LPEM, ESPCI Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC University of Paris 6, 10 rue Vauquelin, Paris F-75005, France.
Centre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud, Universités Paris-Saclay, C2N-Marcoussis, Marcoussis 91460, France.
Nat Commun. 2017 Feb 27;8:14549. doi: 10.1038/ncomms14549.
How small can superconductors be? For isolated nanoparticles subject to quantum size effects, P.W. Anderson in 1959 conjectured that superconductivity could only exist when the electronic level spacing δ is smaller than the superconducting gap energy Δ. Here we report a scanning tunnelling spectroscopy study of superconducting lead (Pb) nanocrystals grown on the (110) surface of InAs. We find that for nanocrystals of lateral size smaller than the Fermi wavelength of the 2D electron gas at the surface of InAs, the electronic transmission of the interface is weak; this leads to Coulomb blockade and enables the extraction of electron addition energy of the nanocrystals. For large nanocrystals, the addition energy displays superconducting parity effect, a direct consequence of Cooper pairing. Studying this parity effect as a function of nanocrystal volume, we find the suppression of Cooper pairing when the mean electronic level spacing overcomes the superconducting gap energy, thus demonstrating unambiguously the validity of the Anderson criterion.
超导体能有多小？对于受量子尺寸效应影响的孤立纳米粒子，P.W.安德森在 1959 年推测，只有当电子能级间距 δ 小于超导能隙 Δ 时，超导性才可能存在。在这里，我们报告了在 InAs (110) 表面生长的超导铅 (Pb) 纳米晶的扫描隧道光谱研究。我们发现，对于横向尺寸小于 InAs 表面二维电子气费米波长的纳米晶，界面的电子传输较弱；这导致库仑阻塞，并能够提取纳米晶的电子附加能量。对于较大的纳米晶，附加能量显示超导奇偶效应，这是库珀配对的直接结果。研究这种奇偶效应作为纳米晶体积的函数，我们发现当平均电子能级间距超过超导能隙时，库珀配对受到抑制，从而明确证明了安德森判据的有效性。
