Anomalous Non-Hydrogenic Exciton Series in 2D Materials on High-κ\kappa Dielectric Substrates

Engineering of the dielectric environment represents a powerful strategy to control the electronic and optical properties of two-dimensional (2D) materials without compromising their structural integrity. Here we show that the recent development of high-$\kappa$ 2D materials present new opportunities for dielectric engineering. By solving a 2D Mott-Wannier exciton model for WSe$_2$ on different substrates using a screened electron-hole interaction obtained from first principles, we demonstrate that the exciton Rydberg series changes qualitatively when the dielectric screening within the 2D semiconductor becomes dominated by the substrate. In this regime, the distance dependence of the screening is reversed and the effective screening increases with exciton radius, which is opposite to the conventional 2D screening regime. Consequently, higher excitonic states become underbound rather than overbound as compared to the Hydrogenic Rydberg series. Finally, we derive a general analytical expression for the exciton binding energy of the entire 2D Rydberg serie

The Computational 2D Materials Database: High-Throughput Modeling and Discovery of Atomically Thin Crystals

We introduce the Computational 2D Materials Database (C2DB), which organises a variety of structural, thermodynamic, elastic, electronic, magnetic, and optical properties of around 1500 two-dimensional materials distributed over more than 30 different crystal structures. Material properties are systematically calculated by state-of-the art density functional theory and many-body perturbation theory (G$_0\!$W\!_0 and the Bethe-Salpeter Equation for $\sim$200 materials) following a semi-automated workflow for maximal consistency and transparency. The C2DB is fully open and can be browsed online or downloaded in its entirety. In this paper, we describe the workflow behind the database, present an overview of the properties and materials currently available, and explore trends and correlations in the data. Moreover, we identify a large number of new potentially synthesisable 2D materials with interesting properties targeting applications within spintronics, (opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily accessible overview of the rapidly expanding family of 2D materials and forms an ideal platform for computational modeling and design of new 2D materials and van der Waals heterostructures.Comment: Add journal reference and DOI; Minor updates to figures and wordin

Rehabilitation of torture survivors in five countries: common themes and challenges

<p>Abstract</p> <p>Background</p> <p>Torture continues to be a global problem and there is a need for prevention and rehabilitation efforts. There is little available data on torture survivors from studies designed and conducted by health professionals in low income countries. This study is a collaboration between five centres from Gaza, Egypt, Mexico, Honduras and South Africa who provide health, social and legal services to torture survivors, advocate for the prevention of torture and are part of the network of the International Rehabilitation Council for Torture Victims (IRCT).</p> <p>Methods</p> <p>Socio-demographic, clinical and torture exposure data was collected on the torture survivors attending the five centres at presentation and then at three and six month follow-up periods. This sample of torture survivors is presented using a range of descriptive statistics. Change over time is demonstrated with repeated measures analysis of variance.</p> <p>Results</p> <p>Of the 306 torture survivors, 23% were asylum seekers or refugees, 24% were socially isolated, 11% in prison. A high level of traumatic events was experienced. 64% had suffered head injury whilst tortured and 24% had ongoing torture injury problems. There was high prevalence of symptoms of anxiety, depression, post traumatic stress as well as medically unexplained somatic symptoms. The analysis demonstrates a modest drop in symptoms over the six months of the study.</p> <p>Conclusions</p> <p>Data showed that the torture survivors seen in these five centres had high levels of exposure to torture events and high rates of clinical symptoms. In order to provide effective services to torture survivors, health professionals at torture rehabilitation centres in low income countries need to be supported to collect relevant data to document the needs of torture survivors and to evaluate the centres' interventions.</p

Two-dimensional ferroelectrics from high throughput computational screening

Abstract We report a high throughput computational search for two-dimensional ferroelectric materials. The starting point is 252 pyroelectric materials from the computational 2D materials database (C2DB) and from these we identify 63 ferroelectrics. In particular we find 49 materials with in-plane polarization, 8 materials with out-of-plane polarization and 6 materials with coupled in-plane and out-of-plane polarization. Most of the known 2D ferroelectrics are recovered by the screening and the far majority of the predicted ferroelectrics are known as bulk van der Waals bonded compounds, which makes them accessible by direct exfoliation. For roughly 25% of the materials we find a metastable state in the non-polar structure, which may imply a first order transition to the polar phase. Finally, we list the magnetic pyroelectrics extracted from the C2DB and focus on the case of VAgP2Se6, which exhibits a three-state switchable polarization vector that is strongly coupled to the magnetic excitation spectrum