Explanation for the surface dependence of the tunneling gap in Nb3Sn

An explanation is proposed for the experimental results of Hoffstein and Cohen on tunneling into superconducting Nb₃Sn. In these experiments a strong dependence of the gap on the orientation of the surface was found. This dependence is explained here in terms of the linear chain model; a 100] surface cutting only one family of niobium atom linear chains, while a 111] surface cutting all three families of linear chains.