Lattice constant and cohesive energy are basic properties in the design of materials and devices. However, due to neglect of long-range van der Waals (vdW) interactions, density functional approximations (DFAs) often yield unusually large errors for ionic solids and heavy metals. Here, we propose a model for the dynamically screened vdW correction, including the leading order as well as higher-order contributions. The striking feature of this model is that important screening effects and higher-order contributions are properly considered and that its contribution to the short-range part is removed by a novel damping function for the avoidance of double counting. As a result, the model dramatically reduces the error of the DFA-GGA in lattice constant and cohesive energy. We also find that the three-body interactions are small, due to the screening effects. These observations greatly improve our fundamental understanding of vdW interactions and enhance the applicability of efficient semilocal DFAs.

${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ is an overdoped analog of hole-doped layered cuprates. Here we show via ab initio calculations that Ce-doped ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ $\left({\mathrm{Pr}}_3{\mathrm{CeNi}}_3{\mathrm{O}}_8\right)$ has the same electronic structure as the antiferromagnetic insulating phase of parent cuprates. We find that substantial Ce doping should be thermodynamically stable and that other 4+ cations would yield a similar antiferromagnetic insulating state, arguing this configuration is robust for layered nickelates of low-enough valence. The analogies with cuprates at different $d$ fillings suggest that intermediate Ce-doping concentrations near 1/8 should be an appropriate place to search for superconductivity in these low-valence Ni oxides.

The design of next generation $\beta$-type titanium implants requires detailed knowledge of the relevant stable and metastable phases at temperatures where metallurgical heat treatments can be performed. Recently, a standard specification for surgical implant applications was established for Mo-Ti alloys. However, the thermodynamic properties of this binary system are not well known and two conflicting descriptions of the $\beta$-phase stability have been presented in the literature. In this study, we use ab initio calculations to investigate the Mo-Ti phase diagram. These calculations predict that the $\beta$ phase is stable over a wide concentration range, in qualitative agreement with one of the reported phase diagrams. In addition, they predict stoichiometric compounds, stable at temperatures below $300{}^{\circ }\mathrm{C}$, which have not yet been detected by experiments. The resulting solvus, which defines the transition to the $\beta$-phase solid solution, therefore occurs at lower temperatures and is more complex than previously anticipated.

Pressure-induced superconductivity and structural phase transitions in phosphorus (P) are studied by resistivity measurements under pressures up to 170 GPa and by fully ab initio crystal structure exploration and superconductivity calculations up to 350 GPa. Two distinct superconducting transition temperature $\left(T_C\right)$ vs pressure $\left(P\right)$ trends at low pressure have been reported more than 30 years ago, and we are able to devise a consistent explanation founded on thermodynamically metastable phases of black phosphorus. Our experimental and theoretical results form a single, consistent picture which not only provides a clear understanding of elemental P under pressure but also sheds light on the longstanding and unsolved anomalous superconductivity trends. Moreover, at higher pressures we predict a similar scenario of multiple metastable structures which coexist beyond their thermodynamical stability range. We observe that all the metastable structures systematically exhibit larger transition temperatures than the ground-state structures, indicating that the exploration of metastable phases represents a promising route to design materials with improved superconducting properties.

We report the growth of ${\mathrm{PbTiO}}_3$ thin films by molecular-beam epitaxy utilizing continuous codeposition. In addition to the requirements from thermodynamics, whether the resulting film is single-phase ${\mathrm{PbTiO}}_3$ or not at a particular temperature depends strongly on the film growth rate and the incident fluxes of all species, including titanium. We develop a simple theory for the kinetics of lead oxidation on the growing film surface and find that it qualitatively explains the manner in which the adsorption-controlled growth window of ${\mathrm{PbTiO}}_3$ depends on lead flux, oxidant flux, and titanium flux. We successfully apply the kinetic theory to the dependence of the growth of ${\mathrm{BiFeO}}_3$ on oxidant type and surmise that the theory may be generally applicable to the adsorption-controlled growth of complex oxides by MBE.

The stacking effect on the $E_{2g}^1$ and $A_{1g}$ vibrational modes of mechanically exfoliated ${\mathrm{MoS}}_2$ samples supported on ${\mathrm{SiO}}_2$ was investigated by nonresonant Raman spectroscopy at high-pressure conditions. The split of $E_{2g}^1$ and $A_{1g}$ modes were observed for single- and bilayered samples whereas such splitting disappears for higher numbers of layers. The differences on the $E_{2g}^1$ pressure coefficients allowed us to interpret the observed splitting as due to the presence of two types of regions corresponding to a high and a low conformation of ${\mathrm{MoS}}_2$ to the substrate roughness. The difference in the pressure coefficient appears then as due to the biaxial stress introduced via the substrate compression. Such effects were not observed for the $A_{1g}$ mode due to its vibration symmetry. This out-of-plane vibration is mainly affected by the normal stress that corresponds to pressure transmitted by the pressure-transmitting medium.

The electronic and optical properties of the paradigmatic F4TCNQ-doped pentacene in the low-doping limit are investigated by a combination of state-of-the-art many-body ab initio methods accounting for environmental screening effects, and a carefully parametrized model Hamiltonian. We demonstrate that while the acceptor level lies very deep in the gap, the inclusion of electron-hole interactions strongly stabilizes dopant-semiconductor charge transfer states and, together with spin statistics and structural relaxation effects, rationalize the possibility for room-temperature dopant ionization. Our findings reconcile available experimental data, shedding light on the partial vs. full charge transfer scenario discussed in the literature, and question the relevance of the standard classification in shallow or deep impurity levels prevailing for inorganic semiconductors.

Tuning magnetic properties of perovskite thin films is a central topic of recent studies because of its fundamental significance. In this work, we demonstrated the modification of the magnetism of $\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{C}{\mathrm{a}}_{0.1}\mathrm{Co}{\mathrm{O}}_3$ (LCCO) thin films by introducing a stripelike superstructure in a controllable manner using electron beam irradiation (EBI) in a transmission electron microscope. The microstructure, electronic structure, strain change, and origin of magnetism of the LCCO thin films were studied in detail using aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations based on density functional theory. The results indicate that the EBI-induced unit cell volume expansion accompanies the formation of oxygen vacancies and leads to the spin state transition of Co ions. The low spin state of $\mathrm{C}{\mathrm{o}}^{4+}$ ions depress the stripelike superstructure, while higher spin states of Co ions with lower valences are conductive to the formation of “dark stripes”. Our work clarifies the origin of magnetism of epitaxial LCCO thin films, benefiting a comprehensive understanding of correlated physics in cobalt oxide thin films.

Polar compensation can play an important role in the determination of interfacial electronic and magnetic properties in oxide heterostructures. Using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, bulk magnetometry, and transport measurements, we find that interfacial charge redistribution via polar compensation is essential for explaining the evolution of interfacial ferromagnetism in ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ superlattices as a function of ${\mathrm{LaNiO}}_3$ layer thickness. In insulating superlattices (four unit cells or less of ${\mathrm{LaNiO}}_3$), magnetism is dominated by Ni–Mn superexchange, while itinerant electron-based Mn–Mn double exchange plays a role in thicker metallic superlattices. X-ray magnetic circular dichroism and resonant x-ray scattering show that Ni–Mn superexchange contributes to the magnetization even in metallic superlattices. This Ni–Mn superexchange interaction can be explained in terms of polar compensation at the ${\mathrm{LaNiO}}_3–{\mathrm{CaMnO}}_3$ interface. These results highlight the different mechanisms responsible for interfacial ferromagnetism and the importance of understanding compensation due to polar mismatch at oxide-based interfaces when engineering magnetic properties.

Grain boundaries (GBs) in crystalline materials are traditionally described by five crystallographic angles, which are assumed to fully define the GB structure and energy. It has recently been realized that variations in the atomic density $\lambda$ in the GB region can drastically alter the GB structure and cause transformations between different GB phases. Here we extend the previous studies of Cu $\mathrm{\Sigma }5$ GBs by computing the structures and energies of a set of [001] symmetrical tilt GBs over the entire angular range by allowing arbitrary variations in $\lambda$. The results confirm the existence of stable and metastable phases in all GBs studied here. There are three types of structural units that can describe all GB structures obtained in this work. The work demonstrates that $\lambda$ should be added to the description of GBs as an extra thermodynamic parameter that helps predict the GB phases and transformations among them.

The influence of Sb content, substrate type, and cap layers on the quantum anomalous Hall effect observed in V-doped (Bi,Sb)${}_2{\mathrm{Te}}_3$ magnetic topological insulators is investigated. Thin layers showing excellent quantization are reproducibly deposited by molecular beam epitaxy at growth conditions affecting a compromise between controlled layer properties and high crystalline quality. The Sb content can be reliably determined from the in-plane lattice constant measured by x-ray diffraction, even in thin layers. This is the main layer parameter to be optimized in order to approach charge neutrality. Within a narrow range at about 80% Sb content, the Hall resistivity shows a maximum of about $10\mathrm{k}\mathrm{\Omega }$ at 4 K and quantizes at mK temperatures. Under these conditions, thin layers grown on Si(111) or InP(111) and with or without a Te cap exhibit quantization. The quantization persists independently of the interfaces between cap, layer, and substrate, the limited crystalline quality, and the degradation of the layer proving the robustness of the quantum anomalous Hall effect.

Measurements of the anisotropic properties of single crystals play a crucial role in probing the physics of new materials. Determining a growth protocol that yields suitable high-quality single crystals can be particularly challenging for multicomponent compounds. Here we present a case study of how we refined a procedure to grow single crystals of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ from a high temperature, quaternary liquid solution rich in iron and arsenic (“FeAs self-flux”). Temperature dependent resistance and magnetization measurements are emphasized, in addition to the x-ray diffraction, to detect intergrown ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, ${\mathrm{CaFe}}_2{\mathrm{As}}_2$, and ${\mathrm{KFe}}_2{\mathrm{As}}_2$ within what appear to be single crystals. Guided by the rules of phase equilibria and these data, we adjusted growth parameters to suppress formation of the impurity phases. The resulting optimized procedure yielded phase-pure single crystals of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. This optimization process offers insight into the growth of quaternary compounds and a glimpse of the four-component phase diagram in the pseudoternary $\mathrm{FeAs}\text{−}{\mathrm{CaFe}}_2{\mathrm{As}}_2\text{−}{\mathrm{KFe}}_2{\mathrm{As}}_2$ system.

${\mathrm{CrCl}}_3$ is a layered insulator that undergoes a crystallographic phase transition below room temperature and orders antiferromagnetically at low temperature. Weak van der Waals bonding between the layers and ferromagnetic in-plane magnetic order make it a promising material for obtaining atomically thin magnets and creating van der Waals heterostructures. In this work we have grown crystals of ${\mathrm{CrCl}}_3$, revisited the structural and thermodynamic properties of the bulk material, and explored mechanical exfoliation of the crystals. We find two distinct anomalies in the heat capacity at 14 and 17 K confirming that the magnetic order develops in two stages on cooling, with ferromagnetic correlations forming before long-range antiferromagnetic order develops between them. This scenario is supported by magnetization data. A magnetic phase diagram is constructed from the heat capacity and magnetization results. We also find an anomaly in the magnetic susceptibility at the crystallographic phase transition, indicating some coupling between the magnetism and the lattice. First-principles calculations accounting for van der Waals interactions also indicate spin-lattice coupling, and find multiple nearly degenerate crystallographic and magnetic structures consistent with the experimental observations. Finally, we demonstrate that monolayer and few-layer ${\mathrm{CrCl}}_3$ specimens can be produced from the bulk crystals by exfoliation, providing a path for the study of heterostructures and magnetism in ultrathin crystals down to the monolayer limit.

An appealing mechanism for inducing multiferroicity in materials is the generation of electric polarization by a spatially varying magnetization that is coupled to the lattice through the spin-orbit interaction. Here we describe the reciprocal effect, in which a time-dependent electric polarization induces magnetization even in materials with no existing spin structure. We develop a formalism for this dynamical multiferroic effect in the case for which the polarization derives from optical phonons, and compute the strength of the phonon Zeeman effect, which is the solid-state equivalent of the well-established vibrational Zeeman effect in molecules, using density functional theory. We further show that a recently observed behavior—the resonant excitation of a magnon by optically driven phonons—is described by the formalism. Finally, we discuss examples of scenarios that are not driven by lattice dynamics and interpret the excitation of Dzyaloshinskii-Moriya-type electromagnons and the inverse Faraday effect from the viewpoint of dynamical multiferroicity.