Plane-wave Approach Research Articles

Ab initio study of the structure, elastic, and electronic properties of Ti3(Al1\u2212nSin)C2 layered ternary compounds

The MAX phase materials such as layered ternary carbides that simultaneously exhibit characteristics of metallic and ceramic materials have received substantial interest in recent years. Here, we present a systematic investigation of the electronic, structural stabilities, and elastic properties of Ti3(Al1−nSin)C2 (n = 0,1) MAX phase materials using the ab initio method via a plane-wave pseudopotential approach within generalized-gradient-approximations. The computed electronic band structures and projected density of states show that both Ti3SiC2 and Ti3AlC2 are metallic materials with a high density of states at the Fermi level emanating mainly from Ti-3d. Using the calculated elastic constants, the mechanical stability of the compounds was confirmed following the Born stability criteria for hexagonal structures. The Cauchy pressure and the Pugh’s ratio values establish the brittle nature of the Ti3SiC2 and Ti3AlC2 MAX phase materials. Due to their intriguing physical properties, these materials are expected to be suitable for applications such as thermal shock refractories and electrical contact coatings.

Heavy neutrino-antineutrino oscillations in quantum field theory

It has been proposed that the coherent propagation of long-lived heavy neutrino mass eigenstates can lead to an oscillating rate of lepton number conserving (LNC) and violating (LNV) events, as a function of the distance between the production and displaced decay vertices. We discuss this phenomenon, which we refer to as heavy neutrino-antineutrino oscillations, in the framework of quantum field theory (QFT), using the formalism of external wave packets. General formulae for the oscillation probabilities and the number of expected events are derived and the coherence and localisation conditions that have to be satisfied in order for neutrino-antineutrino oscillations to be observable are discussed. The formulae are then applied to a low scale seesaw scenario, which features two nearly mass degenerate heavy neutrinos that can be sufficiently long lived to produce a displaced vertex when their masses are below the W boson mass. The leading and next-to-leading order oscillation formulae for this scenario are derived. For an example parameter point used in previous studies, the kinematics of the considered LNC/LNV processes are simulated, to check that the coherence and localisation conditions are satisfied. Our results show that the phenomenon of heavy neutrino-antineutrino oscillations can indeed occur in low scale seesaw scenarios and that the previously used leading order formulae, derived with a plane wave approach, provide a good approximation for the considered example parameter point.

Effect of spin-orbital coupling on the electronic, mechanical, thermoelectric, and vibrational properties of XPtBi (X = Sc and Y): A first-principles study

Heusler compounds constitute a versatile class of materials, as evidenced by their numerous technological applications. Herein, we report a comparative study of the electronic, mechanical, vibrational, and thermoelectric properties of half-Heusler compounds XPtBi (X = Sc and Y) based on first-principles calculations, with and without spin-orbital coupling (SOC). The plane-wave pseudo-potential approach within density functional theory (DFT) and the Boltzmann transport equations have been used to compute these properties. SOC has a considerable effect on the electronic band structures of XPtBi (X = Sc and Y), leading to band inversion without breaking any symmetry. This band inversion results in the emergence of a topological phase in ScPtBi and YPtBi. Phonon dispersion curves and phonon densities of state have been calculated for these compounds, with and without SOC, for comparison. The calculations also reveal that, with the inclusion of SOC, these materials have improved thermoelectric properties, with figures of merit (ZT) of 0.41 for ScPtBi and 0.35 for YPtBi at 1200 K. Hence, these are promising thermoelectric materials for use at higher temperatures.

Electronic structure of tantalum dichalcogenide using Compton scattering technique and density functional theory

Compton spectroscopy measurements on tantalum dichalcogenide TaS0.5Se1.5 using a 100 mCi Am-241 γ-ray Compton spectrometer are presented. Experimental Compton profile (CP) is compared with those computed using linear combination of atomic orbitals (LCAO) method within the density functional theory (DFT) and hybrid functionals of Hartree-Fock and DFT. Hybrid functional namely WC1LYP based CP is found in better accordance with the experimental CP. The relative nature of bonding in TaS0.5Se1.5 and TaSSe is interpreted by scaling the experimental CP on equal-valence-electron-density and Mulliken's population analysis. Origin of metallic behaviour of TaS0.5Se1.5 is discussed in terms of energy bands and density of states. It is seen that the 5d states of Ta dominate around the Fermi level leading to metallic character of the compound. Fermi surface topology and optical response like dielectric constants, absorption coefficients, etc. of TaS0.5Se1.5 are also described using modified Becke-Johnson theory of full-potential linearized augmented plane wave approach.

Investigation of the structural, electronic, optical, elastic, and thermodynamic properties of the zinc blende Ga1-xAlxAs1-yPy quaternary alloys: A DFT-Based simulation

In this work, the effect of the composition on the structural, electronic, elastic, optical and thermodynamic properties of the Ga1-xAlxAs1-yPy quaternary alloys are investigated using the full-potential augmented plane wave plus local orbitals (FP-APW + lo) approach in the density functional theory framework as embodied in the WIEN2k computational package. The fundamental physical properties of the cubic Ga1-xAlxAs1-yPy quaternary alloys, such as lattice constant, bulk modulus, energy band structure and elastic constants, are predicted for the first time. The obtained results show that the Ga1-xAlxAs1-yPy quaternary alloys are direct band gap semiconductors, while their parents; GaP, AlAs, and AlP binary compounds are indirect band gap semiconductors. It is found that the studied materials are of brittle character and ionic bonding nature. The static dielectric constant and static refractive index of the considered alloys are calculated and compared with the obtained results using empirical models. Pressure and temperature dependencies of some thermodynamic parameters of the title quaternary alloys are also investigated and discussed.

Microwave-Assisted Synthesis, Characterization and Modeling of CPO-27-Mg Metal-Organic Framework for Drug Delivery.

The coordination polymer CPO-27-Mg was rapidly synthesized under microwave irradiation. This material exhibits a sufficiently high drug loading towards aspirin (~8% wt.) and paracetamol (~14% wt.). The binding of these two molecules with the inner surface of the metal-organic framework was studied employing the Gaussian and Plane Wave approach of the Density Functional Theory. The structure of CPO-27-Mg persists after the adsorption of aspirin or paracetamol and their desorption energies, being quite high, decrease under solvent conditions.

Analysis of a photonic crystal temperature sensor based on Z-shaped ring resonator

A 2D photonic crystal (PC) sensor based on optical filter has been designed using a PC ring resonator (PCRR). The sensor used a 2D-PCRR triangular lattice with circular silicon rods placed in air background. We employed a ring resonator with Z-shaped core in the investigated structure, which is designed as temperature sensor and investigated theoretically along with high quality factor, transmission and sensitivity parameters. For sensing analysis, the 2D finite-difference time-domain method and the plane-wave expansion approach have been applied. A temperature variation from 0 to 20 °C causes a defect mode shifting and the channel filter wavelength shift was used as sensing mechanism. Using the number of the functionalized rods N = 36 at the temperature of 5 °C, the quality factor, the temperature sensitivity, the refractive index sensitivity and the detection limit of the studied structure were found to be 57,104, 96 pm/°C, 400 nm/RIU and 6.16 × 10–6 RIU, respectively.

A Reciprocity Method of Analysis for Gap Waveguide Slot Antennas

This paper introduces analytical solutions to gap waveguide slot antenna structures. The reciprocity method is used for the analysis by exploiting Green’s function of a bed of nails structure employing a homogenized plane-wave spectral-domain approach. Green’s function is found using parallel plate waveguide and surface admittance concepts. The spectral surface admittance of the pin structure is calculated using asymptotic boundary condition (ABC), and the unknown antenna slot currents are solved using moment methods. The structure is designed to have a cutoff frequency of 10–15 GHz, and the central operating frequency of the slot is 12.5 GHz. This approach is applied to the specific geometry of a propagating ridge gap waveguide single transverse broad wall slot antenna structure. The equivalent series impedance and far zone fields and patterns are calculated, and the results are compared with a full-wave commercial software, and they are proved to be accurate.

FandMcenters in alkali halides: A theoretical study applying self-consistent dielectric-dependent hybrid density functional theory

Point defects significantly change electronic properties of alkali halides and thereby enhance their reactivity. However, both the experimental and theoretical description of defects such as the $F$ center and the $M$ center are still far from complete, in particular for the less common bromides. A self-consistent dielectric-dependent global hybrid and plane-wave approach is employed for a comparative theoretical study of the electronic properties of NaCl, KCl, NaBr, and KBr bulk and (100) surface, both perfect and defective. For these systems, a zero-point renormalization was calculated to account for electron-phonon interaction and enhance comparability with the experiment. We focus on anion vacancy defects, the so-called $F$ and $M$ centers. The methodology employed is capable of reproducing measured defect level energies, electronic band gaps, ionization energies, and electron affinities within experimental errors. A general trend of the $F$ center defect level energy with respect to the lattice parameter is found. The results for both the $F$ and the $M$ center of KCl agree with findings from magnetic resonance experiments. The defect orbitals are analyzed and virtual states of the defect electron are identified.

A Novel All-Optical Sensor Design Based on a Tunable Resonant Nanocavity in Photonic Crystal Microstructure Applicable in MEMS Accelerometers

In view of the large scientific and technical interest in the MEMS accelerometer sensor and the limitations of capacitive, resistive piezo, and piezoelectric methods, we focus on the measurement of the seismic mass displacement using a novel design of the all-optical sensor (AOS). The proposed AOS consists of two waveguides and a ring resonator in a two-dimensional rod-based photonic crystal (PhC) microstructure, and a holder which connects the central rod of a nanocavity to a proof mass. The photonic band structure of the AOS is calculated with the plane-wave expansion approach for TE and TM polarization modes, and the light wave propagation inside the sensor is analyzed by solving Maxwell’s equations using the finite-difference time-domain method. The results of our simulations demonstrate that the fundamental PhC has a free spectral range of about 730 nm covering the optical communication wavelength-bands. Simulations also show that the AOS has the resonant peak of 0.8 at 1.644µm, quality factor of 3288, full width at half maximum of 0.5nm, and figure of merit of 0.97. Furthermore, for the maximum 200nm nanocavity displacements in the x- or y-direction, the resonant wavelengths shift to 1.618µm and 1.547µm, respectively. We also calculate all characteristics of the nanocavity displacement in positive and negative directions of the x-axis and y-axis. The small area of 104.35 µm2 and short propagation time of the AOS make it an interesting sensor for various applications, especially in the vehicle navigation systems and aviation safety tools.

Modified Becke-Johnson calculations using norm-conserving pseudopotential plane-wave approach: Systematic analysis

The modified Becke-Johnson potential for exchange and local density approximation for correlation (mBJLDA) approach is proved to provide a highly improved description of the bandgap. This work presents the results of a systematic investigation of the mBJLDA electronic band structure calculations using the norm-conserving pseudopotential (PP) approach, as implemented in the ABINIT code. The considered systems represent different classes of semiconductors and insulators. Moreover, all-electron (AE) mBJLDA potential and screened hybrid functional YS-PBE0 calculations are also performed for comparison purposes. We show that the mBJLDA@PP bandgaps obtained using typical PP's (i.e., valence and semicore electrons are allowed to relax) are generally speaking smaller than that of the mBJLDA@AE approach. This artifact cannot be attributed to a small “c” parameter of the mBJLDA, since we found that this parameter is quite insensitive to the adopted computational method. Interestingly, the number of electrons treated as valence, especially for the cations, can be used as an extra adjustable parameter to improve the mBJLDA@PP bandgaps. In particular, the best agreement with the mBJLDA@AE bandgaps is obtained by including the outer core p electrons as valence. The optimal PP's for mBJLDA@PP calculations are transferable. A similar agreement is also achieved for the binding energies of the semicore d electrons (in the IIB-VI and IIIB-V compounds), upper valence bandwidths and electron effective masses. Sound explanations of the mBJLDA@PP results for the considered electronic structure properties are provided.

Ab initio prediction of the structural, electronic and optical behavior of novel combinations of ternary perovskite oxides ATiO3 (A = Rb, Cs, Fr) using the Hubbard ‘U’ correction for optoelectronic devices

The structural and optoelectronic properties of ATiO3 (A = Rb, Cs, and Fr) perovskite oxides in the cubic phase are investigated using an ab initio method based on density functional theory. The exchange correlation function is determined by the full-potential linearized augmented plane wave approach. RbTiO3 shows an indirect band gap energy of 2.43 eV, whereas direct band gap energies of 1.90 eV and 1.66 eV are calculated for CsTiO3 and FrTiO3 compounds, respectively. The electronic density of states reveals that d states from A-cations (Rb, Cs, and Fr), along with Ti-d states, are located in the conduction band. The charge density demonstrates the ionic nature of the A-cations (Rb, Cs and Fr), while oxygen atoms share isolines with Ti atoms, leading to covalent bonds between them. Optical analyses show that the considered compounds have good optical conductivity and absorptivity when high-energy photons are incident on the surface of the materials, in particular showing minimal reflectivity. In view of these results, we consider that perovskites belong to a semiconductor category of materials suitable for application in photovoltaic and optoelectronic devices.

Density functional study on structural and optoelectronic properties of cubic $$\hbox {Mg}_{x}\hbox {Zn}_{1-x}\hbox {S}_{y}\hbox {Se}_{1-y}$$ semiconductor quaternary alloys

In the case of technologically important quaternary alloys, structural and optoelectronic properties have been calculated with density functional theory (DFT)-based full-potential linearised augmented plane-wave (FP-LAPW) approach. The Perdew–Burke–Ernzerhof generalised gradient approximation (PBE-GGA) for structural properties and both the modified-Becke–Johnson (mBJ) and Engel and Vosko GGA (EV-GGA) for optoelectronic properties are employed to calculate the respective exchange-correlation potentials. Each specimen within the quaternary system is a direct band-gap – semiconductor. The lattice constant decreases, while bulk modulus and band gap increase nonlinearly with increasing anionic (S) concentration at each cationic (Mg) concentration . On the other hand, nonlinear increment in lattice constant and band gap, but decrement in bulk modulus is found with increase in cationic concentration at each anionic concentration . Calculated contour maps for lattice constants and energy band gaps would be useful in fabricating new quaternary alloys with preferred optoelectronic features. Optical properties of the specimens within the quaternary system show several interesting features. , 4p and 4p optical excitations contribute intense peaks in each spectrum. The composition dependence of each calculated zero-frequency limit shows opposite trend, while each calculated critical point shows similar trend of composition dependence of band gap. Moreover, calculations suggest the possibility of growth of several cubic quaternary specimens on GaAs and InP substrates.

Plane-wave approach to the exact van der Waals interaction between colloid particles.

The numerically exact evaluation of the van der Waals interaction, also known as Casimir interaction when including retardation effects, constitutes a challenging task. We present a new approach based on the plane-wave basis and demonstrate that it possesses advantages over the more commonly used multipole basis. The rotational symmetry of the plane-sphere and sphere-sphere geometries can be exploited by means of a discrete Fourier transform. The new technique is applied to a study of the interaction between a colloid particle made of polystyrene or mercury and another polystyrene sphere or a polystyrene wall in an aqueous solution. Special attention is paid to the influence of screening caused by a variable salt concentration in the medium. It is found that, in particular for low salt concentrations, the error implied by the proximity force approximation is larger than usually assumed. For a mercury droplet, a repulsive interaction is found for sufficiently large distances, provided that screening is negligible. We emphasize that the effective Hamaker parameter depends significantly on the scattering geometry on which it is based.

Trajectory Surface Hopping Nonadiabatic Molecular Dynamics with Kohn-Sham ΔSCF for Condensed-Phase Systems.

We present an efficient approach for surface hopping-based nonadiabatic dynamics in the condensed phase. For the systems studied, a restricted Kohn-Sham orbital formulation of the delta self-consistent field (ΔSCF) method was used for efficient calculation of excited electronic states. Time-dependent density functional theory (DFT) is applied to aid excited-state SCF convergence and provide guess electronic state densities. Aside from that the Landau-Zener procedure simplifies the surface hopping between electronic states. By utilizing the combined Gaussian and plane waves approach with periodic boundary conditions the method is easily applicable to full atomistic DFT simulations of condensed-phase systems and was used to study the nonradiative deactivation mechanism of photoexcited diimide in water solution.

Structural and elastic properties, Vickers hardness and Debye temperature of (B1) BP: DFT study

A theoretical study of the structural parameters and elastic constants of boron phosphide (BP) compound with cubic rocksalt structure has been carried out using ab-initio density functional theory (DFT) and density functional perturbation theory (DFPT) calculations based on the plane-wave and pseudopotential (PW-PP) approach. Elastic anisotropy factors, Cauchy pressure, inverted Pugh’s ratio, aggregate mechanical moduli (shear modulus, Young's modulus and Poisson's ratio), Vickers hardness HV, elastic wave velocity as well as the Debye temperature θD and the melting point have been also calculated. Our obtained results are in general in good agreement with other data of the literature. The deviation between our obtained value (4.225 Ã…) of the lattice constant and the theoretical value (4.282 Ã…) of the literature is around 1.33%, while that between our obtained value (169.7 GPa) of the bulk modulus and the theoretical one (171 GPa) is only around 0.77%. The calculated values of HV and θD were found at around 30.5 GPa and 1254 K (1314.4 K), respectively. Â

Theoretical investigation of the structural, electronic, and elastic properties of TM3In(TM = Pd and Pt) intermetallic compounds

We present a systematic study of the structural, elastic, and electronic properties of TM3In(TM = Pd and Pt) intermetallic compounds using density functional theory via a full-potential linearized augmented plane wave approach in the context of the Perdew–Burke–Ernzerhof generalized gradient approximation. We investigate the elastic constants, partial and total density of states (DOS), and charge density. Our structural study shows that the compounds have a cubic crystal with space group Pm-3m in phase I. In addition, we have calculated the formation energy and the DOS at the Fermi level [N(Ef)]. The results indicate that Pd3In is more stable than Pt3In.

The impact of point defects on the optical and electrical properties of cubic ZrO2

First-principles calculations based on the full-potential linear augmented plane wave approach are applied herein to investigate the electronic, optical, and electrical properties of intrinsic point defects in cubic ZrO2. The exchange–correlation potential is treated using the recent modified Becke–Johnson potential (TB-mBJ) proposed by Tran and Blaha in addition to the generalized gradient approximation (GGA), which successfully corrects the bandgap problem found in a wide range of materials when using the GGA alone. The energy of formation of the point defects is calculated after the optimization of atomic positions. The results indicate that the ZrO antisite has the lowest energy of formation and could be the acceptor defect responsible for the p-type conductivity of undoped cubic ZrO2. Besides, the TB-mBJ method is used to calculate various optical coefficients, including the complex refractive index and the absorption and transmittance coefficients. Furthermore, the electrical properties of each type of point defect are investigated by applying Boltzmann transport theory.

CP2K: An electronic structure and molecular dynamics software package - Quickstep: Efficient and accurate electronic structure calculations.

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post-Hartree-Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension.

High-Frequency Nonlinear Doppler Contrast-Enhanced Ultrasound Imaging of Blood Flow.

Current methods for in vivo microvascular imaging (<1 mm) are limited by the tradeoffs between the depth of penetration, resolution, and acquisition time. Ultrasound Doppler approaches combined at elevated frequencies (<7.5 MHz) are able to visualize smaller vasculature and, however, are still limited in the segmentation of lower velocity blood flow from moving tissue. Contrast-enhanced ultrasound (CEUS) has been successful in visualizing changes in microvascular flow at conventional diagnostic ultrasound imaging frequencies (<7.5 MHz). However, conventional CEUS approaches at elevated frequencies have met with limited success, due, in part, to the diminishing microbubble response with frequency. We apply a plane-wave acquisition combined with the non-linear Doppler processing of ultrasound contrast agents at 15 MHz to improve the resolution of microvascular blood flow while compensating for reduced microbubble response. This plane-wave Doppler approach of imaging ultrasound contrast agents also enables simultaneous detection and separation of blood flow in the microcirculation and higher velocity flow in the larger vasculature. We apply singular value decomposition filtering on the nonlinear Doppler signal to orthogonally separate the more stationary lower velocity flow in the microcirculation and higher velocity flow in the larger vasculature. This orthogonal separation was also utilized to improve time-intensity curve analysis of the microcirculation, by removing higher velocity flow corrupting bolus kinetics. We demonstrate the utility of this imaging approach in a rat spinal cord injury model, requiring submillimeter resolution.