Evidence for ferromagnetic spin-pairing superconductivity in UGe2: A 73Ge-NQR study under pressure
Abstract
We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe2 via 73Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure (P). Measurements of the nuclear spin-lattice relaxation rate (1/T1) have revealed an unconventional nature of superconductivity such that the up-spin band is gapped with line nodes, but the down-spin band remains gapless at the Fermi level. This result is consistent with that of a ferromagnetic spin-pairing model in which Cooper pairs are formed among ferromagnetically polarized electrons. The present experiment has shed new light on a possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.