Effect of a dimensional crossover on the upper critical field of practical Nb{endash}Ti alloy superconductors
- Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, Wisconsin 53706 (United States)
Multilayers of a Nb{sub 0.37}Ti{sub 0.63} alloy, a chief material of superconducting magnet technology, and a Cu{sub 0.95}Sn{sub 0.05} or Cu{sub 0.70}Ni{sub 0.30} alloy exhibit a dimensional crossover with a decreasing bilayer period {Lambda}=d{sub N}+d{sub S}, where d{sub S}=3d{sub N}. Cusps of H{sub c2}({theta}) and square root H{sub c2{parallel}}(T) develop when {Lambda}{le}40 nm, which indicate a crossover to 2D behavior from 3D behavior seen at {Lambda}=60 nm. Full proximity coupling of Cu{endash}Sn layers for {Lambda}=13 nm restores isotropic angular dependence, but with sharply lower H{sub c2} values. By contrast, proximity coupling was suppressed by magnetic Cu{endash}Ni layers, and 2D behavior was retained while T{sub c} fell below 4 K for {Lambda}{lt}20 nm. The data are consistent with numeric results obtained by Takahashi and Tachiki [Phys. Rev. B {bold 33}, 4620 (1986)] when the variation of the Bardeen{endash}Cooper{endash}Schrieffer pairing potential is the primary cause of the dimensional crossover. Since practical Nb{endash}Ti conductors have a layered nanostructure, this result suggests that a dimensional crossover should also be found in wires. However, the 3D{endash}2D crossover occurs when {Lambda} is much greater than the separation of the flux lines at high field (10{endash}20 nm) and above the range where optimum flux pinning is found. This implies that a 2D state (for insulating or magnetic layers) or a 2D strongly coupled state (for normal metals) exists when flux pinning is strongest. These implications are discussed in the context of practical Nb{endash}Ti wires used in superconducting magnet technology. {copyright} {ital 1999 American Institute of Physics.}
- OSTI ID:
- 687987
- Journal Information:
- Journal of Applied Physics, Journal Name: Journal of Applied Physics Journal Issue: 10 Vol. 86; ISSN JAPIAU; ISSN 0021-8979
- Country of Publication:
- United States
- Language:
- English
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