FISHTAIL OR PEAK EFFECT DUE TO PROXIMITY IN SUPERCONDUCTOR WITH NORMAL INCLUSIONS

The possible rise of the peak (or fishtail) effect due to the proximit y-based mechanism is discussed for a superconductor with normal inclus ions. The proximity induces only a slight variation of the order param eter near the normal inclusion-superconductor (NS) interface at low ma gnetic field and this results in a small pinning force. The supercondu ctivity induced in normal inclusions decays at higher magnetic fields and the pinning force at the NS interface increases. As a result, the peak (or fishtail) may be observed in the magnetic field dependence of the critical current. It is shown that both the characteristic size o f a normal inclusion and the coherence length in it should be larger t han (or at least comparable to) the coherence length in the supercondu ctor to provide the,clearly pronounced peak effect. This condition ass ures that the characteristic magnetic field of the superconductivity d ecay in normal inclusions is smaller than the upper critical field H(c 2) in the superconductor. Therefore the pinning force at low magnetic fields is small as compared to that at high fields. The approximate ex pression for the elementary pinning force is found in the framework of the Ginzburg-Landau theory and the critical current is calculated for the two-dimensional model system formed by a periodic array of superc onducting and normal layers. The obtained results may give some insigh t into the nature of the peak effect in both low- and high-temperature superconductors.