Relativistic and retardation effects in the two--photon ionization of hydrogen--like ions

The non-resonant two-photon ionization of hydrogen-like ions is studied in second-order perturbation theory, based on the Dirac equation. To carry out the summation over the complete Coulomb spectrum, a Green function approach has been applied to the computation of the ionization cross sections. Exact second-order relativistic cross sections are compared with data as obtained from a relativistic long-wavelength approximation as well as from the scaling of non-relativistic results. For high-Z ions, the relativistic wavefunction contraction may lower the two-photon ionization cross sections by a factor of two or more, while retardation effects appear less pronounced but still give rise to non-negligible contributions.Comment: 6 pages, 2 figure

Imaging the antiparallel magnetic alignment of adjacent Fe and MnAs thin films

The magnetic coupling between iron and alpha - MnAs in the epitaxial system Fe/MnAs/GaAs(001) has been studied at the sub-micron scale, using element selective x-ray photoemission electron microscopy. At room temperature, MnAs layers display ridges and grooves, alternating alpha (magnetic) and beta (non-magnetic) phases. The self-organised microstructure of MnAs and the stray fields that it generates govern the local alignment between the Fe and alpha - MnAs magnetization directions, which is mostly antiparallel with a marked dependence upon the magnetic domain size

A Ga-doped SnO2 mesoporous contact for UV stable highly efficient perovskite solar cells

Increasing the stability of perovskite solar cells is a major challenge for commercialization. The highest efficiencies so far have been achieved in perovskite solar cells employing mesoporous TiO2 (m-TiO2). One of the major causes of performance loss in these m-TiO2-based perovskite solar cells is induced by UV- radiation. This UV instability can be solved by replacing TiO2 with SnO2; thus developing a mesoporous SnO2 (m-SnO2) perovskite solar cell is a promising approach to maximise efficiency and stability. However, the performance of mesoporous SnO2 (m-SnO2) perovskite solar cells has so far not been able to rival the performance of TiO2 based perovskite solar cells. In this study, for the first time, high-efficiency m-SnO2 perovskite solar cells are fabricated, by doping SnO2 with gallium, yielding devices that can compete with TiO2 based devices in terms of performance. We found that gallium doping severely decreases the trap state density in SnO2, leading to a lower recombination rate. This, in turn, leads to an increased open circuit potential and fill factor, yielding a stabilised power conversion efficiency of 16.4%. The importance of high-efficiency m-SnO2 based perovskite solar cells is underlined by stability data, showing a marked increase in stability under full solar spectrum illumination