Low Temperature Transport and Magnetic Properties of SmB₆

Abstract

We present results of transport and magnetic properties of three single--crystalline samples of the intermediate valence small-gap semiconductor SmB6 at low temperatures. The received resistivity dependences of the samples below 0.5 K exhibit an activated behavior with an energy gap of a few mK. The temperature dependences of the magnetic susceptibility show an increase below 15 K which can be accounted for by impurities, by bare Sm3+ ions or by a small amount of in-gap magnetic 4f 5 5d1 states.PACS numbers: 75.20.Ηr, 71.28. -+ d Samarium hexaboride is a typical representative of intermediate valence semiconductors and is considered as an example of a Kondo insulator [1][2][3][4]. Almost three decades after the first investigations on this material [5], a large number of detailed and comprehensive studies of SmB 6 have been performed. However, many fundamental properties of SmB 6 such as the electrical conductivity and the behavior of magnetic susceptibility at lower temperatures are not fully understood. The resistivity p(T) is slightly temperature dependent down to 50 K below which it starts to increase exponentially [1,5] due to the opening of a hybridization gap in the density of states. Between 15 K and 5 K it can be described by an exponential increase p(T) α exp(-Δ 0 /kΒ Τ), with an activation energy Δ 0 of a few meV [1,[5][6][7][8][9]. However, there exists disagreement about the temperature dependence of p below about 4 K and about the origin of the observed residual conductivity. The residual conductivity has been interpreted in terms of the minimum conductivity [7], as a Wigner lattice formation [9], or attributed to surface states [10]. Thermally activated behavior and hopping transport have been proposed to account for the temperature dependence [5,9] in this temperature range. The magnetic susceptibility reveals the characteristic features of an intermediate valence compound: a Curie-Weiss like susceptibility at temperatures above 100 K indicating a local-moment behavior, and a temperature independent susceptibility at low temperatures due to a nonmagnetic 4f 5 5d1 configuration between localized (419)

Surface and electronic structure of SmB through scanning tunneling microscopy

“…In line with earlier measurements [7,13,46] it exhibits Curie-Weiss-like and van Vleck contributions (due to its intermediate valence) for T 100 K, and non-magnetic behaviour with an enlarged DOS at E F at lowest temperatures. The upturn at lowest temperatures which is typically assigned to paramagnetic impurities [7,46,47] is comparatively tiny for this SmB 6 sample indicating excellent sample quality. Nonetheless, one might speculate that the small difference in χ between zero-field and field cooled measurement may be due to this tiny amount of impurities.…”

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

“…(1) 8 band, which is strongly renormalized. Its position in energy indicates that this peak resides inside the (indirect) Kondo hybridization gap and therefore, it is often referred to as in-gap state [17,52,122,133,[182][183][184][185]. As will be argued below, its weight likely contains contributions beyond a simple co-tunneling model.…”

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

“…Its position in energy indicates that this peak resides inside the Kondo hybridization gap and therefore, it is often referred to as in‐gap state. [ 17,52,122,133,182–185 ] As will be argued below, its weight likely contains contributions beyond a simple co‐tunneling model. This peak certainly manifests the properties of the smaller gap of a few meV observed in studies of thermal activation energies, [ 37,49,72,86,111,166,186 ] which is understood in terms of an indirect bulk gap [ 45,64,187 ] (cf.…”