Very high residual resistivity ratios of heteroepitaxial superconducting niobium films on MgO substrates

We report residual resistivity ratio (RRR) values (up to RRR-541) measured in thin film Nb grown on MgO crystal substrates, using a vacuum arc discharge, whose 60?160 eV Nb ions drive heteroepitaxial crystal growth. The RRR depends strongly upon substrate annealing and deposition temperatures. X-ray diffraction spectra and pole figures reveal that, as the crystal structure of the Nb film becomes more ordered, RRR increases, consistent with fewer defects or impurities in the lattice and hence longer electron mean free path. A transition from Nb(110) to purely Nb(100) crystal orientation on the MgO(100) lattice occurs at higher temperature.