First-principles Band Structure Calculations Research Articles

N-Type Ohmic contact and p-type Schottky contact of monolayer InSe transistors.

Owing to their few lateral dangling bonds and enhanced gate electrostatics, two-dimensional semiconductors have attracted much attention for the fabrication of channels in next-generation field-effect transistors (FETs). Herein, combining first-principle band structure calculations with more precise quantum transport simulations, we systematically explore the interface properties between monolayer (ML) indium selenide (InSe) and a sequence of common electrodes in an FET. The ML InSe band structure is damaged by Sc, Au, Cr, Pt, and Pd electrodes but identifiable in contact with Ag, Cu, In, graphene and ML O-terminated Cr2C electrodes. A lateral n-type Schottky contact is generated with Sc, Au, Cr, Pt, Pd, and ML graphene electrodes owing to Fermi level pinning originating from the metal-induced gap states, which feature a pinning factor of 0.32. Luckily, a highly desirable lateral n-type Ohmic contact is generated with the Ag, Cu, and In electrodes. The calculated contact polarity is in agreement with the available experimental results using Au, Cr, ML graphene, Ag, and In as electrodes. Remarkably, a lateral p-type Schottky contact is generated with ML O-terminated Cr2C despite the very high work function of ML InSe. Therefore, this study offers a deeper understanding of ML InSe device interfaces and instructions for the design of ML InSe transistors.

Emergence of undulating surface band upon oxygen adsorption of Fe thin film on W(110)

We have performed high-resolution angle-resolved photoemission spectroscopy of oxygen-adsorbed Fe thin films on W(110) to elucidate the evolution of electronic states upon surface oxidation. After oxygen adsorption of 0.25 monolayer onto Fe films, we found a surface state in the vicinity of the Fermi level which exhibits an undulating energy band dispersion. Our first-principles band-structure calculation indicates that this state is attributed to the spin-polarized minority band arising from the hybridization between Fe 3d and O 2p orbitals. The present result suggests that the observed surface state is related to the unusual magnetic properties of oxidized Fe surfaces.

Impact of Mismatch Angle on Electronic Transport Across Grain Boundaries and Interfaces in 2D Materials

We study the impact of grain boundaries (GB) and misorientation angles between grains on electronic transport in 2-dimensional materials. Here we have developed a numerical model based on the first-principles electronic bandstructure calculations in conjunction with a method which computes electron transmission coefficients from simultaneous conservation of energy and momentum at the interface to essentially evaluate GB/interface resistance in a Landauer formalism. We find that the resistance across graphene GBs vary over a wide range depending on misorientation angles and type of GBs, starting from 53 Ω μm for low-mismatch angles in twin (symmetric) GBs to about 1020 Ω μm for 21° mismatch in tilt (asymmetric) GBs. On the other hand, misorientation angles have weak influence on the resistance across MoS2 GBs, ranging from about 130 Ω μm for low mismatch angles to about 6000 Ω μm for 21°. The interface resistance across graphene-MoS2 heterojunctions also exhibits a strong dependence on misorientation angles with resistance values ranging from about 100 Ω μm for low-mismatch angles in Class-I (symmetric) interfaces to 1015 Ω μm for 14° mismatch in Class-II (asymmetric) interfaces. Overall, symmetric homo/heterojunctions exhibit a weak dependence on misorientation angles, while in MoS2 both symmetric and asymmetric GBs show a gradual dependence on mismatch angles.

Theoretical band structure of the superconducting antiperovskite oxide [formula omitted]SnO

In order to investigate the position of the strontium deficiency in superconductive Sr3−xSnO, we synthesized and measured X-ray-diffraction patterns of Sr3−xSnO (x∼0.5). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on Sr3−xSnO (x=0, 0.5) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of Sr3−xSnO in case of x∼0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr3SnO, where the mixing of the valence p and conduction d orbitals are considered to be small.

Electronic structure and optical properties of Ag2HgSnSe4: First-principles DFT calculations and X-ray spectroscopy studies

Complex study of the electronic structure of the Ag2HgSnSe4 compound employing both experimental and theoretical methods was carried out in the present work. In particular, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+ ion-irradiated surfaces of Ag2HgSnSe4 single crystal have been measured. The present X-ray photoelectron spectroscopy (XPS) results indicate that Ag2HgSnSe4 single crystal surface is sensitive with respect to Ar+ ion-irradiation. Particularly, Ar+ ion bombardment of the single crystal surface causes significant modification in topmost surface layers resulting in abrupt decreasing the content of Hg atoms in the layers. The X-ray emission band representing the energy distribution of the Se 4p states was measured and compared with the XPS valence-band spectrum provided that a common energy scale is used. Results of such comparison reveal that the main contributions of the Se 4p states occur at the top and in the upper portion of the valence band. The experimental findings were confirmed by first-principles band-structure calculations performed in the present work employing the augmented plane wave+local orbitals (APW+lo) method as realized in the WIEN2k package. The calculations indicate that the Hg 5d and Ag 4d states are the principal contributors to the bottom and the central part of the Ag2HgSnSe4 valence band, respectively, while its upper part is dominated by contributions of the Se 4p states. Regarding the occupancy of the conduction band, the present APW+lo calculations manifest that its bottom is dominated by the unoccupied Sn 5s states. The principal optical characteristics of Ag2HgSnSe4 are elucidated by the calculations.

Topological Hopf and Chain Link Semimetal States and Their Application to Co_{2}MnGa.

Topological semimetals can be classified by the connectivity and dimensionality of the band crossings in momentum space. The band crossings of a Dirac, Weyl, or an unconventional fermion semimetal are zero-dimensional (0D) points, whereas the band crossings of a nodal-line semimetal are one-dimensional (1D) closed loops. Here we propose that the presence of perpendicular crystalline mirror planes can protect three-dimensional (3D) band crossings characterized by nontrivial links such as a Hopf link or a coupled chain, giving rise to a variety of new types of topological semimetals. We show that the nontrivial winding number protects topological surface states distinct from those in previously known topological semimetals with a vanishing spin-orbit interaction. We also show that these nontrivial links can be engineered by tuning the mirror eigenvalues associated with the perpendicular mirror planes. Using first-principles band structure calculations, we predict the ferromagnetic full Heusler compound Co_{2}MnGa as a candidate. Both Hopf link and chainlike bulk band crossings and unconventional topological surface states are identified.

Semimetallic behavior in Heusler-type Ru2TaAl and thermoelectric performance improved by off-stoichiometry

We report a study of the temperature-dependent electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) in Heusler-type ${\mathrm{Ru}}_{2}\mathrm{TaAl}$, to shed light on its semimetallic behavior. While the temperature dependence of the electrical resistivity exhibits semiconductorlike behavior, the analysis of low-temperature specific heat reveals a residual Fermi-level density of states (DOS). Both observations can be realized by means of a semimetallic scenario with the Fermi energy located in the pseudogap of the electronic DOS. The NMR Knight shift and spin-lattice relaxation rate show activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap in ${\mathrm{Ru}}_{2}\mathrm{TaAl}$. From the first-principles band structure calculations, we further provide a clear picture that an indirect overlap between electron and hole pockets is responsible for the formation of a pseudogap in the vicinity of the Fermi level of ${\mathrm{Ru}}_{2}\mathrm{TaAl}$. In addition, an effort for improving the thermoelectric performance of ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ has been made by investigating the thermoelectric properties of ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$. We found significant enhancements in the electrical conductivity and Seebeck coefficient and marked reduction in the thermal conductivity via the off-stoichiometric approach. This leads to an increase in the figure-of-merit $ZT$ value from $6.1\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{\ensuremath{-}4}$ in ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ to $3.4\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{\ensuremath{-}3}$ in ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$ at room temperature. In this respect, a further improvement of thermoelectric performance based on ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ through other off-stoichiometric attempts is highly probable.

Electronic structure and thermoelectric properties of narrow-band-gap intermetallic compound Al2Fe3Si3

We investigated the electronic structure and thermoelectric properties of a ternary intermetallic compound Al2Fe3Si3 composed only of abundant elements. First-principles band structure calculations implied that the target compound Al2Fe3Si3 forms a narrow band gap of a few hundred meV near the Fermi level, indicating that both p- and n-type thermoelectric properties can be obtained by shifting the chemical potential. We present the thermoelectric properties of the narrow-band-gap intermetallic compound Al2Fe3Si3, which was synthesized by arc melting and spark plasma sintering. The conduction type could be controlled through changing the Al/Si ratio. The measured Seebeck coefficients were +55 and −90 μV K−1 for p- and n-type materials, respectively. The obtained power factor was 370–400 μW m−1 K−2 in a mid-temperature region for both p- and n-type materials. Because of the low crystal symmetry of Al2Fe3Si3, the phonon thermal conductivity at 300 K exhibited a relatively low value of 4.5–5.5 W m−1 K−1, despite being composed of relatively light elements. The estimated dimensionless figure of merit was found to be less than 0.04, which should be largely enhanced for practical applications.

Precursor synthesis and magnetic properties of Cd1-xFexO (0 ≤ x ≤ 0.07) polycrystalline solid solutions

Polycrystalline solid solutions Cd1-xFexO (0 < x ≤ 0.07) have been synthesized by the precursor method using formate of the composition Cd1-xFex(HCOO)2·2H2O as a precursor. For the production of Cd1-xFexO, the precursor was annealed in air at a maximal temperature of 400 °C. At x ≥ 0.075, an impurity CdFe2O4 ferrite phase with cubic spinel structure was detected in the samples. The examination of the magnetic properties of Cd1-xFexO at 5 and 300 K revealed that they are ferromagnets, whose magnetization increases with the concentration of iron both at low and room temperature reaching the maximal values in samples with х = 0.07. In order to determine the degree of oxidation and the spin state of iron atoms, the main interatomic interactions giving rise to ferromagnetism as well as to estimate the Curie temperatures, we also performed first-principle calculations of the electronic band structure. They showed the existence of chemical bonding between adjacent iron atoms leading to the appearance of ferromagnetism by direct exchange. The value of the Curie temperature calculated by the mean field method on the basis of the first-principle calculations is 507 K, which is in reasonable agreement with the experimental value of 560 K.

Ternary Weyl semimetal NbIrTe4 proposed from first-principles calculation

Based on first-principles band-structure calculations, we predict that an inversion-breaking, stoichiometric single crystal ${\mathrm{NbIrTe}}_{4}$ is a Weyl semimetal candidate. Without spin-orbit coupling (SOC), we find that there are only eight Weyl points in the ${k}_{z}=0$ plane. The separation between Weyl points of opposite chiralities is determined by the magnitude of the band inversion. When considering the SOC, ${\mathrm{NbIrTe}}_{4}$ hosts 16 Weyl points in the whole Brillouin zone. Actually, there are only three irreducible Weyl points and two of them are type-II Weyl points. Finally, the surface-state Fermi arc structures on (001) surface are also obtained. The results on the surface correspond with the properties in the bulk.

Magnetotransport properties of the single-crystalline nodal-line semimetal candidatesCaTX(T=Ag,Cd;X=As,Ge)

Topological semimetals are characterized by protected crossings between conduction and valence bands. These materials have recently attracted significant interest because of the deep connections to high-energy physics, the novel topological surface states, and the unusual transport phenomena. While Dirac and Weyl semimetals have been extensively studied, the nodal-line semimetal remains largely unexplored due to the lack of an ideal material platform. In this paper, we report the magneto-transport properties of two nodal-line semimetal candidates CaAgAs and CaCdGe. First, our single crystalline CaAgAs supports the first "hydrogen atom" nodal-line semimetal, where only the topological nodal-line is present at the Fermi level. Second, our CaCdGe sample provides an ideal platform to perform comparative studies because it features the same topological nodal line but has a more complicated Fermiology with irrelevant Fermi pockets. As a result, the magnetoresistance of our CaCdGe sample is more than 100 times larger than that of CaAgAs. Through our systematic magneto-transport and first-principles band structure calculations, we show that our CaTX compounds can be used to study, isolate, and control the novel topological nodal-line physics in real materials.

Band splitting and Weyl nodes in trigonal tellurium studied by angle-resolved photoemission spectroscopy and density functional theory

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on trigonal tellurium consisting of helical chains in the crystal. Through the band-structure mapping in the three-dimensional Brillouin zone, we found a definitive evidence for the band splitting originating from the chiral nature of crystal. A direct comparison of the band dispersion between the ARPES results and the first-principles band-structure calculations suggests the presence of Weyl nodes and tiny spin-polarized hole pockets around the H point. The present result opens a pathway toward studying the interplay among crystal symmetry, band structure, and exotic physical properties in chiral crystals.

Crystallographic, optical, and electronic properties of (Cu,Li)InS2 system

To investigate the effect of a lithium doping in chalcopyrite-type CuInS2 on its crystallographic and optical properties, we prepared (Cu,Li)InS2 samples by a mechanochemical process and sequential heating at 550°C in H2S gas. The single-phase chalcopyrite-type (Cu1−xLix)InS2 solid solution could be obtained for the samples with 0.0≤x≤0.10. The lattice constants a and c of tetragonal chalcopyrite-type (Cu1−xLix)InS2 solid solution increase with increasing Li content. The band gap energy of the (Cu1−xLix)InS2 solid solution linearly increased from 1.44eV of CuInS2 (x=0.0) to 1.54eV of (Cu0.90Li0.10)InS2 (x=0.10). We performed first-principles band structure calculations for chalcopyrite-type (Cu1−xLix)InS2, using the HSE06 nonlocal screened hybrid density functional. Theoretically, the band-gap energies of (Cu1−xLix)InS2 increase with increasing Li content. The energy levels of the valence band maxima (VBMs) were estimated from the ionization energy measured by photoemission yield spectroscopy. The ionization energy of chalcopyrite-type (Cu1−xLix)InS2 solid solutions increased from 5.45eV of CuInS2 (x=0.0) to 5.79eV of (Cu0.90Li0.10)InS2 (x=0.10). The energy level of the VBM of the (Cu1−xLix)InS2 solid solution considerably decreases with increasing Li content, x. The conduction band minimum (CBM) level of the (Cu1−xLix)InS2 solid solution also decreases with increasing Li content. The Li-doping in CuInS2 is expected to be useful for decreasing the VBM of CuInS2 absorber and increasing the band-gap energy of CuInS2 absorber without increasing the CBM level.

Electronic reconstruction of hexagonal FeS: a view from density functional dynamical mean-field theory

We present a detailed study of correlation- and pressure-induced electronic reconstruction in hexagonal iron monosulfide, a system which is widely found in meteorites and one of the components of Earth’s core. Based on a perusal of experimental data, we stress the importance of multi-orbital electron-electron interactions in concert with first-principles band structure calculations for a consistent understanding of its intrinsic Mott–Hubbard insulating state. We explain the anomalous nature of pressure-induced insulator-metal-insulator transition seen in experiment, showing that it is driven by dynamical spectral weight transfer in response to changes in the crystal-field splittings under pressure. As a byproduct of this analysis, we confirm that the electronic transitions observed in pristine FeS at moderated pressures are triggered by changes in the spin state which causes orbital-selective Kondo quasiparticle electronic reconstruction at low energies.

Magnetic evolution of itinerant ferromagnetism and interlayer antiferromagnetism in cerium doped LaCo2P2 crystals

ThCr2Si2-type phosphide ACo2P2 (A=Eu, La, Pr, Nd, Ce) has the same structure as iron arsenides, but their magnetic behaviors are quite distinct. In this paper, we grew a series of La1−xCexCo2P2 single crystals (x=0.0 to1.0), made structural and magnetic characterizations. We found the introduction of cerium induces a rapid decrease of c-axis and a change from ferromagnetic to antiferromagnetic states. Compared to other trivalent doped compounds, the enhancement of ferromagnetism with doping is suppressed and the transition from ferromagnetism to antiferromagnetism appear earlier. By employing first-principles band-structure calculations, we identify the increase of Ce valence suppress the itinerant ferromagnetism and leading to formation of P-P bonding with the shortening of c-axis. The bonding effectively drives an increase of interlayer antiferromagnetic interaction, eventually leads to antiferromagnetic ordering of cobalt in high-doping region.

Topological Nonsymmorphic Metals from Band Inversion

We introduce the notion of a band-inverted, topological semimetal in two-dimensional nonsymmorphic crystals. This notion is materialized in the monolayers of MTe$_2$ (M $=$ W, Mo) if spin-orbit coupling is neglected. We characterize the Dirac band touching topologically by the Wilson loop of the non-Abelian Berry gauge field. An additional feature of the Dirac cone in monolayer MTe$_2$ is that it tilts over in a Lifshitz transition to produce electron and hole pockets, a type-II Dirac cone. These pockets, together with the pseudospin structure of the Dirac electrons, suggest a unified, topological explanation for the recently-reported, non-saturating magnetoresistance in WTe$_2$, as well as its circular dichroism in photoemission. We complement our analysis and first-principle bandstructure calculations with an $\textit{ab-initio}$-derived-derived tight-binding model for the WTe$_2$ monolayer.

Room-temperature magnetic topological Weyl fermion and nodal line semimetal states in half-metallic Heusler Co2TiX (X=Si, Ge, or Sn).

Topological semimetals (TSMs) including Weyl semimetals and nodal-line semimetals are expected to open the next frontier of condensed matter and materials science. Although the first inversion breaking Weyl semimetal was recently discovered in TaAs, its magnetic counterparts, i.e., the time-reversal breaking Weyl and nodal line semimetals, remain elusive. They are predicted to exhibit exotic properties distinct from the inversion breaking TSMs including TaAs. In this paper, we identify the magnetic topological semimetal states in the ferromagnetic half-metal compounds Co2TiX (X = Si, Ge, or Sn) with Curie temperatures higher than 350 K. Our first-principles band structure calculations show that, in the absence of spin-orbit coupling, Co2TiX features three topological nodal lines. The inclusion of spin-orbit coupling gives rise to Weyl nodes, whose momentum space locations can be controlled as a function of the magnetization direction. Our results not only open the door for the experimental realization of topological semimetal states in magnetic materials at room temperature, but also suggest potential applications such as unusual anomalous Hall effect in engineered monolayers of the Co2TiX compounds at high temperature.

Vertex Corrections for Positive-Definite Spectral Functions of Simple Metals.

We present a systematic study of vertex corrections in a homogeneous electron gas at metallic densities. The vertex diagrams are built using a recently proposed positive-definite diagrammatic expansion for the spectral function. The vertex function not only provides corrections to the well known plasmon and particle-hole scatterings, but also gives rise to new physical processes such as the generation of two plasmon excitations or the decay of the one-particle state into a two-particle-one-hole state. By an efficient Monte Carlo momentum integration we are able to show that the additional scattering channels are responsible for a reduction of the bandwidth, the appearance of a secondary plasmon satellite below the Fermi level, and a substantial redistribution of spectral weights. The feasibility of the approach for first-principles band-structure calculations is also discussed.

The effects of Cr and Si additions and deposition conditions on the structure and properties of the (Zr-Ti-Nb)N coatings

In this study, (Zr-Ti-Nb)N, (Zr-Ti-Cr-Nb)N and (Zr-Ti-Cr-Nb-Si)N nitride coatings were obtained using a well-developed vacuum arc deposition. The systematical investigations demonstrate that the chemical composition, microstructure, and properties of the coatings intimately rely on the deposition parameters (pressure of working gas and substrate bias). Effects of Cr and Si additions on microstructure and mechanical properties of the (Zr-Ti-Nb)N coatings have been investigated using scanning electron microscopy (SEM) equipped with energy dispersive spectrum (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscope (TEM), hardness measurements and adhesion testing. First-principles band-structure calculations and Gibbs-Rosenbaum triangle representation have been used to investigate the elemental and phase compositions in nitride coatings. The multi-component (Zr-Ti-Cr-Nb-Si)N and (Zr-Ti-Nb)N coatings are found to be a simple face-centered cubic (FCC) solid solution. For the coatings without Si, the structure is mainly composed of TiN fcc phase and Cr2N trigonal modification. The hardness values were in the ranges (24–42GPa). The (Zr-Ti-Nb)N, (Zr-Ti-Cr-Nb)N coatings provided the best adhesive strength in different conditions. The (Zr-Ti-Cr-Nb-Si)N coatings exhibited the worst adhesive strength, which may be attributed to the relative low hardness.

First principles calculation of thermoelectric parameters of Monolayer- and Bilayer-Graphene and Heterostructures of Graphene and h-BN

Electrical conductivity and Seebeck coefficient were calculated using Boltzmann transport theory based on first-principles bandstructure calculations for the monolayer- and bilayer-graphene. We also present calculations of the thermoelectric parameters of Graphene/h- BN/Graphene sandwiched heterostructures. These results will be discussed in the light of existing experimental data.