Selective coupling of coherent optical phonons in YBa2Cu3O7 with electronic transitions

We investigate coherent lattice dynamics in optimally doped YBa2Cu3O7−δ driven by ultrashort (∼ 12 fs) near infrared (NIR) and near ultraviolet (NUV) pulses. Transient reflectivity experiments, performed at room temperature and under moderate (<0.1 mJ/cm2) excitation fluence, reveal phonon modes related to the O(2,3) bending in the CuO2 planes and to the apical O(4) stretching at frequencies between 10 and 15 THz, in addition to the previously reported Ba and Cu(2) vibrations at 3.5 and 4.5 THz. The relative coherent phonon amplitudes are in stark contrast to the relative phonon intensities in the spontaneous Raman scattering spectrum excited at the same wavelength. This contrast indicates mode-dependent contributions of the Raman and non-Raman mechanisms to the generation of the coherent phonons. The particularly intense amplitude of the coherent Cu(2) phonon, together with its initial phase, supports its generation to be dominated by non-Raman mechanism involving charge transfer within the CuO2 plane. By contrast, the coherent out-of-phase O(2,3) bending mode is unproportionally weak compared with its Raman counterpart, suggesting that the charge transfer is ineffective in generating such an "asymmetric" atomic displacement. When the pump light has the polarization component normal to the CuO2 plane, the coherent O(4) mode is strongly enhanced compared to the in-plane excitation, probably by the charge transfer from the apical oxygen to the Cu-O chains. Our findings demonstrate that the charge transfer excitations in YBa2Cu3O7−δ strongly contribute to the electron-phonon coupling on a femtosecond timescale.