Ubiquitous Spin Freezing in the Superconducting State of UTe2

Abstract In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either a quantum of energy from a crystal lattice vibration (phonon) or by long-range interactions such as spin fluctuations. The novel superconductor UTe2 is a rare material wherein electrons form pairs in a unique spin-triplet state with potential topological properties. Theoretically, spin-triplet superconductivity in UTe2 may be explained in terms of pairing mediated by either ferromagnetic or antiferromagnetic fluctuations. Experimentally, the magnetic properties of UTe2 remain enigmatic. Here we report muon spin rotation/relaxation (μSR) measurements on independently grown UTe2 single crystals that exhibit either a single or double phase transition in the specific heat near the onset of superconductivity. In the absence of an applied magnetic field, we observe an inhomogeneous distribution of magnetic fields in a sizeable volume fraction of all samples studied. The growth in the volume of the magnetic regions is halted by the onset of superconductivity at the critical temperature Tc. Upon further cooling, slow fluctuations of the local fields persist until a disordered spin frozen state appears below about one tenth of Tc. The μSR results are consistent with the formation of magnetic clusters in UTe2 due to the influence of disorder on long-range electronic correlations or geometrical magnetic frustration associated with the ladder-like U sublattice structure. Our findings suggest that inhomogeneous magnetic clusters are responsible for the ubiquitous residual linear term and low-temperature upturn in the temperature dependence of the specific heat in UTe2 below Tc. The omnipresent magnetic inhomogeneity has implications for the interpretation of other low-temperature experimental observations in the superconducting state of UTe2.