A multiscale model is developed to simulate the climb-assisted glide of edge dislocations anchored by a random distribution of nanosized vacancy clusters. Atomic-scale simulations allowed us to characterize the interactions between an edge dislocation and nanovoids as a function of their sizes and shapes. The atomic-scale data were used to calibrate the parameters of an elastic line model, which we employed to evaluate the average glide distance of a dislocation with realistic dimensions. To complete our scheme, a standard model for the climb velocity of edge dislocations was enhanced with atomic-scale inputs in order to determine the deformation rate expected through the climb-assisted glide. Our predictions made for the archetypical case of Al are in good agreement with experiments of different types, i.e., tensile deformation tests and steady creep tests.

We report x-ray photon correlation spectroscopy (XPCS) experiments on a series of concentrated oil-in-water nanoemulsions with varying droplet volume fraction subjected to in situ steady-state large-amplitude oscillatory shear (LAOS). The shear strain causes periodic echoes in the x-ray speckle patterns that lead to peaks in the intensity autocorrelation function. Above an onset strain amplitude that depends on nanoemulsion concentration, the peaks become attenuated, signaling spatially heterogeneous, shear-induced droplet dynamics. These dynamics include irreversible rearrangements among the droplets that occur in some regions of the nanoemulsions during a given shear cycle and residual strain-like displacements in those regions that do not rearrange. The wave-vector dependence of the peak attenuation indicates a power-law distribution in the size of regions undergoing shear-induced rearrangement that is similar to that observed previously in LAOS-XPCS measurements on concentrated nanocolloidal gels. The values of the onset strains for rearrangement correlate with the concentration-dependent macroscopic yielding behavior of the nanoemulsions. Specifically, they occur below the strains at which the nanoemulsions become effectively fluidized and, except for the lowest-concentration nanoemulsion in the study, significantly above the threshold strain for nonlinear rheological response.

In this paper, we combine density functional theory data with a thermodynamic and a kinetic model to determine the total concentration of hydrogen implanted in the subsurface of tungsten exposed to a hydrogen flux. The subsurface hydrogen concentration is calculated given a flux of hydrogen, a temperature of implantation, and the energy of the incoming hydrogen ions as independent variables. This global model is built step by step; an equilibrium between atomic hydrogen within bulk tungsten and a molecular hydrogen gas phase is first considered, and the calculated solubility is compared with experimental results. Subsequently, a kinetic model is used to determine the chemical potential for hydrogen in the subsurface of tungsten. Combining both these models, two regimes are established in which hydrogen is preferentially trapped either at interstitial sites or in vacancies. We deduce from our model that the existence of these two regimes is driven by the temperature of the implanted tungsten sample; above a threshold or transition temperature is the interstitial regime, and below is the vacancy regime in which supersaturated layers form within tenths of an angstrom below the surface. A simple analytical expression is derived for the coexistence of the two regimes depending on the implantation temperature, the incident energy, and the flux of the hydrogen ions which we use to plot the corresponding phase diagram.

Many dopants are incorporated into ${\mathrm{LiNbO}}_3$ (LNO) to modify the physical properties, particularly many optical properties. To understand these changes, the substitution site for the dopant must be known. Here we provide a detailed EXAFS study of the local environment about the metal (M) ion dopants indium, erbium, and hafnium in LNO, using crystals grown in both stoichiometric and congruent forms. In primarily substitutes at the Li site in congruent material; Er also substitutes mostly on the Li site with a small fraction roughly 10% on the Nb site—the substitution sites for Er do not appear to change much between stoichiometric and congruent LNO. For Hf, approximately half the dopant substitutes on Li and half on Nb. In all cases, some charge compensating defects are required (self-compensating schemes are not consistent with the data), and can occupy several nearby sites in the crystal leading to many slightly different distributions, some of which may be metastable. For each dopant, changes in the the M-O shell approximately follow the ionic radius of the dopant ion; in addition, the Nb neighbors about M on a Li site are shifted away from the dopant, with large displacements for some dopants such as Er. Comparisons with other experiments and existing calculations are discussed.

Charge transfer in oxide heterostructures can be tuned to promote emergent interfacial states, and accordingly, has been the subject of intense study in recent years. However, accessing the physics at these interfaces, which are often buried deep below the sample surface, remains difficult. Addressing this challenge requires techniques capable of measuring the local electronic structure with high-resolution depth dependence. Here, we used linearly polarized resonant x-ray reflectometry (RXR) as a means to visualize charge transfer in oxide superlattices with single unit cell precision. From our RXR measurements, we extract valence depth profiles of ${\mathrm{SmTiO}}_3\left(\mathrm{SmTO}\right)/{\mathrm{SrTiO}}_3$ (STO) heterostructures with STO quantum wells varying in thickness from five SrO planes down to a single SrO plane. At the polar-nonpolar SmTO/STO interface, an electrostatic discontinuity leads to approximately half an electron per areal unit cell transferred from the interfacial SmO layer into the neighboring STO quantum well. We observe this charge transfer as a suppression of the $t_{2g}$ absorption peaks that minimizes contrast with the neighboring SmTO layers at those energies and leads to a pronounced absence of superlattice peaks in the reflectivity data. Our results demonstrate the sensitivity of RXR to electronic reconstruction in single unit cell layers, and establish RXR as a powerful means of characterizing charge transfer at buried oxide interfaces.

Among the numerous exotic phenomena exhibited by honeycomb antiferromagnets, linear magnetoelectricity in niobiates and tantalates of an $A_4{M}_2{\mathrm{O}}_9$ formula has been attracting a lot of attention. The magnetic frustration, first exploited in $\alpha \text{−}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3$ in the 1960s, can be tuned in the “429” compounds of which structures derive from corundum, by varying the $3d$ orbitals filling (from $3d^5$ to $3d^7$ as A goes from $\mathrm{M}{\mathrm{n}}^{2+}$ to $\mathrm{C}{\mathrm{o}}^{2+}$) and/or the size of the $d^0$M cation ($\mathrm{N}{\mathrm{b}}^{5+}$ or $\mathrm{T}{\mathrm{a}}^{5+}$). $\mathrm{F}{\mathrm{e}}_4\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_9$ illustrates the physics richness of these compounds, as it exhibits a rare combination of multiferroism and linear magnetoelectricity associated to different magnetic field dependences of the electrical polarization.

The development of reliable and highly energy efficient multiferroic nanosystems, which can function at room temperature, is key for the design of ultralow-power magnetoelectric devices. Here, we report electrically controlled magnetic domain wall motion and magnetization switching in $\mathrm{BaTi}{\mathrm{O}}_3\text{/}\mathrm{C}{\mathrm{o}}_{50}\mathrm{F}{\mathrm{e}}_{50}$ microstructures, at room temperature. The perfect one-to-one connection between the ferroelectric domain pattern of the $\mathrm{BaTi}{\mathrm{O}}_3$ crystal and the ferromagnetic state of the CoFe microstructures, which relies on a strain-induced magnetic anisotropy modification, is the cause of the observed magnetoelectric effect. As a result, the observed electrically driven magnetization switching is highly reliable, independent of the shape and size of the microstructures. This is a key factor that makes the studied multiferroic system very promising for integration in real-world magnetoelectric devices.

We present the Co-Gd composition dependence of the spin-Hall magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) for ferrimagnetic $\mathrm{C}{\mathrm{o}}_{100-x}\mathrm{G}{\mathrm{d}}_x/\mathrm{Pt}$ bilayers. With Gd concentration $x$, its magnetic moment increasingly competes with the Co moment in the net magnetization. We find a nearly compensated ferrimagnetic state at $x=24$. The AMR changes sign from positive to negative with increasing $x$, vanishing near the magnetization compensation. On the other hand, the SMR does not vary significantly even where the AMR vanishes. These experimental results indicate that very different scattering mechanisms are responsible for AMR and SMR. We discuss a possible origin for the alloy composition dependence.

We investigate a range of possible materials containing the supermolecular form of hydrogen comprising 13 ${\mathrm{H}}_2$ molecules arranged in an icosahedral arrangement. This supermolecule consists of freely rotating 12 ${\mathrm{H}}_2$ molecules in an icosahedral arrangement, enclosing another freely rotating ${\mathrm{H}}_2$ molecule. To date, this supermolecule has only been observed in a compound with iodane (HI). The extremely high hydrogen content suggests possible application in hydrogen storage so we examine the possibility of supermolecule formation at ambient pressures. We show that ab initio molecular dynamics calculations give a good description of the known properties of the $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$ material, and we make predictions of the existence of the related compounds $\mathrm{Xe}{\left({\mathrm{H}}_2\right)}_{13}$, $\mathrm{HBr}{\left({\mathrm{H}}_2\right)}_{13}$, and $\mathrm{HCl}{\left({\mathrm{H}}_2\right)}_{13}$, including a symmetry-breaking phase transition at low temperature. The icosahedral ${\left({\mathrm{H}}_2\right)}_{13}$ supermolecule remains stable in all these compounds. This suggests ${\left({\mathrm{H}}_2\right)}_{13}$ is a widespread feature in hydrogen compounds and that appropriately sized cavities could hold hydrogen supermolecules even at low pressure. The structure of the supermolecule network is shown to be independent of the compound at equivalent density.

We report on a bottom-up approach via molecular-beam epitaxy to grow high-quality crystalline two-dimensional films of ${\mathrm{PdTe}}_2$ on ${\mathrm{SrTiO}}_3$(001), and investigation on the electronic and superconducting properties therein by using scanning tunneling microscopy and transport experiments in combination with first-principles calculations. We observed a transition from the narrow-gap semiconducting phase in the monolayer to the metallic phase in the multilayer films. Importantly, all the multilayer films exhibited robust superconductivity, whose transition temperatures in the ultrathin limit were obviously higher than expected from the empirical inverse-of-thickness dependence and could be further enhanced through magnesium intercalation. Our first-principles studies revealed thickness-tuned band topology caused by the significant orbital-dependent interlayer coupling, which explained the experimentally observed thickness-dependent behaviors of atomic and electronic properties. Combined with the prediction of topologically nontrivial states introduced by honeycomb lattice of $p$ orbitals, ${\mathrm{PdTe}}_2$ thin films are promising material candidates for exploring the interplay between superconductivity and topology in the two-dimensional limit.

First-principles calculations based on density-functional theory were performed to investigate heterostructures of group-III monochalcogenides (GaS, GaSe, InS, and InSe) and the effects of incommensurability on their electronic structures. We considered two heterostructures: GaS/GaSe, which has a lattice mismatch of 4.7%, and GaSe/InS, with a smaller mismatch of 2.1%. We computed the cost of having commensurate structures, and we also examined the potential energy landscape of both heterostructures in order to simulate the realistic situation of incommensurate systems. We found that a commensurate heterostructure may be realized in GaSe/InS as the interaction energy of this system with the monolayers assuming the average lattice constant is smaller than the interaction energy of an incommensurate system in which each layer keeps its own lattice constant. For GaS/GaSe, on the other hand, we found that the incommensurate heterostructure is energetically more favorable than the commensurate one, even when taking into account the energetic cost due to the lack of proper registry between the layers. Since the commensurate condition requires that one (or both) layer(s) is (are) strained, we systematically investigated the effect of strain on the band gaps and band-edge positions of the monolayer systems. We found that, in all monolayers, the conduction-band minimum is more than two times more sensitive to applied strain than the valence-band maximum; this was observed to strongly affect the band alignment of GaS/GaSe, as it can change from type-I to type-II with a small variation in the lattice constant of GaS. The GaSe/InS heterostructure was found to have a type-II alignment, which is robust with respect to strain in the range of $-2\%$ to $+2\%$.

We present an approach to control resistive switching in metal-ferroelectric contacts using a radially symmetric electric field. In ferroelectrics with significant polarization along the corresponding field lines, the field above a critical threshold will induce polarization discontinuity, a corresponding nanoscale volume of space charge, and a conducting junction. We demonstrate this principle using nanoscale polarization switching of a conventional (001)-oriented thin film of $\mathrm{BiFe}{\mathrm{O}}_3$. Without any optimization, the conducting state created in this regime of resistive switching exhibits local currents of $\sim 1\text{–}10\mathrm{nA}$, approaching the $\sim 100\mathrm{nA}$ threshold required for device implementation [J. Jiang et al., Nat. Mater. 17, 49 (2018)]. The corresponding electronic function is that of a volatile resistive switch, which is directly compatible with neuristor functionality that encodes the functioning basis of an axon [M. D. Pickett et al., Nat. Mater. 12, 114 (2013)]. Phase-field modeling further reveals that in the strongly charged local configuration, $\mathrm{BiFe}{\mathrm{O}}_3$ locally undergoes a rhombohedral-tetragonal (R-T) phase transition, in part due to substantial piezoelectric expansion of the lattice. The estimated local charge density can be as high as $\sim {10}^{21}\mathrm{c}{\mathrm{m}}^{-3}$, which would be extremely difficult to achieve by conventional doping approaches without altering other material properties. Therefore, this method for creating stable and reproducible strongly charged ferroelectric junctions enables more systematic studies of their physical properties, such as the possibility of structural and electronic phase transitions, and it can lead to new ferroelectric devices for advanced information functions.

We demonstrate emergent antiferromagnetic interactions in strained thin films of the mixed valence manganite $(\mathrm{Eu},\mathrm{Sr}){\mathrm{MnO}}_3$. Although the composition studied, ${\mathrm{Eu}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$, will nominally yield a ferromagnetic phase in the bulk, we observe significant suppression of the saturation magnetization in films under both tensile and compressive strain. Despite the magnetization suppression, muon spin rotation spectrometry and polarized neutron reflectometry reveal uniform magnetic ordering in these films. Neutron diffraction and x-ray absorption spectroscopy demonstrate high-temperature G-type antiferromagnetic order in films under tensile strain, in contrast to the A- and C-type orders that are found in the phase diagrams of bulk $(\mathrm{Eu},\mathrm{Sr}){\mathrm{MnO}}_3$ and other mixed-valence manganites. These probes also show that compressive strain results in a uniform but suppressed magnetization suggestive of moment canting resulting from competition between ferromagnetism and antiferromagnetism.

A quantitative geometric predictor for the dimensionality of magnetic interactions is presented. This predictor is based on networks of superexchange interactions and can be quickly calculated for crystalline compounds of arbitrary chemistry, occupancy, or symmetry. The resulting data are useful for classifying structural families of magnetic compounds. We have examined compounds from a demonstration set of 42 520 materials with $3d$ transition metal cations. The predictor reveals trends in magnetic interactions that are often not apparent from the space group of the compounds, such as triclinic or monoclinic compounds that are strongly 2D. We present specific cases where the predictor identifies compounds that should exhibit competition between 1D and 2D interactions, and how the predictor can be used to identify sparsely populated regions of chemical space with as-yet-unexplored topologies of specific $3d$ magnetic cations. The predictor can be accessed for the full list of compounds using a searchable front end and further information on the connectivity, symmetry, valence, and cation-anion and cation-cation coordination can be freely exported.

We have quantified substitutional impurity concentrations in synthetic diamond crystals down to sub-parts-per-billion levels. The capture lifetimes of electrons and excitons injected via photoexcitation were compared for several samples with different impurity concentrations. Based on the assessed impurity concentrations, we have determined the capture cross section of electrons for boron impurity, ${\sigma }_{\mathrm{A}}=1\times {10}^{-14}{\mathrm{cm}}^2$, and that of excitons for nitrogen impurity, ${\sigma }_{\mathrm{D}}^{\mathrm{ex}}=3\times {10}^{-14}{\mathrm{cm}}^2$. The general tendency of the mobility values for different carrier species is successfully reproduced by including carrier scattering by impurities and excitons.

When a swift heavy ion (SHI) penetrates amorphous $\mathrm{Si}{\mathrm{O}}_2$, a core/shell (C/S) ion track is formed, which consists of a lower-density core and a higher-density shell. According to the conventional inelastic thermal spike (iTS) model represented by a pair of coupled heat equations, the C/S tracks are believed to form via “vaporization” and melting of the $\mathrm{Si}{\mathrm{O}}_2$ induced by SHI (V-M model). However, the model does not describe what the vaporization in confined ion-track geometry with a condensed matter density is. Here we reexamine this hypothesis. While the total and core radii of the C/S tracks determined by small angle x-ray scattering are in good agreement with the vaporization and melting radii calculated from the conventional iTS model under high electronic stopping power $\left(S_e\right)$ irradiations (>10 keV/nm), the deviations between them are evident at low-$S_e$ irradiation (3–5 keV/nm). Even though the iTS calculations exclude the vaporization of $\mathrm{Si}{\mathrm{O}}_2$ at the low $S_e$, both the formation of the C/S tracks and the ion shaping of nanoparticles (NPs) are experimentally confirmed, indicating the inconsistency with the V-M model. Molecular dynamics (MD) simulations based on the two-temperature model, which is an atomic-level modeling extension of the conventional iTS, clarified that the “vaporlike” phase exists at $S_e\sim 5$ keV/nm or higher as a nonequilibrium phase where atoms have higher kinetic energies than the vaporization energy, but are confined at a nearly condensed matter density. Simultaneously, the simulations indicate that the vaporization is not induced under 50-MeV Si irradiation $(S_e\sim 3$ keV/nm), but the C/S tracks and the ion shaping of nanoparticles are nevertheless induced. Even though the final density variations in the C/S tracks are very small at the low stopping power values (both in the simulations and experiments), the MD simulations show that the ion shaping can be explained by flow of liquid metal from the NP into the transient low-density phase of the track core during the first ∼10 ps after the ion impact. The ion shaping correlates with the recovery process of the silica matrix after emitting a pressure wave. Thus, the vaporization is not a prerequisite for the C/S tracks and the ion shaping.

Reduced ceria (${\mathrm{CeO}}_{2-x}$) has a rich phase diagram comprising a variety of bulk phases with stoichiometries ranging from ${\mathrm{CeO}}_2$ to ${\mathrm{Ce}}_2{\mathrm{O}}_3$ as the reduction conditions are increased. Some observed reduced ceria (111) surface reconstructions are terminations of stable bulk phases, such as $\sqrt{7}\times \sqrt{7}$, whereas others are not associated with any known stable phases, namely, $\sqrt{7}\times 3,3\times 3$, and $\sqrt{3}\times \sqrt{3}$. In this paper, we analyze different periodic reconstructions of reduced ceria (111) surfaces within a theoretical framework. We describe the surfaces of stable bulk phases and elucidate the structure of the observed periodicities not corresponding to terminations of known stable bulk phases. Extending the $3\times 3$ and $\sqrt{3}\times \sqrt{3}$ terminations into a bulk structure, we find a new quasistable ${\mathrm{Ce}}_3{\mathrm{O}}_5$ bulk phase, which explains both periodicities. On the contrary, the $\sqrt{7}\times 3$ surface structure cannot be extended into a bulk but stabilizes as a very thin overlayer. We also analyze other reported bulk structures with stoichiometry close to the ${\mathrm{Ce}}_3{\mathrm{O}}_5$ one, namely, the $C$-type ${\mathrm{Ce}}_2{\mathrm{O}}_{3+\delta }$ bulk phase, which cannot describe the observed terminations. Our goal is to provide a unified picture to describe the road map of ${\mathrm{CeO}}_{2-\delta }\left(111\right)$ surface reconstructions and, when possible, their relationship to reduced ceria bulk phases.

Conventional linear elasticity theory predicts the strain fields of a dislocation core to diverge, whereas it is known from atomistic simulations that core strains should remain finite. We present an analytical solution to a generalized, variational Peierls-Nabarro model of edge dislocation displacement fields that features a finite core width and correct isotropic elastic behavior at large distances away from the core. We derive an analytical expression for the dislocation core radius, representing the convergence radius of the linear elasticity far-field expansion. The strain fields are in qualitative agreement with atomistic simulations of $\frac{1}{2}\left[111\right]\left(10\overline{1}\right)$ edge dislocations in bcc tungsten and iron. The treatment is based on the multistring Frenkel-Kontorova model that we reformulate as a generalized Peierls-Nabarro model using the principle of least action.

Electric-field switching of polarization is the building block of a wide variety of ferroelectric devices. In turn, understanding the factors affecting ferroelectric switching and developing routes to control it are of great technological significance. This work provides systematic experimental evidence of the role of defects in affecting ferroelectric-polarization switching and utilizes the ability to deterministically create and spatially locate point defects in $\mathrm{PbZ}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}{\mathrm{O}}_3$ thin films via focused-helium-ion bombardment and the subsequent defect-polarization coupling as a knob for on-demand control of ferroelectric switching (e.g., coercivity and imprint). At intermediate ion doses ($0.22–2.2\times {10}^{14}\mathrm{ions}{\mathrm{cm}}^{-2}$), the dominant defects (isolated point defects and small clusters) show a weak interaction with domain walls (pinning potentials from $200–500\mathrm{K}\mathrm{MV}{\mathrm{cm}}^{-1}$), resulting in small and symmetric changes in the coercive field. At high doses ($0.22–1\times {10}^{15}\mathrm{ions}{\mathrm{cm}}^{-2}$), on the other hand, the dominant defects (larger defect complexes and clusters) strongly pin domain-wall motion (pinning potentials from 500 to $1600\mathrm{K}\mathrm{MV}{\mathrm{cm}}^{-1}$), resulting in a large increase in the coercivity and imprint, and a reduction in the polarization. This local control of ferroelectric switching provides a route to produce novel functions; namely, tunable multiple polarization states, rewritable pre-determined 180° domain patterns, and multiple zero-field piezoresponse and permittivity states. Such an approach opens up pathways to achieve multilevel data storage and logic, nonvolatile self-sensing shape-memory devices, and nonvolatile ferroelectric field-effect transistors.