The superconducting gap structure of recently discovered heavy fermion superconductor PrOs4Sb12 was investigated by using thermal transport measurements in magnetic field rotated relative to the crystal axes. We demonstrate that a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state, giving a clear indication of the presence of two distinct superconducting phases with twofold and fourfold symmetries. We infer that the gap functions in both phases have a point node singularity, in contrast to the familiar line node singularity observed in almost all unconventional superconductors.

We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (µSR) measurements. Zero-field µSR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field µSR is nearly flat approaching T = 0 K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal symmetry breaking in SrPtAs are allowed. Out of these, a dominantly d + id (chiral d-wave) order parameter is most consistent with our experimental data. Transition metal pnictides have attracted considerable scientific interest as they present the second largest family of superconductors after the cuprates [1]. All superconductors of this family share one common structural feature: superconductivity takes place in a square lattice formed by the transition metal elements. Very recently superconductivity with a T c of 2.4 K has been discovered in SrPtAs [2], which has a unique and attractive structural feature: It crystallizes in a hexagonal structure with weakly coupled PtAs layers forming a honeycomb lattice. SrPtAs supports three pairs of split Fermi surfaces, two of which are hole-like and centered around the Γ-point with a cylindrical shape extended along the k z direction and together host only about 30% of the density of states. The remaining 70% of the density of states are hosted by the third pair of split Fermi surfaces that is electron-like, centered around the K and K ′

We discuss the polar Kerr effect in a chiral p-wave ͑p x + ip y -wave͒ superconductor. It is found that the off-diagonal component of a current-current correlation function is induced by impurity scattering in the chiral p-wave condensate, and a nonzero Hall conductivity is obtained using the Kubo formula. We estimate the Kerr rotation angle by using this impurity-induced Hall conductivity and compare it with experimental results ͓J. Xia et al., Phys. Rev. Lett. 97, 167002 ͑2006͔͒. Recently, the quasi-two-dimensional ͑quasi-2D͒ superconductor Sr 2 RuO 4 with T c = 1.5 K has attracted considerable attention and it has been investigated extensively. 1 It is plausible that the order parameter in Sr 2 RuO 4 has the spin triplet p x Ϯ ip y -wave symmetry. One of the fascinating properties of this state is the spontaneous breaking of parity in a twodimensional ͑2D͒ sense ͑p x → p x and p y → −p y ͒ and in a timereversal symmetry due to the presence of nonzero chirality characterized by l z = Ϯ 1, where l z is the z component of the relative orbital angular momentum of the Cooper pair.The polar Kerr effect ͑PKE͒, in which the direction of polarization of reflected linearly polarized light is rotated, has been known as an effective tool for understanding ferromagnetism. 2 Because of the analogy between ferromagnetic order-for instance, with s z = 1 and chiral pair condensation with l z = 1-it is naively expected that the PKE is induced in the chiral p-wave state at zero field. In fact, the PKE has been observed in the superconducting state of Sr 2 RuO 4 . 3 Up-to-date theoretical reports on the PKE in the chiral p-wave state are given in Refs. 4 and 5, in which interesting mechanisms have been proposed by the field theoretical approach; however, obtained results of the Kerr rotation angle are considerably smaller than the experimental results.Therefore, it is crucial to elucidate the fundamental nature of the PKE in the chiral p-wave superconductor. We will show that a Kerr rotation angle comparable to that obtained experimentally is obtained by taking into account nonmagnetic and short-ranged impurity scatterings of quasiparticles in a chiral p-wave condensate. It is also found that this impurity-induced PKE is suppressed or is zero for any superconducting state other than the chiral p-wave state. 6 This result is contrary to the naive analogy with a ferromagnet, since the effect is not proportional to chirality analogous to magnetization but suppressed in higher chirality states with l z = Ϯ 2, Ϯ 3,¯. The natural unit ប = c = k B = 1 is used throughout this Rapid Communication.We reviewed the phenomenology of the PKE in timereversal symmetry-breaking superconducting systems, which is an extension of the argument for itinerant ferromagnetic systems. 2,8 We also refer to the discussion of anyon superconductivity. 9 Let us suppose that z Ͼ 0 is empty, z Ͻ 0 is filled by the superconductor, and incident light is linearly polarized and propagating along the z direction perpendicular to the superconducting plane with a w...

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