Evidence for uniform coexistence of ferromagnetism and unconventional superconductivity in UGe2. A 73Ge-NQR study under pressure

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 Px ∼ 1.2 GPa. The superconductivity exhibiting a maximum value of Tsc=0.7 K at Px∼ 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 decrease without the coherence peak below Tsc. 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. (author)