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This book summarizes the most recent and compelling experimental results for complex oxide interfaces. The results of this book were obtained with the cutting-edge photoemission technique at highest energy resolution. Due to their fascinating properties for new-generation electronic devices and the challenge of investigating buried regions, the book chiefly focuses on complex oxide interfaces.
The crucial feature of exploring buried interfaces is the use of soft X-ray angle-resolved photoemission spectroscopy (ARPES) operating on the energy range of a few hundred eV to increase the photoelectron mean free path, enabling the photons to penetrate through the top layers – in contrast to conventional ultraviolet (UV)-ARPES techniques. The results presented here, achieved by different research groups around the world, are summarized in a clearly structured way and discussed in comparison with other photoemission spectroscopy techniques and other oxide materials. They are complemented and supported by the most recent theoretical calculations as well as results of complementary experimental techniques including electron transport and inelastic resonant X-ray scattering.

### Chapter 1. Introduction: Interfaces as an Object of Photoemission Spectroscopy

Abstract
In this short introductory chapter, basic concepts of photoemission techniques will be given. In particular, the importance of some parameters like probing depth, energy and momentum resolution will be tackled by comparing photoemission experiments in different photon energy ranges from ultraviolet to soft and hard X-rays. Buried system i.e. interfaces could be probed only by using high energy photoemission. Apart from the band structure resolved in electron momentum k, the photoemission technique directly probes the electron spectral function encoding information about how particles are dressed by their interactions with the remainder of the system. Many body effects and electron correlation can in this way be accessed, in particular, the electron-phonon interaction affecting electron mobility. Finally, the instrumental development of photoemission is described in connection with its scientific perspective.

### Chapter 2. The / Interface: The Origin of the 2D Electron Liquid and the Fabrication

Abstract
This chapter discusses the formation of the 2D electron liquid at the LAO/STO interface. The first part presents the theoretical proposals aimed at explaining the origin of the charge at the interface. The second part focuses on the importance of the growth techniques and parameters like temperature, oxygen pressure, post-deposition annealing and their influence on the electronic transport properties.
S. Gariglio, C. Cancellieri

### Chapter 3. Transport Properties of TMO Interfaces

Abstract
Phenomena that are absent of bulk TMO compounds can emerge at their interfaces when they are grown on top of each-other. A prototypical example of such emerging states is found at the $$\text {LaAlO}_{3}/\text {SrTiO}_{3}$$ interface, which also attracted most of the initial interest for this new field of research (in the TMO context). Here we review some properties of this peculiar interface as investigated by transport measurements allowing the studies of different effects such as magnetism, superconductivity or Rashba effect; hence indirectly accessing the band structures studied by the methods presented in the rest of the book.
A. M. R. V. L. Monteiro, A. D. Caviglia, N. Reyren

### Chapter 4. ARPES Studies of Two-Dimensional Electron Gases at Transition Metal Oxide Surfaces

Abstract
High mobility two-dimensional electron liquids (2DELs) underpin today’s silicon based devices and are of fundamental importance for the emerging field of oxide electronics. Such 2DELs are usually created by engineering band offsets and charge transfer at heterointerfaces. However, in 2011 it was shown that highly itinerant 2DELs can also be induced at bare surfaces of different transition metal oxides where they are far more accessible to high resolution angle resolved photoemission (ARPES) experiments. Here we review work from this nascent field which has led to a systematic understanding of the subband structure arising from quantum confinement of highly anisotropic transition metal d-states along different crystallographic directions. We further discuss the role of different surface preparations and the origin of surface 2DELs, the understanding of which has permitted control over 2DEL carrier densities. Finally, we discuss signatures of strong many-body interactions and how spectroscopic data from surface 2DELs may be related to the transport properties of interface 2DELs in the same host materials.
Siobhan McKeown Walker, Flavio Y. Bruno, Felix Baumberger

### Chapter 5. Photoelectron Spectroscopy of Transition-Metal Oxide Interfaces

Abstract
Transition metal oxides exhibit a plethora of intrinsic functionalities like superconductivity, magnetism or multiferroicity. To put these to practical use requires the integration of suited oxide materials within thin film structures where the active regions with switchable and tunable physical properties often are the very interfaces. Fundamental knowledge on the chemical and electronic interface structure is key to design target properties for working devices. Here we will show that photoelectron spectroscopy is a powerful tool to obtain such kind of information if high enough photon energies in the soft and hard X-ray regime are employed to enhance the probing depth and hence get access to the electronic structure of buried layers and interfaces.
M. Sing, R. Claessen

### Chapter 6. Electrons and Polarons at Oxide Interfaces Explored by Soft-X-Ray ARPES

Abstract
Soft-X-ray ARPES (SX-ARPES) with its enhanced probing depth and chemical specificity allows access to fundamental electronic structure characteristics—momentum-resolved spectral function, band structure, Fermi surface—of systems difficult and even impossible for the conventional ARPES such as three-dimensional materials, buried interfaces and impurities. After a recap of the spectroscopic abilities of SX-ARPES, we review its applications to oxide interfaces, focusing on the paradigm LaAlO3/SrTiO3 interface. Resonant SX-ARPES at the Ti L-edge accentuates photoemission response of the mobile interface electrons and exposes their dxy-, dyz- and dxz-derived subbands forming the Fermi surface in the interface quantum well. After a recap of the electron-phonon interaction physics, we demonstrate that peak-dip-hump structure of the experimental spectral function manifests the Holstein-type large polaron nature of the interface charge carriers, explaining their fundamentally reduced mobility. Coupling of the charge carriers to polar soft phonon modes defines dramatic drop of mobility with temperature. Oxygen deficiency adds another dimension to the rich physics of LaAlO3/SrTiO3 resulting from co-existence of mobile and localized electrons introduced by oxygen vacancies. Oxygen deficiency allows tuning of the polaronic coupling and thus mobility of the charge carriers, as well as of interfacial ferromagnetism connected with various atomic configurations of the vacancies. Finally, we discuss spectroscopic evidence of phase separation at the LaAlO3/SrTiO3 interface. Concluding, we put prospects of SX-ARPES for complex heterostructures, spin-resolving experiments opening the totally unexplored field of interfacial spin structure, and in-operando field-effect experiments paving the way towards device applications of the reach physics of oxide interfaces.
Vladimir N. Strocov, Claudia Cancellieri, Andrey S. Mishchenko

### Chapter 7. Standing-Wave and Resonant Soft- and Hard-X-ray Photoelectron Spectroscopy of Oxide Interfaces

Abstract
We discuss several new directions in photoemission that permit more quantitatively studying buried interfaces: going to higher energies in the multi-keV regime; using standing-wave excitation, created by reflection from either a multilayer heterostructure or atomic planes; tuning the photon energy to specific points near absorption resonances; and making use of near-total-reflection geometries. Applications to a variety of oxide and spintronic systems are discussed.
Slavomír Nemšák, Alexander X. Gray, Charles S. Fadley

### Chapter 8. Ab-Initio Calculations of TMO Band Structure

Abstract
We review the fundamental aspects related to ab-initio band structure calculations for the $$\text {SrTiO}_3/\text {LaAlO}_3$$ interface, analyzing capabilities and limits of the most advanced approaches, using available experiments as a reference. In particular, we discuss accuracy and failures for what concern the description of electronic, transport, and thermoelectric properties of oxide heterostructures. Despite evident shortcomings, our overview assesses the usefulness and the satisfying quality of ab-initio methods as an efficient approach for oxide heterostructure design and analysis.
A. Filippetti

### Chapter 9. Dynamical Mean Field Theory for Oxide Heterostructures

Abstract
Transition metal oxide heterostructures often, but by far not always, exhibit strong electronic correlations. State-of-the-art calculations account for these by dynamical mean field theory (DMFT). We discuss the physical situations in which DMFT is needed, not needed, and where it is actually not sufficient. By means of an example, $$\text {SrVO}_3/\text {SrTiO}_3$$, we discuss step-by-step and figure-by-figure a density functional theory (DFT) + DMFT calculation. The second part reviews DFT + DMFT calculations for oxide heterostructure focusing on titanates, nickelates, vanadates, and ruthenates.
O. Janson, Z. Zhong, G. Sangiovanni, K. Held

### Chapter 10. Spectroscopic Characterisation of Multiferroic Interfaces

Abstract
In this chapter we discuss the capabilities of X-ray photoemission and absorption spectroscopies for the investigation of the electronic, magnetic and electric properties of multiferroic materials and heterostructures. As complementary techniques providing element selective information on both occupied and empty states, their combination delivers a comprehensive picture of the chemical state of individual species, magnetic moments, bulk and surface band structure, and local atomic environment at the interface between dissimilar materials. By directly probing the electronic structure at the atomic level, unique insights can be learned about the mechanisms responsible for the magnetoelectric couplings in this fascinating class of materials.
M.-A. Husanu, C. A. F. Vaz

### Chapter 11. Oxides and Their Heterostructures Studied with X-Ray Absorption Spectroscopy and Resonant Inelastic X-Ray Scattering in the “Soft” Energy Range

Abstract
Soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have become essential experimental tools for the investigations the complex physics of transition metal oxide (TMO) heterostructures. XAS has been long used to determine the valence, the orbital and magnetic properties of transition metals. More recently, linear and circular dichroism in XAS have been widely applied to determine the crystal field splitting, the atomic orbital and spin moments, and the magnetic order of 3d-states, in bulk sample, in thin films and at atomically-sharp interfaces. Although less common, RIXS is also gaining popularity for its capability of accessing local and collective excitations at a time; the recent technical advances have been established RIXS as an important method for the determination of the electronic and magnetic properties of TMOs. This chapter is a brief review of the salient XAS and RIXS results on TMO and TMO heterostructures published in the last 15 years.
M. Salluzzo, G. Ghiringhelli