Theory of Tunneling Spectroscopy of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">d</mml:mi></mml:math>-Wave Superconductors

A tunneling theory for a normal metal--insulator-- $d$-wave superconductor junction is presented. In contrast to the $s$-wave superconductor, the tunneling conductance spectra strongly depend on the tunneling direction relative to the crystalline axes, and do not always represent the bulk density of states. Zero-bias conductance peaks are expected in $\mathrm{ab}$-plane tunneling. The present theory systematically explains various experimental results in the tunneling spectroscopy of high- ${T}_{c}$ superconductors.