Magnetic resonance probing of ground state in the mixed valence correlated topological insulator SmB.

Introducing of topological insulator concept for fluctuating valence compound - samarium hexaboride - has recently initiated a new round of studies aimed to clarify the nature of the ground state in this extraordinary system with strong electron correlations. Here we discuss the data of magnetic resonance in the pristine single crystals of SmB measured in 60 GHz cavity experiments at temperatures 1.8-300 K. The microwave study as well as the DC resistivity and Hall effect measurements performed for the different states of SmB [110] surface prove definitely the existence of the layer with metallic conductivity increasing under lowering temperature below 5 K. Four lines with the g-factors g ≈ 2 are found to contribute to the ESR-like absorption spectrum that may be attributed to intrinsic paramagnetic centers on the sample's surface, which are robust with respect to the surface treatment. The temperature dependence of integrated intensity I(T) for main paramagnetic signal is found to demonstrate anomalous critical behavior I(T) ~ (T - T) with characteristic temperature T  = 5.34 ± 0.05 K and exponent ν = 0.38 ± 0.03 indicating possible magnetic transition at the SmB [110] surface. Additional resonant magnetoabsorption line, which may be associated with either donor-like defects or cyclotron resonance mode corresponding to the mass m  ~ 1.2m, is reported.