Effect of oxygen disorder on superconductivity-induced self-energy effects in impurity-free YBa2Cu3O7−δ

Solid State Communications, Printed in Great Britain.

Effect

of oxygen

Vol. 80, No. 8, pp. 643-647,

dirorder

V.G. Hadjiev’, Max-Planck-Institut

1991.

on superconductivity-induced YBasCusOr-6

C. Thomsen,

self-energy

A. Erbr , G. Miiller-Vogtr,

fiir Festkiirperforschung, (Received

0038-1098/91$3.00+.00 Pergamon Press plc

effects

M.R. Koblischkat,

Heisenbergstrasse

in impurity-free

and M. Cardona

1, D-7000 Stuttgart,

Germany

Sept. 24, 1991 by T.P. Martin)

We investigated various areas of impurity-free YBa&usOr_6 single crystals with Raman spectroscopy. A definite correlation between the appearance of the grouptheoretically forbidden chain-related Raman lines at 232 and 598 cm-’ and the superconductivity-induced self-energy effects observed for the 340 cm-’ &,-like mode was established: Disorder in the chains in oure crvstals has a similar effect as the introduction of impurity atoms, both leiding than increased gap value for 2A/kT, as deduced from the phonon self-energy effects. The absence of the forbidden chain modes in the Raman spectra may be used ss an extremely sensitive quality criterion of impurity-free crystals. The two sets of experimental data might be reconciled. if one considers the influence of the imnuritv atoms which stern from the crucible and are mea&t ih larger amount in single crystals than in ceramic pellets of the former. because of the higher rowth temperature Indeed, it has been s%own [lo] that the broadening of the 340 cm-’ line is svstematicallv reduced with increasing Au concentration in YB~&u~-~Au,O~_J. Although there is a sli ht increase in T, with Au substisup resses the tution for Cu (131, w%icb simultaneously broadening, a large increase of the linewidt % below T, seems to be a si ature of an impurity-free YBa&usOr_~ we give further evispecimen. In t8!is communication dence that the broadening below T, of the &,-like Raman line at 340 cm-’ is an intrinsic feature, not only for impurity-free [lo] but also for highly oxygen-ordered YBa&rnOT_r crvstals with 6 N 0. We arzue this bv establish& a correlation between the apcearance of the defect-induced Raman lines at 232 and 598 cm-’ and the temperature dependence of the linewidth of the 340 cm-’ line below T,.

Despite the availability of sizable single crystals of high- temperature superconductors [ 1) and an obvious trend towards a consolidation of experimental data [2], discrepancies among these data still exist. They concern not onlv the values of narticular nhvsical ouantities, such as-the ratio of gap’ to transition temperature 21, but also 2A/kT, determined by different methods those obtained by the same probe on di erent specimens [5,6].

d

Amoug the spectroscopic methods for the gap determination, a recent development [3-51 exploits the superconductivity-induced changes in the self-energy C of k’ = 0 optical phonons via electron-phonon coupling. The self-energy effects for a given coupling constant are expected to be strongest when a phonon energy is in resonance with the gap [3]. The most drastic changes in the phonon self-energy due to superconductivity have been observed for the 340 cm-’ &,-like that the phonon mode in YBa&usOr_6 [7,8] implyin energy is close to the gap. Below 8, all experiments provided

the sample

In our experiments we used YBa&usOr_6 single crystals grown in ZrOs and SnOs crucibles, in order to minimrze the effect of imnuritv contamination. We also measured a representatiie pellet and a thin granular film prepared by laser evaporation both featuring a large broadening of the 340 cm-’ line below T, [5,10], as well as a 10% Au doped pellet, which shows a sharpening of this mode [lo].

is

the changes in the linewidth (-Im(C)) of the 340 cmRaman line. In a series of experiments, a tremendous broadening of the 340 cm-’ line has been observed below T, for polycrystalline ram les [5,9], thin films [lo], and twinned crystals [8,1 l] whife another group reported 6,121 just the opposite behavior, a sharpening of this Iine, for untwinned single crystals. These results naturally lead to different values of 2A/kT, for polycrystalline and single crystal samples, 4.95 [5] and 6.8 - 7.7 [6], respectively.

The crystal, further denoted in the text as A-crystal was grown using the eutectic composition of the BaCuOsCuO sidesystem as a flux. After slow cooling the sepabefore soration of the crystals was done by decantin lidification of the flux. YrOs-stabilized Zr 8 s crucibles were used. Subsequently the crystals were annealed in flowing oxygen at 450°C for 10 days in order to raise the oxygen content.

* on leave from the Faculty of Physics, Sofia University, 5 Ivanov Blvd., BG-1126 Sofia, Bulgaria. + Kristall-und Materiallabor, UniversitBt Karlsruhe, Kaiserstr. 12, D-7500 Karlsruhe, Federal Republic of Germany. t Max-Plan&Institut Wr Metallforschung, Heisenbergstr. 1, D-7000 Stuttgart 80, Federal Republic of Germany.

The crystal so prepared was cut and ground into a rectangular shape and placed in a hotstage polarization 643

644

Vol. 80, No. 8

SUPERCONDUCTIVITY-INDUCED SELF-ENERGY EFFECTS

microscope under uniaxial pressure (100 N / m m ~ using a press described elsewhere [14]. By heating in nowing oxygen to a temperature of up to 450°C we were able to detwin the crystal within 10 minutes, which is favorable because a short heat treatment does not affect the oxygen content of the crystal. A few residual twins were seen with an optical microscope under polarized light. However, for this crystal, we collected P~rnan spectra only from the detwinned area denoted as DT-A. The second crystal - B was grown in a (Zn, Ti/doped SnO2 crucible by a slow-cooling method I1,151. For this crystal we took spectra from different representative areas: U T - B abbreviates a large untwinned area, HT-B and U T / H T - B stand for heavily twinned and closely neighboring untwinned areas, respectively. Here, for untwinned areas, we consider optical domains recognized to be twin free by means of polarized optical microscopy [16]. Besides, due to the specific polarization anisotropy of the spectra, R a m a n spectroscopy itselfcan distinguish well between untwinned and twinned parts of a given crystal [17].

The wet-chemical analysis of a piece of the A-crystal and of a crystal from the same batch as the B-crystal gave no detectable concentration of impurities. In particular, Zr atoms were not found in the A-crystal to within the experimental error (for the crystal mass used) of 0.I w t % Zr or 0.007 Zr atoms per YBa~Cu30¢_~ formula unit. The D C (SQUID) susceptibility measurements showed, that the two crystals are high-quality superconductors both with Tc = 91 K and 90 -10% transition widths of 2 K and 2.5 K for A- and B- crystals, respectively, in a field of 12 Gauss. All Ram.an measurements were performed using a microprobe technique on the setups described before ,18], equiped with microscopes and an optical liquidelium microcryostat. In this work we use the Porto notation for representing the differentscattering geometries. The symbols z, y, z stand for polarizations or directions along a, b, and c axis, respectively, z' and y' denote [110] and [II0] directions.

DT-A Ba C ~

E-I

We summarize the results displayed in Figs. 1 3 as follows: (i) No extra peaks under resonance were seen for the heavily twinned area HT-B and the untwinned strip near this area UT/HT-B, both in the B

out-ofphase AO2.3

i

Defect mode .

100 UT-B

'~

200

r

~,UT/HT-B

200F"TB

L

100l[

/~

t

A

l+zly,

v'~,' I

I

I

I

[

I

2O0 40O 6O0 Raman Shift (cm -I) Fig. 1 Polarized Rsman spectra of different samples: DT-A - detwinned area in the A-crystal (ZrO2 crucible); UT-B - large untwinned area, HT-B h e a v i l y twinned area, mad UT/HT-B untwinned area neighboring the heavy twinned one, all in t h e B-crystal (SnO2 crucible).

DT-A

~

The most striking feature of the z(I/,y)~' polarized spectra shown in Fig. I, is that, although all samples have almost equal oxygen content, as indicated by the frequency of the 04 peak, some of them exhibit two additional peaks at 232 and 598 c m - L These peaks are seen only under light polarized along 1/, i.e along the C u O chains. Indeed, given that the two peaks are absent in the (z, z) spectrum (not shown here), it follows from Fig. 2 that these modes can be exclted only if the electric-fieldvector of the incident light has a component along the C u O chains. In Fig. 1 we display the spectra excited with the 5682 ~, (2.18 eV) Kr + line, where the 232 and 598 crn-I peaks are strongest due to a resonant enhancement recently reported [19]. The strength of the peaks weakens significantlyfor an excitation with 5309 A (2.34 eV - Fig. 2), but stillpersists for A = 5145 ~, (2.41 eV - Fig. 3). Fig. 3 (upper spectrum) also demonstrates that the sample which does not have the two extra modes at resonance, does not show them either for relatively high power density out of resonance.

;k = 5682 ,& Defect mode

'00i-

k = 5309 A

A

II

z(y.y)~-

~'= 7

I

.~'50 -

~

r" E

z(x,x)~

II

. _ _ ~ j ~

z(x',

y')~

10 z(y', y')~

50

10

I

i

l

I

I

I

200 400 600 Raman Shift (cm -1) Fig. 2 Polarized Raxnan spectra of the D T - A sample taken from the ab crystal plane for the main polarization geometries. single crystal. The same result was obtained for both the undoped pellet and the thin film (ii)The two peaks are characteristicof the detwinned area D T - A and the large untwinned area U T - B in B-crystal, as well as for Au-doped pellets.

Vol. 80, No. 8

SUPERCONDUCTIVITY-INDUCED SELF-ENERGY EFFECTS UT/HT-B

~. = 5 1 4 5 A

z(y, y)~. 200 ~,1.7 mW

'~= -1

100

r~ 200 1.7 mW

>,

100

~

70 0.5 mW

20

I

[ 200

I

I 400

Raman

Shift

I

I 600

( c m -1)

Fig. 3 Rare.an spectra for excitations with different laser power. The laser spot was -'-2 pm. For the green 5145 ~ laser excitation, the peaks at 232 and 598 cm -1 disappear in the background for low power.

(a)

~_ [HT-B

z(x', y')~

--..

'E 30 o

;>,- 25

250

"O

?

300

350

400

Raman Shift (cm "])

.c: 20 ._1

.j/ 15

~o "

o~

oi-6..........0. ...........o............................ o Q DT-A , UT-B

Tc

° UT/HT-B 0 HT-B --)-~_L

~"342

(b)

~___~:

Eo340

~ 338 336 -I

o" 334 G)

/ ,4,~

Tc

,';- 332 100

200

300

Temperature (K)

Fig. 4 Temperature dependence of the linewidths (a) and frequencies of the 340 cm-* phonon for different samples measured in Blg-z(z/, I{)~' geometry. The inset shows that the effect of broadening of the line is equally strong for twinned and untwinned areas.

645

The dependence of linewidth and frequency of the 340 cm -1 phonon on temperature is given in Fig. 4. These data were deduced by fitting the 340 cm -1 peak intensity distribution to a Fano-type line shape [5]. All the samples show a softening of the 340 cm -1 mode below T: (Fig. 4b), as expected for superconducting crystals, although the degree of softening varies. The temperature dependence of the phonon linewidth is, however, very intriguing: the samples featuring no extra peaks at 232 and 598 cm -1 exhibit the tremendous broadening of the 340 cm -1 line below Tc reported for ceramic samples [5], while a sharpening is seen for those specimens which have additional peaks. The inset in Fig. 4 shows that the large broadening is equally pronounced in a heavily twinned and a neighboring untwinned part of the crystal. Note that the softening correlates with the broadening/shaxpening: it is larger for samples exhibiting broadening. From the results in Figs. 1 and 4 for the impurityfree crystals we find a definite correlation between the appearance of the two extra Raman peaks at 232 and 598 c m -l and the change of temperature dependence of the 340 c m -I phonon linewidth below Tc • The extra two modes belong to group-theoretically Ramanforbidden vibrations. In Fig. I we denoted the Raman lines according to their generally accepted assignment [17]. For the orthorhombic YBa~CuaOT_6 crystal structure there are SAg and 5(B2~ + Bs,) R a m a n modes allowed by symmetry . The Ag modes normally apone to two orders stronger of magnitude in the an spectra than the B2g and Bag modes [20]. For the scattering geometries used for the spectra in Fig. l, only the five Ag modes should be seen: the B2g and Bag modes occur only in (z,z) and (z,I/) polaxizations. One of the Ag modes, namely the 02-03 plane oxygen out-of-phase mode at 340 c m -t has special selection rules (see Fig. 2) reminiscent of the Bin mode involving the same vibrations in the corresponding tetragonal structure (i.e. YBaaCuaOs). The C u O chains contribute only to the/~ = 0 infrared-activeodd-~paxity modes 2(B1u + B2u + Bsu). From the unique potaxization propertiesof the 232 and 598 c m -l modes (Fig. 2), it follows that they stem from orthorhombic structural elements, i.e.from the C u O chains. In the double-chain material YBa2Cu4Os, the copper and oxygen atoms of the chain are no longer in a center of inversion and their vibrations become also Raman active. The frequencies for Cul and 01, observed in the Raman spectra of YBa2Cu4Os, axe 250 and 610 cm - l [18]. Thus we are confident that the two extra peaks in YBa2CusOT-6 at 232 and 598 cm -1 arise from vibrations of the CuO chains [17].

~

Modes, with similar frequencies, were also seen in oxygen deficient YBa2CusOT-~ (6 _< 0.5) [21,22]. On the other hand, no sign of these modes has been observed in the resonant Raman study of tetragonal YBa2CusO6a (see the x(y,y)~ spectrum for 2.18 eV laser excitation in Fig.l in [23]). Consistently with the findings that the modes at 232 and 598 c m -l are seen in YBa2CusO¢_6 for 6 < 0.5 and disappear when 5 -* I, we conclude that the two modes arise from forbidden (Jr-active)vibrations of the chain atoms which become R a m a n active via the introduction of defects in the chains. While it is reasonable to assign these modes to in-phase and out-of phase Cul-O1 vibrations 5], the mechanism for their excitation is still unclear. he selective electronic resonance for these modes as opposed to R a m a n allowed modes [19] (see also Figs. I

~

646

SUPERCONDUCTIVITY-INDUCED SELF-ENERGY EFFECTS

3), is not seen in the dielectricfunction as determined by ellipsometry data [24], and suggests transitions involving localizedcenters. They may be rather complex and formed by Cul, O1 and possibly 04, as well as the vacancies at the O1, 04 sites and occupied 05 sites. In YBa2CusOT-#, various types of oxygen order may be realizedfor a fixed value of 5. For 5 < 0.2,the fractional site occupancy of Ol and 05 sites can he changed by the final heat treatment [25].and is reflectedin a different a-b anisotropy of specimens with otherwise the same 5. Slow cooling should lead to higher anisotropy, i.e. to better ordering of the C u O chains[25]. The chain modes allowed by the introduction of defects should decay (the respective Raman lines have finitelinewidths) differently depending on the concentration of the defects. Such an influence may be recognized in the different linewidths of the 232 and 598 c m -l modes for D T - A and UT-B samples in Fig. I. One might expect the linewidth of the Raman-forbidden chain modes to increase and their intensityto decrease with increasing disorder in the chains and/or 5 ---,I. For chains with 5 ---,0 the intensitiesof the lines vanish with ordering. -

What is the origin of the different degrees of ordering in the C u O chains? W e have observed areas of perfectly ordered chains in heavily twinned and untwinned parts of the crystal produced by very slow final cooling. In a model for twin formation [26], the oxygen-depleted twin-boundary zones serve as the primary oxygen-diffusion paths from the grain surface toward the interiorof the grains. Thus the inner part of narrow twins and the area close to a region with a high density of twin boundaries are easilyenriched with oxygen and, under proper thermal treatment, may realize perfect order in the C u O chains. A similar argument holds for the thin granular film studied, where the grain boundaries take the place of the twinn boundaries in crystals. Ram.an scattering, however, probes only the surface layer of ~-500 ~ thickness [17,24]. One may argue that this layer could be oxygenated directly by diffusion from the surface and thus the discussed processes should not be important for a R a m a n probe. In fact, all specimen shown in Fig. I have almost equal oxygen content according to the usual R a m a n criterion for the frequency of the 04 mode at ,,,500 c m -I [27]. They differ,however, in the degree of ordering of the chains. These findings suggest that for the purpose of producing higher order in the surface layer accessibleto a R a m a n probe it is necessary for this process to begin deep in the bulk. Having established that disorder in the C u O chains causes the appearance of two particular,normally Raman-forbidden modes, we discuss further the possible reasons for the superconductivity-induced broadening/ sharpening of the 340 crn-l peak in impurity-freecrystals with perfect chains. R a m a n scattering from phonons takes place via electrons and can proceed only through electron-phonon coupling. There are several processes that can contribute to the experimentally observed phonon linewidth "/ ( H W H M ) . The most comm o n contribution 7o to 3' comes from the temperaturedependent anharrnonic decay of the Rarnan-active phonon into two phonons [28]. In the high-T= materials phononic R a m a n scatteringis superimposed on a broad electronic continuum of so far unknown detailed origin. W h e n the two scattering finalstates axe coherent, as is the case of YBa2Cu307-# in the normal state, a antum interferencearises,resultingin an asymmetric o-type) line shape [29]. In the approximation of a continuum the linewidth acquires an additionalcontribution xpg ~ [29], p and g being the electronicden-

Vol. 80, No. 8

sity of states and electron-phonon matrix element for the two scattering sources. It is essentialto note that the value of "/deduced from a fitwith a Fano-type line shape is actually 7 = "Yo+ 7fPV2 i.e. 7 always includes ~rpV2. Below To, changes in "y are expected mainly due to two mechanisms. According to the Zeyher and Zwicknagl's calculations [3], phonon self-energy changes appear below Tc due to the strong intera~:tion of optical phonons with superconducting quasipartides. They predict a broadening of the Raman lines for phonons with energies above the gap because of relaxation via pair-breaking. They cannot expl~in, however, a sharpening of the 340 cm -I line observed [6,12] because of the implicit assumption of k conservation and Ak ~ 0 transfer in the Rarnan process. Violation of these assumptions is required for a finite density of electronic excitations p in the normal state. In this case p may decrease below T~ for w < 2A, narrowing the line with respect to its normal-state value. For ~a > 2A, the phonon experiences the additional density-of-states pile-up in p from the opening of the gap, and thus it broadens.

The broadening of the 340 cm -1 peak in perfectchain crystals below T= may alternatively arise from an interaction of this phonon with the electronic continuum involving transitions between states of the chains and planes. The states contributing to the transition may be sensitive to disorder or alloying. Indeed, both the electronic continuum and the strength of the Fano interference (uymmetry of the line) decrease with increasing oxygen deficiency and perhaps with oxygen disorder in the chains. Besides, the tremendous broadening of the 340 cm -l line is always accompanied by a strong asymmetry (the inset in Fig. 4), while the sharpened line is closer to a Lorenzian [6]. Finally, the main findings in this work fit well the picture previously proposed to explain a similar effect due to the presence of impurities such as gold [I0]. Since the variation of the oxygen content in YBaaCusOv-6 can change drastically the physical properties of this compound from a superconductor to an insulator,the disorder in the C u O chains may have an effect similar to the introduction of impurities. It may smear the gap anisotropy, leading to increase in the minimal gap to which the R a m a n modes have been proposed to couple [10]. This finding, together with the previous results [10],reconciles the experimental data on superconductivity-induced self-energy effects in YBa2Cu3OT-s availableup to now. The gap anisotropy is snmared by the presence of impurities and by disorder in the C u O chains. The latteris not surprisingsince the chain atoms have large weight at the Fermi surface

[3o].

In conclusion, we have established a correlationbetween the appearance of the symmetry-forbidden Ram a n peaks at 232 and 598 c m -t, both arising from the CuO-chaln vibrations, and the self-energy effects below T= in impurity-free YBa2Cu3Ov-~. Disorder in the chains of pure crystalshas the same effectas the introduction of impurity atoms [10]: both lead to a sharpening of the 340 crn-l B1e-likepeak in YBa~CusOr_~ below To, with the consequence of increasing the value for 2A/kT= deduced from the self-energy effects. In addition to the frequency of the 04 mode usually employed as a measure of the oxygen content in YBa~Cu3Ov-#, we propose the absence of the yellow-resonance-enhanced chmn modes to be a complementary criterionfor highquality, oxygen-ordered, and impurity-free samples of this compound.

Vol. 80, No. 8

SUPERCONDUCTIVITY-INDUCED SELF-ENERGY EFFECTS

We thank B. Friedl for fruitful discussions, O. Bureach for the wet-chemical analysis and R. Kremer for magnetic susceptibility measurements. The expert technical help from H. Hirt, M. Siemers and P. Wurster is highly appreciated. One of us (V.G.H.) acknowl-

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