Intermediate state: a new look at an old story*

One of the central problems of superconductivity is magnetic structure of vortices and elicitation of microscopic parameters from parameters of the mixed state (MS) in type-II superconductors. Similar problem, i.e. magnetic structure of normal (N) domains and elicitation of the microscopic parameters from parameters of the intermediate state (IS) in type-I materials, is the longest standing problem of superconductivity advanced by Landau in 1930s. We will report on our recent study of the IS in a high purity indium films using magneto-optical imaging, and transport and magnetization measurements. The least expected observation is that the magnetic flux density in N-domains can be as small as nearly 40\% of the thermodynamic critical field $H_c$. This fact contradicts and hence overthrows a paradigm stating that the N-phase is unstable in the fields less than $H_c$. We will present a new theoretical model of the IS for the first time consistently addressing this and $all$ other properties of the IS. Moreover, our model, based on rigorous thermodynamics of the equilibrium flux structure, allows for quantitative determination of the domain-wall parameter and the coherence length. Possible impact of our model on the vortex structure will be discussed.