Abstract
A myriad of fascinating properties have been proposed for unconventional superconductors in the presence of a strong magnetic field. Among the possible exotic superconducting (SC) phases, a spatially nonuniform SC state originating from the paramagnetism of conduction electrons has become a subject of particular interest after the pioneering work by Fulde and Ferrell and as well Larkin and Ovchinnikov (FFLO) in the mid-1960's. We present here ultrasound and NMR studies of the quasi-2D heavy-fermion superconductor CeCoIn_5 with extremely large Pauli paramagnetic susceptibility, which is believed to host a FFLO state. We are able to establish clear evidences of the spatially-inhomogeneous SC state at high field and low temperatures, precisely as expected in a FFLO state.
