We present orientation dependent magnetic measurements on ordered arrays of parallel Sn nanowires for diameters $D\approx 6–170\mathrm{nm}$. For $D⩾50\mathrm{nm}$ clear superconducting transitions could be detected below $T_c=3.7\mathrm{K}$ attributed to superconductivity of the individual nanowires. Decreasing the wire diameters from bulk values to $50\mathrm{nm}$ strongly reduces the diamagnetism (by a factor of ${10}^4$) and enhances the critical field by a factor of 45. For magnetic fields parallel and perpendicular to the wire axis we detect an anisotropy in the diamagnetic screening that is enhanced with decreasing $D$ far above the Ginzburg-Landau prediction. This behavior is attributed to size-dependent anisotropy in the electronic mean free path.

• Received 28 June 2006

DOI:https://doi.org/10.1103/PhysRevB.75.014510