Exotic Superconductivity in the Coexistent Phase of Antiferromagnetism and Superconductivity in CeCu₂(Si_0.98Ge_0.02)₂: A Cu-NQR Study under Hydrostatic Pressure

Abstract

We report a pressure (P) effect on CeCu₂(Si_0.98Ge_0.02)₂ where an antiferromagnetic (AFM) order at T_N∼0.75 K coexists with superconductivity below T_c∼0.4 K. At pressures exceeding P=0.19 GPa, the AFM order is suppressed, which demonstrates that the sudden emergence of AFM order due to the Ge doping is ascribed to the intrinsic lattice expansion. The exotic superconductivity at P=0 GPa is found to evolve into a typical heavy-fermion one with a line-node gap above P=0.91 GPa. We highlight that the anomalous enhancement in nuclear spin–lattice relaxation rate 1⁄T₁ that follows a T₁T = const. behavior well below T_c at P=0 GPa is characterized by the persistence of low-lying magnetic excitations, which may be inherent to the coexistent state of antiferromagnetism and superconductivity.