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

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

A smart pixel detector is being developed for time‐resolved crystallography for biological and material science applications. Using the pixel detector presented here, the Laue method will enable the study of the evolution of structural changes that occur within a protein as a function of time. The x‐ray pixellated detector is assembled to the integrated circuit through a bump bonding process. Within a pixel size of 150×150 mm2, a low‐noise preamplifier‐shaper, a discriminator, a 3‐bit counter and the readout logic are integrated. The readout, based on the column architecture principle, will accept hit rates above 5×108/cm2/s with a maximum hit rate per pixel of 1 MHz. This detector will allow time‐resolved Laue crystallography to be performed in a frameless operation mode, without dead time. Target specifications, architecture, and preliminary results on a 8×8 prototype are presented.

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A 2D smart pixel detector for time‐resolved crystallography

The work of the photocathode R&D program at LBNL is presented. The quantum efficiency (QE) of Cu(111) is measured for different impinging light angles with photon energies just above the work function. We observe that the vectorial photoelectric effect, an enhancement of the quantum efficiency due to illumination of light with an electric vector perpendicular to the sample surface, is stronger in the more surface sensitive regime. This can be explained by a contribution to photoemission due to the variation of the electromagnetic potential at the surface. The contributions of bulk and surface electrons can then be determined. Angle-resolved photoemission data let us obtain the dispersion relation of the surface state, which in turn allows us to determine the thermal emittance of the electrons emitted from the surface.

Photocathode R&D Program at LBNL

The ALS, a third‐generation synchrotron light source at Berkeley Lab, has been operating for almost a decade and is generating forefront science by exploiting the high brightness of a third‐generation source in three areas: (1) high resolving power for spectroscopy; (2) high spatial resolution for microscopy and spectromicroscopy; and (3) high coherence for experiments such as speckle. However, the ALS was one of the first third‐generation machines to be designed, and accelerator and insertion‐device technology have significantly changed since its conception. As a result, its performance will inevitably be outstripped by newer, more advanced sources. To remain competitive and then set a new standard, the performance of the ALS, in particular its brightness, must be enhanced. Substantial improvements in brightness and current have always been feasible in principle, but they incur the penalty of a much reduced lifetime, which is totally unacceptable to our users. Significant brightness improvements can be realized in the core soft x‐ray region by going to top‐off operation, where injection would be quasi‐continuous and the lifetime objections disappear. In top‐off mode with higher average current, a reduced vertical emittance and beta function, and small‐gap permanent‐magnet or superconducting insertion devices, one to two orders of magnitude improvement in brightness can be had in the soft x‐ray range. These improvements also extend the high energy range of the undulator radiation beyond the current limit of 2000 eV. Descriptions of the upgrade and the important new science achievable are presented.

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The Advanced Light Source Upgrade

Investigation of magnetic materials using a new x-ray photoemission electron microscope

One of the principal industry standard means of measuring surface and near surface wafer contamination is the total reflection X-ray fluorescence (TXRF). A TXRF experiment using synchrotron radiation is carried out at the Advance light source (ALS) beamline 10.32 to investigate the performance of synchrotron radiation based approaches for this application. The detection limit of 4.9×109 atoms/cm2 is achievable for the 3-d elements with the present ALS TXRF beamline 10.32. This is due to the greatly improved signal to background in case of the synchrotron radiation TXRF. Furthermore, there is a path to improving the synchrotron case to reach a detection limit of 7.8 × 107 atoms/cm2.

Howard A. Padmore

Papers

A 2D smart pixel detector for time‐resolved crystallography

Photocathode R&D Program at LBNL

The Advanced Light Source Upgrade

Investigation of magnetic materials using a new x-ray photoemission electron microscope

Total reflection X-ray fluorescence spectroscopy using synchrotron radiation for trace impurity analysis of silicon wafer surfaces