Neutron-irradiation effects in LaO0.9F0.1FeAs superconductor

The effect of atomic disorder induced by neutrons irradiation on superconducting and normal state properties of polycrystalline LaFeAsO_0.9F_0.1 was investigated. The irradiation of the sample by a moderate neutron fluence F = 1.6*1019 cm^-2 at Tirr = 50 +- 10 C leads to the suppression of superconductivity which recovers almost completely after annealing at temperatures Tann < 750 C. It is shown that the reduction of superconducting transition temperature Tc under atomic disordering is not determined solely by the value of Hall concentration nH, i.e. doping level, but is governed by the reduction of electronic relaxation time. This behavior can be described qualitatively by universal Abrikosov-Gorkov equation which presents evidence on the anomalous type of electrons pairing in Fe-based superconductors.Comment: 8 pages, 11 figure

Superconductivity in iron silicide Lu2Fe3Si5 probed by radiation-induced disordering

Resistivity r(T), Hall coefficient RH(T), superconducting temperature Tc, and the slope of the upper critical field -dHc2/dT were studied in poly- and single-crystalline samples of the Fe-based superconductor Lu2Fe3Si5 irradiated by fast neutrons. Atomic disordering induced by the neutron irradiation leads to a fast suppression of Tc similarly to the case of doping of Lu2Fe3Si5 with magnetic (Dy) and non-magnetic (Sc, Y) impurities. The same effect was observed in a novel FeAs-based superconductor La(O-F)FeAs after irradiation. Such behavior is accounted for by strong pair breaking that is traceable to scattering at non-magnetic impurities or radiation defects in unconventional superconductors. In such superconductors the sign of the order parameter changes between the different Fermi sheets (s+- model). Some relations that are specified for the properties of the normal and superconducting states in high-temperature superconductors are also observed in Lu2Fe3Si5. The first is the relationship -dHc2/dT ~ Tc, instead of the one expected for dirty superconductors -dHc2/dT ~ r0. The second is a correlation between the low-temperature linear coefficient a in the resistivity r = r0 + a1T, which appears presumably due to the scattering at magnetic fluctuations, and Tc; this correlation being an evidence of a tight relation between the superconductivity and magnetism. The data point to an unconventional (non-fononic) mechanism of superconductivity in Lu2Fe3Si5, and, probably, in some other Fe-based compounds, which can be fruitfully studied via the radiation-induced disordering.Comment: 7 pages, 8 figure

Localization effects in radiationally disordered high-temperature superconductors: Theoretical interpretation

Theoretical interpretation of recent experiments on radiationally disordered high-temperature superconductors is presented, based on the concepts of mutual interplay of Anderson localization and superconductivity. Microscopic derivation of Ginzburg-Landau coefficients for the quasi-two-dimensional system in the vicinity of localization transition is given in the framework of the self-consistent theory of localization. The 'minimal metallic conductivity' for the quasi-two-dimensional case is enhanced due to a small overlap of electronic states on the nearest neighbor conducting planes. This leads to a stronger influence of localization effects than in ordinary (three-dimensional) superconductors. From this point of view even the initial samples of high-temperature superconductors are already very close to Anderson transition. Anomalies of H(c2) are also analyzed, explaining the upward curvature of H(c2)(T) and apparent independence of dH(c2)/dT (T = T(sub c)) on the degree of disorder as due to localization effects. Researchers discuss the possible reasons of fast T(sub c) degradation due to the enhanced Coulomb effects caused by the disorder induced decrease of localization length. The appearance and growth of localized magnetic moments is also discussed. The disorder dependence of localization length calculated from the experimental data on conductivity correlates reasonably with the theoretical criterion for suppression of superconductivity in the system with localized electronic states