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 ph onons and electronic excitations in an YBa2Cu3O7 single crystal. Our r esults 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 a t temperatures below T(c). The calculated dimensionless coupling const ants lambda(j) of these modes are 0.3 and 0.45, respectively. The esti mated value of the total coupling constant lambda congruent-to 3 is hi gh enough to account for the high T(c) value. A dramatic increase in t he bare oscillator strength of these phonons in the superconducting ph ase of YBa2Cu3O7 is interpreted in terms of phonon-mediated charge tra nsfer between CuO2 plane and CuO chain through the BaO plane.