بررسي ابتدا به ساكن مغناطش، ديناميك شبكه و توپولوژي چگالي بار الكتروني پنيكتيدهاي caN و caAs

چكيده انگليسي :

Ab inito Study of Magnetism Lattice Dynamics and Topology of Electron Charge Density of Pnictides CaN and CaAs Seyed Mojtaba Rezaei Sani s rezaeisani@ph iut ac ir 04 03 2011 Department of Physics Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language PersianSupervisor Prof H Akbarzadeh akbarzad@cc iut ac irAbstractConventional magnetism occurs in systems which contain transition metal or rare earth ions with par tially filled d or f shells In practice all systems which have been studied or used in devices in thepast contain transition metals or rare earth ions Recent experimental and or theoretical observationsof magnetism in systems with no valance d or f electron challenge our classical understanding of mag netism These novel magnetic materials are categorized in two groups in the first group it is crystalatoms that carry magnetic moment while in the other group the magnetic moment is distributed onthe lattice defects From the first group it is theoretically predicted that compounds of group IA andIIA with IV and V in some structural phases are ferromagnetic half metals which can be candidatesfor spintronics application In this project we employ density functional theory and ultrasoft pseudopotential calculation to studymagnetism in binary compounds CaN and CaAs Regarding the structure of analogous magnetic ma terials and experimental results of CaAs synthesis we have considered two cubic structures rocksalt RS and zinc blende ZB and four hexagonal structures NiAs wurtzite WZ anti NiAs and NaO The calculated results show that CaN in cubic NiAs and wurtzite structures and CaAs only in zinc blende phase have ferromagnetic ground states with a magnetic moment of 1 B Electronic structureanalysis of these materials indicates that magnetism originates from anion p states Existence of flatp bands and consequently high density of states at the Fermi level of magnetic structures give risesto Stoner spin splitting and spontaneous ferromagnetism Larger exchange interaction in 2p orbitalof nitrogen respect to 4p orbtial of arsenic causes that CaN has more structures with ferromagneticground state than CaAs Slight hybridization with calcium d states increase density of p states at theFermi level and enhance ferromagnetism in system The stability of all systems is studied from boththermodynamical and lattice dynamical point of view We discuss that although all structures havenegative heat of formation except NaO phase other structures possess imaginary phonon frequenciesindicating dynamical instabilities To find out the reason of instability topology of electron chargedensity is investigated in all structures According to the obtained result the tendency of anion atomsto get close together and make covalent bonds is the origin of the observed dynamical instability Anions in NaO phase have the shortest distance from each other among the all phases and specialtopology of charge density of this phase provide the strongest covalent bonds between anions in re spect to the other phases Keywords CaN CaAs Ferromagnetism Half metal p orbital magnetism Stoner criterion Lattice dynamics Phonon frequency Topology of electron charge density

رضائي ثاني، مجتبي

بررسي ابتدا به ساكن مغناطش، ديناميك شبكه و توپولوژي چگالي بار الكتروني پنيكتيدهاي caN و caAs

فرو مغناطيس , نيم فلز , مغناطش اوربيتال p , معيار استونر , فركانس فونوني