In this letter, we examine the effect of Coulomb interactions in the normal region of a normalsuperconducting (N/S) mesoscopic structure, here the change from an attractive to a repulsive coulombic interaction, at the N/S interface, causes a shift in the order parameter phase. We show that this shift has a pronounced effect on Andreev bound states and demonstrate that the effect on Andreev scattering of non-zero order-parameter tails, can be used to probe the sign of the interaction in the normal region.Recent advances in material technology have enabled the fabrication of normal/superconducting (N/S) mesoscopic hybrid structures with well defined dimensions and interfaces [1][2][3][4]. Due to the proximity of the normal material to the superconductor the pairing field f (x) = ψ ↑ (x)ψ ↓ (x) , in the normal region, decays to zero on the scale of a coherence length ξ [5]. During the past decade this proximity effect has been extensively investigated both experimentally and theoretically (see for example [6][7][8][9]).In contrast to the pairing field f (x), the effective electron-electron interaction, V (x), changes abruptly at the S/N interface, from an attractive interaction in the superconductor to either zero, a much diminished attractive interaction or to a repulsive interaction, in the normal (N) material. Consequently the order parameter, ∆ = V (x)f (x), of a s-wave superconductor also changes abruptly at the S/N interface, as shown in figure 1. To date theoretical research into the transport properties of N/S interfaces have mainly considered the order parameter in the normal region to be zero (figure 1a) [10,11].In this letter we consider the effect of an attractive or repulsive electron-electron interaction (V (x) = 0), as shown in 1b and 1c. The difference between figures 1b and 1c is the π phase shift in ∆(x), induced by the cross-over from an attractive to a repulsive interaction at the N/S interface. In what follows we examine how this phase shift affects the transport properties associated with Andreev bound states.To investigate this regime we adopt a general scattering approach to dc transport, which was initially developed to describe phase-coherent transport in dirty mesoscopic superconductors [12]. For simplicity in this letter, we focus solely on the zero-voltage, zero-temperature conductance, for the structure shown in figure 2. In the linear-response limit, at zero temperature, the conductance of a phase-coherent structure may be calculated from the fundamental current voltage relationship [13,14],The above expression relates the current I i from a normal reservoir i to the voltage differences (v j − v), where v = µ/e and the sum is over the 2 normal leads connected to the scattering region. The a ij 's are linear combinations of the normal and Andreev scattering coefficients and in the absence of superconductivity satisfy where G is the conductance in units ofh . As noted in [15] the various transmission and reflection coeffcients can be computed by solving the Bogoliubov -de Gennes equatio...