Band Structure Theory Research Articles

Operation and Design of van der Waals Tunnel Transistors: A 3-D Quantum Transport Study

We propose a model Hamiltonian for van der Waals tunnel transistors (vdW-TFETs) relying on few physical parameters calibrated against density functional theory (DFT) band structure calculations. Based on this model, we develop a fully 3-D nonequilibrium Green’s function simulator including electron-phonon scattering, and we investigate some fundamental aspects and design challenges related to vdW-TFETs based on single-layer MoS2 and WTe2. In particular, we devote a specific analysis to the impact of top gate alignment and back-oxide thickness on the device performance. Our results suggest that the vdW-TFETs can provide very small values of subthreshold swing (SS) and fairly good ON-state current. However, these devices also pose specific design challenges related to the geometrical features of gated regions, and their ultimate SS may be lower limited by inelastic phonon scattering.

Supersymmetry protected topological phases of isostatic lattices and kagome antiferromagnets

I generalize the theory of phonon topological band structures of isostatic lattices to frustrated antiferromagnets. I achieve this with a discovery of a many-body supersymmetry (SUSY) in the phonon problem of balls and springs and its connection to local constraints satisfied by ground states. The Witten index of the SUSY model demands the Maxwell-Calladine index of mechanical structures. "Spontaneous supersymmetry breaking" is identified as the need to gap all modes in the bulk to create the topological isostatic lattice state. Since ground states of magnetic systems also satisfy local constraint conditions (such as the vanishing of the total spin on a triangle) I identify a similar SUSY structure for many common models of antiferromagnets including the square, triangluar, kagome, pyrochlore nearest neighbor antiferromagnets, and the $J_2 = J_1/2$ square lattice antiferromagnet. Remarkably, the kagome family of antiferromagnets is the analog of topological isostatic lattices among this collections of models. Thus, a solid state realization of the theory of phonon topological band structure may be found in frustrated magnetic materials.

One-step theory of two-photon photoemission

A theoretical frame for two-photon photoemission is derived from the general theory of pump-probe photoemission, assuming that not only the probe but also the pump pulse is sufficiently weak. This allows us to use a perturbative approach to compute the lesser Green function within the Keldysh formalism. Two-photon photoemission spectroscopy is a widely used analytical tool to study non-equilibrium phenomena in solid materials. Our theoretical approach aims at a material-specific, realistic and quantitative description of the time-dependent spectrum based on a picture of effectively independent electrons as described by the local-density approximation in band-structure theory. To this end we follow Pendry's one-step theory of the photoemission process as close as possible and heavily make use of concepts of multiple-scattering theory, such as the representation of the final state by a time-reversed low-energy electron diffraction state. The formalism is fully relativistic and allows for a quantitative calculation of the time-dependent photocurrent for moderately correlated systems like simple metals or more complex compounds like topological insulators. An application to the Ag(100) surface is discussed in detail.

Nonequilibrium carrier dynamics in transition metal dichalcogenide semiconductors

When exploring new materials for their potential in (opto)electronic device applications, it is important to understand the role of various carrier interaction and scattering processes. In atomically thin transition metal dichalcogenide semiconductors, the Coulomb interaction is known to be much stronger than in quantum wells of conventional semiconductors like GaAs, as witnessed by the 50 times larger exciton binding energy. The question arises, whether this directly translates into equivalently faster carrier–carrier Coulomb scattering of excited carriers. Here we show that a combination of ab initio band-structure and many-body theory predicts Coulomb-mediated carrier relaxation on a sub-100 fs time scale for a wide range of excitation densities, which is less than an order of magnitude faster than in quantum wells.

Equation of State of Nickel in WDM Region

AbstractResults of theoretical calculations and experimental measurements of the equation of state (EOS) in the region of warm dense matter (WDM) are discussed and applied to nickel. The thermodynamic properties of nickel and its phase diagram are calculated with the use of multi‐phase EOS model. Theoretical calculations of thermodynamic properties of the solid, liquid, and plasma phases, and of the critical point, are compared with results of experiments. The analysis deals with thermodynamic properties of solid nickel at T = 0 and 298 K from different band–structure theories, static compression experiments in diamond anvil cells, and the information obtained in shock‐wave experiments. Thermodynamic data in the liquid and plasma states, resulting from traditional thermophysical measurements, ”exploding wire” experiments, and evaluations of the critical point are presented. Numerous shock‐wave experiments for nickel have been done to measure shock adiabats of crystal and porous samples and release isentropes. These data are analyzed in a self‐consistent manner together with all other available data at high pressure. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Density Functional Theory Modeling of Low-Loss Electron Energy-Loss Spectroscopy in Wurtzite III-Nitride Ternary Alloys.

In the present work, the dielectric response of III-nitride semiconductors is studied using density functional theory (DFT) band structure calculations. The aim of this study is to improve our understanding of the features in the low-loss electron energy-loss spectra of ternary alloys, but the results are also relevant to optical and UV spectroscopy results. In addition, the dependence of the most remarkable features with composition is tested, i.e. applying Vegard's law to band gap and plasmon energy. For this purpose, three wurtzite ternary alloys, from the combination of binaries AlN, GaN, and InN, were simulated through a wide compositional range (i.e., Al x Ga1-x N, In x Al1-x N, and In x Ga1-x N, with x=[0,1]). For this DFT calculations, the standard tools found in Wien2k software were used. In order to improve the band structure description of these semiconductor compounds, the modified Becke-Johnson exchange-correlation potential was also used. Results from these calculations are presented, including band structure, density of states, and complex dielectric function for the whole compositional range. Larger, closer to experimental values, band gap energies are predicted using the novel potential, when compared with standard generalized gradient approximation. Moreover, a detailed analysis of the collective excitation features in the dielectric response reveals their compositional dependence, which sometimes departs from a linear behavior (bowing). Finally, an advantageous method for measuring the plasmon energy dependence from these calculations is explained.

A New Mathematical Model for Calculating the Electronic Coupling of a B-DNA Molecule

The charge transport properties of DNA have made this molecule very important for use in nanoscale electronics, molecular computing, and biosensoric devices. Early findings have suggested that DNA can behave as a conductor, semiconductor, or an insulator. This variation in electrical behavior is attributed to many factors such as environmental conditions, base sequence, DNA chain length, orientation, temperature, electrode contacts, and fluctuations. To better understand the charge transport characteristics of a DNA molecule, a more thorough understanding of the electronic coupling between base pairs is required. To achieve this goal, two mathematical methods for calculating the electronic interactions between base pairs of a DNA molecule have been developed, which utilize the concepts from Molecular Orbital Theory (MOT) and Electronic Band Structure Theory (EBST). The electronic coupling characteristics of a B-DNA molecule consisting of two Guanine-Cytosine base pairs have been examined for variation in the twist angle between the base pairs, the separation between base pairs, and the separation between base molecules in a given base pair, for both the HOMO and LUMO states. Comparison of results to published literature reveals similar outcomes. The electronic properties (metallic, semi-conducting, insulating) of a B-DNA molecule are also determined.

Very large strain gauges based on single layer MoSe2 and WSe2 for sensing applications

Here, we propose a strain gauge based on single-layer MoSe2 and WSe2 and show that, in these materials, the strain induced modulation of inter-valley phonon scattering leads to large mobility changes, which in turn result in highly sensitive strain gauges. By employing density-functional theory bandstructure calculations, comprehensive scattering models, and the linearized Boltzmann equation, we explain the physical mechanisms for the high sensitivity to strain of the resistivity in single-layer MoSe2 and WSe2, discuss the reduction of the gauge factor produced by extrinsic scattering sources (e.g., chemical impurities), and propose ways to mitigate such sensitivity degradation.

Linear and nonlinear optical response in 3-nitroaniline: A DFT study for comparison between molecular and bulk phases

We have studied the electronic structure and optical responses of both molecular and bulk phases of 3-nitroaniline (m-NA) using density functional theory (DFT). We have developed our calculations within the framework of band structure theory for both phases. The results of current simulation are also useful for comparison between solids and vapors. Findings show that the crystalline m-NA has an indirect bandgap which is smaller than its molecular counterpart. Due to the weak intermolecular interactions, the crystalline band structure shows very low dispersions and hence the crystalline spectra are very similar to those of molecules, especially at lower energies. This resemblance is extended to lower wavelengths in linear regime, but limited to higher wavelengths in nonlinear one. This study shows that the substituent groups play major roles in the band structure and charge transfer excitations. The optical spectra show higher intensity and more splitting, especially in nonlinear regime, when we go from molecular phase to bulk phase. Findings show that the electron energy loss structures in vapor phase are very close to [Formula: see text], hence energy loss spectroscopy is a direct way for measuring dielectric function of vapors. According to our results, the crystalline phase exhibits plasmon resonances at much higher energies compared to those of vapor phase. For example, the energy loss spectrum of m-NA molecule shows plasmon peak around 17[Formula: see text]eV which is about 10[Formula: see text]eV lower than the crystalline counterpart. The comparison between nonlinear spectra and the linear spectra (as functions of both [Formula: see text] and 2[Formula: see text]) reveals the significant resemblance between linear and nonlinear structures in both phases. This study shows that the molecular-based models for investigating optical properties of organic crystals are only suitable for low energy regions. Finally, our simulation reproduces the experimental results very well.

Stochastic Approach to Phonon-Assisted Optical Absorption.

We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling MonteCarlo scheme, and we obtain temperature-dependent line shapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature.

The mechanism of charge density wave in Pt-based layered superconductors: SrPt2As2 and LaPt2Si2.

The intriguing coexistence of the charge density wave (CDW) and superconductivity in SrPt2As2 and LaPt2Si2 has been investigated based on the ab initio density functional theory band structure and phonon calculations. We have found that the CDW instabilities for both cases arise from the q-dependent electron-phonon coupling with quasi-nesting feature of the Fermi surface. The band structure obtained by the band-unfolding technique reveals the sizable q-dependent electron-phonon coupling responsible for the CDW instability. The local split distortions of Pt atoms in the [As-Pt-As] layers play an essential role in driving the five-fold supercell CDW instability as well as the phonon softening instability in SrPt2As2. By contrast, the CDW and phonon softening instabilities in LaPt2Si2 occur without split distortions of Pt atoms. The phonon calculations suggest that the CDW and the superconductivity coexist in [X-Pt-X] layers (X = As or Si) for both cases.

Correlation-Driven Topological Fermi Surface Transition in FeSe.

The electronic structure and phase stability of paramagnetic FeSe is computed by using a combination of ab initio methods for calculating band structure and dynamical mean-field theory. Our results reveal a topological change (Lifshitz transition) of the Fermi surface upon a moderate expansion of the lattice. The Lifshitz transition is accompanied with a sharp increase of the local moments and results in an entire reconstruction of magnetic correlations from the in-plane magnetic wave vector, (π,π) to (π,0). We attribute this behavior to a correlation-induced shift of the van Hove singularity originating from the d(xy) and d(xz)/d(yz) bands at the M point across the Fermi level. We propose that superconductivity is strongly influenced, or even induced, by a van Hove singularity.

Band Structure Engineering by Substitutional Doping in Solid-State Solutions of [5-Me-PLY(O,O)]2B(1-x)Be(x) Radical Crystals.

We report the substitutional doping of solid-state spiro-bis(5-methyl-1,9-oxido-phenalenyl)boron radical ([2]2B) by co-crystallization of this radical with the corresponding spiro-bis(5-methyl-1,9-oxido-phenalenyl)beryllium compound ([2]2Be). The pure compounds crystallize in different space groups ([2]2B, P1̅, Z = 2; [2]2Be, P2₁/c, Z = 4) with distinct packing arrangements, yet we are able to isolate crystals of composition [2]2B(1-x)Be(x), where x = 0-0.59. The phase transition from the P1̅ to the P2₁/c space group occurs at x = 0.1, but the conductivities of the solid solutions are enhanced and the activation energies reduced for values of x = 0-0.25. The molecular packing is driven by the relative concentration of the spin-bearing ([2]2B) and spin-free ([2]2Be) molecules in the crystals, and the extended Hückel theory band structures show that the progressive incorporation of spin-free [2]2Be in the lattice of the [2]2B radical (overall bandwidth, W = 1.4 eV, in the pure compound) leads to very strong narrowing of the bandwidth, which reaches a minimum at [2]2Be (W = 0.3 eV). The results provide a graphic picture of the structural transformations undergone by the lattice, and at certain compositions we are able to identify distinct structures for the [2]2B and [2]2Be molecules in a single crystalline phase.

"Missing magnetism" puzzle solved

Plutonium To explain plutonium's complex structural properties, conventional band structure theories typically invoke magnetism, in stark contrast with experiment. Janoschek et al. used neutron spectroscopy and found that the magnetism in plutonium is not “missing” but dynamic: the fingerprint

Ab initiotheory of superconductivity in a magnetic field. II. Numerical solution

We numerically investigate the Spin Density Functional theory for superconductors (SpinSCDFT) and the approximated exchange-correlation functional, derived and presented in the preceding paper I. As a test system we employ a free electron gas featuring an exchange-splitting, a phononic pairing field and a Coulomb repulsion. SpinSCDFT results are compared with Sarma, the Bardeen Cooper and Schrieffer theory and with an Eliashberg type of approach. We find that the spectrum of the superconducting Kohn-Sham SpinSCDFT system is not in agreement with the true quasi particle structure. Therefore, starting from the Dyson equation, we derive a scheme that allows to compute the many body excitations of the superconductor and represents the extension to superconductivity of the G0W0 method in band structure theory. This superconducting G0 W0 method vastly improves the predicted spectra.

Plasmon Enhanced Internal Photoemission in Antenna-Spacer-Mirror Based Au/TiO₂ Nanostructures.

Emission of photoexcited hot electrons from plasmonic metal nanostructures to semiconductors is key to a number of proposed nanophotonics technologies for solar harvesting, water splitting, photocatalysis, and a variety of optical sensing and photodetector applications. Favorable materials and catalytic properties make systems based on gold and TiO2 particularly interesting, but the internal photoemission efficiency for visible light is low because of the wide bandgap of the semiconductor. We investigated the incident photon-to-electron conversion efficiency of thin TiO2 films decorated with Au nanodisk antennas in an electrochemical circuit and found that incorporation of a Au mirror beneath the semiconductor amplified the photoresponse for light with wavelength λ = 500-950 nm by a factor 2-10 compared to identical structures lacking the mirror component. Classical electrodynamics simulations showed that the enhancement effect is caused by a favorable interplay between localized surface plasmon excitations and cavity modes that together amplify the light absorption in the Au/TiO2 interface. The experimentally determined internal quantum efficiency for hot electron transfer decreases monotonically with wavelength, similar to the probability for interband excitations with energy higher than the Schottky barrier obtained from a density functional theory band structure simulation of a thin Au/TiO2 slab.

First-principles calculations of two cubic fluoropervskite compounds: RbFeF3 and RbNiF3

We present first-principles calculations of the structural, elastic, electronic, magnetic and optical properties for RbFeF3 and RbNiF3. The full-potential linear augmented plan wave (FP-LAPW) method within the density functional theory was utilized to perform the present calculations. We employed the generalized gradient approximation as exchange-correlation potential. It was found that the calculated analytical lattice parameters agree with previous studies. The analysis of elastic constants showed that the present compounds are elastically stable and anisotropic. Moreover, both compounds are classified as a ductile compound. The calculations of the band structure and density functional theory revealed that the RbFeF3 compound has a half-metallic behavior while the RbNiF3 compound has a semiconductor behavior with indirect (M–Γ) band gap. The ferromagnetic behavior was studied for both compounds. The optical properties were calculated for the radiation of up to 40eV. A beneficial optics technology is predicted as revealed from the optical spectra.

First-Principles Calculation of Atomic Forces and Structural Distortions in Strongly Correlated Materials

We introduce a novel computational approach for the investigation of complex correlated electron materials which makes it possible to evaluate interatomic forces and, thereby, determine atomic displacements and structural transformations induced by electronic correlations. It combines ab initio band structure and dynamical mean-field theory and is implemented with the linear-response formalism regarding atomic displacements. We apply this new technique to explore structural transitions of prototypical correlated systems such as elemental hydrogen, SrVO3, and KCuF3.

Imprints of spin-orbit density wave in the hidden-order state of URu2Si2

The mysterious second order quantum phase transition, commonly attributed to the `hidden-order' (HO) state, in heavy-fermion metal URu2Si2 exhibits a number of paradoxical electronic and magnetic properties which cannot be associated with any conventional order parameter. We characterize and reconcile these exotic properties of the HO state based on a spin-orbit density wave order (SODW), constructed on the basis of a realistic density-functional theory (DFT) band structure. We quantify the nature of the gapped electronic and magnetic excitation spectrum, in agreement with measurements, while the magnetic moment is calculated to be zero owing to the spin-orbit coupling induced time-reversal invariance. Furthermore, a new collective mode in the spin-1 excitation spectrum is predicted to localize at zero momentum transfer in the HO state which can be visualized, for example, by electron spin resonance (ESR) at zero magnetic field or polarized inelastic neutron scattering measurements. The results demonstrate that the concomitant broken and invariant symmetries protected SODW order not only provides insights into numerous nontrivial hidden-order phenomena, but also offers a parallel laboratory to the formation of a topologically protected quantum state beyond the quantum spin-Hall state and Weyl semimetals.

Photocatalytic degradation of bisphenol A using Ti-substituted hydroxyapatite

Ti-substituted hydroxyapatite (TiHAP) is a new photocatalyst with high adsorption capacity and photocatalytic activity. The morphology and structure of TiHAP were characterized using transmission electron microscopy, X-ray diffraction, ultraviolet-visible spectrophotometry, and the zeta potential. The adsorption and photocatalytic degradation of bisphenol A (BPA, an environmental endocrine disrupting chemical) over TiHAP and P25 TiO 2 photocatalysts were studied using liquid chromatography-mass spectrometry. The influences of fulvic acid and Fe 3+ ions on the BPA degradtion rate were analyzed. The adsorption of BPA on TiHAP and TiO 2 obeyed the Langmuir adsorption equation. TiHAP exhibited much higher adsorption capacity and photocatalytic degradation activity of BPA than TiO 2 . Fulvic acid and Fe 3+ showed different effects on the photocatalytic activity of TiHAP and TiO 2 films. These were explained by band structure theory, the electron transfer path, and optical absorption capacity. The results are useful for the application of TiHAP in the photocatalytic degradation of environmental endocrine disrupting chemicals. TiHAP film showed an enhanced photocatalytic activity than P25 TiO 2 film for degradation of bisphenol (BPA), an important kind of environmental endocrine disrupting chemicals.