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RISS 인기검색어
- 검색키워드
- 저자 : Walko, Robert (검색결과 3 건)
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Structure and faceting of low-symmetry metal surfaces
Walko, Donald Alan University of Illinois at Urbana-Champaign 2000 해외박사(DDOD)
X-ray diffraction has, within the past two decades, developed into a powerful analytical tool for determining the atomic structure of crystalline surfaces. The dual advantages of high intensity and high resolution have made this technique thrive at synchrotron radiation sources. The experiments in this thesis extend surface x-ray diffraction to metal surfaces with particularly low symmetry: the multilayer surface structure of α-Ga(010), complicated by the low symmetry of the bulk, and Cu(115), a regularly stepped surface which spontaneously facets when exposed to oxygen. The equations describing surface x-ray diffraction are derived, with attention paid to assumptions made during the derivation and limitations of the technique. We have determined the surface structure of α-Ga(010) near its melting point using x-ray diffraction. Due to the low symmetry of the α-Ga bulk structure, two distinct bulk truncations of the (010) surface are possible. Of these two ways, we find the true surface is formed by cutting through dimer bonds (i.e., between metallic bilayers). Cu(001) vicinal surfaces facet when exposed to O. We have studied this process on Cu(115), which transforms from a clean surface to 104 and 113 facets, using surface x-ray diffraction. Unlike α-Ga(010), the low symmetry of these surfaces is due to their high Miller indices, i.e., the stepped nature of the surfaces. The Cu(115) surface exhibits a complex interlayer relaxation accounted for by basic elasticity theory; the vertical displacements of the three surface atoms correlate to those of the subsurface atoms directly below. Besides being instrumental in determining the static structure of surfaces and facets, surface x-ray diffraction allows us to noninvasively observe, <italic> in situ</italic>, the evolution of the faceting surface. We find that the faceting is driven by the formation of O/Cu(104) facets: O exposure induces spinodal decomposition of the (115) surface into (104) and (014) facets, which form spontaneously, and also disordered, stepped facets, whose orientation gradually changes from (115) to (113) as the (104) facets grow. We identify three temperature regimes which have qualitatively different faceting processes, shedding light on the temperature dependence of the equilibrium crystal shape for part of the O-covered Cu system. Throughout this thesis, comparisons are made with results obtained by other surface-sensitive techniques. The complimentarity of these techniques is worth emphasizing; despite the power of surface x-ray diffraction in solving crystal structures, its ability to interpret and explain the properties of these surfaces is greatly enhanced by microscopy, spectroscopy, and other diffraction techniques, as well as theoretical and numerical studies. (Abstract shortened by UMI.).
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Walkoe, Iver ProQuest Dissertations & Theses University of Minn 2019 해외박사(DDOD)
In this paper, we extend Lax-Phillips' discreteness of pseudo-cuspforms, in the style of Colin de Verdiere's use of the Friedrichs self-adjoint extension of a restriction of the Laplace-Beltrami operator, as opposed to the use of semigroup methods. We use this to prove meromorphic continuation of Eisenstein series in the Q-rank one case, again following Colin de Verdiere, as opposed to the semigroup-oriented viewpoint of Lax-Phillips and W. Mueller.
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Walko, Robert ProQuest Dissertations & Theses The Ohio State Uni 2022 해외박사(DDOD)
Material systems that combine magnetism and topology have garnered intense interest recently due to predictions of a variety of phenomena such as the quantum anomalous Hall effect, chiral topological edge states, magnetoelectric effects, and Weyl semimetal and axion insulator phases. In this dissertation scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are used to investigate the structural and electronic properties of several materials that exist at the intersection of topology and magnetism such as topological insulators, van der Waals layered materials, two-dimensional magnets, and intrinsic magnetic topological insulators.Specifically, STM was used to confirm the growth quality and properties of thin films grown by molecular beam epitaxy including Bi2Se3, SnSe2, MnSe, and Fe3GeTe2. In the van der Waals heterostructure SnSe2/Bi2Se3 a moire pattern was observed which was found to be correlated to a set of localized electronic states in STS measurements. STM was also used to provide feedback for the growth of the heterostructure MnSe/Bi2Se3, which led to successful growth of partial monolayers of MnSe. In Fe3GeTe2, a magnetic field dependent Kondo lattice behavior was observed as well as a ferromagnetic hysteresis loop using spin-polarized STM.In addition, this work reports the first STM study of the recently experimentally verified intrinsic antiferromagnetic topological insulator (AFM TI) MnBi2Se4. Its atomic and layered properties are found to reasonably match theoretical predictions. Two different terminations of its layered structure are observed on the surface with distinct electronic properties. In-gap states are observed near some step edges which could be related to predicted topological edge states. Another AFM TI, the related material MnBi2Te4, was also studied in which nanoscale surface manipulation using an STM tip was demonstrated.
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