Muon Spin Rotation/Relaxation Studies of the Kondo-Insulator SmB6

Synopsis

The intermediate-valence compound SmB6 is a well-known Kondo insulator, in which the hybridization of itinerant 5d electrons with localized samarium (Sm) 4f electrons leads to the opening of an insulating gap in the bulk. Yet in contrast to expectations for a Kondo insulator, SmB6 exhibits a low-temperature resistivity plateau due to the presence of metallic surface states, as well as low-temperature bulk magnetism of unknown origin.  The presence of low-temperature bulk magnetism in SmB6 is also seemingly at odds with recent studies that suggest SmB6 is a topological insulator, with time-reversal symmetry protected metallic surface states emerging from a fully insulating hybridized bulk band structure.

This thesis is an investigation of the bulk magnetic properties of SmB6 via magnetic susceptibility and muon spin rotation/relaxation (μSR) measurements. The samples studied were grown by an Al-flux method. The bulk magnetic susceptibility measurements show a Curie-Weiss behaviour above 100 K, indicative of the presence of paramagnetic Sm-4f moments.  A magnetic-field dependent upturn in the temperature dependence of the magnetic susceptibility observed below 15 K is attributed to the presence of paramagnetic impurities and/or Sm vacancies. The μSR results collectively provide evidence for the existence of bulk magnetic excitations at temperatures below 20 K. In particular, evidence is provided for the existence of a bulk magnetic excitation on the order of 1 meV, compatible with the existence of magnetic in-gap states and in particular a low-energy spin exciton. In addition, underlying low energy (~ 100 nev) weak magnetic fluctuations are observed to persist down to at least 0.024 K. Similar forms of magnetism are observed in the candidate topological Kondo insulator YbB12, suggesting a common origin. Even so, intrinsic defects cannot be ruled out as a potential source of the low-temperature magnetism in SmB6.

Keywords: SmB6; Kondo insulator; magnetic excitation