The generation of harmonics of the voltage response is considered when an ACcurrent is applied through a superconducting film above T_c. It is shown thatalmost at all temperatures the mechanism of the temperature oscillationscreated by the AC current and the temperature dependence of the resistancedominates over the isothermal nonlinear electric conductivity. Only in a narrowcritical region close to T_c the latter is essential for the generation of theharmonics. A detailed investigation of harmonics generation provides anaccurate method for measuring the thermal boundary conductance between the filmand the insulating substrate. The critical behaviour of the third harmonic willgive a new method for the determination of the lifetime of metastable Cooperpairs above T_c. The comparison of the calculated fifth harmonics of thevoltage with the experiment is proposed as an important test for theapplicability of the employed theoretical models.
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