Atomistic investigation of surface characteristics and electronic features at high-purity FeSi(110) presenting interfacial metallicity

Abstract

Iron silicide (FeSi) is a fascinating material that has attracted extensive research efforts for decades, notably revealing unusual temperature-dependent electronic and magnetic characteristics, as well as a close resemblance to the Kondo insulators whereby a coherent picture of intrinsic properties and underlying physics remains to be fully developed. For a better understanding of this narrow-gap semiconductor, we prepared and examined FeSi(110) single-crystal surfaces of high quality. Combined insights from low-temperature scanning tunneling microscopy and density functional theory calculations (DFT) indicate an unreconstructed surface termination presenting rows of Fe–Si pairs. Using high-resolution tunneling spectroscopy (STS), we identify a distinct asymmetric electronic gap in the sub-10 K regime on defect-free terraces. Moreover, the STS data reveal a residual density of states in the gap regime whereby two in-gap states are recognized. The principal origin of these features is rationalized with the help of the DFT-calculated band structure. The computational modeling of a (110)-oriented slab notably evidences the existence of interfacial intragap bands accounting for a markedly increased density of states around the Fermi level. These findings support and provide further insight into the emergence of surface metallicity in the low-temperature regime.

Probing FeSi, a d -electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves

“…The observation that the normalized electrical resistance R ( T )/ R (120 K) below 19 K shows a dependence on the dimensions of the FeSi specimens along with the absence of any features in the specific heat at 19 K, indicated the presence of a CSS ( 1 ). Recent research employing scanning tunneling microscopy on high-quality FeSi single crystals supports the existence of surface conductivity of FeSi and further illustrates the similarity of its correlated electron properties to those of SmB 6 ( 42 ). Electrical transport, magnetization, and polarized neutron reflectometry measurements on FeSi nanofilms revealed a ferromagnetic metallic surface state closely related to the “Zak phase” of the bulk band topology of FeSi ( 43 ).…”

Composition dependence of the specific heat of FeSi

“…Recently, the emergence of a high-mobility surface conduction channel at low temperatures was inferred from the electrical transport properties of a series of single crystals of FeSi prepared under systematic variation of the initial iron content using the optical floating-zone technique [19][20][21][22][23]. This observation was corroborated by means of measurements on thin needles grown from tin flux [24] as well as high-resolution tunneling spectroscopy that revealed two in-gap states in the low-temperature regime of the samples grown by the floating-zone technique [25]. The surface-to-bulk ratios of the charge carrier densities and mobilities observed in the transport properties compare quantitatively with values observed in topological insulators such as Bi 2 Te 3 [19,20,26].…”

A Noble‐Metal‐Free Spintronic System with Proximity‐Enhanced Ferromagnetic Topological Surface State of FeSi above Room Temperature

“…Recent discovery of spin–orbit coupled surface state of FeSi and its spintronic functionality showed an ideal example that may overcome the above technological issues. [ 19 ] While FeSi is a well‐known strongly correlated insulator, [ 20 ] its surface state was revealed to constitute metallic and ferromagnetic bands [ 19,21,22 ] confined to a thickness of 0.35 nm. In particular, as a consequence of its bulk‐band topology with nearly quantized Zak phase, [ 19,23–26 ] surface charges are asymmetrically distributed with respect to the surface atomic layer ( Figure a), causing out‐of‐plane potential gradient and hence Rashba‐type spin splitting of 35 meV at the Fermi energy.…”

“…Recent discovery of spin-orbit coupled surface state of FeSi and its spintronic functionality showed an ideal example that may overcome the above technological issues. [19] While FeSi is a well-known strongly correlated insulator, [20] its surface state was revealed to constitute metallic and ferromagnetic bands [19,21,22] Strongly spin-orbit coupled states at metal interfaces, topological insulators, and 2D materials enable efficient electric control of spin states, offering great potential for spintronics. However, there are still materials challenges to overcome, including the integration into advanced silicon electronics and the scarce resources of constituent heavy elements of those materials.…”