Orientational field dependence of low-lying excitations in the mixed state of unconventional superconductors

Orientational field dependence of the zero-energy density of states (ZEDOS) is calculated for superconductors with the polar state (line node), axial state (point node), and three-dimensional d-wave state. Depending on the gap topology and the relative field direction the field dependencies of ZEDOS sensitively differ, providing us a useful and practical method to identify the gap topology. It is also demonstrated that for d-wave state the field rotation in the basal plane shows a sizable oscillation (~3%) of ZEDOS. This is directly measurable in low-T specific heat experiment in the mixed state.