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Photoinduced second-harmonic generation of YBa2Cu3O7- delta single crystals

Complex investigations of the photoinduced changes in YBa2Cu3O7- delta ( delta >0.5) single crystals have been carried out using the time-resolved dependences of the photocurrent, photoresistivity, photovoltage and optical second-harmonic generation (SHG). Simultaneously the spectroscopic parameters were monitored by time-resolved Raman and UV-visible reflection spectra. A time delay shift between the SHG and photocurrent dependences was found. The contributions of two different mechanisms in the formation of the superconducting behaviour were unambiguously shown. It was clearly shown that only by simultaneously taking into account the anharmonicity due to the specific electron-phonon interaction together with the traditional Anderson metal-insulator transition we can clarify the main physical mechanisms of the high-temperature superconductivity. Magnetic fields of up to 1 T suppress the SHG signal. The different delay times for the electronic and phonon spectra indicate the essential role of the electron-phonon anharmonic coupling in the high-temperature superconductivity. Some different approaches to explain the observed phenomena are discussed.