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We report on muon spin relaxation measurements of the 4f 2 -based heavy-fermion superconductor filled-skutterudite PrOs4Sb12. The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature, providing unambiguous evidence for the breaking of time-reversal symmetry in the superconducting state. A discussion is made on which of the spin or orbital component of Cooper pairs carries a nonzero momentum.PACS numbers: 74.70. Tx, 76.75.+i, 74.70.Dd, 74.25.Ha Many unconventional superconducting (SC) states, as in Ce-and U-based heavy-fermion (HF) compounds or high-T c cuprates, appear in close proximity to magnetic instabilities when a certain parameter (pressure, atomic doping, or oxygen content) is varied [1,2,3]. This fact strongly suggests that the attractive interactions binding electrons into Cooper pairs are mediated by magnetic moment fluctuations. As another possible pairing glue, fluctuations of quadrupole moments-distorted shapes of the electronic clouds of ions, are theoretically considered to be possible [4,5]. An interesting question to be addressed is what is the nature of superconductivity in such a system. For this study, Pr-based compounds with a 4f 2 configuration are likely candidates, since nonmagnetic but quadrupolar active low-energy levels can be realized due to the crystalline-electric-field (CEF) effect; in 5f systems, CEF levels are less clear due to the tendency to be itinerant.One promising candidate material for this study is the recently found superconductor PrOs 4 Sb 12 [6], which is to date the only known Pr-based HF superconductor, with a superconducting transition temperature of 1.82 K (hereafter referred to as T c1 ). The estimated electronic specific heat coefficient γ = 350 − 700 mJ/K 2 mol [6,7] and the enhanced cyclotron-effective masses [8] reflect the existence of strong electron correlations. Specific heat (C), magnetic susceptibility (χ), and inelastic neutron studies provide evidence that PrOs 4 Sb 12 has a nonmagnetic ground state and a magnetic triplet excited state separated by ∆E CEF /k B = 8 K [7,9], which is 5 times larger than T c1 . In the temperature-versus-magneticfield (T -vs-H) phase diagram, a field-induced ordered phase (µ 0 H 4 T) [9] appears close to the superconducting phase (the upper critical field µ 0 H c2 = 2.2 T). It was recently proven to be an antiferro-quadrupolar ordered phase by elastic neutron scattering measurements [10]. This fact strongly indicates that quantum quadrupole fluctuations of the Pr ions play an important role in realizing the HF superconductivity in PrOs 4 Sb 12 , considering that the T -vs-H phase diagram is analogous to those for the HF and cuprate systems, where a magnetically ordered phase exists close to the SC phase in the T -vs-pressure, -atomic-doping, or -oxygen-content phase diagram. This scenario is further supported by the enhanced T c1 compared to T c = 0.74 K for a 4f 0 reference compound LaOs 4 Sb 12 [8,11].The remarkable unconventional SC properties ...

When electrons pair in a superconductor, quasi-particles develop an acute sensitivity to different types of scattering potential that is described by the appearance of coherence factors in the scattering amplitudes. Although the effects of coherence factors are well established in isotropic superconductors, they are much harder to detect in their anisotropic counterparts, such as high-superconducting-transition-temperature cuprates. We demonstrate an approach that highlights the momentum-dependent coherence factors in Ca 2– x Na x CuO 2 Cl 2 . We used Fourier-transform scanning tunneling spectroscopy to reveal a magnetic-field dependence in quasi-particle scattering interference patterns that is sensitive to the sign of the anisotropic gap. This result is associated with the d -wave coherence factors and quasi-particle scattering off vortices. Our technique thus provides insights into the nature of electron pairing as well as quasi-particle scattering processes in unconventional superconductors.