ELECTRON PHONON INTERACTIONS OF RAMAN ACTIVE PHONONS IN YBA2CU3O7-Y

Polarized Raman measurements have been carried out on twinned single crystals of YBa2Cu3O7-y. The temperature dependence of the frequencies and linewidths of the 340, 435 and 500 cm-1 phonons have been investigated for temperatures from 15 to 300 K. Both the frequency and linewidth of the 340 cm-1 phonon have been found to decrease as the sample temperature is reduced from about 90 K (T(c)) to 15 K whereas the frequencies and linewidths of the 435 and 500 cm-1 modes both increase as the temperature is lowered through this same interval. For T > T(c) the 500 cm-1 phonon exhibits an anisotropic coupling to the electronic excitations in the crystal but all evidence of this anisotropy vanishes when the sample temperature is reduced below T(c). The superconductivity induced changes in the frequency and linewidth of all three modes have been compared to the predictions of the strong-coupling model of Zeyher and Zwicknagl for high temperature superconductors. Good agreement between theory and measurements is obtained if it is assumed that the superconducting gap parameter is 2-DELTA = 370 +/- 20 cm-1 which corresponds to a ratio 2-DELTA/k(B)T(c)=5.9 +/- 0.3. This result is compared to previously published values for 2-DELTA.