Higher harmonics of ac voltage response in narrow strips of YBa2Cu3O7 thin films: Evidence for strong thermal fluctuations

We report on measurements of higher harmonics of the ac voltage response in strips of YBa2Cu3O7 thin films as a function of temperature, frequency and ac current amplitude. The third (fifth) harmonic of the local voltage is found to exhibit a negative (positive) peak at the superconducting transition temperature and their amplitudes are closely related to the slope (derivative) of the first (Ohmic) harmonic. The peaks practically do not depend on frequency and no even (second or fourth) harmonics are detected. The observed data can be interpreted in terms of ac current induced thermal modulation of the sample temperature added to strong thermally activated fluctuations in the transition region.