Model for a strongly correlated insulator: FeSi

Emergence of spin-orbit coupled ferromagnetic surface state derived from Zak phase in a nonmagnetic insulator FeSi

FeSi is a nonmagnetic narrow-gap insulator, exhibiting peculiar charge and spin dynamics beyond a simple band structure picture. Those unusual features have been attracting renewed attention from topological aspects. Although the surface conduction was demonstrated according to size-dependent resistivity in bulk crystals, its topological characteristics and consequent electromagnetic responses remain elusive. Here, we demonstrate an inherent surface ferromagnetic-metal state of FeSi thin films and its strong spin-orbit coupling (SOC) properties through multiple characterizations of two-dimensional conductance, magnetization, and spintronic functionality. Terminated covalent bonding orbitals constitute the polar surface state with momentum-dependent spin textures due to Rashba-type spin splitting, as corroborated by unidirectional magnetoresistance measurements and first-principles calculations. As a consequence of the spin-momentum locking, nonequilibrium spin accumulation causes magnetization switching. These surface properties are closely related to the Zak phase of the bulk band topology. Our findings propose another route to explore noble metal–free materials for SOC-based spin manipulation.

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

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.

Evidence for a conducting surface ground state in high-quality single crystalline FeSi

We report anomalous physical properties of high-quality single-crystalline FeSi over a wide temperature range of 1.8-400 K. The electrical resistivity ρ(T) can be described by activated behavior with an energy gap Δ = 57 meV between 150 and 67 K, below which the estimated energy gap is significantly smaller. The magneto-resistivity and Hall coefficient change sign in the vicinity of 67 K, suggesting a change of dominant charge carriers. At ∼19 K, ρ(T) undergoes a cross-over from semiconducting to metallic behavior which is very robust against external magnetic fields. The low-temperature metallic conductivity depends strongly on the width/thickness of the sample. In addition, no indication of a bulk-phase transition or onset of magnetic order is found down to 2 K from specific heat and magnetic susceptibility measurements. The measurements are consistent with one another and point to complex electronic transport behavior that apparently involves a conducting surface state in FeSi at low temperatures, suggesting the possibility that FeSi is a 3D topological insulator.

Thermoelectricity in correlated narrow-gap semiconductors

We review many-body effects, their microscopic origin, as well as their impact on thermoelectricity in correlated narrow-gap semiconductors. Members of this class-such as FeSi and FeSb₂-display an unusual temperature dependence in various observables: insulating with large thermopowers at low temperatures, they turn bad metals at temperatures much smaller than the size of their gaps. This insulator-to-metal crossover is accompanied by spectral weight-transfers over large energies in the optical conductivity and by a gradual transition from activated to Curie-Weiss-like behaviour in the magnetic susceptibility. We show a retrospective of the understanding of these phenomena, discuss the relation to heavy-fermion Kondo insulators-such as Ce₃Bi₄Pt₃ for which we present new results-and propose a general classification of paramagnetic insulators. From the latter, FeSi emerges as an orbital-selective Kondo insulator. Focussing on intermetallics such as silicides, antimonides, skutterudites, and Heusler compounds we showcase successes and challenges for the realistic simulation of transport properties in the presence of electronic correlations. Further, we explore new avenues in which electronic correlations may contribute to the improvement of thermoelectric performance.

The pressure induced insulator to metal transition in FeSb2

The evolution of the ground state properties of FeSb(2) has been investigated via temperature (4.2-300 K), magnetic field (0-12 T) and pressure (0-8.8 GPa) dependent electrical resistivity studies. The temperature dependence of the resistivity follows activated behavior in the high temperature (HT) regime (T > 60 K), while variable range hopping (VRH) dictates the transport in the intermediate temperature (IT) regime (10 K > T > 45 K) and power law behavior is observed in the low temperature (LT) regime (T < 10 K). The pressure profoundly affects the resistivity in all the temperature regimes. The energy gap (Δ) extracted in the HT regime initially increases with pressure and then decreases, while the VRH parameter T(0) deduced in the IT regime is seen to decrease monotonically and vanish beyond 5 GPa leading to an insulator to metal transition (MIT) on account of delocalization of the electronic states in the gap. The analysis of the logarithmic derivative of the conductivity indicates the MIT to occur at ~6 GPa. The magnetoresistivity is found to be positive. The analysis of the resistivity behavior under pressure and magnetic field indicates that the former induces delocalization, while the latter tends to assist localization of the defect states inside the gap of FeSb(2).

Signatures of electronic correlations in iron silicide

The intermetallic FeSi exhibits an unusual temperature dependence in its electronic and magnetic degrees of freedom, epitomized by the cross-over from a low-temperature nonmagnetic semiconductor to a high-temperature paramagnetic metal with a Curie-Weiss-like susceptibility. Many proposals for this unconventional behavior have been advanced, yet a consensus remains elusive. Using realistic many-body calculations, we here reproduce the signatures of the metal-insulator cross-over in various observables: the spectral function, the optical conductivity, the spin susceptibility, and the Seebeck coefficient. Validated by quantitative agreement with experiment, we then address the underlying microscopic picture. We propose a new scenario in which FeSi is a band insulator at low temperatures and is metalized with increasing temperature through correlation induced incoherence. We explain that the emergent incoherence is linked to the unlocking of iron fluctuating moments, which are almost temperature independent at short timescales. Finally, we make explicit suggestions for improving the thermoelectric performance of FeSi based systems.

Evidence for itineracy in the anticipated Kondo insulator FeSi: a quantitative determination of the band renormalization

A comparison of high-resolution, angle-resolved photoemission spectroscopy (ARPES) data with ab initio band-structure calculations by density functional theory for the anticipated Kondo insulator FeSi shows that the experimental dispersions can quantitatively be described by an itinerant behavior provided that an appropriate self-energy correction is included, whose real part describes the band renormalization due to interactions of the Fe 3d electrons. The imaginary part of the self-energy, on the other hand, determines the linewidth of the quasiparticle peaks in the ARPES data. We use a model self-energy which consistently describes both the renormalized single-particle dispersion and the energy-dependent linewidth of the Fe 3d bands. These results are clear evidence that FeSi is an itinerant semiconductor whose properties can be explained without a local Kondo-like interaction.