The critical magnetic field in the intermediate state of a Pippard superconductor*

One of the fundamental problems of unconventional superconductivity is the magnetic structure of vortices. In order to contribute to the solution of this problem and to address number of unsolved questions of the intermediate state (IS) we undertook an experimental and theoretical study of IS in an extreme type-I superconductor focusing on the critical field of the IS-N (normal) transition. Results shed new light on the structure and evolution of the magnetic flux density in normal domains and may lead to new insights in the structure of vortices in type-II materials. A 2.5 $\mu$m thick indium film with mean free path 11 $\mu$m was placed in the superconducting vector (3D) magnet. Magneto-optical images were taken simultaneously with measurements of the sample resistance using a small low frequency AC current. The equilibrium domain structure of the IS state was investigated as a function of independently controlled in- and out-of-plane magnetic fields and/or DC transport current applied to the sample. The observed critical field varied in a range from 100\% down to 40\% of the thermodynamic critical field. A theoretical model based on the classical Landau laminar structure quantitatively accounts for the experimental results for both ordered and disordered domain patterns.