First-principles Band Structure Research Articles

Electronic structure evolution of single bilayer Bi(1 1 1) film on 3D topological insulator Bi2SexTe3−x surfaces

The electronic state evolution of single bilayer (1BL) Bi(1 1 1) deposited on three-dimensional (3D) Bi2SexTe3−x topological insulators at x = 0, 1.26, 2, 2.46, 3 is systematically investigated by angle-resolved photoemission spectroscopy (ARPES). Our results indicate that the electronic structures of epitaxial Bi films are strongly influenced by the substrate especially the topmost sublayer near the Bi films, manifesting in two main aspects. First, the Se atoms cause a stronger charge transfer effect, which induces a giant Rashba-spin splitting, while the low electronegativity of Te atoms induces a strong hybridization at the interface. Second, the lattice strain notably modifies the band dispersion of the surface bands. Furthermore, our experimental results are elucidated by first-principles band structure calculations.

Crystallographic and optical properties and band structures of CuInSe2, CuIn3Se5, and CuIn5Se8 phases in Cu-poor Cu2Se–In2Se3 pseudo-binary system

We prepared CuInSe2 and Cu-poor Cu–In–Se (CIS) phases such as CuIn3Se5 and CuIn5Se8 in the composition of (1 − x)Cu2Se–(x)In2Se3 with 0.5 ≤ x ≤ 1.0. The crystal structure of the sample changed from chalcopyrite-type CuInSe2 to hexagonal CuIn5Se8 through stannite-type CuIn3Se5 with increasing x (decreasing Cu/In ratio). The band-gap energies of Cu-poor CIS samples, i.e., CuIn3Se5 (1.17 eV) and CuIn5Se8 (1.22–1.24 eV), are larger than that of chalcopyrite-type CuInSe2 (0.99 eV). The energy levels of the valence band maxima (VBMs) were estimated from the ionization energy by photoemission yield spectroscopy (PYS) measurements. The energy levels of the VBMs of the Cu-poor CIS samples decrease rapidly with decreasing Cu/In ratio. The ionization energy of stannite-type CuIn3Se5 is 0.4 eV larger than that of chalcopyrite-type CuInSe2. The ionization energy of CuIn5Se8 is 0.1–0.3 eV larger than that of CuIn3Se5. These results show that the energy position of the VBM from the vacuum level of Cu-poor CIS phases, such as CuIn3Se5 and CuIn5Se8, is deeper than that of CuInSe2. To understand the electronic structure of Cu-poor CIS compounds, we performed first-principles band structure calculations on stannite-type CuIn5Se8 and a reference compound, tetragonal chalcopyrite-type CuInSe2, using the HSE06 nonlocal screened hybrid density functional. The calculated band-gap energy of tetragonal stannite-type CuIn5Se8 (1.19 eV) is larger than that of chalcopyrite-type CuInSe2 (0.94 eV).

Electron-hole asymmetry, Dirac fermions, and quantum magnetoresistance inBaMnBi2

We report two-dimensional quantum transport and Dirac fermions in BaMnBi2 single crystals. BaMnBi2 is a layered bad metal with highly anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, small cyclotronmass, and the first-principles band structure calculations indicate the presence of Dirac fermions in Bi square nets. Quantum oscillations in the Hall channel suggest the presence of both electron and hole pockets, whereas Dirac and parabolic states coexist at the Fermi level.

Electronic localization and bad-metallicity in pure and electron-doped troilite: A local-density-approximation plus dynamical-mean-field-theory study of FeS for lithium-ion batteries

Iron sulfides are promising candidates for the next generation of rechargeable lithium-ion battery materials. Motivated thereby, we present a detailed study of correlation- and doping-induced electronic reconstruction in troilite. Based on local-density-approximation plus dynamical-mean-field-theory, we stress the importance of multi-orbital Coulomb interactions in concert with first-principles band structure calculations for a consistent understanding of intrinsic Mott-Hubbard insulating state in FeS. We explore the anomalous nature of electron doping-induced insulator-bad metal transition, showing that it is driven by orbital-selective dynamical spectral weight transfer. Our results are relevant for understanding charge dynamics upon electrochemical lithiation of iron monosulfides electrode materials for lithium-ion batteries.

Topological fate of edge states of single Bi bilayer on Bi(111)

We address the topological nature of electronic states of step edges of Bi(111) films by first-principles band structure calculations. We confirm that the dispersion of step-edge states reflects the topological nature of underlying films, which become topologically trivial at a thickness larger than eight bilayers. This result clearly conflicts with recent claims that the step-edge state at the surface of a bulk Bi(111) crystal or a sufficiently thick Bi(111) film represents nontrivial edge states of the two-dimensional topological insulator phase expected for a very thin Bi(111) film. The trivial step-edge states have a gigantic spin splitting of one-dimensional Rashba bands and substantial intermixing with electronic states of the bulk, which might be exploited further.

Glass-like Lattice Thermal Conductivity and Thermoelectric Properties of Incommensurate Chimney-Ladder Compound FeGeγ

We have characterized the thermoelectric properties of FeGeγ, which is one of the promising Nowotny chimney-ladder compounds. A glass-like low lattice thermal conductivity below 1.0 W m–1 K–1 is observed around 373 K because of its complex structural nature, i.e., incommensurate structure. A first-principles band structure calculation implies that a narrow band gap of ∼0.2 eV is formed near the Fermi level for a hypothetical composition of Fe2Ge3 with a Ru2Sn3-type structure, leading to a large power factor of 1.90 mW m–1 K–2 near 600 K. The maximum dimensionless figure-of-merit of 0.57 is attractive as a starting point; calculation using the Boltzmann transport equation under a constant relaxation time approximation predicts that a further enhancement of ZT exceeding unity at 600–700 K can be achieved by optimizing the valence electron count per transition metal and further reduction of the lattice thermal conductivity.

Superconductivity in an Effective Model Derived from Wannier Orbitals for an Organic Conductor (TMTSF)2ClO4

We perform first-principles band calculations and derive an effective tight-binding model for a quasi-one-dimensional organic conductor (TMTSF)2ClO4. The first-principles band structures show four bands near the Fermi level. Since four TMTSF molecules are present in the unit cell, we regard a TMTSF molecule as a site and derive an effective tight-binding model exploiting the maximally localized Wannier orbitals. The outer Fermi surface almost contacts the inner one as in the previous studies. We introduce the on-site repulsive interaction and deal with the electron correlation applying the two-particle self-consistent (TPSC) method. The diagonalized spin susceptibility takes large values at the nesting vector between the outer and inner Fermi surfaces. Assuming a pairing mechanism mediated by the spin fluctuations, it is found that the sign of the superconducting gap in the spin-singlet channel changes between the outer and inner Fermi surfaces.

Pressure-driven semiconducting-semimetallic transition in SnSe.

In this work, we report the pressure-dependent electrical transport and structural properties of SnSe. In our experiments an electronic transition from a semiconducting to semimetallic state was observed at 12.6 GPa, followed by an orthorhombic to monoclinic structural transition. Hall effect measurements indicate that both the carrier concentration and mobility vary abnormally accompanied by the semimetallic electronic transition. First-principles band structure calculations confirm the semiconducting-semimetallic transition, and reveal that the semimetallic character of SnSe can be attributed to the enhanced coupling of Sn-5s, Sn-5p, and Se-3p orbitals under compression that results in the broadening of energy bands and subsequently the closure of the band gap. The pressure modulated variations of electrical transport and structural properties may provide an approach to improving the thermoelectric properties of SnSe.

The origin of electronic band structure anomaly in topological crystalline insulator group-IV tellurides

AbstractGroup-IV tellurides have exhibited exotic band structures. Specifically, despite the fact that Sn sits between Ge and Pb in the same column of the periodic table, cubic SnTe is a topological crystalline insulator with band inversion, but both isovalent GeTe and PbTe are trivial semiconductors with normal band order. By performing first-principles band structure calculations, we unravel the origin of this abnormal behaviour by using symmetry analysis and the atomic orbital energy levels and atomic sizes of these elements. In group-IV tellurides, the s lone pair band of the group-IV element is allowed by symmetry to couple with the anion valence p band at the L-point, and such s–p coupling leads to the occurrence of bandgap at the L-point. We find that such s–p coupling is so strong in SnTe that it inverts the band order near the bandgap; however, it is not strong enough in both GeTe and PbTe, so they remain normal semiconductors. The reason for this is the incomplete screening of the core of the relatively tight-binding Ge 4s orbital by its 3d orbitals and the large atomic size and strong relativistic effect in Pb, respectively. Interestingly, we also find that the rhombohedral distortion removes the inversion symmetry and the reduced s–p coupling transforms the α-SnTe back to a normal semiconductor. Our study demonstrates that, in addition to spin–orbital coupling, strain and interface dipole fields, inter-orbital coupling is another effective way to engineer the topological insulators.

Low-energy model and electron-hole doping asymmetry of single-layer Ruddlesden-Popper iridates

We study the correlated electronic structure of single-layer iridates based on structurally-undistorted Ba$_2$IrO$_4$. Starting from the first-principles band structure, the interplay between local Coulomb interactions and spin-orbit coupling is investigated by means of rotational-invariant slave-boson mean-field theory. The evolution from a three-band description towards an anisotropic one-band ($J=1/2$) picture is traced. Single-site and cluster self-energies are used to shed light on competing Slater- and Mott-dominated correlation regimes. We reveal a clear asymmetry between electron and hole doping, notably in the nodal/anti-nodal Fermi-surface dichotomy at strong coupling. Electron-doped iridates appear comparable to hole-doped cuprates due to the different sign of the next-nearest-neighbor hopping $t'$.

Electronic structure of β-Ga2O3 single crystals investigated by hard X-ray photoelectron spectroscopy

By combination of hard X-ray photoelectron spectroscopy (HAXPES) and first-principles band structure calculations, the electronic states of β-Ga2O3 were investigated to deepen the understanding of bulk information for this compound. The valence band spectra of HAXPES presented the main contribution from Ga 4sp, which are well represented by photoionization cross section weighted partial density of states. The experimental data complemented with the theoretical study yield a realistic picture of the electronic structure for β-Ga2O3.

Two-Dimensional Valley Electrons and Excitons in Noncentrosymmetric 3R−MoS2

We find that the motion of the valley electrons -- electronic states close to the ${\rm K}$ and ${\rm K'}$ points of the Brillouin zone -- is confined into two dimension when the layers of MoS$_{2}$ follow the 3R stacking, while in the 2H polytype the bands have dispersion in all the three dimensions. According to our first-principles band structure calculations, the valley states have no interlayer hopping in 3R-MoS$_{2}$, which is proved to be the consequence of the rotational symmetry of the Bloch functions. By measuring the reflectivity spectra and analyzing an anisotropic hydrogen atomic model, we confirm that the valley excitons in 3R-MoS$_{2}$ have two-dimensional hydrogen-like spectral series, and the spreads of the wave function are smaller than the interlayer distance. In contrast, the valley excitons in 2H-MoS$_{2}$ are well described by the three-dimensional model and thus not confined in a single layer. Our results indicate that the dimensionality of the valley degree of freedom can be controlled simply by the stacking geometry, which can be utilized in future valleytronics.

Monitoring surface resonances onCo2MnSi(100)by spin-resolved photoelectron spectroscopy

The magnitude of the spin polarization at the Fermi level of ferromagnetic materials at room temperature is a key property for spintronics. Investigating the Heusler compound Co$_2$MnSi a value of 93$\%$ for the spin polarization has been observed at room temperature, where the high spin polarization is related to a stable surface resonance in the majority band extending deep into the bulk. In particular, we identified in our spectroscopical analysis that this surface resonance is embedded in the bulk continuum with a strong coupling to the majority bulk states. The resonance behaves very bulk-like, as it extends over the first six atomic layers of the corresponding (001)-surface. Our study includes experimental investigations, where the bulk electronic structure as well as surface-related features have been investigated using spin-resolved photoelectron spectroscopy (SR-UPS) and for a larger probing depth spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The results are interpreted in comparison with first-principles band structure and photoemission calculations which consider all relativistic, surface and high-energy effects properly.

Robust flat bands inRCo5(R=rareearth) compounds

The mechanism to realize the peculiar flat bands generally existing in RCo5 (R=rare earth) compounds is clarified by analyzing the first-principles band structures and the tight-binding model. These flat bands are constructed from the localized eigenstates, the existence of which is guaranteed by the partial cancelation between the intersite hopping amplitudes among the Co-3d states at the Kagome sites and those between the Kagome and honeycomb sites. Their relative positions to other bands can be controlled by varying the lattice parameters keeping their dispersion almost flat, which suggests the possibility of flat-band engineering.

Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of ∼0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.

Angle-resolved and resonant photoemission spectroscopy study of the Fermi surface reconstruction in the charge density wave systemsCeTe2andPrTe2

The electronic structures of a charge density wave (CDW) system $R{\mathrm{Te}}_{2}$ $(R=\text{Ce},\text{Pr})$ have been investigated by employing angle-resolved photoemission spectroscopy (ARPES) and the first-principles band structure method. The $R$ $4f$ hybridization peak $(4{f}^{n}{c}^{m\ensuremath{-}1})$ in the $R$ $4f$ PES spectrum is located deeper in ${\mathrm{PrTe}}_{2}$ than in ${\mathrm{CeTe}}_{2}$ and $R$ $4f$ spectral intensity near ${E}_{F}$ is much weaker in ${\mathrm{PrTe}}_{2}$ than in ${\mathrm{CeTe}}_{2}$, implying the importance of the hybridization between Ce $4f$ and Te(1) $5p$ electrons. For both ${\mathrm{CeTe}}_{2}$ and ${\mathrm{PrTe}}_{2}$, the metallic states crossing the Fermi level $({E}_{F})$ are observed below the CDW transition temperature, indicating the existence of the partially ungapped Fermi surfaces (FSs). The zigzag features having the fourfold rotational symmetry are observed near the $X$ point in the FS of ${\mathrm{CeTe}}_{2}$, but not in the FS of ${\mathrm{PrTe}}_{2}$. The tight-binding model calculations show that the zigzag FS features in ${\mathrm{CeTe}}_{2}$ can be described as the CDW-induced FS reconstruction due to the $4\ifmmode\times\else\texttimes\fi{}4$ CDW supercell structure. The effect of the linear dichroism is observed in ARPES, suggesting that the ${E}_{F}$-crossing states have mainly the in-plane orbital character. The photon-energy maps for the near-${E}_{F}$ states exhibit the straight vertical dispersions for both ${\mathrm{CeTe}}_{2}$ and ${\mathrm{PrTe}}_{2}$, demonstrating the dominant two-dimensional character in $R{\mathrm{Te}}_{2}$ $(R=\text{Ce},\text{Pr})$.

Giant thermoelectric effect in graphene-based topological insulators with heavy adatoms and nanopores.

Designing thermoelectric materials with high figure of merit ZT = S(2)GT/Ktot requires fulfilling three often irreconcilable conditions, that is, the high electrical conductance G, small thermal conductance Ktot, and high Seebeck coefficient S. Nanostructuring is one of the promising ways to achieve this goal as it can substantially suppress lattice contribution to Ktot. However, it may also unfavorably influence the electronic transport in an uncontrollable way. Here, we theoretically demonstrate that this issue can be ideally solved by fabricating graphene nanoribbons with heavy adatoms and nanopores. The adatoms locally enhance spin-orbit coupling in graphene thereby converting it into a two-dimensional topological insulator with a band gap in the bulk and robust helical edge states, which carry electrical current and generate a highly optimized power factor S(2)G per helical conducting channel due to narrow boxcar-function-shaped electronic transmission (surpassing even the Mahan-Sofo limit obtained for delta-function-shaped electronic transmission). Concurrently, the array of nanopores impedes the lattice thermal conduction through the bulk. Using quantum transport simulations coupled with first-principles electronic and phononic band structure calculations, the thermoelectric figure of merit is found to reach its maximum ZT ≃ 3 at low temperatures T ≃ 40 K. This paves a way to design high-ZT materials by exploiting the nontrivial topology of electronic states through nanostructuring.

Transition between high-spin and low-spin states in Mn-doped MgO

Using first-principle band structure methods, we investigate the electronic and magnetic properties of Mn-doped MgO in rock salt structure. Two approaches have been compared: the local spin-density approximation (LSDA) and the LSDA + U with a Hubbard-like Coulomb term. In both approximations, by reducing the lattice parameter, a transition from the high-spin () to a low-spin () state has been found. The high-spin state is insulating, whereas the low-spin state corresponds to a half-metallic ferromagnet if the Jahn–Teller effect can be avoided.

Design of laser-coupled honeycomb optical lattices supporting Chern insulators

We introduce an explicit scheme to realize Chern insulating phases employing cold atoms trapped in a state-dependent optical lattice and laser-induced tunneling processes. The scheme uses two internal states, a ground state and a long-lived excited state, respectively trapped in separate triangular and honeycomb optical lattices. A resonant laser coherently coupling the two internal states enables hopping between the two sublattices with a Peierls-like phase factor. Although laser-induced hopping by itself does not lead to topological bands with non-zero Chern numbers, we find that such bands emerge when adding an auxiliary lattice that perturbs the lattice structure, effectively turning it at low energies into a realization of the Haldane model: A two-dimensional honeycomb lattice breaking time-reversal symmetry. We investigate the parameters of the resulting tight-binding model using first-principles band structure calculations to estimate the relevant regimes for experimental implementation.

Pressure-induced topological quantum phase transition in Sb2Se3

Based on the first-principles band structure calculations, we investigate the effects of hydrostatic pressure on the conventional insulator (CI) Sb_2Se_3 and predict that it undergoes a topological quantum phase transition from a CI to a nontrivial topological insulator at a critical pressure value. The pressure-induced topological quantum phase transition is confirmed by calculating the evolution of the bulk energy gap as a function of pressure, the inversion of energy band structure, and the Z_2 topological invariant, as well as the existence of the Dirac-like topological surface states. Our predictions can be tested by both spectroscopy and transport experiments.