Understanding and controlling the formation and dynamics of ferroelastic domains can be key to enhance metal halide perovskite device performance, but established methods lack spatial control at the level of single domains. Here, the authors induced the formation of ferroelastic domains in CsPbBr${}_3$ nanowires using an atomic force microscope tip, and studied the structural changes using nanofocused x-ray diffraction with a 60-nm beam. The applied stress locally induced lattice tilts that define room temperature-stable ferroelastic domains, which spread spatially and terminated at {112}-type domain walls. While pristine regions show an orthorhombic (004) reflection; regions exposed to higher forces exhibit {220}-type reflections.

Electrostatic gating with ionic liquids, contributing to the accumulation of high surface charge carriers, has been often exploited in thin-film transistors. However, the intrinsic liquid nature of ionic liquids inhibits them from constituting a practical platform for thin-film devices. To this end, lithium-ion solid electrolytic substrates, often used for battery technology, offer similar benefits as ionic-liquids, with the added advantage of solid-state compatibility. In this work, the authors explore a lithium-ion solid electrolytic substrate for direct growth of MoS${}_2$ by chemical vapor deposition method. A near ideal subthreshold swing around 65 mV/dec with field-effect mobility values of 42-49 cm${}^2$V${}^{-1}$s${}^{-1}$ has been achieved with the devices on as-grown crystal, back-gated by the solid electrolyte.

Interesting electronic properties of a-few-degree-twisted bilayer graphene are caused by the formation of a flat band due to the interlayer interaction and electronic correlation. The authors quantitatively investigated the band structure of wide 3${}^{\mathrm{deg}}$-twisted bilayer graphene with clean interface by angle-resolved photoelectron spectroscopy, and compare the results with those of a band calculation using a recently-developed band unfolding method. The observed band structure indicates strong interlayer coupling that renormalizes the band structure including partial flat band features and gap formation. The observed band structure is in good agreement with the calculated results, indicating the importance of the interlayer coupling for the band renormalization.

Sodium-ion batteries have emerged as a promising, cost-effective energy storage solution. Critical to the success of these technologies is the efficient transport of ions within the battery electrodes. Here, the authors use first-principles techniques to examine sodium diffusion in a canonical sodium-ion battery cathode material, revealing a new, unconventional mechanism in which the mobile sodium ions are confined to boundaries between otherwise immobile, ordered regions. They provide evidence of a diffusion mechanism that occurs via the collective motion of the boundaries through the material. This behavior is dramatically different from that which is seen in analogous lithium-ion battery materials, and may extend to related candidate electrode materials for beyond-lithium-ion batteries. These mechanistic insights have important implications for the rate of sodium diffusion, which impacts battery charge/discharge speed.

Defect engineering plays an important role in tailoring the electronic transport properties of van der Waals materials. Methods reported so far mainly rely on the $exsitu$ engineering of defect type and concentration, hindering the realization of new types of device functionalities associated with defect engineering. Here, the authors report temperature-sensitive spatial redistribution of defects in PdSe${}_2$ thin flakes through scanning tunneling microscopy. The spatial characteristics of defect distribution is strongly related to the electronic transport properties such as anisotropic carrier mobility and phase coherent length, indicating a different avenue for creating novel device functionalities based on $insitu$ modulation of defect distribution.

The authors demonstrate a computational scheme of systematically designing new magnetic electrides derived from known conventional solids. Intuitively, we think of localized electrons attached to ions in a conventional solid. But there is an exceptional class of solids, where some electrons localize at the empty space in between ions: electrides. These materials can be used as catalysts, or in the case they are magnetic, for spintronic devices. Only few magnetic electrides are confirmed, so the authors have implemented state-of-the-art simulations of many interacting particles to computationally predict new magnetic electrides. The key is to start from known materials and tweak them just enough by removing or substituting elements. With this scheme they successfully predicted 30 nonmagnetic and 28 magnetic electrides. Topological phases, which are based on a kind of classification of matter, are revealed in the predicted electrides, highlighting the intimate relation between electrides and topological materials.

Invar and anti-Invar are materials having anomalously low and high thermal expansion coefficients, respectively. In the case of Invar, this is related to magneto-volume fluctuations occurring between a large-volume-high-moment state and an energetically higher-lying small-volume-low-moment state. For anti-Invar it is the opposite. For 3d metals and alloys, the occurrence of both effects is governed by the valence-electron-concentration, $e/a$. The authors provide a face-lift for Invar alloys by showing that they can also be tailored as 3d high-entropy alloys just by choosing the proper valence-electron-concentration - in this case ($e/a)=$ 8.7 electrons/atom. The study thus presents a method to identify new alloy variants that could combine the functional properties of Invar with beneficial features that have been identified for high-entropy alloys, such as high mechanical strength and excellent corrosion resistance.

The authors extend the ionic radii database of Shannon’s seminal work using machine learning regression. The developed consolidated table will allow prediction of material properties with high accuracy by considering the definite ionic radius value based on the oxidation state and coordination number. The work is relevant to the evolving material informatics field and has applications in many related fields.

Understanding the domain structure of two-dimensional materials is of paramount importance for the design of next generation microelectronic devices. Here, the authors employ a Dion–Jacobson layered oxide as a model system to study the ferroelectric domain structure with atomic scale analysis. They reveal the existence of a unit-cell-thick ferroelectric domain size as well as both 180° and 90° domain walls in a free-standing ferroelectric oxide. This may suggest ways to achieve unit-cell-thick domain structures and shed light on promising material solutions for emerging nonvolatile high-density memories and synaptic devices.

At the beginning of 2021, eight Physical Review journals began publishing Letters which are intended for the accelerated publication of important new results targeted to the specific readership of each journal.

APS Editor in Chief, Michael Thoennessen, discusses a new opportunity for communicating authors to include their pronouns together with their contact email in order to promote a more respectful, inclusive, and equitable environment.

Physical Review Applied and Physical Review Materials are pleased to present the Collection on Two-Dimensional Materials and Devices, highlighting one of the most interesting fields in Applied Physics and Materials Research. Papers belonging to this collection will be published throughout 2020. The invited articles, and an editorial by the Guest Editor, David Tománek, are linked in the Collection.

Guest Editor David Tománek introduces a collection of papers in Physical Review Applied and Physical Review Materials on two-dimensional materials and devices, in a snapshot of the leading edge of this hot field.

A discussion of the focus on materials related research in the Physical Review journals.