Hisashi Kotegawa, Yusuke Tomita, Hideki Tou, Yoshikazu Mizuguchi, Hiroyuki Takeya, Yoshihiko Takano
We report ^{77}Se-NMR results down to sufficiently low temperature under magnetic fields parallel to both the ab plane and the c axis in a paramagnetic/superconducting (PM/SC) phase of K_xFe_{2-y}Se_2. The observation of anisotropy in the orbital part of Knight shift gives a consequence that anisotropy of its spin part increases on approaching the transition temperature. Anisotropy of Korringa relation suggests that the presence of the weak spin fluctuations with finite wave vector q, which induce the magnetic fluctuations along the ab plane at the Se site. Such fluctuations do not correspond to the stripe (\pi,0) correlation of Fe moment observed in many Fe-based superconductors, and rather give no contradictory to weak (\pi,\pi) correlation. Field dependence of nuclear spin-lattice relaxation rate 1/T_1 shows that field-independent T_1T ~ const. behavior observed at low temperatures for H // ab is attributed to the pair breaking by impurity scattering. The estimated residual density of states is ~6 % of that of the normal state, which is comparable to those of many Fe-based superconductors with nodeless SC gap. Temperature dependence of 1/T_1 is also reproduced well by nodeless models with isotropic two gaps or an anisotropic single gap. The obtained gap-magnitude in the isotropic two-gap model is comparable to those obtained by the angle-resolved photoemission spectroscopy experiments.
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http://arxiv.org/abs/1206.1756
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