INTERACTION OF AXIAL INFRARED-ACTIVE PHONONS WITH ELECTRONIC EXCITATIONS AND PHONON-MEDIATED CHARGE-TRANSFER IN YBA2CU3O7-X

We report on an infrared study of the resonance interaction between phonons and electronic excitations in an YBa2Cu3O7 single crystal. Our results show that two transverse axial phonon modes (omega congruent-to 300 cm-1 and omega congruent-to 550 cm-1) are strongly coupled to the c-polarized electronic continuum resulting in Fano-type line shapes at temperatures below T(c). The calculated dimensionless coupling constants lambda(j) of these modes are 0.3 and 0.45, respectively. The estimated value of the total coupling constant lambda congruent-to 3 is high enough to account for the high T(c) value. A dramatic increase in the bare oscillator strength of these phonons in the superconducting phase of YBa2Cu3O7 is interpreted in terms of phonon-mediated charge transfer between CuO2 plane and CuO chain through the BaO plane.