Abstract:

We report high magnetic field (H) neutron scattering investigations of  the superconducting phase diagram of the clean heavy fermion  superconductor CeCoIn5. By elastic neutron scattering we studied the  connection between the superconducting and magnetic order parameters  in CeCoIn5. Using small angle neutron scattering we explored the H- dependence of the vortex lattice structure and the form factor |F|2  throughout the superconducting phase diagram. Superconductivity in  CeCoIn5 has several unconventional aspects to it: The d-wave  superconductivity is in competition with antiferromagnetic order, as  suggested by the presence of a magnetic quantum critical point located  at the upper critical field Hc2 determined by the Pauli effect. For  fields applied in the ab-plane a second superconducting phase has been  identified in CeCoIn5. By elastic neutron scattering we found that  within this phase, CeCoIn5 adopts a multicomponent ground state which  simultaneously carries cooperating magnetic and superconducting  orders. A symmetry analysis of the coupling between the magnetic order  and the superconducting gap function suggests that the superconducting  order parameter of CeCoIn5 has a non-vanishing momentum.
The strong spin effect in CeCoIn5 is also responsible for the unusual  field dependence of |F|2 observed with small angle neutron scattering.  By applying H along c, as well as in the ab-plane we find that |F|2  increases continuously almost all the way up to Hc2. This finding is  in contrast to that normally observed in type-II superconductors,  where |F|2 decreases with increasing H. It suggests a departure from  the Abrikosov-Ginzburg-Landau paradigm, where the properties of the  vortex state can be described by the coherence length ξ and the  penetration depth λ.

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[2] M. Kenzelmann et al., Science 321, 1652 (2008).