Ubiquitous Spin Freezing in the Superconducting State of UTe2

TL;DR

This study reveals pervasive magnetic cluster freezing in UTe$_2$, a superconductor with potential topological spin-triplet pairing, affecting its low-temperature thermodynamic properties and complicating the understanding of its intrinsic behavior.

Contribution

The paper provides experimental evidence of magnetic inhomogeneity and cluster freezing in UTe$_2$, highlighting its impact on the superconductor's low-temperature properties and the interpretation of its pairing mechanism.

Findings

Magnetic clusters in UTe$_2$ freeze into a disordered state at low temperatures.

Magnetic inhomogeneity correlates with residual linear specific heat term.

Magnetic freezing influences the interpretation of UTe$_2$'s intrinsic properties.

Abstract

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe$_2$ is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe$_2$ are unclear. By way of muon spin rotation/relaxation ($\mu$SR) measurements on independently grown UTe$_2$ single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat ($C$) and the low-temperature upturn in $C/T$ versus $T$. The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe$_2$.