Table Of ContentPhenomenological 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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