Electronic Band Dispersion Research Articles

Double-atomic layer of Tl on Si(111): Atomic arrangement and electronic properties

Metastable double-atomic layer of Tl on Si(111) has recently been found to display interesting electric properties, namely superconductivity below 0.96 K and magnetic-field-induced transition into an insulating phase intermediated by a quantum metal state. In the present work, using a set of experimental techniques, including low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, in a combination with density-functional-theory calculations, we have characterized atomic and electronic properties of the Tl double layer on Si(111). The double Tl layer has been concluded to contain ∼ 2.4 monolayer of Tl. A top Tl layer has a ‘1 × 1’ basic structure and displays 6 × 6 moiré pattern which originates from various residence sites of Tl atoms. Upon cooling below ∼ 140 K, the 6 × 6 moiré pattern changes to that having a 63×63 periodicity. However, the experimentally determined electron band dispersions show a 1 × 1 periodicity. The calculated band structure unfolded into the 1 × 1 surface Brillouin zone reproduces well the main features of the photoelectron spectra.

Hole-phonon coupling effect on the band dispersion of organic molecular semiconductors

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (> 1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.

Dispersion and density of states of phonons and electrons in an α-B12 crystal determined from first principles

Using the DFT method, we study the phonon properties of an α-B12 rhombohedral crystal in the basis set of plane waves and its electronic structure in the localized basis set of Gaussians. It follows from the phonon dispersion that the crystal possesses a dynamical stability. The effective Born charges, the oscillator strengths, the transverse–longitudinal splitting, and the dielectric functions of dipole modes are calculated. We show that charge transfer from polar to equatorial atoms takes place in a В12 icosahedron, while В–В bonds have predominantly a covalent character. In the density of states of acoustic modes, we reveal a structure that can manifest itself in the spectra of disordered boron compounds. From the dispersion of electronic bands, the occurrence of an indirect energy gap follows. The overlap of partial densities implies the hybridization of s and p electronic states in boron atoms.

Nontrivial topology of cubic alkali bismuthides

We report an ab initio study of the effect of pressure on vibrational and electronic properties of ${\mathrm{K}}_{3}\mathrm{Bi}$ and ${\mathrm{Rb}}_{3}\mathrm{Bi}$ in the cubic $Fm\overline{3}m$ structure. It is shown that the high-temperature cubic phase of ${\mathrm{K}}_{3}\mathrm{Bi}$ and ${\mathrm{Rb}}_{3}\mathrm{Bi}$ is dynamically unstable at $T=0$ but can be stabilized by pressure. The electronic spectra of alkali bismuthides are found to possess the bulk band touching at the Brillouin zone center and an inverted spin-orbit bulk band structure. Upon hydrostatic compression the compounds transform from the topologically nontrivial semimetal (${\mathrm{K}}_{3}\mathrm{Bi}$)/metal (${\mathrm{Rb}}_{3}\mathrm{Bi}$) into a trivial semiconductor (metal) with a conical Dirac-type dispersion of electronic bands at the point of the topological transition. In ${\mathrm{K}}_{3}\mathrm{Bi}$ the dynamical stabilization occurs before the system undergoes the topological phase transition.

Band structure, electronic and optical features of Tl4SnX3 (X = S, Te) ternary compounds for optoelectronic applications

Abstract In the present work, details of the crystal growth (Bridgeman method) and optical reflection spectroscopy for two single crystals (X = S, Te) were reported. Detailed ab-initio DFT calculations of the structural, electronic and optical properties of the crystals were performed for a deeper understanding of the experimentally obtained optical data. The special attention is paid to the electron charge density distribution, carrier mobility, optical reflectivity spectra and finally the anisotropy of the corresponding optical functions were studied. Were established a relations with carrier mobility, the optical anisotropy and electronic band structures for the titled crystals and compared with the optical reflectance spectra, covering the range of principal inter-band transitions. The origin of the calculated bands and effects of the anion substitution (Se by Te) on the crystal properties are studied. The role of screening effects is discussed by a comparison of the GGA and LDA approaches, including the electronic band dispersion and effective masses anisotropy. The possible changes of the inter-band transition are considered. The relation between the space charge density distribution and optical features is established. Finally was explained the origin of the observed discrepancies between the experimental and theoretical data.

Surface Electronic Structure of Hybrid Organo Lead Bromide Perovskite Single Crystals

The electronic structure and band dispersion of methylammonium lead bromide, CH3NH3PbBr3, has been investigated through a combination of angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES), as well as theoretical modeling based on density functional theory. The experimental band structures are consistent with the density functional calculations. The results demonstrate the presence of a dispersive valence band in MAPbBr3 that peaks at the M point of the surface Brillouin zone. The results also indicate that the surface termination of the CH3NH3PbBr3 is the methylammonium bromide (CH3NH3Br) layer. We find our results support models that predict a heavier hole effective mass in the region of −0.23 to −0.26 me, along the Γ (surface Brillouin center) to M point of the surface Brillouin zone. The surface appears to be n-type as a result of an excess of lead in the surface region.

Spatial confinement model applied to phonons in disordered graphene-based carbons

Analyzing the various bands in Raman spectra, mostly the G (optical carbon–carbon mode) and D (defect-related double resonance process) bands, is powerful at characterizing defects in disordered graphene-based carbons. The crystallite size La could be estimated from the ID/IG (intensity) or AD/AG (area) ratio, but with large uncertainties. Using the spatial confinement model (SCM) of the phonons has fully explained the linewidth variation for the D band, but could not explain the linewidth variation for the G band over the whole La range. Indeed, if the latter for large, Bernal-stacked graphenes is due to the Kohn anomaly and can be explained considering phonon dispersion curves, turbostratically-stacked graphenes with small La generate an additional broadening of the G band not accounted by the phonon-related SCM only. The realistic SCM proposed here explains the way the G band shape is related to La. The model was also validated by successfully duplicating the evolution of the D (mostly related to the electronic band dispersion) and D′ band linewidths with decreasing La down to 5 nm. The work aims to be a guide for future computational works on Raman spectra of graphene-based carbon materials.

Pressure-induced topological phases of KNa2Bi.

We report an ab initio study of the effect of hydrostatic pressure and uniaxial strain on electronic properties of KNa2Bi, a cubic bialkali bismuthide. It is found that this zero-gap semimetal with an inverted band structure at the Brillouin zone center can be driven into various topological phases under proper external pressure. We show that upon hydrostatic compression KNa2Bi turns into a trivial semiconductor with a conical Dirac-type dispersion of electronic bands at the point of the topological transition while the breaking of cubic symmetry by applying a uniaxial strain converts the compound into a topological insulator or into a three-dimensional Dirac semimetal with nontrivial surface Fermi arcs depending on the sign of strain. The calculated phonon dispersions show that KNa2Bi is dynamically stable both in the cubic structure (at any considered pressures) and in the tetragonal phase (under uniaxial strain).

Multiple Coexisting Dirac Surface States in Three-Dimensional Topological Insulator PbBi₆Te₁₀.

By means of angle-resolved photoemission spectroscopy (ARPES) measurements, we unveil the electronic band structure of three-dimensional PbBi6Te10 topological insulator. ARPES investigations evidence multiple coexisting Dirac surface states at the zone-center of the reciprocal space, displaying distinct electronic band dispersion, different constant energy contours, and Dirac point energies. We also provide evidence of Rashba-like split states close to the Fermi level, and deeper M- and V-shaped bands coexisting with the topological surface states. The experimental findings are in agreement with scanning tunneling microscopy measurements revealing different surface terminations according to the crystal structure of PbBi6Te10. Our experimental results are supported by density functional theory calculations predicting multiple topological surface states according to different surface cleavage planes.

Ab initio calculations of magnetic properties of the interstitially doped YFe11Mo compound

The recent increase in the number of studies of RFe11–xMx compounds is related to their promising application as permanent magnets. However, the insufficiently high value of the Curie temperature TC of these compounds is a barrier to their widespread use. The increase in the Curie temperature of these compounds is achieved by doping with the light nonmetallic atoms such as hydrogen, nitrogen, and carbon. In this paper, it is shown numerically that this doping leads to drastic changes of the electronic band dispersions in a wide energy region around the Fermi level. This in turn changes values of the magnetic moments of ions and Heisenberg exchange interaction parameters. The values of ab initio calculated magnetic moments and direct exchange interaction parameters make it possible to calculate the Curie temperatures for both parent and nitrogen-doped compounds within the mean-field approach to the Heisenberg model in the sample of YFe11Mo, a typical representative of the R(Fe,M)12L class. Theoretical values of TC obtained for YFe11Mo and YFe11MoN (514 and 723 K respectively) are consistent with experimental ones (472 and 664 K) with an accuracy of 10%. Also, the calculated increase in TC upon nitrogenization (about 200 K) is in good agreement with the experimental data.

Charge ordering in stoichiometric FeTe: Scanning tunneling microscopy and spectroscopy

We use scanning tunneling microscopy and spectroscopy to reveal a unique stripy charge order in a parent phase of iron-based superconductors in stoichiometric FeTe epitaxy films. The charge order has unusually the same---usually half---period as the spin order. We also found highly anisotropic electron band dispersions being large and little along the ferromagnetic (crystallographic $b$) and antiferromagnetic ($a$) directions, respectively. Our data suggest that the microscopic mechanism is likely of the Stoner type driven by interatomic Coulomb repulsion ${V}_{ij}$, and that ${V}_{ij}$ and charge fluctuations, so far much neglected, are important to the understanding of iron-based superconductors.

Highly efficient conductance control in a topological insulator based magnetoelectric transistor

The spin-momentum interlocked properties of the topological insulator (TI) surface states are exploited in a transistor-like structure for efficient conductance control in the TI-magnet system. Combined with the electrically induced magnetization rotation as part of the gate function, the proposed structure takes advantage of the magnetically modulated TI electronic band dispersion in addition to the conventional electrostatic barrier. The transport analysis coupled with the magnetic simulation predicts super-steep current-voltage characteristics near the threshold along with the GHz operating frequencies. Potential implementation to a complementary logic is also examined. The predicted characteristics are most suitable for applications requiring low power or those with small signals.

Pentagonal monolayer crystals of carbon, boron nitride, and silver azide

In this study, we present a theoretical investigation of structural, electronic, and mechanical properties of pentagonal monolayers of carbon (p-graphene), boron nitride (p-B2N4 and p-B4N2), and silver azide (p-AgN3) by performing state-of-the-art first principles calculations. Our total energy calculations suggest feasible formation of monolayer crystal structures composed entirely of pentagons. In addition, electronic band dispersion calculations indicate that while p-graphene and p-AgN3 are semiconductors with indirect bandgaps, p-BN structures display metallic behavior. We also investigate the mechanical properties (in-plane stiffness and the Poisson's ratio) of four different pentagonal structures under uniaxial strain. p-graphene is found to have the highest stiffness value and the corresponding Poisson's ratio is found to be negative. Similarly, p-B2N4 and p-B4N2 have negative Poisson's ratio values. On the other hand, the p-AgN3 has a large and positive Poisson's ratio. In dynamical stability tests based on calculated phonon spectra of these pentagonal monolayers, we find that only p-graphene and p-B2N4 are stable, but p-AgN3 and p-B4N2 are vulnerable against vibrational excitations.

Stable half-metallic monolayers of FeCl2

The structural, electronic, and magnetic properties of single layers of Iron Dichloride (FeCl2) were calculated using first principles calculations. We found that the 1T phase of the single layer FeCl2 is 0.17 eV/unit cell more favorable than its 1H phase. The structural stability is confirmed by phonon calculations. We found that 1T-FeCl2 possess three Raman-active (130, 179, and 237 cm−1) and one infrared-active (279 cm−1) phonon branches. The electronic band dispersion of the 1T-FeCl2 is calculated using both gradient approximation of Perdew-Burke-Ernzerhof and DFT-HSE06 functionals. Both functionals reveal that the 1T-FeCl2 has a half-metallic ground state with a Curie temperature of 17 K.

Dispersion of electronic bands in intermetallic compound LiBe and related properties

Based on the all-electron full-potential linearized augmented plane wave within density functional theory calculations dispersion of the electronic band structure, total and the angular momentum resolved projected density of states, the shape of Fermi surface, the electronic charge density distribution and the optical response of the intermetallic LiBe compound are performed. Seeking the influence of the different exchange correlations on the ground state properties of the intermetallic LiBe, calculations are performed within four types of exchange correlations, namely the local density approximation, general gradient approximation, Engel–Vosko generalized gradient approximation and the modified Becke–Johnson potential. It has been found that replacing the exchange correlations exhibit insignificant influence on the bands dispersion, density of states and hence the optical properties. The obtained results suggest that there exists a strong hybridization between the states resulting in covalent bonds. The Fermi surface is formed by two bands and the center of the Fermi surface is formed by holes. The electronic charge density distribution confirms that the charge is attracted toward Be atoms and the calculated bond lengths are in good accordance with the available experimental data. To get deep insight into the electronic structure, the optical properties are investigated and analyzed in accordance with the calculated band structure and the density of states.

Two-dimensional mineral [Pb2BiS3][AuTe2]: high-mobility charge carriers in single-atom-thick layers.

Two-dimensional (2D) electronic systems are of wide interest due to their richness in chemical and physical phenomena and potential for technological applications. Here we report that [Pb2BiS3][AuTe2], known as the naturally occurring mineral buckhornite, hosts 2D carriers in single-atom-thick layers. The structure is composed of stacking layers of weakly coupled [Pb2BiS3] and [AuTe2] sheets. The insulating [Pb2BiS3] sheet inhibits interlayer charge hopping and confines the carriers in the basal plane of the single-atom-thick [AuTe2] layer. Magneto-transport measurements on synthesized samples and theoretical calculations show that [Pb2BiS3][AuTe2] is a multiband semimetal with a compensated density of electrons and holes, which exhibits a high hole carrier mobility of ∼1360 cm(2)/(V s). This material possesses an extremely large anisotropy, Γ = ρ(c)/ρ(ab) ≈ 10(4), comparable to those of the benchmark 2D materials graphite and Bi2Sr2CaCu2O(6+δ). The electronic structure features linear band dispersion at the Fermi level and ultrahigh Fermi velocities of 10(6) m/s, which are virtually identical to those of graphene. The weak interlayer coupling gives rise to the highly cleavable property of the single crystal specimens. Our results provide a novel candidate for a monolayer platform to investigate emerging electronic properties.

Electronic band dispersion of graphene nanoribbons via Fourier-transformed scanning tunneling spectroscopy

Atomically precise armchair graphene nanoribbons of width $N=7$ (7-AGNRs) are investigated by scanning tunneling spectroscopy (STS) on Au(111). The analysis of energy-dependent standing wave patterns of finite length ribbons allows, by Fourier transformation, the direct extraction of the dispersion relation of frontier electronic states. Aided by density functional theory calculations, we assign the states to the valence band, the conduction band and the next empty band of 7-AGNRs, determine effective masses of $0.42\pm 0.08\,m_e$, $0.40\pm 0.18\,m_e$ and $0.20\pm 0.03\,m_e$, respectively, and a band gap of $2.37\pm 0.06$ eV.

Origin of the band dispersion in a metal phthalocyanine crystal

Understanding the crystal structure and electronic states of the organic semiconductor is of fundamental importance for developing the materials for the organic electronics. However, the theoretical treatment of organic semiconductors remains challenging, as the semilocal density functional theory fails to describe the dispersion forces accurately. We use van der Waals inclusive density functionals to study the zinc phthalocyanine polymorphs. It is found that the structure and energetics are well described with the van der Waals density functional, and as a result, the electronic band structure is nicely reproduced. Furthermore, we reveal that the distance between the molecules and the molecule tilt angle are important factors that determine the electronic band dispersion.

Dirac Cones in two-dimensional conjugated polymer networks.

Linear electronic band dispersion and the associated Dirac physics has to date been limited to special-case materials, notably graphene and the surfaces of three-dimensional (3D) topological insulators. Here we report that it is possible to create two-dimensional fully conjugated polymer networks with corresponding conical valence and conduction bands and linear energy dispersion at the Fermi level. This is possible for a wide range of polymer types and connectors, resulting in a versatile new family of experimentally realisable materials with unique tuneable electronic properties. We demonstrate their stability on substrates and possibilities for doping and Dirac cone distortion. Notably, the cones can be maintained in 3D-layered crystals. Resembling covalent organic frameworks, these materials represent a potentially exciting new field combining the unique Dirac physics of graphene with the structural flexibility and design opportunities of organic-conjugated polymer chemistry.

Pressure-induced isostructural phase transition in CaB4

The structural and electrical properties of CaB4 under high pressure have been studied by angle dispersive X-ray diffraction, in situ Hall effect measurement, and first-principles calculations. An abnormal change of the c/a ratio was observed around 12 GPa in the experiments and backed by theory calculation, indicating an isostructural phase transition. Unlike other materials, the analysis of the electronic and phonon band structure doesn't reveal an electronic topological transition or phonon softening supporting this phenomenon. While the subtle changes in the electron band dispersions at the Fermi surface shown by the measurements of the resistivity, Hall coefficient, carrier concentration, mobility, and temperature-dependent resistivity can explain the anomaly in the c/a ratio. The study of electrical transport properties provides strong support for the occurrence of the isostructural phase transition in CaB4.