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.
@article{arxiv.0705.2944,
title = {Evidence for ferromagnetic spin-pairing superconductivity in UGe$_2$: A $^{73}$Ge-NQR study under pressure},
author = {A. Harada and S. Kawasaki and H. Mukuda and Y. Kitaoka and Y. Haga and E. Yamamoto and Y. Onuki and K. M. Itoh and E. E. Haller and H. Harima},
journal= {arXiv preprint arXiv:0705.2944},
year = {2009}
}