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.

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The charge degrees of freedom in several different organic charge transfer salts display slow or glassy dynamics. In order to gain insight into this behaviour, we obtain the low energy theory for an extended dimer Hubbard model, taking into account the occupations of sites on neighbouring dimers. We take a classical limit of the resulting effective model of coupled spins and dimers and study it using classical Monte Carlo simulations. We find that frustration induced by intra-and inter-dimer interactions leads to glassiness in the charge degress of freedom in the absence of ordering of the spin degrees of freedom. Our results may have relevance to experimental observations of relaxor ferroelectric behaviour in the dynamics of organic charge transfer salts.

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