Experimental evidence for ferromagnetic spin-pairing superconductivity emerging in U Ge2: A Ge73 -nuclear-quadrupole-resonance study under pressure

We report that a different type of superconducting order parameter has been realized in the ferromagnetic states in U Ge2 via Ge73 -nuclear-quadrupole- resonance 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 light on the possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.