FIRST PRINCIPLES STUDY OF TRANSPORT AND MAGNETIC PROPERTIES IN CERTAIN CORRELATED MATERIALS

Abstract

Strongly correlatedelectronicsystemsshowawiderangeofveryinteresting and complexphenomenaandthereforetheyhavedrawnalotofattentionamong the researchersforlastfewdecades.Thesalientfeatureofthesematerialsisthe existence ofvariouscompetingstates.Outofmanyfascinatingpropertiesshown bythesematerials,colossalmagnetoresistance,high-temperaturesuperconductivity, heavyfermionicbehavior,metal-insulatortransition,charge-,orbital-andmagnetic- orders arethemostintenselystudiedbytheresearchers.Althoughnumberoftheo- retical andexperimentalapproacheshavebeenusedtoexplorethebasicunderlying mechanismofthevariousexoticproperties,understandingofthesesystemsisstill one ofthehardestproblemstilldate.Theclassofsystemsfallingunderthecategory of stronglycorrelatedsystemsinclude3d,4d(transitionmetal),4f(rareearthtran- sition metal)electronswhicharemostlytransitionmetaloxides(TMO)andrare earth transitionmetaloxides(R-TM-O).Inthisthesiswehaveexploredfewcor- related systemsnamelyrareearthchromatessuchasDyCrO4, GdCrO4, rareearth orthoferrites suchasNdFe0:5Mn0:5O3, dopedTitaniumandvanadiumoxidessuchas LaTi1ð€€€xVxO3 (x=0.25,0.5,0.75,1.0) andLuVO3. First-principledensityfunctional theory (DFT)calculationshavebeenperformedtostudyvariousphysicalproperties, suchasspinreorientation,magneticorder,orbitalorder,metal-insulatortransition, chargetransferintheseoxides.WehaveusedtwoverypopularDFTcodestoper- form thesecalculations:oneisVASPwhichusesprojectoraugmentedwave(PAW) methodandotherisWIEN2kwhichusesfullpotentiallinearizedaugmentedplane wavemethod.The rstprinciplecalculationshavebeenperformedthroughvari- i ii ous approximationssuchasLSDA,LSDA+U,LSDA+U+SO,GGA,GGA+Uand GGA+U+SO. FurtherWannier90andWien2wanniercodehavebeenalsoused along withdensityfunctionaltheory(DFT)topreciselyobtaintheorbitalchar- acters andtheiroverlapsatatomicsites.Thepresentthesisisdividedintosix chapters.Chapter1givesabriefintroductiontothephysicsofstronglycorrelated systems andrichphysicalphenomenaincertainTMOandR-TM-O.Thesecond part ofthischaptercontainsabriefideaofthetheoreticaltechniques(DFTalong with wien2wanniercode)thatwehaveused.Chapter2presentsthedetailedstudy of natureoftransportgapandmagneticorderinDyCrO4 and GdCrO4 systems. Both thezirconandscheelitephasesoftheDyCrO4 and GdCrO4 are studied.Here wehaveclassi edtheDyCrO4 and GdCrO4 oxidestobeMott-Hubbardorcharge transfer insulatorfromourGGA+Ucalculations.Inchapter3wehavestudiedthe spin reorientationinNdFe0:5Mn0:5O3 from ourab-intiocalculations.FromOurcal- culations we ndthatintheabsenceofanynetNdmoment,thepreferreddirection of Fe/Mnspinsis b with G-typearrangementforhightemperaturephase(below TN). The alignmentofspinschangestothe c-axis forlowtemperaturephase(belowspin reorientationtransition)afterinclusionofNdspins.ThissuggeststhatitstheNd 4f-Mn(Fe)3d interactionwhichplaysaroleinreorientationofthespinsatlowtem- perature.Inchapter4wehavestudiedLaTi1ð€€€xVxO3 for x=0,0.25,0.5,0.75,1.For x=0.5 casewemadecertaininterestingobservations.Chargetransferdrivenmetal to insulatortransitionwasobservedandfurthermore,thischargetransferfromTi to Vionsandtheconsequentmetal-insulatortransitionwerefoundtobeorientation dependent.Inchapter5wehavestudiedthemagneticandorbitalorderinLuVO3 from our rstprinciplesbasedDFTcalculationsalongwiththelocalizedWannier orbitals. Fromtheanalysisofwannierizedbandcharacterswewereabletoextract the natureoforbitalorderintheboth(highandlowtemperature)structuralphases of LuVO3. OrbitalorderisobservedtobeG-typeinthehightemperaturephase while itisC-typeinthelowtemperaturephase.Chapter6includesthesummary and futuredirectionsofourwork.