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

TL;DR

This study provides evidence of a unique ferromagnetic spin-pairing superconductivity in UGe₂, revealing an unconventional order parameter with spin-dependent gap structures, mediated by ferromagnetic fluctuations, through $^{73}$Ge-NQR experiments under pressure.

Contribution

It demonstrates the realization of a novel superconducting state with spin-polarized pairing in UGe₂ using NQR under pressure, highlighting the role of ferromagnetic fluctuations.

Findings

Up-spin band is gapped with line nodes.

Down-spin band remains gapless.

Superconductivity is mediated by ferromagnetic spin-density fluctuations.

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.