Neutron Dark-Field Imaging of the Domain Distribution in the Intermediate State of Lead

The intermediate state (IS) of a type-I superconductor is characterized by coexistence of Meissner phase and normal conducting phase. Experiments on the topology of the IS show a variety of universal domain patterns which are also seen in various other physical, chemical or even biological systems on various length and time scales. The possibility to easily tune the domain structure of the IS by a variation of magnetic field or temperature ideally qualifies type-I superconductors as general model systems for the investigation of domain nucleation and distribution. However, the experimental observation of the IS domain structure was up to now restricted to either thin films or surfaces. We demonstrate how neutron grating interferometry (nGI) probes the IS domain distribution in the interior of a bulk single-crystalline lead sample. By means of nGI, we are able to visualize the field penetration process into the superconductor as well as the hysteretic behavior of the intermediate state morphology. Finally, the impact of nGI for investigations on bulk domain nucleation is discussed, as this technique is applicable on many other systems that reveal a phase separation on a micrometer length scale.