Microscopic parameters of a type-II superconductor measured by small-angle neutron scattering

A necessary condition for understanding and predicting the properties of any material is knowledge of microscopic parameters which control these properties in a state of thermodynamic equilibrium. One can show (see, e.g., Ref.\,\cite{VK_book}), that in superconductors these parameters are the radius of the orbital motion of electrons bound in Cooper pairs $R_0$ and the radius of the field-induced currents $r_i$ caused by precession of the pairs; one more parameter, associated with $r_i$, is the number density of Cooper pairs $n_{cp}$. In this paper we report on the first measurements of these parameters in a type-II superconductor (niobium) by SANS (small-angle neutron scattering). Other approaches to measuring the microscopic parameters are considered. Our work suggests novel avenues for studying superconductivity, important for disclosing its mechanisms in superconductors of all kinds.