Browsing by Author "Koç, H."

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    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, 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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    Elastic and optical properties of sillenites: First principle calculations
    (Taylor & Francis, 2020-04) Koç, H.; Palaz, S.; Şimşek, Ş.; Mamedov, Amirullah M.; Özbay, 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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    Elastic, electronic, and optical properties of NaSnX (X=Sb, Bi, As): first principle calculations
    (IOP, 2019) Koç, H.; Mamedov, Amirullah M.; Özbay, Ekmel
    In the present work, the structural, mechanical, electronic and optical properties of NaSnX (X=Sb, B, As) compounds have been investigated by means of first principles calculations. The generalized gradient approximation has been used for modeling the exchangecorrelation effects. It has been observed that the calculated lattice parameters are in good agreement with the experimental and theoretical lattice parameters. Bulk modulus, shear modulus, Young's modulus Poisson's ratio, sound velocities and the Debye temperatures using the calculated elastic constants for NaSnSb, NaSnBi and NaSnAs compounds have been obtained. The electronic band structure and the projected density of states corresponding to the electronic band structure have calculated.and interpreted. The obtained electronic band structure for NaSnSb and NaSnBi compounds are metallic in nature, and the NaSnAs compound is also a narrow semiconductor. Based on the obtained electronic structures, we further calculated the frequency-dependent dielectric function, the energy-loss function, optical conductivity, and reflection along the x- and z- axes.
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    ItemOpen Access
    Electronic and elastic properties of the multiferroic crystals with the Kagome type lattices -Mn3V2O8 and Ni3V2O8: First principle calculations
    (Taylor & Francis, 2019-08-16) Koç, H.; Palaz, S.; Mamedov, Amirullah M.; Özbay, Ekmel
    The electronic, mechanical, and optical properties of the Kagome staircase compounds, Mn3V2O8 and Ni3V2O8, have been investigated using the VASP (Vienna ab-initio Simulation Program) that was developed within the density functional theory (DFT). The spin polarized generalized gradient approximation has been used for modeling exchange-correlation effects. The electronic band structures for both compounds and total and partial density of states corresponding to these band structures have been calculated. Spin up (spin down) Eg values for Mn3V2O8 and Ni3V2O8 compounds are 0.77 eV indirect (3.18 direct) and 1.58 eV indirect (0.62 eV) direct, respectively. The band gaps of both compound is in the d-d character. Bulk modulus, shear modulus, Young's modulus, Poisson's ratio, anisotropic factors, sound velocity, and Debye temperature were calculated and interpreted.
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    Electronic properties of spin excitation in multiferroics with a spinel structure: first principles calculation
    (Taylor & Francis, 2019-06-04) Koç, H.; Palaz, S.; Mamedov, Amirullah M.; Özbay, Ekmel
    In the present work, the structural, electronic and mechanical properties of LiVCuO4 and LiCu2O4 spinel type multiferroics have been investigated by means of first principles calculations. The spin polarized generalized gradient approximation has been used for modeling exchange-correlation effects. The structural optimization of these multiferroics compounds has been performed by using VASP-code, and the lattice parameters and magnetic moments have been calculated. From our calculation, it has been determined that the LiVCuO4 compound is a narrow band gap semiconductor, while the LiCu2O4 compound is metallic in nature. Considering the spin states from the electronic band structure and density of the state (DOS) of the LiVCuO4 compound, it has been identified that Eg=1.87 eV for spin up and Eg=0.37 eV for spin down. The second-order elastic constants have been calculated, and the other related quantities have also been estimated in the present work.
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    ItemOpen Access
    Incommensurate phase transition and electronic properties of BaMnF4
    (IOP, 2019) Palaz, S.; Şimşek, Ş.; Koç, H.; Babayeva, R.; Mamedov, Amirullah M.; Özbay, 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 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.
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    A retrospective comparison of allogeneic peripheral blood stem cell and bone marrow transplantation results from a single center: a focus on the incidence of graft-vs.-host disease and relapse
    (Elsevier, 1999) Üstün, C.; Arslan, Ö.; Beksaç, M.; Koç, H.; Gürman, G.; Özçelik, T.; Yilmaz, B.; İlban, O.; Akan, H.; Özcan, M.; Demirer, T.; Uysal, A.; Konuk, N.; Arat, M.; Dilek, İ.; Çelebi, H.; Coskun, H. S.
    To detect the effect of the stem cell source, allogeneic peripheral blood stem cell transplantations (alloPBSCTs) performed between 1995 and 1997 from human leukocyte antigen (HLA)-identical siblings in 40 patients with acute and chronic hematological disorders were compared with a historical group of 40 patients with similar variables who had received allogeneic bone marrow transplants (alloBMTs) between 1993 and 1995. Patients in both groups were identical except that both the recipient and the donor ages were, on average, higher in the alloPBSCT group (26 vs. 36 [p = 0.005] and 27 vs. 32 [p = 0.024], respectively). Patients received similar therapy excluding posttransplant granulocyte colony-stimulating factor administration (97% in alloBMT vs. 12.5% in alloPBSCT). The median time to reach neutrophil counts >0.5×109/L and platelet counts >20×109/L was 13 and 14 days, respectively, in patients receiving alloPBSCTs compared with 19 and 27 days in patients receiving alloBMTs (p = 0.0014 and p = 0.0002). The alloPBSCT group required similar transfusions of red blood cells or platelets. The incidence of grade II-IV acute graft-vs.-host disease (aGVHD) was similar in both groups. However, chronic GVHD (cGVHD) of all grades developed in 78.1% of patients in the alloPBSCT group after a median follow-up period of 12.5 (range 0.5-34) months. In alloBMT recipients, cGVHD of all grades developed in 21.4% after a median follow-up period of 38 (range 0.5-62) months (p = 0.00001). Day 100 transplant-related mortality was also similar: 20% (8 of 40) in the alloBMT patients and 17.5% (7 of 40) in the alloPBSCT group. Although not statistically significant, a relatively higher relapse rate occurred in the alloBMT group (21.4 vs. 10.7%). The estimated disease-free survival in month 24 was 51.3% for alloBMT and 54.6% for alloPBSCT, and the estimated overall survival in month 24 was 56.1% for alloBMT and 64.6% for alloPBSCT. In conclusion, this retrospective comparison suggests that alloPBSCT from HLA-identical donors is associated with faster engraftment, fewer transfusions, and no greater incidence of aGVHD, but a high incidence of cGVHD.
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    ItemOpen Access
    SbSI based photonic crystal superlattices: band structure and optics
    (IOP, 2015) Şimsek, Ş.; Koç, H.; Palaz, S.; Oltulu, O.; Mamedov, Amirullah M.; Özbay, 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 SbSI based layers in air background. We have theoretically calculated the photonic band structure and optical properties of SbSI based PC superlattices. 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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    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, 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, 2019) Şimsek, Ş.; Koç, H.; Mamedov, Amirullah M.; Özbay, 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 A3Mn2O7 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 structural, elastic, electronic, and optical properties of Li₂RuO₃ compound in different structures
    (Zhonghua Minguo Wuli Xuehui,Physical Society of the Republic of China, 2024-05-29) Koç, H.; Mamedov, Amirullah M.; Özbay, Ekmel
    The structural, mechanical, and electronic properties of $Li_2RuO_3$ compound in $C2/c$, $C2/m$, and $P2_1/m$ structures are investigated using spin polarized generalized gradient approximation (GGA) for the exchange-correlation energy. It has been observed that the lattice parameter values ob tained as a result of the optimization are in good agreement with the experimental and theoretical values. The second-order elastic constants were calculated, and the other related quantities (bulk modulus, shear modulus, the Young’s modulus, Poisson’s ratio, anisotropy factor, sound veloc ities, and Debye temperature) have also been estimated. The electronic band structures of $Li_2RuO_3$ compound and the partial density of states corresponding to these band structures have been calculated. All three of these structures are semi-metallic. The spin-down states are metallic and the spin-up states are semiconducting. Finally, effective masses, the linear photon-energy dependent dielectric functions, and the energy-loss functions are also calculated and interpreted.
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    The structural, elastic, electronic, and optical properties of the rhombohedral La₂NiMnO₆ double perovskite compound at different temperatures
    (Elsevier BV, 2024-11-26) Kaya, F.; Koç, H.; Mamedov, Amirullah M.; Özbay, Ekmel
    This study investigates the structural, mechanical, electronic, and optical properties of the rhombohedral $La_2NiMnO_6$ (LNMO) perovskite compound at different temperatures using density functional theory. The calculation employed the spin-polarized generalized gradient approximation (GGA). The calculated lattice parameter values agree (less than 1.2 % difference) with experimental values. We determined the mechanical properties using elastic constants calculated from the stress-strain relationship. The elastic constants obtained satisfy the Born stability criteria for the rhombohedral structure. Electronic band structure calculations revealed that LNMO exhibits the suitable narrow band gap semiconductor character in the spin-polarized state. Finally, the real and imaginary parts of the dielectric function and other optical functions along the x and z directions are calculated and interpreted.