Identifying novel functional materials with desired key properties is an important part of bridging the gap between fundamental research and technological advancement. In this context, high-throughput calculations combined with data-mining techniques highly accelerated this process in different areas of research during the past years. The strength of a data-driven approach for materials prediction lies in narrowing down the search space of thousands of materials to a subset of prospective candidates. Recently, the open-access organic materials database OMDB was released providing electronic structure data for thousands of previously synthesized three-dimensional organic crystals. Based on the OMDB, we report about the implementation of a novel density of states similarity search tool which is capable of retrieving materials with similar density of states to a reference material. The tool is based on the approximate nearest neighbor algorithm as implemented in the ANNOY library and can be applied via the OMDB web interface. The approach presented here is wide ranging and can be applied to various problems where the density of states is responsible for certain key properties of a material. As the first application, we report about materials exhibiting electronic structure similarities to the aromatic hydrocarbon p-terphenyl which was recently discussed as a potential organic high-temperature superconductor exhibiting a transition temperature in the order of 120 K under strong potassium doping. Although the mechanism driving the remarkable transition temperature remains under debate, we argue that the density of states, reflecting the electronic structure of a material, might serve as a crucial ingredient for the observed high $T_{\mathrm{c}}$. To provide candidates which might exhibit comparable properties, we present 15 purely organic materials with similar features to p-terphenyl within the electronic structure, which also tend to have structural similarities with p-terphenyl such as space group symmetries, chemical composition, and molecular structure. The experimental verification of these candidates might lead to a better understanding of the underlying mechanism in case similar superconducting properties are revealed.

Thin films and $p-n$ junctions for solar cells based on the absorber materials $\mathrm{Cu}(\mathrm{In},\mathrm{G}\mathrm{a}){\mathrm{Se}}_2$ and ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ were investigated as a function of depth using implanted low energy muons. The most significant result is a clear decrease of the formation probability of the ${\mathrm{Mu}}^{+}$ state at the heterojunction interface as well as at the surface of the $\mathrm{Cu}(\mathrm{In},\mathrm{G}\mathrm{a}){\mathrm{Se}}_2$ film. This reduction is attributed to a reduced bonding reaction of the muon in the absorber defect layer at its surface. In addition, the activation energies for the conversion from a muon in an atomiclike configuration to a anion-bound position are determined from temperature-dependence measurements. It is concluded that the muon probe provides a measurement of the effective surface defect layer width, both at the heterojunctions and at the films. The CIGS surface defect layer is crucial for solar-cell electrical performance and additional information can be used for further optimizations of the surface.

Two-dimensional materials have attracted tremendous attention for their fascinating electronic, optical, chemical, and mechanical properties. However, the band gaps of most reported two-dimensional (2D) materials are smaller than 2.0 eV, which has greatly restricted their optoelectronic applications in the blue and ultraviolet range of the spectrum. Here, we propose a stable trisulfur dinitride (${\mathrm{S}}_3{\mathrm{N}}_2$) 2D crystal that is a covalent network composed solely of S-N $\sigma$ bonds. The ${\mathrm{S}}_3{\mathrm{N}}_2$ crystal is dynamically, thermally, and chemically stable, as confirmed by the computed phonon spectrum and ab initio molecular dynamics simulations. GW calculations show that the ${\mathrm{S}}_3{\mathrm{N}}_2$ crystal is a wide, direct band-gap (3.92 eV) semiconductor with a small-hole effective mass. In addition, the band gap of ${\mathrm{S}}_3{\mathrm{N}}_2$ structures can be tuned by forming multilayer ${\mathrm{S}}_3{\mathrm{N}}_2$ crystals, ${\mathrm{S}}_3{\mathrm{N}}_2$ nanoribbons, and ${\mathrm{S}}_3{\mathrm{N}}_2$ nanotubes, expanding its potential applications. The anisotropic optical response of the 2D ${\mathrm{S}}_3{\mathrm{N}}_2$ crystal is revealed by GW–Bethe-Salpeter-equation calculations. The optical band gap of ${\mathrm{S}}_3{\mathrm{N}}_2$ is 2.73 eV and the exciton binding energy of ${\mathrm{S}}_3{\mathrm{N}}_2$ is 1.19 eV, showing a strong excitonic effect. Our result not only marks the prediction of a 2D crystal composed of nitrogen and sulfur, but also underpins potential innovations in 2D electronics and optoelectronics.

Interfacial characteristics are critical to various properties of two-dimensional (2D) materials such as band alignment at a heterojunction and nucleation kinetics in a 2D crystal. Despite the desire to harness these enhanced interfacial properties for engineering new materials, unexpected phase transitions and defects, unique to the 2D morphology, have left a number of open questions. In particular, the effects of configurational anisotropy, which are difficult to isolate experimentally, and their influence on interfacial properties are not well understood. In this work, we begin to probe this structure-thermodynamic relationship, using a rotating magnetic field to generate an anharmonic interaction potential in a 2D system of paramagnetic particles. At low magnetic field strengths, weakly interacting colloidal particles form non-close-packed, fluidlike droplets, whereas, at higher field strengths, crystallites with hexagonal ordering are observed. We examine spatial and interfacial properties of these 2D colloidal clusters by measuring the local bond orientation order parameter and interfacial stiffness as a function of the interaction strength. To our knowledge, this is the first study to measure the tunable interfacial stiffness of a 2D colloidal cluster by controlling particle interactions using external fields.

The giant birefringence of layered h-BN was demonstrated by analyzing the interference patterns in reflectance and transmittance measurements in the mid-infrared to the deep ultraviolet energy range. The refractive index for polarization perpendicular to the $c$ axis is much higher than the refractive index for polarization parallel to the $c$ axis, and it displays a strong increase in the ultraviolet range that is attributed to the huge excitonic effects arising from the unique electronic structure of h-BN. Thus, h-BN is shown to exhibit a giant negative birefringence that ranges from $-0.7$ in the visible to $-2$ in the deep ultraviolet close to the band gap. The electronic dielectric constants for polarization perpendicular and parallel to the $c$ axis were determined to be ${\varepsilon }_{\infty }^{\perp }=4.95$ and ${\varepsilon }_{\infty }^{\parallel }=2.86$, respectively. The anisotropy we find in high-quality h-BN is significantly larger than proposed in previous experimental studies but in excellent agreement with ab initio calculations.

The recent discovery of the extreme magnetoresistance (XMR) in the nonmagnetic rare-earth monopnictides $\mathrm{La}X$ ($X$ = P, As, Sb, Bi,), a recently proposed new topological semimetal family, has inspired intensive research effort in the exploration of the correlation between the XMR and their electronic structures. In this work, using angle-resolved photoemission spectroscopy to investigate the three-dimensional band structure of LaBi, we unraveled its topologically nontrivial nature with the observation of multiple topological surface Dirac fermions, as supported by our $\mathit{ab} \mathit{initio}$ calculations. Furthermore, we observed substantial imbalance between the volume of electron and hole pockets, which rules out the electron-hole compensation as the primary cause of the XMR in LaBi.

Using a real-space implementation of pseudopotentials within the density-functional theory, we show that the magnetization of a ${\mathrm{Co}}_3\mathrm{N}$ cluster with a recently discovered atomic structure is significantly affected by nitrogen dopants. In a ${\mathrm{Co}}_3\mathrm{N}$ cluster with the hexagonal $P{6}_3/mmc$ structure, N dopants promote spin polarization for the Co-$3d$ electrons and the dopants themselves make additional contributions to net magnetic moment. These two factors enhance the total magnetic moment of a hexagonal ${\mathrm{Co}}_3\mathrm{N}$ cluster, which can be as strong as bulk iron. In contrast, N dopants in a ${\mathrm{Co}}_3\mathrm{N}$ cluster with the rhombohedral $R\overline{3}c$ structure degrade magnetic moment and the dopants are magnetically “inert,” which results in lower total magnetic moments in rhombohedral ${\mathrm{Co}}_3\mathrm{N}$ clusters. These changes in magnetic moment originate from differences in the orbital hybridization between the Co-$3d$ and N-$2p$ states. We also examine how the magnetization of a ${\mathrm{Co}}_3\mathrm{N}$ cluster depends on a N content. We find that the total magnetic moment of a hexagonal ${\mathrm{Co}}_3{\mathrm{N}}_{1+x}$ cluster with $-0.15\le x\le 0.15$ is tunable and can be enhanced further by controlling the amount of nitrogen dopants.

The ab initio calculation of quasiparticle (QP) energies is a technically and computationally challenging problem. In condensed matter physics, the most widely used approach to determine QP energies is the $GW$ approximation. Although the $GW$ method has been widely applied to many typical semiconductors and insulators, its application to more complex compounds such as transition metal oxide perovskites has been comparatively rare, and its proper use is not well established from a technical point of view. In this work, we have applied the single-shot $G_0{W}_0$ method to a representative set of transition metal oxide perovskites including $3d$ (${\mathrm{SrTiO}}_3, {\mathrm{LaScO}}_3, {\mathrm{SrMnO}}_3, {\mathrm{LaTiO}}_3, {\mathrm{LaVO}}_3, {\mathrm{LaCrO}}_3, {\mathrm{LaMnO}}_3$, and ${\mathrm{LaFeO}}_3$), $4d$ (${\mathrm{SrZrO}}_3, {\mathrm{SrTcO}}_3$, and ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$), and $5d$ (${\mathrm{SrHfO}}_3, {\mathrm{KTaO}}_3$, and ${\mathrm{NaOsO}}_3$) compounds with different electronic configurations, magnetic orderings, structural characteristics, and band gaps ranging from 0.1 to 6.1 eV. We discuss the proper procedure to obtain well-converged QP energies and accurate band gaps within single-shot $G_0{W}_0$ by comparing the conventional approach based on an incremental variation of a specific set of parameters (number of bands, energy cutoff for the plane-wave expansion and number of k points) and the basis-set extrapolation scheme [J. Klimeš et al., Phys. Rev. B 90, 075125 (2014)]. Although the conventional scheme is not supported by a formal proof of convergence, for most cases it delivers QP energies in reasonably good agreement with those obtained by the basis-set correction procedure and it is by construction more useful for calculating band structures. In addition, we have inspected the difference between the adoption of norm-conserving and ultrasoft potentials in $GW$ calculations and found that the norm violation for the $d$ shell can lead to less accurate results in particular for charge-transfer systems and late transition metals. A minimal statistical analysis indicates that the correlation of the $GW$ data with the density functional theory gap is more robust than the correlation with the experimental gaps; moreover, we identify the static dielectric constant as alternative useful parameter for the approximation of $GW$ gap in high-throughput automatic procedures. Finally, we compute the QP band structure and spectra within the random phase approximation and compare the results with available experimental data.

Doping is a common route to reducing nanowire transistor on-resistance but it has limits. A high doping level gives significant loss in gate performance and ultimately complete gate failure. We show that electrolyte gating remains effective even when the Be doping in our GaAs nanowires is so high that traditional metal-oxide gates fail. In this regime we obtain a combination of subthreshold swing and contact resistance that surpasses the best existing $p$-type nanowire metal-oxide semiconductor field-effect transistors (MOSFETs). Our subthreshold swing of 75 mV/dec is within $25\%$ of the room-temperature thermal limit and comparable with $n-\mathrm{InP}$ and $n-\mathrm{GaAs}$ nanowire MOSFETs. Our results open a new path to extending the performance and application of nanowire transistors, and motivate further work on improved solid electrolytes for nanoscale device applications.

Nonstoichiometric uranium dioxide experiences a shrinkage of its lattice constant with increasing oxygen content, in both the hypostoichiometric and the hyperstoichiometric regimes. Based on first-principles calculations within the density functional theory $\text{(DFT)}+U$ approximation, we have developed a point defect model that accounts for the volume of relaxation of the most significant intrinsic defects of ${\mathrm{UO}}_2$. Our point defect model takes special care of the treatment of the charged defects in the equilibration of the model and in the determination of reliable defect volumes of formation. In the hypostoichiometric regime, the oxygen vacancies are dominant and explain the lattice constant variation with their surprisingly positive volume of relaxation. In the hyperstoichiometric regime, the uranium vacancies are predicted to be the dominating defect,in contradiction with experimental observations. However, disregarding uranium vacancies allows us to recover a good match for the lattice-constant variation as a function of stoichiometry. This can be considered a clue that the uranium vacancies are indeed absent in ${\mathrm{UO}}_{2+x}$, possibly due to the very slow diffusion of uranium.

Dynamic instabilities, stabilized by anharmonic interactions in cubic and tetragonal halide perovskites at high temperature, play a role in the electronic structure and optoelectronic properties of halide perovskites. In particular, inorganic and hybrid perovskite materials undergo structural phase transitions associated with octahedral tilts of the metal-halide octahedra. We investigate the structural instabilities present in inorganic $\mathrm{Cs}M{X}_3$ perovskites with Pb or Sn on the metal site and Br or I on the $X$ site. Defining primary order parameters in terms of symmetry-adapted collective displacement modes and secondary order parameters in terms of symmetrized Hencky strain components, we unravel the coupling between octahedral tilt modes and macroscopic strains as well as the role of $A$-site displacements in perovskite phase stability. Symmetry-allowed secondary strain order parameters are enumerated for the 14 unique perovskite tilt systems. Using first-principles calculations to explore the Born-Oppenheimer energy surface in terms of symmetrized order parameters, we find coupling between octahedral tilting and $A$-site displacements is necessary to stabilize $Pnma$ ground states. Additionally, we show that the relative stability of an inorganic halide perovskite tilt system correlates with the volume decrease from the high-symmetry cubic phase to the low-symmetry distorted phase.

We have performed direct molecular dynamics (MD) simulations of heteroepitaxial vapor deposition of $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ films on nonpolar $\left(11\overline{2}0\right)$ wurtzite-GaN surfaces to investigate strain relaxation by misfit-dislocation formation. The simulated growth is conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN substrate. We apply time-and-position-dependent boundary constraints to affect the appropriate environments for the vapor phase, the near-surface solid phase, and the bulklike regions of the growing layer. The simulations employ a newly optimized Stillinger-Weber In-Ga-N system interatomic potential wherein multiple binary and ternary structures are included in the underlying density-functional theory and experimental training sets to improve the treatment of the In-Ga-N related interactions. To examine the effect of growth conditions, we study a matrix of 63 different MD-growth simulations spanning seven $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$-alloy compositions ranging from $x=0.0$ to $x=0.8$ and nine growth temperatures above half the simulated melt temperature. We found a composition dependent temperature range where all kinetically trapped defects were eliminated, leaving only quasiequilibrium misfit and threading dislocations present in the simulated films. Based on the MD results obtained in this temperature range, we observe the formation of interfacial misfit and threading dislocation arrays with morphologies strikingly close to those seen in experiments. In addition, we compare the MD-observed thickness-dependent onset of misfit-dislocation formation to continuum-elasticity-theory models of the critical thickness and find reasonably good agreement. Finally, we use the three-dimensional atomistic details uniquely available in the MD-growth histories to directly observe the nucleation of dislocations at surface pits in the evolving free surface.

Heterostructures of wurtzite based devices have attracted great research interest because of the tremendous success of GaN in light emitting diodes (LED) industry. High-quality GaN thin films on inexpensive and lattice matched ZnO substrates are both commercially and technologically desirable. Intrinsic wetting conditions, however, forbid such heterostructures as the energy of ZnO polar surfaces is much lower than that of GaN polar surfaces, resulting in 3D growth mode and poor crystal quality. Based on first-principles calculations, we propose the use of surfactant hydrogen to dramatically alter the growth mode of the heterostructures. Stable H-involved surface configurations and interfaces are investigated with the help of our newly developed modelling techniques. The temperature and chemical potential dependence of our proposed strategy, which is critical in experiments, is predicted by applying the experimental Gibbs free energy of ${\mathrm{H}}_2$. Our thermodynamic wetting condition analysis is a crucial step for the growth of GaN on ZnO, and we find that introducing H will not degrade the stability of ZnO substrate. This approach will allow the growth of high-quality GaN thin films on ZnO substrates. We believe that our new strategy may reduce the manufactory cost, improve the crystal quality, and improve the efficiency of GaN-based devices.

The stability, structure, and properties of carbonate minerals at lower mantle conditions have significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron x-ray diffraction in a diamond anvil cell coupled with direct laser heating of ${\mathrm{CaCO}}_3$ using a ${\mathrm{CO}}_2$ laser, we identify a crystalline phase of the material above 40 GPa—corresponding to a lower mantle depth of around 1000 km—which has first been predicted by ab initio structure predictions. The observed $s{p}^2$ carbon hybridized species at 40 GPa is monoclinic with $P{2}_1/c$ symmetry and is stable up to 50 GPa, above which it transforms into a structure which cannot be indexed by existing known phases. A combination of ab initio random structure search (AIRSS) and quasiharmonic approximation (QHA) calculations are used to re-explore the relative phase stabilities of the rich phase diagram of ${\mathrm{CaCO}}_3$. Nudged elastic band (NEB) calculations are used to investigate the reaction mechanisms between relevant crystal phases of ${\mathrm{CaCO}}_3$ and we postulate that the mineral is capable of undergoing $s{p}^2-s{p}^3$ hybridization change purely in the $P{2}_1/c$ structure—forgoing the accepted postaragonite $Pmmn$ structure.

The stability of graphene nanoribbons (GNRs) against oxidation is critical for their practical applications. Here we study both the thermal stability and the oxidation process of the ambient-exposed armchair GNRs with a width of seven carbon atoms (7-aGNR), grown on an Au(111) surface. The atomic scale evolution of the armchair edges and the zigzag ends of the aGNRs after annealing at different temperatures are revealed by using scanning tunneling microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. We observe evidence that the zigzag ends start to be oxidized and decomposed at 180 °C, while the armchair edges are intact at 430 °C but become oxidized at 520 °C. Two different oxygen species are identified at the armchair edges, namely the hydroxyl pair and the epoxy bonding motif with one oxygen bonded to two edge carbons. These oxidization species modify the electronic properties of the pristine 7-aGNRs, with a band-gap reduction from 2.6 to 2.3 eV and 1.9 eV for the hydroxyl pair- and epoxy-terminated edges, respectively. These findings demonstrate the oxidation stability of both the zigzag and armchair edges of GNRs, and they provide an opportunity to harness the high density of edge atoms in applications such as GNR-based high-temperature oxygen sensors.

We performed magnetic field and frequency tunable electron paramagnetic resonance spectroscopy of an ${\mathrm{Er}}^{3+}$-doped ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal by observing the change in flux induced on a direct current superconducting quantum interference device (dc SQUID) loop of a tunable Josephson bifurcation amplifer. The observed spectra show multiple transitions which agree well with the simulated energy levels, taking into account the hyperfine and quadrupole interactions of ${}^{167}\mathrm{Er}$. The sensing volume is about 0.15 pl, and our inferred measurement sensitivity (limited by external flux noise) is approximately $1.5\times {10}^4$ electron spins for a 1-s measurement. The sensitivity value is two orders of magnitude better than similar schemes using a dc SQUID switching readout.

We show that the Gaussian Approximation Potential (GAP) machine-learning framework can describe complex magnetic potential energy surfaces, taking ferromagnetic iron as a paradigmatic challenging case. The training database includes total energies, forces, and stresses obtained from density-functional theory in the generalized-gradient approximation, and comprises approximately 150,000 local atomic environments, ranging from pristine and defected bulk configurations to surfaces and generalized stacking faults with different crystallographic orientations. We find the structural, vibrational, and thermodynamic properties of the GAP model to be in excellent agreement with those obtained directly from first-principles electronic-structure calculations. There is good transferability to quantities, such as Peierls energy barriers, which are determined to a large extent by atomic configurations that were not part of the training set. We observe the benefit and the need of using highly converged electronic-structure calculations to sample a target potential energy surface. The end result is a systematically improvable potential that can achieve the same accuracy of density-functional theory calculations, but at a fraction of the computational cost.

We investigated the metal-insulator transition for epitaxial thin films of the perovskite ${\mathrm{CaFeO}}_3$, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard x-ray photoelectron spectroscopy, soft x-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe sites undergo no observable spectroscopic change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for ${\mathrm{CaFeO}}_3$ and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by $\sim 5–10\%$ in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.

The influence of elastic strain on the lithium vacancy formation and migration in bulk ${\mathrm{LiCoO}}_2$ is evaluated by means of first-principles calculations within density functional theory (DFT). Strain dependent energies are determined directly from defective cells and also within linear elasticity theory from the elastic dipole tensor $\left(G_{ij}\right)$ for ground state and saddle point configurations. We analyze finite size effects in the calculation of $G_{ij}$, compare the predictions of the linear elastic model with those obtained from direct calculations of defective cells under strain, and discuss the differences. Based on our data, we calculate the variations in vacancy concentration and mobility due to the presence of external strain in bulk ${\mathrm{LiCoO}}_2$ cathodes. Our results reveal that elastic in-plane and out-of-plane strains can significantly change the ionic conductivity of bulk ${\mathrm{LiCoO}}_2$ by up to several orders of magnitude and thus strongly affect the performance of Li-secondary batteries.

Theoretical evidence of the existence of 12 inequivalent Dirac cones at the vicinity of the Fermi energy in monolayered ${\mathrm{ZrB}}_2$ is presented. Two-dimensional ${\mathrm{ZrB}}_2$ is a mechanically stable $d$- and $p$-orbital compound exhibiting a unique electronic structure with two Dirac cones out of high-symmetry points in the irreducible Brillouin zone with a small electron-pocket compensation. First-principles calculations demonstrate that while one of the cones is insensitive to lattice expansion, the second cone vanishes for small perturbation of the vertical Zr position. Internal symmetry breaking with external physical stimuli, along with the relativistic effect of spin-orbit coupling, is able to remove selectively the Dirac cones. A rational explanation in terms of $d$- and $p$-orbital mixing is provided to elucidate the origin of the infrequent Dirac cones in a flat structure. The versatility of transition-metal $d$ orbitals combined with the honeycomb lattice provided by the B atoms yields particular features in a two-dimensional material.