Evidence for Uniform Coexistence of Ferromagnetism and Unconventional Superconductivity in UGe₂: A ⁷³Ge-NQR Study under Pressure

Abstract

We report on the itinerant ferromagnetic superconductor UGe₂ through ⁷³Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum signals a first-order transition from the low-temperature (T) and low-P ferromagnetic phase (FM2) to high-T and high-P one (FM1) around a critical pressure of P_x∼1.2 GPa. The superconductivity exhibiting a maximum value of T_sc=0.7 K at P_x∼1.2 GPa, was found to take place in connection with the P-induced first-order transition. The nuclear spin–lattice relaxation rate 1⁄T₁ has probed the ferromagnetic transition, exhibiting a peak at the Curie temperature as well as a decrease without the coherence peak below T_sc. These results reveal the uniformly coexistent phase of ferromagnetism and unconventional superconductivity with a line–node gap. We remark on an intimate interplay between the onset of superconductivity and the underlying electronic state for the ferromagnetic phases.