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High Pressure Study of High-Temperature Superconductors

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High Pressure Study of High-Temperature Superconductors High Pressure Study of High-Temperature Superconductors Von der Fakultät Mathematik und Physik der Universität Stuttgart zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung Vorgelegt von Sofia-Michaela Souliou aus Thessaloniki (Griechenland) Hauptberichter: Prof. Dr. Bernhard Keimer Mitberichter: Prof. Dr. Martin Dressel Tag der mündlichen Prüfung: 29 September 2014 Max-Planck-Institut für Festkörperforschung Stuttgart 2014 Contents Zusammenfassung in deutscher Sprache9 Introduction 13 1 High-Temperature Superconductors 17 1.1 Cuprate superconductors . 17 1.1.1 Crystal Structure and Carrier Doping . 17 1.1.2 Electronic Structure . 20 1.1.3 Phase diagram . 24 1.1.4 The YBa2Cu3O6+x and YBa2Cu4O8 systems . 37 1.1.5 High Pressure Effects . 43 1.2 Iron-based Superconductors . 47 1.2.1 Crystal Structure and Superconductivity . 47 1.2.2 Electronic Structure . 49 1.2.3 Phase Diagram . 52 1.2.4 The REFeAsO "1111" family . 57 1.2.5 High Pressure Effects . 59 2 Raman scattering 63 2.1 Overview of Inelastic Light Scattering . 63 2.2 Phononic Raman scattering . 65 2.2.1 Classical description of phononic Raman scattering . 65 2.2.2 Quantum mechanical description of phononic Raman scattering . 68 2.2.3 Phonon Interactions . 71 2.3 Phononic Raman scattering under high pressure . 75 2.4 The Raman spectrum of high-Tc cuprate and iron-based superconductors . 77 2.4.1 Raman modes of YBa2Cu3O6+x .................. 77 2.4.2 Raman modes of YBa2Cu4O8 ................... 84 2.4.3 Raman modes of REFeAsO1−xFx .................. 86 2.5 Analysis of the phononic Raman data . 89 5 Contents 3 Experimental Techniques 93 3.1 High Pressures and Low Temperatures . 93 3.1.1 High Pressure Techniques: The Diamond Anvil Cell . 93 3.1.2 Low Temperature Techniques: The Cryostat . 100 3.2 Raman and x-ray Diffraction Setups . 102 3.2.1 Raman Scattering Setups . 102 3.2.2 X-ray Diffraction Setup . 105 4 Raman study of YBa2Cu3O6+x 107 4.1 Ambient Pressure Results . 107 4.1.1 Introduction . 107 4.1.2 Experimental Details . 108 4.1.3 Doping Dependence . 109 4.1.4 Scattering Geometry Dependence . 112 4.1.5 Resonance effects . 114 4.1.6 Temperature Dependence . 116 4.1.7 Discussion . 117 4.2 High Pressure Results . 124 4.2.1 Introduction . 124 4.2.2 Experimental Details . 125 4.2.3 Room temperature measurements . 125 4.2.4 Low temperature measurements . 129 4.2.5 Discussion . 131 5 Raman study of YBa2Cu4O8 133 5.1 Ambient Pressure Results . 133 5.1.1 Introduction . 133 5.1.2 Experimental Details . 134 5.1.3 Low Temperature measurements . 134 5.1.4 Discussion . 135 5.2 High Pressure Results . 138 5.2.1 Introduction . 138 5.2.2 Experimental Details . 140 5.2.3 Room Temperature measurements . 141 5.2.4 Low Temperature measurements . 143 5.2.5 Discussion . 154 6 SmFeAsO under Pressure 159 6.1 Introduction . 159 6.2 Experimental Details . 160 6.3 X-ray Diffraction Study . 160 6.3.1 X-ray Diffraction Data . 160 6 Contents 6.3.2 Discussion . 162 6.4 Raman Study . 166 6.4.1 Structural transition: the Eg modes . 166 6.4.2 Magnetic transition: the A1g and B1g modes . 169 6.4.3 Discussion . 176 Summary 179 Acknowledgements 183 Bibliography 185 7 Zusammenfassung in deutscher Sprache Die vorliegende Arbeit beschäftigt sich experimentell mit dem Einfluss von hohem, ex- ternen Druck auf Hochtemperatursupraleiter. Die Dynamik von Struktur- und Gitterpara- metern verschiedener Vertreter der cuprat- und eisenbasierten Supraleitern wurde unter präzise kontrolliertem, hohem hydrostatischen Druck und tiefen Temperaturen mittels Ramanspektroskopie und Röntgendiffraktion untersucht. In den komplexen Phasendiagrammen dieser Systeme beeinflussen externe Parame- ter, wie z.B. chemische Dotierung und hohe Drücke, das Wechselspiel zwischen den unterschiedlichen, konkurrierenden Grundzuständen empfindlich. Hoher hydrostatischer Druck ist ein idealer, "sauberer" Parameter, da die Komplexität, die durch chemische Sub- stitution oft eingebracht wird, vermieden wird und parallel dazu Messungen an chemisch dotierten Proben einen direkten Vergleich der beiden Parameter - Druck und Dotierung - auf die Phasenübergänge in diesen Systemen ermöglichen. Die Gitter-Schwingungen des Hochtemperatur-Supraleiters YBa2Cu3O6+x wurden sy- stematisch mittels Raman-Spektroskopie als Funktion von Dotierung (x = 0.95, 0.75, 0.60, 0.55, und 0.45) und externem Druck untersucht. Bei Normaldruck und tiefen Tem- peraturen, jedoch noch deutlich oberhalb des Übergangs zum supraleitenden Zustand, beobachten wir für unterdotierte Proben neue Raman-aktive Phononen zusätzlich zu den gemäß der Gruppen-Theorie erwarteten Raman-Moden. In den Raman-Spektren von op- timal dotierten Proben wurden keine entsprechenden neuen Phononen beobachtet. Die neuen Phononen sind in Resonanz mit dem eingestrahlten roten Laserlicht (1.96 eV) und besitzen gemäß den Raman-Auswahlregeln Ag-Symmetrie. Das Einsetzen der neuen Raman-Moden unterhalb bestimmter Temperaturen und ihre Dotierungsabhängigkeit le- gen einen Zusammenhang mit der inkommensurablen Ladungsdichtewelle (CDW) nahe, die kürzlich durch Methoden der Synchrotron- Röntgenstreuung in unterdotierten Cupra- ten entdeckt wurde [1,2]. Im CDW-Zustand würde die Rückfaltung der ursprünglichen YBa2Cu3O6+x Phononen-Dispersion in die neue Brillouin-Zone zu neuen optische Mo- den am Γ-Punkt führen und ihre Beobachtung in den Raman-Spektren ermöglichen. Unter hohem Druck und innerhalb des von uns untersuchten Bereichs (von 2 bis 12 GPa) zeigen unsere Raman-Messungen an hochgeordnetem, unterdotiertem YBa2Cu3O6:55 keine der neuen Raman-Phononen, die bei Normaldruck beobachtet wurden. Hierfür gibt 9 Zusammenfassung in deutscher Sprache es zwei mögliche Ursachen. Erstens könnte dies, im Rahmenkonzept der CDW, mit ei- ner druckinduzierten Unterdrückung der Ladungsordnung zusammenhängen. Dies wird in einem Szenario mit konkurrierenden Phasen erwartet und ist in Übereinstimmung mit früheren Beobachtungen in La1:875Ba0:125CuO4 mit Streifenordnung [3,4]. Andererseits könnte unsere Beobachtung auf druckinduziertes Relaxationsverhalten der Sauerstoffa- tome innerhalb der sauerstoffdefizitären Cu-O-Ketten zurückzuführen sein, worauf auch ältere Magnetisierungsmessungen bei hohen Drücken hinweisen [5]. Ferner beobachten wir in unseren Hochdruck-Daten eine ausgeprägte, druckinduzierte Rotverschiebung der Resonanzbedingungen für Raman-Moden, die auf Defekte in den Ketten zurückgehen. Eine analoge druckinduzierte Verschiebung der Resonanzbedingungen für die neuen Mo- den würde auch ihr Ausbleiben bei hohen Drücken erklären. Ramanmessungen unter hohem Druck und tiefen Temperaturen wurden an dem un- terdotierten Supraleiter YBa2Cu4O8 durchgeführt, der, aufgrund der stöchiometrischen Zusammensetzung, nicht durch das komplexe Zusammenspiel von Sauerstofffehlstellen und druckinduziertem Relaxationsverhalten beeinflusst wird. Dabei wird unterhalb der kritischen Temperatur eine deutliche Renormalisierung einiger Ramanmoden beobach- tet, die eine Folge von Veränderungen in der Phononeigenenergie durch die Öffnung der supraleitenden Bandlücke ist. Die markanteste hiervon ist die B1g-ähnliche Buckling- Mode. Das Ausmaß dieser Renormalisierung nimmt mit dem Druck stark zu, ähnlich dem Einfluss von Lochdotierung in YBa2Cu3O6+x. In YBa2Cu3O6+x ist die Phononrenorma- lisierung im unterdotierten Bereich schwach und steigt in Richtung optimaler Dotierung an, wo sie vergleichbar ist mit der Renormalisierung in YBa2Cu4O8 bei 10 GPa. Bei ei- nem Druck von etwa 10 GPa geht das System durch einen reversiblen, druckinduzierten, strukturellen Phasenübergang zu einer nicht-punktsymmetrischen Struktur über, in der die CuO2-Ebenen fast unbeeinflusst bleiben, während die doppelten Cu-O-Ketten kolla- bieren (Raumgruppe Imm2) [6]. Der Strukturübergang spiegelt sich durch das Auftreten neuer Moden eindeutig in den Ramandaten wider. Dadurch ist es uns möglich das (P,T)- Phasendiagramm im Detail abzubilden und die Grenze zwischen den beiden Phasen zu bestimmen. In der neuen Phase wird die Renormalisierung der Buckling-Mode vollstän- dig unterdrückt. Zugleich werden in keinem anderen Raman-aktivem Phonon Anoma- lien beobachtet. Ab-initio-Berechnungen, die sehr genau die neue Struktur und die zu- gehörigen, experimentell bestimmten Phononfrequenzen reproduzieren, zeigen, dass die Kopplung zwischen der Buckling-Mode und dem elektronischen System nicht signifikant durch den Phasenübergang beeinflusst wird. Die Abwesenheit von Phononrenormali- sierungen bei gleichzeitiger Anwesenheit beträchtlicher Elektronen-Phononen-Kopplung weist darauf hin, dass, im Gegensatz zu älteren Transportmessungen [7], YBa2Cu4O8 unter einem hydrostatischen Druck von über 10 GPa nicht mehr supraleitend ist. Abschließend untersuchten wir die strukturellen Eigenschaften und Gitterschwingun- gen von SmFeAsO unter hohem Druck. Dieses ist ein Vertreter der "1111"-Familie (Raum- gruppe P4/nmm) der eisen-basierten Supraleiter, in der die Supraleitung für gewöhnlich entweder durch die Substitution von O durch F/H oder durch den Einsatz hoher Drücke 10 im undotierten Material hervorgerufen wird, dessen magnetischer Übergang durch eine Änderung der Struktur von tetragonal zu orthorhombisch begleitet wird. Beides wird nor- malerweise durch das Auftreten der Supraleitung unterdrückt. Während der magnetische Übergang in dem System von SmFeAsOxF1−x bereits bei niedriger Dotierung komplett unterdrückt wird, zeigten strukturelle Untersuchungen entweder die allmähliche Verdrän- gung der orthorhombischen Verzerrung [8] oder ihre Beibehaltung über einen weiten Be- reich
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