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 effects for type-II superconductors in general orientations. Using previous results, we speculate that the relation among the lower critical field , the thermodynamic critical field and the upper critical field , namely , is still approximately valid for general experimental orientations. More precisely, the magnitudes of and are consistent with the formulae given for bulk superconductors when the typical size of the superconducting specimen is greater than the coherence length , provided the penetration depth is substituted by a modified value . Consequently, the criterion for observing type-II superconducting magnetic behaviors also becomes , although the London equation is still governed by . In the second part of the paper, we shall also suggest an experiment, based on the fact of . of superconducting thin films in parallel magnetic field being much higher than that of bulk superconductors, to measure precisely, especially for extreme type-II superconductors, without influence of the penetration barrier.