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 PrOs₄ Sb₁₂ and URu₂ Si₂ in order to examine their superconducting gap structures. In the filled skutterudite PrOs₄ Sb₁₂, a clear fourfold angular dependence is observed with H rotated in the (1 0 0) planes, whose field variation suggests gap minima along [1 0 0] directions. Interestingly, a reversed angular oscillation of C (H, θ) has been observed in the normal state above H_{c 2} which continues to grow as the high-field antiferroquadrupole phase is approached. In URu₂ Si₂, 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 a- and c-axes indicates the existence of point nodes along the [0 0 1] direction.