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    Band gap and optical transmission in the Fibonacci type one-dimensional A5B6C7 based photonic crystals
    (Wiley-V C H Verlag Gmbh, 2015) Simsek, Sevket; Koc, Husnu; Palaz, Selami; Oltulu, Oral; Mamedov, Amirullah M.; Ozbay, Ekmel
    In this work, we present an investigation of the optical properties and band structure calculations for the photonic crystal structures (PCs) based on one-dimensional (1D) photonic crystal. Here we use 1D A(5)B(6)C(7) (A:Sb; B:S, Se; C:I) based layers in air background. We have theoretically calculated photonic band structure and optical properties of A(5)B(6)C(7) (A:Sb; B:S, Se; C:I) based PCs. In our simulation, we employed the finite-difference time domain (FDTD) technique and the plane wave expansion method (PWE) which implies the solution of Maxwell equations with centered finite-difference expressions for the space and time derivatives.
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    Complete photonic band gaps in Sn2P2X6 (X = S, Se) supercell photonic crystals
    (Taylor & Francis Ltd, 2020) Simsek, Sevket; Palaz, Selami; Koc, Husnu; Mamedov, Amirullah M.; Ozbay, Ekmel
    In 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.
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    Complex Band Structure and Dispersion Relation of Acoustic Waves in Piezoelectric Based Topological Phononic Crystals
    (META Conference, 2019) Palaz, Selami; Ozer, Zafer; Simsek, Sevket; Koc, Husnu; Mamedov, Amirullah M.; Ozbay, Ekmel
    In present work, the acoustic band structure of a 2D phononic crystal (PC) containing piezoelectric materials (Bi12XO20, X=Si, Ge, Ti; LiTaO3) were investigated by the finite element method. 2D PC with triangular and honeycomb lattices composed of piezoelectric cylindrical rods are in the air and liquid matrix. The existence of stop bands are investigated for the waves of certain frequency ranges. This phononic band - forbidden frequency range - allows sound to be controlled in many useful ways in structures. These structure can be used as sonic filters, waveguides or resonant cavities. The calculated phonon dispersion results indicate the existence of full acoustic modes in the proposed structure along the high symmetry points. We have also calculated the band structures of the different types of unit cells that are yielded by space group symmetry operations of the triangular resonators. The results show that these acoustic metamaterials with Helmholtz resonators can be used successfully to reduce the Dirac cone frequencies. Dirac cone frequency decreases gradually with increasing filling ratio, which indicates a possible way to control wave propagation on the subwavelength scale. Numerical simulation results show that acoustic metamaterials can behave like zero-refractive-index media and can be applied to acoustic tunneling. © 2019, META Conference. All rights reserved.
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    Elastic and optical properties of sillenites: First principle calculations
    (Taylor & Francis Ltd, 2020) Koc, Husnu; Palaz, Selami; Simsek, Sevket; Mamedov, Amirullah M.; Ozbay, Ekmel
    In 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.
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    Incommensurate Phase Transition and Electronic Properties of BaMnF4
    (Iop Publishing Ltd, 2019) Palaz, Selami; Simsek, Sevket; Koc, Husnu; Babayeva, Rena; Mamedov, Amirullah M.; Ozbay, Ekmel
    We 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 t(2g) bands and occupied lower-Hubbard e(g) 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.
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    Mechanical, electronic, and optical properties of the A4B6 layered ferroelectrics: ab initio calculation
    (Wiley-V C H Verlag Gmbh, 2015) Koc, Husnu; Simsek, Sevket; Palaz, Selami; Oltulu, Oral; Mamedov, Amirullah M.; Ozbay, Ekmel
    We have performed a first principles study of the structural, elastic and electronic properties of orthorhombic SnS and GeS compounds using the density functional theory within the local density approximation. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, Debye temperature, and hardness have also been estimated in the present work. All of the calculated modulus and Poisson's ratio for SnS were less than the same parameters for GeS. Our calculations have discovered the large anisotropy of elastic parameters in the (100) and (010)-planes for both compounds. The band structures of orthorhombic SnS and GeS have been calculated along high symmetry directions in the first Brillouin zone (BZ). The calculation results for the band gap of Sn(Ge) S gave E-g=0.256 eV (0.852 eV) and has an indirect character for an interband transition. The real and imaginary parts of dielectric functions and (by using these results) the optical constant such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant were calculated. All of the principal features and singularities of the dielectric functions for both compounds were found in the energy region between 2 eV and 20 eV.
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    Optical Properties of the Narrow-Band Ferroelectrics: First Principle Calculations
    (Taylor & Francis Ltd, 2015) Koc, Husnu; Simsek, Sevket; Mamedov, Amirullah M.; Ozbay, Ekmel
    Based on density functional theory, we have studied the electronic, and optical properties of narrow-band ferroelectric compounds - (Ge,Sn) Te. Generalized gradient approximation has been used for modeling exchange-correlation effects. The lattice parameters of the considered compounds have been calculated. The calculated electronic band structure shows that GeTe and SnTe compounds have a direct forbidden band gap of 0.742 ev and 0.359 eV. The real and imaginary parts of dielectric functions and therefore, the optical functions such as energy-loss function, as well as the effective number of valance electrons and the effective optical dielectric constant are all calculated. Our structural estimation and some other results are in agreement with the available experimental and theoretical data.
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    Sn2P2X2 (X=S, Se) as novel materials for phononic crystals
    (META Conference, 2019) Palaz, Selami; Ozer, Zafer; Simsek, Sevket; Koc, Husnu; Mamedov, Amirullah M.; Ozbay, Ekmel
    [No abstract available]
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    Strain effects and electronic structures of narrow band P-R ferroelectrics: First principles calculation
    (Taylor & Francis Ltd, 2019) Bozdag, Nedim; Koc, Husnu; Simsek, Sevket; Mamedov, Amirullah M.; Ozbay, Ekmel
    In 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.
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    Structural, Electronic, and Mechanical Properties of A3Mn2O7 (A=Sr, Ca): Ab Initio Calculation
    (Taylor & Francis Ltd, 2019) Simsek, Sevket; Koc, Husnu; Mamedov, Amirullah M.; Ozbay, Ekmel
    In the present study, the structural, electronic, optical, and mechanical properties of the Ruddlesden-Popper type oxide compounds are investigated by means of density functional theory. In our calculation, spin polarized electron band structures and density of the state were identified by adding to the spin contribution of the Mn-atom. For A(3)Mn(2)O(7) compounds, the real and imaginary parts of the dielectric function and other optical properties, such as energy loss function, effective number of valence electrons, and effective optical dielectric constant, were calculated accordingly. In addition, the bulk modules, shear modules, Young's modulus and Poisson ratios, anisotropy factors, sound velocities, and Debye temperatures for these compounds were calculated too.
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    The Mechanical, Electronic and Optical Properties of Sn2P2S6 Compound in Different Phases
    (Taylor & Francis Ltd, 2021) Koc, Husnu; Palaz, Selami; Simsek, Sevket; Mamedov, Amirullah M.; Ozbay, Ekmel
    In present paper, the structural, mechanical, and electronic properties of the Sn2P2S6 compound under different pressures by the density functional methods in the generalized gradient approximation have been examined in the ferroelectric (Pc) and paraelectric (P2_1/c) phases. The lattice parameters, mechanical properties, electronic bands structures and partial density of states for both phases are presented and analyzed. The nonlinear optical properties and electro-optic effects of Sn2P2S6-Pc have been studied by the density functional theory in the local density approximation. Our structural estimation and some other results are in agreement with the available experimental and theoretical data. We present calculations of the frequency-dependent complex dielectric function (omega) and the second harmonic generation response coefficient chi ((2)) (-2 omega, omega, omega) over a large frequency range. The electronic linear electro-optic susceptibility chi ((2)) (-omega, omega, 0) is also evaluated below the band gap. These results are based on a series of the LDA calculation. The results for chi ((2)) (-omega, omega, 0) are in agreement with the experiment below the band gap and those for chi ((2)) (-omega, omega, 0) are compared with the experimental data where available.