Blue phosphorene (BlueP) is considered as a promising two-dimensional (2D) material for future-(opto)electronic applications. Monolayer BlueP is often fabricated on Au(111) single-crystal surfaces. It has been suggested that small P domains form a lateral network with Au adatoms. Previous studies attempting to use BlueP/Au(111) as a substrate for the deposition of fullerene indicated that the former is susceptible to structural deterioration, unlike most other 2D materials in this regard. Here, we investigate 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA) monolayers and multilayer films deposited on BlueP/Au(111). We monitor the film growth in real time using in situ optical differential reflectance spectroscopy with the substrate kept either at room temperature or at $\approx 120{}^{\circ }\mathrm{C}$. Furthermore, we examine the epitaxial alignment between PTCDA and BlueP by means of distortion-corrected low-energy electron diffraction and low-temperature scanning tunneling microscopy. Our data clearly reveal that PTCDA exhibits a highly ordered monolayer structure with a herringbone packing motif, whereas the domain orientations differ from all previously reported structures of PTCDA directly on Au(111). Importantly, we find no indications for adsorption-induced reordering or degradation of BlueP/Au(111). At elevated temperatures, PTCDA is observed to form multilayer islands covering only a fraction of the available PTCDA monolayer surface on BlueP/Au(111), and this Stranski-Krastanov growth mode bears similarities to literature results for PTCDA multilayers on pristine coinage metal surfaces.

The elastic and structural properties of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ and $\alpha \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ are investigated from first principles. The full elastic tensors and elastic moduli of both phases at $0\mathrm{K}$ are computed in the framework of semilocal density-functional theory. We determine mechanical instabilities of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ by evaluating the full stiffness tensor under load for a range of hydrostatic pressure values. While a phase transition from the $\beta$ to $\alpha$ phase is found to be energetically favored at $2.6\mathrm{G}\mathrm{Pa}$, we show that the $\beta$ phase is mechanically unstable only for much higher pressures $(>30\mathrm{G}\mathrm{Pa})$, which agrees well with experimental results. Our employed approach is based on the Born stability criterion, is independent of crystal symmetry, and thus can be readily applied to different materials.

Mechanistic analysis of oxygen transfer in an atomic-scale structure is important for various applications, such as the three-way catalysts for treating ${\mathrm{NO}}_x$ in automotive exhaust and solid oxide fuel cell electrolyte materials. It has been shown that the $n=2$ Ruddlesden-Popper phases ${\mathrm{Sr}}_3{\mathrm{Fe}}_2{\mathrm{O}}_{7.0-\delta }$ have superior oxygen storage capacity compared to the conventional ${\mathrm{CeO}}_2\text{−}{\mathrm{ZrO}}_2$. Atomic structure, magnetic ordering, charged defects arising due to oxygen vacancies, and the effect of lattice parameters on the ionic transport in ${\mathrm{Sr}}_3{\mathrm{Fe}}_2{\mathrm{O}}_{7.0-\delta }$ within the oxygen content of 5.75 to 7.0 were investigated by using $\mathrm{DFT}+U$ calculations. In the case of ${\mathrm{Sr}}_3{\mathrm{Fe}}_2{\mathrm{O}}_{6.0-7.0}$, the oxidation reaction energies were found to be almost independent of the initial oxygen content. We investigated the activation energies of oxygen diffusion by using the equilibrium and compressively strained lattice parameters for ${\mathrm{Sr}}_3{\mathrm{Fe}}_2{\mathrm{O}}_{6.5}$. The activation energies were found to be nearly proportional to the volume of the unit cell; namely, as the volume of the unit cell increases, the activation energy decreases. It was found that in the oxygen diffusion process the activation energy increases as the bond length between iron and oxygen decreases and the bond length is minimum in the transition state, indicating that the iron-oxygen repulsive interaction is the origin of the activation energy. Moreover, in addition to the dominant migration pathway in ${\mathrm{Sr}}_3{\mathrm{Fe}}_2{\mathrm{O}}_{6.0-7.0}$, a new migration pathway in ${\mathrm{Sr}}_3{\mathrm{Fe}}_2{\mathrm{O}}_{5.75-6.0}$ was found.

The growth of metallic dendrites during the electrodeposition and solidification of metal films is a formidable scientific and industrial problem. It is mostly known for hindering the use of high energy-density solid metal electrodes in rechargeable batteries. Over a century ago, it was experimentally shown that metallic dendrites may be mitigated under the action of mass advection in the liquid adjacent to a solid metal surface which grows out of a melt. Similar observations were reported for electrochemical deposition four decades ago and very recently. However, these insights proved inconclusive; dendrites appear to grow to different extents in many advection-based deposition systems, such as flow batteries, augmented finger batteries, and convective solidification. Here, we scrutinize the contribution of high Peclet metal ion convection in a dilute solution to the growth rate of metal dendrites, emphasizing convective effects near the metal surface. We employ an ideal model system for the isothermal flow of an ion solution along a duct where ions desorb from one solid surface of the duct, transport through the solution, and adsorb (undergo deposition) onto the other side. To account for the limit where metal-ion transport through the solution is the bottleneck for the rate of metal deposition (contributions to dendrite mitigation from near equilibrium surface phenomena are assumed small), we employ the perspective of kinetic stability to elucidate the contribution of ion convection to the mitigation of dendrite growth. We show that while the convection of metal ions appears to enhance dendrite growth — an intuitive insight — it also enhances the growth of the bulk solid metal on which the dendrites grow — a well-known phenomenon. We demonstrate that for a fixed growth rate of the bulk solid metal, the rate of growth of dendrites is smaller in the presence of convective flow compared to the case where ion transport is solely by diffusion. The difference in the relative rate of growth in the presence and absence of convective flow may span orders of magnitude, which explains the absence of significant dendrites in corresponding experiments.

Precise control of molecular-beam epitaxy deposition conditions has enabled the fabrication of chemically and structurally abrupt ${\mathrm{Eu}}_{1-x}{\mathrm{La}}_x\mathrm{O}/\mathrm{SrO}/\mathrm{Si}$ interfaces by interposing as little as half a monolayer of SrO between the La-doped EuO and silicon. The interfaces were analyzed by scanning transmission electron microscopy-electron energy loss spectroscopy; no reaction products were detected. These abrupt interfaces might enable direct ohmic injection of spin-polarized currents from the doped half-metallic semiconductor EuO into silicon.

Thermally stimulated motion of micron-sized eutectic PtGe droplets on Ge(110) has been studied in situ mainly by photoemission electron microscopy, low-energy electron microscopy, and spatially resolved low-energy electron diffraction. In line with earlier studies of eutectic AuSi, PtSi, AuGe, and PtGe clusters on, respectively, various Si and Ge substrates we find that the motion toward regions at higher temperature is driven by the entropy gain of substrate atoms which become constituents of the droplet during its journey. At ∼1100 K, i.e., well above the bulk eutectic temperature, the direction is governed solely by the local thermal gradient, irrespective of eventual crystalline preferences. Access to the diffusivity of the host material (in this case Ge) inside the eutectic droplets shows that this is well above one order of magnitude higher than expected if it was rate limiting for the velocity of the moving droplets. This excludes a significant gradient of the (Ge) concentration inside the droplet and disqualifies dissolution-diffusion-deposition flow as the driving force for motion of the droplets on the surface, as assumed widely hitherto, to explain surface diffusion of eutectic droplets on surfaces. In addition, the interface between the droplet and the surface appears flat and we find no indication for “endotaxy.” The droplets make direct contact with the flat Ge substrate through atomic steps, which are abundantly present at the interface. The droplets are surrounded by a $\mathrm{Pt}{\mathrm{Ge}}_3$ wetting layer with an ordered (2 × 1) structure. Dissolution of the edges of the wetting layer at the leading edge of the droplet with an activation energy of 2.2 eV is identified as the rate limiting step for its motion.

The infinite layer structure type has been known to host high-temperature superconductivity since the discovery of ${\mathrm{Ca}}_{0.86}{\mathrm{Sr}}_{0.14}{\mathrm{CuO}}_2$, yet little progress has been made to synthesize many analogs. Here, using ${\mathrm{SrFeO}}_x$ as a prototype system, we explore the thermodynamic obstacles behind the scarcity of $3d$ elements adopting the infinite layer structure type. In this context, synthetic considerations to achieve the $AB{\mathrm{O}}_3$ to $AB{\mathrm{O}}_2$ transformation are discussed. Specifically, we demonstrate that the conventionally reported topochemical reduction can result in hydride incorporation into $\mathrm{SrFe}{\mathrm{O}}_2$, causing a decrease in the magnetic ordering temperature of the infinite layered oxide. First-principles simulations further confirm that the incorporation of H is necessary for stabilizing the $\mathrm{SrFe}{\mathrm{O}}_2$ phase by decreasing the thermodynamic cost of individual steps required to transform $\mathrm{SrFe}{\mathrm{O}}_3$ into $\mathrm{SrFe}{\mathrm{O}}_2$, and is the driving factor behind the changes in magnetic exchange interactions that ultimately change the Néel temperature ($T_{\mathrm{N}}$). Additionally, inspired by recent reports of superconductivity in another low-dimensional oxide ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$, ${\mathrm{Sr}}_{0.95}{\mathrm{Nd}}_{0.05}{\mathrm{FeO}}_2$ was synthesized via a more traditional topochemical reduction procedure. Both physical characterization and accompanying density-functional theory calculations show that this $A$-site doping can have similar effects on $A{\mathrm{FeO}}_2$ stability and magnetic ordering temperatures as with the incorporation of hydrogen. Ultimately, these results suggest that charge doping either through the incorporation of H or $A$-site substitution may be fruitful routes in tuning stability and magnetic properties, with direct consequences on superconducting behavior.

We report combined x-ray diffraction (XRD) and electrical transport studies of the van der Waals (vdW) insulator ${\mathrm{NiPS}}_3$ to elucidate a pressure-induced insulator-to-metal transition (IMT). On application of quasihydrostatic pressure, two successive structural transitions occur at 10 and 29.4 GPa, as evidenced by the XRD and resistivity measurements. The concomitant IMT with a monoclinic-to-trigonal structural transition near 29.4 GPa turns out to be a common feature of the ${\mathrm{MPS}}_3$ family (M=transition metals). Under uniaxial-like pressure, the critical pressure for IMT is drastically reduced to $P=12.5$ GPa. These results showcase that the IMT is susceptible to strain and hydrostatic environments and that pressure offers a new venue to control a dimensional crossover in layered vdW materials.

We used density functional theory to assess the stability of nonpolar tetragonal $(P{4}_2/nmc) \mathrm{Hf}{\mathrm{O}}_2\left(110\right)$ reconstructed surfaces and the effect of polarization on the stability of the corresponding surfaces of polar orthorhombic $\left(Pca{2}_1\right) \mathrm{Hf}{\mathrm{O}}_2\left(001\right)$. The models consisted of nine-formula-unit-thick slabs with two-formula-unit surface unit cells. We determined an oxygen-terminated surface reconstruction to be the most stable surface for the nonpolar ${\mathrm{HfO}}_2$ slab, with one oxygen atom per formula unit on both sides of the slab (1.0-O/1.0-O). For the same surface composition, the ferroelectric displacements in the polar orthorhombic slab result in band bending that leads to the migration of charge carriers to the surface (with sign opposite to that of the surface polarization charge) which metallizes the surface to eliminate or reduce the net out-of-plane dipole. Ionic passivation also is effective at screening the polarization charge, which therefore alleviates band bending, leading to stabilization. This is achieved via a nonstoichiometric surface reconstruction, in which the most stable positively polarized side is oxygen-terminated with 1.5 oxygen atoms per formula unit, while the negatively polarized side has one oxygen atom per formula unit (P+:1.5-O/P−:1.0-O). This work establishes a link between the stability of the surface reconstruction and the ferroelectric polarization in ${\mathrm{HfO}}_2$, which is important for the technological need to control ferroelectric performance at the nanoscale.

The spin structure and transport behavior of B20-ordered CoSi nanomagnets are investigated experimentally and by theoretical calculations. B20 materials are of interest in spin electronics because their noncentrosymmetric crystal structure favors noncoplanar spin structures that yield a contribution to the Hall effect. However, stoichiometric bulk CoSi is nonmagnetic, and combining magnetic order at and above room temperature with small feature sizes has remained a general challenge. Our CoSi nanoclusters have an average size of 11.6 nm and a magnetic ordering temperature of 330 K. First-principle calculations and x-ray circular dichroism experiments show that the magnetic moment is predominantly confined to the shells of the clusters. The CoSi nanocluster ensemble exhibits a topological Hall effect, which is explained by an analytical model and by micromagnetic simulations on the basis of competing Dzyaloshinskii-Moriya and intra- and intercluster exchange interactions. The topological Hall effect is caused by formation of chiral spin textures in the shells of the clusters, which exhibit fractional skyrmion number and are therefore termed as paraskyrmions (closely related to skyrmion spin structures). This research shows how nanostructuring of a chiral atomic structure can create a spin-textured material with a topological Hall effect and a magnetic ordering temperature above room temperature.

The Mott-insulating rare-earth titanates ($R{\mathrm{TiO}}_3$, with $R$ being a rare-earth ion) are an important class of materials that encompasses interesting spin-orbital phases as well as ferromagnet-antiferromagnet and insulator-metal transitions. The growth of these materials has been plagued by difficulties related to overoxidation, which arises from a strong tendency of ${\mathrm{Ti}}^{3+}$ to oxidize to ${\mathrm{Ti}}^{4+}$. We describe our efforts to grow sizable single crystals of ${\mathrm{YTiO}}_3, {\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ ($x\le 0.25$), and ${\mathrm{Y}}_{1-y}{\mathrm{Ca}}_y{\mathrm{TiO}}_3$ ($y\le 0.35$) with the optical traveling-solvent floating-zone technique. We present sample characterization via chemical composition analysis, magnetometry, charge transport, neutron scattering, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism to understand macroscopic physical property variations associated with overoxidation. Furthermore, we demonstrate a good signal-to-noise ratio in inelastic magnetic neutron scattering measurements of spin-wave excitations. A superconducting impurity phase, found to appear in Ca-doped samples at high doping levels, is identified as TiO.

We investigate the energy landscape of 2D CuS, CuSe, and CuTe compounds using a global evolutionary algorithm in combination with density functional theory. Four low-lying energy ${\mathrm{Cu}}_2{X}_2$ (X=S, Se, Te) slabs with $P\overline{3}m1$, $P4/mmm$, $P4/nmm$, and $Pmmn$ symmetries have been identified on their respective potential energy surfaces. Three structures present tetrahedral ${\mathrm{Cu}X}_4$ motifs which pave the 2D slabs, while the latter is based on shared ${\mathrm{Cu}}_4{X}_2$ octahedra. The viability of each phase was examined by looking at dynamical, thermodynamic, and thermal stability criteria. We find that 2D copper monosulfide crystallizes in the $P\overline{3}m1$ phase, reminiscent of the covalent slab found in the $\mathrm{Ca}{\mathrm{Al}}_2{\mathrm{Si}}_2$ prototype. Two polymorphs of 2D copper selenide with symmetries $P\overline{3}m1$ and $P4/mmm$ are close in energy. The latter ${\mathrm{Cu}}_2{\mathrm{Se}}_2$ slab is made of fused ${\mathrm{Cu}}_4{\mathrm{Se}}_2$ square bipyramids with electronegative Se atoms in apical positions. Finally, the ground-state 2D CuTe has a $Pmmn$ space group containing distorted ${\mathrm{CuTe}}_4$ tetrahedra with some Te–Te bonding. The electronic and bond analyses show that all of these predicted 2D CuX phases are metallic with ionocovalent bonds.

In this work, we report the spontaneous formation of superlattice structures in nominal InGaN films grown by plasma-assisted molecular beam epitaxy. A 700-nm-thick self-assembled ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}/\mathrm{GaN}$ superlattice with excellent structural quality was achieved. Strain was studied as a possible driving force for the formation of self-assembled superlattice (SASL) structure by growth of InGaN on ZnO substrate using similar growth conditions. The SASL structures were optically characterized using photoluminescence spectroscopy. Structural characterization was conducted via transmission electron microscopy and atom probe tomography. High-resolution x-ray diffraction (XRD) and XRD reciprocal space map were utilized to determine the average composition and the degree of relaxation of InGaN films. We propose that the vertical phase separation observed in the SASL structure is caused by high-temperature growth and intensified by strain. This work provides a method for engineering strain and growth of thick InGaN films for a variety of applications including solar cells and photodetectors.

Oxygen vacancies play significant roles in various properties of oxide materials. Therefore, insights into the oxygen vacancies can facilitate the discovery of better oxide materials. To achieve this, we developed codes for high-throughput point-defect calculations and applied them to characterize oxygen vacancies in 937 oxides. From the resulting large dataset, we analyzed the vacancy structures and formation energies and constructed machine-learning regression models to predict vacancy formation energies. We have found that the vacancy formation energies are predicted using the random forest regression models with accuracies of 0.27–0.44 eV depending on the charge states. Analyses of the importance of the descriptors show that the formation energies of the neutral vacancies are mainly determined by the orbital characteristics of the conduction-band minima, the oxide stability, and the band gaps, whereas those of the doubly charged defects are determined by factors related to electrostatic energy. These codes and datasets are publicly available, and a graphical user interface is available to analyze the calculation results.

Elemental two-dimensional (2D) materials in monolayer form are attractive due to their simplicity and indigenous properties. In the present study, we investigate the influence of biaxial strain on group VI elemental 2D material of tellurium, which has been discussed for its potential in various applications. Considering realistic estimates of strain that may be imposed to monolayer Te through van der Waals heterostructing with other known 2D materials, we demonstrate that the structural, electrical, and thermal transport properties can get strongly influenced by strain. Importantly, through strain engineering, the thermoelectric performance of elemental 2D Te in $p$-type doping can be made comparable to that of the known binary or ternary layered compound at room temperature, and can outperform the known binary or ternary layered compounds at high to moderate temperature. The $ZT$ of monolayer Te is found to reach a value of 6.07 at 800 K under tensile strain, being larger than 2 for temperature greater than 400 K. Our study provides a way to tune the thermoelectric properties of 2D Te for future applications.

Topological materials often exhibit remarkably linear nonsaturating magnetoresistance (LMR), which is both of scientific and technological importance. However, the role of topologically nontrivial states in the emergence of such a behavior has eluded clear demonstration in experiments. Here, by reducing the coupling between the topological surface states (TSS) and the bulk carriers, we controllably tune the LMR behavior in ${\mathrm{Pt}}_{1-x}{\mathrm{Au}}_x\mathrm{LuSb}$ into distinct plateaus in Hall resistance, which we show arise from a quantum Hall phase. This allowed us to reveal how smearing of the Landau levels, which otherwise gives rise to a quantum Hall phase, results in an LMR behavior due to strong interaction between the TSS with a positive $g$ factor and the bulk carriers. We establish that controlling the coupling strength between the surface and the bulk carriers in topological materials can bring about dramatic changes in their magnetotransport behavior. In addition, our work outlines a strategy to reveal macroscopic physical observables of TSS in compounds with a semimetallic bulk band structure, as is the case in multifunctional Heusler compounds, thereby opening up opportunities for their utilization in hybrid quantum structures.

Spin reorientation transitions and other related magnetic phenomena, which owe their origin to the complex interplay between multiple magnetic sublattices, have long attracted scientific attention both from the perspective of fundamental curiosity and technological applications. In this study, combining first principles calculations together with finite temperature Monte Carlo simulations, we explore the possible origins of reorientation transition of Cr spins in ${\mathrm{NdCrO}}_3$. We construct a ${\mathrm{NdCrO}}_3$ specific magnetic model, consisting of symmetric superexchange interactions between magnetic ions, as well as their magnetic anisotropy. We show that the observed spin reorientation in ${\mathrm{NdCrO}}_3$ arises out of a delicate balance between $\mathrm{Nd}\text{−}\mathrm{Cr}$ magnetic exchange interactions, single ion anisotropy of Nd spins, and single ion anisotropy of Cr spins. Moreover, though our model does not take into consideration the effect of antisymmetric and anisotropic-symmetric magnetic exchanges, the qualitative as well as quantitative agreement of the theoretically derived and the experimentally observed spin-reorientation transition in ${\mathrm{NdCrO}}_3$ confirms the merit of our proposed microscopic model. Our results also propose a hitherto unobserved collective magnetic ordering in the Nd sublattice, which is challenging to detect as it is an extreme low temperature phenomena, which therefore calls for further investigations.

The changing thermal conductivity of an irradiated material is among the principal design considerations for any nuclear reactor, but at present few models are capable of predicting these changes starting from an arbitrary atomistic model. Here we present a simple model for computing the thermal diffusivity of tungsten, based on the conductivity of the perfect crystal and resistivity per Frenkel pair, and dividing a simulation into perfect and athermal regions statistically. This is applied to highly irradiated microstructures simulated with molecular dynamics. A comparison to experiments shows that simulations closely track observed thermal diffusivity over a range of doses from the dilute limit of a few Frenkel pairs to the high-dose saturation limit at three displacements per atom (dpa).

Metal halide perovskite semiconductors have outstanding optoelectronic properties. Although these perovskites are defect-tolerant electronically, defects hamper their long-term stability and cause degradation. Density functional theory (DFT) calculations are an important tool to unravel the microscopic structures of defects, but results suffer from the different approximations used in the DFT functionals. In the case of metal halide perovskites, qualitatively different results have been reported with different functionals, either predicting vacancy or interstitial point defects to be most dominant. Here, we conduct a comprehensive comparison of a wide range of functionals for calculating the equilibrium defect formation energies and concentrations of point defects in the archetype metal halide perovskite, ${\mathrm{MAPbI}}_3$. We find that it is essential to include long-range van der Waals interactions in the functional, and that it is vital to self-consistently optimize structure and volume of all compounds involved in the defect formation. For calculating equilibrium formation energies of point defects in ${\mathrm{MAPbI}}_3$ and similar metal halide perovskites, we argue that the exact values of the chemical potentials of the species involved, or of the intrinsic Fermi level, are not important. In contrast to the simple Schottky or Frenkel pictures, we find that the dominant defects are MA and I interstitials, and Pb vacancies.