Texture in the superconducting order parameter of CeCoIn₅ revealed by nuclear magnetic resonance

We present a ¹¹⁵In NMR study of the quasi-two-dimensional heavy-fermion superconductor CeCoIn₅ believed to host a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. In the vicinity of the upper critical field and with a magnetic field applied parallel to the ab plane, the NMR spectrum exhibits a dramatic change below T*(H) which well coincides with the position of reported anomalies in specific heat and ultrasound velocity. We argue that our results provide the first microscopic evidence for the occurrence of a spatially modulated superconducting order parameter expected in a FFLO state. The NMR spectrum also implies an anomalous electronic structure of vortex cores.