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

We report on the itinerant ferromagnetic superconductor UGe 2 through 73 Ge-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/ T 1 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.