μ+ Knight shift in UTe2: Evidence for relocalization in a Kondo lattice

The local magnetic susceptibility of the spin-triplet superconductor UTe2 has been investigated by positive muon (μ+) Knight shift measurements in the normal state. Three distinct μ+ Knight shift components are observed for a magnetic field applied parallel to the c axis. Two of these exhibit a breakdown in the linear relationship with the bulk magnetic susceptibility (χ) below a temperature T∗∼30K, which points to a gradual emergence of a correlated Kondo liquid. Below Tr∼12K linearity is gradually restored, indicating relocalization of the Kondo liquid quasiparticles. The third Knight shift component is two orders of magnitude larger, and despite the c-axis alignment of the external field, scales with the a axis χ above Tr∼12K. We conjecture that this component is associated with magnetic clusters and the change in the temperature dependence of all three Knight shift components below Tr is associated with a change in magnetic correlations. Our findings indicate that prior to the onset of superconductivity the development of the itinerant heavy-electron fluid is halted by a gradual development of local U 5f-moment fluctuations.