Cihan Kürkçü

The finite-size scaling study of four-dimensional Ising model in the presence of external magnetic field

The four-dimensional ferromagnetic Ising model in external magnetic field is simulated on the Creutz cellular automaton algorithm using finite-size lattices with linear dimension 4 ≤ L ≤ 8. The critical temperature value of infinite lattice, Tcχ(∞)=6,680(1) obtained for h = 0 agrees well with the values Tc(∞)≈ 6.68 obtained previously using different methods. Moreover, h = 0.00025 in our work also agrees with all the results obtained from h = 0 in the literature. However, there are no works for h ≠ 0 in the literature. The value of the field critical exponent (δ = 3.0136(3)) is in good agreement with δ = 3 which is obtained from scaling law of Widom. In spite of the finite-size scaling relations of |ML(t)| and χL(t) for 0 ≤ h ≤ 0.001 are verified; however, in the cases of 0.0025 ≤ h ≤ 0.1 they are not verified.

Hidrojen depolama malzemeleri için MgH2'nin yapısal ve elektronik özellikleri

Bu calismada, ab-initio teknigi kullanilarak MgH 2 ’nin yuksek basinc etkisi altindaki davranisi arastirildi. Siesta programi ile yurutulen simulasyonlar boyunca MgH 2 ’nin bir yuksek basinc fazina rastlandi. Bu faz, Pnnm olarak belirlendi. Bu calismada elde edilen faz gecislerinin deneysel sonuclarla uyumunu arastirmak icin toplam enerji ve entalpi hesaplamalari yapildi. Bu hesaplamalar sonucu faz degisiminin 6 GPa civarinda gerceklestigi sonucuna varildi. Bu sonucun literatur ile uyum icinde oldugu goruldu. Ayrica MgH 2 ’nin elektronik ozellikleri de incelendi. MgH 2 ’nin elde edilen her iki fazi icin de yari iletken ozellikte oldugu goruldu.

Theoretical calculations of high-pressure phases of NiF2: An ab initio constant-pressure study

We have studied the structural properties of the antiferromagnetic NiF2 tetragonal structure with P42/mnm symmetry using density functional theory (DFT) under rapid hydrostatic pressure up to 400 GPa. For the exchange correlation energy we used the local density approximation (LDA) of Ceperley and Alder (CA). Two phase transformations are successfully observed through the simulations. The structures of XF2-type compounds crystallize in rutile-type structure. NiF2 undergoes phase transformations from the tetragonal rutile-type structure with space group P42/mnm to orthorhombic CaCl2-type structure with space group Pnnm and from this orthorhombic phase to monoclinic structure with space group C2/m at 152 GPa and 360 GPa, respectively. These phase changes are also studied by total energy and enthalpy calculations. According to these calculations, we perdict these phase transformations at about 1.85 and 30 GPa.

I4/mmm phase of MgF2: An ab initio molecular dynamics study

A constant pressure ab initio technique is applied in order to study the structural response of magnesium fluoride (MgF2) under the hydrostatic pressure up to 800 GPa. The orthorhombic-to-tetragonal phase transformation of MgF2 is obtained at 720 GPa. This phase transition is also analyzed from the total energy and enthalpy calculations, and it is found that this phase change should occur around 260 GPa. As far as we know, this tetragonal phase has not been obtained in any previous studies.