| Superconductivity was observed in the Pr-filled skutterudite heavy fermion system PrOs4Sb12 in 2002. Symmetry considerations, based on the Landau theory of second order phase transitions, were used to derive all possible superconducting states in this material. It was found that a three-component order parameter that belongs to the Tu irreducible representation of the tetrahedral point group Th best describes superconductivity in PrOs4Sb 12. Two different superconducting phases have been observed in PrOs 4Sb12; the lower temperature B phase occupies the bulk of the phase diagram, while the higher temperature A phase is found in a narrow region below Hc2, and possibly does not exist at all. The proposed gap function in the A phase is unitary and has two point nodes in the [00 +/- 1] directions. In the B phase, the proposed gap function is nonunitary and the lower branch has four point nodes in the [+/-alpha, 0, +/-beta] directions. Hence, thermodynamic and transport properties of a nonunitary superconducting state are analyzed. It is shown that the conductivity tensor has inequivalent diagonal components due to the off-axis nodal positions of the B phase. Moreover, nonuniversal conductivities (impurity scattering dependent) have been obtained. A semi-classical approach based on the Doppler shift of the quasiparticle energy is used to calculate the oscillatory past of the magnetic field-angle dependent density of states and thermal conductivity. All possible point node configurations for superconducting phases of a tetrahedral superconductor are considered. |
