J. P. Buban

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta).

The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

The atomic origins of reduced critical currents at [001] tilt grain boundaries in YBa2Cu3O7−δ thin films

Abstract Grain boundaries have long been known to have a deleterious and irreproducible effect on the transport properties of high- T c oxide superconductors, particularly in the high-angle regime where an exponential decrease in critical current has been reported. We demonstrate, through a combination of atomic resolution Z-contrast imaging and bond valence sum analysis, that it is the atomic structure of the grain boundary that dominates this behavior. [001] tilt grain boundaries in thin-film YBa 2 Cu 3 0 7− δ are composed of arrays of dislocations in defined sequences. The resulting strain fields seriously perturb the local electronic structure, leading to a non-superconducting zone at the grain boundary. The width of this zone increases linearly with misorientation angle, naturally explaining the observed exponential decrease in critical current. In addition, the widely varying J c measurements for a given grain boundary misorientation can be naturally explained by the facetting of the grain boundary plane.

The influence of atomic structure on the formation of electrical barriers at grain boundaries in SrTiO3

An experimental atomic resolution analysis of an undoped Σ5 36° [001] tilt grain boundary in SrTiO3 shows that the structure contains incomplete oxygen octahedra. These incomplete octahedra act as effective oxygen vacancies and lead to a fixed, positive boundary charge. Annealing the boundary in the presence of MnO2 does not change the atomic structure of the boundary plane, and results in a high concentration of Mn3+ (acceptor) enrichment at the specific Ti4+ locations in closest proximity to the effective oxygen vacancies. This result can be explained in terms of standard charge compensation models and indicates that the formation of electrical barriers at oxide grain boundaries may be influenced by the atomic structure of the boundary plane.

Co-doped La0.5Sr0.5TiO3−δ: Diluted magnetic oxide system with high Curie temperature

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La0.5Sr0.5TiO3−δ). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (<∼2%), but develops inhomogeneity at higher cobalt concentrations. The films range from being opaque metallic to transparent semiconducting depending on the oxygen pressure during growth and are yet ferromagnetic.

Investigating the atomic scale structure and chemistry of grain boundaries in high-Tc superconductors

Abstract The short superconducting coherence length in high-Tc materials makes them extremely susceptible to the deleterious effect of atomic scale defects. Perhaps the most important of these defects for large-scale technological applications, are grain boundaries. Here we describe an atomic resolution investigation of structural and chemical changes that occur at grain boundaries in high-Tc materials using scanning transmission electron microscopy (STEM). STEM is ideally suited to this analysis, as atomic resolution Z-contrast images and electron energy loss spectra (EELS) can be acquired simultaneously. This permits a direct correlation between the structural images and the local electronic structure information in the spectrum. From this detailed experimental characterization of the grain boundaries, simple theoretical models can be derived that allow the structure-property relationships in high-Tc superconductors to be inferred. Results obtained from YBa2Cu3O7−δ and (Bi/Pb)2Sr2Ca2Cu3O10 show that there is a charge depletion zone formed at grain boundaries. This charge depletion zone can act as a tunnel barrier to the flow of superconducting charge carriers and appears to increase in width with increasing misorientation angle. The magnitude of the critical current across grain boundaries in high-Tc materials predicted from these models is in excellent agreement with the widely reported electrical transport results.

Simulating the oxygen K-edge spectrum from grain boundaries in ceramic oxides using the multiple scattering methodology

In this paper we demonstrate the use of the multiple scattering methodology to interpret oxygen K-edge spectra from both the bulk and grain boundaries in a variety of ceramic oxides. The experimental electron energy loss spectra (EELS) used in this study, were obtained from a dedicated scanning transmission electron microscope (STEM). Using the STEM to obtain the spectra has the advantage that each spectrum can be acquired with atomic spatial resolution. While the energy resolution is limited to approximately 0.8 eV, and the angular integration in the microscope apertures precludes momentum resolved spectroscopy, this unprecedented spatial resolution allows the electronic structure at individual defect sites to be determined. Additionally, as the microscope can also provide an atomic resolution image of the defect, the relationship between the atomic structure of the defect and its local electronic structure can be determined. In practice, this is achieved by using the structure observed in the image to build the real space atomic cluster for multiple scattering simulations. Detailed interpretation of the simulations of oxygen K-edge spectra from bulk MgO, CaO, SrTiO3, TiO2, MnO2, Mn3O4, Mn2O3 and MnO are presented. In addition, the simulations from grain boundaries in TiO2 (undoped) and SrTiO3 (undoped and Mn doped) are discussed in relation to quantifying the changes in the local electronic structure that are a direct consequence of the defect structure. The simulations are used to make interpretations of the structure-property relationships at these grain boundaries.

The Electronic Structure of Pristine and Doped (100) Tilt Grain Boundaries in SrTiO3

To understand the electronic properties of doped grain boundaries, we reviewed the atomic scale techniques currently available to study the electronic structure at pristine SrTiO3 grain boundaries. The knowledge gained from the pristine boundaries is used to interpret experimental and theoretical results from a Mn doped Σ5 SrTiO3 grain boundaries. Mn atoms are shown to preferentially substitute at specific Ti sites at the grain boundary core. Furthermore, the formal oxidation state of the Mn atoms at the grain boundary core was found to be reduced compared to the Mn atoms substituting for Ti in the bulk. This change of valence did not, however, significantly affect the atomic structure of the grain boundary, as determined by Z-contrast imaging and electron energy-loss spectroscopy, which revealed similar fine-structure features at both the doped and pristine grain boundary. We conclude, therefore, that composition and atomic structure have different effects on the local electronic structure and should be treated separately in any segregation and electrical conductivity models for grain boundaries.

Correlating Atomic Scale Experimental Observations to Develop Three-Dimensional Structural Models for Grain Boundaries in Oxides

: Determining the three-dimensional atomic structure of grain boundaries is a crucial first step toward understanding how these defects control the overall bulk properties of materials. In this report we discuss the correlation of experimental atomic resolution Z-contrast images and electron energy loss spectrometry (EELS) to achieve this goal. Initial structural analysis is afforded through empirical bond-valence potentials. This structure is then refined using multiple scattering analysis of the energy loss spectra. These techniques are demonstrated in the analysis of a 27 degrees MgO [001] tilt grain boundary. Through this analysis, we were able to determine specific atomic locations of Ca dopants found present at this grain boundary.

Auto-encoders for Noise Reduction in Scanning Transmission Electron Microscopy

In recent years, machine learning has been applied to a wide variety of fields with increasing success. With the current availability of cheap computing power and the advent of deep learning, the applications of several powerful neural network architectures, such as convolutional neural networks, recurrent neural networks and auto-encoding neural networks, have shown remarkable accuracy in classifying various data sets [1]. In particular, the auto-encoder (AE) architecture excels at recovering signal from noisy, or corrupted input data [2]. In the scanning transmission electron microscope (STEM), images are often plagued by various sources of noise. This is especially problematic for imaging beam sensitive materials where the scan speed is exceptionally fast and/or the beam current is very low [3]. Here, we investigate the applicability of using AEs for noise reduction in STEM images.