Dependence of the excitation wavelength on the Raman-active phonons of YBa2Cu3O7

PhysicaC200 (1992) 315-322 North-Holland

Dependence of the excitation wavelength on the Raman-active phonons of YBa2Cu307 Search for Landau damping

in single-domain

crystals

K.F. McCarty, J.E. Schirber and D.R. Boehme Sandia National Laboratories, Livermore, CA 94551 and Albuquerque, NM 87185. USA

H.B. Radousky Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

J.Z. Liu and R.N. Shelton Department of Physics, University of California, Davis, CA 95616, USA

Received 1 July 1992

In single-domain crystals of YBa#&O,, we examine the dependence of phonon linewidth on wavevector by varying the wavelength of the exciting laser. In three crystals, we find the linewidths of the Raman-active phonons at 120 and 150 cm-’ to be nearly invariant for excitation wavelengths between 647 and 4 13 nm. That is, we find no broadening of the phonon linewidths with increasing phonon wavevector (decreasing excitation wavelength), and thus no evidence of Landau damping in our crystals. In addition, the correlation between the intensity of the chain-related Raman feature at -232 cm-’ and the temperature dependence of the B,,-like phonon at 340 cm-’ has been investigated. In all three crystals, we find a net sharpening of the 340 cm-’ phonon below r, and essentially no intensity for the - 232 cm-’ mode under resonance conditions.

1. Introduction Recently Fried1 et al. [ 1 ] measured the phonon linewidths of a YBa2Cu307_-x film (T,(R=O) =87 K) as a function of wavelength of the exciting laser. Unlike the 440 and 500 cm- ’ phonons, the 120 and 150 cm-’ phonons were found to broaden with increasing wavevector (decreasing excitation wavelength). Their interpretation of this effect was that of Landau damping. That is, for wavevectors above some onset value determined by the maximum Fermi velocity of the electrons, the low-frequency phonons decay into single-electron excitations. It is significant that the Raman-scattering properties of YBaZCu307_-xare much more sensitive to the details of sample origin and processing history than is T,.The temperature dependence of the B,,like phonon at 340 cm-’ is most dramatically affected. In different samples with T,of nominally 90 K, the superconductivity-induced effects observed

for this phonon range from a large net broadening to a large sharpening [ 21 between T> T, and T+O [3,4]. Altendorf et al. [ 51 have recently examined a series of YBa2Cu307_-x crystals with varying oxygen content and found a net broadening for XX 0.05 and a net sharpening for 0.05 5x5 0.15. Hadjiev et al. [ 6 ] have observed large variations in the 340 cm-’ phonon for different regions within the same crystal. They correlated the behavior of the 340 cm-’ phonon with the intensity of a formally-forbidden chain-related mode at 232 cm-‘. The 340 cm-’ phonon broadened below T, when the 232 cm-’ mode was absent, and the 340 cm-’ phonon sharpened when the 232 cm- ’ mode was present [ 61. In three single-domain crystals, we have varied the excitation wavelength to determine how the linewidths of the Raman-active phonons vary as a function of phonon wavevector. We find the linewidths of the 120 and 150 cm-’ phonons to be nearly invariant for excitation wavelengths between 647 and

0921-4534/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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K.F. McCarty et al. /Dependence qfexcitation wavelength on Raman-active phonons

413 nm. That is, we find no evidence of Landau damping in our YBazCu30, crystals. In addition, the correlation between the intensity of the chain-related Rarnan feature at -232 cm-’ and the temperature dependence of the phonon at 340 cm-’ has been investigated [ 61. In all three crystals, we find a net sharpening of the 340 cm-’ phonon below T, and essentially no intensity for the - 232 cm-’ mode under resonance conditions. 10Oe 2. Experimental facts Crystals of YBa,Cu30, were grown in either gold or zirconia (ZrOz) crucibles [ 7,8], and characterized in a SQUID magnetometer and by X-ray diffraction (XRD). The (001) Bragg diffraction peaks with I= 1O-14 were averaged to determine a c-axis lattice parameter. The zirconia-grown crystal, referred to as YBa&u,O,-Zr-An, has been previously studied [ 81. The crystal was mechanically detwinned and then annealed for 5 days at 450°C in flowing oxygen before Raman analysis. YBazCu30,Zr-An had a superconducting transition temperature of 9 1 K and a 1O-90% transition width of 2-3 K (see fig. 1 of ref. [ 8 ] ). From the measured c-axis lattice parameter of 11.676 A, we estimate an oxygen stoichiometry of O,.,, [ 9, lo]. Two gold-grown crystals were annealed in flowing oxygen at 450°C for 5 days and then mechanically detwinned. One crystal, referred to as YBazCu30,Au-#30, was further annealed at 420°C for 6 days, followed by 1 day at 385°C. This crystal exhibited an onset of diamagnetism above 9 1 K in an applied field of 10 Oe, with a lo-90% transition width of about 2 K (see fig. 1). XRD analysis showed the caxis lattice parameter to be 11.7 11 8, after annealing. The second gold-grown crystal, referred to as YBazCu30,-Au-HPO, was annealed in 3 kbar of O2 at 400°C for 12 h after detwinning. The c-axis parameter was 11.722 8, before and 11.7 16 A after the high-pressure treatment. The gold-grown crystals are known to be nearly saturated with gold (i.e., YBazCu2.9,,Au0.,,0,) [ 2,8], which expands the c-axis relative to gold-free crystals [ 111. Unfortunately, this complicates the determination of an oxygen content from the XRD analysis. However, by assuming that the slope of the c-axis length versus oxygen content

T(K) Fig. 1. Zero-field-cooled (ZFC) magnetization analysis in a SQUID magnetometer of twinfree YBa&u,O,-Au-#30 (open circles) and YBa2Cu,0,-Au-HP0 (filled circles) crystals grown in gold crucibles. Crystals aligned with the a-b planes along the magnetic field of 10 Oe.

relationship is the same as in gold-free YBazCu30,_, [ 91, we can estimate the change in the oxygen content of YBa&u,O,-Au-HP0 due to the high-pressure oxygen treatment. Then, the decrease of the caxis by 0.006 A corresponds to increasing the oxygen content by Ax= 0.04. The YBazCuJO,-Au-HP0 crystal exhibited an onset of diamagnetism above 92 K in an applied field of 10 Oe, with a lo-90% transition width of about 2 K (see fig. 1). Overall, the high-pressure oxygen treatment had little effect on the superconducting properties; before the highpressure treatment, the crystal exhibited an onset of diamagnetism at 93 K in an applied field of 20 Oe, with a lo-90% transition width of about 2.5 K. For the crystal before high-pressure annealing, both the electronic-continuum scattering [ I 1 ] and the temperature dependence of the phonon linewidths [2] have been previously reported. All crystal surfaces were cleaned of potential surface contaminants by etching in a solution of 1% bromine in methanol. Raman measurements were made at room temperature in a 180” backscattering geometry using the 647, 568, 53 1, 476, and 4 13 nm lines of a krypton ion laser. Scattering geometries y(zz)y and x(zz)X were employed to create phonons with wavevectors parallel to the b and a axes, respectively. The magnitude [ 121 of the phonon wavevector q is proportional to the refractive index

K. F. McCarty et al. /Dependence

of excitation wavelength on Raman-active phonons

( nL) of YBazCu@_, at the laser wavelength and is inversely proportional to the vacuum wavelength (A,) of the laser: q=4mL/AL. The observed linewidths were corrected for instrumental broadening [ 13 1, taking the instrumental response as the width of the exciting laser line (half widths equal to 0.7, 1.1, 1.5, 1.3, and 2.1 cm-’ forthe 647, 568, 531,476, and 4 13 nm laser lines, respectively). Raman measurements of the 340 cm-’ phonon as a function of temperature were made using 5 14.5 nm excitation for the YBa2Cu30,-Au-HP0 and YBa#&O,-Au#30 crystals, and 520.8 nm excitation for the YBa2Cu30,-Zr-An crystal.

3. Results 3.1. Wavelength dependence of the phonon linewidths Figure 2 illustrates the linewidths of the 120, 150, 440, and 500 cm-’ phonons of the YBa2Cu30,-ZrAn crystal using 413, 476, and 647 nm excitation.

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Figure 3 shows the linewidths of the 120 and 150 cm-’ phonons as a function of laser wavelength for all three single-domain crystals. There is little variation in the linewidths of the 120 and 150 cm-’ phonons as a function of laser wavelength. Specifically, no broadening is observed upon going to shorter excitation wavelengths. This is true for wavevectors qlla and qllb. Figure 3 also illustrates the results of Fried1 et al. [ 11, where the large broadening of the 120 cm-’ phonon upon changing from 531 nm excitation to 476 nm excitation was ascribed to Landau damping. For the YBa$&,O,-Zr-An crystal, the precise spot at which the wavelength-dependent measurements (see figs. 2 and 3 ) were made had been previously examined as a function of temperature. The results for the 340 cm-’ phonon (see fig. 7(a) of ref. [8]) show a significant sharpening between T> T, and T%4 K. This is in contrast to the thin-film samples of Fried1 et al. [ 1 ] in which Landau damping was postulated to occur in the 120 and 150 cm-’ phonons; they found the 340 cm-’ phonon to have a very large net broadening below T,. In our present study

175 350

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I

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Shift

I

_I_

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(cm-‘)

Fig. 2. Raman spectra from YBa#&,O,-Zr-An crystal in a y(zz)y geometry (phonon wavevector qjlb) using 413, 476, and 647 nm radiation. The instrument-corrected linewidths (FWHM) are determined from fits to Lorentzian lineshapes for the 120, 150, and 440 cm-’ phonons, and to a Fano lineshape for the 500 cm-’ phonon. Spectra are offset for clarity.

K.F. McCarty et al. /Dependence

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of excitation wavelength on Raman-activephonons

citing and scattered radiation are polarized along the Cu-0 chains (y=b direction). The 232 cm-’ mode is resonantly enhanced, with maximum intensity occurring for excitation near 2.2 eV ( - 564 nm excitation ) [ 141. Since at low temperature the 232 cm- ’ mode can be optically bleached by photons above 2.2 eV in energy, the presence of an optically-induced metastable state has been suggested [ 141. The mode has been attributed to an IR-active mode that derives Raman intensity due to oxygen defects in the Cu-0 chains [ 61. Alternatively, it has been suggested that the mode is an intrinsic feature of YBaZCu307 [ 141. Figure 4 shows spectra from each of the crystals with polarization along the Cu-0 chains (i.e., a (yy) polarization geometry) under resonance conditions (568.2 nm excitation). In all crystals, there is at most only the slightest hint of the 232 cm-’ mode. This observation suggests that the feature is not intrinsic

150~cm-’ phonon

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‘E

0 16 120~cm

8

:

400

’ phonon

I 500 550 Laser Wavelength (nm)

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650

Fig. 3. Linewidths (FWHM ) and frequencies of the 120 and I50 cm-’ phonons as a function of laser excitation wavelength. The circles are for the YBarCurO,-Au-HP0 crystal, the squares for the YBa#&O,-Au-#30 crystal, the triangles for YBa$u,O,-ZrAn, and the crosses are the thin film data of Fried1 et al. (qllab) [ 11. Open symbols represent phonon wavevectors along the a direction (qlla) and filled symbols represent wavevectors along the b direction (411b).

of gold-grown crystals, the 340 cm- ’ phonon was not measured as a function of temperature at the precise spot of the wavelength dependent measurements. However, such measurements at different spots (see below) and all previous measurements [ 2,7 ] of goldgrown crystals have shown a sharpening of the 340 cm- ’ phonon below T,. 3.2. Relationship between chain-related mode at

232 cm-’ and behavior of the 340 cm-’ phonon In the Raman scattering of YBa2Cu@_-x, a peak at -232 cm-’ is sometimes observed when the ex-

0

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Raman Shift (cm-‘) Fig. 4. Raman spectra of YBa&urO,-Au-HPO, YBazCu,07-Au#30, YBazCu307-Zr-An crystals using 568.2 nm laser radiation. Essentially no Cu-0 chain-related modes at - 232 cm-’ are observed in any of the crystals. Regions analyzed are the same as in the temperature-dependent measurements of fig. 5. Polarization of the incident and scattered light was parallel to the Cu-0 chains (JjY).

K.F. McCarty et al. /Dependence

_fi

18-

‘5 ;5

of excitation wavelength on Raman-active phonons

- _F 18-

-

0

oo

16-

?j 14e 12-

0

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14-

12YBazCu30,-Au-HP0 (x’y’) -

I 0

-

-E - J

-

0

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-c N :16- -

O0

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*Ed

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loo 150 200 250 300 350 Temperature (K)

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10

q

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0

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YBazCu90,-Au-#30 (yy) I

I

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100 150 200 250 300 350 Temperature (K)

Fig. 5. Linewidths (full width) and Fano asymmetry factors of the 340 cm-’ phonon of the YBa&u,O,-Au-HPO, YBa2Cug0,-Au-#30, YBa#&O,-Zr-An crystals. Solid lines are guides to the eye. Data obtained with polarization of the incident and scattered light parallel to the Cu-0 chains (yy), or with polarization of the incident light at 90” to the scattered light and both polarizations at 45” with respect to the chains (x’y’ ).

to either the YBa2Cu30, structure [ 141, or to golddoped YBa2Cu307 [ 61. The presence of the -232 cm-’ feature has also been correlated with the sharpening of the 340 cm-’ phonon [ 6 1. For the same points on the three crystals where the intensity of the 232 cm-’ mode was examined (see fig. 4)) fig. 5 shows the temperature dependence of the 340 cm-’ phonon. Importantly, in all three crystals, we find a net sharpening of the 340 cm-’ phonon between T, and T-+0 and an absence of the - 232 cm- ’ mode. This is in sharp contrast to the work of Hadjiev et al. [ 61, where the 232 cm- ’ feature was by far the most intense peak in spectra of samples that showed a net sharpening of the 340 cm-’ phonon below T,. Between T> T, and T-0, the 340 cm-’ phonon softened by 6 cm-’ for the zirconia-grown crystal, versus about 4.5 cm-’ for the two gold-grown crystals. Figure 5 also shows the Fano q factor, a measure

of the asymmetry of the lineshape, for the 340 cm-’ phonon of the three crystals. At all temperatures, the 340 cm- ’ phonon of the zirconia-grown crystal is less symmetric (smaller (q I) than the same phonon in the gold-grown crystals. While the asymmetry of the phonon in the zirconia-grown crystal changes little below T,, the phonon in the gold-grown crystals becomes significantly more symmetric below T,.

4. Discussion In YBaZCu307_n the variability of the temperature dependence of the phonon linewidths and frequencies to small changes in processing conditions is marked [ 5 1. Apparently, such variability also extends to the dependence of the phonon linewidths on excitation wavelength: in single crystals, we find the linewidths of the 120 and 150 cm-’ phonons to vary

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ofexcitation wavelength on Raman-active phonons

little for excitation between 647 and 413 nm. In contrast, Fried1 et al. [ 1 ] found the 120 and 150 cm-’ phonons to broaden considerably upon changing from 531 nm to 476 nm excitation. The magnitude of the broadening they observed is surprising; at the largest phonon wavevector, it was about two times that predicted from the local-density approximation calculations [ 11. The magnitude of the phonon wavevector (q) at which intraband electronic transitions begin to contribute to phonon linewidth is determined by the maximum electron velocity (u,,,) and the phonon frequency: w= q v,,,,, [ 15 1. The variable behavior of the 120 and 150 cm-’ phonons as a function of excitation wavelength suggests that there are underlying electronic structure differences between different samples. It is noted that the Landau damping was observed in films in which the 340 cm-’ phonon broadened greatly below T,, while in our crystals, this phonon has a net sharpening below T,. Altendorf et al. [ 5 ] have recently examined a series of YBazCu30,_, crystals with varying oxygen content and found a net broadening of the 340 cm-’ phonon below T, for ~~0.05 and a net sharpening for 0.05 ~~~0.15. The YBazCuXO,-Zr-An crystal (see fig. 5 ) looks very similar to crystal B of Altendot-f et al. [ 5 ] in both the behavior of the linewidth of the 340 cm-’ phonon and the magnitude of softening below T, ( - 6 cm- ’ ). The gold-grown crystals look very similar to their crystal C, the optimally doped crystal with the highest T, and with oxygen content of about 06.95. The amount of softening is smaller ( - 4 cm-’ ) than in the crystals with higher oxygen content but lower T,. In the present study, there are discernible differences between the gold-grown and zirconia-grown crystals in the extent of sharpening and softening of the 340 cm- ’ phonon (see fig. 5 ). There are also differences in the lineshape symmetry below Tc: for goldgrown crystals, the lineshape becomes significantly more symmetric below T,, while the asymmetry for the zirconia-grown crystal changes little (see fig. 5 ). This can be interpreted as a significant reduction below T, of electronic states at 340 cm- ’ for the goldgrown crystals [ 7 1. Compared to the present study, previous analysis of zirconia-grown crystals [ 7,8 ] showed a larger amount of net sharpening and a smaller amount softening ( - 4 cm-’ ) of the 340

cm- ’ phonon below T,. Consistently, the Fano q factor for these previous analyses indicates that the phonon lineshape becomes more symmetric abruptly below T, (see fig. 7 (a) of ref. [ 8 ] and fig. 2 of ref. [ 71). In one aspect, the literature regarding the 340 cm-’ phonon is consistent: when it greatly broadens below T,, it also becomes less symmetric and has the largest softening ( - 8 cm-’ ) [ 1,36,16,17]. Conversely, when the phonon sharpens significantly below T,, it becomes more symmetric, and has a smaller amount of softening ( - 4 cm- ’ ) [2,5,7,8,18-211. The results of Altendorf et al. [ 5 ] are intriguing in that the highest T, was obtained for an oxygen content of 06.95, and T, actually decreased as the oxygen content approached O,.‘,. Such “overdoping” of YBaZCu307_,~ has also been observed in thin films grown in low pressures of oxygen and has been attributed to Ba2+ substitution on the Y3+ sites [ 221. For optimal doping within this view, the oxygen content needs to be below 0, in order to compensate for the additional hole doping from the antisite disorder. An alternative explanation of “overdoping” in YBa2Cu307_-x is the two-gap model of Kresin et al. [ 23 1, where T, can be depressed as the perfection of the Cu-0 chains improves. Here, there is a proximity effect between the intrinsically superconducting planes and the “normal” chains [ 23 1. This proximity effect, which depresses T,, is enhanced when the chains are highly conductive (i.e., few vacancies) [ 231. In our gold-grown crystals, it appears that they cannot be “overdoped” with oxygen in that annealing in 3 kbar of O2 at 400’ C at most decreased T, by about 1 K. Consistently, this treatment [24] had little effect upon the linewidth of the 340 cm-’ phonon and the magnitude of the phonon softening below T, (compare fig. 5 to figs. 1 (a) of ref. [ 7 ] and 7 (b ) of ref. [ 8 ] ). The effect of gold doping upon T, can be viewed in two ways: in the two-gap model [ 231, gold substitution on the Cu chain sites decreases the conductivity of the chains [ 111, which decreases the proximity tunneling between the planes and chains, which in turn diminishes the reduction of T, from the proximity effect. Alternatively, gold doping results in Au3+ substitution on the Cu2+ chains sites [ 111, which decreases the net hole doping of the planes. This decreased doping then precludes any “overdoping” and the resulting depres-

K.F. McCarty et al. /Dependence

of excitation wavelength on Raman-active phonons

sion of T, even as oxygen content approaches 0,. In our single domain crystals, we find essentially no intensity for the chain-related mode at 232 cm-’ and we find that the 340 cm-’ phonon sharpens below T,. This is in marked contrast to the results of Hadjiev et al. [6], where the 232 cm-’ mode was the strongest spectral feature in crystals where the 340 cm-’ phonon sharpened below T,. While the relative intensity of the 232 cm-’ mode may be an indication of the perfection of the Cu-0 chains, the presence of the mode is clearly not a necessary condition for the 340 cm-’ phonon to sharpen below T,. The Stuttgart group has used experimental determinations [ 4,171 of phonon self-energy effects in YBazCu30, and calculations within a strong coupling (1~ 2.9) Eliashberg formalism [ 25 ] in order to determine quantitatively the energy gap. To explain the variable behavior of the 340 cm-’ phonon, the Stuttgart group then suggested that the energy gap of the a-b plane was highly anisotropic, with a minimum below 320 cm-’ and a maximum above 570 cm- ’ [ 261. Combescot [ 27 ] has shown that within Eliashberg theory, the energy gap becomes isotropic in the strong coupling limit [ 28 1. More to the point, Combescot also demonstrates that for Az 3, any gap anisotropy must be reasonably small. Therefore, it is inconsistent to determine the energy gap based upon strong-coupling Eliashberg theory [4] and to postulate a highly anisotropic gap [ 261.

5. Summary In three single-domain crystals of YBa$&O,, we find the linewidths of the 120 and 150 cm-’ phonons to be esstentially invariant for excitation wavelengths between 4 13 and 647 nm. Thus, we find no evidence of Landau damping in our crystals, unlike a previous study of thin films [ 11. This variable behavior of the 120 and 150 cm-’ phonons as a function of excitation wavelength, along with the variable temperature dependence of the phonon linewidths, suggests that there are underlying electronic structure differences between different samples. In the same three crystals, we find a net sharpening of the 340 cm-’ phonon below T, and essentially no intensity for the chain-related mode at 232 cm-’ under resonance conditions. Therefore, the

321

presence of the 232 cm-’ mode in a sample is not a necessary condition for the 340 cm-’ phonon to sharpen below T,.

Acknowledgements The work at SNL was supported by the USDOE, Office of Basic Energy Sciences, Division of Material Sciences under contract DE-AC04-76DP00789. The work at LLNL and UC-Davis was performed under the auspices of the USDOE under contract W-7405ENG-48. Research at UC-Davis also supported by the NSF under Grant #DMR 90-2 1029.

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[ 151 O.K. Andersen, AI. Liechtenstein, 0. Rodriguez, 1.1. Mazin, 0. Jepsen, V.P. Antropov, 0. Gunnarsson and S. Gopalan, PhysicaC 185-189 (1991) 147. [ 161 M. Krantz, H.J. Rosen, R.M. Macfarlane and V.Y. Lee, Phys. Rev. B 38 ( 1988) 4992. [ 171 C. Thomsen, M. Cardona, B. Friedl, C.O. Rodriguez, 1.1. Mazin and O.K. Andersen, Solid State Commun. 75 ( 1990) 219. [ 18 ] K.F. McCarty, J.Z. Liu, R.N. Shelton and H.B. Radousky, Phys. Rev. B 42 (1990) 9973. [ 191 E. Altendorf, J. Chrzanowski, J.C. Irwin, A. O’Reilly and W.N. Hardy, Physica C 175 ( 199 1) 47. [20] E. Altendorf, J.C. Irwin, R. Liang and W.N. Hardy, Solid State Commun. 80 ( 199 1) 627. [21] K.-M. Ham, J.-T. Kim, R. Sooryakumar and T.R. Lemberger, Phys. Rev. B ( 1991), submitted. [22] R. Feenstra, D.K. Christen, C.E. Klabunde and J.D. Budai, Phys. Rev. B 45 (1992) 7555.

phonons

[ 231 V.Z. Kresin, S.A. Wolf and G. Deutscher, Physica C 191 (1992) 9. [ 241 While having little effect on the linewidth and softening of the 340 cm-’ phonon, the high-pressure oxygen treatment did measurably change the intensity of the chain-related mode at 232 cm-‘. Using 568.2 nm excitation and (yy) polarization, the 232 cm-’ mode was weakly observed before the HP0 anneal, being about t as intense as the 120 cm-’ phonon. After the anneal, the 232 cm-’ mode was not observed (see fig. 4). [25] R. Zeyher and G. Zwicknagl, Z. Phys. B Cond. Matt. 78 (1990) 175. [26] C. Thomsen, B. Friedl, M. Cieplak and M. Cardona, Solid State Commun. 78 ( 199 1) 727. [27] R. Combescot, Phys. Rev. Lett. 67 (1991) 148. [ 281 As noted by Combescot (ibid), this does not prevent there being another gap structure that arises from a different type of electron, i.e., the chain electrons in YBa,Cu,O,_,