Interaction of axial infrared-active phonons with electronic excitations and phonon-mediated charge transfer in YBa 2Cu 3O 7− x

Abstract

We report on an infrared study of the resonance interaction between phonons and electronic excitations in an YBa 2Cu 3O 7 single crystal. Our results show that two transverse axial phonon modes ( ω≅300 cm −1 and ω≅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 λ j of these modes are 0.3 and 0.45, respectively. The estimated value of the total coupling constant λ≅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 YBa 2Cu 3O 7 is interpreted in terms of phonon-mediated charge transfer between CuO 2 plane and CuO chain through the BaO plane.