ANOMALOUS MAGNETIC BEHAVIORS OF THE CUPRATE SUPERCONDUCTORS - MEISSNER EFFECT, HC1 AND HC2

In order to elucidate whether the superconductivity of the cuprate HTSC is "normal" or "abnormal", systematic studies have been performed on the Meissner effect, lower and higher critical fields. The Meissner fraction is strongly dependent on the size of sample and on the field, which is explained by a novel model considering the sharp increase in the vortex pinning force while the flux expulsion process proceeds as T is lowered. Another new method has been proposed to determine H(c1) from the magnetization curve above H(c1), based on which the abrupt upturn of the H(c1)-T curve frequently reported in the low T region has been attributed to the error induced by the strong pinning. The ambiguity associated with the H(c2)-T curve is discussed in terms of the "giant fluctuation" of superconductivity.