Evidence for ferromagnetic spin-pairing superconductivity in UGe${}_2$: A ${}^{73}$Ge-NQR study under pressure

Abstract

We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe${}_2$ via ${}^{73}$Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure ($P$). Measurements of the nuclear spin-lattice relaxation rate $(1/T_1)$ 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.

5 pages, 5 figures

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