Fizik Bilim Dalı

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Temperature-dependent absorption edge and photoconductivity of Tl 2In2S3Se layered single crystals
(2013) Güler, İpek; Ambrico, M.; Ligonzo, T.; Gasanly, N.M.; 101531
Temperature variation of indirect band gap of Tl2In 2S3Se layered single crystals were obtained by means of absorption and photoconductivity measurements. The temperature coefficient of -7.1 × 10-4 eV/K from absorption measurements in the temperature range of 10-300 K in the wavelength range of 520-1100 nm and -5.0 × 10-4 eV/K from PC measurements in the temperature range of 132-291 K in the wavelength range of 443-620 nm upon supplying voltage V = 80 V were obtained. From the analysis of dark conductivity measurements in the temperature range of 150-300 K, conductivity activation energy was obtained as 0.51 eV above 242 K. The degree of the disorder, the density of localized states near Fermi level, the average hopping distance and average hopping energy of Tl2In2S3Se crystals were found as, 1.9 × 105 K, Nf = 4 × 1020 cm -3eV-1, 29.1 Å and 24.2 meV in the temperature range of 171-237 K, respectively. Activation energy of hopping conductivity at T = 171 K was obtained as 41.3 meV and the concentration of trapping states was found as 1.6 × 1019 cm-3.
Parallel data reduction techniques for big datasets
(2013) Özdoğan, Cem; Özdoğan, Cem; Watson, Dan; 40569
Data reduction is perhaps the most critical component in retrieving information from big data (i.e., petascale-sized data) in many data-mining processes. The central issue of these data reduction techniques is to save time and bandwidth in enabling the user to deal with larger datasets even in minimal resource environments, such as in desktop or small cluster systems. In this chapter, the authors examine the motivations behind why these reduction techniques are important in the analysis of big datasets. Then they present several basic reduction techniques in detail, stressing the advantages and disadvantages of each. The authors also consider signal processing techniques for mining big data by the use of discrete wavelet transformation and server-side data reduction techniques. Lastly, they include a general discussion on parallel algorithms for data reduction, with special emphasis given to parallel waveletbased multi-resolution data reduction techniques on distributed memory systems using MPI and shared memory architectures on GPUs along with a demonstration of the improvement of performance and scalability for one case study.
Optical analysis of TlInS2xSe2(1-x) mixed crystals
(2014) Güler, İpek; 101531
The ellipsometry measurements were carried out on TlInS2xSe2(1-x) mixed crystals in the spectral range of 1.5-6.0 eV at room temperature. The refractive index, extinction coefficient, real and imaginary parts of dielectric function were found as a result of ellipsometric measurements. The energies of interband transitions (critical point energies) of the TlInS2xSe2(1-x) mixed crystals were obtained by means of the second derivative of the real and imaginary parts of dielectric function. The variation of the critical point energies with the isomorphic anion substitution that is sulfur for selenium atoms was established.
Thermoluminescence in gallium sesquisulfide single crystals: usual and unusual heating rate dependencies
(2018) Guler, I.; Isik, M.; Gasanova, L.; Mahammadov, A.; Gasanly, N.
Thermoluminescence (TL) experiments were conducted for Ga2S3 crystals to obtain information about trapping parameters. TL measurements were performed from 10 to 300 K with varying heating rates in the range of 0.2–0.8 K/s. Two TL glow peaks centered at 44 K (peak A) and 91 K (peak B) were observed at heating rate of β = 0.5 K/s. For peak A, TL intensity decreased whereas that for peak B increased with elevating the heating rates that means anomalous heating rate occurred for peak B. TL glow curves were analyzed using initial rise method to find activation energies of traps. Distribution of trap centers was investigated using Tmax − Tstop method. Quasi-continuous distributions with increasing activation energies from 40 to 135 meV and 193 to 460 meV were attributed to trap centers A and B, respectively.
Analysis of the specific heat and the free energy of [N(CH3)(4)](2)ZnBr4 close to the ferro-paraelastic phase transition
(2019) Kiracı, Ali; 42475
A power-law formula deduced from the Ising model was used to analyze the temperature dependence of the specific heat and the Gibbs free energy of [N(CH3)(4)](2)ZnBr4 compound in the vicinity of the phase transition temperature of T-C = 287.2 K. Obtained values of the critical exponents from the Gibbs free energy were consistent with that predicted from 2-d potts model ( = 0.3), while obtained values of from the specific heat in both ferroelastic and paraelastic phases were consistent with that predicted from the mean field theory ( = 0) in the vicinity of the phase transition temperature. This is an indication of that [N(CH3)(4)](2)ZnBr4 compound undergoes a second order type phase transition. Also, the enthalpy () and the entropy () of this crystal were calculated in terms of the extracted values of the critical exponent in both ferroelastic and paraelastic phases.
Raman wavenumbers calculated as a function of pressure from the mode Gruneisen parameter of PZT (x=0.48) ceramic close to the monoclinic-cubic transition
(2019) Kiracı, Ali; 42475
The isothermal mode Grüneisen parameter γT(P) of some Raman modes in PbZr1-xTixO3 (PZT, x=0.48) were calculated as a function of pressure by means of the observed pressure-dependent volume data of PZT (x=0.48) crystal from the literature at room temperature of 298K. Those calculated values of γT(P) were then used to compute the pressure dependence of the Raman modes in PZT (x=0.48) ceramic studied here. The observed and calculated values of the Raman wavenumbers in PZT were in good agreement, which indicates that the isothermal mode Grüneisen parameter can also be used to predict the pressure-dependent wavenumbers of some other perovskite-type crystals. Additionally, the pressure dependence of the thermodynamic quantities such as isothermal compressibility κT, thermal expansion αP and the specific heat CP-CV of PZT (x=0.48) ceramic were predicted at constant temperature of 298K. Here, the experimentally measurable thermodynamic quantities calculated for PZT (x=0.48) ceramics provide theoretically a significant opportunity for testing.
Damping constant and the inverse relaxation time calculated as a function of pressure using the X-ray diffraction data close to the cubic-tetragonal phase transition in SrTiO3
(2019) Kiracı, Ali; Kiracı, Ali; 42475
The damping constant is calculated as a function of pressure at room temperature using the normalized intensity as an order parameter near the cubic-tetragonal phase transition in SrTiO3. The observed X-ray diffraction data are used for the normalized intensities to calculate the damping constant () from the pseudospin-phonon (PS) coupled model and the energy fluctuation (EF) model, which is fitted to the observed FWHM data from the literature for comparison. Using the calculated values, the pressure dependence of the inverse relaxation time () is predicted close to the cubic-tetragonal phase transition in SrTiO3. Our calculated damping constant from both models explains the observed FWHM satisfactorily and our prediction of the inverse relaxation time can also be compared with the experimental measurements when they are available in the literature.
Order-disorder transition in the ferroelectric LiTaO3
(2019) Kiracı, Ali; Yurtseven, Hamit; 42475
The temperature dependences of the damping constant and the relaxation time are calculated by using the Raman frequencies of a* and the lowest A(1) (TO) phonons in the ferroelectric phase close to the ferroelectric-paraelectric transition in LiTaO3 (T-C = 963 K). Both calculations are performed by considering the frequency as an order parameter for the pseudospin-phonon (PS) and the energy fluctuation (EF) models using the observed data from the literature. Values of the activation energies of this crystal are also deduced by using both models in this crystal. Our results show that the PS and EF models can describe the observed behavior adequately for the order-disorder transition in LiTaO3.
Structural and optical properties of thermally annealed thallium indium disulfide thin films
(2020) Güler, İpek; Gasanly, N.; 101531
Structural and optical properties of thallium indium disulfide (TlInS2) thin films, deposited by thermal evaporation technique and thermally annealed at different temperatures, were analyzed. Crystallite size, dislocation density and lattice strain of the thin films were found from X-ray diffraction experiments. The atomic compositions of the films were determined from energy dispersive spectroscopy analysis. Surface morphology of the films was analyzed using atomic force microscopy. From room temperature transmittance spectrum, the band gap energies of the films were identified. The decrease in band gap energies of the films with the annealing temperature up to 300 degrees C was observed due to increase in crystallite size and decrease in lattice strain. From Raman measurements, it was observed that the Raman shifts of the films were well correlated with those of TlInS2 bulk crystal.
Thermoluminescence characterization of (Ga2Se3)(0.25) - (Ga2S3)(0.75) single crystal compounds
(2020) Güler, İpek; Güler, İpek; Gasanly, N. M.; 101531
Ga2Se3 and Ga2S3 compounds take attention due to their potential applications in photovoltaics. Defects and impurities may affect the quality of optoelectronic devices. Therefore, it is worthwhile to determine the parameters (activation energy, order of kinetics, frequency factor) of traps associated with the defects and/or impurities. The aim of the present paper is to investigate the trapping parameters of (Ga2Se3)(0.25) - (Ga2S3)(0.75) single crystal which is one of the member of (Ga2Se3)(x) - (Ga2S3)(1-x) mixed crystals. For this purpose, thermoluminescence (TL) experiments were performed on (Ga2Se3)(0.25) - (Ga2S3)(0.75) single crystals in the 10-300 K region. TL spectra were also recorded using various heating rates in between 0.2 and 1.0 K/s and stopping temperatures from 30 to 60 K to get the detailed information about the characteristics of the trapping parameters. TL glow curves exhibited the overlapped peaks. The stopping temperature experimental data indicated that traps present quasi-continuous distribution within the band gap. Initial rise method analyses were applied to get the activation energies of quasi-continuously distributed revealed traps. Thermal activation energies of distributed traps were found as increasing from 108 to 246 meV as stopping temperature was increased from 30 to 60 K. The structural characteristics (lattice constants and atomic compositions of constituent elements) of used compound were also reported in the present study.
The Important Role of N(2)(CH3)(4) Ion in the Phase-Transition Mechanism of [N(CH3)(4)](2)ZnBr4
(2020) Kiracı, Ali
The chemical shift of the N(2)(CH3)(4) ion, which has been found to exhibit the similar anomalous behavior of the monoclinic angle $\Delta \beta $ , was related to the order parameter to evaluate the temperature dependence of the linewidth (damping constant) for N-14 nuclear magnetic resonance spectrum of this crystal in terms of the dynamic Ising models, namely the pseudospin-phonon-coupled (PS) and the energy fluctuation (EF) models. The results from both PS and EF models were successful to explain the abnormal behavior of the linewidth in the vicinity of the phase-transition temperature of ${T}_{C}= {287.6}$ K, when compared with the observed linewidth of the transverse acoustic soft mode in this crystal. As an extension of this work, the N-14 relaxation time and the values of the activation free energy were calculated as a function of temperature. The results indicate that the ferroelastic-paraelastic phase transition in this compound is of the order-disorder type.
Analysis of the specific heat and the free energy and calculation of the entropy and the internal energy of [N(CH3)(4)](2)MnBr4 close to the phase transition
(2021) Kiracı, Ali; 42475
The critical behavior of the specific heat and the Gibbs free energy of [N(CH3)(4)](2)MnBr4 was analyzed using the 'sing model close to the phase transition temperature of T-c = 276.5 K. Obtained value of alpha=0.02 from the Gibbs free energy and from the specific heat approximately 2.0 K and 1.4 K, respectively, below Tc (ferroelastic phase) and also deduced value of alpha= 0.04 from the specific heat approximately 0.3 K above T-c (paraelastic phase) can be compared with that predicted from mean field theory (alpha = 0). Also, the entropy and the internal energy of this crystal were predicted.
A phenomenological study on ferroelastic KH3(SeO3)(2) and KD3(SeO3)(2)
(2021) Kiracı, Ali; 42475
The anomalous behavior of the Brillouin frequency shift omega of the soft acoustic mode for KH3(SeO3)(2) and KD3(SeO3)(2), and also the volume thermal expansion proportional to(V) and the specific heat C of KH3(SeO3)(2) have been analyzed for various temperature intervals close to the phase transition temperatures TC at around 211 K and 302 K, respectively. Our extracted values of the critical exponent from C and proportional to(V) of KH3(SeO3)(2) within the temperature intervals of vertical bar T -T-C vertical bar < 7 K are nearly zero, while the critical exponent extracted from the Brillouin frequency shift data of both KH3(SeO3)(2) and KD3(SeO3)(2) within the same temperature interval of vertical bar T -T-C vertical bar < 7 K decrease very rapidly toward to zero as the phase transition temperatures are approached. Our results indicate that the critical behavior of omega, proportional to(V) and C of both KH3(SeO3)(2) and KD3(SeO3)(2) can be described with the same critical exponent within the temperature interval of vertical bar T -T-C vertical bar < 2 K.
A phenomenological study on ferroelectric β-glycine
(2021) Kiracı, A; 42475
The anomalous behavior of the specific heat for (Formula presented.) -glycine was analyzed in terms of the compressible Ising model approximately 5 K below and above the ferrelectric-paraelectric phase transition temperature of TC = 252 K. The obtained value of the critical exponent (Formula presented.) = 0.12 in the ferroelectric phase (T < TC) was consistent with that predicted from the 3-d Ising model ((Formula presented.) = 0.13), while the obtained value of (Formula presented.) = 0.32 in the paraelectric phase (T > TC) was consistent with that predicted from the 2-d potts model ((Formula presented.) = 0.30). Some thermodynamic quantities such as the internal energy, the entropy and the free energy of (Formula presented.) -glycine were then predicted in terms of these extracted values of the critical exponents close to the phase transition temperature of TC. Our calculated values of the thermodynamic quantities are in good agreement with the observed data.
Phenomenological approaches on the Nd3+ doped ferroelectric LaBGeO5
(2021) Kiracı, Ali; Kiracı, Ali; Yurtseven, Hamit
Two phenomenological models, namely the compressible Ising model and Landau model, have been used to analyze the specific heat and the dielectric constant data, respectively for the pure and Nd3+ doped LaBGeO5 (LBG) crystals. The critical exponent of the specific heat was extracted in both ferroelectric and paraelectric phases of the crystals studied here within the temperature intervals of (Formula presented.) The extracted values of the critical exponent were then used to predict some thermodynamic quantities such as the enthalpy, entropy, and the Gibbs free energy. Regarding the analysis of dielectric constant data within the framework of the Landau theory, the observed values of the birefringence were associated with the order parameter below the transition temperature Tc. The Landau coefficients (Formula presented.) and (Formula presented.) were determined. Our results indicate a second-order phase transition mechanism.
Phenomenological Study of Manganese Antimonite Close to the Néel Temperature
(2021) Kiracı, Ali
The anomalous behavior of the frequency $f$ and specific heat ${C_p}$ data for the new layered trigonal ($P\bar{3}1m$) form of manganese antimonite (MnSb2O6) was analyzed by means of the power-law relations with the critical exponent $\alpha $ in the vicinity of the Néel temperature of ${T_{\rm{N}}}$= 8.0 K. While the extracted values of $\alpha $ from both $f$ and ${C_p}$ below $T_{\rm{N}}$ (the same value of 0.06) match exactly the value 1/16 (= 0.06) predicted from the three-dimensional (3-D) Ising model, the extracted value of 0.65 from the ${C_p}$ data above $T_{\rm{N}}$ is much higher than the predicted value of 1/8 (= 0.13) from the 3-D Ising model. As an extension of this work, the temperature dependence of the muon-spin relaxation rate (damping constant) $\lambda $ of MnSb2O6 was calculated from the pseudospin-phonon-coupled (PS) model and the energy fluctuation (EF) model below $T_{\rm{N}}$. Our results are in good agreement with the data. In addition, the activation energy was calculated from the predicted values of $\lambda $ from both PS and EF models for MnSb2O6. Our results indicate an order-disorder-type transition at $T_{\rm{N}}$= 8.0 K for MnSb2O6.
Investigation of the anomalous behavior of the linewidth (damping constant) for the Raman Ag modes in SrSnO3 ceramic
(2022) Kiracı, Ali; 42475
The anomalous behavior of the linewidth (damping constant) for the Raman Ag modes of 223 cm−1 and 260 cm−1 that contribute to the phase transition mechanism of SrSnO3 ceramic close to the phase transition temperatures of TC1= 650 K and TC2= 530 K, respectively, was calculated as function of temperature from the pseudospin-phonon coupled (PS) and the energy fluctuation (EF) models below (T < TC) and above (T > TC) the phase transition temperatures of TC1 and TC2. For this calculation, the frequency shift of these modes were associated as the order parameter and disorder parameter below and above TC, respectively. Our results are in good agreement with the observed linewidth of these 223 cm−1 and 260 cm−1 Raman Ag modes. In addition, these two models (PS and EF) were used to deduce the values of the activation energy for SrSnO3 ceramic below and above the phase transition temperatures of TC1 and TC2.
Exploring the linear and nonlinear optical behavior of (TlInS2)0.75(TlInSe2)0.25: Insights from ellipsometry measurements
(2023) Güler, İpek; Güler, İpek; Gasanly, N.; 101531
The search for layered structured new semiconductor materials with remarkable optical properties has become a driving force, especially for materials science. Tl2In2S3Se [(TlInS2)0.75(TlInSe2)0.25], a fascinating compound, holds great promise for advanced photonic and optoelectronic applications. In the present study, the linear and nonlinear optical properties of Tl2In2S3Se layered single crystals were studied by ellipsometry measurements. The variation of refractive index, extinction coefficient, absorption coefficient and skin depth with energy were investigated. Applying the derivative analysis technique to the absorption spectrum, indirect bandgap was found as 2.19 eV. The refractive index data was analyzed considering single-effective-oscillator model. The lattice dielectric constant, plasma frequency, carrier density to the effective mass ratio and zero-frequency refractive index were found. Moreover, the change in optical conductivity with energy yielded to determine the direct bandgap as 2.40 eV. The optical parameters of nonlinear refractive index, first-and third-order nonlinear susceptibilities were also reported.
Optical characterization of (TlInS2)0.5(TlInSe2)0.5 crystal by ellipsometry: linear and optical constants for optoelectronic devices
(2023) Guler, İ.; Isik, M.; Gasanly, N.; 101531
TlInSSe [(TlInS2)0.5(TlInSe2)0.5] crystals have garnered significant attention as promising candidates for optoelectronic applications due to their exceptional optoelectrical characteristics. This study focused on investigating the linear and nonlinear optical properties of TlInSSe layered single crystals through ellipsometry measurements. The X-ray diffraction analysis revealed the presence of four distinct peaks corresponding to a monoclinic crystalline structure. In-depth analysis was conducted to examine the variations of refractive index, extinction coefficient, and complex dielectric function within the energy range of 1.25–6.15 eV. By employing derivative analysis of the absorption coefficient and utilizing the Tauc relation, the indirect and direct bandgap energies of TlInSSe crystals were determined to be 2.09 and 2.26 eV, respectively. Furthermore, this research paper presents findings on oscillator energy, dispersion energy, Urbach energy, zero and high frequency dielectric constants, plasma frequency, carrier density to effective mass ratio, nonlinear refractive index, and first-order and third-order nonlinear susceptibilities of TlInSSe crystals.
Growth and temperature tuned band gap characteristics of NaBi(MoO4)2 single crystal
(2023) Isik, M.; Güler, İ.; Gasanly, N.M.; 101531
Structural and optical properties of double sodium-bismuth molybdate NaBi(MoO4)2 semiconductor compound was investigated by x-ray diffraction, Raman and transmission experiments. From the x-ray diffraction experiments, the crystal that has tetragonal structure was obtained. Vibrational modes of the crystal were found from the Raman experiments. Transmission experiments were performed in the temperature range of 10-300 K. Derivative spectroscopy analysis and absorption spectrum analysis were performed to get information about the change in band gap energy of the crystal with temperature. It was observed that the band gap energies of the crystal at different temperatures obtained from these techniques are well consisted with each other. By the help of absorption spectrum which was obtained from transmission measurements performed at varying temperatures, absolute zero value of the band gap and average phonon energy as 3.03 ± 0.02 eV and E p h = 24 ± 0.2 meV, respectively. Moreover, based on absorption spectrum analysis the Urbach energy of the crystal was obtained as 0.10 eV.

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