Search

We present a novel machine learning method for predicting the baryonic properties of dark matter only subhalos from N-body simulations. Our model is built using the extremely randomized tree (ERT) algorithm and takes subhalo properties over a wide range of redshifts as its input features. We train our model using the IllustrisTNG simulations to predict blackhole mass, gas mass, magnitudes, star formation rate, stellar mass, and metallicity. We compare the results of our method with a baseline model from previous works, and against a model that only considers the mass history of the subhalo. We find that our new model significantly outperforms both of the other models. We then investigate the predictive power of each input by looking at feature importance scores from the ERT algorithm. We produce feature importance plots for each baryonic property, and find that they differ significantly. We identify low redshifts as being most important for predicting star formation rate and gas mass, with high redshifts being most important for predicting stellar mass and metallicity, and consider what this implies for nature vs nurture. We find that the physical properties of galaxies investigated in this study are all driven by nurture and not nature. The only property showing a somewhat stronger impact of nature is the present-day star formation rate of galaxies. Finally we verify that the feature importance plots are discovering physical patterns, and that the trends shown are not an artefact of the ERT algorithm., Comment: Accepted to MNRAS, 15 pages, 8 figures, 2 tables, Main Figure is Fig 5

Summary In this paper, we discuss the application of the maximum entropy method to atomic resolution Z-contrast images acquired in a scanning transmission electron microscope. Zcontrast is an incoherent imaging technique, and can be described as a convolution between an object function (the real-space map of the columnar scattering cross-section to high angles) and a point spread function (the effective electron probe). As such, we show that the technique is ideally suited to maximum entropy analysis which can, given an electron probe distribution, retrieve the ‘most likely’ Z-contrast object function. Using both simulated and experimentally acquired data, we explore the capabilities of maximum entropy analysis when applied to atomic resolution Z-contrast images, drawing conclusions on both the range of applicability of the technique and the nature of the retrieved crystal structures. Ultimately, we show the way in which the combination of Z-contrast imaging with maximum entropy analysis can be used to yield important information on unexpected atomic structures.

The size and orientation dependent micromagnetic structures of epitaxial Fe(001) thin film elements with in-plane anisotropy are reported. A transition from single domain to multidomain remanent states is observed upon reducing the element size beneath $\ensuremath{\sim}50\ensuremath{\mu}\mathrm{m}$, indicating that the in-plane dipolar field becomes competitive with the magnetocrystalline anisotropy at this size. Because of this competition, distinct micromagnetic structures arise according to the orientation of the element edges. The epitaxial elements are of high structural quality allowing the micromagnetic behavior to be controllably modified.

The scanning transmission electron microscope is capable of forming an electron probe of atomic dimensions, allowing incoherent imaging and microanalysis to be achieved at atomic resolution. Images formed using elastically scattered electrons show strong atomic number ( Z ) contrast and reveal atomic column locations without the need for preconceived structure models. Column positions and scattering power can be directly retrieved by a maximum entropy analysis. The Z -contrast image also allows the probe to be positioned with atomic precision over selected atomic columns or planes, making possible atomic-resolution electron energy loss spectroscopy. Interfacial structure and bonding can both be determined directly from experimental data, often leading to unexpected insights. In SrTiO 3 , grain boundary structural units were identified, and found to be reconstructed, an efficient way to overcome the problem of like-ion repulsion in ionic materials. At a CdTe-GaAs [001] interface, 60° dislocations were directly identified to be of glide type, while in the case of Lomer dislocations, an unexpected core structure was found comprising a fourfold ring structure. With these techniques, dislocation core structure and impurity segregation may be studied experimentally at the atomic scale.

The atomic structures of 25° and 45° asymmetric [001] tilt boundaries in SrTiO3 have been determined experimentally using the unique combination of high-resolution Z-contrast imaging and simultaneous electron-energy-loss spectroscopy in the scanning transmission electron microscope. These complementary techniques permit the direct location of the cation columns at the boundary to an accuracy of 0.2A and the coordination of the O atoms to be investigated with atomic resolution. Using bond-valence sum calculations, the structural and electronic results were combined to produce models for the grain-boundary structures. These calculations highlighted the presence of half-occupied columns at both grain boundaries as an important feature for reducing the grain-boundary energy associated with these ionic materials. In addition, the grain-boundary structure units identified in the experimental images were used to predict structures for other misorientations by the application of the principles of continu...

Z-contrast STEM using an annular detector can provide an intuitively interpretable, column-by-column, compositional map of crystals. Incoherent imaging reduces dynamical effects to second order so that the map directly reflects the positions of the atomic columns and their relative high-angle scattering power. This article outlines how these characteristics arise, presents some examples of the insights available from a direct image, and discusses recent developments of atomic-resolution microanalysis, direct structure retrieval by maximum entropy analysis, and Z-contrast imaging at 1.4 Å resolution using a 300-kV STEM.

The scanning transmission electron microscope (STEM) provides a route for the determination of interface structure and bonding directly from experimental data. Through an annular detector, Z-contrast images reveal atomic column locations without prior knowledge. The incoherent nature of such images allows a direct structure inversion through a maximum entropy analysis. The Z-contrast image also facilitates atomic-resolution spectroscopy by allowing the probe to be positioned with atomic precision. With this combination of atomic-resolution imaging and spectroscopy, structural units for [001] tilt grain boundaries in SrTiO{sub 3} were identified. All units revealed the presence of half-filled columns, an efficient way to overcome the problem of like-ion repulsion in ionic materials. With the 1.3 {angstrom} probe of the 300-kV STEM, an unexpected core structure has been found for Lomer dislocations at a CdTe/ GaAs [001] interface, while 60{degrees} dislocations were directly identified to be of glide type.

Incoherent Z-contrast imaging uses a high-angle annular detector to collect only highly local, incoherently generated scattering with the result that images become dependent on intensities, not phases. No model structures are required for a first-order structure determination, and the images remain intuitively interpretable even at interfaces. Under suitable conditions, incoherently generated inelastic scattering may be collected simultaneously with a large-aperture axial spectrometer, and, by using the Z-contrast image to locate the scanning transmission electron microscope (STEM) probe over selected atomic columns, can provide an atomic-resolution chemical analysis. This is demonstrated with reference to an epitaxial CoSi2/Si(100) interface, achieving a 2.7 A spatial resolution. Recent insights into the growth and relaxation of strained Si-Ge epitaxial films are described, highlighting the role of stress concentrations, and contrasting the case of a free surface with that of a surface constrained by an oxide layer.

The Microstructural Physics group at the Cavendish Laboratory is actively involved in a considerable number of research projects which cover a broad range of materials science. In this paper, we describe briefly several such projects, with particular emphasis given to the application of parallel-detection electron energy loss spectroscopy (PEELS) on a scanning transmission electron microscope (STEM) to the analysis of materials such as stainless steels, catalysts, and high temperature superconductors. In addition, we describe a number of related projects that are currently being carried out in the group, particularly those which utilise and develop novel STEM imaging and analytical techniques. © 1993 Wiley-Liss, Inc.

Nanometre-scale copper-rich precipitates in irradiated ferritic pressure vessel weld surveillance samples have been imaged directly in the field emission gun scanning transmission electron microscope. This is achieved by using the atomic number sensitivity of the high-angle elastically scattered electron signal. Microanalysis after location using this technique indicates that the precipitates contain phosphorus and manganese as well as copper. Preliminary results show that the smallest (sub-2 nm) precipitates to be observed have a high content of phosphorus and manganese relative to copper while in samples with larger precipitate sizes, in which thermal effects have been significant, copper predominates.

A strategy is presented for determining sublattice polarity at defects in compound semiconductors. Core structures of 60-degree and Lomer dislocations in the CdTe/GaAs(001) system have been obtained by the application of maximum-entropy analysis to Z-contrast images (Z is atomic number) obtained in a 300-kilovolt scanning transmission electron microscope. Sixty-degree dislocations were observed to be of the glide type, whereas in the case of Lomer dislocations, both a symmetric (Hornstra-like) core and an unexpected asymmetric structure made up of a fourfold ring were seen.

The optical arrangement of the high-resolution scanning transmission electron microscope (STEM) allows incoherent imaging conditions to be established simultaneously for both elastically and inelastically scattered electrons, with good detection efficiencies. For zone axis illumination and localized inelastic transitions, both images show a resolution governed by the incident probe intensity profile, with maximum intensity occurring when the probe is located over the atomic columns. The high intensity elastic signal may therefore be used as an atomic resolution reference image for the low intensity inelastic signal, allowing spectroscopy to be achieved from selected atomic columns or planes. The potential of this approach in a 300-kV STEM is discussed. This technique was used to determine the interfacial structure in SrTiO{sub 3} and ZrO{sub 2}/NiO.

Incoherent Z-contrast imaging in the scanning transmission electron microscope allows atom column positions to be deduced directly from the experimental image, including locations where the column separation is less than the resolution limit. Maximum entropy analysis applied to the incoherent image locates the high-Z columns to an accuracy of ±0.2 A. Oxygen coordination at the boundary plane can be deduced by high spatial resolution electron energy loss spectroscopy, and approximate column positions determined by simple bond-valence sum calculations. Observations of 25° (Σ=85), 36° (Σ=5) and 67° (Σ=13) [001] symmetric tilt grain boundaries in SrTiO3 bicrystals show that “half columns” are a ubiquitous feature of grain boundary structural units. The observed structural units can be combined to produce structural models for symmetric tilt boundaries over a 0–90° range. The Σ=17 (410), Σ=5 (310), and Σ=5 (210) are found to be favored boundaries and the structures of all the other tilt boundaries are comprised of these units combined with Σ=1 (100) and Σ=1 (110) structural units. All the proposed boundary models show continuity of grain boundary structure over the entire misorientation range. The Σ=17 (410) structural unit is asymmetric which induces microfacetting on all boundaries less than the Σ=5, 36.87° misorientation.

An atomic structure model for a 25 degrees [001] symmetric tilt grain boundary in SrTiO(3) has been determined directly from experimental data with the use of high-resolution Z-contrast imaging coupled with electron energy loss spectroscopy. The derived model of the grain boundary was refined by bond-valence sum calculations and reveals candidate sites for dopant atoms in the boundary plane. These results show how the combined techniques can be used to deduce the atomic structure of defects and interfaces without recourse to preconceived structural models or image simulations.

This paper presents directly interpretable atomic resolution images of dislocation structures at interfaces in CdTe/GaAs(001) systems. This is achieved using the technique of Z-contrast imaging in a 300 kV scanning transmission electron microscope in conjunction with maximum entropy image analysis. In addition to being used to further the understanding of the relation between growth conditions and exhibited properties, the data presented provides direct information on the atomic arrangements at dislocation cores.

In the electroceramic SrTiO3 the grain boundary atomic structure governs a variety of electrical properties such as non-linear I-V characteristics. An understanding of this atomic structure-property relationship for individual grain boundaries requires a technique which probes both composition and chemical bonding on an atomic scale. Atomic structure models for [001] tilt boundaries in SrTiO3 bicrystals have been determined directly from experimental data, by combining high-resolution Z-contrast imaging to locate the cation columns at the boundary, with simultaneous electron energy loss spectroscopy to examine light element coordination at atomic resolution. In this paper we compare and contrast the grain boundary structure models of symmetric and asymmetric boundaries in SrTiO3.

By averaging phase correlations between scattered electrons a high angle detector in the scanning transmission electron microscope (STEM) can provide an incoherent, Z-contrast image at atomic resolution. Phase coherence is effectively destroyed through a combination of detector geometry (transverse incoherence) and phonon scattering (longitudinal incoherence). Besides having a higher intrinsic resolution, incoherent imaging offers the possibility of robust reconstruction to higher resolutions, provided that some lower frequency information is present in the image. This should have value for complex materials and regions of complex atomic arrangements such as grain boundaries. Direct resolution of the GaAs sublattice with a 300kV is demonstrated.

The Microstructural Physics group at the Cavendish Laboratory is actively involved in a considerable number of research projects which cover a broad range of materials science. In this paper, we describe briefly several such projects, with particular emphasis given to the application of parallel-detection electron energy loss spectroscopy (PEELS) on a scanning transmission electron microscope (STEM) to the analysis of materials such as stainless steels, catalysts, and high temperature superconductors. In addition, we describe a number of related projects that are currently being carried out in the group, particularly those which utilise and develop novel STEM imaging and analytical techniques.

By applying the technique of Z-contrast imaging to the study of a GaAs/AlGaAs multilayer using a newly developed 300kV scanning transmission electron microscope, we show that it is possible to directly observe the interlocking group III and group V sub-lattices on a column-by-column level. In addition to the direct observation of structural polarity in the [110] orientation, we show that, by using a maximum entropy approach to image processing, the experimentally acquired data can provide direct information on interface structures at atomic resolution.

The study of materials by electron microscopy has traditionally been divided into two sub-fields, imaging and microanalysis. While both of these have advanced to the level of atomic sensitivity, a complete characterization of a materials structure, composition and bonding requires a correlation between the two. This correlation has been achieved through the combination of Z-contrast imaging and electron energy loss spectrocopy (EELS) in the scanning transmission electron microscope (STEM). For the highly coherent and convergent probe in the STEM, the scattering to a high-angle detector (40- 150 mrad) shows an intensity which is proportional to the Z2-dependence of the Rutherford scattering cross-section. The resultant "Z-contrast" image, which is generated point-by-point as the probe is scanned over the surface of the specimen, can be described as a simple convolution of the probe intensity profile, P2eff (R), and a specimen object function O(R,t), This incoherent description of image intensity holds for crystalline materials in zone-axis orientations where dynamical diffraction manifests itself as a columnar channeling effect.

High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted intuitively without the need for preconcieved atomic structure models. Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale. The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface. In this paper we use the complimentary techniques of high resolution Z-contrast imaging and PEELS to investigate the atomic structure and chemistry of a 25 degree symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.Figure 1(a) shows a Z-contrast image of a symmetric region of the tilt boundary. The brightest spots in the image correspond to the increased scattering power of the Sr atomic columns (Z=38) with theless bright spots corresponding to the Ti atomic columns (Z=22). The lighter O atomic columns are notvisible in a Z-contrast image.

The stress rupture properties of high nickel austenitic alloys and austenitic stainless steels used in the nuclear industry are strongly affected under irradiation by the formation of helium from boron by means of an n-α reaction. As the congregation and growth of helium bubbles (typically 20-50nm diameter) at grain boundaries can cause embrittlement in stressed steels, it is important to gain an understanding of embrittlement mechanisms in the material. Consequently, an accurate measurement of quantities such as bubble pressure and metal surface energy is highly desirable. However, this can only be achieved if the number of helium atoms per unit volume is known. In this work, we report on the use of parallel-detection electron energy loss spectroscopy (PEELS) on a VG HB501 scanning transmission electron microscope (STEM) as a powerful technique for characterising helium bubbles in irradiated steels. In addition, we suggest a simple analytical process that could be used to quantify the amount of helium present in each bubble.

By employing the technique of Z‐contrast imaging in a 300 kV scanning transmission electron microscope, we show that it is possible to obtain directly interpretable, atomic resolution images of interface structures in semiconductor materials. With particular emphasis on the study of the CdTe/GaAs(001) system, we demonstrate that such an approach enables the direct observation of the sublattice in compound semiconductors and, as a direct consequence of this, facilitates the observation of atomic arrangements (in some cases unexpected) at dislocation cores on the column‐by‐column level. In addition, we demonstrate that the technique can also be applied to the study of grain boundary structures, by showing examples taken from boundaries in Si and SrTiO3.

X‐ray microanalysis in a scanning transmission electron microscope can be employed to determine concentration profiles with a spatial resolution of ≲2 nm across layer interfaces in III‐V compound semiconductor multilayer structures. Here we describe its application to the InGaAs/InP multilayer system grown by metalorganic chemical vapor deposition at atmospheric pressure. The results show that even when the material is structurally perfect, there can be a significant As and Ga content beyond the boundaries of the InGaAs layers. Furthermore, the composition profiles can show a marked asymmetry relating to the growth direction. The results are consistent with previous imaging observations but provide quantitative information with high spatial resolution on individual element distributions. This combination of analytical attributes is not available using any other assessment technique.

This paper is concerned with the influence of different estimation procedures on the data for real output by industry group of a number of O.E.C.D. countries. The authors have examined the methods and indicators used in preparing sector real output data and have tried to assess the effect of the different methods on the recorded changes of sector real output. The data for real output, employment and productivity are compared for the different sectors and countries. The comparison between sectors lays particular emphasis on the dichotomy between the services and non-service sectors of the economy. In this comparison as well as in inter-country comparison it is seen that the data are influenced to a considerable extent by different methods. The survey of estimation methods also shows the incidence of use of double deflation techniques and other methods in the different countries, and the extent to which quality change, output specification and valuation problems are reflected in the different methods.

The macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models (1). Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale (2). The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface (3). In this paper we use the complimentary techniques of high resolution Zcontrast imaging and PEELS to investigate the atomic structure and chemistry of a 25° symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.

Theo scanning transmission electron microscope can now routinely produce a probe of atomic dimensions, 2.2Å at 100 kV and 1.3Å at 300 kV. This capability opens up a new strategy for determining the structure and bonding of materials at the atomic scale, which is particularly advantageous for complex interfaces such as grain boundaries. The Z-contrast image allows the high-Z column locations to be determined by direct inspection, and quantified through maximum entropy methods to a positional accuracy of currently ~0.2Å. With the same electron probe, located to sub-Ångstrom precision with reference to the Z-contrast image, parallel detection EELS can determine light element coordination from core edge fine structure, in principle around a single atomic column.The attractive features of this approach are the increased resolution associated with incoherent imaging, 0.44 Cs1/4 λ3/4 compared with 0.66 Cs1/4 λ3/4 for phase contrast imaging, and the highly local nature of the image, which makes possible a greatly extended regieme of intuitive image interpretation, and also permits column-by-column spectroscopy without beam broadening.

In seeking to understand society, sociologists in this set sought to understand social institutions. Here sociologists consider the policy and planning implications of attempts to respond to and meet social needs by the Church, Civil Service, Industry and Voluntary Organizations. This set contains such classic works as Penelope Hall's 'Social Services of Modern England' and Peter Nokes' 'The Professional Task in Welfare Practice'.