S. -H. Baek, L. Harnagea, S. Wurmehl, B. Büchner, H. -J. Grafe
$^{75}$As NMR investigation of a single crystal of superconducting LiFeAs is presented. The Knight shift and the in situ ac susceptibility measurements as a function of temperature and external field reveal two superconducting (SC) transition temperatures, each of which is associated with its own upper critical field. Strikingly, the Knight shift maintains its normal state value in the SC state at high temperatures, whereas it drops abruptly entering the other state at low temperatures indicating the occurrence of spin-singlet pairing. We discuss that the anomalous SC state at high temperatures emerges due to the extreme sensitivity of the system stemming from its inherent closeness to a critical instability toward ferromagnetism.
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http://arxiv.org/abs/1211.1594
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