NMR Study of peculiar electronic state in the vortex core of anisotrpic superconducttors

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In the Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state, a pair-breaking due to a Pauli effect is reduced by the formation of new pairing state(k,-k+q) in the superconducting (SC) state. As a result, a new SC state with spatially-oscillating SC order parameter and spin polarization should appear.[1] A newly-discovered CeCoIn_5 has aroused great interest, as recent measurements revealed that a second order phase transition takes place within the SC state in the vicinity of H_<c2> with H parallel to the ab-plane.[2] Recent experimental results make the FFLO scenario a very appealing one for CeCoIn_5. Particularly, In^<115>-NMR spectra exhibited a dramatic change in the vicinity of H_<c2>(T) and with a magnetic field applied parallel to the ab-plane. Based on an analysis with a spatially-modulated SC energy gap, a clear evidence of the FFLO state in high field parallel to the ab-plane is established.[3] The 2D nature of CeCoIn_5 is believed to be essential for the formation of the FFLO state, … More because the strong reduction of the orbital pair breaking in a parallel field and the nesting properties of the quasi-2D Fermi surface are expected to stabilize the FFLO state. Thus, a clarification whether the FFLO state can exist in the case of H//ab is important for understanding the nature of the FFLO state in CeCoIn_5. We here direct our attention to the SC state of CeCoIn_5 in a perpendicular field (H⊥ab) as well as in a parallel field (H//ab), and present evidences of the FFLO state in both cases as well.[4]NMR spectra at the Co, In(1), and In(2) sites have been measured in a wide frequency range for H⊥ab. Upon entering the new SC state at high field and low temperature region, NMR spectra change dramatically below T (H_c) in the case of H⊥ab as well as H//ab. NMR signals at two well-separated positions persists down to the lowest temperature (70~mK), which indicates the appearance of a nordal plane structure of superconducting order parameter. We find the critical field H_<FFLO> separating the FFLO and the BCS state lies in a very narrow range between 4.65 T and 4.70 T below T_c(H).We have obtained discrepancies of the field dependence of the Knight shift among In(1), In(2) and Co site. We have reveal strong Pauli paramagnetic effects on the temperature and field dependences of the Knight shift both in the region of low temperature and in the vicinity of the first order transition at H_<c2>. Less