Study on filled-skutterudite arsenide superconductors

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The filled skutterudite phosphides and antimonides have been actively studied because the compounds show wide variety of strongly correlated electron behaviors such as unconventional superconductivity, multipole ordering or Kondo semiconductor. However, filled skutterudite arsenides were not studied so far because a preparation of samples is quite difficult. We have developed the experimental technique of "in situ" observation for crystallization under high temperatures and high pressures using synchrotron radiation. Using the technique, we determined the synthesis conditions for several filled skutterudite arsenide and we have succeeded in synthesizing high quality single-phase samples of the filled skutterudite compounds CeRu_4As_<12>, PrRu_4As_<12>, SmFe_4As_<12> at high temperatures and high pressures. CeRu_4As_<12> exhibits a semiconducting behavior, with small activation energy of 50K. The magnetic and thermal properties suggest that CeRu_4As_<12> is a Kondo semiconductor. We measured specific heat of filled skutterudite superconductor PrRu_4As_<12> (Tc=2.3K) prepared under high pressure in detail. A systematic comparison among Pr-based filled skutterudite compounds and their La-based references reveals that crystalline electric field split of 4f levels play an essential role for superconducting properties in the Pr based-filled skutterudite superconductor. New compound SmFe_4As_<12> shows ferrimagnetism.