HARMONIC-GENERATION BY FIELD MODULATION OF THE MICROWAVE COMPLEX IMPEDANCE OF HIGH-TC SUPERCONDUCTORS

The microwave transmission through thin superconducting films of BiSrCaCuO and YBaz.CuO was studied in the presence of an oscillating magnetic field, H1 cos(2πf{hook}0t), and a dc magnetic field, H0. The oscillating field generates many even harmonics (kf{hook}0, k=2, 4, 6,...) in the low frequency spectrum of the transmitted microwave for H0=0. For H0≠0 additional odd harmonics appear. We demonstrate the presence of irreversibilities in the harmonic signal and "memory" effect in the harmonic generation below a certain temperature, Tirr. The harmonic generation is explained by the non-linear response of the microwave transmission (through the non-linear response of the complex resistivity) to a periodic applied field. Following Portis et al., we show the dependence of the complex resistivity on the magnitude of the applied field and suggest a simple model. The model can account for (a) the harmonic generation, (b) the interesting oscillatory dependence of the even harmonic signal on H0 and (c) the disappearance of all harmonics except the first harmonic for H0≫H1. The different onset temperatures for (a) the harmonic generation and (b) the change in the microwave transmittance may suggest two superconducting temperatures associated with the intergrain region, TcJ, and the grains themselves, Tcg, as suggested by theoretical models for granular superconductors. For our BiSrCaCuO thin film we find Tcg=(102±2)K, and TcJ=(92±3)K. © 1990.