5 research outputs found
Deep Data Analysis of Conductive Phenomena on Complex Oxide Interfaces: Physics from Data Mining
Spatial variability of electronic transport in BiFeO<sub>3</sub>–CoFe<sub>2</sub>O<sub>4</sub> (BFO–CFO) self-assembled heterostructures is explored using spatially resolved first-order reversal curve (FORC) current voltage (IV) mapping. Multivariate statistical analysis of FORC-IV data classifies statistically significant behaviors and maps characteristic responses spatially. In particular, regions of grain, matrix, and grain boundary responses are clearly identified. <i>k</i>-Means and Bayesian demixing analysis suggest the characteristic response be separated into four components, with hysteretic-type behavior localized at the BFO–CFO tubular interfaces. The conditions under which Bayesian components allow direct physical interpretation are explored, and transport mechanisms at the grain boundaries and individual phases are analyzed. This approach conjoins multivariate statistical analysis with physics-based interpretation, actualizing a robust, universal, data-driven approach to problem solving, which can be applied to exploration of local transport and other functional phenomena in other spatially inhomogeneous systems
Oxygen Control of Atomic Structure and Physical Properties of SrRuO<sub>3</sub> Surfaces
Complex oxide thin films and heterostructures have become one of the foci for condensed matter physics research due to a broad variety of properties they exhibit. Similar to the bulk, properties of oxide surfaces can be expected to be strongly affected by oxygen stoichiometry. Here we explore the coupling between atomic structure and physical properties of SrRuO<sub>3</sub> (SRO), one of the most well-studied oxide materials. We perform a detailed <i>in situ</i> and <i>ex situ</i> experimental investigation of the surfaces of SRO thin films using a combination of scanning tunneling microscopy (STM), X-ray and ultraviolet photoelectron spectroscopy, SQUID magnetometry, and magnetotransport measurements, as well as <i>ab initio</i> modeling. A number of remarkable linear surface reconstructions were observed by STM and interpreted as oxygen adatoms, favorably adsorbed in a regular rectangular or zigzag patterns. The degree of oxygen coverage and different surface patterns change the work function of the thin films, and modify local electronic and magnetic properties of the topmost atomic layer. The <i>ab initio</i> modeling reveals that oxygen adatoms possess frustrated local spin moments with possible spin-glass behavior of the surface covered by adsorbed oxygen. Additionally, the modeling indicates presence of a pseudo gap on the topmost SrO layer on pristine SrO-terminated surface, suggesting possibility for realization of a surface half-metallic film
Correlating Electronic Transport to Atomic Structures in Self-Assembled Quantum Wires
Quantum wires, as a smallest electronic conductor, are
expected
to be a fundamental component in all quantum architectures. The electronic
conductance in quantum wires, however, is often dictated by structural
instabilities and electron localization at the atomic scale. Here
we report on the evolutions of electronic transport as a function
of temperature and interwire coupling as the quantum wires of GdSi<sub>2</sub> are self-assembled on Si(100) wire-by-wire. The correlation
between structure, electronic properties, and electronic transport
are examined by combining nanotransport measurements, scanning tunneling
microscopy, and density functional theory calculations. A metal–insulator
transition is revealed in isolated nanowires, while a robust metallic
state is obtained in wire bundles at low temperature. The atomic defects
lead to electron localizations in isolated nanowire, and interwire
coupling stabilizes the structure and promotes the metallic states
in wire bundles. This illustrates how the conductance nature of a
one-dimensional system can be dramatically modified by the environmental
change on the atomic scale
Surface Control of Epitaxial Manganite Films <i>via</i> Oxygen Pressure
The trend to reduce device dimensions demands increasing attention to atomic-scale details of structure of thin films as well as to pathways to control it. This is of special importance in the systems with multiple competing interactions. We have used <i>in situ</i> scanning tunneling microscopy to image surfaces of La<sub>5/8</sub>Ca<sub>3/8</sub>MnO<sub>3</sub> films grown by pulsed laser deposition. The atomically resolved imaging was combined with <i>in situ</i> angle-resolved X-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr of O<sub>2</sub> leads to mixed-terminated film surfaces, with B-site (MnO<sub>2</sub>) termination being structurally imperfect at the atomic scale. A relatively small reduction of the oxygen pressure to 20 mTorr results in a dramatic change of the surface structure leading to a nearly perfectly ordered B-site terminated surface with only a small fraction of A-site (La,Ca)O termination. This is accompanied, however, by surface roughening at a mesoscopic length scale. The results suggest that oxygen has a strong link to the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also pronounced: Ca surface segregation is decreased with oxygen pressure reduction
Dimensionality Controlled Octahedral Symmetry-Mismatch and Functionalities in Epitaxial LaCoO<sub>3</sub>/SrTiO<sub>3</sub> Heterostructures
Epitaxial strain provides a powerful
approach to manipulate physical properties of materials through rigid
compression or extension of their chemical bonds via lattice-mismatch.
Although symmetry-mismatch can lead to new physics by stabilizing
novel interfacial structures, challenges in obtaining atomic-level
structural information as well as lack of a suitable approach to separate
it from the parasitical lattice-mismatch have limited the development
of this field. Here, we present unambiguous experimental evidence
that the symmetry-mismatch can be strongly controlled by dimensionality
and significantly impact the collective electronic and magnetic functionalities
in ultrathin perovskite LaCoO<sub>3</sub>/SrTiO<sub>3</sub> heterojunctions.
State-of-art diffraction and microscopy reveal that symmetry breaking
dramatically modifies the interfacial structure of CoO<sub>6</sub> octahedral building-blocks, resulting in expanded octahedron volume,
reduced covalent screening, and stronger electron correlations. Such
phenomena fundamentally alter the electronic and magnetic behaviors
of LaCoO<sub>3</sub> thin-films. We conclude that for epitaxial systems,
correlation strength can be tuned by changing orbital hybridization,
thus affecting the Coulomb repulsion, U, instead of by changing the
band structure as the common paradigm in bulks. These results clarify
the origin of magnetic ordering for epitaxial LaCoO<sub>3</sub> and
provide a route to manipulate electron correlation and magnetic functionality
by orbital engineering at oxide heterojunctions