High-field state of the flux-line lattice in the unconventional superconductor CeCoIn₅

Ultrasound velocity measurements of the unconventional superconductor CeCoIn₅ with extremely, large Pauli paramagnetic susceptibility reveal an unusual structural transformation of the flux-line lattice (FLL) in the vicinity of the upper critical field. The transition field coincides with that at which heat capacity measurements reveal a second-order phase transition. The lowering of the sound velocity at the transition is consistent with vortex segmentation and a crossover to quasi-two-dimensional FLL pinning. These results provide strong evidence that the high-field state is the Fulde-Ferrel-Larkin- Ovchinikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices.