Session J5: Focus Session: Interfaces in Complex Oxides - Electronic Structure

Angle resolved photoemission spectroscopy of the electronic structure of complex oxide interfaces

Engineering interfaces between complex oxides has proven to be a powerful technique for tuning their electronic and magnetic properties. To fully understand how these interfaces can control these electronic properties, one requires advanced spectroscopic tools to uncover their electronic structure. Angle-resolved photoemission spectroscopy (ARPES) is the leading tool for probing energy and momentum resolved electronic structure. To understand the physics of these complex oxide interfaces, we have developed an approach which combines state-of-the-art oxide molecular beam epitaxy with high-resolution ARPES. As one example, we describe our work on oxide superlattices ([LaMnO$_{3}$]$_{2n}$ / [SrMnO$_{3}$]$_{n}$) with alternating LaMnO$_{3}$ and SrMnO$_{3}$ blocks. Our ARPES measurements reveal that as a function of separation between the LaMnO$_{3}$-SrMnO$_{3}$ interfaces, the interfacial quasiparticle states evolve from a quasi-three-dimensional ferromagnetic metal, to a two-dimensional spin-polarized electron liquid, and ultimately to a pseudogapped ferromagnetic insulator with increasing superlattice thickness, n. I will also describe our work on other oxide thin films SrTiO$_{3}$ and Sr$_{2}$TiO$_{4}$-based interfaces where the quasiparticle interactions can be tuned as a function of dimensionality, and work on cuprate and ruthenate thin films.   

Spectral and spatial redistribution at the LaAlO$_{3}$/SrTiO$_{3}$ interface

A conductive two-dimensional electron liquid (2DEL) appears at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface for a LAO thickness of n$\geq$4 unit cells. Despite the tremendous research interest, many questions regarding the origin and characteristics of the 2DEL have to be addressed yet. In particular, this includes the electronic structure of the ``buried interface.'' Optical second harmonic generation (SHG) is an ideal tool for studying interfaces, since it is sensitive to the interfacial symmetry breaking along the stacking direction. Using SHG with frequency-tunable amplified femtosecond laser pulses we obtain information about the structural reorganization of the interfacial STO conduction band for SHG photon energies up to 6.2 eV. A massive spectral weight redistribution is present for n$\geq$3, indicating a global reorganization of the conduction band structure. At low temperatures the spectral resolution is enhanced which alleviates the distinction of interfacial electronic transitions. Additionally, we discuss the modification of the interfacial states by variation of the STO surface termination. Our data are supported by a theoretical framework based on symmetry selection rules that allows a specific assignment of interfacial O(2p)-Ti(3d) transitions to the SHG spectrum.   

Electronic properties of LaAlO$_3$/SrTiO$_3$ superlattices: interface charge density, Fermi surfaces, plasmon and optical spectra

We present calculations of the charge density profile, Fermi surface topology, plasmon and optical excitation spectra of the electron gas at the LaAlO$_3$/SrTiO$_3$ interface. The calculations are based on a self-consistent Hartree/RPA approximation and a tight binding parametrization of the band structure. The subband occupancy is determined as a function of polar discontinuity magnitude and dielectric constant. The number of occupied xy bands changes significantly whereas for all reasonable polar discontinuity and dielectric constant profiles only one yz and one xz bands are occupied. These yz and xz band give dominant contributions to the long-distance tail of the interface charge. The plasmon and optical absorption spectra are determined. The results are compared with experimental data.   

Direct observations of the band alignment of LaAlO3/SrTiO3 using scanning tunneling microscopy

In this work, by using cross-sectional scanning tunneling microscopy, local and direct evidence of the electronic information across the LaAlO3/SrTiO3 hetero-interfaces are investigated. A combination of scanning tunneling spectroscopy and analysis with atomic resolution across the hetero-interface reveals how the oppositely charged atomic planes undergo electronic reconstructions and introduce a built-in electric field across the polar LaAlO3 thin films grown on SrTiO3 substrates. With the consideration of the tip-induced band bending effect, the magnitude of the built-in field across LaAlO3, the band bending on SrTiO3 side, and the decay length of the band downshift of SrTiO3 side at the hetero-interface are directly observed.   

Multi-orbital subband structure of transition-metal oxide 2DEGs

We demonstrate the creation and control of two-dimensional electron gases (2DEGs) in the 3d and 5d transition metal oxides SrTiO$_3$ and KTaO$_3$. These 2DEGs, of the form usually generated by interface engineering, are created here at the bare oxide surfaces. This permits their detailed spectroscopic investigation using angle-resolved photoemission (ARPES), and we employ this to directly image the d-orbital subband structure of the 2DEGs. We find that quantum confinement lifts the degeneracy of the bulk band structure, driving orbital ordering of the 2DEG. We measure the resulting ladder of light d$_{xy}$ subbands, which co-exist at lower binding energies with heavy $d_{xz/yz}$-derived states. The electronic structure revealed by ARPES is in quantitative agreement with our model tight-binding calculations. While the strong spin-orbit coupling of KTaO$_3$ promotes substantial orbital mixing, our calculations predict only a small Rashba splitting of the 2DEG states, consistent with our experimental measurements where any spin splitting is too small to be resolved. The polar nature of the KTaO$_3(100)$ surface plays a striking role in mediating formation of the 2DEG as compared to non-polar SrTiO$_3(100)$, reminiscent of the polar catastrophe at LaAlO$_3$/SrTiO$_3$ interfaces.   

A Universal Critical Density Underlying the Physics of Electrons at the LaAlO$_{3}$/SrTiO$_{3}$ Interface

The two-dimensional electron system formed at the interface between the insulating oxides LaAlO$_{3}$ and SrTiO$_{3}$ exhibits ferromagnetism, superconductivity, and a wide range of unique magnetotransport properties. A key challenge is to find a unified microscopic mechanism that underlies these emergent phenomena. Here we show that a universal Lifshitz transition between d-orbitals lies at the core of the observed transport phenomena in this system. Our measurements find a critical electronic density at which the transport switches from single to multiple carriers. This density has a universal value, independent of the LaAlO$_{3}$ thickness and electron mobility. The characteristics of the transition, its universality, and its compatibility with spectroscopic measurements establish it as a transition between d-orbitals of different symmetries. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed universal transition to a range of reported magnetotransport properties. Interestingly, we also find that the maximum of the superconducting transition temperature occurs at the same critical transition, indicating a possible connection between the two phenomena. Our observations demonstrate that orbital degeneracies play an important role in the fascinating behavior observed so far in these oxides. Ref: arxiv.org/abs/1110.2184   

Localization of the Two-dimensional Electron Gas in LaAlO$_3$/SrTiO$_3$ Heterostructures

We use low temperature magnetotransport measurements to compare the quasi 2-dimensional electron gas (2DEG) at the LaAlO$_3$/SrTiO$_3$ interface in heterostructures grown on (LaAlO$_3$)$_{0.3}$-(Sr$_2$AlTaO$_3$)$_{0.7}$ (LSAT) substrates to the 2DEG at the LaAlO$_3$/single crystal SrTiO$_3$ interface. All heterostructures were grown by pulsed laser deposition with \emph{in-situ} reflection high-energy electron diffraction. For the samples on LSAT, we find that increasing the carrier concentration by growing at lower oxygen partial pressures changes the conductivity mechanism, from strongly localized transport at low carrier concentrations to metallic conductivity with indications of weak localization at higher concentrations. We interpret this as an increasing occupation of Ti 3\emph{d} bands of layers near the interface, changing the spatial extent of the conduction region and its susceptibility to localization by disorder and point defects at the interface. On the other hand, the 2DEG of similarly grown LaAlO$_3$ on single crystal SrTiO$_3$ shows metallic behavior and low temperature measurements display Kohler scaling of the out-of-plane magnetoresistance, consistent with classical orbital transport.   

Localized electron states and phase separation at the LaAlO$_3$/SrTiO$_3$ interface

Even though the 2D electron gas at the polar interfaces of LAO/STO has been studied extensively, an explanation for the observed magnetic centers or the coexistence of magnetism and superconductivity is still lacking. Earlier density-functional calculations have indicated the presence of multi-bands and two types of electrons at the interface [1]. Here we propose that a combination of lattice disorder, octahedral rotations, and Jahn-Teller distortion can lead to some of these electrons to be localized near the interface and form lattice polarons. Evidence for this is presented from detailed density-functional calculations, which indicate that the energy gain associated with JT distortion and impurity or disorder induced local potentials can offset the kinetic energy cost of localization. Our model studies [2] also show the possible existence of a phase separation, thereby providing a natural explanation for the coexistence of magnetism and superconductivity [3].\\[4pt] [1] Z. S. Popovi\'{c}, S. Satpathy and R. M. Martin {\it Phys. Rev. Lett.}, {\bf 101}, 256801 (2008)\\[0pt] [2] B. R. K. Nanda and S. Satpathy {\it Phys. Rev B}, {\bf 83}, 195114 (2011)\\[0pt] [3] L. Li {\it et. al.} {\it Nature Physics}, {\bf 7}, 762 (2011); J. A. Bert {\it et. al.} {\it ibid}, 767 (2011);   

Electronic structure, Fermi surfaces, and electron-phonon coupling in La-doped Sr$_{2}$TiO$_{4}$ and SrTiO$_{3}$

Sr$_{2}$TiO$_{4}$ is a quasi-two-dimensional Ruddlesden-Popper structure analogue to SrTiO$_{3}$, and is isostructural with the cuprate parent compound La$_{2}$CuO$_{4}$. Although the electronic structure of SrTiO$_{3}$ has been well-explored due to its importance in oxide electronics, little is known about the electronic properties of Sr$_{2}$TiO$_{4}$. To investigate this, we synthesized epitaxial La doped Sr$_{2}$TiO$_{4}$ and SrTiO$_{3}$ films on (100) LSAT substrates by molecular beam epitaxy (MBE) and investigated the electronic structure using angle-resolved photoemission spectroscopy (ARPES). The electronic structure of 5{\%} La doped Sr$_{2}$TiO$_{4}$ shows a single electron like band with mostly Ti-3d$_{xy}$ character dispersing across the Fermi surface which corresponds well with LDA calculations. This is in contrast to doped SrTiO$_{3}$ where all three t2g bands are degenerate. We also observed signatures of strong electron-phonon coupling in the quasi-two-dimensional Sr$_{2}$TiO$_{4}$ materials which appear to be absent in three-dimensional SrTiO$_{3}$.   

Theory of SrTiO3 2DEGs

There has been much recent interest in oxide 2DEGS - in particular those involving SrTiO3. In spite of the simple conduction band structure of the parent material, there has been no quantitative understanding of the physical properties of these systems. We have built a model, informed by experiment, that is sufficiently realistic to describe defect free surface/interface states in SrTiO3. The model is able to account for non-local dielectric screening and the orbital dependence of quantum confinement effects. Preliminary results of the model will be compared with experiment.   

Conducting band structure in LaTiO3/SrTiO3 interfaces

Oxide interfaces between insulating hosts show unexpected conducting carriers, which can be useful for next-generation electronic applications. However, the fundamental understanding of the conducting interfaces remains elusive. Here we report \textit{in situ} angle-resolved photoemission spectroscopy (ARPES) studies in the LaTiO3/SrTiO3 heterostructures, of which layer thicknesses were precisely prepared by pulsed laser deposition in the BL7.0, ALS. We found that the interface generates a high-density electron gas over few unit cells from the junction. We further discuss the orbital characteristics of the interface electronic states with comparison to the recent theoretical calculations. Based on the unit-cell layer resolved electronic structure of the LaTiO3/SrTiO3 interface, we discuss the conducting carriers comparing to the LaAlO3/SrTiO3 interfaces.