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In this work, a first-principles study of Na2O and K2O monolayers and their Janus structure has been carried out using density functional theory calculations based on the projector-augmented plane wave method. Good structural stability is confirmed by phonon dispersion curves and formation enthalpy. The formation of the NaKO monolayer starting from either Na2O or K2O is analyzed via the surface formation energy method. The experimental feasibility with the exothermic synthesis process is demonstrated, mostly in Na-poor or K-poor conditions. Na2O and NaKO monolayers are direct gap semiconductors with band gaps of 0.97 (1.97) and 0.51 (1.21) eV, respectively, as calculated using the PBE (HSE06) functional. Meanwhile, the K2O monolayer exhibits an indirect gap semiconductor character with an energy gap of 0.52 (1.28) eV. The ionic nature is also confirmed by means of the projected density of states and charge density difference. In addition, the 2D materials studied herein exhibit good absorption in a wide energy range from the visible to ultraviolet region with large absorption coefficients of the order of 105 (cm−1), indicating their potential to be applied in optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this work, we investigate the structural, electronic, and optical properties of the HfSe2 and Janus HfSSe monolayers, and their lateral heterostructures (LHSs) (HfSe2)4−n–(HfSSe)n (n = 1, 2, and 3) using first‐principles calculations. Stability of these two‐dimensional (2D) materials is examined through phonon dispersion curves and cohesive energy. Besides, small heat of formation suggests the feasibility of experimental synthesis of the LHSs. Calculated electronic band structures indicate an indirect gap semiconductor nature of all materials at hand, where the energy gap decreases slightly from (HfSe2)4–(HfSSe)0 to (HfSe2)0–(HfSSe)4 as a result of the increase of Coulomb effect. Moreover, the LHSs formation makes possible the indirect–direct gap transition, which can be achieved by applying proper external strain to the lattice. Energy gap of the H22‐system shows a strong strain‐dependence, increasing with tensile strain and decreasing with compressive strain. The semiconductor–metal transition may be induced at strain strengths of −6% and −8%. Optical calculations indicate a wide absorption band from visible to ultraviolet regime with large absorption coefficient. Our findings introduce new 2D LHSs with tunable properties for application in optoelectronic nano devices. [ABSTRACT FROM AUTHOR]

Exploration of new half-metallic materials for spintronic applications has drawn great attention from researchers. In this work, we investigate the structural, electronic, and magnetic properties of the NaMgO3 perovskite in the bulk and (001) surface conformations. The results show the half-metallic nature of bulk NaMgO3 generated by insulator spin-up channels with a large band gap of 6.08 eV and metallic spin-down channels. A total magnetic moment of 3 (μB) is obtained, which is produced mainly by O atoms with a local magnetic moment of 0.94 (μB). Once the bulk is cleaved along the (001) direction, atomic relaxation takes place to reach an equilibrium, where all constituent atoms exhibit an inward movement. Interestingly, the half-metallicity is retained from the bulk to the (001) surface conformation. The effects of slab termination and thickness on the surface energy, stability, band edges, spin-up energy gaps, and magnetic anisotropy will be also analyzed in detail. The results presented herein introduce the NaMgO3 perovskite as a promising half-metallic material to generate spin current in spintronic devices. [ABSTRACT FROM AUTHOR]

In this work, two new compounds, namely CaF3 and CaF2N, are designed by incorporating fluorine and nitrogen into the CaF2 fluorite structure. The newly created materials show good thermodynamic and structural stability. CaF2 is an insulator with a band gap of 10.92 eV, exhibiting non‐magnetic feature. Under the incorporation effects, CaF3 becomes a ferromagnetic half‐metallic material with insulator spin‐up state with an energy gap of 11 eV and metallic spin‐down state. While CaF2N presents the magnetic insulator characteristic with an energy gap of 9.21 and 5.74 eV in the spin‐up and spin‐down channels, respectively. Consequently, diverse magnetic properties with total spin magnetic moments of 1 μB of CaF3 and 3 μB of CaF2N are obtained, being produced mainly by spin‐up state of the incorporated atoms. Interestingly, the population analysis indicates that Ca2+ ion remains its oxidation state in newly created systems. Unusual properties of CaF3 and CaF2N materials provide the possibility of harvesting the visible and far‐ultraviolet light with the absorption coefficient reaching values of 30 (104/cm) and 65.6 (104/cm), respectively. Theoretical predictions in this study suggest that the designed materials may be prospective candidates for use in spintronic and optoelectronic devices, applications that have not been done accounting for the pristine CaF2 compound yet. [ABSTRACT FROM AUTHOR]

In this work, the pressure effects on the electronic, magnetic, thermoelectric, and thermodynamic properties of the half‐metallic full‐Heusler Mn2CoSi compound are investigated using spin‐polarized first‐principles calculations. The material used is half‐metallic, with the spin‐up alignment being metallic and semiconductivity found in the spin‐down state, creating the perfect spin polarization of 100%. Ferromagnetic and spin‐flip energy gaps have values of 0.887 and 0.415 eV, respectively. The half‐metallicity is quite robust under high pressures. The magnetic properties of Mn2CoSi follows the Slater‐Pauling rule, Mt = Z − 24, being produced mainly by the Mn and Co atoms, while the Si contribution is negligible. Thermoelectric properties' calculations indicate that the metallic behavior generates quite a small figure of merit, while large values close to unity can be obtained with the semiconductor nature. Finally, the Mn2CoSi thermodynamic properties, including the Gibbs free energy, thermal expansion coefficient, bulk modulus, heat capacity, Grüneisen parameter, and Debye temperature, under different temperatures (up to 1000 K) and pressures (up to 50 K) are determined and discussed in detail. [ABSTRACT FROM AUTHOR]

Calculation including the electron correlation effects is reported for the ground 1 1S and lowest triplet 1 3S state energies of the confined helium atom placed at the center of an impenetrable spherical box. While the adopted wave-functional treatment involves optimization of three nonlinear parameters and 10, 20, and 40 linear coefficients contained in wave functions expressed in a generalized Hylleraas basis set that explicitly incorporates the interelectronic distance r12, via a Slater-type exponent and through polynomial terms entering the expansion, the Kohn-Sham model employed here uses the Perdew and Wang exchange-correlation functional in its spin-polarized version within the local-density approximation (LDA) with and without the self-interaction correction. All these calculations predict a systematic increase in the singlet-triplet energy splitting toward the high confinement regime, i.e., when the box radius is reduced. By using the variational results as benchmark, it is found that the LDA underestimates the singlet-triplet energy splitting, whereas the self-interaction correction overestimates such a quantity. [ABSTRACT FROM AUTHOR]

Mn adsorption on the GaAs(111)-(1×1)B surface electronic and magnetic properties are investigated using first principles total energy calculations within the periodic spin polarized density functional theory. Results show that one Mn atom adsorption on top of the surface drives to an interstitial Mn atom. The interstitial atom is bonded to three first monolayer As atoms forming a chain-like structure. This stable structure has a ferromagnetic behavior with a Mn magnetic moment of ∼ 3.98 μ B. The Mn coverage increase yields a stable structure with the creation of an interstitial Mn layer, which in turn gives rise to a GaMnAs like alloy at the surface. The layer displays a ferromagnetic characteristic with magnetic moment of ∼ 4.40 μ B. A surface formation energy analysis of all these stable structures shows that one Mn interstitial monolayer and a Mn interstitial atom may coexist. Density of states shows that there is an important contribution of the Mn atoms to the occupied and unoccupied states. The DOS of one Mn monolayer is metallic with the most important contribution around the Fermi level coming from the Ga-p and Mn-d orbitals. [ABSTRACT FROM AUTHOR]

Doping ZnO with Eu results in the hybridization of the f atomic orbitals of the Eu with the bands of the host material. The strongest interaction turns out to be with the oxygen s- p band. This hybridization manifests itself as a Kondo-like resonance, namely, the appearance of a narrow resonant f band close to the Fermi level. Our aim in the present work is to calculate the electronic structure of ZnO:Eu in which the lanthanide ion replaces one of the Zn ions inducing a geometric relaxation on the system. We have used two reliable crystal electronic structure codes for our calculation. The program WIEN2k [Blaha, P., et al. WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties, 2001] allows us to include the effects of the f the GGA + U model (generalized gradient approximation plus U) for the relaxed configuration. On the other hand, the program CRYSTAL06 [] allows us to monitor the charge of the Eu ion using a judicious mix of the Hartree-Fock nonlocal exchange term with the density functional exchange-correlation potential. Results are presented for the density of states (both the total and the partial contribution from the f electrons), as well as maps of the spin density around the Eu ion, both clearly showing the presence of the resonant f bands. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]