Anomalous flux line lattice in CeCoIn5

Flux line lattice (FLL) of a quasi-two-dimensional heavy fermion superconductor CeCoIn5 in perpendicular field was investigated by small-angle neutron scattering technique. A square FLL was observed in magnetic fields up to 3 T and at 4 T a splitting of diffraction spots was observed, indicating reorganization of the FLL toward a triangular (rhombic) FLL. The FLL form factor is nearly field-independent up to 4 T. This is in sharp contrast to conventional superconductors, in which the form factor rapidly decays with increasing magnetic field. This behavior of the form factor, which indicates the anomalous field distribution around the vortex cores, can be attributed to strong Pauli paramagnetic effects and to field-tuned quantum critical point effects.