There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Multiferroics: progress and prospects in thin films.

Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green’s-function equations, starting with a one-electron propagator and considering the electron-hole Green’s function for the response. Key ingredients are the electron’s self-energy S and the electron-hole interaction. A good approximation for S is obtained with Hedin’s GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of S. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green’s functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green’s functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress.

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Electronic excitations: density-functional versus many-body Green's-function approaches

Although the field of polymer solar cell has seen much progress in device performance in the past few years, several limitations are holding back its further development For instance, current high-efficiency (>90%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor Here we report the achievement of high-performance (efficiencies up to 108%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes The uncovered aggregation and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility

/pdf/aggregation-and-morphology-control-enables-multiple-cases-of-37iznlawg4.pdf

Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells.

Laser Heated Diamond Anvil Cell at the Advanced Light Source

An elliptically bent mirror of total length 1.25 m has been developed at the Advanced Light Source (ALS) for focusing soft x-rays. The mirror is used to produce a small, high flux density illuminated field of view for a Photo-Emission Electron Microscope. The requirement to collect the maximum horizontal aperture with the need to highly demagnify the source leads to a mirror with a wide range of curvatures along the surface. This combined with the need to produce a low slope error surface at a reasonably low cost has required us to develop a mirror based on the controlled bending of a flat substrate. This is an extension of several other mirror projects at the ALS where controlled bending of glass and metal substrates has been used in micro-focusing applications. Those mirrors however are a maximum of 200 mm long, and in this paper we describe the new challenges we have faced and the solutions we have adopted in developing a long and highly elliptical mirror. The mirror described here is manufactured from a low carbon steel (1006) which is capable of good dimensional stability, it is electroless nickel plated for polishing, and is bent into an elliptical shape by the application of unequal couples. We describe the mirror fabrication process, the mechanical details of the bending mechanism and the experimentally measured slope error from an ellipse. The final mirror has an rms roughness of 6 angstroms (rms), a full aperture (1.1 m) slope error of 14 (mu) rad (rms), and a slope error of < 3 (mu) rad when optimized over approximately 2/3 of the required optical length (0.917 m).

Construction and performance of a 1-m-long elliptically bent steel mirror

Towards Ultra-Smooth Alkali Antimonide Photocathode Epitaxy

Multialkali antimonide photocathodes have been shown to be excellent electron sources for a wide range of applications because of high quantum efficiency, low emittance, good lifetime, and fast response. In recent years, synchrotron X-ray methods have been used to study the growth mechanism of K2CsSb photocathodes. The traditional sequential growth of K2CsSb has been shown to result in rough surface, which will have an adverse impact on the emittance of the electron beam. However, coevaporation of alkali metals on the evaporated Sb layer and sputter deposition may offer a route to solving the roughness problem. Recent studies on K2CsSb grown by these methods are presented and surface roughness is determined by X-ray reflectivity (XRR) and results are compared.

Improving the Smoothness of Multialkali Antimonide Photocathodes: An In-Situ X-Ray Reflectivity Study

This paper presents preliminary data on analyses of selected materials using two state-ofthe- art XPS systems: the Physical Electronics Inc. (PHI, Eden Prairie, MN) Quantum 2000 instrument and the microXPS beamline (7.3.2.1) at the Advanced Light Source (ALS). This research compares and contrasts relevant performance characteristics of the two systems including elemental and chemical state detection sensitivity, imaging capabilities including lateral resolution and useful image fields, role of X-ray dose damage to surface, analysis speed as well as analytical throughput.

Howard A. Padmore

Papers

Laser Heated Diamond Anvil Cell at the Advanced Light Source

Construction and performance of a 1-m-long elliptically bent steel mirror

Towards Ultra-Smooth Alkali Antimonide Photocathode Epitaxy

Improving the Smoothness of Multialkali Antimonide Photocathodes: An In-Situ X-Ray Reflectivity Study

State-of-the-art x-ray photoelectron spectroscopy (XPS): conventional and synchrotron x-ray sources for micro-xps