Most superconductors subjected to a magnetic field respond by the formation of a mixed state where the material is threaded by a regular lattice of Abrikosov vortices each carrying one quantum of magnetic flux.  For more than half a century, the phenomenological Ginzburg-Landau theory based on the concept of characteristic length scales has provided a surprisingly good description of the Abrikosov vortex lattice state.

Here we report small-angle neutron scattering measurements of the vortex lattice (VL) in the heavy fermion superconductor CeCoIn₅.  Since its discovery, a plethora of interesting phenomena has been observed in this material.  Among these are one of the highest critical temperatures (T_c = 2.3 K) in any heavy fermion superconductor, d-wave pairing symmetry, a paramagnetically limited upper critical field which is first-order at low temperatures, and field- and pressure-induced quantum-critical points and non-Fermi liquid behavior.  Finally, several bulk measurements indicate a phase transition to a non-uniform (Fulde-Ferrell-Larkin-Ovchinnikov) superconducting state just below H_c2 at low temperatures.

We find that the magnetic field dependence of the VL scattered intensity in CeCoIn₅ show a striking departure from the usual exponential decrease, which is  inconsistent with the notion of characteristic length scales and thus marks a qualitative departure from the Abrikosov-Ginzburg-Landau paradigm.  We speculate that this anomalous field dependence arise due to strong paramagnetic effects in this material, combined with the proximity of the superconducting state to a quantum-critical point

If time permits a comparison will be made with measurements on TmNi₂B₂C in the paramagnetic state above T_N.