The cosmic-ray energy spectrum above 10 18.5 eV is reported using the updated data set of the Akeno Giant Air Shower Array (AGASA) from February 1990 to October 1997. The energy spectrum extends beyond 10 20 eV and the energy gap between the highest energy event and the others is being filled up with recently observed events. The spectral shape suggests the absence of the 2.7 K cutoff in the energy spectrum or a possible presence of a new component beyond the 2.7 K cutoff.PACS numbers: 98.70. Sa, 96.40.Pq, 96.40.De Typeset using REVT E X

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Various kinds of energetic particles are irradiated into iron-based superconductors, and their effects on the critical current density (J c ) and vortex dynamics have been systematically studied. It is found that J c is enhanced and vortex dynamics is strongly suppressed by energetic particles having a sufficient energy deposition rate, similar to the case of high temperature cuprate superconductors. The enhancement of J c , in general, persists up to much higher irradiation doses than in cuprates. However, details of the effect of irradiation depend on the kind of ion species and their energies. Even with the same ions and energies, the effect is not universal for different kinds of iron-based superconductors. The correlated nature of defects created by heavy-ion irradiation is confirmed by the angular dependence of irreversible magnetization.

The effect of proton irradiation in Ba(Fe 0.93 Co 0.07 ) 2 As 2 single crystals is reported. We analyze temperature dependence of the current density and normalized flux relaxation rate in the framework of the collective-creep model. The glassy exponent and barrier height for flux creep are directly determined by Maley's method. Our model functions for barrier height and critical current density in the absence of flux creep are explained by the superposition of δT c and δl pinnings. We also approach true critical current density by means of the generalized inversion scheme, and the obtained result is in reasonable agreement with our model function. The proton-irradiation effect on temperature dependence of the current density and normalized relaxation rate can be summarized as doubling of the barrier height at the beginning of flux creep. PACS number(s): 74.25. Wx, 74.25.Uv, 74.25.Sv, 74.70.Xa ) 1.5 J with flux creep J c by GIS J c δl-pinning J c δT c -pinning Ba(Fe 0.93 Co 0.07 ) 2 As 2 pristine 094527-5

We report the study on superconducting pairing mechanism of FeSe via the pair-breaking effects induced by H + -irradiation combined with low-temperature specific heat measurements. A multigap structure with nodes or gap minima is suggested in a clean FeSe by the specific heat results. The suppression of critical temperature Tc with increasing the defect density manifests a two-step behavior. When the increase in the residual resistivity is small, ∆ρ0 < ∼4.5 µΩcm, Tc is gradually suppressed with increasing the density of scattering centers, suggesting the presence of symmetryunprotected nodes or gap minima. However, for ∆ρ0 > ∼4.5 µΩcm, Tc is almost independent of the scattering, which indicates that the nodes or gap minima are lifted and the order parameter becomes almost isotropic without sign change. Thus, the superconductivity in FeSe is found to be realized in symmetry-unprotected nodal or highly anisotropic s++ state.