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.

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The intermediate-valence compound SmB6 is a well-known Kondo insulator, in which hybridization of itinerant 5d electrons with localized 4f electrons leads to a transition from metallic to insulating behavior at low temperatures. Recent studies suggest that SmB6 is a topological insulator, with topological metallic surface states emerging from a fully insulating hybridized bulk band structure. Here we locally probe the bulk magnetic properties of pure and 0.5 % Fe-doped SmB6 by muon spin rotation/relaxation (µSR) methods. Below 6 K the Fe impurity induces simultaneous changes in the bulk local magnetism and the electrical conductivity. In the low-T insulating bulk state we observe a temperature-independent dynamic relaxation rate indicative of low-lying magnetic excitations driven primarily by quantum fluctuations.Topological insulators are exotic quantum states of matter characterized by an electrically insulating bulk and topologically-protected metallic surface states. Due to an interplay of strong correlations and strong spinorbit coupling of the 4f electrons, SmB 6 is predicted to develop a non-trivial Z 2 topological insulating state. 1 Angle-resolved photoemission 2 and point-contact spectroscopy 3 measurements show that the crossover from the bulk high-T metallic state to the low-T Kondo insulating phase occurs gradually over a fairly wide temperature range (30 K < T < 110 K). Transport measurements show that surface electrical conduction occurs below T ∼ 5 to 6 K with a resistance that saturates at lower temperature. 4-6 The low-T conduction arises from twodimensional states 7 that occur in the hybridization gap exclusively at the surface, 3,4,8 as expected for metallic surface states of topological origin. 9 Yet the ground state of SmB 6 is still unclear, in part because not all bulk properties at low T are that of a conventional band-gapped insulator. Despite the loss of bulk electrical conduction, quantum oscillations consistent with a bulk Fermi surface have been observed, 10 and the low-temperature specific heat exhibits a significant bulk residual T -linear term typical of a metallic state. 11 Recently, it has been argued that there is some residual bulk electrical conductivity in SmB 6 below 4 K. 12 There also exists significant bulk acconduction arising from low-energy states in the Kondo gap. 13Nuclear magnetic resonance (NMR) Knight shift and spin-lattice relaxation rate (1/T 1 ) measurements, 14 bulk magnetic susceptibility, 15 Raman spectroscopy, 16,17 and inelastic neutron scattering (INS) 18,19 studies of SmB 6 reveal the emergence of bulk in-gap bound states of a different origin below T ∼ 20-30 K. The sharp dispersive magnetic excitations observed at 14 meV within the hybridization gap by INS have been attributed to a bulk collective spin exciton resonance mode due to residual antiferromagnetic (AFM) quasiparticle interactions. 20,21 These bound magnetic quasiparticle states are robust due to the protection provided by the hybridization gap, and there is evidence that the spin excitons co...

We report low temperature muon spin relaxation (µSR) measurements of the high-transitiontemperature (Tc) cuprate superconductors Bi2+xSr2−xCaCu2O 8+δ and YBa2Cu3O6.57, aimed at detecting the mysterious intra-unit cell (IUC) magnetic order that has been observed by spin polarized neutron scattering in the pseudogap phase of four different cuprate families. A lack of confirmation by local magnetic probe methods has raised the possibility that the magnetic order fluctuates slowly enough to appear static on the time scale of neutron scattering, but too fast to affect µSR or nuclear magnetic resonance (NMR) signals. The IUC magnetic order has been linked to a theoretical model for the cuprates, which predicts a long-range ordered phase of electron-current loop order that terminates at a quantum crictical point (QCP). Our study suggests that lowering the temperature to T ∼ 25 mK and moving far below the purported QCP does not cause enough of a slowing down of fluctuations for the IUC magnetic order to become detectable on the time scale of µSR. Our measurements place narrow limits on the fluctuation rate of this unidentified magnetic order.PACS numbers: 74.25.Ha, 76.75.+i An enduring and central open question concerning cuprate superconductors is the nature of the mysterious pseudogap regime above T c . Achieving an understanding of the pseudogap (PG) has long been viewed as key to understanding high-T c superconductivity. A clue to the origin of the PG has come from spinpolarized neutron diffraction studies that have detected the onset of an unusual three-dimensional (3-D), longrange IUC magnetic order at a temperature concomitant with the PG onset temperature T * in YBa 2 Cu 3 O 6+x (Y123), HgBa 2 CuO 4+δ (Hg1201) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212). 1-7 This finding provides evidence for a change in symmetry at T * associated with the onset of a novel type of order, which is supported by other kinds of measurements that indicate the the PG is related to a true phase transition. 9-12 The magnetic order observed by polarized neutron diffraction is described by staggered out-of-plane magnetic moments that diminish in magnitude from the underdoped to optimally-doped regime. 7,8 A similar mysterious magnetic order is also observed in x = 0.085 La 2−x Sr x CuO 4 (LSCO), 13 although it is short-range, two-dimensional, and onsets at a temperature far below T * . The latter is also the case in underdoped YBa 2 Cu 3 O 6.45 -suggesting a potential competition with Cu spin density wave order at low doping.The magnetic structure and the hole-doping dependence of the onset temperature of the IUC magnetic order are somewhat compatible with a model derived from a three-band Hubbard model, which attributes the PG to a time-reversal symmetry breaking phase consisting of a pattern of circulating electron currents that preserve translational symmetry. 14 With increased hole doping the transition temperature of the circulating-current (CC) ordered phase is reduced towards zero, terminating at a QCP within the superconducting phase n...

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