Intermediate State: Do We Really Know It Well?
Rich physics of the intermediate state (IS) in type-I superconductors have been recognized already at the first consideration of superconductivity as a new thermodynamic state of matter (Gorter and Casimir, 1934). Classic works on the IS include the works by F. London (1936), Peierls (1936), Landau (1937), and Ginzburg and Landau (1950).
It may sound unexpected but many fundamental problems of this state are still open. Among those are the problems of the magnetic structure of the normal (N) domains, of the critical field of the IS-N transition and of extraction of microscopic parameters from properties of the IS. The same problems are among the central problems of the contemporary research on type-II superconductors. Recently, being attracted by the richness of this phenomenon and stimulated by revealed inconsistencies between values of microscopic parameters measured in the Meissner state and those following from historically seminal data obtained from the IS (played important role in establishing Ginzburg-Landau theory in the first place, M. Tinkham), we conducted an experimental study of the IS focusing on the equilibrium magnetic flux structure, the flux density and the critical field**.
The least expected observation is that the flux density in N domains can be less than half of the thermodynamic critical field Hc. This fact contradicts and hence overthrows a long established paradigm stating that the N phase is unstable at the field less than Hc. I will present results of this study and introduce a new theoretical model consistently describing these and all other properties of the IS measured in independent experiments. I will also discuss relevance of this model to studies of magnetic structure of the vortex state.
*This work was supported by NSF.
** V. Kozhevnikov, R. J. Wijngaarden, J. de Wit and C. Van Haesendonck, PRB 89, 100503 (2014).