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Ultrafast Electron Dynamics and the Role of Screening

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Ultrafast Electron Dynamics and the Role of Screening Ultrafast electron dynamics and the role of screening Im Fachbereich Physik der Freien Universit¨at Berlin eingereichte Dissertation Daniel Wegkamp November 2014 This work was performed between October 2009 and November 2014 in the Department of Physical Chemistry (Professor Martin Wolf) at the Fritz Haber Institute of the Max Planck Society. Berlin, November 2014 Erstgutachter: Prof. Dr. Martin Wolf Zweitgutachter: Prof. Dr. Martin Weinelt Drittgutachter: Dr. Robert A. Kaindl Datum der Disputation: 11.05.2015 Abstract This thesis focuses on the ultrafast dynamics of electronic excitations in solids and how they are influenced by the screening of the Coulomb interaction between charged particles. The impact of screening on electron dynamics is manifold, ranging from modifications of electron-electron scattering rates over trapping of excess charges to massive renormalisation of electronic band structures. The timescales of these dynam- ical processes are directly accessible by femtosecond time-resolved photoemission and optical spectroscopy. Three exemplary systems are investigated to shed light onto these fundamental processes: Vanadiumdioxide undergoes a phase transition from a monoclinic insulator to a rutile metal. Apart from temperature, doping and other influences, the insulator- to-metal transition can also be driven by photoexcitation. This, in the past, gave rise to a controversy about the timescales of structural and electronic transition and raised the question which of them constitutes the driving mechanism. Using time- resolved photoelectron spectroscopy, it is shown that the electronic band gap of the insulator collapses instantaneously with photoexcitation and without any structural involvement. The reason is a change of screening due to the generation of photoholes. At the same time, the symmetry of the lattice potential changes, as seen by coherent phonon spectroscopy. This potential change is likely to initiate the structural phase transition from monoclinic to rutile structure. However, the initial non-equilibrium situation can be described by a metallic electronic structure with the atoms still in the monoclinic lattice positions. The SrTiO3/vacuum interface exhibits a two-dimensional electron gas (2DEG), which is delocalised within the surface plane, but localised perpendicular to it. The lower dimensionality changes the form of the screened Coulomb interaction and the phase space within the 2DEG, leading to modified hot carrier lifetimes. These are in- vestigated by time-resolved photoemission spectroscopy: The predicted 2D behaviour is confirmed and two distinct final states within the unoccupied electronic band structure are discovered. Furthermore, the population of the 2DEG is transiently increased by photoexcitation from localised in-gap states into the 2DEG. A different type of screening by dipole moments in amorphous ice layers, is exploited to stabilise and trap electrons within the polar medium in front of a metal surface. Thereby, the mean free path of low energy electrons in amorphous ice is estimated. Moreover, the trapped electrons are used to drive a chemical reaction: A persistent modification of the surface electronic structure of the ice layer is explained via the ‘dielectron hydrogen evolution reaction’. Understanding the role of screening in these systems allows to explain seemingly unrelated effects, like trapping of excess electrons in ice and the insulator-to-metal transition in VO2, within the same concept. I Kurzfassung Diese Arbeit besch¨aftigt sich mit ultraschneller Dynamik elektronischer Anregungen in Festk¨orpern und dem Einfluss der Abschirmung der Coulomb-Wechselwirkung zwischen geladenen Teilchen. Die Auswirkungen dieser Abschirmung sind vielf¨altig und reichen von der Modifizierung von Elektron-Elektron-Streuraten uber¨ das Einfangen von Uber-¨ schussladungen bis hin zur vollst¨andigen Renormierung der elektronischen Bandstruk- tur. Zeitaufgel¨oste optische und Photoelektronenspektroskopie kann die Femtosekun- den Zeitskala dieser dynamischen Prozesse direkt erfassen. Drei Beispielsysteme werden untersucht, um solche grundlegenden Prozesse zu beleuchten: Vanadiumdioxid weist einen Phasenubergang¨ von monoklinem Isolator zu rutilem Metall auf. Neben Temperatur, Dotierung und weiteren Einflussen¨ kann man den Phasenubergang¨ auch durch Anregung mit Licht induzieren. Dies fuhrte¨ zu einer kon- troversen Diskussion uber¨ die Zeitskalen des strukturellen und elektronischen Phasen- ubergangs¨ und warf die Frage auf, welcher der beiden die treibende Kraft darstellt. Durch zeitaufgel¨oste Photoelektronenspektroskopie wird gezeigt, dass sich die Band- lucke¨ des Isolators sofort mit der Photoanregung schließt, ohne dass die Gitterstruktur beteiligt ist. Der Grund ist eine Anderung¨ der Abschirmung der Coulomb-Wechsel- wirkung durch photoinduzierte L¨ocher. Die Spektroskopie von koh¨arenten Phononen zeigt, dass sich gleichzeitig das Gitterpotential ¨andert. Diese Potential¨anderung initiiert vermutlich den Phasenubergang¨ von monokliner zu rutiler Struktur. Die anf¨angliche Ungleichgewichtssituation kann dennoch durch eine metallische elektronische Struk- tur beschrieben werden, w¨ahrend die Atome sich an ihren monoklinen Gitterpl¨atzen befinden. An der SrTiO3/Vakuum-Grenzfl¨ache existiert ein zwei-dimensionales Elektronengas (2DEG), welches entlang der Oberfl¨ache delokalisiert, senkrecht dazu jedoch lokalisiert ist. Die Form der abgeschirmten Coulomb-Wechselwirkung und der Phasenraum im 2DEG sind durch die niedrige Dimensionalit¨at modifiziert, was zu ver¨anderten Lebens- dauern von heißen Ladungstr¨agern fuhrt.¨ Diese Lebensdauern werden mit zeitaufgel¨oster Photoelektronenspektroskopie untersucht: Das vorhergesagte 2D-Verhalten wird best¨a- tigt und es werden zwei ausgepr¨agte Endzust¨ande in der unbesetzten elektronischen Bandstruktur entdeckt. Weiterhin wird die Besetzung des 2DEG durch Photoanregung aus lokalisierten Defektzust¨anden in der Bandlucke¨ vorubergehend¨ erh¨oht. Eine andere Art der Abschirmung mit Hilfe von Dipolmomenten in einer amorphen Eisschicht, wird ausgenutzt um Elektronen in einem polaren Medium vor einer Metall- oberfl¨ache zu stabilisieren und einzufangen. Hierbei wird die mittlere freie Wegl¨ange von niederenergetischen Elektronen in amorphem Eis abgesch¨atzt. Weiterhin werden die eingefangenen Elektronen dazu benutzt eine chemische Reaktion anzutreiben: Eine langlebige Anderung¨ der elektronischen Oberfl¨achenstruktur der Eisschicht wird durch die sogenannte ”dielectron hydrogen evolution reaction” erkl¨art. Das Verst¨andnis der Abschirmung der Coulomb-Wechselwirkung in diesen Systemen erlaubt es, scheinbar unverwandte Effekte, wie den Einfang von Uberschusselektronen¨ in Eis oder den Phasenubergang¨ von Isolator zu Metall in VO2, im Rahmen des gleichen Konzeptes zu erkl¨aren. III CONTENTS Contents Abstract I Deutsche Kurzfassung III 1 Introduction 1 2 Questions and Background 5 2.1 Electron interactions and screening in the investigatedsystems ..... 6 2.2 Crystallographic and electronic phase transition in VO2 ......... 9 2.3 Surface 2D electron gas in insulating SrTiO3 ............... 15 2.4 Electron solvation and trapping in amorphous solid water ........ 19 3 Experimental details and techniques 23 3.1 Thefemtosecondlasersystem . 23 3.1.1 Commercial oscillator and amplifier . 23 3.1.2 Noncollinear third and fourth harmonic generation . ...... 26 3.1.3 Adaptive white-light continuum compression for broadband op- ticalprobing ............................. 28 3.2 Photoelectron spectroscopy . 34 3.2.1 One- and three-step-model of photoemission . 35 3.2.2 Angle-resolved photoemission spectroscopy (ARPES) ....... 37 3.2.3 Matrix elements and selection rules . 39 3.2.4 Unoccupied band structure and excited state dynamics ..... 40 3.2.5 Two-photon photoemission . 40 3.2.6 Time- and angle-resolved photoelectron spectroscopy....... 42 3.2.7 Dataanalysis ............................. 43 3.2.8 Experimental setup for TR photoemission spectroscopy ..... 46 3.3 Opticalpump-probespectroscopy . 47 3.3.1 Optical response, dielectric function and band structure ..... 47 3.3.2 Transient reflectivity upon photo-excitation . ...... 48 3.3.3 Coherent phonon generation and detection . 49 4 Instantaneous band gap collapse in VO2: Photoinduced change of screening 53 4.1 Probing the potential energy landscape of VO2 .............. 53 4.1.1 Temperature induced phase transition in VO2/Si(100) . 54 4.1.2 Non-equilibrium dynamics of the insulating monoclinic phase . 56 4.1.3 Non-equilibrium dynamics of the rutile metallic phase ...... 59 4.1.4 Strongphotoexcitation. 62 4.1.5 The dynamics of the photoexcited state . 74 4.1.6 Conclusion: Ultrafast symmetry loss of monoclinic VO2 ..... 79 4.2 Ultrafast metallisation of VO2 ........................ 81 4.2.1 VO2/Al2O3(0001) sample for photoelectron spectroscopy . 81 4.2.2 Thermal insulator-to-metal transition . 83 4.2.3 Photo-induced change of electronic density of states ....... 88 V CONTENTS 4.2.4 The dynamics of the photo-induced insulator-to-metal transition 93 4.2.5 Relaxation dynamics in the transient metallic phase . 106 4.2.6 Summary: Instantaneous band gap collapse in VO2 ........ 109 4.3 A new picture of the photo-induced, ultrafast phase transition in VO2 . 110 5 Screening and electron dynamics in two-dimensions: The SrTiO3/vacuum interface 113 5.1 VacuumcleavedandpolishedSTO . 113 5.2 Unoccupied states at the STO(001) surface . 115 5.3 HotelectrondynamicsinSTO . 122 5.4 Pump-induced dynamics of the occupied band structure . ....... 126
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