Imaging of Quantum Mechanical Effects in Superconductors by Means of Polarized Neutron Radiography

The most prominent macroscopic quantum mechanical effects in superconductivity are beside the zero electrical resistance the Meissner effect (magnetic field expulsion) and magnetic flux trapping (pinning) in materials in their superconducting state. Their i vestigations are still subject of many publications because of the lack of consistent models or a theory which describes flux domains or flux trapping in e.g. type-I superconductors. The spin of the neutron interacts with magnetic fields which can be visualized by radiography (and tomography) using polarized neutrons. It could be shown that both the Meissner effect and flux trapping in high purity lead samples (type-I super-conductor) occur different for the crystalline and polycrystalline samples, respectively. The trapped flux of an uni- form external magnetic field applied to the lead samples is for T < Tc not uniform distributed but centred and squeezed around the rod axis of the samples. Calculations c ncerning magnetic flux trapping agree perfectly with experimental images if a Gaussian shaped B-field is assumed. Disc-shaped different surface treated niobium samples (type-II superconductor) behave different con erning Meissner phase and intermediate state. Both seem to suppress nearly completely but different the Meissner state causing different flux trapping in the sample in the intermediate state. These macroscopic Quantum mechanical effects could be visualized by radiographies with polarized neutrons yielding a deep look at phase transition in superconductivity and based on them trapped fields could be calculated and quantified.