Superconducting vortices in CeCoIn5: Toward the Pauli-limiting field

Many superconducting materials allow the penetration of magnetic fields in a mixed state in which the superfluid is threaded by a regular lattice of Abrikosov vortices, each carrying one quantum of magnetic flux. The phenomenological Ginzburg- Landau theory, based on the concept of characteristic length scales, has generally provided a good description of the Abrikosov vortex lattice state. We conducted neutron- scattering measurements of the vortex lattice form factor in the heavy- fermion superconductor cerium- cobalt- indium ( CeCoIn5) and found that this form factor increases with increasing field- opposite to the expectations within the Abrikosov- Ginzburg- Landau paradigm. We propose that the anomalous field dependence of the form factor arises from Pauli paramagnetic effects around the vortex cores and from the proximity of the superconducting state to a quantum critical point.