STM Studies of the Proximity Effect in Superconducting Wires with Artificial Pinning Centers

Abstract

Scanning tunneling microscopy and spectroscopy were applied to study the proximity effect with nanometer spatial resolution. The measurements were conducted on novel superconducting wires, which consist of an ordered array of submicron diameter normal metal filaments (Cu, Ni) embedded in a superconducting (NbTi) matrix. Spatially resolved information about the local density of states is obtained by taking I-V curves simultaneously with topographic images. Our data provide direct evidence on the interplay between the normal and superconducting constituents in the vicinity of the interface between them.PACS numbers: 74.50.+r, 73.40.Gk, 61.16.Ch The mutual effect of a normal metal (N) in good electrical contact with a superconductor (S), a phenomenon known as the proximity effect (PE), has been studied for over three decades [1,2]. A new interest in the subject has arisen lately, because of technological advancements that allow to conduct experiments which reveal novel mesoscopic effects [3][4][5]. One of the main points of interest is the microscopic variations of the superconducting condensate and the pair potential near the N-S boundary, and the way they influence the local density of states (DOS). Most of the experiments that probe the PE dealt so far with macroscopic properties (resistivity, magnetization, etc.), averaging out local information. Tunneling spectroscopy has also been extensively employed, but using relatively large area tunnel junctions where tunneling was performed perpendicular to the N-S interface [2].Local studies of electronic properties were made possible by the development of scanning tunneling microscopy (STM). Some high spatial resolution tunneling spectroscopy measurements of the PE have already been performed [6][7][8]. However, in these works tunneling occurred through (or near) very thin N islands deposited on top of a bulk superconductor [6], or in granular samples where adjacent grains (425)