When a type-II superconductor is subjected to a magnetic field it 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 (VL) state.
In this talk I will report on extensive small-angle neutron scattering (SANS) measurements of the VL in the heavy fermion superconductor CeCoIn 5 , which show a dramatic departure from the Abrikosov-Ginzburg-Landau behavior. This material has attracted great interest, because it shows strong Pauli paramagnetic effects and also a close proximity of superconductivity to a quantum critical point. At low temperature, the transition to the normal state is first-order, showing that the superconductivity is suppressed by coupling of the field to the anti-parallel spins of the singlet Cooper pair (the Pauli effect) rather than the more common coupling to the orbital motion of Cooper pairs in the mixed state (the orbital effect). Recently, a magnetically ordered phase existing only within the superconducting mixed state was discovered, which occupies the same high field, low temperature region of the superconducting phase diagram as proposed for a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state.
I will present results of SANS measurements with the magnetic field applied both perpendicular and parallel to the basal plane, focusing on the field- and temperature-dependence of the VL form factor, which is a measure of the spatial modulation of the field in the mixed state. For temperatures up to 1.25 K, we observe effects of vortex core paramagnetism resulting in an increase of the form factor with field. Close to H c2 the field contrast decreases again, which is attributed to an expansion of the vortex cores due to paramagnetic suppression of Cooper pairing. The vortex core expansion also provides a possible explanation for the unusual evolution of the vortex lattice symmetry observed CeCoIn 5 . At higher temperatures, a gradual crossover towards a more conventional Abrikosov-Ginzburg-Landau mixed state behavior is observed.