Exploring Epitaxial Structures for Electrically Pumped Perovskite Lasers: A Study of CsPb(Br,I)3 Based on the Ab Initio Bethe-Salpeter Equation.

Halide perovskites are widely used as components of electronic and optoelectronic devices such as solar cells, light-emitting diodes (LEDs), optically pumped lasers, field-effect transistors, photodetectors, and γ-detectors. Despite this wide range of applications, the construction of an electrically pumped perovskite laser remains challenging. In this paper, we numerically justify that mixing two perovskite compounds with different halide elements can lead to optical properties suitable for electrical pumping. As a reference, the chosen model material was CsPbBr3, whose performance as a part of lasers has been widely recognised, with some Br atoms substituted by I at specific sites. In particular, a strong enhancement of the low-energy absorption peaks has been obtained using the ab initio Bethe-Salpeter equation. Based on these results, we propose specific architectures of ordered doping that could be realised by epitaxial growth. Efficient light emission from the bottom of the conduction band is expected.

Role of inorganic cations in the excitonic properties of lead halide perovskites.

We theoretically investigate lead iodide perovskites of general formula APbI3 for a series of metallic cations (namely Cs+, Rb+, K+, Na+ and Li+) by means of density functional theory, the GW method and the Bethe-Salpeter equation including spin-orbit coupling. We demonstrate that the low-energy edges (up to 1.3 eV) of the absorption spectra are dominated by weakly bound excitons, with binding energies Eb of ∼ 30-80 meV, and the corresponding intensities increase as metallic cations become lighter. The middle parts of the spectra (1.8-2.4 eV), on the other hand, contain optical dipole transitions comprising more confined excitons (Eb ∼ 150-200 meV) located at PbI3. These parts of the spectra correspond to the optical-gain wavelengths which are experimentally achieved in optically pumped perovskite lasers. Finally, the higher energy parts, from about 2.8 eV (LiPbI3) to 4.3 eV (CsPbI3), contain optical transitions with very confined excitons (Eb ∼ 220-290 meV) located at halide atoms and the empty states of the metallic cations.

Wannier90 as a community code: new features and applications.

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.

Gapless edge states in (C,O,H)-built molecular system with p-stacking and hydrogen bonds.

The gapless edge states have been found in a 2D molecular system built with light atoms: C,O,H. This prediction is done on the basis of combined density functional theory (DFT) and tight-binding calculations. The system does not exhibit any effect of the spin-orbit coupling (SOC), neither intrinsic nor Rashba type. The band structure and the edge states are tuned with a strength of the p-stacking and O...H interactions. The elementary cell of this noncovalent structure, does not have the 3D inversion or rotational symmetry. Instead, the system transforms via a superposition of two reflections: with respect to the xz and xy mirror planes, both containing the non-periodic direction. This superposition is equivalent to the inversion in the 2D subspace, in which the system is periodic. The energy gap obtained with the DFT method is 0.11 eV, and largely opens (above 1 eV) with the GW and hybrid-DFT approaches. The bands inversion is partial, i.e. the Bloch states are mixed, with the "swapping" and "non-swapping" atomic contributions.

New memory devices based on the proton transfer process.

Memory devices operating due to the fast proton transfer (PT) process are proposed by the means of first-principles calculations. Writing  information is performed using the electrostatic potential of scanning tunneling microscopy (STM). Reading information is based on the effect of the local magnetization induced at the zigzag graphene nanoribbon (Z-GNR) edge-saturated with oxygen or the hydroxy group-and can be realized with the use of giant magnetoresistance (GMR), a magnetic tunnel junction or spin-transfer torque devices. The energetic barriers for the hop forward and backward processes can be tuned by the distance and potential of the STM tip; this thus enables us to tailor the non-volatile logic states. The proposed system enables very dense packing of the logic cells and could be used in random access and flash memory devices.

Contacts for organic switches with carbon-nanotube leads.

We focus on two classes of organic switches operating due to the photo- or field-induced proton transfer (PT) process. By means of first-principles simulations, we search for the atomic contacts that strengthen diversity of the two swapped current-voltage (I-V) characteristics between two tautomers. We emphasize that the low-resistive contacts do not necessarily possess good switching properties. Very often, the higher-current flow makes it more difficult to distinguish between the logic states. Instead, the more resistive contacts multiply a current gear to a larger extent. The low- and high-bias work regimes set additional conditions, which are fulfilled by different contacts: (i) in the very low-voltage regime, the direct connections to the nanotubes perform better than the popular sulfur contacts, and (ii) in the higher-voltage regime, the best are the peroxide (-O-O-) contacts. Additionally, we find that the switching-bias value is not an inherent property of the conducting molecule, but it strongly depends on the chosen contacts.

CVD formation of graphene on SiC surface in argon atmosphere.

We investigate the microscopic processes leading to graphene growth by the chemical vapor deposition of propane in an argon atmosphere at the SiC surface. Experimentally, it is known that the presence of argon fastens the dehydrogenation processes at the surface, at high temperatures of about 2000 K. We perform ab initio calculations, at zero temperature, to check whether chemical reactions can explain this phenomenon. Density functional theory and supporting quantum chemistry methods qualitatively describe formation of the graphene wafers. We find that the 4H-SiC(0001) surface exhibits a large catalytic effect in the adsorption process of hydrocarbon molecules, this is also supported by preliminary molecular dynamics results. The existence of the ArH(+) molecule, and an observation from the Raman spectra that the negative charge transfers into the SiC surface, would suggest that presence of argon atoms leads to a deprotonization on the surface, which is necessary to obtain a pure carbon adlayer. But the zero-temperature description shows that the cold environment is insufficient to promote argon-assisted surface cleaning.

The PRINTO criteria for clinically inactive disease in juvenile dermatomyositis.

OBJECTIVES: To develop data-driven criteria for clinically inactive disease on and off therapy for juvenile dermatomyositis (JDM). METHODS: The Paediatric Rheumatology International Trials Organisation (PRINTO) database contains 275 patients with active JDM evaluated prospectively up to 24 months. Thirty-eight patients off therapy at 24 months were defined as clinically inactive and included in the reference group. These were compared with a random sample of 76 patients who had active disease at study baseline. Individual measures of muscle strength/endurance, muscle enzymes, physician's and parent's global disease activity/damage evaluations, inactive disease criteria derived from the literature and other ad hoc criteria were evaluated for sensitivity, specificity and Cohen's κ agreement. RESULTS: The individual measures that best characterised inactive disease (sensitivity and specificity >0.8 and Cohen's κ >0.8) were manual muscle testing (MMT) ≥78, physician global assessment of muscle activity=0, physician global assessment of overall disease activity (PhyGloVAS) ≤0.2, Childhood Myositis Assessment Scale (CMAS) ≥48, Disease Activity Score ≤3 and Myositis Disease Activity Assessment Visual Analogue Scale ≤0.2. The best combination of variables to classify a patient as being in a state of inactive disease on or off therapy is at least three of four of the following criteria: creatine kinase ≤150, CMAS ≥48, MMT ≥78 and PhyGloVAS ≤0.2. After 24 months, 30/31 patients (96.8%) were inactive off therapy and 69/145 (47.6%) were inactive on therapy. CONCLUSION: PRINTO established data-driven criteria with clearly evidence-based cut-off values to identify JDM patients with clinically inactive disease. These criteria can be used in clinical trials, in research and in clinical practice.

Exchange interactions and Tc in rhenium-doped silicon: DFT, DFT + U and Monte Carlo calculations.

Interactions between rhenium impurities in silicon are investigated by means of the density functional theory (DFT) and the DFT + U scheme. All couplings between impurities are ferromagnetic except the Re-Re dimers which in the DFT method are nonmagnetic, due to the formation of the chemical bond supported by substantial relaxation of the geometry. The critical temperature is calculated by means of classical Monte Carlo (MC) simulations with the Heisenberg Hamiltonian. The uniform ferromagnetic phase is obtained with the DFT exchange interactions at room temperature for the impurities concentration of 7%. With the DFT + U exchange interactions, the ferromagnetic clusters form above room temperature in MC samples containing only 3% Re.

The Paediatric Rheumatology International Trials Organisation provisional criteria for the evaluation of response to therapy in juvenile dermatomyositis.

OBJECTIVE: To develop a provisional definition for the evaluation of response to therapy in juvenile dermatomyositis (DM) based on the Paediatric Rheumatology International Trials Organisation juvenile DM core set of variables. METHODS: Thirty-seven experienced pediatric rheumatologists from 27 countries achieved consensus on 128 difficult patient profiles as clinically improved or not improved using a stepwise approach (patient's rating, statistical analysis, definition selection). Using the physicians' consensus ratings as the "gold standard measure," chi-square, sensitivity, specificity, false-positive and-negative rates, area under the receiver operating characteristic curve, and kappa agreement for candidate definitions of improvement were calculated. Definitions with kappa values >0.8 were multiplied by the face validity score to select the top definitions. RESULTS: The top definition of improvement was at least 20% improvement from baseline in 3 of 6 core set variables with no more than 1 of the remaining worsening by more than 30%, which cannot be muscle strength. The second-highest scoring definition was at least 20% improvement from baseline in 3 of 6 core set variables with no more than 2 of the remaining worsening by more than 25%, which cannot be muscle strength (definition P1 selected by the International Myositis Assessment and Clinical Studies group). The third is similar to the second with the maximum amount of worsening set to 30%. This indicates convergent validity of the process. CONCLUSION: We propose a provisional data-driven definition of improvement that reflects well the consensus rating of experienced clinicians, which incorporates clinically meaningful change in core set variables in a composite end point for the evaluation of global response to therapy in juvenile DM.