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As a real-space technique, atomic-resolution STEM imaging contains both amplitude and geometric phase information about structural order in materials, with the latter encoding important information about local variations and heterogeneities present in crystalline lattices. Such phase information can be extracted using geometric phase analysis (GPA), a method which has generally focused on spatially mapping elastic strain. Here we demonstrate an alternative phase demodulation technique and its application to reveal complex structural phenomena in correlated quantum materials. As with other methods of image phase analysis, the phase lock-in approach can be implemented to extract detailed information about structural order and disorder, including dislocations and compound defects in crystals. Extending the application of this phase analysis to Fourier components that encode periodic modulations of the crystalline lattice, such as superlattice or secondary frequency peaks, we extract the behavior of multiple distinct order parameters within the same image, yielding insights into not only the crystalline heterogeneity but also subtle emergent order parameters such as antipolar displacements. When applied to atomic-resolution images spanning large (~0.5 × 0.5 μm2) fields of view, this approach enables vivid visualizations of the spatial interplay between various structural orders in novel materials.
We demonstrate the Kerr-effect induced by the electric field of single-cycle THz pulses in the relaxor ferroelectrics potassium-tantalum niobate KTa1-xNbxO3 (KTN) and K1-yLiyTa1-xNbxO3 (KLTN). We find a slow orientational relaxation with a time constant of 6 ps in KTN with x=1.8% at room temperature, that decreases upon approaching the transition temperature.
This is a copy of the slides presented at the meeting but not formally written up for the volume.
The interface electronic structure of transition metal oxides has attracted considerable attention in recent years. Given the versatile physical properties of these materials, and the interest in developing novel functional devices unattainable by conventional semiconductors, understanding interface barrier formation and control is a central issue. Among various interface structures, single interface junctions are the simplest structure for their study, and their transport properties can provide much of the essential information for depiction of the interface electronic structure. Recently, magnetic field dependent current-voltage (I-V) and capacitance-voltage (C-V) characteristics were reported for a junction between oxygen deficient manganite (La0.7Sr0.3MnO3-ä) and 0.01wt% Nb doped SrTiO3 (Nb:STO). By contrast, no magnetic field dependence was observed for the stoichiometric junction (La0.7Sr0.3MnO3 / Nb:STO) . La0.7Sr0.3MnO3-ä is a ferromagnetic metal with a Curie temperature lower than the stoichiometric counterpart, and Nb:STO is an n-type band semiconductor. The magnetic field dependence of the I-V and C-V are contrary to the expectation from the Zeeman effect . To elucidate the origin of the magnetic field dependence observed, we used internal photoemission (IPE) to directly probe the evolution of the Schottky barrier height. In this technique, the reverse junction current is monitored while the sample is illuminated with monochromatic light of varying wavelength. IPE is a direct, reliable method for the determination of Schottky barrier heights since the measurement is free from external electric field, eliminating any complexity associated with bias dependent effects observed in the case of I-V and C-V. In addition, its compatibility with external magnetic field makes this method especially desirable for investigating interface electronic structures involving magnetically sensitive materials. Junctions with different oxygen stoichiometry were grown by pulsed laser deposition of a La0.7Sr0.3MnO3 target on Nb:STO substrates, and their I-V, C-V and IPE characteristics were measured under magnetic field. Magnetic field dependence was observed by all three techniques for La0.7Sr0.3MnO3-ä / Nb:STO, whereas they were field independent for La0.7Sr0.3MnO3 / Nb:STO. In La0.7Sr0.3MnO3-δ / Nb:STO, the barrier height determined from IPE measurements significantly decreased with magnetic field, which is consistent with the I-V and C-V results. The origin of the magnetic field dependent Schottky barrier height will be discussed.  N. Nakagawa et al., App. Phys. Lett. 87, 241919 (2005).
The following article is based on the Outstanding Young Investigator Award presentation given by Harold Y. Hwang of the University of Tokyo on March 29, 2005, at the Materials Research Society Spring Meeting in San Francisco. Hwang was cited for “innovative work on the physics of transition-metal oxides and the atomic-scale synthesis of complex oxide heterostructures.” Perovskite oxides range from insulators to superconductors and can incorporate magnetism as well as couple to phonon instabilities. The close lattice match between many perovskites raises the possibility of growing epitaxial thin-film heterostructures with different ground states that may compete or interact. The recent development of superconducting Josephson junctions, magnetic tunnel junctions, ferroelectric memory cells, and resistive switching can be considered examples within this new heteroepitaxial family. In this context, Hwang presents his studies of electronic structure at atomically abrupt interfaces grown by pulsed laser deposition. Some issues are generic to all heterointerfaces, such as the stability of dopant profiles and diffusion, interface states and depletion, and interface charge arising from polarity discontinuities. A more unusual issue is the charge structure associated with Mott insulator/band insulator interfaces. The question is, how should one consider the correlated equivalent of band bending? This semiconductor concept is based on the validity of rigid single-particle band diagrams, which are known to be an inadequate description for strongly correlated electrons. In addition to presenting an interesting scientific challenge, this question underlies the attempts to develop new applications of doped Mott insulators in device geometries.
We report 12 patients with Sturge-Weber-Dimitri disease treated surgically between January 1975 and December 1987. Hemispherectomy was performed on ten, two others underwent occipital lobectomy for intractable seizures. All operations were performed between the ages of 3 months and 20 months, except in two at age 8 and 9 years. The onset of seizures in all was between 2 and 8 months of age, except for two at 15 months. There were no postoperative deaths. Postoperative shunt procedures were required in 3 out of 12 (25%). Postoperative seizure control for one year or more was achieved in 11 out of 12 patients. The remaining patient is on medication with no seizures, but follow-up is less than a year. Intellectual deterioration was not seen after surgery except in 2 patients who had late operations. We conclude that patients with Sturge-Weber-Dimitri syndrome who have intractable seizures in the first 6 months of life and unilateral hemispheric involvement should be considered for early resection of the involved hemisphere.
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