Magnetic resonance probing of ground state in the mixed valence correlated topological insulator SmB6

Abstract

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 SmB6 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 SmB6 [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)ν with characteristic temperature T* = 5.34 ± 0.05 K and exponent ν = 0.38 ± 0.03 indicating possible magnetic transition at the SmB6 [110] surface. Additional resonant magnetoabsorption line, which may be associated with either donor-like defects or cyclotron resonance mode corresponding to the mass mc ~ 1.2m0, is reported.

SmB6: A material with anomalous energy distribution function of charge carriers?

“…The key assumptions of this scenario, namely (i) presence of valence fluctuations (VFs), (ii) existence of an impurity band (IB), and (iii) the Fermi level lying in the IB, are in agreement with theoretical studies of impurity states in mixed-valent materials [23,25,26], studies of electrodynamic properties of SmB 6 in the low-frequency regime [6], optical conductivity studies of SmB 6 [27,28], observation of presence of variable-range hopping transport in SmB 6 at lowest temperatures [7], and many other experimental observations reported for SmB 6 (see e.g. [6,[27][28][29] and references therein). As explained in [18], the energy of an impurity in the semiconductor with an IB that contains metallic ions in two different valency states (e.g.…”

“…Nevertheless in can be still expected that such change of E 0 will be associated with change of physical properties of the system in a certain temperature region. As possible (or perhaps accidental) analogy with the above provided picture we would like to mention very recent ESR studies of SmB 6 [29]. The ESR signal observed at T = 5 K was not more observed at T = 6 K [29]; moreover, the critical behavior of the integrated ESR intensity, I(T ) ∼ (T * − T ) ν , with characteristic temperature T * = 5.34 ± 0.05 K and exponent ν = 0.38 ± 0.03 was observed, and the authors [29] mentioned a possibility of "some abrupt structural/magnetic transition" to explain experimental results and emphasized the importance of re-considering "5 K anomaly" in context of modern topological physics of SmB 6 .…”

An STM perspective on hexaborides: Surface states of the Kondo insulator SmB$_6$

“…In consequence, several studies have been conducted to compare the impact of non-magnetic and magnetic impurities on the surface states [102,149,150,207,208] for insight into the surface states. It should also be noted that SmB 6 doped with Gd or Eu was also used for electron spin resonance (ESR) investigations [209,210]. Moreover, Gd-and Fe-substituted samples were investigated in Ref.…”

An STM Perspective on Hexaborides: Surface States of the Kondo Insulator SmB6

“…It should also be noted that SmB 6 doped with Gd or Eu was also used for electron spin resonance (ESR) investigations. [209,210] Moreover, Gd-and Fe-substituted samples were investigated in ref. [128] to identify the 2 × 1 reconstructed surface.…”