N coordination chemistry in diluted InGaAs nitride layers

GaAsN and InGaAsN semiconductor alloys with a small amount of nitrogen, so called dilute nitrides, constitute a novel compounds family with applications in telecom lasers and very efficient multijunction solar cells. The incorporation of N, which has a much larger electronegativity and smaller atomic size compared to As, induces a strong structural distortion in the InGaAs coordination chemistry, which will also affect the material electronic structure and band-gap. In particular, the nearest-neighbour bonding configuration of the N in InGaAsN has proven its influence on the band-gap. Our ARXPS results demonstrate that a higher growth temperature favour the formation of In-N bonds.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. - MINECO through TEC2011-28639-C02-02 and TEC2014-54260-C3-3-P - Wroclaw University of Technology statutory gran

3D Topological Semimetal Phases of Strained α\alpha-Sn on Insulating Substrate

$\alpha$-Sn is an elemental topological material, whose topological phases can be tuned by strain and magnetic field. Such tunability offers a substantial potential for topological electronics. However, InSb substrates, commonly used to stabilize $\alpha$-Sn allotrope, suffer from parallel conduction, restricting transport investigations and potential applications. Here, the successful MBE growth of high-quality $\alpha$-Sn layers on insulating, hybrid CdTe/GaAs(001) substrates, with bulk electron mobility approaching 20000 cm$^2$V$^{-1}$s$^{-1}$ is reported. The electronic properties of the samples are systematically investigated by independent complementary techniques, enabling thorough characterization of the 3D Dirac (DSM) and Weyl (WSM) semimetal phases induced by the strains and magnetic field, respectively. Magneto-optical experiments, corroborated with band structure modeling, provide an exhaustive description of the bulk states in the DSM phase. The modeled electronic structure is directly observed in angle-resolved photoemission spectroscopy, which reveals linearly dispersing bands near the Fermi level. The first detailed study of negative longitudinal magnetoresistance relates this effect to the chiral anomaly and, consequently, to the presence of WSM. Observation of the $\pi$ Berry phase in Shubnikov-de Haas oscillations agrees with the topologically non-trivial nature of the investigated samples. Our findings establish $\alpha$-Sn as an attractive topological material for exploring relativistic physics and future applications.Comment: Main text: 35 pages, 7 figures; Supplementary Materials: 22 pages, 12 figure

Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of melting. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state.Comment: main manuscript 33 pages, 9 figures; supplemental material 19 pages, 13 figures - all in one fil

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm². Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm²) and liquid state (54 mJ/cm²), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes

Synthesis of core–shell silver–platinum nanoparticles, improving shell integrity

Silver–platinum core–shell (Ag@Pt) nanoparticles have been synthesized using various methods. In the case of Ag@Pt nanoparticles synthesized by the standard method based on the galvanic replacement reaction between Ag seeds and PtCl42−, transmission electron microscopy micrographs revealed well visible core–shell structure. However, electrochemical experiments showed that relatively large amount of silver can be easily stripped off from such nanoparticles. Significant improvement on the integrity of the deposited platinum shell can be achieved when nanoparticles are synthesized by the seeded growth reaction including reduction of PtCl42− with ascorbic acid at room temperature. To obtain pinhole-free platinum layers (where Ag oxidation is not observed) relatively large amount of platinum must be deposited. For example, to cover 11 nm Ag seeds, the number of moles of platinum in the formed Ag@Pt nanoparticles must be at least equal to the number of moles of Ag. It was also found that a similar seeded growth reaction may be used to form pinhole-free Au@Pt nanoparticles. The electrochemical behaviour of those two systems (Ag@Pt and Au@Pt nanoparticles) towards CO stripping was rather different. While the CO-stripping on Au@Pt occurred at typical potentials and without a significant reconstruction of the original surface, CO stripping voltammograms on Ag@Pt were very unusual and exhibited both exceptionally strong binding of CO to the surface, and such a reconstruction of the surface that silver atoms were no longer compactly covered by platinum.This project was financed from the funds of the National Science Centre (Poland) allocated on the basis of the decision number DEC-2011/01/B/ST4/00581 and co-financed by the European Regional Development Fund within the Innovative Economy Operational Programme 2007–2013 No POIG.02.01-00-14-032/08. S.W. is thankful for the support from the Foundation for Polish Science MPD Programme co-financed by the EU European Regional Development Fund