Compressed sensing electron tomography using adaptive dictionaries: a simulation study

Electron tomography (ET) is an increasingly important technique for examining the three-dimensional morphologies of nanostructures. ET involves the acquisition of a set of 2D projection images to be reconstructed into a volumetric image by solving an inverse problem. However, due to limitations in the acquisition process this inverse problem is considered ill-posed (i.e., no unique solution exists). Furthermore reconstruction usually suffers from missing wedge artifacts (e.g., star, fan, blurring, and elongation artifacts). Compressed sensing (CS) has recently been applied to ET and showed promising results for reducing missing wedge artifacts caused by limited angle sampling. CS uses a nonlinear reconstruction algorithm that employs image sparsity as a priori knowledge to improve the accuracy of density reconstruction from a relatively small number of projections compared to other reconstruction techniques. However, The performance of CS recovery depends heavily on the degree of sparsity of the reconstructed image in the selected transform domain. Prespecified transformations such as spatial gradients provide sparse image representation, while synthesising the sparsifying transform based on the properties of the particular specimen may give even sparser results and can extend the application of CS to specimens that can not be sparsely represented with other transforms such as Total variation (TV). In this work, we show that CS reconstruction in ET can be significantly improved by tailoring the sparsity representation using a sparse dictionary learning principle

The Influence Of Eighteenth-century British Landscape Aesthetics On Narrative And Pictorial Responses To The British North American North And West 1769-1872

This study undertakes a consideration of a century of travel writing by Britons who explored, surveyed, traded, hunted, prospected, botanized, and established missions in the British North American North and West between 1769 and 1872. Its particular concern is how Britons employed the principles and conventions of eighteenth-century British landscape aesthetics to describe and depict northern and western terrain.;An aesthetic mode of perceiving nature, as has been argued by perceptual geographers, art historians, and literary critics, constitutes one way that a society forms it understanding of reality. For the eighteenth- and early-nineteenth-century Briton, the Sublime and the Picturesque were the aesthetic modes of perception by which he described nature. When he travelled, these made up his aesthetic baggage. Their application to new lands told him where he stood aesthetically in relation to home landscapes just as his measurements of latitude and longitude told him where he stood spatially in relation to Greenwich.;Chapter I traces the formulation of the Sublime and the Picturesque through British aesthetic philosophy, painting, landscape gardening, and poetry. Chapter II deals with the journals of British mariners who searched for a Northwest Passage from 1819 to 1859, and includes discussions of works by Arctic fur traders. Chapter III treats the journals of Britons who travelled in the West. In each case, an attempt has been made to compile complete bibliographies of works published by the travellers or by modern editors.;Each work\u27s narrative and pictorial descriptions of landscape are considered for their adherence to and departure from the conventions of the Sublime and the Picturesque. As well, historical and biographical details of the author\u27s situation are considered when they impinge on his response to landscape. Although the understanding of the Sublime and the Picturesque was widespread, it was affected by other concerns. The fur trader looked for landscape qualities that promised the presence of beaver in a terrain. The prospector, the artist, the hunter, and the mariner sought particular landscape qualities as well. Taken together, the journals compile an aesthetic map of two regions in modern Canada

Redistribution, state trading enterprises and ‘politically optimal’ tariffs

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.State trading enterprises (STEs) are widely used and can be viewed as instruments of trade policy. We analyse two aspects: the first is their potential trade distorting effect; the second is how they modify the case for the ‘politically optimal’ tariff. We show that the STE can reduce the need for a tariff designed for domestic redistribution. This result introduces some ambiguity about how STEs are interpreted: from a multilateralist perspective, they should be dealt with in the same way as other non-tariff barriers; from a nationalistic perspective, they can reduce the need for ‘politically optimal.’ State trading enterprises (STEs) are widely used and can be viewed as instruments of trade policy. We analyse two aspects: the first is their potential trade distorting effect; the second is how they modify the case for the ‘politically optimal’ tariff. We show that the STE can reduce the need for a tariff designed for domestic redistribution. This result introduces some ambiguity about how STEs are interpreted: from a multilateralist perspective, they should be dealt with in the same way as other non-tariff barriers; from a nationalistic perspective, they can reduce the need for ‘politically optimal.

Correction of EELS dispersion non-uniformities for improved chemical shift analysis

We outline a simple routine to correct for non-uniformities in the energy dispersion of a post-column electron energy-loss spectrometer for use in scanning transmission electron microscopy. We directly measure the dispersion and its variations by sweeping a spectral feature across the full camera to produce a calibration that can be used to linearize datasets post-acquisition, without the need for reference materials. The improvements are illustrated using core excitation electron energy-loss spectroscopy (EELS) spectra collected from NiO and diamond samples. The calibration is rapid and will be of use in all EELS analysis, particularly in assessments of the chemical states of materials via the chemical shift of core-loss excitations

Determining the Anisotropic Exchange Coupling of CrO_2 via First-Principles Density Functional Theory Calculations

We report a study of the anisotropic exchange interactions in bulk CrO_2 calculated from first principles within density functional theory. We determine the exchange coupling energies, using both the experimental lattice parameters and those obtained within DFT, within a modified Heisenberg model Hamiltonian in two ways. We employ a supercell method in which certain spins within a cell are rotated and the energy dependence is calculated and a spin-spiral method that modifies the periodic boundary conditions of the problem to allow for an overall rotation of the spins between unit cells. Using the results from each of these methods, we calculate the spin-wave stiffness constant D from the exchange energies using the magnon dispersion relation. We employ a Monte Carlo method to determine the DFT-predicted Curie temperature from these calculated energies and compare with accepted values. Finally, we offer an evaluation of the accuracy of the DFT-based methods and suggest implications of the competing ferro- and antiferromagnetic interactions.Comment: 10 pages, 13 figure

‘Fair trade’ coffee and the mitigation of local oligopsony power

In recent years, there has been considerable growth in ‘fair trade’ markets for several commodities, most notably coffee. We argue that coffee is grown under conditions that might well subject growers to the market power of downstream intermediaries (processors). Using an approach designed to evaluate the effects of state trading enterprises on trade and welfare, we develop an oligopsony model of intermediaries. In this model, fair-trade processors optimise a welfare function that includes the producer surplus of coffee growers. This concern for growers' welfare among some processing firms helps to alleviate the market power distortion. We calibrate the model to price data reported by a fair-trade organisation and consider the counterfactual removal of fair-trade behaviour by processors. As expected, the income of coffee growers (in aggregate) is reduced, though the effects are quite small