Behavior of higher harmonics of the granular HTSC response to a low-frequency magnetic field

A theoretical explanation is proposed for the experimental results on the behavior of higher harmonics of the granular HTSC response to a variable magnetic field of frequencies ～10² Hz. The theory explains the periodicity in the dependence of the harmonics amplitudes on a static external magnetic field; the dependence of the period on the harmonics number and on the amplitude of the variable magnetic field; the existence of the threshold in the dependence of the odd-harmonics amplitudes on the amplitude of the variable magnetic field; and the possible formation of pairs for these dependences for the neighboring odd harmonics. It is shown that the experimental dependences can be explained without a detailed analysis of microprocesses in individual Josephson loops only by treating HTSC as a macroscopic medium characterized by the dependence Φ(H) of the magnetic flux on the external magnetic field, which is typical of type II superconductors.