Experiments on the Critical Behavior of Type-II Superconductors in High Magnetic Fields

Abstract

For two different reasons type-II superconductors have attracted great interest during the last few years in experimental work as well as in theoretical considerations. At first there was mainly a practical reason, because all materials for superconducting coils are type-II superconductors. Thus work on the fabrication of these materials and the improvement of their properties was important and initiated many experiments. But then the fundamental scientific aspect took over. New theories appeared and many experimental results were found and checked with these theories. The main feature of a type-II superconductor is the existence of a mixed state of superconducting and normal material for each temperature T below the critical temperature T c . The mixed state exists between the lower critical field H c1(T) and the upper critical field H c2(T) In the region between the two critical field values flux penetrates the type-II superconductor. For some groups of type-II superconductors the upper critical fields are very high, especially for compounds of the W and NaCl-type structure. Experiments will be described and the results will be compared with recent theory for these materials.