Superconductivity and Structure of Multilayered Nb/Ti Thin Films

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Abstract
Multilayered Nb/Ti thin films were made by a dual dc- and rf-sputtering technique. The samples with modulation wavelength λ ranging from 2000 Å down to 20 Å were prepared. X-ray diffractometry shows that the as-deposited multilayer modulation is sinusoidal in the shorter wavelength, while nearly square in the longer wavelength. Irrespective of the wavelength λ, the Nb is bcc with stacking of (110) parallel to the substrate, and the Ti is either the stacking of hcp(00.2) or bcc(110). The superconducting transition temperature T c is close to that for bulk Nb (9.3 K) in the region λ> 1000 Å, while T c decreases to 4 K with decreasing λ in the region 50< λ< 1000 Å. Dependence of the transition temperature on the wavelength is in good agreement with that observed by Qian et al . and is explained by the proximity effect according to Werthamer theory.