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The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO3 (STO) and GdTiO3 (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells (u.c.) STO/3 u.c. of GTO](20) using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.

The electronic structure of Heusler alloys having mixed magnetic phases, comprising of vicinal anti-ferromagnetic and ferromagnetic orders, is of great significance. We present the results of an electronic structure study on Ni_(x)Cu_(1−x)MnSb Heusler alloys, using Mn-2p core-level photoemission spectroscopy. Room temperature data in the paramagnetic phase reveal a non-monotonic variation of both electron correlation strength and conduction-band hybridization such that the former enhances while the latter weakens for compositions showing a mixed phase relative to compositions at the phase boundaries to the ordered phases. The results suggest a possible electronic driving force for settling mixed-magnetic phases.

Applied physics letters 102, 062907 (2013). doi:10.1063/1.4792525, Electroresistance in ferroelectric tunnel junctions is controlled by changes in the electrostaticpotential profile across the junction upon polarization reversal of the ultrathin ferroelectricbarrier layer. Here, hard X-ray photoemission spectroscopy is used to reconstruct the electricpotential barrier profile in as-grown Cr/BaTiO3(001)/Pt(001) heterostructures. Transportproperties of Cr/BaTiO3/Pt junctions with a sub-lm Cr top electrode are interpreted in terms oftunneling electroresistance with resistance changes of a factor of 30 upon polarization reversal.By fitting the I-V characteristics with the model employing an experimentally determined electricpotential barrier we derive the step height changes at the BaTiO3/Pt (Cr/BaTiO3) interfaceþ0.42(0.03) eV following downward to upward polarization reversal., Published by American Institute of Physics, Melville, NY

We present a detailed study of the electronic structure and band alignment in LaAlO3/SrTiO3 oxide heterostructures by hard x-ray photoelectron spectroscopy. Our spectroscopic measurements find no evidence for the strong potential gradient within the polar LaAlO3 film predicted by band theory. Due to the high interface sensitivity of the method, we are further able to determine the valence band offset between the LaAlO3 film and the SrTiO3 substrate, which is found to be independent of the number of LAO overlayers. Finally, we discuss several explanations for the apparent absence of the built-in field in ex situ prepared LaAlO3/SrTiO3 heterostructures.

A double‐crystal, UHV‐compatible monochromator for XAFS applications at bending magnet beamlines has been designed. It uses two crystal sets, Si(111) and (311), on a common central rotation axis driven by an ex‐vacuo goniometer. All mechanical and electrical components are mounted on a 400 mm UHV flange which is attached to a compact vacuum chamber. The first crystals are water cooled using connector‐ and bellowless tubing through the fluidic sealed feedthrough of the central rotation. The first crystal set is mounted off‐axis and can be translated vertically to keep the fixed exit condition. The second crystal set uses small crystals of the same size as the first. In order to accept the reflected beam of the first crystal at small Bragg angles, it is tangentially translated along the beam. The angle can be varied from 5° to 55.5° resulting in a total energy range 2.4 – 43.4 keV for Si(111)/(311). Crystal sets are interchangeable by translating the vacuum chamber. Angle encoding is achieved by a Renishaw incremental optical encoder in vacuo.

Resonant electronic spectroscopy consists in measuring a non-radiative decay process (Auger or autoionization process) excited with photon energies around an absorption edge. The resonant spectra carry information both on the nature of the electronic transitions near the absorption edge by scanning the very first empty orbitals above the Fermi level (through the absorption process), and, on the other hand, on the atomic electronic configuration through the lineshape of the observed decay process. In this paper, after a quick review of the pioneering works in this field, we show that resonant measurements and their theoretical modeling can be used to precisely interpret complex absorption spectra. Hence, Ce resonant 3d photoemission spectra allow, in the Ce-L3 edge, to attribute the two white lines to 4f1 and 4f0 final state electronic configurations and to evidence the presence of a hidden 4f2 structure in the rise of the absorption threshold. Quadrupolar transitions towards localized 3d orbitals are also detected and quantitatively characterized in the titanium K-edge prepeaks in TiO2, thanks to angular dependent resonant Ti-KLL Auger measurements.

Physical review / B 67(3), 035122 (2003). doi:10.1103/PhysRevB.67.035122, Published by APS, Woodbury, NY

Physical review / B 57(11), 6422 - 6426 (1998). doi:10.1103/PhysRevB.57.6422, The evolution of the Pd Auger electron emission resulting in an open 3d and 4d shell, respectively, was experimentally studied as a function of photon energy across the L3 threshold. Both for the core-core-core (L3−M4,5M4,5) and the core-valence-valence (L3−N4,5N4,5) processes the observed resonance behavior is interpreted in terms of radiationless resonant Raman scattering. In the threshold regime the Auger electron emission traces the photon energy and the linewidth exhibits a sublifetime narrowing. Around the L3 threshold the L3−N4,5N4,5 emission partly overlaps with the valence-band satellite. The accurate intensity normalized constant initial state data reveal a Fano-type modulation for the main valence band. However, no such behavior is obtained for the satellite region if the resonant threshold process in the L3−N4,5N4,5 Auger channel is taken properly into account., Published by Inst., Woodbury, NY