Spectroscopy of Complex Oxide Interfaces

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.

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.

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.

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.

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 d_xy-, d_yz- and d_xz-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.

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.

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.

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.

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.

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.