polarized raman scattering from y1−xprxba2cu3o7 (0.1 ≤ x≤1) single crystals

Physica C 191 (1992) 477-484 North-Holland PHYSICA

Polar ized R a m a n scattering from _xPrxBa2Cu307 (0.1 _<x_< 1 ) single crystals

M.N. Iliev and G.A. Zlateva Faculty of Physics, Sofia University, BG- 1126 Sofia, Bulgaria

P. Nozar Faculty c~¢Mathernatics and Physics, Charles University, 121 16 Prague 2, Czecho-Slovakia

P. Stastny International Institute of TechnologT and New Materials/ICTP. HTS Laboratory, PO Box 586, 34100 Trieste, Italy

Received 31 October 1991 Revised manuscript received 6 December 1991

The polarized Raman spectra of Y: _xPrxBa2Cu307 _~ single crystals (~ ~ 0. 0.1 -< x-< 1 ) were measured at room temperature in various scattering configurations allowing one to distinguish the diagonal (.-~) and no~-~ 4,,real (B2~ and 33,) modes. The (zz)- Raman spectra, corresponding to incident and scattered radiation polarized parallel to the c-axis, are v,:rv similar to that of YBaECU30~. The (xx)-soectra, however, differ from those of both orthorhombic and tetragonal YBazCu307 ~ I he variations with x of the non-diagonal Bzg, B3g counterparts give evidence for a nearly constant anisotropy over the w~olc substitutional range.

l . In~oducfion

In contrast to the other RBa2Cu3OT_~ (R=rare - earth or Y) compounds that are superconducting with Tc~90 K, PrBa2Cu3OT_~ is a non-supercon- ductor even at T~0 , c ~ 0 . The system Y l_xPrxBaECU3OT_# is particularly interesting as one can follow the variation of its properties in both superconducting (x<0 .5 ) and non-superconducting (x> 0.5) regions.

The Raman spectra of Yl_xPrxBa2Cu307_~ have been studied by Radousky et al. [ 1 ] and In-Sang Yang et al. [ 2 ]. It has been found that three out of the five diagonal Raman-active modes change significantly as a function of Pr concentration. The Ag mode near 500 cm-~ corresponding to vibrations along the c-axis of the apical oxygens O (4) (see fig. 1 ) hardens by 15-20 cm-I , while the B~¢like mode at 340 c m - ' assigned to the out~of-CuO~-plane out- of-phase vibrations of O(2) and O (3) atoms softens by more than 40 cm -~ in going from x = 0 to x = 1. The Ag Ba-mode at 120 cm-" has also been found to

.-., 01

Fig. 1. Elementary cell of Y, _xPrxBa2Cu307.

harden by 8-10 c m- ~ in going from Y-123 to Pr-123. It has been suggested by Radousky et al. [ 1 ] that

the dominant effect of the Pr substitution on the ,~ 500 cm- ~ and ~ 340 c m - ~ modes is caused by the increased ionic size of the Pr ion relative to the Y

0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

478 M.N. lliev et aL / Polarized Raman scattering

ion. However, on the basis of comparison of changes with x of the interionic distances and frequencies of the ~ 120 cm- t and ~ 500 cm- i Raman modes in Yt_~-PrxBa2Cu307_,~ and Yl_xEuxBa2Cu3OT_6. In- Sang Yang et al. [2,3] have drawn the conclusion that the changes of these two modes in Yt _xPrxBazCu307_,~ are rather due to a localization of holes at Ba-sites thus giving an explanation for the suppression of superconductivity at x>__ 0.5. The latter assumption is consistent with the results of X-ray photo-emmission spectroscopy [4 ].

In this work we report the Raman spectra of single crystals of Y~ _xPr,BazCu30-_6 (0 < x < 1, ~ 0) for all important scattering configurations. The polarization properties of the five A~ Raman modes are compared with those in orthorhombic and tetragonal YBa2Cu307_6. At least four non-diagonal modes (B,g or B3g) modes were also observed and the variations of their frequencies with x were established.

2. Samples: preparation and characterization

man spectra taken at polarizations parallel or per- pendicular to these edges.

The X-ray diffraction analysis of the as obtained pellets showed that the crystal structure is definitely orthorhombic at x < 0.7. At higher Pr-concentrations the diffraction patterns could be fitted assuming either orthorhombic (Pmmm) or tetragonal (P4/ mmm) structure. The variations of the lattice parameters with x are shown in fig. 3 (a). The decrease of c at x>0.7 correlates with the appearance of a small amount of the "'minor" BaCuO2 phase. Ten- tatively, the latter indicates that Pr could substitute for part of the Ba atoms in Yt - ~PrxBa2Cu3OT-~.

The SEM analysis (Philips 515, EDAX 9100, size of the electron spot 0.15/am) confirmed that x var- ies within less than 5% for different grains within a pellet and corresponds to the nominal value. The oxygen content (7-3) as determined by iodometric titration decreases slightly with x (fig. 3 (b)) but remains very close to 7. The results of AC susceptibility measurements for x = 0.1 and x = 0.3 (fig. 3 (c) ) also show that the Tc-values correspond to a high oxygen col~.cnt.

The starting materials were pure powders of Y203, BaCO3C, CuO and Pr6Ol 1, heated for several hours at 1000 ° in order to remove chemically bounded water. The appropriate amounts of these materials corresponding to the desired stoichiometry were mixed in an agate mortar with iso-propanol, calcined three times for 12 h at 850, 860 and 910°C with in- termediate grinding and then pressed into pellets.

The pellets were sintered in oxygen at 950, 970 and 930°C following a heat treatment procedure illus- trated in fig. 2.

The visual observation of the polished surface of the pellets under an optical microscope showed that the average size of the microcrystals constituting the pellet was 10-20 ~tm. In polarized light most of the microcrystals in the samples with x < 0.6 and part of those in the samples with 0.7<x_<0.9 exhibited twinning. It is not clear, however, whether the rest of the grains are untwinned or the twinning takes place at a submicron level. The elongated projec- tions of the microcrystals were often of the form of rectangles with longer sides corresponding to a direction within the a-b plane. The latter assignment is confirmed by comparison of the shapes of the Ra-

3. Experimental

The micro-Raman spectra were measured at room temperature using a multichannel triple spectrome- ter (Microdil 28) equipped with a microscope. An objective of 100 × magnification was used for both focusing the laser beam (2L=488.0 nm) and col- lecting the scattered light. The laser power at the focus (2-3 ~tm diameter of the spot) was kept below 0.2 mW in order to avoid overheating.

Following a procedure described elsewhere [ 5 ] it was possible to select microcrystals with zx and xy orientation on the polished surface of each Y1- ~PrxBa2Cu307 pellet, where z is a direction that nearly coincides with the c-axis and x and y are two orthorhombic directions within the a-b plane.

The Raman spectra were measured in the most in- formative scattering polarization configurations, namely y(zz).~ and y ( x x ) ~ (where the Ag modes are allowed) as well as y ( x z ) y and y ( z x ) ~ (allowed for the B2g and Bag modes). It was observed that some A s lines shift towards lower wave numbers when excited with the same laser power along x instead of

M.N. lliev et al. / Polarized Raman scattering 479

T[°C} 28h

1000- 15h 950o C r____~ 970 oc

80o- _ i 02~oC 600-

t.00-

200-

l O - - - I I I / I I I I I I I | I I I [

20 t,0 0 20 /,0 60 ' 8'0 0 20 TIME[hours]

3 5 h

~0

930 % 33 h.__

750% \

} I 1 I ! 1 60 80 100

Fig. 2. Heat treatment procedures used during sintering the samples

......... 1 1 7 5

o °

z 3 9 . ~ 4~

38~" t ,,t+ " V~

a ] •

3 8 ~ ! •

3 3 8{, * . . . . . . . . . I O 0 0 5 1 (a) x ( P r c o n t e n t l

b < - 0 40 ~ H.~ = 10 Oe :

× o 6 o 4 ~, ~ 1o Hz i

- o 80 ! j I O0

1 2 0 ~ 0 50

(C) TEMPERATURE (K)

100

7°7i T i ~ 7 0 4

c700 ~_~ T T ~ " ij

c ~ 6 9 3 4 . . . . . . . . . 0 0 0 5 1 0

x ( P r c o n t e n t )

(b)

0 0 0 i

0 2 0 ~ [ x : 0 3 :

~: - 0 4 0

0 6 0

- 0 80

- I O0 i . . . ~ . , e

1 2 o ~ r 50

TEMPERATURE (K)

I 0 0

Fig. 3. (a) Lattice parameters of Yl_xPrJ~a2Cu307_& as esti- mated from the X-ray diffraction data. The squares for x=0,9 and 1.0 show the a = b values corresponding to a tetragonal (P4/ mmm) structure. (b) Oxygen content as determined by iodometric titration. (c) AC magnetic susceptibility for samples with x=0.1 and x=0.3.

the z direction. In order to verify that the latter effect is not related to variations of the temperature at the spot due to the different values of the anisotropic ab- sorption coefficient the spectra were measured also

in the y ( x + z, x ) 9 and y ( x + v, z)37 polarization configurations. The latter spect~_~ which are the super- positions y ( x x ) O + y ( z x k 9 and y ( x z ) y + y ( z z ) y , respectively, could be obtained by rotation of only the analysor and :L~,clbre all phonon frequencies correspond to the same temperature of the sample. The shift of the phonon frequencies at higher laser power when the xx and zz spectra are compared was confirmed in these experiments.

The acquisition t imes for the diagonal ( x x and zz) and non-diagonai ( x z and zx) configurations were up to 1000 s and 3000-5000 s, respectively. The spectral width was ~ 3 cm - j .

4. Results and discussion

The Raman spectra of Yl_xPr~Ba2Cu307 in y(zz )~ , y ( x x ) g and y ( x z ) 9 are shown in figs. 4, 5 and 6, respectively. In figs. 4 and 5 the spectra o for - thorhombic YBa2Cu307 (T¢=92 K) and of tetragonal non-superconducting YBa2Cu306.2 single crystals are also included for comparison. The intensity scale of fig. 5 is enlarged by a factor 3 with respect to that of fig. 4 with the exception of the case of YBa2Cu3Or.2 where the two scales coincide. In fig. 7 the spectra for various scattering polarizations are compared for x = 0.2. The variations of phonon frequencies with x are summarized in fig. 8.

480 M.N. lliev et al. /Polarized Raman scattering

[ i v(zz)~ i } r

! /\,

Y1 _ x P r x B et2 Cu:3 0 ?

' , . : , -.-o~ /-~!/\)k\ I

I ~ - . _ _ ~ - - ° ~ t l ~11 \ \ ' < l

~ 6 o 4 0 0 6 o o

R a m a n shift (era -I )

Fig. 4. Variations with x of the Raman spectra of Yl-~Pr~Ba2Cu307 in y ( z z )y scattering configuration. The laser power at the focus (spot diameter ~ 1-2 ~m) was 0.15 m W (3,L=488.0 rim).

4. I. Diagonal R a m a n modes

The results of figs. 4,5 and 8 are in agreement with those of Radousky et al. [ 1 ] and In-Sang Yang et al. [ 2 ] concerning the variation with x of the frequencies of the five diagonal modes seen in a parallel ( z z

or x x ) scattering configuration. However, the two- mode behavior for the out-of-phase out-of-plane mode (B~g-like mode), reported in ref. [2] was not confirmed in our experiments. We suggest that the nonmonotonical behavior of phonon frequencies versus x in ref. [2 ] is rather due to an erroneous de- termination of the value of x.

Increase of the laser power results in a weak shift of most of the lines towards lower wave numbers. As example, the Raman lines of the x = 0.8 sample, seen in the zz spectrum at 132, 155, 439 and 524 c m - ' for 0.05 mW excitation, shift to 127, 152.5, 437 and 521 cm- ~, respectively, when excited with 0.15 roW. Another effect is an increase of the relative intensity of the line at 570-580 c m - ' , seen in xx, x z and zx

polarizations. Both effects are reversible (at least up to 0.3 mW) which is evidence that the laser heating does not change the oxygen content.

y ( x x ) y

YBa2Cu307

! t Y 1 - x P r x B a 2 C u 3 0 7

G - ~ . _ - k x = l O

i - -

200 400 6 0 0

R a r n a n s h i f t ( c r r a - 1 )

Fig. 5. Variations with x of the Raman spectra of Yl _xPrxBa2Cu3Ov in y ( xx ) y scattering configuration. The laser power at the foc ' s (spot diameter ~ 1 - 2 ~tm) was 0.15 mW (AL=488.0 nmj .

Another effect pronounced at higher laser power is the appearance of some modes, e.g. those of Cu (2) and 0 ( 4 ) stretching vibrations, at different positions in the zz and x x spectra of Y~_xPrxBa2Cu307. At a fixed laser power the z z - x x splitting increases with Pr-concentration (see fig. 8). A z z - x x splitting is observed for the Ba-mode even with the lowest laser power used. Due to the weakness of the x x compo- nent, however, it is not clear, to what extent the z z -

x x splitting in this case is affected by the laser power. The comparison of the polarized spectra of

Y~_xPrxBazCu307 with those of orthorhombic superconducting and tetragonal non-superconducting YBa2Cu3OT_a shows that over the whole range of substitution (0.1_<x_<l) the y ( z z ) y spectra of Yl_xPrxBazCu307 are very similar to that of YBa2Cu307 (fig. 4). It follows from fig. 5, however, that the y ( x x ) y spectra of Yj _xPrxBa2Cu307 differ from those of both YBa2CuaO7 and YBa2Cu306 .2 .

Indeed, the y ( x x ) y spectrum of YBaECU307 is dom- inated by the Ba, CU(2) and 0 ( 4 ) lines, which are of comparable intensity, while for tetragonal non-superconducting YBa2Cu306.2 the Ba and 0 ( 4 ) lines are suppressed in favour of a strong enhancement of

M.N. lliev et al. / Polarized Raman scattering 481

Y ~ - x P r B a z c u a 0 7

y(xz)y

, ~ ~=o~

x = 0 . 3

d

i

- .

._o.o

x 0 9 ~ ,

200 4 0 0 600

R a r n a n s h i f t (ore-- 1) Fig. 6. Variations with x of the Raman spectra of Y= _=Pr~Ba_,CusO7 in y (xz)y scanenng configuration. The laser power at the focus (spot diameter ~ 1-2 gm) was 0.15 mW (2L=488.0 nm) .

the Cu (2) ( 140 c m - ~ ), O (2), O ( 3 ) -out-of-phase ( 340 cm- ' ) and O ( 2 ), O ( 3 )-in-phase ( 450 c m - ~ ) modes. For Y~ _xPrxBa2Cu307 the Ba and 0 ( 4 ) lines are much weaker than in the case of YBa2CusO7 and cannot be detected at x>0.5. Their suppression, however, is not accompanied by an increase of the modes related to the atoms of the Cu (2 ) -O (2), O (3) layers.

The Stokes Raman efficiency Sis for scattering by an optical phonon is given by [ 6 ]:

2

Sisoc (0)-1"2) 4 ~ = (to-K2)4l ~iXis(to, -Q) i'-, (1)

where i and s are the polarization-directions of the incident and the scattered light, to and ~ (t2<<to) are the laser znd phonon frequencies, respectively, Xi= the complex electric susceptibility, and Q the phonon normal coordinate. Theretbre, the different Raman line intensities at various scattering configurations, or their changes upon cation substitution

v

~>~

Yo.s Pro .2 Ba2Cy30?

I ~ .~ - =-,xz)y/\

< , ,

200 400 600

R a m a n shift (¢m-- I )

Fig. 7. Raman spectra of Yo sPro.2Ba2CusO, in y(zz).R y(xx)y, y ( x z ) y and y(zx))~ scattering configurations. The spectra are shifted for clarity. The intensity scale for the y (,:x)?Y spectrum is enlarged by a factor 3 and those of ttae y( x z ).f and y(zx)j~ spectra by a factor 10.

or oxygen removal reflect the corresponding changes in the second-order (derivative) susceptibility 8Xis ( to, .Q).

Recently, the absolute values of Szz and S~/ry for the five Ag phonon modes of YBa2CusO7 have been measured within the laser photon energy range 1.8 eV<hto<2.7 eV [7,8]. In ref. [7] the measured Ra- man efficiencies have been compared to those obtained from first-principles local density approximation (LDA) calculations. Good agreement between theory and experiment has been found not only in the absolute phonon intensities but also in the shape of the resonance profiles. Only interband contributions to the polarizability have been ac- counted for in the evaluation of the real and ima- ginary parts of the complex dielectric tensor ~is= 1 +Xis.

Within the latter approximation, the changes of the relative intensities of the Ag modes upon Pr substitution are related to changes of the band structure

482 M.N. lliev et al. / Polarized Raman scattering

,,ot. If 00, . . . . . . l i d II

300 " % 55°

500

= • y(xx)y IN y(zx)y

! o y(zz)y)a y(xz)y

ml t ~ l Im y(zx) ;

I I I

150 1 c ~ I 4 0 0 -

% I 100 . . . . . ~ . . . . . [ 350 - , . . . . . . . . I . . . . . . . . .

0 0.5 05 1 0 0.5 0.5 X (Pr content)

Fig. 8. Variations with x of the Raman line frequencies in the spectra obtained with an excitation laser power of0.15 mW (laser spot diameter = 1-2 ~tm. AL=488.0 nm) at different scattering geometries.

and electron-phonon interaction around the Fermi energy. The following experimental observations, however, could hardly be interpreted associating the second-order susceptibility with phonon-modulated interband electronic transitions alone. ( 1 ) Pr substitution changes significantly the relative Raman intensities in the x x spectra, whereas in the zz spectra they remain practically unchanged. (2) The substitution of Pr for Y and Fe for Cu [ 9 ] has the same effect. (3) The drastic changes in the x x spectra of Y,_.~PrxBa2Cu307 occur in the narrow range between x = 0 and x=0.1, whereas the changes at 0.1 _<x_< 1.0 are much more moderate.

In principle, the Raman intensities could be influ-

enced also by the existence o f free carriers. At low temperatures the Y,_,Pr~BazCu307 (x>0 .5 ) is an anfiferromagnetic insulator [ 10]. Free carriers o f relatively high concentration, however, are definitely present at room temperature over the whole substitutional range, as indicated by both resistivity and Hall measurements [ 11,12 ]. The semilog plot of p (T) versus 1 /T for PrBazCu307 shows strong de- viations from linearity [ 11 ] and cannot be fit with a single temperature-activated mechanism. This may i adicate the presence of more than one mechanism for conductivity in the normal state.

In the three-dimensional case, free carriers of high concentration screen the incident and scattered radiation, thus reducing the scattering volume. The effect o f two-dimensional conductivity on the Raman intensities seems to be more complicated and de- pendent on the scattering configuration. A contin- uation of polarized Raman studies for the Y, _xPrxBaaCu307 series (in particular for x > 0.5) at low temperatures will be extremely valuable to distinguis v between the effects of interband transitions ami free carriers on the Raman intensities.

Although the relative intensity of the peak at 580 era- ' increases with laser power, its height remains nearly unchanged if the spectra are normalized to the laser power. At the same power density the peak is stronger for oxygen-deficient Yl_xPr.,-Ba2Cu3OT_a samples. A detailed analysis of the experimental results and a discussion concerning the origin of this peak will be given elsewhere [ 13 ]. We will only men- tion here, that, following our preliminary results, the 580 cm -~ peak is relatcd to a maximum in the phonon density-of-states and corresponds to the Ra- man peak of amorphous YBa2Cu307 reported by Be- lousov et al. [ 14 ].

4.2. Non-diagonal R a m a n modes

The new spectral lines that appear in the y ( x z ) y and y ( z x ) y spectra at 130-138, 189-206, 295-315 and 565-580 c m - ' (figs. 4 to 6) are at least by an order of magnitude less intense than those observed in the y ( z z ) y geometry. While extremely weak, these peaks were reproducibly observed in all samples under study. These peaks exhibit the polarization de- pendence expected for B2g or B3g phonons in the or- therhombic structure.

M.N. lliev et aL / Polarized Raman scattering 483

There are several reports on the non-diagonal B2g and B3g modes in orthorhombic RBa2Cu307_a compounds [ 15-17 ]. Gasparov et al. [ 15 ] has assigned the 210 cm -~ and 310 cm - t lines of YBa2Cu307_o tO B2g and B3g vibrations of O ( 4 ) atoms along the a- and b-axis, respectively. Two weaker lines pronounced at 340 and 580 c m - ~ have been assigned to B2g and B3g modes o f O(2 ) and 0 ( 3 ) . In a more re- cent study McCarty et al. [ 16 ] established that in YBa2Cu307_aB2g and B3g phonon pairs are reproducibly found at 70 and 83 cm-~, 142 and !40 c m - J, 210 and 303 c m - ' and 579 and 526 c m - L In RBa2Cu307_a ( R = T m , Er, Dy, Gd, Eu, Sm, Nd and Pr) the frequency of the 210 cm -~ (B2g) line decreases roughly linearly with the ionic radius of the rare earth R [ 17 ].

Since, as a rule, B2g and B3g lines were observed together in our zx and xz spectra, the conclusion could be drawn that the grains under study were mi- crotwinned rather than untwinned.

The ve~, weak Raman features at 130-t38 c m - could straightforwardly be assigned to a superposi- tion o f B2g and B3g modes, involving motions pri- marily of Cu(2) atoms along the a- or b-axis. The calculations of lattice dynamics [ 18 ] for the frequencies o f these modes in YBa,Cu307 give values of 142 c m - t and 137 c m - t, practically coinciding with the experimentally observed ones (142 c m - and 140 c m - ' ). Upon substitution of Pr for Y the a and b parameters increase and, hence, the observed softening of the modes is reasonable.

The B2g-B3g pair assigned to O (4) vibrations along a and b is observed over the whole range 0.1 <x_< 1 (figs. 6 and 8). With increasing Pr content the relatively narrow B2g line shifts from 205-208 c m - ' to 188-190 cm -~ and decreases in intensity, most strongly between x = 0.1 and x = 0.5. The position and the intensity of the B3g line could not be determined as precisely because of its broadness and inevitable appearance in the same spectral range of the B~g-like mode through polarization leakage. Nevertheless, both the existence of the Bsg mode and its softening by 5-15 c m - ~ with replacement of Pr for Y are un- ambiguous. It is not clear, however, why the intensity versus x changes differ for the B2g and Bag lines.

The finding that the large splitting of the O(4)-related B2g-B3g counterparts in Y~_.,.Pr,-Ba_,CusOT_a slightly increases with x varying between 0 and 1,

while the a-b anisotropy as determined by X-ray and neutron diffraction decreases or disap0ears gives evidence that on a microscopic scale the structure of Y~_ ,.PrxBa2CusOT_ a remains ~rthorhombic, the a/b rat~:, ~eing nearly independent on x. Recently [9], similar behavior of this B2g-Bsg pair was observed also for the system YBa2(Cul_xFex)307_a (0.02_< x_< 0.15 ), which (following the X-ray and neutron diffraction data) undergoes an O - T struc- tural transition at x ~ 0.05.

Of the rest structure~ pronounced in y(zx)f, and y(xz)y geometries only the peak at 565-580 cm -~ for certain represents a B2g or B~, mode. Although another peak of comparabIe intensity is present in the same frequency range (at 589 cm -~ ), the two peaks are clearly distinguished in some spectra. Such a peak of B2g symme!r3 ~ has bee~ observed at 579 era- ~ in YBa2Cu307 [ 15 ] and ha~ ~een assigned to out-of-phase vibrations of 0 ( 2 ) and 0 ( 3 ) atoms along the b direction.

5. Conclusion

Over the whole range of substitution (0.1 < x < 1 ) the Raman spectra of YI-xPGBa2Cu3OT-a ( ~ 0 ) , in particular for incident polarization parallel to the a--h plane, differ from the well known spectra of both orlhorhombic and tetragonal YBazCu307_ a compounds. The variations with x of the non-diagonal B2g, B3g counterparts give evidence for a nearly constant anisotropy over the whole range of substitution.

Acknowledgements

Fruitful discussions with V. Hadjiev and M. Geor- giev are highly appreciated. We thank V. Skumriev for AC magnetic susceptibility measurements, E. Di- nolova for SEM analysis and L. Georgieva for her expert technical help. The work was supported by the Bulgarian National Foundation for Science.

References

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484 M.N. lliev et aL / Polarized Raman scattering

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polarized raman scattering from y1−xprxba2cu3o7 (0.1 ≤ x≤1) single crystals

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