Coexistence of antiferromagnetism and superconductivity in heavy-fermion systems

We report the novel pressure (P)-temperature (T) phase diagrams of antiferromagnetism (AFM) and superconductivity (SC) in CeRhIn₅, CeIn₃, and CeCu₂Si₂ revealed by the nuclear quadrupole resonance measurement. In the itinerant helical magnet CeRhIn₅, we found that the Néel temperature T_N is reduced at P ≥ 1.23 GPa with an emergent pseudogap behavior. The coexistence of AFM and SC is found in a narrow P range of 1.63-1.75 GPa, followed by the onset of SC with line-node gap over a wide P window 2.1-5 GPa. In CeIn₃, the localized magnetic character is robust against the application of pressure up to P ∼ 1.9GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. In CeCu₂Si₂, the SC and AFM coexist on a microscopic level once its lattice parameter is expanded. We remark that the underlying marginal AFM state is due to collective magnetic excitations in the superconducting state in CeCu₂Si₂. An interplay between AFM and SC is discussed on the SO(5) scenario that unifies AFM and SC. We suggest that the SC and AFM in CeCu₂Si₂ have a common mechanism.