Combined photo- and electroreflectance of multijunction solar cells enabled by subcell electric coupling

Electric coupling between subcells of a monolithically grown multijunction solar cell in short circuit allows their simultaneous and independent characterization by means of photo- and electroreflectance. The photovoltage generated by selective absorption of the pump beam in a given subcell during photoreflectance measurements results in reverse biasing the complementary subunits at the modulation frequency set on the pump illumination. Such voltage bias modulation acts then as external perturbation on the complementary subcells. The spectral separation of the different subcell absorption ranges permits the probe beam to record in a single spectrum the response of the complete device as a combination of photo- and electroreflectance, thereby providing access for diagnosis of subcells on an individual basis. This form of modulation spectroscopy is demonstrated on a GaInP/GaAs tandem solar cell.Comment: 5 pages, 4 figures. This article has been accepted by Appl. Phys. Lett. After it is published, it will be found at https://doi.org/10.1063/1.506260

Electron spin relaxation in graphene with random Rashba field: Comparison of D'yakonov-Perel' and Elliott-Yafet--like mechanisms

Aiming to understand the main spin relaxation mechanism in graphene, we investigate the spin relaxation with random Rashba field induced by both adatoms and substrate, by means of the kinetic spin Bloch equation approach. The charged adatoms on one hand enhance the Rashba spin-orbit coupling locally and on the other hand serve as Coulomb potential scatterers. Both effects contribute to spin relaxation limited by the D'yakonov-Perel' mechanism. In addition, the random Rashba field also causes spin relaxation by spin-flip scattering, manifesting itself as an Elliott-Yafet--like mechanism. Both mechanisms are sensitive to the correlation length of the random Rashba field, which may be affected by the environmental parameters such as electron density and temperature. By fitting and comparing the experiments from the Groningen group [J\'ozsa {\it et al.}, Phys. Rev. B {\bf 80}, 241403(R) (2009)] and Riverside group [Pi {\it et al.}, Phys. Rev. Lett. {\bf 104}, 187201 (2010); Han and Kawakami, {\it ibid.} {\bf 107}, 047207 (2011)] which show either D'yakonov-Perel'-- (with the spin relaxation rate being inversely proportional to the momentum scattering rate) or Elliott-Yafet--like (with the spin relaxation rate being proportional to the momentum scattering rate) properties, we suggest that the D'yakonov-Perel' mechanism dominates the spin relaxation in graphene. The latest experimental finding of a nonmonotonic dependence of spin relaxation time on diffusion coefficient by Jo {\it et al.} [Phys. Rev. B {\bf 84}, 075453 (2011)] is also well reproduced by our model.Comment: 13 pages, 9 figures, to be published in New J. Phy

A multiply substituted G-H loop from foot-and-mouth disease virus in complex with a neutralizing antibody: A role for water molecules

The crystal structure of a 15 amino acid synthetic peptide, corresponding to the sequence of the major antigenic site A (G-H loop of VP1) from a multiple variant of foot-and-mouth disease virus (FMDV), has been determined at 2·3 resolution. The variant peptide includes four amino acid substitutions in the loop relative to the previously studied peptide representing FMDV C-S8c1 and corresponds to the loop of a natural FMDV isolate of subtype C1. The peptide was complexed with the Fab fragment of the neutralizing monoclonal antibody 4C4. The peptide adopts a compact fold with a nearly cyclic conformation and a disposition of the receptor-recognition motif Arg-Gly-Asp that is closely related to the previously determined structure for the viral loop, as part of the virion, and for unsubstituted synthetic peptide antigen bound to neutralizing antibodies. New structural findings include the observation that well-defined solvent molecules appear to play a major role in stabilizing the conformation of the peptide and its interactions with the antibody. Structural results are supported by molecular-dynamic simulations. The multiply substituted peptide developed compensatory mechanisms to bind the antibody with a conformation very similar to that of its unsubstituted counterpart. One water molecule, which for steric reasons could not occupy the same position in the unsubstituted antigen, establishes hydrogen bonds with three peptide amino acids. The constancy of the structure of an antigenic domain despite multiple amino acid substitutions has implications for vaccine design

Tailoring the microstructure by a proper electric current control in flash sintering: The case of barium titanate

Flash sintering is arousing growing interest because high-density ceramics can be obtained at lower temperatures and shorter dwell times than conventional sintering. However, not only temperature and dwell times should be controlled during flash sintering but also parameters such as the electric field and electric current should be considered. Controlling all the parameters during the processing allows comprehensive control of the microstructure and, consequently, functional properties can be improved. In this work, it is evidenced that an exhaustive control of the flash electric current is a crucial factor for tailoring the microstructure of BaTiO3 ceramics. The results reveal that the most suitable way to control the sintering process is by using nonlinear current profiles because better densification and improved grain growth is achieved. Although the results focus on BaTiO3, this work offers a new pathway to tailor the microstructure of flash sintered ceramics, which may be extended to other materials

Particle size effect on the microstructure and the aging process of flash-sintered barium titanate from micro and nanopowders

Flash sintering is a novel sintering technique that allows high-density ceramics to be obtained at lowtemperatures and using short dwell times, thus providing an energy-efficient alternative to conventionalsintering. The microstructure of flash-sintered samples can be fine-tuned by a proper control ofelectrical parameters such as current density, electric field, and current profile, yielding significantimprovements of functional properties. The starting powder should also be carefully selected sincebetter sintering results are reported for smaller green grain sizes. However, this work evidences timeevolution of electrical properties of flash-sintered BaTiO3 ceramics from submicron powders. Theresults reveal that these transformations greatly depend on powder grain size and can be furtheradjusted with an adequate selection of electric power profiles. This work provides new insights intoongoing phenomena during field-assisted sintering, such as grain growth and defect formationdynamics. Although the results focus on BaTiO3, it offers a new pathway to tailor the microstructure offlash-sintered ceramics, which may be extended to other electronic materials

Morphological and Genetic Analysis of Four Color Morphs of Bean Leaf Beetle

Bean leaf beetle (BLB), Cerotoma trifurcata (Forster; Coleoptera: Chrysomelidae), exhibits considerable color variation but little is known about the underlying genetic structure and gene flow among color phenotypes. Genetic and morphological variation among four color phenotypes—green with spots (G+S), green without spots (G-S), red with spots (R+S) and red without spots (R-S)—were analyzed using amplified fragment length polymorphisms (AFLP) and morphometrics, respectively. AFLP generated 175 markers that showed ≥80% polymorphism. Analysis of molecular variance (AMOVA) indicated that genetic variation was greatest within phenotypes (82.6–84.0%); gene flow among the four phenotypes was relatively high (Nm = 3.82). The dendrogram and STRUCTURE analysis indicated some population divergence of G-S from the other phenotypes. Morphological parameters were similar among phenotypes except that R+S showed significant differences in weight and body-length. Canonical variables 1 and 2, based on average morphometric characters, accounted for 98% of the total variation; some divergence was indicated between G+S and R+S from each other and from the G-S/R-S BLB color morphs. The pattern of genetic variation indicated potential divergence of G-S and G+S from each other and from R-S and R+S. Although these results indicate that the four different color morphs are not genetically or reproductively isolated, there is some genetic differentiation/structure and morphological dissimilarity suggesting weak/incomplete isolation