Experimental study of the magnetic field distribution and shape of domains near the surface of type-I superconductors in the intermediate state

The importance of accounting for the inhomogeneity of the magnetic field distribution and roundness of domain walls near the surface of type-I superconductors in the Intermediate State (IS) for forming the equilibrium flux structure was demonstrated by Landau eight decades ago. Further studies confirmed this prediction and extended it to all equilibrium properties of the IS. Here we report on direct measurements of the field distribution and shape of domains near the surface of high-purity type-I (indium) films in perpendicular field using Low-Energy muon Spin Rotation spectroscopy. We found that at low applied fields (in about half of the IS field range) the field distribution and domains' shape agrees with that proposed by Tinkham. However for high fields our data suggest that reality can differ from theoretical expectations. In particular, the width of the superconducting laminae can expand near the surface leading to formation of a maximum in the static magnetic field in the current-free space outside the sample. We speculate that the apparent contradiction of our observations with classical electrodynamics is due to the inapplicability of the standard boundary conditions to the vicinity of an "active" superconductor.