Intertwined degrees of freedom in the series Nd(1-x)Ce(x)CoIn(5)

Strongly correlated electron systems are quantum materials that reveal a deep intertwining between different electronic charge, orbital, spin and lattice degrees of freedom. The interaction among them can stabilize ground states that feature novel collective phenomena and that potentially contribute to the development of future technical applications, if they are understood on a microscopic level. Particular complex quantum phenomena occur in systems containing rare earth elements, where the conduction electrons either screen or couple the magnetic moments of partially filled electronic $f$-states. The subtle balance between these energy scales yields strong electronic fluctuations that trigger a rich diversity of ground states, including unconventional superconductivity, antiferromagnetism or correlated insulating, metallic and topological protected states. CeCoIn$_5$ is a model heavy-fermion $d$-wave superconductor that is believed to be mediated by magnetic fluctuations. Superconductivity is Pauli limited and features an additional phase at very low temperatures and large magnetic fields. This so-called Q-phase reveals magnetic order that only survives inside the superconducting condensate and directly couples to it. Here, we show that the substitution of the local-moment element Nd for Ce in CeCoIn$_5$ tunes the hybridization between the 4$f$-electrons and the conduction band, such that the system is driven into an antiferromagnetic state arising from a small Fermi surface. We demonstrate that the Q-phase is stable under a small perturbation represented by a Nd doping of 5%. The high-field phase is separated from a low-field antiferromagnetic state via a magnetic instability that may origin from a field-induced quantum phase transition. Intriguingly, both phases display an identical magnetic symmetry, which prevents the emergence of a primary order parameter of magnetic nature in the Q-phase. The detailed investigation of the magnetic order in the two phases shows that the spin-density modulation directions are affected differently by a rotation of the magnetic field inside the tetragonal plane. While the anisotropic spin susceptibility in both phases arises from intertwined spin and orbital degrees of freedom, the coupling between superconductivity and magnetism is altered in the high-field state. These results suggest that the field-induced quantum phase transition triggers the emergence of an auxiliary superconducting order parameter in the Q-phase that couples magnetic order with $d$-wave superconductivity. In contrast, we suggest that magnetism and superconductivity are decoupled in the low-field phase. This conclusion is based on the investigation of the low-energy excitation spectrum of Nd$_{0.05}$Ce$_{0.95}$CoIn$_5$ at zero field. We observe magnetic fluctuations that are related to the superconducting condensate, but which are not affected by magnetic order. We suggest that the superconducting resonance consists of Ising-like fluctuations along the direction of static magnetic order