The Van Hove singularity as a source of anomalies in NIN and SIN tunneling experiments

The differential conductance in NIN and SIN junctions is computed using the BCS tunneling hamiltonian. The role of the Van Hove singularity and its departure far from the Fermi energy is studied for s, s_x2+y2 and d_x2-y2 symmetries. A shift in the singularity far from the Fermi energy leads to a loss of the even parity of the density of electronic states and then to the breakdown of the particle-hole symmetry. As a consequence, an asymmetry with respect to zero bias is obtained with a dip on the side of negative biases. This feature is due only to the presence of the Van Hove singularity and is obtained regardless of the gap symmetry considered.