Study of the FFLO state with Anomalous Vortices in CeCoIn5

准二维重费米子超导体CeCoIn_5由于表现出许多令人着迷的超导性质，继续引起人们的极大兴趣。CeCoIn5是一种非常纯净的d波对称性高κ超导体。CeCoIn_5在高场下的超导电性被泡利顺磁效应破坏，表现为上临界场H_c2处的一级相变。这种高场低温相(HL相)与Pauli极限超导电性密切相关，被认为是Fulde-Ferrell-Larkin-Ovchinnikov(FFLO)态。CeCoIn5中FFLO态的存在得到了几个实验的支持。然而，最近在平行场中的核磁共振和中子数据显示了HL相的长程静态磁序。值得注意的是，当超导电性在H_c2处消失时，这种无公度的自旋密度波序消失，这表明磁性和奇异超导电性是紧密交织在一起的。在这项研究中，为了加深我们对奇异超导电性和磁性如何在CeCoIn5中相互作用的理解，我们测量了三个不同In位的核磁共振谱。核磁共振测量表明，在平行场中的HL相中出现了空间分布的正常准粒子区。顺磁磁化强度和低能准粒子能级密度的场演化可以用与FFLO二级相变形成节面有关的序参量来描述。核磁共振谱还表明，空间均匀的SDW与FFLO节面共存。随着磁场方向与a轴夹角的增大，奇特的HL相被抑制，并在θ~20°范围内消失。我们的结果为FFO态的形成提供了有力的证据，它是通过与静态磁序共存的二级相变而形成的。

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.