Small Angle Scattering of Neutrons in a Superconductor in the Meissner and Mixed States.

By definition, the S (superconducting) state is the state which properties are determined by the presence of stable and movable Cooper pairs (CP). The mobility of CPs allows them to organize into a perfectly ordered lattice of microwhirls formed by the field induced currents in CPs, thereby reducing their entropy down to zero. The latter entails zero temperature of the pairs’ ensemble, one of the consequences of which is zero resistivity of the S state [1,2]. Coming from this hypothesis, an estimated lattice parameter (diameter of microwhirls which is equal to the quadrupled London penetration depth) is ~ 100 nm; hence potentially it can be measured by SANS; the main challenge in such measurements is a low contrast of the ordered current structure on a background of disordered currents and nuclei at zero field. Here we will report on results of the first SANS probing of a superconductor in the Meissner state performed on a single crystal Nb disc in a parallel field using non-polarized neutrons. The field range of the reported experiment also covers the mixed state.



[1] V. Kozhevnikov, in Encyclopedia of Condensed Matter Physics, v.2, 644 (Elsevier, 2024).

[2] V. Kozhevnikov, Electrodynamics of Superconductors (CRC Press, 2024).