Longitudinal spin fluctuations driving field-reinforced superconductivity in UTe${}_2$

Our measurements of ${}^{125}$Te NMR relaxations reveal an enhancement of electronic spin fluctuations above ${\mu }_0{H}^{\ast }\sim 15$ T, leading to their divergence in the vicinity of the metamagnetic transition at ${\mu }_0{H}_m\approx 35$ T, below which field-reinforced superconductivity appears when a magnetic field ($H$) is applied along the crystallographic $b$ axis. The NMR data evidence that these fluctuations are dominantly longitudinal, providing a key to understanding the peculiar superconducting phase diagram in $H\mathrm{\parallel }b$, where such fluctuations enhance the pairing interactions.