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

TL;DR

This study reveals that longitudinal spin fluctuations in UTe$_2$ are enhanced by magnetic fields, driving field-reinforced superconductivity near a metamagnetic transition, offering insights into the pairing mechanism in this unconventional superconductor.

Contribution

It demonstrates the role of longitudinal spin fluctuations in promoting superconductivity in UTe$_2$ under high magnetic fields, a novel insight into its phase diagram.

Findings

Spin fluctuations diverge near the metamagnetic transition.

Longitudinal fluctuations are dominant and linked to superconductivity.

Field-reinforced superconductivity occurs below the metamagnetic transition.

Abstract

Our measurements of $^{125}$Te NMR relaxations reveal an enhancement of electronic spin fluctuations above $\mu_0H^*\sim15$ T, leading to their divergence in the vicinity of the metamagnetic transition at $\mu_0H_m\approx35$ 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 $H\|b$, where such fluctuations enhance the pairing interactions.