Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were carried out for the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transitions are reported to occur at T_<Curie>〜3K and T_S〜0.8K [Huy et al.: Phys. Rev. Lett. 99 (2007) 067006], in order to investigate the coexistence of ferromagnetism and superconductivity as well as the normal-state and SC properties from a microscopic point of view. From the nuclear spin-lattice relaxation rate 1/T_1 and Knight-shift measurements, we confirm that ferromagnetic fluctuations that possess a quantum critical character are present above T_<Curie> and also the occurrence of a ferromagnetic transition at 2.5K in our polycrystalline sample. The magnetic fluctuations in the normal state show that UCoGe is an itinerant ferromagnet similar to ZrZn_2 and YCo_2. The onset SC transition is identified at T_S〜0.7K, below which 1/T_1 arising from 30% of the volume fraction starts to decrease due to the opening of the SC gap. This component of 1/T_1, which follows a T^3 dependence in the temperature range 0.3-0.1K, coexists with the magnetic components of 1/T_1 showing a √<T> dependence below T_S. From the NQR measurements in the SC state, we suggest that the self-induced vortex state is realized in UCoGe.
社団法人日本物理学会の論文
2008-02-15