Band Structure Approach Research Articles

Ab initio based study of the diamagnetism of diamond, silicon and germanium

Diamond, silicon and germanium are widely used technological materials, whose electron structures are tabulated and understood very well. Yet the detailed picture of various diamagnetic contributions and their interplay with the paramagnetic effect, especially at the ab initio level, due to the complexity of this problem remains largely unknown. In our study, based on the ab initio electron band structure approach, all possible magnetic contributions, including the Langevin (Larmor) diamagnetism and the Van Vleck paramagnetism accounted in the second order of the perturbation treatment, are analyzed quantitatively. The Van Vleck contribution has been adapted to the band structure calculations by performing integration over k-points. Also, the partial diamagnetic contributions of core and valence electrons are obtained and discussed in detail. Of all terms there is only one — the Langevin diamagnetism of the valence electrons, which is not well defined quantitatively due to the contribution from the interstitial region. We discuss possible approaches to partitioning this contribution among neighboring crystal sites, based on the Bader charge separation and introducing the area of depleted electron density, excluded from the integration. Under various assumptions for the interstitial region, we have found that the magnetic susceptibilities for diamond, silicon and germanium lie in the range -13.4/-9.3, -4.7/-4.0 and -8.4/-8.8 (volume values, in units of 10−7), correspondingly. In general, this is consistent with the available experimental values, albeit the numerical uncertainty typically reaches tens of percent.

Significant k-point selection scheme for computationally efficient band structure based UTB device simulations

The accurate calculation of channel electrostatics parameters in ultra-thin body devices requires self-consistent solution of the Poisson’s equation and the full-band structure of the thin channel. For silicon channel, the full-band structure is obtained using the semi-empirical tight-binding model. To make this approach computationally tractable for a wide range of channel thicknesses, in terms of time and resource, only significant k-points in the irreducible Brillouin zone need to be considered. In this work, we present a scheme for precisely identifying the significant k-points based on Fermi–Dirac probability and show that the band-structure approach using those significant k-points can be applied over a wide range of channel thicknesses, oxide thicknesses, device temperatures and different channel orientations. The benchmarking of the obtained channel electrostatics parameters is performed with the results from accurate full-band structure simulations showing excellent agreement (maximum error within ) along with significant reduction in computational time.

Proposal and analysis of ultra-high amplitude-sensitive refractive index sensor by thick silicon multi-slot sub-wavelength Bragg grating waveguide

In this paper, we proposed an ultra-high amplitude-sensitive multi-slot sub-wavelength Bragg grating (MS-SW BG) waveguide sensor on silicon on insulator (SOI) platform with a thick silicon waveguide (WG) and analyzed the design and performances. The MS-SW BG sensor with a thick waveguide supports multi-transverse modes for the first and second modes, whose stopbands (SBs) have different center wavelengths and bandwidths to form a mid-transmission band (MTB) between these SBs. By adjusting sensor parameters, the MTB can be made sharp and steep, and these spectral profiles shift in response to a refractive index change in the surrounding medium. Due to the sharp spectral feature of the MTB, ultra-high amplitude-sensitive performance can be realized. The sensor length is as short as about 10 μm for a grating period count of 21. With an additional increase of the period count the transmission peak count in the MTB region increases, and the shortest wavelength peak among them exhibits steeper slope which is suitable for higher sensitivity. The appearance of the MTB is explained by coupled mode theory and band structure approach. The structural design is investigated from viewpoints of WG thickness, WG width, duty ratio of the grating and so on to demonstrate an ultra-high amplitude sensitivity of 5000 RIU−1 with transmission loss limit up to 3 dB.

Ab initio investigation of strain effects on the physical properties of PdVTe alloy

The investigations of the strain effects on magnetism, elasticity, electronic, optical, and thermodynamic properties of PdVTe half-Heusler alloy are carried out using the most accurate methods to obtain electronic band structure (i.e., the full-potential linearized augmented plane wave plus a local orbital (FP-LAPW + lo) approach). The analysis of the band structures and the density of states reveals half-metallic behavior with a small indirect band gap Eg of 0.51 eV around the Fermi level for the minority spin channels. The study of magnetic properties led to the predicted value of total magnetic moment µtot = 3µB, which nicely follows the Slater–Pauling rule µtot = Zt – 18. Several optical properties are calculated for the first time and the predicted values are in line with the Penn model. It is shown from the imaginary part of the complex dielectric function that the investigated alloy is optically metallic. The variations of thermodynamic parameters calculated using the quasi-harmonic Debye model agree well with the results predicted by the Debye theory. Moreover, the dynamical stability of the investigated alloy is computed by means of the phonon dispersion curves, the density of states, and the formation energies. Finally, the analysis of the strain effects reveals that PdVTe alloy preserves its ferromagnetic half-metallic behavior, it remains mechanically stable, the ionic nature dominates the atomic bonding, and the thermodynamic and the optical properties keep the same features in a large interval of pressure.

Propagation and attenuation of Rayleigh and pseudo surface waves in viscoelastic metamaterials

The development of seismic metamaterials has attracted much research interest in the past decade. Efforts have been made by using experimental and theoretical approaches to isolate buildings and structures susceptible to elastic surface wave damage. However, most seismic metamaterials were designed without considering the viscoelastic effect that widely exists in nature. In this work, we investigate the propagation and attenuation of the Rayleigh and pseudo surface waves (PSWs) in two types of viscoelastic seismic metamaterials, namely, pillared and inclusion-embedded metamaterials, by analyzing the complex band structures and transmission spectra. The complex band structure developed in this work reveals for the first time the existence of PSWs and their propagation properties in inclusion-embedded metamaterials at the surface. These PSW modes are hidden in the traditional ω(k) technique, therefore showing the usefulness of the complex band structure approach. Introducing viscosity to the substrate of both types of seismic metamaterials will enhance the attenuation of both the Rayleigh wave and PSW. For inclusion-embedded metamaterials, the viscoelastic effect in the soft coating layer can have a specific influence only on the PSW. PSWs show advantages to minimize the relative attenuating effect in general. The results in this work will open up great possibilities for designing and optimizing seismic metamaterials in practice.

Ising-like models for stacking faults in a free electron metal.

We propose an extension of the axial next nearest neighbour Ising (ANNNI) model to a general number of interactions between spins. We apply this to the calculation of stacking fault energies in magnesium—particularly challenging due to the long-ranged screening of the pseudopotential by the free electron gas. We employ both density functional theory (DFT) using highest possible precision, and generalized pseudopotential theory (GPT) in the form of an analytic, long ranged, oscillating pair potential. At the level of first neighbours, the Ising model is reasonably accurate, but higher order terms are required. In fact, our ‘ ANNNI model’ is slow to converge—an inevitable feature of the free electron-like electronic structure. In consequence, the convergence and internal consistency of the ANNNI model is problematic within the most precise implementation of DFT. The GPT shows the convergence and internal consistency of the DFT bandstructure approach with electron temperature, but does not lead to loss of precision. The GPT is as accurate as a full implementation of DFT but carries the additional benefit that damping of the oscillations in the ANNNI model parameters are achieved without entailing error in stacking fault energies. We trace this to the logarithmic singularity of the Lindhard function.

Analysis in reciprocal space of the band-pass filter effect in uniform and non-uniform grating couplers

A detailed study on the origin of the bandwidth reduction in the Coupling- Efficiency (CE) spectrum for non-uniform grating couplers (nu-GCs) due to a band-pass filter effect is reported. This effect is deeply investigated through Finite Difference Time Domain simulations (FDTD) in order to look at the spatial Fourier Transform (sFT) of the nu-GC’s emission, which gives the energies and the wavevectors sustained by the overall structure. We show: how to evaluate the bandwidth from the sFT and how the band-pass filter effect arises from the sFT affecting the CE spectrum giving a physical insight on the bandwidth’s reduction. Moreover, we also point out how the concept of photonic crystal bandstructure can be applied for such nu-GCs and used as a powerful tool to describe the dispersion of the light inside these non periodic structures. Finally, both the sFT and the bandstructure approaches are shown to give the same outcomes in terms of the bandwidth evaluation and to be in agreement with the CE spectra’s behaviours. The entire analysis is proposed also for uniform-GC in order to directly compare the two different types of structures.

Synthesis, growth and characterization of a long-chain π-conjugation based methoxy chalcone derivative single crystal; a third order nonlinear optical material for optical limiting applications

Organic material (chalcone derivative 4-[(1E)-3-(4-methoxyphenyl)-3-oxoprop-1-en-1-yl]phenyl 4-methylbenzene-1-sulfonate {4MPMS}) has been grown by solvent evaporation technique. The structural properties have been carried out by powder XRD and single crystal XRD data. The linear optical properties (band gap, extinction co-efficient, optical conductivity and complex dielectric constant) were studied by UV-VIS-NIR spectrometer. In addition, defect states were identified and analyzed by creation of color center mechanism using photoluminescence spectroscopy. In Thermal analysis, crystal (4MPMS) is found to be stable up to 158 °C. Hence it may be used for the fabrication of device below the melting point. The electrical properties (dielectric constant and dielectric loss factor) have been carried out. The electronic polarizability is the major contribution for the high frequency dielectric constant values. Therefore, using theoretical aspects, the electronic polarizability values were calculated using Clausius–Mossotti relation as well as band structure approach of Penn model. Laser damage threshold for 4MPMS crystal was determined by using Nd:YAG laser with 532 nm. The nonlinear optical absorption coefficient and third order nonlinear optical susceptibility (χ(3)) values are reasonably larger than those of the other chalcone derivative crystals. High second order molecular hyperpolarizability enhances the third order optical nonlinear efficiency. Increased χ(3) and coupling factor (less than 1/3) indicates that the optical nonlinearity is electronic in origin. The behavior of nonlinear optical absorption curve confirms that the mechanism belongs to reverse saturable absorption (RSA).

First-principles study of the impact of the atomic configuration on the electronic properties of AlxGa1−xN alloys

We employ first-principles calculations in the formalism of standard and hybrid density functional theory to study the electronic and structural properties of wurtzite ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ alloys. We address the discrepancies observed in literature regarding essential electronic properties of these alloys and we investigate the dependence of these properties on the atomic ordering and composition. We show that the bowing parameter is significantly affected by the atomic ordering, ranging from zero to strong downward bowing. The effects of atomic ordering of the alloys on their band offset with respect to the pure phases are also investigated. Finally, using the effective band structure approach, we study the electronic band structure of the random alloys.

Ab Initio Calculations of Transport Properties of Vanadium Oxides

The temperature-dependent transport properties of vanadium oxides have been studied near the Fermi energy using the Kohn–Sham band structure approach combined with Boltzmann transport equations. V2O5 exhibits significant thermoelectric properties, which can be attributed to its layered structure and stability. Highly anisotropic electrical conduction in V2O5 is clearly manifested in the calculations. Due to specific details of the band structure and anisotropic electron–phonon interactions, maxima and crossovers are also seen in the temperature-dependent Seebeck coefficient of V2O5. During the phase transition of VO2, the Seebeck coefficient changes by 18.9 µV/K, which is close to (within 10% of) the observed discontinuity of 17.3 µV/K.

Brillouin zone unfolding method for effective phonon spectra

Thermal properties are of great interest in modern electronic devices and nanostructures. Calculating these properties is straightforward when the device is made from a pure material, but problems arise when alloys are used. Specifically, only approximate bandstructures can be computed for random alloys and most often the Virtual Crystal Approximation (VCA) is used. Unfolding methods [T. B. Boykin, N. Kharche, G. Klimeck, and M. Korkusinski, J. Phys.: Condens. Matt. 19, 036203 (2007).] have proven very useful for tight-binding calculations of alloy electronic structure without the problems in the VCA, and the mathematical analogy between tight-binding and valence-force-field approaches to the phonon problem suggest they be employed here as well. However, there are some differences in the physics of the two problems requiring modifications to the electronic structure approach. We therefore derive a phonon alloy bandstructure (vibrational mode) approach based on our tight-binding electronic structure method, modifying the band-determination method to accommodate the different physical situation. Using the method, we study In$_x$Ga$_{1-x}$As alloys and find very good agreement with available experiments.

Spin-resolved momentum densities: probing orbitals in magnetic oxides

Studies of spin-resolved electron momentum densities involve the measurement of the so-called magnetic Compton profile. This is a one-dimensional projection of the electron momentum distribution of only those electrons that contribute to the spin moment of a sample. The technique is applicable to ferri- and ferromagnetic materials. Since electrons originating from different atomic orbitals have specific momentum densities, it is often possible to determine the origin of the magnetism present. Typically, interpretation requires the use of electronic structure calculations using molecular orbital and band structure approaches. The profile is obtained experimentally via the inelastic "Compton" scattering of high energy X-rays. For the experiments discussed here, the high energy beamlines at the ESRF and SPring-8 synchrotron X-ray sources were used, where we have a cryomagnet which can provide a sample environment with applied magnetic fields up to 9 Tesla, at temperatures from 1.3K to 600K. In this talk, we discuss our combined experimental and theoretical study of the spin density of the low-dimensional frustrated metamagnet Ca3Co2O6. The spin moment, measured using magnetic Compton scattering, confirms the existence of a large unquenched Co orbital moment (1.310.1 μB). With regards to the orbital occupation, we have performed molecular orbital calculations on the active trigonal CoO6cluster in order to determine which Co 3d orbitals are responsible for the observed electronic and magnetic behaviour and the observed orbital moment, and revealing the existence a oxygen spin moment of approximately 0.9 μB. Electronic structure calculations with a Hubbard U energy term give Compton profiles which are in good agreement with our experimental data. The magnetic Compton profile exhibits oscillations, which are well described, and their frequency in momentum space corresponds to the real-space inter-cobalt site bond length.

Synchrotron charge density studies of chemical bonding in the polymorphs of FeS2

The experimental charge density (CD) distributions in both polymorphs of the photovoltaic compound iron-disulphide (FeS2; cubic pyrite and orthorhombic marcasite) will be described.[1] The CDs are determined by multipole modelling using synchrotron X-ray diffraction data collected at 10 K on extremely small single crystals (<10 mu) thus minimizing the influence of systematic errors such as absorption, extinction and TDS, and exploiting experiences gained from our recent synchrotron studies of CoSb3.[2] The analysis of the charge density in both polymorphs of FeS2 provides an opportunity to see how the different geometries affect local atomic properties, such as 2-center chemical bonding, atomic charges and d-orbital populations. In particular, the data and the resulting multipole models enable us to link the atomic-centered view that emerges from the multipole analysis with the band structure approach. This is carried out by combination with results from periodic calculations on the compounds in the experimental geometries using WIEN2k, thereby providing unambiguous answers to a number of unsolved issues regarding the nature of the bonding in FeS2. The chemical bonding will be characterized by topological analyses showing that the Fe-S bonds are polar covalent bonds, with only minor charge accumulation but significantly negative energy densities at the bond critical points. Using the IAM as reference, density is found to accumulate in-between the atoms, supporting a partial covalent bonding description. The homopolar covalent S-S interaction is seemingly stronger in pyrite than in marcasite, determined not only from the shorter distance but also from all topological indicators. Integrated atomic (Bader) charges show significantly smaller values than those estimation based on crystal-field theory of Fe2+, S-1. In connection with this, the experimentally derived d-orbital populations on Fe are found to deviate from the commonly assumed full t2g set, empty eg set, and they fit very well with the theoretical individual atomic orbitals projected density of states showing a higher dxy participation in the valence band in marcasite compared with pyrite. Thus, the differences between the two polymorphic compounds are directly reflected in their valence density distributions and d-orbital populations.

A band structure study on inter-wall conductance of double-walled carbon nanotubes

Recent advances in etching agents and techniques on the fabrication of double-walled carbon nanotubes (DWCNTs) have introduced various techniques and devices based on nanotube technology. The present study evaluates the inter-shell interaction of DWCNT through density functional theory (DFT) and charge transfer model. The results showed that the band structure of a DWCNT is determined by super positioning of the band structures of the two individual CNT constituting the DWCNT. In next step, interwall conductance the electronic properties of DWCNT were studied through the band-structure approach. Our assessments have shown that at lower potentials the amount of intershell leakage current is negligible.

Hour-glass magnetic spectrum in a stripeless insulating transition metal oxide

An hour-glass-shaped magnetic excitation spectrum appears to be a universal characteristic of the high-temperature superconducting cuprates. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. Recently, an hour-glass spectrum has been observed in an insulating cobaltate, thus favouring the charge stripe scenario. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge-ordered regime of La(2-x)Sr(x)CoO(4). The search for charge stripe ordering reflections yields no evidence for charge stripes in La(1.6)Sr(0.4)CoO(4), which is supported by our phonon studies. With the observation of an hour-glass-shaped excitation spectrum in this stripeless insulating cobaltate, we provide experimental evidence that the hour-glass spectrum is neither necessarily connected to charge stripes nor to band structure effects, but instead, probably intimately coupled to frustration and arising chiral or non-collinear magnetic correlations.

First-principles band structure and FLEX approach to the pressure effect on Tc of the cuprate superconductors

High-temperature cuprate superconductors have been known to exhibit significant pressure effects. In order to fathom the origin of why and how Tc is affected by pressure, we have recently studied the pressure effects on Tc adopting a model that contains two copper d-orbitals derived from first-principles band calculations, where the dz2 orbital is considered on top of the usually considered dx2-y2 orbital. In that paper, we have identified two origins for the Tc enhancement under hydrostatic pressure: (i) while at ambient pressure the smaller the hybridization of other orbital components the higher the Tc, an application of pressure acts to reduce the multiorbital mixing on the Fermi surface, which we call the orbital distillation effect, and (ii) the increase of the band width with pressure also contributes to the enhancement. In the present paper, we further elaborate the two points. As for point (i), while the reduction of the apical oxygen height under pressure tends to increase the dz2 mixture, hence to lower Tc, here we show that this effect is strongly reduced in bi-layer materials due to the pyramidal coordination of oxygen atoms. As for point (ii), we show that the enhancement of Tc due to the increase in the band width is caused by the effect that the many-body renormalization arising from the self-energy is reduced.

What we can learn from magnetic Compton scattering: application to the determination of spin polarization

Studies of spin-resolved electron momentum densities involve the measurement of the so-called magnetic Compton profile. This is a one-dimensional projection of the electron momentum distribution of only those electrons that contribute to the spin moment of a sample. The technique is applicable to ferri- and ferromagnetic materials. The profile is obtained via the inelastic 'Compton' scattering of high energy X-rays. Since electrons originating from different atomic orbitals have specific momentum densities, it is often possible to determine the origin of the magnetism present. Typically, interpretation requires the use of electronic structure calculations using molecular orbital and band structure approaches. Here, we highlight the application of the technique to the determination of the Fermi level spin polarization, the knowledge of which is important to the development of novel spintronic materials.

Analysis of plasmonic properties of heavily doped semiconductors using full band structure calculations

Surface plasmon polaritons (SPPs) and localized surface plasmon (LSP) resonances are not limited to noble metals. Any material with a substantial amount of free carriers will support surface plasma oscillations which, when coupled to an electromagnetic field, will result in surface plasmon polaritons and localized surface plasmon resonances in confined systems. Utilizing a full band structure approach, we analyze the plasmonic properties of several heavily doped semiconductors. We present rigorous quantum mechanical calculations of the plasma frequency, and study in detail its dependence on impurity doping concentration. Results are presented for silicon, germanium, gallium arsenide, zinc oxide, and gallium nitride. For silicon and zinc oxide, the surface plasmon resonance frequency is calculated for a large range of doping concentrations and we study the dispersion of surface plasmon polaritons on thin films. The investigated properties of heavily doped semiconductors hold promises for several interesting applications within plasmonics.

Majorana modes and complex band structure of quantum wires

We describe Majorana edge states of a semi-infinite wire using the complex band structure approach. In this method the edge state at a given energy is built as a superposition of evanescent waves. It is shown that the superposition can not always satisfy the required boundary condition, thus restricting the existence of edge modes. We discuss purely 1D and 2D systems, focusing in the latter case on the effect of the Rashba mixing term.

Two-Dimensional Transition Metal Dichalcogenide Alloys: Stability and Electronic Properties.

Using density-functional theory calculations, we study the stability and electronic properties of single layers of mixed transition metal dichalcogenides (TMDs), such as MoS2xSe2(1-x), which can be referred to as two-dimensional (2D) random alloys. We demonstrate that mixed MoS2/MoSe2/MoTe2 compounds are thermodynamically stable at room temperature, so that such materials can be manufactured using chemical-vapor deposition technique or exfoliated from the bulk mixed materials. By applying the effective band structure approach, we further study the electronic structure of the mixed 2D compounds and show that general features of the band structures are similar to those of their binary constituents. The direct gap in these materials can continuously be tuned, pointing toward possible applications of 2D TMD alloys in photonics.