Arsen Soukiassian

Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices

Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

Quantification of Internal Electric Fields and Local Polarization in Ferroelectric Superlattices

Oxide heterostructure superlattices constitute a new family of materials with tunable ferroelectric properties. While theoretical models predict the presence of nanosized ferroelectric domains in these films, they had not been observed as the magnitude of the response functions challenges the limits of experimental detection. Here, a new protocol in a precise variant of piezoforce microscopy is used to image domains in BaTiO(3)/SrTiO(3) superlattices. Comparison of experimentally determined polarization to predictions of phase-field calculations is in quantitative agreement. Additionally, a combination of theory and experiment is used to determine the magnitude of internal electric field within the thin film, in a procedure that can be generalized to all ferroelectric films.

Towards artifact-free atomic-resolution elemental mapping with electron energy-loss spectroscopy

Atomic-resolution elemental maps of materials obtained using energy-loss spectroscopy in the scanning transmission electron microscope (STEM) can contain artifacts associated with strong elastic scattering of the STEM probe. We demonstrate how recent advances in instrumentation enable a simple and robust approach to reduce such artifacts and produce atomic-resolution elemental maps amenable to direct visual interpretation. The concept is demonstrated experimentally for a (BaTiO3)8/(SrTiO3)4 heterostructure, and simulations are used for quantitative analysis. We also demonstrate that the approach can be used to eliminate the atomic-resolution elastic contrast in maps obtained from lower-energy excitations, such as plasmon excitations.

Hybrid reflections from multiple x-ray scattering in epitaxial oxide films

In numerous symmetric θ-2θ scans of phase-pure epitaxial complex oxide thin films grown on single-crystal substrates, we observe x-ray diffraction peaks that correspond to neither the film nor the substrate crystal structure. These peaks are the result of multiple, sequential diffraction events that occur from both the film and the substrate. The occurrence of so-called “hybrid” reflections, while described in the literature, is not widely reported within the complex oxide thin-film community. We describe a simple method to predict and identify peaks resulting from hybrid reflections and show examples from epitaxial complex oxide films belonging to three distinct structural types.

Infrared spectroscopy of strained BaTiO3/SrTiO3 superlattices on scandate substrates

Polarized infrared reflectance of ferroelectric [(BaTiO)/(SrTiO)] superlattices grown on DyScO, TbScO, EuScO and SmScOsubstrates by reactive molecular beam epitaxy was measured. Infrared spectra (30–3000 cm) were taken independently from the (110) cut of bare substrate and the substrate covered with superlattice in two polarizations: along the -axis and perpendicular to it. The phonon parameters and dielectric function of the superlattice were extracted using a fitting procedure to study the influence of the strain induced by different substrates. The phonon frequency dependence on the substrate-induced strain was varying from 75 to 95 cm. The superlattice static permittivity was estimated from that data, which varies from 150 to 270 depending on substrate and polarization. The highest permittivity was obtained for the EuScO substrate.

Properties of single RuO 2 layer embedded in SrTiO 3

A two-dimensional ferromagnet is an interesting system both for basic understanding of magnetism and for spintronic applications. However, making such a system is not easy as ferromagnetism becomes more and more unstable with reducing the thickness of a ferromagnet. For example, in oxide systems such as SrRuO3, La1-xSrxMnO3, etc, ferromagnetism is lost around the thickness of a few unit cells [1, 2]. Recent theoretical studies have shown that a single SrRuO3 layer can exhibit half-metallicity when it is sandwiched between SrTiO3 lattices [3, 4]. The conduction electrons are predicted to exist only in RuO2 layers, which means that the system is a two-dimensional half-metal. In this work, in order to demonstrate the theoretical prediction, we have fabricated high-quality SrRuO3/SrTiO3 superlattices using molecular beam epitaxy. Magnetic and transport measurements show a clear signature of ferromagnetism and conductivity.

Infrared Spectroscopy of Nanoscopic Epitaxial BaTiO3/SrTiO3 Superlattices

Artificially layered ferroelectric superlattices have enormous appeal from both a technological and a fundamental standpoint. However, it is difficult to measure and monitor their properties on nan...