Physica B 263—264 (1999) 691—694

Two-level-like anomalous phonon scattering in La Sr CuO
\V V
and La Sr MnO
\V V
Hiroyuki Fujishiro*, Manabu Ikebe
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan

Abstract
Thermal conductivities i of high-T cuprate La Sr CuO (0)x)0.30) and manganese oxides La Sr MnO
\V V
\V V
(0.11)x)0.30) have been measured and phonon scattering mechanisms have been analyzed. In both systems phononscattering is remarkably enhanced around a peculiar Sr concentration x, at which structural transformations occur. Thetunneling motion of oxygen atoms related to the structural transformation is suggested to be the main origin of theenhancement.  1999 Elsevier Science B.V. All rights reserved.
Keywords: Thermal conductivity; Phonon scattering; Oxide superconductors; Manganese oxides

Among (2 1 4) cuprate superconductors,
La Sr CuO system with the so-called T-phase
\V V
structure shows much smaller i values in comparison to Nd Ce CuO system with T-phase struc\V V
ture. The existence of apical oxygen atoms at bothsides of a unit Cu—O square and the structural
instability between the high-temperature tetragonal (HTT) and the low-temperature orthorhombic(LTO) phase seem to be the origin of the smaller
i values of the T-phase [1]. As materials related tothe high-T cuprates, manganese oxides with perov
skite structure, R A MnO (R is a rare-earth\V Vtrivalent ion such as La and A a divalent ion suchas Sr and Ca) have recently attracted renewed andextended interest because of several dramatic properties such as the colossal magnetoresistance. In* Corresponding author. Fax: #81-19-621-6373; e-mail:fujishiro@iwate-u.ac.jp.

La Sr MnO , MnO octahedra network exist
\V V

similar to CuO octahedra in La Sr CuO [2].
\V V
In this paper, we report the thermal conductivities
i of La Sr CuO (0)x)0.30) oxide supercon\V V
ductors and La Sr MnO (0.11)x)0.30) man\V V
ganese oxides. The phonon scattering mechanisms
operating in both systems are analyzed.
Specimens of La Sr CuO (0)x)0.30) (LSCO)
\V V
and La Sr MnO (0.11)x)0.30) (LSMO) were
\V V
fabricated by a standard solid-state reaction
method. Detailed fabricating procedures were reported elsewhere [1,3]. The X-ray diffraction analysis confirmed that all samples were in a single phase
except for x"0.30 of LSCO which showed a traceof impurity phases. The density of the samples washigher than 90% and 85% of each ideal density for
LSCO and LSMO, respectively. The thermal conductivity i measurement was made between
10—300 K by a continuous heat flow method with
an automatic measuring system [4].

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 4 4 - 6

692

H. Fujishiro, M. Ikebe / Physica B 263—264 (1999) 691—694

The heat conduction in conductors is due to bothelectrons (i ) and phonons (i ). For the LSCOsystem, we assumed that the electron contribution
in the superconducting state, i , follows the theory
proposed by Kadanoff and Martin with the d-waveenergy gap anisotropy, D"D cos 2 [5], and theelectron contribution in the normal state, i , fol
lows the Wiedemann—Franz law. Fig. 1 shows thetemperature dependence of the phonon thermal
conductivity i ("i!i ) of LSCO for various Srconcentrations. The pretty large i values of thepure La CuO sample (i "80 mW/cm K) aredrastically diminished by substituting La by Sratoms up to x"0.20 and then i again increases
with further increase of x. The reduction of i is
more pronounced in the low temperature region(T(40 K). In order to analyze the phonon scattering mechanisms, we calculated i (T ) on the basis
of the relaxation time approximation [6]. In thisanalyses, the phonon scattering rate q\ is assumed

to be given by the sum of contributions of various
scattering centers,q\"q\#s¹x#p¹x#E¹xg(x, y)Hx#ºx exp ! " #K¹x tanh .
b¹2





(1)

Here, x is the reduced phonon frequency and parameters q\, s, p and E represent the phonon scattering strength due to grain boundaries, sheet-likeFig. 1. The temperature dependence of the phonon thermal
conductivity i ("i!i ) and the calculated fitting curves for
i (¹) of LSCO for various Sr concentrations (0)x)0.30).
faults, point defects and electrons, respectively. Theterm ºx exp(!H /b¹) (b"2.0, H : Debye tem""perature) is a standard form of the phonon umklapp processes. g(x, y)"q /q is the ratio of the phonon—electron scattering rate in the normal andsuperconducting state, which depends on the energy gap through the parameter y"D(¹)/k ¹. The
last term K¹x tanh(x/2) stands for the scatteringby two-level tunneling states [7], which becomesdominant at low temperatures. The fitting curves
for i (¹) are also shown in Fig. 1 and the para
meters determined in the fitting processes are summarized in Table 1. It is worthwhile to notice that
the K value increases with increasing x up tox"0.20 and then drastically decreases with thefurther increase of x. We propose that the enhancement of the K value comes from the local structural
instability connected with the instability of the lowtemperature orthorhombic (LTO) phase against
the high temperature tetragonal (HTT) phase. The
structural phase boundary between LTO and HTT
was reported to exist at x&0.21 [8]. We infer thatthe dynamical tunneling motion of apical oxygens
is the most important origin of the K term phonon
scattering, because the apical oxygen plays a keyrole in the LTO to HTT transition.
Fig. 2 shows the temperature dependence of thethermal conductivity i(¹) of La Sr MnO for\V Vvarious Sr concentrations x. The i value forx(0.17 samples is suppressed to low values andthen drastically increases with the increase of the Srconcentration X from x"0.18 to x"0.20. In thisLSMO system, the phase boundary of the structural transformation between high temperaturerhombohedral and low temperature orthorhombic
lattice exists at X"0.17—0.18 [2]. The structural
instabilities may suppress the i values througha similar oxygen tunneling process as
La Sr CuO because the orthorhombic to\V Vrhombohedral transition at x&0.18 is also relatedto the tilting of MnO octahedra and shifts ofoxygen sites play a major role. The local minima ofi at T"240 K for x"0.155 and T"270 K forx"0.17 result from the enhanced phonon scattering due to ferromagnetic transition and the dynamical nature of the phonon scattering around theferromagnetic transition was pointed out in a previous paper [9].

H. Fujishiro, M. Ikebe / Physica B 263—264 (1999) 691—694
Table 1
Summary of the parameters determined in the fitting processes in La693

Sr CuO and La Sr MnO systems
\V V
\V VSample

xq\ (s\)

s(K\s\)p(K\s\)

E(K\s\)

U(s\)

K(K\s\)

LSCO

00.10
0.15
0.20
0.25
0.30

9.9;10
3.2;10
6.4;10
6.5;10
7.0;10
7.8;10

1.9;10
5.0;10
5.5;10
1.8;10
9.0;10
5.5;10

2.0;10
1.4;10
1.5;10
1.1;10
1.3;10
6.6;10

03.6;10
2.5;10
1.4;10
1.2;10
09.9;10
00002.3;10

1.1;10
2.2;10
2.5;10
3.5;10
2.2;10
0LSMO

0.135
0.170.180.200.30

1.0;10
9.5;10
9.1;10
6.4;10
4.0;10

1.1;10
9.5;10
9.1;10
4.3;10
4.4;10

2.8;10
6.2;10
5.5;10
1.3;10
1.1;10

000001.1;10
9.5;10
9.1;10
3.1;10
1.9;10

2.1;10
1.0;10
1.8;10
6.4;10
4.0;10Fig. 2. The temperature dependence of the thermal conductivity
i(¹) of La Sr MnO for various Sr concentrations
\V V(0.11)x)0.30).

Fig. 3. The calculated fitting curves for i (¹) of typicalLa Sr MnO samples.
\V V

The i behavior of LSMO was analyzed in thesame way as that of LSCO system. Since i is verysmall for the LSMO samples as estimated fromthe electrical resistivity values [3], we regarded themeasured i as i . In this case the coefficient of theelectron term E in the phonon scattering rate q\ in
Eq. (1) was assumed to be zero. The typical fitting
curves for i (T ) are shown in Fig. 3 and the para
meters determined in the fitting processes are also
summarized in Table 1. For x)0.20, the phonon
scattering strength represented by the K term remarkably increases. Thus the main origin of thephonon scattering enhancement in LSMO at lowtemperatures can also be explained by the K term
contribution. Thus the phonon scattering in both
La Sr CuO and La Sr MnO systems is\V V
\V V
heavily enhanced near the structural instabilities
and a dynamic phonon scattering mechanism suchas two-level-like tunnelings of oxygen atoms is likely to be at work commonly in La Sr CuO and\V VLa Sr MnO systems.
\V V
The authors wish to thank Mr. T. Mikami ofIwate University for his assistance in the calculation

694

H. Fujishiro, M. Ikebe / Physica B 263—264 (1999) 691—694

of the phonon thermal conductivity. This work is
partially supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science,Sports and Culture (No. 09555018).
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[2] A. Urushibara et al., Phys. Rev. B 51 (1995) 14 103.

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[6] L. Tewordt, T. Wölkhausen, Solid State Commun. 75 (1990)
515.
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[9] M. Ikebe, H. Fujishiro, Y. Konno, J. Phys. Soc. Japan 67(1998) 1083.