Giant transverse magnetic fluctuations at the edge of re-entrant superconductivity in UTe$_{2}$

TL;DR

This study reveals giant transverse magnetic fluctuations at the edge of re-entrant superconductivity in UTe₂, suggesting quantum critical fluctuations may mediate pairing in this unconventional superconductor.

Contribution

The paper introduces magnetotropic susceptibility measurements revealing large transverse magnetic fluctuations in UTe₂, not detectable by conventional magnetization, linked to its re-entrant superconductivity.

Findings

Large decrease in magnetotropic susceptibility indicates increased transverse magnetic susceptibility.

Superconducting phases surround the region of enhanced susceptibility in the phase diagram.

Maximum transverse susceptibility occurs near the critical end point of a metamagnetic transition.

Abstract

UTe$_{2}$ exhibits the remarkable phenomenon of re-entrant superconductivity, whereby the zero-resistance state reappears above 40 tesla after being suppressed with a field of around 10 tesla. One potential pairing mechanism, invoked in the related re-entrant superconductors UCoGe and URhGe, involves transverse fluctuations of a ferromagnetic order parameter. However, the requisite ferromagnetic order - present in both UCoGe and URhGe - is absent in UTe$_{2}$, and magnetization measurements show no sign of strong fluctuations. Here, we measure the magnetotropic susceptibility of UTe$_{2}$ across two field-angle planes. This quantity is sensitive to the magnetic susceptibility in a direction transverse to the applied magnetic field - a quantity that is not accessed in conventional magnetization measurements. We observe a very large decrease in the magnetotropic susceptibility over a broad range of field orientations, indicating a large increase in the transverse magnetic susceptibility. The three superconducting phases of UTe$_{2}$, including the high-field re-entrant phase, surround this region of enhanced susceptibility in the field-angle phase diagram. The strongest transverse susceptibility is found near the critical end point of the high-field metamagnetic transition, suggesting that quantum critical fluctuations of a field-induced magnetic order parameter may be responsible for the large transverse susceptibility, and may provide a pairing mechanism for field-induced superconductivity in UTe$_{2}$.