Effect of a dimensional crossover on the upper critical field of practicalNb-Ti alloy superconductors

Multilayers of a Nb0.37Ti0.63 alloy, a chief material of superconducting ma gnet technology, and a Cu0.95Sn0.05 or Cu0.70Ni0.30 alloy exhibit a dimensi onal crossover with a decreasing bilayer period Lambda=d(N)+d(S), where d(S )=3d(N). Cusps of H-c2(theta) and square root H-c2 parallel to(T) develop w hen Lambda less than or equal to 40 nm, which indicate a crossover to 2D be havior from 3D behavior seen at Lambda=60 nm. Full proximity coupling of Cu -Sn layers for Lambda=13 nm restores isotropic angular dependence, but with sharply lower H-c2 values. By contrast, proximity coupling was suppressed by magnetic Cu-Ni layers, and 2D behavior was retained while T-c fell below 4 K for Lambda < 20 nm. The data are consistent with numeric results obtai ned by Takahashi and Tachiki [Phys. Rev. B 33, 4620 (1986)] when the variat ion of the Bardeen-Cooper-Schrieffer pairing potential is the primary cause of the dimensional crossover. Since practical Nb-Ti conductors have a laye red nanostructure, this result suggests that a dimensional crossover should also be found in wires. However, the 3D-2D crossover occurs when Lambda is much greater than the separation of the flux lines at high field (10-20 nm ) and above the range where optimum flux pinning is found. This implies tha t a 2D state (for insulating or magnetic layers) or a 2D strongly coupled s tate (for normal metals) exists when flux pinning is strongest. These impli cations are discussed in the context of practical Nb-Ti wires used in super conducting magnet technology. (C) 1999 American Institute of Physics. [S002 1-8979(99)09322-6].