rf surface resistance measurements of binary and ternary niobium compounds

The superconducting properties of binary and ternary niobium compounds for rf applications are investigated. The materials under study are niobium nitride (NbN), niobium-titanium nitride (NbTiN), and niobium tin (NbaSn). Preparation techniques of the compounds are discussed. NbN and NbTiN are obtained by thermal reaction of bulk Nb in nitrogen atmosphere. NbaSn has been obtained using a technique originally developed for high-field magnet fabrication and modified to be applied to rf cavity production. The experimental apparatus and measurement technique are described. In particular the raw experimental data have been carefully analyzed to obtain important informations on fundamental material parameters. The effect of field penetration in the superconductor and the influence on the experimental results of the normal metallic substrate on which the superconductor is grown is also considered. The measurements show that NbN and NbaSn are effectively potentially good materials for rf applications due to their low theoretical BCS surface resistance. So far the results obtained with NbTiN are more difficult to interpret because the quality of the material did not allow sufficiently accurate collection of data.