;10
7.8;10
4.5;10
4.2;10
2.9;10

3.1;10
4.0;10
3.8;10
3.3;10
4.8;10

2.7;10
2.6;10
3.3;10
1.5;10
5.2;10

1.0;10
6.6;10
5.2;10
1.4;10
5.3;10

01.8;10
3.5;10
4.7;10
1.1;10

4.3;10
6.5;10
3.2;10
2.9;10
3.2;10M. Ikebe, H. Fujishiro / Physica B 263—264 (1999) 636—639

from the Ministry of Education, Science, Sportsand Culture (No. 09555018).

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