Probing single-unit-cell resolved electronic structure modulations in oxide superlattices with standing-wave photoemission

Abstract

Control of structural couplings at the complex-oxide interfaces is a powerful platform for creating new ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element-and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standingwave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice [Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3]×15 wherein the electronicstructural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depthresolved studies of a wide variety of heteroengineered material systems.3 Rational design and understanding of the electronic properties of new functional materials is a dominant theme in modern experimental and theoretical condensed matter physics and materials science [1][2][3][4][5]. Over the past two decades, epitaxial complex-oxide heterostructuring and interface engineering have emerged as powerful and versatile experimental platforms, enabling the synthesis of electronic, magnetic and structural phases, which are unattainable in bulk crystals or thin films [6][7][8][9][10][11][12]. Concurrently, significant strides in the development and refinement of modern materials theories, including various modalities of density functional theory (DFT) [5,13] and dynamical mean-field theory (DMFT) [14,15], have led to the availability of advanced firstprinciples tools for guiding the synthesis of such heterostructures and interfaces, as well as interpreting experimental results.Engineering structural couplings at the epitaxial interfaces between perovskite oxides is a promising avenue for atomic-level control of the electronic and magnetic properties in such structures [16][17][18]. Recent studies of the isovalent La0.7Sr0.3MnO3/Eu0.7Sr0.3MnO3 (LSMO/ESMO) and La0.5Sr0.5MnO3/La0.5Ca0.5MnO3 (LSMO/LCMO) superlattices revealed that the emerging highly-localized lattice distortions and non-bulk-like rotations of oxygen octahedra can lead to new electronic and magnetic properties, and provide a way to enhance or suppress functional properties, such as electronic bandwidth and ferromagnetism [19][20][21]. Furthermore, varying the thicknesses of individual layers within a superlattice above and below the interfacial coupling lengths (2-8 unit cells) adds a powerful control mechanism for tuning these properties at the unit...

Direct experimental evidence of tunable charge transfer at the LaNiO3/CaMnO3 ferromagnetic interface

“…It is immediately obvious that the Mn/Ca, Ni, and C signals originate from different (vertical within the to the contrasting line shapes (phases) of their respective rocking-curve spectra. The O 1s rocking curve is dominated by the signal from the upper CaMnO 3 layer due to the limited probing depth and, therefore, resembles the Ca and Mn spectra, consistent with prior studies [30,48]. The Bragg features for the thinner (N = 2 u.c.)…”

“…Once the x-ray SW field is established within the sample, it can be translated vertically (perpendicular to the sample's surface) by approximately half of the superlattice period by scanning (rocking) the grazing x-ray incidence angle across the Bragg condition. In a recent soft x-ray study [48], the depth resolution of one cubic perovskite unit cell (approximately 3.8 Å) was demonstrated using the same experimental setup (see Methods section in the Supplemental Material [36]).…”

Layer-resolved many-electron interactions in delafossite PdCoO2 from standing-wave photoemission spectroscopy

“…To overcome this problem, we use the depth specificity of standing-wave X-ray photoemission spectroscopy (SW-XPS), which has been successfully used to study complex oxide quantum materials in the form of multilayers, as well as single crystalline materials. [18][19][20][21][22][23] In single crystals, an X-ray standing wave is created by using Bragg reflection off atomic planes, which results in enhanced photoemission intensity from atoms whose positions coincide with the maxima (or antinodes) of the standing wave (see Figure 1). By measuring the change in the photoemission intensities as function of the incident angle, it is possible to decompose the valence band spectrum into contributions from the individual layers as we demonstrate below.…”