Differential Paramagnetic Effect and Harmonic Generation in an Enclosed Superconductor

Abstract

The low-frequency behavior of a cylindrical enclosed superconductor (superconducting coaxial cable) is discussed, with particular attention to the case in which the inner conductor is in the intermediate state. Preliminary observations of an effect which is equivalent to the familiar differential paramagnetic effect (DPE) are reported. A qualitative explanation is developed by a detailed analysis of the ideal case, which corresponds to the limit in which the phase boundaries follow the ac field perfectly. For the specimen studied, in which the DPE is reduced from the ideal value, a model is proposed based on the assumption that the intermediate state is composed of only a few normal and superconducting regions. In addition to accounting for the lack of reproducibility of the DPE on different trials, this model makes it possible, in principle, to relate the DPE directly to the velocity of propagation of the phase boundaries, although the data are not yet adequate for this purpose. The DPE was observed by means of a coaxial pair of coils wound directly on the inner superconductor. Under conditions such that the boundary motion led to switching of the inner conductor from the intermediate to the normal state or from the intermediate to the superconducting state, a strong harmonic content was observed. A brief analysis of this harmonic content is presented. Phase-boundary motion was observed directly in an auxiliary two-frequency experiment in which the low-frequency boundary motion was monitored by a high-frequency signal.