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    Computational investigations of mechanic, electronic and lattice dynamic properties of yttrium based compounds
    (World Scientific Publ Co Pte Ltd, 2018) Yildiz, Gokcen Dikici; Yildiz, Yasin Gokturk; Selgin, A. L.; Iyigor, Ahmet; Arikan, Nihat
    Electronic, mechanic and lattice dynamic properties of yttrium-based compounds, X3Y, where X = Pd, Pt and Rh were investigated using the density functional theory. The electronic band calculations demonstrated that X3Y compounds are metallic at the cubic crystal structures. The calculated elastic constants using the energy-strain method indicate that the three materials are mechanically stable. The calculated bulk modulus and Young's modulus values suggest that the Pt3Y is stiffer than that of the other two. The type of bonding and ductility in the X3Y compounds were also evaluated based on their B/G ratios, Cauchy pressures (C-12-C-44) and band structure calculations. These compounds were found to be ductile in nature. The density functional perturbation theory was used to derive full phonon frequencies and total and projected phonon density of states. The computed full phonon spectra for X3Y compounds show that these compounds in the L1(2) phase are dynamically stable. Debye temperature and specific heat of these compounds were also calculated and evaluated using quasi harmonic approximation.
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    Effect of Thermocycling on the Structure of Martensite and Kinetics of Martensitic Transformation in Alloy Fe-30% Ni-3% Pd
    (Springer, 2017) Yildiz, Yasin Gokturk; Yildiz, Gokcen Dikici
    The methods of scanning electron microscopy, differential scanning calorimetry and x-ray diffraction are used to study the morphological and thermal characteristics of cooling-induced martensite in alloy Fe - 30% Ni - 3% Pd. It is shown that the kinetics of the martensitic transformation is athermal and the morphology is lamellar. The mean values of the parameters of the austenite and martensite lattices are 0.35704 and 0.28614 nm respectively. The thermocycling leaves the points A (s) and A (f) invariable.
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    Electronic, Elastic, Vibrational and Thermodynamic Properties of HfIrX (X = As, Sb and Bi) Compounds: Insights from DFT-Based Computer Simulation
    (SPRINGER, 2020) Arikan, Nihat; Dikici Yildiz, Gokcen; Yildiz, Yasin Gokturk; Iyigor, Ahmet
    Ab-initio calculations were performed to reveal and thoroughly understand the structural, electronic, elastic, thermodynamic and vibrational properties of HfIrX (X = As, Sb and Bi) compounds in the C1(b) phase. Basic physical characteristics, such as bulk modulus, pressure derivative of bulk modulus, anisotropy factor, shear modulus, Poisson's ratio, Cauchy pressure, elastic constants and Young's modulus were obtained and some of them were compared with those in the literature. Electronic band structure, the density of states and phonon dispersion curves were obtained and compared with current theoretical calculations. It was concluded according to current band structure calculations that the HfIrAs and HfIrBi compounds showed semimetal characteristics, while the HfIrSb compound behaves as a semiconductor. It was determined based on phonon calculations that all three compounds were dynamically stable. Various thermodynamic properties, such as heat capacity, thermal expansion coefficient values and Gruneisen parameter were calculated under constant volume and constant pressure by using Gibbs2 code within the Quasi-harmonic approach, and these results are discussed.
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    Exchange bias effect revealed by irreversible structural transformation between the HCP and FCC structures of Cobalt nanoparticles
    (TAYLOR & FRANCIS LTD, 2020) Yildiz, Yasin Gokturk
    In this study, the exchange bias effect (EBE) revealed by the HCP-FCC structural phase transformation in Cobalt nanoparticles (CoNPs) are modeled and investigated by using the Ising model within the framework of the Effective Field Theory developed by Kaneyoshi. CoNPs are mixed (indistinguishable HCP and FCC) phase at T < 0.36 and T > Tc-FCC. For the non-structural transformation, CoNPs exhibit no exchange bias effect (H-EB = 0.0) and they have normal symmetric coercive field points. But, they have asymmetric coercive field points and they have an exchange bias field for the HCP -> FCC transformation or the inverse (HCP <- FCC). Additionally, an intersection point (Hi) revealed by EBE is obtained between the irreversible M-H curve of the FCC and HCP phase (Hi = Hc1(FCC) = -0.39 and -0.13). Our results show that the exchange bias effect strongly depends on their irreversible structural phase transformations in the materials.
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    First-Principles Study on PdMnSn and PtMnSn Compounds inC1(b)Structure
    (PLEIADES PUBLISHING INC, 2020) Arikan, Nihat; Yildiz, Yasin Gokturk; Yildiz, Gokcen Dikici
    The phase stability, the electronic, mechanic and lattice dynamical properties ofC1(b)-type PdMnSn and PtMnSn compounds were investigated using first principles density functional calculations within the generalized gradient approximation. The computed lattice constants of PdMnSn and PtMnSn compounds were in line with the experimental and theoretical data in the published literature. The elastic constants in theC1(b)structure for PdMnSn and PtMnSn compounds were carried out using the energy-strain method. The computed values of three independent elastic constants, both compounds are mechanically stable in theC1-type crystal structure and met the stability criteria. The electronic structure, total and partial density values of states, and total magnetic moment of these compounds were calculated and the evaluations were carried out by comparing with the existing results. Dynamic properties of PdMnSn and PtMnSn compounds were obtained using the density functional perturbation theory. Both of the compounds were dynamically stable due to the absence of the imaginary phonon frequencies. In addition, it was found that the compounds had a rapid rise in specific heat capacities from 0 to 300 K.
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    Investigation of the Mechanical, Electronic and Phonon Properties of X2ScAl (X = Ir, Os, and Pt) Heusler Compounds
    (KOREAN PHYSICAL SOC, 2020) Arikan, Nihat; Ocak, Hamza Yasar; Yildiz, Gokcen Dikici; Yildiz, Yasin Gokturk; Unal, Rahmi
    In the present study, the second-order elastic constants and the electronic band structures of the X2ScAl (x= Ir, Os, and Pt) compounds crystallized in the L2(1)phase were calculated separately by using theab-initiodensity functional theory. According to the results for the second-order elastic constants, these compounds met the Born mechanical stability criteria. Also, according to the Pugh criteria, they were found to have a ductile structure and to show anisotropic behavior. The microhardneses of the compounds were between 2 and 14 GPa, and the highest hardness was found in the Ir2ScAl (14.290 GPa) compound. In addition, the energy band structures of these compounds were calculated, and the crystals were found to have a metallic bond structure. All the computed data were compared with previously calculated results obtained with different methods. According to the findings obtained in the present study, in terms of its mechanical and electronic behaviors, Ir2ScAl was found to have better physical properties than Os2ScAl and Pt2ScAl. The phonon dispersion curves and their corresponding total and projected densities of states were investigated for the first time by using a linear-response approach in the context of density functional perturbation theory. The frequencies of the optical phonon modes of all compounds at the Gamma point were 4.767, 7.504 and 9.271 THz for Ir2ScAl, 2.761, 7.985 and 9.184 THz for Os2ScAl and 2.012, 5.6952 and 8.118 THz for Pt2ScAl. The heat capacityC(v)at constant volume versus temperature was calculated using a quasi-harmonic approach and the results are discussed.
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    Magnetic properties of the Martensitic transformations with twinned and detwinned
    (Elsevier Science Bv, 2019) Sarli, Numan; Yildiz, Gokcen Dikici; Yildiz, Yasin Gokturk; Yagci, Nermin Kahveci
    The effects of the Austenite (A), twinned Martensite (TM) and detwinned Martensite (DTM) structural transformations on the ferromagnetic properties are investegated by means of the effective field theory developed by Kaneyoshi. We find that the critical Curie temperature of the A, TM and DTM is obtained as T-c(A) < T-c(DTM)< T-c(TM) for H = 0. Thermal magnetization loops are obtained for H not equal 0. The area of the thermal magnetization loops of the A-TM is wider than those of the A-DTM. Paramagnetic Austenite has a phase transition from paramagnetic (PM) phase to ferromagnetic (FM) phase at T-c(TM) and T-c(DTM). The temperature of the As (Austenite start), TMf (twinned Martensite finish) and DTM f (detwinned Martensite finish) increase as the external magnetic field (H) increases. The magnetizations of the A, TM and DTM are the same at low temperature (T < A(s)). The coercive field points are obtained as H-c(A) < H-c(DTM)< H-c(TM). The shearing angle plays very important role in the structural transformations and their ferromagnetic properties, especially in TM.
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    Modeling of the Magnetization and Magnetocaloric Effect in Ni2MnGa Heusler Alloy with the Effective Field Theory
    (Springer/Plenum Publishers, 2022) Yildiz, Yasin Gokturk; Yildiz, Gokcen Dikici
    This study modeled and investigated the magnetocaloric effect in Ni2MnGa Heusler alloy characterized by its magnetic entropy change (Delta S-m) and its refrigerant capacity (RCFWHM) by using the effective field theory. It was found that the peak of the negative magnetic entropy change (Delta S-m(pk)) and the RCFWHM decreased as the external magnetic field increased, whereas the full width half maximum peak (delta T-FWHM) and the temperature (T-pk) of the Delta S-m(pk) both increased. At higher external magnetic fields, delta T-FWHM and T-pk were close to each other, but the Delta S-m(pk) and RCFWHM were separated from each other. The RCFWHM of the corner atoms (Ni1 and Ni3) was higher than those of the central atom (Ni5). This result revealed that corner atoms have an important role in the magnetocaloric properties of Ni2MnGa and other materials with a magnetocaloric effect.
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    Origin of the hardness in the monolayer nanographene
    (Elsevier Science Bv, 2019) Yildiz, Yasin Gokturk
    In this paper, we investigated the lattice distortion displacement effects (LDDE) on magnetizations versus temperature in a monolayer nanographene (MLNG) by using effective field theory developed by Kaneyoshi. To obtain the LDDE, we used the twinning angle (theta) of the zigzag edge of the MLNG and obtain the M-T curves for theta = 10 degrees-180 degrees with 10 steps. We find that MLNG has a minimum Curie temperature (T-C(min) = 2.738) for a certain critical twinning angle (theta(CTA) = 130) of the zigzag edge of the MLNG. The T-C increases (T-C > T-C(min)) as the theta increases or decreases from the theta(CTA) = 130 degrees (both theta < theta(CTA) and at theta > theta(CTA) cases). Since the hardness is directly proportional with the T-C, the hardness of the MLNG increases for both theta < theta(CTA) and at theta > theta(CTA) cases. Thus, the MLNG has a resistance against to the any changing in the theta(CTA). Therefore, we suggest that the origin of the hardness in nanographene is theta(CTA). (C) 2019 Elsevier B.V. All rights reserved.
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    Prediction of the Bain spin memory materials (BSMM) revealed by Kaneyoshi theory
    (TAYLOR & FRANCIS LTD, 2020) Saatci, Buket; Sarli, Numan; Dagdemir, Yilmaz; Yildiz, Yasin Gokturk; Ocak, Hamza Yasar
    In this work, we investigate the spin transformation from ferromagnetic austenite BCT lattice to antiferromagnetic martensite BCC lattice (j(i) = +J -> j(i) = -J or up arrow up arrow ->up arrow down arrow) in Bain transformation (or Bain strain) by using effective field theory which is developed by Kaneyoshi. We find that the spin orientation changes the magnetic properties in the Bain BCT -> BCC transformation. Therefore, we predict that material can memory their spin orientations in the austenite-martensite transformation similar to the shape memory effect in the shape memory materials. Since the austenite-martensite transformation mechanism is first modelled by Bain, we call this spin memory effect as 'Bain spin memory materials (BSMM)'.
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    Spin Induced Quantum Tunneling of the Magnetization
    (World Scientific Publ Co Pte Ltd, 2021) Yildiz, Gokcen Dikici; Yildiz, Yasin Gokturk; Sarli, Numan
    Quantum tunneling of the magnetization (QTM) of the spin-1/2 Vanadium-15 (V15) single molecular magnets (SMMs) is investigated by means of the effective field theory developed by Kaneyoshi. We deal with the two hexagon layers (shell, VS) and one triangular interlayer (core, VC) with the spin-1/2 Ising particles. We find that the V15 has a normal ferromagnetic hysteresis behavior for its all spins are up (parallel). However, V15 has a staircase-like hysteresis behavior that is the signature of the QTM for the spin-down orientation (antiparallel) of the core. QTM appears with the splitting of the magnetizations versus temperature (M-T) and external field (M-H) curves. Namely, QTM occurs from +m(VC1) to -m(VC2)(|m(VC1)|>|m(VC2)|) in the core, from +m(VS1) to +m(VS2) (|m(VS1)|>|m(VS2)|) in the shell (only mVS-H curves at T> 0.1) and from +M1(V15) to +M2(V15) (|M1V15|>|M2V15|) in the total V15. By considering these different splitting behaviors of the magnetizations of the shell, core and total V15 at TT-C, we suggest three different kinds of QTM: 1-ferromagnetic QTM (FMQTM) at TT-C for the shell, core and total V15 SMM. Moreover, it is found that there is a relationship between the QTM and type II and surface superconductivity. The EFT results of QTM in V15 are in good agreement with the other theoretical and experimental results of the V15 and other SMMs.