arXiv subjects

S. Javad Hashemifar

Publications and source records attributed to S. Javad Hashemifar.

18 recordsLinked to original sources

arXiv2024

Driven charge density modulation by spin density wave and their coexistence interplay in SmFeAsO: A first-principles study

We use density functional theory to investigate effects of spin-orbit coupling and single-stripe-type antiferromagnetic (sAFM) ordering on the crystal structure and electronic properties of SmFeAsO. The results indicate that AFM ordering causes the crystal structure transition from tetragonal to orthorhombic, along with increase in the height of As atoms from Fe layer due to magnetostriction. It also leads to the opening of partial band gaps, the emergence of a prominent peak near Fermi energy (EF) in the density of states (DOS) and a reduction in Fermi surface nesting. The study finds a correlation between the calculated area under the DOS curve, spanning from EF to the first peak above it, and the optimal electron-doped concentration required for inducing superconductivity in SmFeAsO. The findings suggest that spin and charge density waves can play important roles in the superconducting mechanism of Fe-based superconductors. Our calculations demonstrate that spin-orbit coupling reduces electronic correlation.

cond-mat.supr-con↗

arXiv2022

A benchmark of first-principles methods for accurate prediction of semiconductor band gaps

The band gap is an important parameter of semiconductor materials that influences several functional properties, in particular optical properties. However, a fast and reliable first-principles prediction of band gaps remains a challenging problem. Standard DFT approximations tend to strongly underestimate band gaps, while the more accurate $GW$ and hybrid functionals are much more computationally demanding and unsuitable for high-throughput screening. In this work, we have performed an extensive benchmark of several approximations with different computational complexity ($G_{0}W_{0}$@PBEsol, HSE06, PBEsol, mBJ, PBEsol$-1/2$, and ACBN0) to evaluate and compare their performance in predicting the band gap of semiconductors. The benchmark is based on 114 binary semiconductors of different compositions and crystal structures, where about half of them have experimental band gaps. We find that, as expected, $G_{0}W_{0}$@PBEsol performs well relative to the experiment, with a noticeable underestimation of the band gaps by about 14% on average. Surprisingly, $G_{0}W_{0}$@PBEsol is followed closely by the much computationally cheaper pseudo-hybrid ACBN0 functional, showing an excellent performance with respect to experimental data. The meta-GGA mBJ functional also performs well relative to the experiment, even slightly better than $G_{0}W_{0}$@PBEsol in terms of mean absolute (percentage) error. The HSE06 and PBEsol$-1/2$ schemes perform overall worse than ACBN0 and mBJ schemes but much better than PBEsol. Comparing the calculated band gaps on the whole data set (including the samples with no experimental band gap), we find that HSE06 and mBJ have excellent agreement with respect to the reference $G_{0}W_{0}$@PBEsol band gaps. Thus, we propose the mBJ band gaps as economic descriptors when developing artificial intelligence models for high-throughput screening of semiconductor band gaps.

cond-mat.mtrl-sci↗

arXiv2022

The overlooked role of band-gap parameter in characterization of Landau levels in a gapped phase semi-Dirac system: the monolayer phosphorene case

Two-dimensional gapped semi-Dirac (GSD) materials are systems with a finite band gap that their charge carriers behave relativistically in one direction and Schrödinger-like in the other. In the present work, we show that besides the two well-known energy bands features (curvature and chirality), the band-gap parameter also play a crucial role in the index- and magnetic field-dependence of the Landau levels (LLs) in a GSD system. We take the monolayer phosphorene as a GSD representative example to explicitly provide physical insights into the role of this parameter in determining the index- and magnetic field-dependence of LLs. We derive an effective one-dimensional Schrödinger equation for charge carriers in the presence of a perpendicular magnetic field and argue that the form of its effective potential is clearly sensitive to a dimensionless band-gap that is tunable by structural parameters. The theoretical magnitude of this effective gap and its interplay with oval shape $k$-space cyclotron orbits resolve the seeming contradiction in determining the type of the quantum Hall effect in the pristine monolayer phosphorene. Our results strongly confirm that the dependence of LLs on the magnetic field in this GSD material is as conventional two-dimensional semiconductor electron gases up to a very high field regime. Using the strain-induced gap modification scheme, we show the field dependence of the LLs continuously evolves into $B^{2/3}$ behavior, which holds for a gapless semi-Dirac system. The highlighted role of the band-gap parameter may affect the consequences of the band anisotropy in the physical properties of a GSD material, including magnetotransport, optical conductivity, dielectric function, and thermoelectric performance.

cond-mat.mes-hall↗

arXiv2021

Al$_2$B$_2$ and AlB$_4$ monolayers: emergence of multiple two-dimensional Dirac nodal line semimetals with novel properties

Topological semimetal phases in two-dimensional (2D) materials have gained widespread interest due to their potential applications in developing nanoscale devices. Despite the prediction of the Dirac/Weyl points in a wide variety of 2D candidates, materials featuring topological nodal lines are still in great scarcity. Herein, we predict two stable thinnest films of aluminum diboride with hyper- and hypo-stoichiometries of Al$_2$B$_2$ and AlB$_4$ as new 2D nonmagnetic Dirac nodal line semimetals (NLSMs) which promise to offer many novel features. Our elaborate electronic structure calculations combined with analytical studies reveal that, in addition to the multiple Dirac points, these 2D configurations host various type-I closed nodal lines (NLs) around the Fermi level, all of which are semimetal states protected by the time-reversal and in-plane mirror symmetries. The most intriguing NL in Al$_2$B$_2$ encloses the K point and crosses the Fermi level with a considerable dispersion, thus providing a fresh playground to explore exotic properties in dispersive Dirac nodal lines. More strikingly, in the case of 2D superconductor AlB$_4$ which exhibits a high transition temperature, we provide the first evidence for a set of 2D nonmagnetic open type-II NLs in weak spin-orbit coupling limit, coinciding with closed type-I NLs near the Fermi level. The coexistence of superconductivity and nontrivial band topology in AlB$_4$ not only makes it a promising material to exhibit novel topological superconducting phases, but also the rather large energy dispersion of type-II nodal lines in this configuration, may offer a distinguished platform for realization of novel topological features in two-dimensional limit.

cond-mat.mes-hall↗

arXiv2021

Novel First-Principles Insights into Graphene Fluorination

Comprehensive first-principles calculations are performed on diverse arrangements of relevant chemical defects in fluorographene to provide accurate microscopic insights into the process of graphene fluorination. The minimum energy paths for the half- and full-fluorination processes are calculated for a better understanding of these phenomena. While experimental observations indicate a much slower rate of the full-fluorination process, compared with the half-fluorination one, the obtained energy profiles demonstrate much enhanced fluorine adsorption after the half-fluorination stage. This ambiguity is explained in terms of significant chemical activation of the graphene sheet after half-fluorination, which remarkably facilitates the formation of chemical contaminants in the system and thus substantially slows down the full-fluorination procedure. After considering the binding energy and durability of the relevant chemical species, including hydrogen, oxygen, and nitrogen molecules and xenon atom, it is argued that oxygen-fluorine ligands are the most likely chemical contaminants opposing the full-fluorination of a graphene sheet. We propose an oxygen desorption mechanism for the atomic description of the full-fluorination procedure in realistic situations. It is argued that the proposed mechanism explains well much enhanced rate of the full-fluorination procedure at elevated temperatures.

cond-mat.mes-hall↗

arXiv2020

Ab initio analysis of some Ge-based 2D nanomaterials

The structural, electronic and dynamical properties of a group of 2D germanium-based compounds, including GeC, GeN, GeO, GeSi, GeS, GeSe, and germanene, are investigated by employing first-principles calculations. The most stable structure of each of these systems is identified after considering the most probable configurations and performing accurate phonon calculations. We introduce a new phase of germanene, which we name the tile germanene, which is significantly more stable than the known hexagonal germanene. We apply the modern modified Becke-Johnson (mBJ) and DFT1/2 schemes to obtain an accurate band structure for our selected 2D materials. It is seen that GeO and GeC exhibit the highest band gaps of more than 3 eV in this group of materials. Moreover, we argue that, in contrast to the semi-metallic nature of hexagonal germanene, the tile germanene is a very good conductor. The band edges of our semiconducting 2D materials are accurately aligned to the vacuum level to address the potential photocatalytic application of this system for water splitting and carbon dioxide reduction. The optical properties, including dielectric functions, refractive index, reflectivity, and Loss function of the samples are investigated in the framework of the Bethe-Salpeter approach.

cond-mat.mtrl-sci↗

arXiv2019

Ab initio investigation of magnetic ordering in the double perovskite Sr$_{2}$NiWO$_{6}$

{\it Ab initio} calculations, GGA/GGA+$U$, are used to propose a spin Hamiltonian for the B-site ordered double perovskite, Sr$_{2}$NiWO$_{6}$. Our results show that the exchange interaction constants between the next nearest neighbors in both intra- and inter- $ab$ plane ($J_2$ and $J_{2c}$) are an order of magnitude larger than the ones between the nearest neighbors ($J_1$ and $J_{1c}$). Employing the Monte Carlo simulation, we show that the obtained Hamiltonian properly describes the finite temperature properties of Sr$_{2}$NiWO$_{6}$. Our {\it ab initio} calculations also reveal a small magnetic anisotropy and non-trivial bi-quadratic interaction between the nearest inter-$ab$ plane neighbors, which play essential roles in stabilizing the type-II anti-ferromagnetic ground state of Sr$_{2}$NiWO$_{6}$.

cond-mat.str-el↗

arXiv2018

First-principles insights into ultrashort laser spectroscopy of molecular nitrogen

In this research, we employ accurate time-dependent density functional calculations for ultrashort laser spectroscopy of nitrogen molecule. Laser pulses with different frequencies, intensities, and durations are applied to the molecule and the resulting photoelectron spectra are analyzed. It is argued that relative orientation of the molecule in the laser pulse significantly influence the orbital character of the emitted photoelectrons. Moreover, the duration of the laser pulse is also found to be very effective in controlling the orbital resolution and intensity of photoelectrons. Angular resolved distribution of photoelectrons are computed at different pulse frequencies and recording times. By exponential growth of the laser pulse intensity, the theoretical threshold of two photons absorption in nitrogen molecule is determined.

physics.atm-clus↗

arXiv2018

Ab-initio study of different structures of CaC: Magnetism, Bonding, and Lattice Dynamics

On the basis of ab-initio pseudopotential calculations, we study structural, magnetic, dynamical, and mechanical properties of the hypothetical CaC ionic compound in the rock-salt (RS), B2, zinc-blende (ZB), wurtzite (WZ), NiAs (NA), anti-NiAs (NA*), and CrB (B33) structures. It is argued that the ZB, WZ, NA, and RS structures are more ionic while the NA*, B2, and B33 structures are more covalent systems. As a result of that, the nonmagnetic B33-CaC is the energetically preferred system, while the more ionic structures prefer a ferromagnetic ground state with high Fermi level spin polarization. The observed ferromagnetism in the more ionic systems is attributed to the sharp partially filled $p$ states of carbon atom in the system. In the framework of density functional perturbation theory, the phonon spectra of these systems are computed and the observed dynamical instabilities of the NA* and B2 structures are explained in terms of the covalent bonds between carbon atoms. The calculated Helmholtz and Enthalpy free energies indicate the highest stability of the B33 structure in a wide range of temperatures and pressures. Among the ferromagnetic structures, RS-CaC and ZB-CaC are reported, respectively, to be the most and the least metastable systems in various thermodynamics conditions. Several mechanical properties of the dynamically stable structures of CaC are determined from their phonon spectra.

cond-mat.mtrl-sci↗

arXiv2018

Effects of strain on electronic and magnetic properties of Co/WS$_2$ junction: a density functional and Monte Carlo study

In this work, density functional computations and Monte Carlo simulations are performed to investigate structural, electronic, magnetic, and thermodynamic properties of Co/WS$_2$ junction, a semiconductor WS$_2$ monolayer covered by a ferromagnetic cobalt monolayer. In addition to a conventional semilocal exchange-correlation functional, three nonlocal functional, including the novel ACBN0 scheme, are applied to obtain reliable electronic and magnetic properties. It is argued that the ACBN0 scheme, is very efficient for first principles description of the Co/WS$_2$ junction. The obtained electronic structures evidence a trustworthy half-metallic gap in the majority spin channel of the lowest energy configuration of the junction, promising for spintronic applications. The obtained magnetic thermodynamics properties from Monte Carlo simulations predict a Curie temperature of about 110\,K, which is far small for device applications of this junction. The electronic and magnetic properties of the system are calculated under various compressive and tensile strains and it is shown that a tensile strain of about 4\% may effectively improve thermal stability of half-metallic ferromagnetism in the Co/WS$_2$ junction.

cond-mat.mtrl-sci↗

arXiv2018

Ab initio simulation of the structure and transport properties of zirconium and ferromagnetic cobalt contacts on the two-dimensional semiconductor WS_2

Using density-functional theory calculations, the atomic and electronic structure of single-layer WS_2 attached to Zr and Co contacts are determined. Both metals form stable interfaces that are promising as contacts for injection of n-type carriers into the conduction band of WS_2 with Schottky barriers of 0.45eV and 0.62eV for Zr and Co, respectively. With the help of quantum transport calculations, we address the conductive properties of a free-standing WS_2 sheet suspended between two Zr contacts. It is found that such a device behaves like a diode with steep I-V characteristics. Spin-polarized transport is calculated for such a device with a floating-gate Co electrode added. Depending on the geometrical shape of the Co gate and the energy of the carriers in WS_2, the transmission of spin majority and minority electrons may differ by up to an order of magnitude. Thus the steep I-V characteristics of the nanoscale device makes it possible to realize a spin filter.

cond-mat.mes-hall↗

arXiv2017

First-principles study of MoS$_2$ and MoSe$_2$ nanoclusters in the framework of evolutionary algorithm and density functional theory

Evolutionary algorithm is combined with full-potential ab-initio calculations to investigate conformational space of (MoS$_2$)$_n$ and (MoSe$_2$)$_n$ (n=1-10) nanoclusters and to identify the lowest energy structural isomers of these systems. It is argued that within both BLYP and PBE functionals, these nanoclusters favor sandwiched planar configurations, similar to their ideal planar sheets. The second order difference in total energy ($Δ_2$E) of the lowest energy isomers are computed to estimate the abundance of the clusters at different sizes and to determine the magic sizes of (MoS$_2$)$_n$ and (MoSe$_2$)$_n$ nanoclusters. In order to investigate the electronic properties of nanoclusters, their energy gap is calculated by several methods, including hybrid functionals (B3LYP and PBE0), GW approach, and $Δ$scf method. At the end, the vibrational modes of the lowest lying isomers are calculated by using the force constants method and the IR active modes of the systems are identified. The vibrational spectra are used to calculate the Helmholtz free energy of the systems and then to investigate abundance of the nanoclusters at finite temperatures.

cond-mat.mtrl-sci↗

arXiv2016

Calculated Structural and Electronic Interactions of the nano dye molecule $\rm Ru(4,\acute{4}-COOH-2,\acute{2}-bpy)_2(NCS)_2$($\rm N3$) with a iodide/triiodide redox shuttle

In this paper, dye sensitized solar cell based on nano dye molecule N3 are investigated by using density functional computations. The main focus is on the N3 dye molecule and corresponding complexes formed at the interface between electrolyte and dye, during the regeneration process. The optimizaed geometry and electronic structure of the molecule and complexs are calculated by using the pseudopotential as well as full-potential techniques. The absorption spectra of metalliferous dye molecule, N3, and its complexes are computed in the framework of time dependent density functional theory. We determine the reaction path of dye regeneration by Nudged Elastic Band (NEB) method. IR spectrum of the N3 dye molecule were also calculated. We found that complexes of N3 dye molecule and transition states formed in reactions, are magnetic.

cond-mat.mtrl-sci↗

arXiv2016

First-principles structure search for the stable isomers of stoichiometric WS2 nano-clusters

In this paper, we employ evolutionary algorithm along with the full-potential density functional theory (DFT) computations to perform a comprehensive search for the stable structures of stoichiometric (WS2)n nano-clusters (n=1-9), within three different exchange-correlation functionals. Our results suggest that n=3, 5, 8 are possible candidates for the low temperature magic sizes of WS2 nano-clusters while at temperatures above 600 Kelvin, n=5 and 7 exhibit higher relative stability among the studied systems. The electronic properties and energy gap of the lowest energy isomers were computed within several schemes, including semilocal PBE and BLYP functionals, hybrid B3LYP functional, many body based DFT+GW approach, and time dependent DFT calculations. Vibrational spectra of the lowest lying isomers, computed by the force constant method, are used to address IR spectra and thermal free energy of the clusters. Time dependent density functional calculation in real time domain is applied to determine the full absorption spectra and optical gap of the lowest energy isomers of the WS2 nano-clusters.

cond-mat.mtrl-sci↗

arXiv2016

Ab-initio investigation of structural, electronic, and optical properties of (5,0) finite-length carbon nanotube

We use density functional computations to study the size effects on the structural, electronic, magnetic, and optical properties of (5,0) finite carbon nanotubes (FCNT), with length in the range of 4-44 Å. It is found that the structural and electronic properties of (5,0) FCNTs, in the ground state, converge at a length of about 30 Å, while the excited state properties exhibit long-range edge effects. We discuss that curvature effects govern the electronic structure of short (5,0) FCNTs and enhance energy gap of these systems, in contrast to the known trend in the periodic limit. It is seen that compensation of curvature effects in two special small sizes, may give rise to spontaneous magnetization. The obtained cohesive energies provide some insights into the effects of environment on the growth of FCNTs. The second-order difference of the total energies reveals an important magic size of about 15 Å. The optical and dynamical magnetic responses of the FCNTs to polarized electromagnetic pulses are studied by time dependent density functional theory. The obtained results show that the static and dynamic magnetic properties mainly come from the edge carbon atoms. The optical absorption properties are described in term of local field effects and characterized by Casida linear response calculations.

cond-mat.mes-hall↗

arXiv2015

First-principles insights into f magnetism, a case study on some magnetic pyrochlores

First-principles calculations are performed to investigate f magnetism in A$_2$Ti$_2$O$_7$ (A=Eu, Gd, Tb, Dy, Ho, Er, Yb) magnetic pyrochlore oxides. The Hubbard U parameter and the relativistic spin orbit correction is applied for more accurate description of the electronic structure of the systems. It is argued that the main obstacle for first-principles study of these systems is the multi-minima solutions of their electronic configuration. Among the studied pyrochlores, Gd$_2$Ti$_2$O$_7$ shows the least multi-minima problem. The crystal electric field theory is applied for phenomenological comparison of the calculated spin and orbital moments with the experimental data.

cond-mat.str-el↗

arXiv2015

Density-functional study of the pure and palladium doped small copper and silver clusters

The size-dependent electronic, structural, magnetic and vibrational properties of small pure cop- per and silver clusters and their alloys with one and two palladium atoms are studied by using full-potential all-electron density functional computations. The stable isomers of these clusters are identified and their theoretical magic numbers are determined via the analysis of the second differ- ence of their minimized energy. We discuss that the doped Pd atoms generally prefer to sit in the high coordination sites of the pure clusters. It is argued that Pd doping influences the structural properties and the two dimensional to three dimensional structural cross over in the small Cu and Ag clusters. The many body based GW correction is applied for more accurate determination of the electron affinity and ionization potential of these systems. Magnetic and vibrational properties of the pure and doped clusters are presented and discussed.

cond-mat.mtrl-sci↗

arXiv2012

First-principles study of structural, electronic and thermodynamic properties of (ZnO)$_n$(n=2-16) clusters

The structural, electronic, and vibrational thermodynamic properties of the (ZnO)$_n$ (n=2-16) clusters are studied using density functional - full potential computations. The results show, small clusters up to $n=9$ stabilize in the 2D ring shape geometries while the larger clusters prefer the 3D cage like structures. The ring to cage structural cross over in ZnO clusters is studied by investigating the behavior of the Zn-O-Zn bond angle, the Zn-O bond strength, and the number of bonds in the systems. It is argued that 12 is the lowest magic number of ZnO clusters at ground state, while finite temperature vibrational excitations enhance the relative stability of the (ZnO)$_9$ cluster and make it a magic system at temperatures above about 170 K. The obtained electronic structure of ZnO clusters before and after applying the many-body GW corrections evidence a size induced red shift originated from the ring to cage structural cross over in these systems. The behavior of the extremal points of electron density of the clusters along with the extrapolated cluster binding energies at very large sizes may be evidences for existence of a metastable structure for large ZnO nanostructures, different with the bulk ZnO structure.

cond-mat.mtrl-sci↗