Unconventional superconductivity in ferromagnetic UGe₂: A ⁷³Ge nuclear magnetic resonance/nuclear quadrupole resonance study

We report ⁷³Ge nuclear magnetic resonance/nuclear quadrupole resonance (NMR/NQR) measurements on the itinerant ferromagnetic superconductor UGe₂ at ambient pressure (P = 0) and P = 1.3 GPa. Measurements of the nuclear spin-lattice relaxation rate 1/T₁ of the ⁷³Ge NMR at P = 0 have revealed a T₁T = constant behaviour well below T_Curie = 52 K, evidencing the presence of a non-zero density of states at the Fermi level for the up-and down-spin bands in the ferromagnetic state. At P = 1.3 GPa, where the ferromagnetic transition is reduced to T_Curie = 26 K, the dependence on temperature (T) of NQR 1/T₁ at zero field has demonstrated the onset of the superconducting transition at T_c = 0.55 K. The lack of a coherence peak in (1/T₁) just below T_c, followed by a T³-like behaviour, provide compelling evidence for the unconventional nature of the superconducting state that coexists with the ferromagnetic state on a microscopic scale in UGe₂.