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Öğe Critical dynamics of cluster algorithms in the random-bond Ising model(Amer Physical Soc, 2024) Kanbur, Ulvi; Vatansever, Zeynep DemirIn the present work, we present an extensive Monte Carlo simulation study on the dynamical properties of the two-dimensional random -bond Ising model. The correlation time tau of the Swendsen-Wang and Wolff cluster algorithms is calculated at the critical point. The dynamic critical exponent z of both algorithms is also measured by using the numerical data for several lattice sizes up to L = 512. It is found for both algorithms that the autocorrelation time decreases considerably and the critical slowing -down effect reduces upon the introduction of bond disorder. Additionally, simulations with the Metropolis algorithm are performed, and the critical slowingdown effect is observed to be more pronounced in the presence of disorder, confirming the previous findings in the literature. Moreover, the existence of the non -self -averaging property of the model is demonstrated by calculating the scaled form of the standard deviation of autocorrelation times. Finally, the critical exponent ratio of the magnetic susceptibility is estimated by using the average cluster size of the Wolff algorithm.Öğe First principles investigation of electronic and magnetic properties of half-metallic Heusler alloys(Karabük Üniversitesi, 2011) Kanbur, Ulvi; Gökoğlu, GökhanBu çalışmada, kübik L21 geometrisine sahip iki yeni yarı-metalik tam-Heusler bileşikleri olan Co2CrAs ve Co2CrSb için elektronik, menyetik ve yapısal özellikler verilmiştir. Hesaplamalar, düzlem dalga sözde-potansiyel metodu ve değiş tokuş korelasyon fonksiyonelleri için spin-polarize genelleştirilmiş gradyan yaklaşımı ışığında yoğunluk fonksiyoneli teorisine dayanmaktadır. Sistemlerin elektronik bant yapıları ve durum yoğunlukları, Fermi düzeyinde aşağı spin elektronik durum yoğunlukarının sıfırlanması ile tam bir spin polarizasyonu oluşturarak yarı-metalik karakter göstermektedir. Her iki sistemin L21 yapısında hesaplanan manyetik momentleri 5.00?B değerindedir ve büyük ölçüde krom atomu kaynaklıdır. Aşağı spin durumlarında enerji boşlukları, iki farklı alt örgüdeki kobalt atomlarının 3d durumları tarafından sınırlandırılmıştır. Sistemlerin oluşma entalpileri negatif değere sahiptir ve kendini oluşturan elementlerin katı bileşikerine bozulmaya karşı kararlılığını gösterir.Öğe Half-metallic magnetism of Co2CrX (X=As, Sb) Heusler compounds: An ab initio study(Elsevier, 2011) Kanbur, Ulvi; Gokoglu, GokhanIn this study, we present the electronic, magnetic, and structural properties of two novel half-metallic full-Heusler compounds, Co2CrAs and Co2CrSb, in cubic L2(1) geometry. The calculations are based on the density functional theory within plane-wave pseudopotential method and spin-polarized generalized gradient approximation of the exchange-correlation functional. The electronic band structures and density of states of the systems indicate half-metallic behavior with vanishing electronic density of states of minority spins at Fermi level, which yields perfect spin polarization. The calculated magnetic moments of both systems in L2(1) structure are 5.00 mu(B), which are largely localized on the chromium site. The energy gaps in minority spin states are restricted by the 3d-states of cobalt atoms on two different sublattices. The formation enthalpies for both structures are negative indicating stability of these systems against decomposition into stable solid compounds. (C) 2010 Elsevier B.V. All rights reserved.Öğe Poly-icosahedral Co-Fe-Pd nanoalloy: Isomerism effect on the structural and magnetic properties from DFT(Elsevier, 2022) Yildirim, Hueseyin; Kanbur, Ulvi; Arslan, HaydarThis study seeks to understand and explain the role of isomerism on the structural and magnetic properties of Co-Fe-Pd nanoalloys. To this end, semiempirical Gupta potential was utilized to identify the first three isomers and the global minimum structures of all compositions, and then these isomers were subject to re-optimization by means of the DFT approach. The structural analysis of Co2FenPd17-n (n = 0-17) nanoalloys revealed that Co atoms prefer locations in the inner sites of the nanoalloys. Fe and Pd atoms occupy the surface for both GM structures and isomers. Our general result shows an increase in the total magnetic moment of the nanoalloys but a decrease in the local magnetic moments of the Co and Fe atoms. Another important result is that the local magnetic moment of the Pd atoms is not zero. These results support the idea that the isomerism effect plays a crucial role in structural and magnetic properties.Öğe Thermal properties of rung-disordered two-leg quantum spin ladders: Quantum Monte Carlo study(Amer Physical Soc, 2020) Kanbur, Ulvi; Polat, Hamza; Vatansever, ErolA two-leg quenched random bond disordered antiferromagnetic spin-1/2 Heisenberg ladder system is investigated by means of stochastic series expansion quantum Monte Carlo (QMC) method. Thermal properties of the uniform and staggered susceptibilities, the structure factor, the specific heat, and the spin gap are calculated over a large number of random realizations in a wide range of disorder strength. According to our QMC simulation results, the considered system has a special temperature point at which the specific heat takes the same value regardless of the strength of the disorder. Moreover, the uniform susceptibility is shown to display the same character except for a small difference in the location of the special point. Finally, the spin gap values are found to decrease with increasing disorder parameter and the smallest gap value found in this study is well above the weak coupling limit of the clean case.
