Superconducting Nanoparticles and Nanowires

Abstract

The properties of macroscopic samples of the classic, phonon-mediated superconductors are now well understood (although the situation is less clear with the high-temperature superconductors). We address the question of how superconductivity is affected if the sample size is reduced to the nanometer scale. We distinguish two cases: nanoparticles and nanowires. First, we shall discuss the case of nanograins, where the small volume of the sample implies a finite spacing δ between the quantized states of the conduction electron gas. As pointed out by Anderson (1959), when the spacing δ becomes larger than the superconducting energy gap, the BCS theory implies that superconductivity should be excluded. We shall briefly survey some of the more accessible theoretical analyses and then describe the experimental results carried out on Al nanoparticles. Secondly, we shall review the experimental and theoretical situation concerning superconducting nanowires