Kramer-Pesch approximation for analyzing field-angle-resolved measurements made in unconventional superconductors: A calculation of the zero-energy density of states

By measuring angular-oscillation behavior of the heat capacity with respect to the applied field direction, one can detect the details of the gap structure. We introduce the Kramer-Pesch approximation (KPA) as a new method to analyze the field-angle-dependent experiments quantitatively. We calculate the zero energy density of states for various combinations of typical Fermi surfaces and superconducting gaps. The KPA yields a merit that one can quantitatively compare theoretical calculations with experimental results without involving heavy numerical computations, even for complicated Fermi surfaces. We show an inadequacy of the frequently-used Doppler-shift technique, which is remedied by application of the KPA.