Tunneling spectroscopy as a probe of c-axis variation of d_a²−b²-wave order parameter

The effect of variation along c axis of the d_a²−b²-wave order parameter on tunneling spectra of normal metal-superconductor junctions with various orientations is theoretically investigated. It is found that the zero-bias conductance peak is virtually unaffected by this variation. In particular, the variation causes a small decrease in the width and does not change the height. On the contrary, the coherence peak of the conductance spectra is affected by this variation. The peak gets wider as the c-axis variation of the order parameter is larger. The width of the coherence peak in tunneling spectrum can thus be used to measure the c-axis variation of the order parameter.