Hawoong Hong

Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices

Lattice distortion due to oxygen octahedral rotations have a significant role in mediating the magnetism in oxides, and recently attracts a lot of interests in the study of complex oxides interface. However, the direct experimental evidence for the interrelation between octahedral rotation and magnetism at interface is scarce. Here we demonstrate that interfacial octahedral rotation are closely linked to the strongly modified ferromagnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices. The maximized ferromagnetic moment in the N=6 superlattice is accompanied by a metastable structure (space group Imcm) featuring minimal octahedral rotations (a(-)a(-)c(-), α~4.2°, γ~0.5°). Quenched ferromagnetism for N<4 superlattices is correlated to a substantially enhanced c axis octahedral rotation (a(-)a(-)c(-), α~3.8°, γ~8° for N=2). Monte-Carlo simulation based on double-exchange model qualitatively reproduces the experimental observation, confirming the correlation between octahedral rotation and magnetism. Our study demonstrates that engineering superlattices with controllable interfacial structures can be a feasible new route in realizing functional magnetic materials.

Elucidation of zeolite microstructure by synchrotron X-ray diffuse scattering

Single-crystal diffuse scattering measurements can now rapidly probe the three-dimensional structure of subtle defects in microporous framework materials. Diffuse scattering data from natural mordenite crystals are shown to exhibit a complex distribution of weak features which have been mapped out using a synchrotron X-ray source and a CCD detector. Comparison with computer-simulated diffuse scattering patterns yields a detailed three-dimensional columnar defect structure and reveals that roughly one third of the mordenites columnar defects cooperate to form a block-mosaic pattern of {110} stacking faults.

Imaging phonons in a fcc Pu-Ga alloy by thermal diffuse x-ray scattering

X-ray thermal diffuse scattering intensity patterns from phonons in a fcc δ-Pu–Ga alloy have been recorded using an 18 keV undulator x-ray beam with a beam diameter of 25 μm. The results are consistent with patterns calculated using the Born–von Karman force constant model of lattice dynamics, and support the pronounced softening of the transverse acoustic branch along the [111] direction observed from inelastic x-ray scattering measurements. This work demonstrates the feasibility of using a “large-grain, small beam” approach to study lattice properties, such as phonon dispersion curves, of materials not readily available in the form of large single crystals.

Reflection surface x-ray diffraction patterns: k-space images

For the past two decades, x-ray diffraction has been utilized for surface structural determination. Unlike reflection high-energy electron diffraction (RHEED) which is a complicated dynamical scattering process, x-ray surface analysis is simple and straightforward due to the kinematic nature of x rays. Using high brilliance x rays from an undulator beamline and a highly sensitive charge coupled device detector, we successfully observed RHEED-like x-ray diffraction patterns. The patterns were recorded during the preparation of Si(111)-(7×7), transformation to Ge/Si(111)-(5×5) and Ge growth. Also, simultaneous measurements of x-ray reflectivity and crystal truncation rods are shown feasible with this technique.

Growth and atomic structure of epitaxial Si films on Ge(111)

Abstract Heteroepitaxial growth by molecular beam epitaxy of thin Si films on the Ge(111) surface was studied. The surface morphology and atomic structure were examined by scanning tunneling microscopy and synchrotron-radiation photoemission. For submonolayer Si coverages on the Ge(111) substrate at room temperature, the impinging Si atoms condense to form small islands. The areas surrounding the islands remain c(2×8). Post annealing or growth at high temperatures causes Si indiffusion and Ge segregation to the surface. Multilayer deposition at high temperatures can be described as a mixed two- and three-dimensional growth. Many small three-dimensional islands are observed on a two-dimensional film. The surface structure of the film shows partial disorder, and the film itself contains numerous defects caused by the lattice mismatch between Si and Ge. The role of the segregated Ge as a surfactant in the growth is discussed.

Self-organization of Pb islands on Si(111) caused by quantum size effects

Growth of metallic Pb islands on Si(111) by vacuum deposition was studied in real time using synchrotron x-ray diffraction. The islands coarsen and order, maintaining a nearly uniform interisland distance but without angular correlation. The resulting interisland structure is akin to a two-dimensional liquid. Over a wide temperature range, the interisland ordering is well correlated with the development of “magic” island heights caused by energy minimization of the Pb electrons. The results demonstrate quantum confinement effects as a driving force for self-organization, as opposed to strain effects that generally govern the formation of semiconductor quantum dot arrays.

Time-resolved reflection surface x-ray diffraction

Methods to collect two-dimensional time-resolved x-ray diffraction patterns from surfaces/interfaces were developed. Reflection surface x-ray diffraction utilizing high brilliance x rays and a charge coupled device can achieve a time resolution as good as one second. Also, two-dimensional maps of reflectivity rocking curves can be recorded fast enough to monitor growth processes. These methods were demonstrated for the study of Ag and Pb films on Si (111)−(7×7) surfaces.

X-ray studies of the growth of smooth Ag films on Ge(111)-c(2×8)

We have performed in situ reflectivity measurements using synchrotron radiation of Ag films deposited on Ge(111) over the thickness range of 3–12 atomic layers. The films deposited at a substrate temperature of 110 K are not well ordered, but become well ordered upon annealing, as evidenced by substantial changes in the x-ray reflectivity data. The thickness distribution for each annealed film, deduced from a fit to the reflectivity data, is remarkably narrow, with just two or three adjacent discrete thicknesses present, despite the large lattice mismatch between Ag and Ge. In some cases, the film thickness is nearly atomically uniform. The results are discussed in connection with recent models and theories of electronic effects on the growth of ultrathin metal films.

Structural determination of the C60Ge(111) interface via X-ray diffraction

Abstract An X-ray diffraction study was performed to determine the nature of the C 60 Ge (111) interface formed by depositing C60 on a Ge(111)−c(2 × 8) surface at room temperature. In-plane k-scans show a (1 × 1) periodicity at the C 60 Ge (111) interface with no trace of the c(2 × 8) reconstruction, indicating that the Ge adatoms on the clean c(2 × 8)-reconstructed surface are displaced. Scans along the (10) rod indicate that these adatoms are transferred from the T4 bonding site to the H3 site after C60 deposition. A model consisting of three relaxed bilayers of Ge and randomly distributed adatoms in the H3 site explains our results.

X-ray diffraction study of the structure of xenon multilayers on single crystal graphite

The structures of xenon multilayers (1 to 44 layers) physisorbed onto a graphite single crystal surface have been studied using x-ray scattering techniques. Both the intra-planar and inter-planar structures could be examined by measurements of the (1 0l) diffraction rods. The (0 0l) diffraction provides direct information about the thickness of the multilayer. We find two principal and surprising results. First, the xenon does not form an infinite number of layers at low temperatures. This disagrees with a number of previous reports which suggest complete wetting of xenon on graphite. Second, the structures of the adsorbed layers turn out to be rather elaborate. Instead of uniform, defect free layers, the xenon multilayers exhibit stacking disorder and a commensurate-incommensurate transition in the first layer.