Study of anisotropic superconductors by means of angle-resolved specific he measurements in magnetic fields

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Field-orientation dependent specific heat C(H,θ) measurement is a powerful tool to probe the nodal structure of unconventional superconductors. The principle of the measurement is based on the theoretical prediction that the zero energy density of states induced in the vortex state vary as a function of the angle between the field and the nodal directions. In this project, we have examined the nodal structure of various anisotropic f electron superconductors CeRu_2, PrOs_4Sb_<12> and URu_2Si_2, which are summarized as follows.1. For CeRu_2, clear fourfold angular oscillation with maxima (minima) along [100] ([110]) directions have been observed. The relative amplitude of the oscillation is a strongly varying function of H and T, and vanishes below 〜0.5 T and 0.3 K. This behavior indicates the presence of a minimum gap of order 〜1 K along the [110] directions, in accordance with the anisotropic s-wave scenario of this system.2. In PrOs_4Sb_<12>, C(H,θ) exhibits the minima (maxima) along [100] ([111]) in the whole field range below Hc2, indicating a deep minima in the gap amplitude along [100]. The relative oscillation amplitude, however, tends to diminish for H << Hc2, suggesting that the gap is fully opened. No evidence for a symmetry change is observed in the superconducting state.3. C(H,θ) of URu_2Si_2 shows no angular dependence for H rotated in the basal plane, indicating that the nodal structures have rotational symmetry with respect to the tetragonal c axis. A marked anisotropy is found in C(H) as a function of H/Hc2 suggesting that the gap nodes are rather localized near the c directions. Pauli paramagnetic depairing effect is evidenced in the high field behavior of C(H), indicating d-wave superconductivity.