Transition to the Mixed State in Lead Films at 4.2°K
Type I and type II superconductors are distinguished by their qualitatively different behaviors in the presence of an applied magnetic field. For a flat plate geometry and perpendicularly applied magnetic fields, magnetic flux must penetrate a superconducting specimen at fields much lower than the critical field in perpendicular fields H T . For a type II superconductor the distribution of flux consists of singly quantized vortices in the “mixed state,” while for a type I superconductor the distribution is a much coarser arrangement of normal and superconducting domains referred to as the “intermediate state”. However, as pointed out by Tinkham,1 for sufficiently thin specimens even materials which are type I in the bulk should exhibit mixed-state rather than intermediate-state behavior. Theoretical and experimental work on thin superconducting films has been extensive and includes the detailed calculation of Lasher2 and the critical field measurements of Cody and Miller,3 whose publications include extensive references to many other papers relevant to the subject. Theory and experiment confirm the essential validity of Tinkham’s results but disagree in several details. No direct microscopic observation of the mixed-state structure in very thin films has been reported, although the intermediate-state structure in thicker films has been studied in some detail.4, 7
KeywordsIntermediate State Critical Field Critical Thickness Thin Superconducting Film Direct Microscopic Observation
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