The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation to the exchange-correlation energy is widely used in materials science. For many solids it somewhat overestimates lattice constants and underestimates cohesive energies. What attraction among the ions is missing in PBE? For ionic solids and heavy metals, a detailed analysis shows that the answer is the van der Waals (vdW) interaction arising from electric charge fluctuations on different ions. Our vdW correction to PBE is a short-range damped and long-range screened pairwise interaction among the ions. We find that the three-body interactions are small due to screening, and that the vdW correction stabilizes the bcc structure of cesium halides over the rocksalt structure.

To elucidate the nature of high-temperature superconductivity in the cuprates, it would be helpful to find other transition metal oxides with cupratelike electronic structures to test theories about the origin of superconductivity. Using $ab$ $initio$ calculations, this paper proposes low-valence layered nickelates as the closest analogue to cuprates. Doping metallic Pr${}_4$Ni${}_3$O${}_8$ with cerium should allow researchers to span the range of the cuprate phase diagram where superconductivity is observed. If this results in a superconducting nickelate, a long-sought goal, it could be a game changer in the field of high-temperature superconductivity.

Epitaxial perovskite oxide thin films and heterostructures are a highly active materials research topic generating both fundamental and applied interest. This manuscript reports the roles of oxidation kinetics and film deposition rate on the growth of phase-pure complex oxides by molecular-beam epitaxy, one of the premier techniques for these materials, in a regime that is traditionally described as being governed by thermodynamics. It is found that oxidation kinetics on the film surface are surprisingly important for the growth of PbTiO${}_3$ and for formulating a simple kinetic theory to interpret experimental observations. The results are also enlightening to predict the conditions for improved growth of BiFeO${}_3$ and other complex oxides by MBE, of high interest to the broad community studying oxide films and heterostructures.

Design and thermal processes development of advanced alloys require the detailed knowledge of phase diagrams, including the phases at temperatures suitable for metallurgical heat treatments. Scientists have traditionally relied on empirical databases that contain considerable gaps for many potentially useful systems. In this work, the authors present a computational methodology, based on $ab$ $initio$ calculations, for rapid investigation of phase diagrams, and apply it to a test case: the Mo-Ti system. The computed diagram agrees with established knowledge at high temperatures, but predicts new compounds as well as an extended stability domain of the useful β-phase solid solution at lower temperatures wider than previously anticipated.

This paper sheds light on the interaction between MoS${}_2$ and SiO${}_2$ substrate under the influence of external pressure. Using Raman measurements, the authors studied the evolution of in-plane and out-of-plane vibrational modes of exfoliated MoS${}_2$ under a high pressure of up to 8 GPa. It turns out biaxial stress is transmitted from substrate to layered MoS${}_2$ via substrate deformation, and the degree of stress transfer depends on the thickness of MoS${}_2$. For example, they observe that $E$${}_{2}g$ and $A$${}_{1}g$ modes shift for all few-layer thick system, and split only for mono- and bilayer MoS${}_2$. The thickness-dependent response is understood in terms of the variation of bending modulus and adhesion properties of MoS${}_2$. The results presented in this paper can aid to the study of strain engineering and straintronics in two-dimensional systems.

Tuning the magnetic properties of perovskite thin films is of great significance for the design of future devices using related materials. The magnetization of LaCoO${}_3$ thin films is closely related with the stripelike superstructures observed in the atomic-scale images. In this work, the authors show that the magnetization of La${}_{0.9}$Ca${}_{0.1}$CoO${}_3$ thin films grown on SrTiO${}_3$ substrate decreases with the substitution of La${}^{3+}$ ions by Ca${}^{2+}$ ions. Interestingly, the magnetization could again increase by introducing the stripelike superstructures in a continuous and controllable manner using electron beam irradiation. These findings not only pave the way for tuning the magnetization of La${}_{0.9}$Ca${}_{0.1}$CoO${}_3$ thin films artificially by electron beam irradiation, but also help to deeply understand the origins of the magnetism of La${}_{0.9}$Ca${}_{0.1}$CoO${}_3$ thin films.

Generating interfacial ferromagnetism in oxide heterostructures is a promising method for creating multifunctional low-dimensional systems. However, our understanding of interfacial ferromagnetism is complicated by competing mechanisms in these oxide systems. Here the authors demonstrate the importance of polar compensation at oxide interfaces using LaNiO${}_3$/CaMnO${}_3$ superlattices as a model system. The interfacial ferromagnetism is explained in terms of the formation of interfacial Ni${}^{2+}$ as a result of polar compensation-driven oxygen vacancies. This gives rise to a ferromagnetic Ni${}^{2+}$-Mn${}^{4+}$ superexchange. These results highlight the importance of polar mismatch in designing interfacial ferromagnetism and will help guide the creation of new ferromagnetic interfaces.

The fascinating behavior of transition metal oxides can change dramatically when they are scaled down to atomic-size dimensions; however, understanding the emergent properties is a major challenge. In this paper, the authors observe the evolution of multiple phase transitions as the thickness is reduced from bulk to the atomic layer limit in NdNiO${}_3$ superlattices. Their measurements demonstrate a separation of the insulating phase from magnetic and charge-ordered phases, which coexist in the bulk, and the emergence of a hidden, unordered insulating phase for a single atomic layer. Modeling shows that the phase manipulation uniquely takes advantage of the effects of 2D confinement and symmetry-breaking at the interface.

Among elemental compounds, the high-pressure superconducting phase diagram of phosphorus is one of the most complex. Resistivity measurements and $ab$ $initio$ superconductivity calculations reported in this paper solve for the first time the open controversies on the anomalous superconducting trends; forming a single, consistent scenario of multiple metastable structures which coexist beyond their thermodynamical stability range. These metastable structures exhibit critical temperatures, which are distinctively higher than the putative ground-state structures, suggesting that the selective stabilization of metastable phases represents a viable strategy to improve superconductivity properties on conventional superconductors.

Doping of semiconductors is central to many electronic devices. However, contrary to inorganic systems, there is still no accepted mechanism for organic semiconductors (OSC) involved in flexible optoelectronic devices. Using a QM/MM implementation of ab initio many-body perturbation theories, combined with an accurate model excitonic Hamiltonian, it is shown that the doping mechanisms in OSC are profoundly original. While the dopant impurity levels can be extremely deep in the gap, room-temperature ionization is made possible thanks to the large electron-hole interaction that stabilizes dopant-to-host charge transfer excitations. As in excitonic solar cells, correlated electron-hole pairs are the key to organic systems.

The polymerization index of polymers is a critical parameter determining many physical properties including solubility and glass transition. Even the most careful chemical synthesis techniques produce a range of polymer sizes. In this paper we show that thermal evaporation, often used to extract dimer and trimer components, can be extended to polymerization indices as high as 13. This new method provides a simple and effective way of producing almost purely monodisperse samples which can in turn be used in tests of the effect of polydispersity on physical properties.

Hybrid metal/organic systems are typically used as models for novel nanoscale interfaces. In particular, the adsorption of molecules on a metal surface implies a rearrangement of electron density, which can be used to modulate the electronic properties of the device. Application of a fully self-consistent Tkatchenko-Scheffler van der Waals density functional demonstrates that these weak interactions can induce large charge rearrangements, leading to nontrivial modifications of interface dipoles, charge transfer phenomena and work functions. Therefore van der Waals interactions should be considered as an additional control parameter in the design of hybrid interfaces with desired electronic properties.