PHYSICAL REVIEW B VOLUME 47, NUMBER 13 1 APRIL 1993-I Temperature dependences of the 340-, 440-, and 500-cm ' Raman modes of YBa&Cu30y for 6. 75 y 57. 0 E. Altendorf, X. K. Chen, and J. C. Irwin Physics Department, Simon Fraser University, Burnaby, British Columbia, Canada V5A IS6 R. Liang and W. N. Hardy Physics Department, University of British Columbia, Vancouver, British Columbia, Canada V6T1Z1 (Received 27 July 1992; revised manuscript received 12 October 1992) The temperature dependences of the linewidths and frequencies of the 340-, 440-, and SOO-cm Raman-active phonons have been measured in lightly twinned single crystals of YBa&Cu30~ with diff'erent oxygen concentrations y. The observed superconductivity-induced changes in the linewidths and frequencies that occur for temperatures below the critical temperature have been found to depend on the oxygen concentration within the range 6. 7 ~y 7. 0, with the most sensitive dependence occur- ring in the 340-cm phonon-linewidth variations near y=7. 0. The linewidth and frequency depen- dences of the 340-cm ' Bjg mode for T & T, suggest that the redistribution of electronic states below T, is very sensitive to the oxygen or hole concentration within this range. Estimates for the superconduct- ing energy gap 2h have been obtained from comparisons of the temperature dependence of the 340-cm mode with strong-coupling theory and these indicate that 2A increases as the oxygen or hole concentra- tion y decreases from y =7.0 where 2d=S. 4 kT, . The linewidth and frequency of the 440-cm mode are, however, relatively insensitive to variations in y and it is suggested that the 440- and SOO-cm phonons couple to a diff'erent set of electrons than does the 340-cm ' mode. I. INTRODUCTION The temperature dependences of the Raman-active 3 modes in YBa2Cu307 have been the subject of numerous investigations in the five years since the discovery of su- perconductivity in this compound. Macfarlane, Rosen, and Seki' observed that in fully oxygenated samples of polycrystalline material the 340-cm ' phonon decreased in frequency by about 2% as the sample was cooled from the critical temperature (T, ) near 90 K to about 10 K. This result was subsequently confirmed in experiments on polycrystalline samples, thin films, and single crys- tals and furthermore it was found that the linewidth of this same mode increased by up to 50% when the crys- tal was cooled below T, . These superconductivity- induced changes in the frequency and linewidth of the 340-cm phonon, in conjunction with its distinctive Fano line shape, ' appear to provide evidence of strong electron-phonon interactions in YBa2Cu3O7. Subsequent- ly it was found that the Raman-active oxygen vibrations at 440 and 500 cm also exhibited superconductivity- induced changes in frequency and linewidth ' ' " Qualitatively one expects that phonons with energies less than the superconducting gap will become narrower' ' as the temperature falls below T, because of a reduction in the number of available decay channels. On the other hand, phonons with energies greater than 2A can decay via a pair-breaking mechanism and hence should broaden' ' as the temperature is reduced below T, . These effects were first observed in neutron-scattering ex- periments' ' on Nb3Sn and Nb. Thompsen et al. " and Friedl, Thomsen, and Cardo- na' utilized the temperature dependences of the frequen- cies (co ) and linewidths (FWHM=2y, ) of the Raman- active phonons to obtain an estimate for the supercon- ducting gap (2b, ) in several polycrystalline RBa2Cu3O7 compounds (R is a rare-earth element, e. g. , Y, Eu, Gd), all of which had a critical temperature T, of about 90 K. They obtained a value' of 26=5. 0kT, by comparing their experimental observations to the predictions of a strong-coupling model. ' The formulation of this model by Zeyher and Zwicknagl' (ZZ) was motivated by the significant softening and broadening of the 340-cm mode, which also implied that its energy was close to 25. Not surprisingly the experimentally measured values for the superconductivity-induced changes in frequency ( b, co„) and linewidth ( 62y ) for the 340-cm ' mode played a more important role in the gap determination of Thomsen and co-workers"' than did those for the 440- and 500-cm ' modes. More recently, however, several experiments ' have been carried out on samples in which the 340-cm linewidth exhibited a very different behav- ior than that observed previously. Altendorf et al. , from experiments on lightly twinned single crystals, found that the 340-cm ' mode did not broaden at all as the temperature was reduced below T„and obtained a value of 2A/kT, = 5. 9 for the gap energy from compar- isons with the ZZ model. ' McCarty et a/. found that the 340-cm ' mode actually narrowed for T & T, and ob- tained an estimate of 6. 8kT, &25 &7. 7kT, for the gap from measurements on untwinned crystals. Thomsen et al. ' have suggested that these quite different results could be attributed to the presence of impurities in the samples. In particular, the crystals studied by Altendorf 47 1993 The American Physical Society