The signature of subsurface Kondo impurities in the local tunnel current

The conductance of a tunnel point-contact in an STM-like geometry having a single defect placed below the surface is investigated theoretically. The effect of multiple electron scattering by the defect after reflections by the metal surface is taken into account. In the approximation of s-wave scattering the dependence of the conductance on the applied voltage and the position of the defect is obtained. The results are illustrated for a model s-wave phase shift describing Kondo-resonance scattering. We demonstrate that multiple electron scattering by the magnetic impurity plays a decisive role in the point-contact conductance at voltages near the Kondo resonance. We find that the sign and shape of the Kondo anomaly depends on the position of the defect.