Impurity Lattice and Sublattice Location by Electron Channeling

A new formulation is presented for the use of crystallographic orientation effects in electron scattering to determine impurity lattice location. The development of electron channeling techniques is reviewed and compared to high energy ion channeling and to the Borrmann effect in x-ray diffraction. The advantages of axial over planar geometry are discussed. Delocalization effects are more serious for quantitative analysis than have generally been believed. The new formulation applies to any crystal lattice and quantitatively includes delocalization effects via c-factors, which have been experimentally determined for diamond structure semiconductors. For sublattice site location this formulation removes the two major approximations of the original ALCHEMI formulation, namely that all the inner shell excitations are perfectly localized, and that all of the impurity atoms occupy distinct crystallographic sites. As an example, we study the location of small perfectly coherent Sb precipitates within the Si lattice

Supercurrent through grain boundaries in the presence of strong correlations

Strong correlations are known to severely reduce the mobility of charge carriers near half-filling and thus have an important influence on the current carrying properties of grain boundaries in the high-$T_c$ cuprates. In this work we present an extension of the Gutzwiller projection approach to treat electronic correlations below as well as above half-filling consistently. We apply this method to investigate the critical current through grain boundaries with a wide range of misalignment angles for electron- and hole-doped systems. For the latter excellent agreement with experimental data is found. We further provide a detailed comparison to an analogous weak-coupling evaluation.Comment: 4 pages, 3 figure

Surface Stress, Morphological Development, and Dislocation Nucleation During SixGe1-x Epitaxy

Utilizing Ge marker layer experiments combined with atomic number contrast (Z-contrast) imaging, we have studied the evolving surface morphology of SixGe1-x alloys during growth by molecular beam epitaxy. The marker layers map out the instability transition between planar two-dimensional (2D) growth and three-dimensional (3D) growth. The transition occurs via the gradual formation of a surface ripple as anticipated from instability theory. However, these undulations rapidly develop into crack-like surface instabilities which we simulate and explain by the mechanism of stress-driven surface diffusion. Finally, we model the large stresses associated with these features within a fracture mechanics formalism. This analysis demonstrates that crack-like instabilities provide ideal candidate sites for the nucleation of misfit dislocations

Suppression of Octahedral Tilts and Associated Changes of Electronic Properties at Epitaxial Oxide Heterostructure Interfaces

Epitaxial oxide interfaces with broken translational symmetry have emerged as a central paradigm behind the novel behaviors of oxide superlattices. Here, we use scanning transmission electron microscopy to demonstrate a direct, quantitative unit-cell-by-unit-cell mapping of lattice parameters and oxygen octahedral rotations across the BiFeO3-La0.7Sr0.3MnO3 interface to elucidate how the change of crystal symmetry is accommodated. Combined with low-loss electron energy loss spectroscopy imaging, we demonstrate a mesoscopic antiferrodistortive phase transition and elucidate associated changes in electronic properties in a thin layer directly adjacent to the interface

Thickness dependence of the exchange bias in epitaxial manganite bilayers

Exchange bias has been studied in a series of La2/3Ca1/3MnO3 / La1/3Ca2/3MnO3 bilayers grown on (001) SrTiO3 substrates by ozone-assisted molecular beam epitaxy. The high crystalline quality of the samples and interfaces has been verified using high-resolution X-ray diffractometry and Z-contrast scanning transmission electron microscopy with electron energy loss spectroscopy. The dependence of exchange bias on the thickness of the antiferromagnetic layer has been investigated. A critical value for the onset of the hysteresis loop shift has been determined. An antiferromagnetic anisotropy constant has been obtained by fitting the results to the generalized Meiklejohn-Bean model.Comment: 7 pages. 8 figure

Spectroscopic imaging of single atoms within a bulk solid

The ability to localize, identify and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information is spatially confined around the scattering atom. Furthermore we show how the depth of the atom within the crystal may be estimated.Comment: 4 pages and 3 figures. Accepted in Phys.Rev.Let