Browsing by Author "Şimşek, Ş."
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Item Open Access Complete photonic band gaps in Sn2P2X6 (X = S, Se) supercell photonic crystals(Taylor & Francis, 2020-04) Şimşek, Ş.; Palaz, S.; Koç, H.; Mamedov, Amirullah M.; Özbay, EkmelIn this work, we present an investigation of the optical properties and band structures for the photonic crystal structures (PCs) based on Sn2P2X6: X = S, Se) with Fibonacci superlattices. The optical properties of PCs can be tuned by varying structure parameters such as the lengths of poled domains, filling factor, and dispersion relation. In our simulation, we employed the finite-difference time domain technique and the plane wave expansion method, which implies the solution of Maxwell equations with centered finite-difference expressions for the space and time derivatives.Item Open Access Elastic and optical properties of sillenites: First principle calculations(Taylor & Francis, 2020-04) Koç, H.; Palaz, S.; Şimşek, Ş.; Mamedov, Amirullah M.; Özbay, EkmelIn the present paper, we have investigated the electronic structure of some sillenites - Bi12MO20 (M = Ti, Ge, and Si) compounds based on the density functional theory. The mechanical and optical properties of Bi12MO20 have also been computed. The second-order elastic constants have been calculated, and the other related quantities have also been estimated in the present work. The band gap trend in Bi12MO20 can be understood from the nature of their electronic structures. The obtained electronic band structure for all Bi12MO20 compounds is semiconductor in nature. Similar to other oxides, there is a pronounced hybridization of electronic states between M-site cations and anions in Bi12MO20. Based on the obtained electronic structures, we further calculate the frequency-dependent dielectric function and other optical functions.Item Open Access Electronic band structure of rare-earth ferroelastics: theoretical investigations(National Institute of Optoelectronics, 2018) Şimşek, Ş.; Uğur, G.; Uğur, Ş.; Mamedov, Amirullah M.; Özbay, EkmelIn the present work, the electronic band structure and optical properties of RE2(MoO4)3 are investigated. The ground state energies and electronic structures were calculated using density functional theory (DFT) within the generalized-gradient approximation (GGA). The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant were also calculated. The main structure element in all our of compounds is the MoO4 tetrahedron. The presence of the MoO4 tetrahedra in the lattice of Gd2(MoO4)3, the similarity of the band structure and optical spectra of Gd2(MoO4)3 to those other tetraoxyanions of molybdenium demonstrate an important role of the MoO4 tetrahedra in the formation of the energy spectrum of Gd2(MoO4)3and other RE2(MoO4)3 compounds. This means that the MoO4 tetrahedra determine the lower edge of the conduction band and the upper edge of the valence band, and the conduction band is split into two subbands. The optical properties of RE2(MoO4)3 are in good agreement with this conclusion and previous experimental data.Item Open Access Incommensurate phase transition and electronic properties of BaMnF4(IOP, 2019) Palaz, S.; Şimşek, Ş.; Koç, H.; Babayeva, R.; Mamedov, Amirullah M.; Özbay, EkmelWe present the ab initio study the electronic, mechanical and structural properties of BaMnF4. We duscuss the trends in the electronic and mechanical properties of BaMnF4 under pressure up to 80 GPa. BaMnF4 belongs to the family of BaMF4-type fluorides (M = Mn, Fe, Co, Ni, Mg, Zn) which share the same orthorhombic structure. The main focus of this study is to elaborate the changes brought about in the electronic and the structural properties by applied pressure. The calculated lattice parameters have been in agreement with the available experimental and theoretical value. Band gap of BaMnF4 in our calculation is about 2.0 eV, separating the empty upper-Hubbard t2g bands and occupied lower-Hubbard eg bands. The total and partial DOS corresponding to the electronic band structure are calculated. Comparative analysis of the results of these calculations shows that the band-gap energy of BaMnF4 decreases with increasing pressure and has a minima value at a critical pressure (appr. 65 GPa), after which it increases again. Some fundamental physical parameters such as elastic constants, bulk modulus, Poisson’s ratio, sound velocities and Debye temperature were calculated and interpreted, too.Item Open Access Photonic band gap of multiferroic-dielectric materials in the IR region: FDTD method(Taylor & Francis, 2019) Palaz, S.; Şimşek, Ş.; Mamedov, Amirullah M.; Özbay, EkmelIn this report, we present an investigation of the optical properties and band structure calculations for the photonic structures based on the multiferroic materials- BaMnF4. We calculate the photonic bands and optical properties of BaMnF4/LiNbO3 based photonic crystal. We study the photonic band gap and optical properties of the photonic structures, numerically analyzed in the IR frequency region by using the FDTD method for various incidence angles, number of periods in the PC and the nature/geometry of the materials.Item Open Access Strain effects and electronic structures of narrow band P-R ferroelectrics: first principles calculation(Taylor & Francis, 2019) Bozdağ, N.; Koç, H.; Şimşek, Ş.; Mamedov, Amirullah M.; Özbay, EkmelIn the present work, the structural, mechanical, electronic and optical properties of the Ruddlesden–Popper(RP) Ba3X2S7 (X = Zr, Hf, Ti) sulfides compounds have been investigated by means of first principles calculations. The generalized gradiend approximation has been used for modeling exchange-correlation effects. It has been observed that the calculated lattice parameters are in good agreement with the experimental values. Bulk modulus, shear modulus, Young’s modulus Poisson’s ratio, and Poisson’s ratio from the calculated elastic constants for Ba3Zr2S7, Ba3Hf2S7, and Ba3Ti2S7 compounds, respectively have been obtained. The obtained electronic band structure for Ba3Zr2S7 and Ba3Hf2S7 compounds are semiconductor in nature, and the Ba3Ti2S7 compound also is metallic. Based on the obtained electronic structures, we further calculated the frequency-dependent dielectric function and other optical functions along the x- and z- axes.