Temel Mühendislik Ana Bilim Dalı

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Calculation of the raman frequency, damping constant (Linewidth) and the relaxation time near the tetragonal-cubic transition in PbTiO3
(Elsevier GMBH, 2017) Kiracı, Ali; Yuetseven, Hamit; 42475
Frequencies, damping constants and the relaxation times of some Raman modes including the two soft modes are calculated as a function of pressure near the tetragonal-cubic transition in PbTiO3. Calculation of the Raman frequencies is performed using the observed volume data from the literature by means of the mode Gruneisen parameter at various pressures. Pressure dependence of the damping constant and the relaxation time is predicted using the pseudospin-phonon coupled model and the energy fluctuation model by considering that the Raman frequency can be taken as the order parameter (spontaneous polarization) for the tetragonal-cubic transition in PbTiO3. Expressions from both models for the damping constants are fitted to the observed Raman linewidths of the two soft modes and for the other Raman modes the damping constant and the relaxation time are predicted close to the transition. We find that damping constants diverge and the inverse relaxation time decreases for the soft modes with increasing pressure near the critical pressure (P-c similar to 11 GPa). The other Raman modes exhibit unusual critical behavior. Our results indicate that the observed behavior of the Raman frequencies can be predicted from the volume data through the mode Gruneisen parameter for the tetragonal-cubic transition in PbTiO3. The damping constant and the relaxation time for the Raman modes can also be predicted adequately using the pseudospin-phonon coupled model and the energy fluctuation model to explain the mechanism of the phase transition between the tetragonal and cubic phases in PbTiO3
Calculation of the raman frequency, damping constant (Linewidth) and the relaxation time near the tetragonal-cubic transition in PbTiO3
(Elsevier GMBH, 2017) Kiracı, Ali; Yurtseven, H.; 42475
Frequencies, damping constants and the relaxation times of some Raman modes including the two soft modes are calculated as a function of pressure near the tetragonal-cubic transition in PbTiO3. Calculation of the Raman frequencies is performed using the observed volume data from the literature by means of the mode Gruneisen parameter at various pressures. Pressure dependence of the damping constant and the relaxation time is predicted using the pseudospin-phonon coupled model and the energy fluctuation model by considering that the Raman frequency can be taken as the order parameter (spontaneous polarization) for the tetragonal-cubic transition in PbTiO3. Expressions from both models for the damping constants are fitted to the observed Raman linewidths of the two soft modes and for the other Raman modes the damping constant and the relaxation time are predicted close to the transition. We find that damping constants diverge and the inverse relaxation time decreases for the soft modes with increasing pressure near the critical pressure (P-c similar to 11 GPa). The other Raman modes exhibit unusual critical behavior. Our results indicate that the observed behavior of the Raman frequencies can be predicted from the volume data through the mode Gruneisen parameter for the tetragonal-cubic transition in PbTiO3. The damping constant and the relaxation time for the Raman modes can also be predicted adequately using the pseudospin-phonon coupled model and the energy fluctuation model to explain the mechanism of the phase transition between the tetragonal and cubic phases in PbTiO3
Damping constant and the relaxation time calculated for the lowest-frequency soft mode in the ferroelectric phase of Cd2Nb2O7
(Elsevier GMBH, 2016) Kiracı, Ali; Yurtseven, H.; 42475
The temperature dependence of the phonon frequency omega(ph) and of the damping constant Gamma(sp) due to pseudospin-phonon coupling of the lowest-frequency soft mode is calculated in the ferroelectric phase near the transition temperature (T-C =196 K) in Cd2Nb2O7. Raman frequency of the soft mode is used as an order parameter which is calculated from the molecular field theory. On that basis, the damping constant is calculated by fitting the expressions from the pseudospin-phonon coupled model and the energy fluctuation model to the observed linewidth from the literature below T-C in Cd2Nb2O7. From our analysis, we find that the molecular field theory is adequate for the soft mode behaviour and that both models are also satisfactory for the divergence behaviour of the damping constant as T-C is approached from the ferroelectric phase in Cd2Nb2O7. Values of the activation energy U are extracted from the temperature dependence of the damping constant (HWHM) of the soft mode in the ferroelectric phase of this crystal. Also, the inverse relaxation time is predicted using the Raman frequency and damping constant close to the T-C in the ferroelectric phase of Cd2Nb2O7, which increases considerably as T-C is approached from the ferroelectric phase, as observed experimentally. (C) 2016 Elsevier GmbH. All rights reserved.
Optical and photoelectrical properties of TlInSSe layered single crystals
(Elsevier GMBH, 2018) Güler, İpek; Gasanly, N.; 101531
Optical and electrical properties of TlInSSe layered single crystals have been studied by means of transmission, reflection and photoconductivity measurements. Transmission and reflection experiments have been carried out from 540 to 1000 nm at room temperature. Derivative analysis was applied to both transmission and reflection spectra and indirect band gap energy was found as 2.06 eV. Photoconductivity measurements have been performed in the temperature range from 245 to 300 K and in the voltage range from 10 to 80 V. From the temperature-dependent photoconductivity measurements, the observed single peak shifted to higher wavelengths with increase of temperature. The increase of photoconductivity with temperature is due to the increase in the mobility of photocarriers that can be explained by Bube model. From X112 method, room temperature indirect band gap of the crystal was also found as 2.06 eV. From voltage-dependent photoconductivity measurements, the peak maximum increased linearly with increase of voltage because of increase of the mobility of charge carriers. Dark current-voltage characteristic of TlInSSe crystal showed the ohmic behavior that means space charge limited current did not exist in the crystal. From the photocurrent with different illumination intensity analysis, the supralinear photoconductivity associated with the two center model was found.
Temperature dependence of the polarization, dielectric constant, damping constant and the relaxation time close to the ferroelectric-paraelectric phase transition in LiNbO3
(Elsevier GMBH, 2017) Kiracı, Ali; Yurtseven, H.; 42475
We calculate the order parameter (spontaneous polarization) and the inverse dielectric susceptibility at various temperatures in the ferroelectric phase of LiNbO3 for its ferroelectric-paraelectric phase transition (T-C =1260 K) using the Landau phenomenological model. For this calculation, the Raman frequencies of the soft optic mode (TO1) are used as the order parameter and the fitting procedure is employed for both the order parameter and the inverse dielectric susceptibility by means of the observed data from the literature. The temperature dependences of the damping constant and the inverse relaxation time are also computed using the pseudospin-phonon coupled model and the energy fluctuation model for the ferroelectric phase of LiNbO3. The activation energy is deduced from the damping constant for both models studied and compared with the k(B)T(C) value of LiNbO3. We find that the order parameter (Raman frequency of the TO1 mode) and the inverse dielectric susceptibility decrease with increasing temperature, as expected from the mean field model. We also find that the damping constant and the inverse relaxation time of this soft mode increases and decreases, respectively, with increasing temperature on the basis of the two models studied in the ferroelectric phase of LiNbO3. This indicates that our method of calculation is satisfactory to describe the observed behaviour of the ferroelectric-paraelectric phase transition in LiNbO3.

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