Perspective on the physics of two-dimensional perovskites in high magnetic field

Two-dimensional (2D) metal halide perovskites consist of atomically thin layers composed of low bandgap metal-halide slabs, surrounded by high bandgap organic ligands, which behave as barriers. In this Perspective, we highlight how the use of large magnetic fields has been an extremely insightful tool to unravel some of the fundamental electronic properties of 2D perovskites. We focus on the combination of magnetoabsorption measurements and theoretical modeling to extract the carrier effective mass, on the use of magnetic field to clarify the fine structure of the exciton manifold, and on how magnetic fields can be helpful to correctly assign side peaks in the complex absorption or photoluminescence spectra displayed by 2D perovskites. We finally point out some challenges which might be successfully addressed by magneto-optical experimental techniques

Non equilibrium anisotropic excitons in atomically thin ReS2_2

We present a systematic investigation of the electronic properties of bulk and few layer ReS$_2$ van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by $\sim$ 20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS$_2$ to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases

Probing the inter-layer exciton physics in a MoS2_2/MoSe2_2/MoS2_2 van der Waals heterostructure

Stacking atomic monolayers of semiconducting transition metal dichalcogenides (TMDs) has emerged as an effective way to engineer their properties. In principle, the staggered band alignment of TMD heterostructures should result in the formation of inter-layer excitons with long lifetimes and robust valley polarization. However, these features have been observed simultaneously only in MoSe$_2$/WSe$_2$ heterostructures. Here we report on the observation of long lived inter-layer exciton emission in a MoS$_2$/MoSe$_2$/MoS$_2$ trilayer van der Waals heterostructure. The inter-layer nature of the observed transition is confirmed by photoluminescence spectroscopy, as well as by analyzing the temporal, excitation power and temperature dependence of the inter-layer emission peak. The observed complex photoluminescence dynamics suggests the presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We show that circularly polarized optical pumping results in long lived valley polarization of inter-layer exciton. Intriguingly, the inter-layer exciton photoluminescence has helicity opposite to the excitation. Our results show that through a careful choice of the TMDs forming the van der Waals heterostructure it is possible to control the circular polarization of the inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters (http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184

The Instrument Set for Generating Fast Adiabatic Passage

The design and construction of a high-performance, low-cost, and easy to assemble adiabatic extension set for homebuilt and commercial spectrometers is described. Described apparatus set was designed for the fast adiabatic passage generation and is based on direct digital synthesizer DDS. This solution gives generator high signal to noise ratio, phase stability even during frequency change which is only possible in expansive commercial high-end hardware. Critical synchronization and timing issues are considered and solutions are discussed. Different experimental conditions and techniques for the measurements are briefly discussed. The proposed system is very flexible and might be used for the measurement of low-frequency nuclear magnetic resonance

Individual, social and environmental determinants of sleep among women : protocol for a systematic review and meta-analysis

Introduction: Sleep is important to promote optimal health and avoid negative health outcomes. Short-duration and low-quality sleep may be more common and more detrimental among women compared with men. Identifying the determinants of behaviour is one of the first steps in designing effective interventions. To our knowledge, no systematic review has identified the individual, social and environmental determinants of sleep among adult women. Methods and analysis: Studies reporting data on adult women from 18 to 64 years of age will be included. On the basis of ecological models of health behaviour and sleep, the types of determinants that will be included in the review are individual (eg, demographic, psychological and behavioural), social (eg, family) and environmental (eg, physical environment and policies) determinants. Observational (cross-sectional and longitudinal) and experimental studies will be included. MEDLINE/PubMed, PsycINFO, CINAHL, EMBASE and Proquest Dissertations and Theses will be investigated. Data will be extracted independently by two reviewers using a standardised data extraction form. The quality of observational studies will be assessed using the National Institute of Health Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies and the quality of experimental studies will be assessed using the Effective Public Health Practice Project Quality Assessment Tool for Quantitative Study. If there is a sufficient number of studies reporting data on a similar determinant among a similar population (k>5), a meta-analysis of the results will be performed with a random-effects model. If between-study heterogeneity is high (I² ≥75%), it will be investigated through sensitivity analyses and meta-regression. Ethics and dissemination: Formal ethical approval is not required as no primary data will be collected. The results will be published in a peer-reviewed journal. This review will provide valuable information to those interested in developing empirically based sleep interventions among women

On the role of visualisation in fisheries management

Environmental change has focused the attention of scientists, policy makers and the wider public on the uncertainty inherent in interactions between people and the environment. Governance in fisheries is required to involve stakeholder participation and to be more inclusive in its remit, which is no longer limited to ensuring a maximum sustainable yield from a single stock but considers species and habitat interactions, as well as social and economic issues. The increase in scope, complexity and awareness of uncertainty in fisheries management has brought methodological and institutional changes throughout the world. Progress towards comprehensive, explicit and participatory risk management in fisheries depends on effective communication. Graphic design and data visualisation have been underused in fisheries for communicating science to a wider range of stakeholders. In this paper, some of the general aspects of designing visualisations of modelling results are discussed and illustrated with examples from the EU funded MYFISH project. These infographics were tested in stakeholder workshops, and improved through feedback from that process. It is desirable to convey not just modelling results but a sense of how reliable various models are. A survey was developed to judge reliability of different components of fisheries modelling: the quality of data, the quality of knowledge, model validation efforts, and robustness to key uncertainties. The results of these surveys were visualized for ten different models, and presented alongside the main case study.Versión del editor1,86

Excitonic Properties of Low-Band-Gap Lead-Tin Halide Perovskites

The MAPb1–xSnxI3 (x = 0–1) (MA = methylammonium) perovskite family comprises a range of ideal absorber band gaps for single- and multijunction perovskite solar cells. Here, we use spectroscopic measurements to reveal a range of hitherto unknown fundamental properties of this materials family. Temperature-dependent transmission results show that the temperature of the tetragonal to orthorhombic structural transition decreases with increasing tin content. Through low-temperature magnetospectroscopy, we show that the exciton binding energy is lower than 16 meV, revealing that the dominant photogenerated species at typical operational conditions of optoelectronic devices are free charges rather than excitons. The reduced mass increases approximately proportionally to the band gap, and the mass values (0.075–0.090me) can be described with a two-band k·p perturbation model extended across the broad band gap range of 1.2–2.4 eV. Our findings can be generalized to predict values for the effective mass and binding energy for other members of this family of materials

Depth concentrations of deuterium ions implanted into some pure metals and alloys

Pure metals (Cu, Ti, Zr, V, Pd) and diluted Pd-alloys (Pd-Ag, Pd-Pt, Pd-Ru, Pd-Rh) were implanted by 25 keV deuterium ions at fluences in the range (1.2{\div}2.3)x1022 D+/m2. The post-treatment depth distributions of deuterium ions were measured 10 days and three months after the implantation using Elastic Recoil Detection Analysis (ERDA) and Rutherford Backscattering (RBS). Comparison of the obtained results allowed to make conclusions about relative stability of deuterium and hydrogen gases in pure metals and diluted Pd alloys. Very high diffusion rates of implanted deuterium ions from V and Pd pure metals and Pd alloys were observed. Small-angle X-ray scattering revealed formation of nanosized defects in implanted corundum and titanium.Comment: 12 pages, 9 figure