SmB 6 : Topological insulator or semiconductor with valence-fluctuation induced hopping transport?

Abstract

We advert to the fact that presence of valence fluctuations (VFs) in semiconductors with in-gap impurity bands unconditionally leads to dynamical changes (fluctuations) of energies of localized impurity states. We provide arguments that in the impurity subnetwork consisting of centers having energy levels fluctuating around the Fermi energy there exist favorable conditions for hops from oc-

SmB 6 nanoparticles: Synthesis, valence states, and magnetic properties

“…These interesting optical absorption properties are well satisfied the demand for reducing solar heat for the windows of automotives and architectures, and have also potential applying for medical care [10][11][12]. More recently, the mixed valence SmB 6 as a multi-functional * E-mail address: tegusph@imnu.edu.cn M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT hexaboride is considered the candidate material for topological Kondo insulators [13][14][15] and its optical absorption properties have become a hot research topic [16]. In addition, the SmB 6 have relatively lower work function of 3.11 eV and that makes it to be a potential thermionic emission material [17].…”

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

“…According to Mott-Ioffe-Regel viewpoint, conventional Boltzmann transport theory becomes meaningless if the characteristic mean free path of the itinerant conduction electrons becomes comparable to, or less than the interatomic spacing [11,12], so the requirement for superunitary scattering implicates that either (i) electrical conductivity is not homogeneous in the volume, i.e. material contains metallic regions forming along the sample a conductive path responsible for electrical conductivity at lowest temperatures [13][14][15][16][17] or (ii) electrical transport at lowest temperatures is realized via a hopping-type transport that in contrast to one in conventional semiconductors has to be temperature non-activated [18].…”

“…The second above mentioned approach is represented * Electronic address: batkova@saske.sk by the scenario of valence-fluctuation induced hopping transport [18] that is applicable also to vacant SmB 6based samples. 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. M e 2+ and M e 3+ ) depends on the distribution of ion valences in the vicinity of this impurity (because of different interaction energy of this impurity with ions of different valency due to their different charge, ionic radii, and magnetic moment).…”

“…M e 2+ and M e 3+ ) depends on the distribution of ion valences in the vicinity of this impurity (because of different interaction energy of this impurity with ions of different valency due to their different charge, ionic radii, and magnetic moment). Changes in distribution of ion valences unconditionally cause corresponding changes of the impurity energies and can be modeled by a rearrangement process (RP) with a characteristic time constant, t r [18]. Thus, the energy E i of the impurity i is not constant in time, but varies within some interval E i,min , E i,max [18].…”