Impurity-induced magnetic droplet in unconventional superconductors near magnetic instability: Application to Nd doped $\mathbf{CeCoIn_5}$

The intricate interplay between magnetism and superconductivity represents one of main challenges in strongly correlated electronic systems. Based on the phenomenological and microscopic modeling, we show that for the Pauli limited $d$-wave superconductors in the vicinity of SDW instability relevant for Nd doped $\mathrm{CeCoIn_5}$, magnetic impurities locally induce droplets of SDW order. At zero field, the Nd impurity spins are along the $c$ axis due to the coupling to the conduction electrons with an easy $c$ axis, besides their own crystal field effect. The in-plane magnetic field cants the impurity moments toward the $ab$ plane, which suppress the droplets of SDW order. At high fields, the long-range SDW inside the superconducting phase is stabilized as a consequence of magnon condensation. Our results are consistent with the recent neutron scattering and thermal conductivity measurements for the Nd doped $\mathrm{CeCoIn_5}$.
Ref: S. -Z. Lin and J. -X. Zhu, arXiv:1705.07174 (2017).