We report low-temperature muon spin relaxation/rotation (µSR) measurements on single crystals of the actinide superconductor UTe2. Below 5 K we observe a continuous slowing down of magnetic fluctuations, which persists through the superconducting transition temperature (Tc = 1.6 K). The temperature dependence of the dynamic relaxation rate down to 0.4 K agrees with the self-consistent renormalization theory of spin fluctuations for a three-dimensional weak itinerant ferromagnetic metal. However, we find no evidence of long-range or local magnetic order down to 0.025 K. Weak transverse-field µSR measurements indicate that the superconductivity coexists with the magnetic fluctuations.

The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry Hentry and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation (µSR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400 • C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure Hentry of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.Recently, to reach high quality factors, a treatment procedure has been established baking cavities at 800 • C and injecting nitrogen gas at the end of this treatment. arXiv:1705.05480v3 [cond-mat.supr-con]

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By means of new muon spin relaxation (µSR) experiments, we disentangle extrinsic and intrinsic sources of low-temperature bulk magnetism in the candidate topological Kondo insulator (TKI) SmB6. Results on Al-flux grown SmB6 single crystals are compared to those on a large floatingzone grown 154 Sm 11 B6 single crystal in which a 14 meV bulk spin exciton has been detected by inelastic neutron scattering (INS). Below ∼ 10 K we detect the gradual development of quasistatic magnetism due to rare-earth impurities and Sm vacancies. Our measurements also reveal two additional forms of intrinsic magnetism: 1) underlying low-energy (∼ 100 neV) weak magnetic moment (∼ 10 −2 µB) fluctuations similar to those detected in the related candidate TKI YbB12 that persist down to millikelvin temperatures, and 2) magnetic fluctuations consistent with a 2.6 meV bulk magnetic excitation at zero magnetic field that appears to hinder surface conductivity above ∼ 4.5 K. We discuss potential origins of the magnetism.

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 UTe2 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 UTe2 are unclear. By way of muon spin rotation/relaxation (μSR) measurements on independently grown UTe2 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 UTe2.