Although Fe-based superconductors are multiorbital correlated electronic systems, previous nuclei magnetic resonance (NMR) measurement suggests that a single spin-fluid model is sufficient to describe its spin behavior. Here, we firstly observed the breakdown of single spin-fluid model in a heavily hole-doped Fe-based superconductor CsFe2As2 by site-selective NMR measurement. At high temperature regime, both of Knight shift and nuclei spin-lattice relaxation at 133Cs and 75As nuclei exhibit distinct temperature-dependent behavior, suggesting the breakdown of single spin-fluid model in CsFe2As2. This is ascribed to the coexistence of both localized and itinerant spin degree of freedom at 3d orbits, which is consistent with orbital-selective Mott phase. However, single spin-fluid behavior is gradually recovered by developing a coherent state among 3d orbits with decreasing temperature. A Kondo liquid scenario is proposed for the low-temperature coherent state. The present work sets strong constraint on the theoretical model for Fe-based superconductors.
@article{arxiv.1705.09885,
title = {Breakdown of single spin-fluid model in heavily hole-doped superconductor CsFe2As2},
author = {D. Zhao and S. J. Li and N. Z. Wang and J. Li and D. W. Song and L. X. Zheng and L. P. Nie and X. G. Luo and T. Wu and X. H. Chen},
journal= {arXiv preprint arXiv:1705.09885},
year = {2018}
}