8034

7469

كارشناسي ارشد

ماده ي چگال

اصفهان: دانشگاه صنعتي اصفهان، دانشكده فيزيك

1391

شش،113ص.: مصور،جدول،نمودار

ص.ع.به فارسي و انگليسي

فرهاد شهبازي

پيمان صاحب سرا

20/7/92

فرهاد فضيله، مجتبي اعلايي

فيزيك

ID7469

به فارسي و انگليسي: قابل رويت در نسخه ديجيتالي

Abstract 3 Recent experiment on spin 2 bilayer honeycomb lattice antiferromagnet shows a spin liquid behavior down to very low temperatures This behavior can be ascribed to the frustration e ect due to competitions between rst and second nearest neighbor antiferromagnet interaction Motivated by the experiment we study Antiferromagnet Heisenberg model using Mean eld theory and Holestein Primakov transformation In mean eld theory we use magnetization as a local eld in a spin system and fourier transform the hamiltonian to nd an intraction matrix By di agonalizing this intraction matrix and choosing the lowest eigenvalue we can deduce the ground state of the system This minimum eigenvalue changes by variation of J2 and J2 we found that two eigevalues determine the phases of the system Although both of eigenvalues have minimum at q 0 they show di erent state for spins One of these minima shows antiferromagnetic phase and the other shows layer antiferromagnetic phase Moreover they have coexistence line which is a line of rst order transition Furthermore degenerate phases have two region due to these two eigen values They have no coexistence line in the phase diagram Through this calculation shows that this system has highly degenerate groud states in both eigenvalues Second nearest neighbor calculation in adjacent layer and draw ing phase diagram of this system in J2 J2 shows that these neighbors also increase frustration in the system In addition rst nearest neighbor calculation in adjacent layer a ect the areas of phases in phase diagram and shift these areas proportional to the magnitude of rst nearest neighbor intraction Moreover by usig Holestein Primakov transformation we can add the quantum e ect of magnon states to the system Classical part of hamiltonian shows the same phase diagram as of which we have for mean eld theory The di erence between these two approach caused by di erent eigenvalues which we have in mean eld theory while in Holestein Primakov approach we have one energy with two parameter to indicate two distinguished phase at q 0 The manganite Bi3 M n4 O12 N O3 which was shows no spin ordering down to low tempratures has coupling constants in antiferromagnetic area of our phase diagram Using Heisenberg hamiltonian we can not explain the frustration e ects for this manganite The quantum e ects or other intrac tion in this hamiltonian may explain the absence of ordering down to low temprature for the manganite

فرهاد شهبازي

پيمان صاحب سرا

فرهاد فضيله، مجتبي اعلايي