Infrared absorption spectroscopy has been used to investigate the complete phonon frequency range (20 cm -1 -700 cm -1 ) at temperatures between 10 K and 300 K. A direct coupling of bulk phonon modes in YBa 2 Cu 3 O 7- delta with the superconductor order parameter is deduced from the critical temperature dependence in the superconducting low temperature phase (T c approximately 92 K). The excess intensity and the frequency shift of the mode near 580 cm -1 are shown to be proportional to the square of a BCS-like order parameter in agreement with the predictions of Hard Mode Spectroscopy.
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Güttler et al. (1991) studied this question.
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