Unconventional superconductivity in the itinerant ferromagnet UGe 2: 73Ge-NQR study under pressure

Abstract

The superconducting characteristics on the itinerant ferromagnetic superconductor UGe 2 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 / T 1 have revealed a power-law like behavior as T n without the coherence peak, whereas at low temperatures, it deviates from the T n 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.