Sandwich and superlattice structures composed of the high critical field superconducting alloy Nb0.53Ti0.47 (where the compositions refer to weight percent) and amorphous Ge were sputter-deposited on sapphire substrates using a novel multi-sputter-gun system. The thicknesses of both the superconducting Nb0.53Ti0.47 and the insulating Ge layers were varied over a wide range in order to study the two-dimensional (2D) and three-dimensional (3D) superconducting properties of these structures. The observed suppression of the measured resistive superconducting transition temperature Tc with increasing sheet resistance is tentatively interpreted using the Maekawa-Fukuyama theory which incorporates contributions from localization and Coulomb interaction effects. The upper critical fields, with the field both normal Hc2⊥ and parallel Hc2∥ to the film, were measured up to 50 kG. Hc2∥ exhibited 2D behavior when the thickness of the superconducting layer was less than 200 Å. The zero temperature values of Hc2∥(0) were estimated using the Rickayzen formula, which is valid in the regime l≪(lξ 0)1/2≪Ds (where l, ξ0, and Ds are the mean free path, pure material coherence length, and film thickness, respectively). The estimated H c2∥(0) peaks at about 400 kG for a superconducting layer thickness of 144 Å. This rather high upper critical field, which is considerably above the bulk value of 145 kG, likely has its origin in flux line accommodation within the insulating Ge layers. Since the Rickayzen expression neglects paramagnetic limiting, the true Hc2∥(0) is expected to be somewhat lower; localization/interaction and spin orbit coupling effects are also neglected in this treatment. The situation is none the less encouraging and suggests multilayer superconductors may have a place in high-field magnet technology.
ASJC Scopus subject areas
- Physics and Astronomy(all)