Novel phase diagram of superconductivity and ferromagnetism in UGe ₂: A ⁷³Ge-NQR study under high pressure

We report on the pressure-induced novel phases of ferromagnetism (FM) and superconductivity (SC) in the itinerant ferromagnet UGe₂ through ⁷³Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum points to a first-order transition from strongly to weakly polarized ferromagnetic phases (SP and WP) around a critical pressure of P*_c ∼ 1.2 GPa. Furthermore, it shows the phase separation into the ferromagnetic and paramagnetic phases around P_c ∼ 1.6 GPa. SC exhibiting a maximum value of the superconducting transition temperature T_sc ∼ 0.7 K at P*_c ∼ 1.2 GPa was found to take place in connection with a P-induced first-order transition from SP to WP. The measurement of 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 FM and unconventional SC with a line-node gap. Around P_c where the FM disappears, it is shown that SC occurs under the background of FM, but not in the paramagnetic phase. We remark on an intimate interplay between the onset of SC and the underlying electronic state for SP and WP.