پديد آورنده :
رحيمي پور، نفيسه
عنوان :
مطالعه ي مدل هاي مغناطيسي بر روي شبكه هاي براوه و غير براوه به روش خودسازگار گاوسي
مقطع تحصيلي :
كارشناسي ارشد
گرايش تحصيلي :
ماده ي چگال
محل تحصيل :
اصفهان: دانشگاه صنعتي اصفهان، دانشكده فيزيك
صفحه شمار :
ده، 79ص.: مصور، نمودار
يادداشت :
ص.ع. به فارسي و انگليسي
استاد راهنما :
فرهاد شهبازي
استاد مشاور :
كيوان آقابابائي ساماني
توصيفگر ها :
ناكامي , مدل هايزنبرگ كلاسيكي , مايع اسپيني , نظم به واسطه ي بي نظمي
تاريخ نمايه سازي :
94/2/13
استاد داور :
فرهاد فضيله، مجتبي اعلايي
تاريخ ورود اطلاعات :
1396/09/26
چكيده انگليسي :
AbstractStrongly correlated magnetic systems have recived graet attention in recent yearsbecause of their remarkable properties But due to the complexity of such sys tems our understanding of their unfamiliar observed properties is still incomplete One of these interesting phenomena is frustration A frustrated system remainsdisorderd at temperatures much less than Curie Weiss temperature Ground statemanifold of this system is macroscopically degenerate and this degenerate groundstate is very sensitive to perturbations In this thesis by concentrating on frus trated systems we use some magnetic models to study Bravias and none Braviaslattices As a rst step in modelling frustrated magnets we introduce the classicalIsing and Heisenberg models with nearest neighbour antiferromagnetic couplings In chepter 2 we introduce Lutinger Tisza LT and Self consistent Gaussian ap proximation SCGA methods LT is a method for nding the ordering wavevectorscharacterizing the lowest energy state for a given set of interactions Jij In SCGAmethod we investigate the collective behaviors of a magnetic system As thermo dynamic information is contained in the partition function we obtain this functionand calculate the correlation function Moreover we can estimate the structure fac tor that is the sum over all sublattice correlation functions Square and simple cubiclattices are studied as Bravias lattice in chapter three First by using Ising model weobtain Self consistent values In order to estimate phase transition temperatureand critical exponent of susceptibility we depict inverse susceptibility and logarit mic susceptibility respectivly Ising model in two dimension was analytically solvedby Onsager so by comparing our data with exact solution s results it can be pre dicted where this method is able to give correct critical temperature and exponent Also we use Heisenberg model to study simple cubic lattice for which the corre lation functions and magnetic susceptibility are obtained and compared with MontCarlo s simulation results In chapter 4 LT and SCGA methods are considered tostudy the classical J1 J2 Heisenberg model in honeycomb and diamond lattices This model on bipartite lattice exhibits N el ordering However if the AF inter actions between the next nearest neighbor nnn are increased with respect to thenearest neighbor nn the frustration effect arises In such situations new phasessuch as ordered phases with coplanar or spiral ordering and disordered phases suchas spin liquids appear Here we nd the spin liquid phases such as ring liquid andpancake liquid in honeycomb lattice Also for diamond lattice we show that thedegeneracy of ground state can be lifted by thermal uctuations through the orderby disorder mechanism Keywords Frustration Classical Heisenberg model Self consistent Gaussianapproximation Spin liquid Order by disorder
استاد راهنما :
فرهاد شهبازي
استاد مشاور :
كيوان آقابابائي ساماني
استاد داور :
فرهاد فضيله، مجتبي اعلايي
