8421

7800

كارشناسي ارشد

ماده چگال

اصفهان: دانشگاه صنعتي اصفهان، دانشكده فيزيك

1392

[هفت]، 78ص.: مصور،جدول،نمودار

ص.ع.به فارسي و انگليسي

فرهاد شهبازي

مجتبي اعلايي

25/10/92

پيمان صاحب سرا، فرهاد فضيله

فيزيك

ID7800

به فارسي و انگليسي: قابل رويت در نسخه ديجيتالي

AbstractGeometrical frustrated systems remain disordered at temperatures mu less than Curie Weisstemperature It has been understood that the ground state manifold of these systems is macro scopically degenerate and this degenerate ground state is very sensitive to perturbations Inthis thesis di erent types of frustration and also theories of geometrically frustrated systemsare considered e behavior of one frustrated material is studied by using Curie Weiss law Since the Coulomb gas occurs only for bipartite la ices these types of la ices are consideredtoo en parent and medial la ices and study pin point behavior are de ned in thesekinds of systems Self consistent Gaussian approximation SCGA is introduced completely Also it is explained that how to calculate self consistent parameter for di erent temperatures It is indicated that how to obtain some thermodynamic functions through this method too Some thermodynamic features for di erent la ices are considered by using SCGA method First of all self consistent values for simple cubic la ice are found and then structure fac tor for di erent wave vectors is shown Correlation function and magnetic susceptibility arecalculated for this la ice too By calculating susceptibility function the phase transition tem perature and critical exponent of susceptibility is obtained on simple cubic la ice whi arecompatible with Monte Carlo s simulation results In addition structure factor for fcc la iceis calculated whi states that there is no unique maximum in gure of this function us the e ect of weak second neighbor ex ange on the appearance of order in face centered cu bic la ice is studied Locations of maxima s of the structure factor depend on the type ofsecond neighbor interaction whether it is antiferromagnetic or ferromagnetic one Moreover antiferromagnetic Heisenberg model is studied on pyro lore la ice e structure factor ofthis la ice is calculated and shown It will be obvious that the structure factor has no uniquemaximum in low temperatures so that pyro lore la ice is frustrated Hence the anisotropicterm is added to the Heisenberg Hamiltonian and its e ect on appearing order is consideredin this la ice Finally XY model on kagome la ice is studied and it has been seen that there isno phase transition in this model en the e ect of adding anisotropic term to Hamiltonianis considered through SCGA method and phase transition temperature and critical exponentof susceptibility is obtained on simple cubic la ice whi are compatible with Monte Carlo ssimulation results Keywords Heisenberg model self consistent Gaussian approximation Coulomb gas pin point behavior

فرهاد شهبازي

مجتبي اعلايي

پيمان صاحب سرا، فرهاد فضيله