Study of the FFLO state with Anomalous Vortices in CeCoIn5

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The quasi-two dimensional heavy fermion superconductor CeCoIn_5 continues to excite great interest, because it shows a number of fascinating superconducting properties. CeCoIn_5 is an extremely clean high-κ superconductor with d-wave symmetry. The superconductivity of CeCoIn_5 at high fields is destroyed by Pauli paramagnetic effect, as evidenced by the first-order phase transition at the upper critical field H_c2. Closely related to the Pauli limited superconductivity, this high-field and low-temperature phase (HL-phase) has been attributed to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The presence of the FFLO state in CeCoIn5 has been supported by several experiments. However, recent NMR and neutron data in parallel field demonstrated a long-range static magnetic order in the HL-phase. Remarkably this incommensurate spin-density wave order vanishes when the superconductivity dies at H_c2, indicating that the magnetism and the exotic superconductivity are closely intertwined.In this study, to improve our understanding of how the exotic superconductivity and magnetism can interact in CeCoIn_5, we measured NMR spectra on the three distinct In sites. ^<115>In NMR measurements demonstrate the emergence of a spatially distributed normal quasiparticle region in the HL-phase in parallel field. The field evolution of the paramagnetic magnetization and low energy quasiparticle DOS can be described well by the order parameter associated with the nodal plane formation via the FFLO second order phase transition. The NMR spectra also reveal that the spatially uniform SDW coexists with the FFLO nodal planes. The exotic HL-phase is suppressed with increasing the angle of the direction of magnetic field with respect to the a-axis, and disappears beyond θ～20°.Our results provide strong evidence for the formation of the FFLO state via the second order phase transition, which coexists with the static magnetic order.