Browsing by Author "Yurtseven, Hamit"

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Analysis of the Specific Heat in the Supercooled Solid Phase of Liquid Crystals
(Taylor & Francis Ltd, 2011) Kilit, Emel; Yurtseven, Hamit
The specific heat Cp is analyzed using the experimental data at various temperatures in the solid phase of cholesteryl myristate according to a power-law formula for the rapidly and slowly cooled solid in the stability limit. We also analyze the temperature dependence of the Cp using the experimental data for the supercooled solid phase of p-azoxyanisole in the stability limit and the stability temperatures are determined for both cholesteryl myristate and p-azoxyanisole. Our results show that the method of analysis given here is adequate to describe the observed behaviour of the Cp at low temperatures in the solid phase.
Calculation of the Dielectric Constant as a Function of Temperature Close To the Smectic A-Smectic B Transition in B5 Using the Mean Field Model
(Hindawi Ltd, 2012) Yurtseven, Hamit; Kilit, Emel
The temperature dependence of the static dielectric constant (epsilon) is calculated close to the smectic A-smectic B (S-A-S-B) transition (T-c = 71.3 degrees C) for the liquid crystal compound B5. By expanding the free energy in terms of the order parameter in the mean field theory, the expression for the dielectric susceptibility (dielectric constant) is derived and is fitted to the experimental data for epsilon which was obtained at the field strengths of 0 and 67 kV/cm from literature. Coefficients in the free energy expansion are determined from our fit for the S-A-S-B transition of B5. Our results show that the observed behaviour of the dielectric constant epsilon close to the S-A-S-B transition in B5 can be described satisfactorily by our mean field model.
Calculation of the Dielectric Constant of a Ferroelectric Liquid Crystal From a Mean Field Model
(Taylor & Francis Ltd, 2011) Kilit, Emel; Yurtseven, Hamit
The static dielectric constant epsilon(perpendicular to) of the ferroelectric liquid crystal 4-(3-methyl-2-chlorobutanoyloxy)-4'--heptyloxybiphenyl (A7) with high spontaneous polarization is calculated as a function of temperature using a mean field model. This calculation is performed close to the smectic A-isotropic liquid (SmA-I) transition for pure optically active compound (T-c = 81.6 degrees C). For this calculation of epsilon(perpendicular to), the free energy of the SmA phase is expanded in terms of the orientational order parameter. and spontaneous polarization P with P-2 psi(2). It is shown here that our model describes adequately the observed behaviour of the temperature dependence of epsilon(perpendicular to) close to the SmA-I transition for A7.
Critical Behaviour of the Dielectric Constant for the Smectic Ac* Transition Under the Electric Field in Ferroelectric Liquid Crystals
(Taylor & Francis Ltd, 2012) Yurtseven, Hamit; Kilit, Emel
The temperature dependencies of the dielectric constant and the tilt angle (order parameter) are calculated under the electric field for the smectic A-smectic C* (AC*) transition in 4-(3-methyl-2-chlorobutanoyloxy)-4'-heptyloxybiphenyl (A7). The dielectric constant is fitted to the experimental data by using our mean field model with the biquadratic coupling between the spontaneous polarisation P and the tilt angle theta. Our results show that the mean field model with the P-2 theta(2) coupling describes satisfactorily the observed behaviour of the AC* transition in A7. It is indicated here that a first-order AC* transition occurring at low electric fields changes its character towards a second order as the electric field increases in A7.
Investigation of the Complex Magnetic Behavior of Ni46.86co2.91mn38.17sn12.06 (At%) Magnetic Shape Memory Alloy at Low Temperatures
(Iop Publishing Ltd, 2022) Yildirim, Oguz; Yuce, Suheyla; Bruno, Nickolaus M.; Dogan, Emel Kilit; Yurtseven, Hamit; Duman, Eyup; Emre, Baris
The magnetic properties, martensitic transformation characteristics, the magnetic field-induced transformation characteristics, and super spin-glass behaviour at low temperature of Ni46.86Co2.91Mn38.17Sn12.06 (at%) magnetic shape memory alloys (MSMAs) were investigated under various magnetic field levels over temperature intervals from 400 K to 10 K. We observe a small magnetization difference during the martensitic transition evidenced with a visible thermal hysteresis. To investigate the magnetic field induced phase fraction, the minimum magnetic field required to start and complete the magnetostructural phase transition is computed. Super-spin glass features in magnetic data are observed that interacting magnetic clusters are frozen below a critical temperature. Magnetization is computed as a function of temperature at various constant fields using molecular field theory. The critical exponent, beta is deduced for the temperature-induced magnetization, which indicates that the MSMA exhibited ferromagnetic ordering during field-cooling and on heating an antiferromagnetic ordering at low temperatures and in low applied magnetic fields. These observations are consistent within the framework of an Ising or Heisenberg model.
Tricritical Behavior of the Smectic-Hexatic Phase Transitions in Binary Mixtures Using the Landau Mean Field Theory
(Taylor & Francis Ltd, 2023) Yurtseven, Hamit; Kilit Dogan, Emel
The smectic-hexatic phase transitions are studied by the Landau mean field theory in binary mixtures, particularly, THI-14 + MPR-6 liquid crystalline system. By considering the phase lines of SmA-Hex B, SmC-SmF (Hex F) and SmF-Hex B as the first order; SmA-SmC and SmC-SmF as the second order, in the presence of the two tricritical points (C-F-B and A-C-B), we calculate the T-X phase diagram of the binary mixture studied. The phase line equations obtained from the free energies of the smectic and hexatic phases, are fitted to the experimental data given in the literature.We find that the Landau mean field theory explains satisfactorily the observed T-X phase diagram for the smectic-hexatic transitions in the binary mixture of THI-14 + MPR-6.