Unconventional superconductivity in the itinerant ferromagnet UGe₂:⁷³Ge-NQR study under pressure

The superconducting characteristics on the itinerant ferromagnetic superconductor UGe₂ have been investigated via the ⁷³Ge-NQR measurements under pressure (P) near the critical pressure P_x where the first-order transition takes place from the low-temperature (T) and low-P ferromagnetic phase (FM2) to high-T and high-P one (FM1). The measurements of nuclear spin-lattice relaxation rate 1 / T₁ have revealed a power-law like behavior as Tⁿ without the coherence peak, whereas at low temperatures, it deviates from the Tⁿ dependence to a T-linear like behavior. These results that depend on the pressure are well understood in terms of non-unitary spin-triplet pairing state where the ferromagnetic up-spin band is gapped, but the down-spin band remains gapless at the Fermi level. We will argue an intimate relationship of the onset of SC with FM1 and FM2.