Framework Of First-principles Calculations Research Articles

Investigation of stabilization and survival of skyrmion vortices in the presence of magnetic field disorder in two-dimensional lattices: a case study for Janus dichalcogenides

Due to the lack of inversion symmetry, very large Dzyaloshinskii–Moriya interaction (DMI) has been reported for a series of Janus monolayers of manganese dichalcogenides within the framework of first-principles calculations (Liang et al 2020 Phys. Rev. B 101 184401). However, from the viewpoint of potential applications, the current ongoing research mainly focuses on the magnetism in pristine two-dimensional (2D) materials exhibiting non-zero DMI, and the effects of disorder in such systems remain an open problem since the influence of randomness may create some drastic effects on the magnetism of low dimensional systems. Here, we present Monte Carlo simulation results regarding the magnetic properties of a 2D manganese based Janus dichalcogenide material MnSTe in the presence of quenched random magnetic fields where the local field variables have been sampled from a Gaussian distribution. For the selected benchmark material, it has been found that the magnetic skyrmion vortexes emerging at (10 K, 3 T) may survive in the presence of weak and moderate quenched randomness, which is important from the viewpoint of technological applications. In both the pristine and random field cases, the stabilization of magnetic skyrmions are achieved by the major contribution of the ferromagnetic exchange energy to the total energy of the system, and the materials exhibiting large DMI/exchange ratios may exhibit resilient magnetic skyrmion vortexes in the presence of weak and moderate amount of randomness.

Spin–orbit coupling tunable electronic properties of [formula omitted]-MoS[formula omitted] and ternary Janus [formula omitted]-MoSSe monolayers: Theoretical prediction

MoS2 can exist in many different polytypes, such as trigonal prismatic, distorted octahedral, or octahedral. The 2H-phase of MoS2 has been extensively studied in recent times, but there are very few studies focusing on the remaining phases, especially the distorted octahedral T′)-phase. Here, we design the MoS2 monolayer and Janus MoSSe structure in the 1T′ phase and study their electronic properties within the framework of first-principles calculations. Both 1T′-MoS2 and 1T′-MoSSe monolayers are confirmed to have structural stability and anisotropic mechanical characteristics. Interestingly, while the 1T′-MoS2 monolayer exhibits metallic characteristics, Janus MoSSe in the 1T′ phase is a semiconductor with a small band gap of 0.04 eV. More interestingly, a tiny bandgap of 0.05 eV has opened up in the 1T′-MoS2 monolayer due to the spin–orbit coupling effect. We also study the mobility of carriers in the semiconductor Janus 1T′-MoSSe monolayer. 1T′-MoSSe monolayer exhibits a high anisotropic carrier mobility due to its high anisotropic crystal structure. Surprisingly, 1T′-MoSSe monolayer possesses extremely high electron mobility, up to 4.58×103 cm2 V−1 s−1. Our findings give a deeper insight into the 1T′-phase of transition metal dichalcogenide and its Janus structure.

Effect of Electron Correlations on the Electronic Structure and Magnetic Properties of the Full Heusler Alloy Mn2NiAl

In this theoretical study, we investigate the effect of electron correlations on the electronic structure and magnetic properties of the full Heusler alloy Mn2NiAl in the framework of first-principles calculations. We investigate the electron correlation effect as employed within hybrid functional (HSE) and also within the DFT+U method with varied values of parameters between 0.9 and 6 eV. The XA-crystal structure was investigated with antiferromagnetic orderings of the magnetic moments of the manganese. It was found that with a growth of the Coulomb interaction parameter, the manganese ions magnetic moment increases, and it reaches the value of 4.15–4.46 μB per Mn. In addition, the total magnetic moment decreases because of the AFM ordering of the Mn ions and a small magnetic moment of Ni. The calculated total magnetic value agrees well with recent experiments demonstrating a low value of magnetization. This experimental value is most closely reproduced for the moderate values of the Coulomb parameter, also calculated in constrained LDA, while previous DFT studies substantially overestimated this value. It is also worth noticing that for all values of the Coulomb interaction parameter, this compound remains metallic in its electronic structure in agreement with transport measurements.

Electronic structure and optical properties of the Ag3SbS3 crystal: experimental and DFT studies.

A high-quality Ag3SbS3 single crystal was grown by the Bridgman-Stockbarger method and its crystalline structure and homogeneity were investigated. The fundamental absorption edge of Ag3SbS3 was studied. The value of the band gap of the studied compound was obtained at the level of 1.91 eV at T = 300 K. The structural, electronic, and optical properties of the Ag3SbS3 crystal were considered within the framework of first-principles calculations using density functional theory (DFT). The structure of the crystal lattice was optimized and its closeness to the experimental one is shown. The band-energy structure of the crystal was calculated revealing that the crystal has a band gap of indirect type with Eg = 0.88 eV for GGA (0.35 eV for LDA). The origin of the energy bands in the crystal was clarified and the nature of the fundamental absorption edge was analyzed using the calculated density of electronic states. The dielectric function (real part ε1(ω) and imaginary part ε2(ω)) and absorption coefficient α(ω) were calculated for two independent directions in the crystal and compared with experimental data. The character and anisotropy of optical functions are analyzed. The high value of the absorption coefficient of the Ag3SbS3 crystal is shown, which makes it a promising material for use as an absorbing layer in photovoltaics.

First-principles and machine-learning study of electronic and phonon transport in carbon-based AA-stacked bilayer biphenylene nanosheets

In this study, the structural, electronic and thermal transport properties of AA-stacked bilayer biphenylene sheet (BPN) are systematically investigated in the framework of first-principles calculations in addition to machine-learning interatomic potential approaches. Optimized geometry of AA-stacked bilayer satisfies all the necessary stability criteria which further infers that structure becomes feasible for experimental design. Similar to monolayer BPN, its bilayer variant also exhibits a metallic band structure. Thermal transport characteristics of the AA-stacked bilayer have been analyzed from thermal conductivity, the Seebeck coefficient, and electrical conductivity variations. The electronic part of the thermal conductivity for AA-stacked bilayer Biphenylene exhibits linearly enhancing character with temperature, whereas lattice contributions possess an inverse function of temperature. Moreover, the negative values of the Grüneisen (γ) parameter for AA-stacked bilayer BPN dictate negative thermal expansion which can have potential application to tailor the thermal expansion coefficient in many practical situations.

Ab initio systematic description of thermodynamic and mechanical properties of binary bcc Ti-based alloys

The effects of the dissolution of 3d, 4d and 5d metals and Al, Ga, Ge, In and Sn in the bcc Ti lattice (β-phase) were studied within the framework of first-principles calculations. Lattice parameters, mixing enthalpies, single-crystal elastic constants C11, C12, C44 and C′, polycrystalline elastic moduli E, G, and plasticity characteristics of bcc Ti-based solid solutions in a wide concentration range up to 50 at% were investigated using the EMTO-CPA method. The PAW-SQS method was used to study the effects of alloying and local atomic relaxations on the indicated properties of Ti-X bcc alloys, where X is a series of 4d elements Nb, Mo, Tc, Ru, Rh, and 5d elements Ta, W, Re, Os, Ir for the set of concentrations: 6.25, 25 and 50 at%. The effect of concentration and periodic dependences on the properties of alloys and the dependence of elastic properties on the electronic structure were analyzed. It is shown that the calculated elastic constant C′ is a convenient and useful parameter for ranking chemical elements according to the relative strength of the stabilizing effect on the β-phase, in addition, the concentration dependences of C′ demonstrate softening with concentration increasing in the case of late transition metals, thereby reflecting the tendency of these metals to form intermetallic compounds with titanium, while early transition metals show a linear grow in stability with increasing concentration.

Electronic structures and physical properties of Mg, C, and S doped g-GaN

Doping can provide hole and electron carriers into g-GaN materials to enhance conductivity and improve photoemission performance of g-GaN-based devices. In this work, we propose 9 × 9 × 1 g-GaN supercell and C, Mg, and S doped g-GaN systems (C and Mg substituting for Ga (CGa and MgGa systems), C and S substituting for N (CN and SN systems)) with different concentrations, and investigate their structural, electronic, electrical, and optical properties in the framework of first-principles calculations. N-type doping is achieved in the CGa and SN g-GaN systems, and p-type doping is achieved in the MgGa and CN g-GaN systems. C substituting for Ga is easier than C substituting for N, and the CGa g-GaN systems are more stability than the CN g-GaN systems. At the ultraviolet of the absorption spectra, the peaks of the doped g-GaN systems blue-shift or red-shift relative to the intrinsic g-GaN, and there are sharp peaks of the doped CGa and SN g-GaN systems at visible range. The work function and the carrier mobility have been modulated by doping and the electrical properties of g-GaN can be tuned effectively, which also promising significant applications in the design of 2D microelectronic devices and the integrated circuit.

Theoretical study of ThO2 by first principles calculations

In this paper, the thermodynamic, structural properties and vibrational spectrum of thorium dioxide have been studied using the Density Functional Perturbation Theory (DFPT) and Density Functional Theory (DFT) in the framework of first principles calculations. Quantum espresso software, which is an open source computing code, has been used in order to compute the kohn-Sham equations to obtain the minimum total energy of crystal. The vibrational spectrum of the thorium dioxide was examined along various symmetrical directions, and the results showed the dynamical stability of the crystal system. The quasi-harmonic Debye-Einstein model as implemented in GIBBS2 Code was used to calculate the thermodynamic properties of thorium dioxide at high temperatures and pressures. The simulation results showed that the Debye temperature of thorium dioxide decreased with increasing temperature at a constant pressure and increased with increasing pressure at a constant temperature. Increasing the Debye temperature indicated an increase in the crystal stiffness and the average sound velocity. It was observed that the volumetric thermal expansion coefficient and gruneisen parameter decreased exponentially with increasing pressure at a constant temperature, while increased with increasing temperature at a constant pressure, indicating an increase in heat transfer in the crystal lattice

Strain engineering the electronic and photocatalytic properties of WS2/blue phosphene van der Waals heterostructures

The effects of −8–8% in-plane uniaxial and biaxial strains on the electronic and photocatalytic activity of tungsten disulfide/blue phosphene (WS2/BlueP) are investigated within the framework of first-principles calculations.

Effect of rare earth on physical properties of Na0.5Bi0.5TiO3 system: A density functional theory investigation

Na0.5(Bi3/4RE1/4)0.5TiO3 (RENBT, RE = Nd, Gd, Dy, and Ho) compounds were investigated in the framework of first-principles calculations using the full potential linearized augmented plane wave (FP-LAPW) method based on the spin-polarized density functional theory implemented in the WIEN2k code. Combined charge density distribution and Ti K-edge X-ray absorption spectra reveal that the RENBT compositions with high polarization values are accompanied by a higher TiO6 distortion, DyNBT, and NdNBT compounds. The effect of the rare-earth elements on the polarization is confirmed experimentally with the collection of the hysteresis loops. The investigation of the electronic properties of the compounds highlights the emergence of a magnetization owing to the 4f orbital effect of the rare-earth elements. Besides, the investigation of the chemical ordering shows a short-range chemical ordering for the pure composition and an increased A-site disorder for dysprosium doped NBT system. The increased disorder may speak for increased relaxor properties in the RE doped compositions.

Effects of Sulfur Doping on Generalized Stacking Fault Energy of Indium Phosphide

Indium phosphide (InP) is one of the most important optoelectronic materials, while stacking faults (SFs), as planar defects, are usually inevitable during the growth of InP possibly due to the low SF energy. As n-type dopants, sulfur atoms are generally used to change the electron concentration of InP-based devices, whereas the effects of sulfur doping on SFs of InP have not been studied in detail. In this work, the generalized stacking fault (GSF) energies of pure and sulfur-doped InP have been investigated by gliding the layers successively in the framework of first principle calculations. The results reveal the stable SF energies of InP are low and extrinsic stacking fault could be seen as the twin embryos in pure InP. Sulfur doping could decrease the GSF energies dramatically due to the lower charge density along In-S bonds than along In-P bonds, which consequently enhances the ability of twinning locally. The preferential segregation of sulfur atoms on SFs or twin boundaries could further promote the thickening of microtwin in InP. These results are of great significance to the understanding of the formation of planar defects in n-type doped III–V compounds.

Electronic structure and band alignment of Blue Phosphorene/Janus ZrSSe heterostructure: A first principles study

Abstract In this work, we construct the BlueP/ZrSSe heterostructure and explore systematically its electronic characteristics and interface features in the framework of first principles calculations. The stacking and electric field effects on the interface characters of BlueP/ZrSSe heterostructure are also considered. We find that the BlueP layer interacts with Janus ZrSSe layer via the weak van der Waals forces, which keeps the BlueP/ZrSSe heterostructure feasible. Both the BlueP/SZrSe and BlueP/SeZrS heterostructures possess indirect semiconductor and exhibits type-I band alignment. Furthermore, electric field can tune the band alignment and switch the BlueP/ZrSSe heterostructure from semiconductor to metal. These findings could provide a helpful guidance for using BlueP/ZrSSe heterostructure in practical applications of nanoelectronics and optoelectronics.

Tri-layered van der Waals heterostructures based on graphene, gallium selenide and molybdenum selenide

In this work, we propose ultrathin trilayered heterostructures (TL-HTSs) of graphene (G), gallium selenide (GaSe), and molybdenum selenide (MoSe2) monolayers and investigate their structural and electronic properties in the framework of first-principles calculations. By calculating the binding energies and interlayer distances and comparing them with those of the typical vdW HTSs, we find that the systems we consider are energetically stable and are characterized by weak vdW interactions. The formation of G, GaSe, and MoSe2 monolayers to form G/GaSe/MoSe2, GaSe/G/MoSe2, and G/MoSe2/GaSe HTSs leads to the opening of a sizable bandgap in graphene at the Dirac point and shows the p-type Schottky contact. Among these kinds of TL-HTSs, the G/GaSe/MoSe2 has many more advantages than the others due to the lowest binding energy of −29.47meV/Å2, the biggest bandgap opening in G of 84.7 meV, and the smallest Schottky barrier height of 0.63 eV. Furthermore, we find that the p-type Schottky contact of G/GaSe/MoSe2 HTS can be turned into an n-type one or into an Ohmic contact when vertical strain or electric field is applied. These results show a potential candidate of the combined HTSs of G, GaSe, and MoSe2 monolayers for developing high speed nanoelectronic and optoelectronic devices.

Ab initio study of magnetic and structural properties of Fe-Ga alloys

The structural and magnetic properties for a series of Fe100-xGax alloys (x = 18 – 30 at.%) are studied in the framework of first-principles calculations and Monte Carlo simulations. The both, general gradient approximation and local density approximation are considered for the exchange-correlation functional. The ground state ab initio calculations are performed for both D03 and L12 crystal structures. It is shown that for general gradient approximation, the optimized lattice parameters and total magnetic moments are found in the better agreement with experimental ones. Using the calculated exchange coupling constants for studied compositions, Curie temperatures are estimated by means of Monte Carlo simulations of Heisenberg Hamiltonian.

Magnetic properties of Fe100−xGax: Ab initio and Monte Carlo study

This work presents a theoretical study of magnetic properties for Fe100-xGax (x = 25, 28.125%) alloys in the framework of first-principles calculations and Monte Carlo simulations. In particular, the zero-temperature ground state properties, magnetocrystalline energy, and magnetic exchange coupling constants for both D03 and L12 structures are calculated. It is shown that for both structures, the magnetocrystalline anisotropy energy change the sign from positive to negative during tetragonal distortion from c/a<1 to c/a>1, respectively. Using the exchange interaction constants within Heisenberg model, the thermomagnetization curves and Curie temperatures for compositions studied are obtained. Calculated data are in a good agreement with experimental ones.

Finite-temperature Gutzwiller approximation from the time-dependent variational principle

We develop an extension of the Gutzwiller approximation to finite temperatures based on the Dirac-Frenkel variational principle. Our method does not rely on any entropy inequality, and is substantially more accurate than the approaches proposed in previous works. We apply our theory to the single-band Hubbard model at different fillings, and show that our results compare quantitatively well with dynamical mean field theory in the metallic phase. We discuss potential applications of our technique within the framework of first principle calculations.

A comparative study for Hydrogen storage in metal decorated graphyne nanotubes and graphyne monolayers

A comparative study for hydrogen storage in metal decorated graphyne nanotubes and graphyne monolayers has been investigated within the framework of first-principle calculations. Our results show that the binding energies of Li, Ca, Sc, Ti on graphyne nanotubes are stronger than that on graphyne monolayers. Such strong binding would prevent the formation of metal clusters on graphyne nanotubes. From the charge transfer and partial density of states, it is found that the curvature effect of nanotubes plays an important role for the strong binding strength of metal on graphyne nanotubes. And the hydrogen storage capacity is 4.82wt%, 5.08wt%, 4.88wt%, 4.76wt% for Li, Ca, Sc, Ti decorated graphyne nanotubes that promise a potential material for storing hydrogen.

Half-metallic ferromagnetism in TM-doped MgH2 hydride

We show that, in addition to its thermodynamic properties that make it a good candidate for hydrogen storage, the MgH2 hydride exhibits interesting magnetic properties when doped with some transition metals (TM). Using the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation in the framework of first-principle calculations, we study the half-metallic ferromagnetic properties of the MgH2 doped with TM: Co, V, Cr, Ti; Mg0.95TM0.05H2. The ferromagnetic state energy is computed and compared with the disordered local moment state energy. We show, from the electronic structure, that doping MgH2 with TM elements can convert the material to a half-metallic with a high wide impurity band and high magnetic moment. We have found that the corresponding Curie temperature is bigger than the room temperature, which is considered as a relevant parameter for spintronic applications. Moreover, the mechanism of the hybridization and the interaction between the magnetic ions are also investigated showing that the double exchange is the underlying mechanism responsible for the magnetism of such materials .

Quantum spin Hall states in graphene interacting with WS2 or WSe2

In the framework of first-principles calculations, we investigate the structural and electronic properties of graphene in contact with as well as sandwiched between WS2 and WSe2 monolayers. We report the modification of the band characteristics due to the interaction at the interface and demonstrate that the presence of the dichalcogenide results in quantum spin Hall states in the absence of a magnetic field.

A theoretical study of electronic and optical properties of SiC nanowires and their quantum confinement effects

We have performed a theoretical study of silicon carbide nanowires (SiCNWs) within the framework of first-principles calculations by incorporating the size effect and hydrogen terminated surface. Specifically, the variation of the energy gap and optical absorption spectra for hydrogen passivated SiCNWs and pristine wires are examined with respect to the wire diameter. All the [001]-orientated SiCNWs derived from the parent zinc-blende (3C) exhibit semiconducting behavior. Our study demonstrates that the saturated 3C-SiCNWs grown along the [001] direction with larger wire sizes are energetically more favorable than the wires with a smaller diameter. Additionally, the energy gaps are reduced with the increment of wire size because of the quantum-confinement effects. The unsaturated SiCNWs possess smaller band gaps than those of saturated ones when the Si- and C-dangling bonds are passivated by hydrogen atoms. Interestingly, the surface terminated by hydrogen atoms substantially alters the onset of absorption as well as the spectrum behavior at upper energies. Moreover, some pronounced fine structures in the absorption peak are conspicuous at the lower energy region of hydrogen saturated SiCNWs as the wire size increases. We find that the distributions of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals are uniform along the wire axis, which reveals that the SiCNWs are exceptional candidates in producing nano-optoelectronic devices.