MAGNETIC, ORBITAL AND TRANSPORT PROPERTIES OF CERTAIN CORRELATED AND FRUSTRATED MATERIALS FROM FIRST PRINCIPLES

Abstract

Strongly correlatedelectronsystems(SCES)consistituteoneofthemost active eldsofresearchincondensedmatterphysicswithmanynewdiscoveriesre- portedregularlysuchasnewhighTcsuperconductors,veryhighmagnetoresistive materials, type-IImultiferroic,andspintronicmaterialsandsoon.Itwasrealized in 1930sand1940sthatindependentparticledescription(orbandtheory)isnot suitable todescribecertaininsulators.In1949,SirNevillMottdemonstratedthat the strongon-siteelectron-electroninteraction(called electroncorrelation) prevents the delocalizationofelectronsthatleadstothisinsulatingstate.Systemswith strong electroncorrelationwherenormalbandtheoryfailsconstitutetheSCES. Compoundshavingpartially lled3dor4dorbitalsasintransitionmetalionsor 4f and5forbitalsasinrareearthfallunderSCES.Thesecompoundsdisplaymany exotic propertiessuchashighTcsuperconductivity,colossalmagnetoresistance, metal-insulator transition,multiferroicity,charge/orbitalorder,Kondoe ect,heavy fermionic behaviorandsoon.Todescribethesesystems,onehastostartwitha many-bodyHamiltonianhavingon-siteelectron-electroninteractionterm{sucha modelwas rstproposedbyHubbardin1963knownasHubbardmodel. Besides strongelectroncorrelation,inSCESitisoftenobservedthatseveraldegrees of freedomsuchasspin,charge,orbitalandlatticehavecompetingenergyscales whichlaylandscapeforlongrangemagnetic,orbital,chargeorderedstatesthrough multiplephasetransitionsasafunctionoftemperature.However,ifallthecom- petinginteractionspresentinthesystemarenotsatis edsimultaneouslytogive rise toauniquegroundstate;thegroundstatebecomeshugelydegenerate.This i ii phenomenon isknownas frustration. Themagneticfrustrationcouldarisedueto the prevalentgeometryofthelatticesuchasintriangularmotifwithIsingspinsat eachvertex.Thistypeisfamiliarlyknownasgeometricalfrustration.Theintrin- sic geometricalfrustrationinsometransitionmetaloxides(TMO)playsacrucial role inemergentexoticbehaviorlikespinice,spinliquids,glassybehavior,mag- netoelectricityetc.Magneticfrustrationcanalsoariseduetocompetingexchange interactionsinnon-geometricallyfrustratedsystemswhichcanleadtonon-collinear magnetic statessuchasspiral,helicaletc.Inthisthesis,wehavestudiedtheas- pectsofbothcorrelationandmagneticfrustrationpresentinasinglesystemthat givesrisetoavarietyofphenomenasuchasnon-collinearmagneticordering,orbital ordering, anisotropicchargetransportetc.inmaterialssuchasrareearthpnictides and spinels.Wehaveemployed rst-principlesdensityfunctionaltheory(DFT) calculations andutilizedDFTcodeslikeWIEN2KandVASP,ELKtoperformcal- culations performedwithinvariousapproximationssuchasLSDA,LSDA+U,GGA, GGA+U andGGA+SO+Uasfoundpertinentfortheproblemconcerned.Further Wannier90andWIEN2WANNIERcodeshavebeenalsousedalongwithdensity functional theory(DFT)topreciselyobtaintheorbitalcharactersandtheirover- laps atatomicsitesandhoppingparameters.Forstudyingtransportproperties,we haveutilizedBoltzTraPandBoltzWanncodes.Wehavealsocheckedforthelattice stabilityofthesystemusingPhonopycode. In Chapter 1, wehaveintroducedthephysicsandtheassociatedphysicalphenom- ena ofstrongcorrelationandmagneticfrustrationindetailalongwiththesystems studied inthisthesissuchasEubasedpnictides,spinelvanadates,rare-earthtetra- boridesasexamples.In Chapter 2, wehavegivenabriefoutlineofthetheoretical techniques(densityfunctionaltheoryalongwithWannier tting,Boltzmanntrans- porttheory,opticalpropertiesandtheoryofphonons)usedinourcalculations.In Chapter 3, wediscusstheelectronic,magnetic,transportandopticalproperties of a122Zintlphasecompoundnamely,EuCd2As2. Thiscompoundhasrecently gained alotofattentionaftertheexperimentalstudywasreportedbyWanget.al iii in whichtheyhaverevealedaninterestinginterplayamongmagnetism,chargetrans- port,andopticalproperties.Weobservedtheelectronicstructureissemimetallic and foundthegroundstateofthesystemtobeA-typeantiferromagnetic(A-AFM) with magneticmomentsalongcrystallographicb-axis.Fromthetransportstudy, wefoundhugeanisotropybetweenout-of-planeandin-planeresistivities( zz= xx ) for A-AFM.Thelargedi erenceinresistivitiesbetweenA-AFMandferromagnetic is indicativeofnegativemagnetoresistanceasseenexperimentally.In-planeoptical re ectivityrevealsthatintrabandcontributionsplayamajorroleinreproducing the experimentalfeaturesinthelow-frequencyregime.In Chapter 4, wehave presentedourstudyonthermoelectric(TE)propertiesofEuCd2As2 compoundand found highSeebeckcoe cientandlowelectronicthermalconductivityforA-AFM case alongz-direction.In Chapter 5, wehavestudiedthenatureofitinerancyand orbital orderinginMnxCo1ð€€€xV2O4 (x= dopingconcentration)anddi erentphase of CoV2O4 using an ab-initio approach.WeobservedtheI41/asymmetryisthe ground stateinCoV2O4. TheWannierorbitalprojectionsofVanadium d bands showthe dxy orbital isalwaysoccupiedandthe dxz and dyz orbitals arealteredin successiveab-planesalongthec-axisforallthecasessuggestsA-typeorbitalor- dering. Fromthetransportproperties,weobservedtheenhancementofelectron's itinerancy withtheincreaseinCoconcentration.In Chapter 6, wehaveexplored the source-freeLSDA+Ufunctionalwithindensityfunctionaltheoryfordescrip- tion ofnon-collinearspintexturesinordertocapturethenon-collinearmagnetic ground stateofthespinelvanadatesAV2O4 (A= Mn,FeandCo).Thetraditional exchange-correlationsfunctionalslikeLSDA/GGA+Uwereunabletoreproducethe experimentallyobservednon-collinearmagneticstructureaswellastheVanadium magnetic moment.Inthis,wedemonstratethatboththenon-collinearityaswellas the magnitudeofVanadiummagneticmoments,arecapturedwhenthesourceterm (i.e., magneticmonopoleterm)isremovedfromtheexchange-correlationmagnetic eld BXC. In Chapter 7, weshowcasethesummaryandconclusionsofthethesis along withabriefoverviewonthefutureprospectsofthesubject.