Abstract
We discuss the mechanism of coherent phonon generation by stimulated ionic Raman scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{6+x}$ to the resonant excitation of apical oxygen motions at 20 THz by midinfrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric $({\mathrm{A}}_{\mathrm{g}})$ Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.
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