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Phenomenological approach to spin fluctuations in itinerant magnets and superconductors from ab initio calculations VonderFakultätMathematikundPhysikderUniversitätStuttgart zurErlangungderWürdeeinesDoktorsderNaturwissenschaften (Dr.rer.nat.)genehmigteAbhandlung vorgelegt von Luciano Ortenzi aus Rom (Italien) Hauptberichter:Prof.Dr.OleKroghAndersen Mitberichter:Prof.Dr.AlejandroMuramatsu Mitberichterin:Dr.LiliaBoeri TagdermündlichenPrüfung:17.Oktober2013 Max-Planck-InstitutfürFestkörperforschung Stuttgart2013 “ExunoVerboomniaetunumloquunturomnia,ethocestPrincipiumquodet loquiturnobis(Ioann.8,25).Nemosineillointelligit,autrectejudicat.Cuiomnia unamsuntetomniaadunumtrahitetomniainunovidetpoteststabiliscorde esseetinDeopacificuspermanere.OveritasDeus,facmeunumtecumincaritate perpetua!” DeimitationeChristiLib.ICap.III,“Dedoctrinaveritatis”. Contents Abbreviations 7 Introduction 9 Itinerantelectronmagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Superconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Investigatedcompounds:anoverview. . . . . . . . . . . . . . . . . . . . . . . 27 Organizationofthethesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1 Theoreticaloverview 39 1.1 Localizedanditinerantelectronmagnetism . . . . . . . . . . . . . . . . 40 1.1.1 Heisenbergmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 1.1.2 Landautheoryofsecondorderphasetransitions . . . . . . . . 45 1.1.3 TheStoner-Wohlfarthmodel. . . . . . . . . . . . . . . . . . . . . 51 1.2 Densityfunctionaltheoryanditsapproximations . . . . . . . . . . . . 54 1.2.1 Densityfunctionaltheory. . . . . . . . . . . . . . . . . . . . . . . 55 1.2.2 LDA,LSDAandGGAFunctionals . . . . . . . . . . . . . . . . . . 58 1.2.3 ExtendedStonertheoryforferromagnetism . . . . . . . . . . . 62 1.3 Effectofspinfluctuationsonthemagneticproperties . . . . . . . . . . 65 1.3.1 Failureofthemean-fielddescription. . . . . . . . . . . . . . . . 65 1.3.2 Dynamicalsusceptibilityandspinfluctuations. . . . . . . . . . 67 1.3.3 Randomphaseapproximationandparamagnons . . . . . . . . 68 1.3.4 Self-consistentrenormalizationtheory. . . . . . . . . . . . . . . 69 1.3.5 Recentdevelopmentsonthespinfluctuationstheoryinitiner- antsystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 1.4 Superconductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 1.4.1 Ginzburg-Landauphenomenologicaltheory . . . . . . . . . . . 72 1.4.2 Microscopictheoryofsuperconductivity . . . . . . . . . . . . . 74 1.4.3 Migdal-Eliashbergtheoryforelectron-phononsuperconductivity 76 1.4.4 Paramagnonsandunconventionalsuperconductivity. . . . . . 79 2 Electron-phononsuperconductivityinAPt P(A=Sr,Ca,La)compounds85 3 2.1 Crystalstructure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 2.2 Electronicstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 2.3 Phonondispersionsandelectron-phononcoupling. . . . . . . . . . . . 97 2.3.1 StrongcouplingandweakcouplingsuperconductivityinAPt P 3 compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 2.4 Migdal-EliashbergTheory . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 2.5 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 2.6 Technicaldetails. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 3 Spinfluctuationsandelectron-phononcouplinginsuperconductingCu1−xBiSO115 3.1 Electronicstructureandweakferromagnetism . . . . . . . . . . . . . . 118 3.2 Phononsandelectron-phononproperties . . . . . . . . . . . . . . . . . 125 3.3 Paramagnonsandelectron-paramagnoncouplingconstant. . . . . . . 131 3.4 PhasediagramofCu1−xBiSO. . . . . . . . . . . . . . . . . . . . . . . . . 134 3.5 Conclusionandoutlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 3.6 TechnicalDetails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 4 PhenomenologicalapproachtotheopticalpropertiesofLaFePO 141 4.1 Crystalandelectronicstructure . . . . . . . . . . . . . . . . . . . . . . . 146 4.2 Intrabandtransitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 4.2.1 TheextendedDrudemodel . . . . . . . . . . . . . . . . . . . . . 151 4.2.2 Themicroscopicmodel . . . . . . . . . . . . . . . . . . . . . . . . 152 4.2.3 Intrabandopticalconductivity. . . . . . . . . . . . . . . . . . . . 158 4.3 Roleofinterbandtransitions . . . . . . . . . . . . . . . . . . . . . . . . . 159 4.4 Originofinterbandtransitions . . . . . . . . . . . . . . . . . . . . . . . . 164 4.5 Phenomenologicalmodelandcomparisonwiththeexperiment. . . . 168 4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 4.7 Technicaldetails. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 5 Towardanab-initioestimateoftheStonerI 175 5.1 MagneticandparamagneticDFTresults . . . . . . . . . . . . . . . . . . 179 5.2 TherenormalizationoftheStonerparameterI→˜I . . . . . . . . . . . 184 5.3 Ab-initiorenormalizationofI:thereducedStonertheory . . . . . . . 189 5.4 Ferromagnetic-paramagnetictransitioninNi Alunderpressure . . . 193 3 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 5.6 Technicaldetails. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 Conclusion 201 Zusammenfassung 207 Listofpublications 225 Acknowledgements 227 Bibliography 239 Abbreviations DFT densityfunctionaltheory DOS densityofstates GGA generalizedgradientapproximation LAPW linearaugmentedplanewave LDA localdensityapproximation LSDA localspindensityapproximation SDW spin-densitywave ARPES angle-resolvedphotoemissionspectroscopy SQUID superconductingquantumInterferencedevice BCS Bardeen,Cooper,Schrieffer RPA randomphaseapproximation EDM extendedDrudeModel VBM valencebandmaximum SCR self-consistentrenormalization EST extendedStonertheory RST reducedStonertheory RBA rigidbandapproximation

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In particular I will focus my attention on the superconducting and normal state properties of the Indeed, given the rich phase diagram of Cu1−x BiSO, in principle one could easily switch we can go skying somewhere, sometimes together (of course you have to teach me.) I also would like to than
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