REMARKS ON SMALL SUPERCONDUCTING SPECIMENS IN MAGNETIC-FIELD AND MEASUREMENTS OF THE LOWER CRITICAL-FIELD

We shall discuss the magnetic behaviors associated with the boundary e ffects for type-II superconductors in general orientations. Using prev ious results, we speculate:that the relation among the lower critical field H-c1, the thermodynamic critical field H-c and the upper critica l field H-c2, namely H-c(2) similar to H-c1 H-c2, is still approximate ly valid for general experimental orientations. More precisely, the ma gnitudes of H-c1, H-c, and H-c2 are consistent with the formulae given for bulk superconductors when the typical size of the superconducting specimen is greater than the coherence length xi, provided the penetr ation depth lambda is substituted by a modified value lambda(m). Conse quently, the criterion for observing type-II superconducting magnetic behaviors also becomes lambda(m) > xi, although the London equation is still governed by lambda. In the second part of the paper, we shall a lso suggest an experiment, based on the fact of H-c1 of superconductin g thin films in parallel magnetic field being much higher than that of bulk superconductors, to measure H-c1 precisely, especially for extre me type-II superconductors, without influence of the penetration barri er.