Size-dependent anisotropic diamagnetic screening in superconducting Sn nanowires

We present orientation dependent magnetic measurements on ordered arrays of parallel Sn nanowires for diameters D≈6--1700.3em0exnm. For D500.3em0exnm clear superconducting transitions could be detected below Tc=3.70.3em0exK attributed to superconductivity of the individual nanowires. Decreasing the wire diameters from bulk values to 500.3em0exnm strongly reduces the diamagnetism (by a factor of 10⁴) 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.