Journal of Low Temperature Physics. Vol. 117, Nos. 5/6. 1999

Enhanced Phonon Scattering below Tc Caused byZn and Ni Substitution in La 1.85 Sr 0.15 CuO 4
Hiroyuki Fujishiro, Manabu Ikebe and Yuzo Shibazaki
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551,
Japan.

The thermal conductivity K has been measured for La2-xSrxCuO4 andLa1.85Sr0.15Cu0.98M0.02O4 (M; Zn, Ni). The characteristic enhancement in K below Tc
was confirmed for La1.85Sr0.15CuO4. The phonon scattering mechanisms wereanalyzed in detail and the electron-phonon coupling constant A was estimated tobe ~0.05. The K enhancement becomes indetectable on Zn or Ni substitution forCu, which indicated survival of the phonon scattering by charge carriers down tolower temperatures as a result of reduced Tc.

PACS number: 74.30.Ek, 65.40.-f, 44.10.+i

1. INTRODUCTION
1-3

The thermal conductivity K of several high Tc cuprates, typically YBa2Cu3O71-3 ,shows a characteristic enhancement below the superconducting transition temperature Tc. In contrast, La2-xMxCuO4 with T-type structure does not clearly show
the corresponding enhancement. In this paper, we present observation of the K
enhancement in La1.85Sr0.15CuO4 and its rapid quenching by a small amount
substitution of Cu by Zn and Ni. The origin of the K enhancement is discussed andthe phonon scattering mechanisms are analyzed in detail.

2.

EXPERIMENTAL

La2-xSrxCuO4 and La1.85Sr0.15Cu0.98M0.02O4 (M; Zn, Ni) samples were prepared bya solid state reaction method. The density of the samples was higher than 90% ofthe ideal one. The K measurement was made between 10K and 150K by a continuous heat flow method and the thermal diffusivity a was measured by anarbitrary heating method simultaneously with K. 511810022-2291/99/1200-1181S16.00/0 C 1999 Plenum Publishing Corporation

H. Fujishiro, M. Ikebe, and Y. Shibazaki

1182

3.

RESULTS AND DISCUSSION

Figure 1 shows the temperature dependence of the electrical resistivity p(T) of
La2-xSrxCuO4 (LSCO) for X=0.10, 0.15 and 0.20. The p values decrease withincreasing Sr concentration X. The superconducting transition temperature Tc(=T c ) is 31K, 38K and 33K for X=0.10, 0.15 and 0.20, respectively. Figure 2
shows the phonon thermal conductivity Kph as a function of temperature T. Kph ofLSCO (X=0.15) shows an enhancement below Tc and takes a weak maximum atT-30K. For LSCO (X=0.20), a trace of the K enhancement below Tc is discernible.
The heat conduction in metals is due to both the electrons (Ke) and the phonons (Kph). We assume that the normal state electronic component Ken follows theWiedemann-Franz law, while that of the superconducting state Kes follows thetheory by Kadanoff and Martin* with the d-wave energy gap A=1.5ABCSCOS2O.3
We also assume a T3 dependence of p(T) below Tc Ke is always much smaller than
total K because of a large residual resistivity p(0) (&0.25mQcm) as estimated by
the extrapolation to 7=0K.
The phonon thermal conductivity Kph (=K-Ke) is given by

where no(=7) is the number of atoms composing the 214 compounds, R the gas
constant ,vt (~3700m/s)7 the average sound velocity and x is the reduced phonon
frequency. Assuming a kind of Matthiesen's rule for the scattering rates, theFig. 1. The temperature dependence of theelectrical resistivity p(T) of La2-xSrxCuO4(X=0.10, 0.15, 0.20).

Fig. 2. Kph vs T of La2-xSrxCuO4. The
clashed line represents the estimatedelectronic component Ke for X=0.15.

Zn and Ni Substituted La 1.85 Sr 0.15 CuO 41183

phonon relaxation time T ph is given byHere, x b is the phonon relaxation time due to grain boundaries and P,S and E referto the strength of the phonon scattering by point defects, sheet-like faults andconduction electrons, respectively.8 KTXtanh(X/2) stands for the scattering due to
two-level like tunneling states, which was suggested to be operative in this system.4,10 We assumed that the dynamical phonon scattering of apical oxygens is an
important origin of the K term, which comes from the phase instability between
LTO and HTT(or LTT) phases. The function, g(x,y)=r e n /r e s , gives the ratio of thephonon scattering by electrons in the normal and the superconducting states,
which depends on the energy gap through the parameter 2/=A(7)/KBT.11
In order to analyze the phonon scattering on the basis of the eqs. (1) and (2),we need the value of the Debye temperature &D. The specific heat C was estimatedfrom the relation C=K/a and an average value of 0D =350K was found to beappropriate for all the samples. The data of the a(T) measurement are presentedin Fig. 3. T b =l b /V s was determined from the average grain size of each sample
observed by a SEM. The results of the fitting for Kph are given by the solid lines inFig. 2. The determined parameters are summarized in Table I. The calculated Kph
can reproduce the measured Kph data satisfactorily.
In Table I, the electron-phonon coupling parameter X is estimated from therelation, k=2a<t>Efttv12 a=4A being the lattice parameter and <t>=5000K, theTable I. Used and determined parameters in the fitting for Kph of the La2-xSrxCuO4 (X=0.10,0.15, 0.20) and La1.85Sr0.15Cu0.98M0.02O4 (M; Zn, Ni) samples.
X=0.10

X=0.15

X=(1.20

X=0.15M=Zn

X=0.15
M=Ni

3.5X108

3.5X108

3.4X108

3.5X108

7.8
3.0X106
8.1X103
1.4X108
3.8X108
0.05

7.7

7.9

7.7X

1.7X108
15.7
2.7X10 6
7.6X103
1.9X107
3.3X108
0.01

1.0X107
5.9X103
7.6X107
5.2X108
0.03

2.8X106
6.0X103
1.4X10 8 *8.9X108(0.05)

2.8X106
5.4X103
1.4X108 *9.9X108(0.05)Tc (K)

31

38

33

1619

Tb(s-1)lb(nm)S (K-2s-1)
P (K-4s-1)£(K-1-1s-1-1)

K(K s )

* E value for X=0.15(M=Zn) and X=0.15(M=Ni) is assumed to be the same as that of thenon-doped La2-xSrxCuO4.

1184

H. Fujishiro, M. Ikebe, and Y. Shibazaki

Fig. 3. Thermal diffusivily a(T) of La2-xSrx
CuO4.The right hand ordinate provides a roughmeasure for the phonon mean free path lph.

Fig. 4. p vs T for La1.85Sr0.15Cu0.98M0.02O4 (M; Zn, Ni).

effective matrix element for the electron hopping in the CuO2 plane. The obtainedl value is 0.05 for La1.85Sr0.15CuO4. The origin of the pretty clear enhancement ofK for this compound is explained as to come from Kph similarly to that of thesintered YBa2Cu3O7-f system (X-0.4 and -0.15 for 90K- and 60K-phase
YBa2Cu3O7-h3).
Figure 4 shows p(T) of La1.85Sr0.15Cu0.98M0.02O4 (M; Zn, Ni) and Fig. 5(a) shows
Kph(T) of the same samples. By substituting Zn (non-magnetic) or Ni (magnetic)
for Cu by only 2%, Tc is drastically depressed. The depression effect is somewhat
more conspicuous for Zn(T c =16K) than Ni (Tc=19K). In Fig. 5(a), the behaviorsof Kph(T) for LSCO (2% Zn) and LSCO (2% Ni) are quite similar and almost indistinguishable. In comparison to Kph(T) of non-substituted La1.85Sr0.15CuO4, Kph(T) ofFig. 5 (a) Kph vs T for La1.85Sr0.15Cu0.98M0.02O4 (M; Zn, Ni). (b) The fitting results for thesamples. The origin of the y axis is shifted by 10mW/cmK for each sample.

Zn and Ni Substituted La1.85Sr0.15CuO41185

Zn and Ni substituted LSCO is larger for 7X35K, which is, however, strongly
surpresscd below 7V35K. The results of the fitting and the determined parameters
are presented by solid lines in Fig. 5(b) and in Table I, respectively. These fittingssuggest that the phonon-electron scattering term (ETxg(x,y) in eq.(2)) remains
effective down to lower temperatures in the Zn and Ni substituted LSCO samples
owing to the depression of Tc.

4.

SUMMARY

The thermal conductivity K(T) of sintered La2-xSrxCuO4 (X=0.10, 0.15, 0.20)
and La1.85Sr0.15Cu0.98M0.02O4 (M; Zn, Ni) has been measured and the phononscattering mechanisms in these compounds have been analyzed.
(1) La1.85Sr0.15CuO4 showed the K enhancement below Tc. The enhancement isattributable to the phonon component Kph, and the electron-phonon coupling
parameter was determined to be X-0.05 from the size of the enhancement.
(2) The large residual electrical resistivity p(0) excluded the electronic origin for
the K enhancement in the present sintered samples.
(3) By substituting Zn or Ni for Cu by 2%, the enhancement in Kph became indetectable. Kph of Zn or Ni substituted LSCO was more strongly depressed than thatof the non-substituted mother compound below T<35K. This is because thephonon scattering by electrons still survives down to lower temperatures owing to
the reduced transition temperature T c caused by the substitution.
(4) The effect of the non-magnetic Zn substitution and that of magnetic Ni
substitution on the thermal conductivity are very similar.

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