The Influence of Epitaxy on the Critical Current Densities of Evaporated Nb Films

Abstract

Critical current densities Jc and corresponding global pinning forces per unit volume Fp (in transverse magnetic field, at k.2K) for high-quality epitaxial single-crystal Nb films are extensively investigated in relation with the crystallinity of the films, compared with those of polycrystalline Nb films. It is observed that the single-crystal films with very smooth surfaces have much higher Jc (above 2×109 A/m2) at low fields than the polycrystalline films with less smooth furfaces. The increase in Jc at low fields for the single-crystal films is found to be caused by the shift of the position of the peak in Fp(b) to lower reduced magnetic field b and the increase in hight of the peak, without producing much change in Fp(b) at high b in perpendicular and parallel field orientations. The high Jc for the single-crystal films may predominantly be due to the existence of surface barriers (predicted by Bean and Livingston) on the films.