Probing the nodal structures of heavy electron superconductors by means of specific heat measurements in magnetic fields

Measurements of the angle-resolved specific heat C(H,θ) have been performed on PrOs 4 Sb 12 and URu 2 Si 2 in order to examine their superconducting gap structures. In the filled skutterudite PrOs 4 Sb 12 , a clear fourfold angular dependence is observed with H rotated in the (100) planes, whose field variation suggests gap minima along [100] directions. Interestingly, a reversed angular oscillation of C(H,θ) has been observed in the normal state above H c2 which continues to grow as the high-field antiferroquadrupole phase is approached. In URu 2 Si 2 , C(H, θ) with H rotated around the tetragonal c-axis suggests that the nodal structure has rotational symmetry. Comparison between the C(H) data for α- and c-axes indicates the existence of point nodes along the [001] direction.