The formation energies of oxygen and silicon impurities have been examined explicitly in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ using hybrid exchange-correlation density-functional theory simulations. Both impurities were initialized in on-site substitutional and off-site DX configurations in a range of charge states. The ${\mathrm{O}}_{\mathrm{N}}^{+1}$ donor was found to always relax into an on-site configuration, and its formation energy is relatively independent of local chemistry (the configuration of Al and Ga atoms surrounding the defect). By contrast, the ${\mathrm{O}}_{\mathrm{N}}^{-1}$ acceptor almost always relaxes into a DX configuration, with a formation energy that is strongly dependent on local chemistry. The differences in formation energy of distinct ${\mathrm{O}}_{\mathrm{N}}^{-1}$ defect configurations are understood through the interplay of two qualitative trends in the types of nearest-neighbor bonds (O-Al or O-Ga), as well as the subtler influence of the lengths of the O-Al bonds. Knowledge of ${\mathrm{O}}_{\mathrm{N}}^{-1}$ formation energies as well as the relative frequencies of sites with different local chemistry allows one to compute the relative site occupancies of ${\mathrm{O}}_{\mathrm{N}}^{-1}$. Because the thermodynamic transition levels associated with different defect configurations are unique, the ${\mathrm{O}}_{\mathrm{N}}$ DX transition is associated with multiple defect levels. ${\mathrm{Si}}_{\mathrm{III}}$, where III represents the group III cation of Al or Ga, provides an interesting counterexample. ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is predicted to be the dominant charge state across the entire band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$, and little dependence of the formation energy on the composition of nearby cation sites was found. This is explained by the fact that the first-nearest neighbors are all of the same species (N), so the local environment is similar to a bulk III nitride, in which on-site ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is stable across the same Fermi level range (i.e., below the band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$). Thus, the trends in the energetics of ${\mathrm{O}}_{\mathrm{N}}$ and ${\mathrm{Si}}_{\mathrm{III}}$ in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ are both determined by the chemistry of the four nearest-neighbor sites surrounding the defect site.

The switching in resistive random-access memory (RRAM) relies on the migration of ions in sputtered amorphous transition metal oxides to both create and annihilate conductive filaments. A comprehensive understanding of ionic diffusion in these materials is therefore critical for optimizing future RRAM devices. While predicting diffusion barriers in crystalline materials is straightforward, the complex energy landscape in disordered materials makes analysis of the effective diffusion barrier inherently more difficult. Using classical molecular dynamics, we examine oxygen diffusion at various temperatures in amorphous tantala $\left({\mathrm{Ta}}_2{\mathrm{O}}_5\right)$, a leading material for RRAM applications. We compare the predicted activation energies for self-diffusion to those from isotope tracer diffusion experiments. We find a diffusion activation energy of 1.55–1.60 eV, which is higher than that measured in isotopic diffusion experiments $\left(1.2\pm 0.1\mathrm{eV}\right)$ on amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$. However, the predicted values are comparable to the activation energy $\left(1.60\pm 0.18\mathrm{eV}\right)$ determined from secondary ion mass spectroscopy analysis of isotopic diffusion experiments of sintered L-${\mathrm{Ta}}_2{\mathrm{O}}_5$ samples. We find that this difference could be due to issues of deposited film porosity, film stoichiometry, or else a preference for the empirical potentials to form nanocrystalline regions.

Ionic liquids form intricate nanostructures, both in the bulk and near charged surfaces. We show that given the ionic positions from molecular simulations, the ionic charges minimize a “spin-glass” Hamiltonian for nearest-neighbor interactions with remarkable accuracy, for both room-temperature ionic liquids and water-in-salt electrolytes. Thus, long-range charge oscillations in ionic liquids result from positional ordering, which is maximized in ionic solids but gradually disappears with added solvent, increased temperature, or by complex molecular structures. As the electrolyte becomes more disordered, geometrical frustration in the spin-glass ground state reduces correlation lengths. Eventually, thermal fluctuations excite the system from its ground state and Poisson-Boltzmann behavior is recovered. More generally, spin-glass ordering arises in any liquid with antiferromagnetic correlations, such as molten salt or the two-dimensional vortex patterns found in superfluids and bacterial turbulence.

${\mathrm{ZnGeN}}_2$ offers a low-cost alternative to InGaN with the potential for band-gap tuning to span the green gap using cation site ordering. The addition of oxygen on the anion site creates an additional degree of electronic tunability. Here, we investigate the structure and optoelectronic properties of an epitaxial ${\mathrm{ZnGeN}}_{2-x}{\mathrm{O}}_x$ thin film library grown by combinatorial co-sputtering on c-${\mathrm{Al}}_2{\mathrm{O}}_3$. Samples exhibit x-ray diffraction patterns and x-ray pole figures characteristic of a wurtzite (cation-disordered) structure with the expected sixfold in-plane symmetry. Transmission electron microscopy reveals a semicoherent interface with periodic dislocations that relieve strain from the large lattice mismatch and also confirms the in-plane and out-of-plane crystallographic orientation. Room-temperature photoluminescence exhibits peaks between 2.4 and 2.8 eV which are consistent with a sharp absorption onset observed by UV-vis spectroscopy. These results demonstrate low-cost synthesis of optically active yet cation disordered ${\mathrm{ZnGeN}}_{2-x}{\mathrm{O}}_x$, indicating a path toward application as a blue-green emitter.

The low-frequency range of vibrational spectra is sensitive to collective vibrations of the lattice. In molecular crystals, it can be decisive in identifying the structure of different polymorphs and, in addition, it plays an important role in the magnitude of the temperature-dependent component of vibrational free-energy differences between these crystals. We study the vibrational Raman spectra and vibrational density of states of different polymorphs of the flexible aspirin and paracetamol crystals based on dispersion-corrected density functional theory, density functional perturbation theory, and ab initio molecular dynamics. We examine the effect of quasiharmonic lattice expansion and compare the results of harmonic theory and the time-correlation formalism for vibrational spectra. Lattice expansion strongly affects the collective vibrations below 300 ${\mathrm{cm}}^{-1}$, but it is significantly less important at higher frequencies, while thermal nuclear motion can be important in the full vibrational range. We also observe that the inclusion or neglect of many-body van der Waals dispersion interactions does not cause large differences in the low-frequency range of Raman spectra or vibrational density of states, provided the lattice constants are fixed. We obtain quantitative agreement with experimental room-temperature Raman spectra below 300 ${\mathrm{cm}}^{-1}$ for all polymorphs studied. Examining the two-dimensional correlations between different vibrations, we find which modes show a larger degree of anharmonic coupling to others, providing a possible route to assess the accuracy of harmonic free-energy evaluations in different cases.

Twin nucleation and growth are prevalent plastic deformation mechanisms in hexagonal close-packed metals such as Mg. For twin growth to occur, the interfaces that border the twinned domain must migrate and the kinetics associated with this process are yet to be fully explained. Thus, the objective of this study is to characterize the relationship between the kinetics of the $\left\{10\overline{1}2\right\}$ twin boundary in pure Mg in the stress-driven regime, and the nucleation, growth, and coalescence of disconnection terraces that serve as the mechanisms for migration. This problem is addressed via atomistic simulations adopting both two- (2D) and three-dimensional (3D) simulation geometries, and a model for the velocity of the $\left\{10\overline{1}2\right\}$ twin boundary as a function of temperature and shear stress is proposed. This study shows that the kinetics of $\left\{10\overline{1}2\right\}$ twin boundary migration must be addressed using 3D models, as 2D simulations do not properly capture disconnection terrace nucleation and growth processes, demonstrated via differences in activation volumes and energies. Importantly, simulations reveal an autocatalytic terrace nucleation mechanism as playing a role in twin growth, where nucleation of a new terrace is dependent on the growth of existing terraces on the twin plane.

Recently, it has been found experimentally that hydrated nacre exhibits a nonlinear mechanical response. While mechanical nonlinearity has been shown to be important in other biological structures, such as spider webs, the implications of mechanical nonlinearity in nacre have not been explored. Here, we show that the nonlinear mechanical response of nacre can be reproduced by an analytical model, which reflects a nacre-like layered structure, consisting of linear-elastic hard sheets glued together by weakly nonlinear-elastic soft sheets. We develop scaling analysis on this analytical model and perform numerical simulations using a lattice model, which is a discrete counterpart of the analytical model. Unexpectedly, we find that the weak nonlinearity in the soft component significantly contributes to enhancing toughness by redistributing the stress at a crack tip over a wider area, and that the weak nonlinearity could be regarded as a result of mechanical optimaization. Beyond demonstrating a mechanism that explains the unusual properties of biological nacre, this study points to a general design principle for constructing tough composites using weak nonlinearity and provides useful guiding principles to develop artificial layered structures mimicking nacre.

Known experiments revealed that various deformation-induced defects in wrought Mg-based alloys indeed accelerate the corrosion rate. In particular, for Mg-based alloys the twinning mode under plastic deformation is the usually and more easily occurring one than other defects. However, to date it has been very difficult to study the specific effect of twin boundaries (TBs) on the corrosion behavior of Mg-based alloys due to the coexistence of other defects in wrought alloy. Here, through first-principles calculations we have theoretically investigated the effects of TBs on the anodic dissolutions of Mg metal. It has been found that the presence of various TBs ($\mathrm{TB}1\left\{10\overline{1}1\right\}\left[1\overline{2}10\right]$, $\mathrm{TB}2\left\{10\overline{1}2\right\}\left[1\overline{2}10\right]$, or $\mathrm{TB}3\left\{10\overline{1}3\right\}\left[1\overline{2}10\right]$) accelerates the corrosion rate and the TB-induced accelerations of the corrosion current density are indeed correlated with the TB interfacial length per area. Physically, the existence of the TBs on the specified surfaces increases the surface energy density, which implies that atoms at various TB-containing surfaces become less stable in energy as compared with the TB-free surface. In addition, we have analyzed that some alloying elements (As, Cd, Hg, Zn, and Sn) in combination with TBs can reduce the anodic dissolution rate, potentially enhancing the corrosion resistance of Mg-based alloys.

The tetradymite compound ${\mathrm{BiSbTeSe}}_2$ is one of the most bulk-insulating three-dimensional topological insulators, which makes it important in topological insulator research. It is a member of the solid-solution system ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Te}}_{3-y}{\mathrm{Se}}_y$, for which the local crystal structure, such as the occupation probabilities of each atomic site, is not well understood. We have investigated the temperature- and polarization-dependent spontaneous Raman scattering in ${\mathrm{BiSbTeSe}}_2$, revealing a much higher number of lattice vibrational modes than predicted by group-theoretical considerations for the space group $R\overline{3}m$ corresponding to an ideally random solid-solution situation. The density-functional calculations of phonon frequencies show a very good agreement with experimental data for parent material ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, where no disorder effects were found. In comparison to ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ the stacking disorder in ${\mathrm{BiSbTeSe}}_2$ causes a discrepancy between theory and experiment. Combined analysis of experimental Raman spectra and density-functional-theory-calculated phonon spectra for different types of atomic orders showed coexistence of different sequences of layers in the material and that those with Se in the center and a local order of Se-Bi-Se-Sb-Te are the most favored.

Room-temperature ferromagnetism was characterized for thin films of $\mathrm{SrT}{\mathrm{i}}_{0.6}\mathrm{F}{\mathrm{e}}_{0.4}{\mathrm{O}}_{3\text{−}\delta }$ grown by pulsed laser deposition on $\mathrm{SrTi}{\mathrm{O}}_3$ and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from $\mathrm{F}{\mathrm{e}}^{2+}$ cations, whereas $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{T}{\mathrm{i}}^{4+}$ did not contribute. Films with the highest magnetic moment ($0.8{\mu }_{\mathrm{B}}$ per Fe) had the highest measured $\mathrm{F}{\mathrm{e}}^{2+}$:$\mathrm{F}{\mathrm{e}}^{3+}$ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies $(\delta =0.19)$. Postgrowth annealing treatments under oxidizing and reducing conditions demonstrated quenching and partial recovery of magnetism respectively, and a change in Fe valence states. The study elucidates the microscopic origin of magnetism in highly Fe-substituted $\mathrm{SrT}{\mathrm{i}}_{1-x}\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_{3\text{−}\delta }$ perovskite oxides and demonstrates that the magnetic moment, which correlates with the relative content of $\mathrm{F}{\mathrm{e}}^{2+}$ and $\mathrm{F}{\mathrm{e}}^{3+}$, can be controlled via the oxygen content, either during growth or by postgrowth annealing.

Dopant atoms captured by vacancies are known to obstruct vacancy-mediated atomic diffusion. Yet, there are cases where the opposite behavior has been reported. Here, Na doping in Cu(In,Ga)${\mathrm{Se}}_2$ is shown to enhance or impede the interdiffusion of In and Ga, depending on the relative concentration of Na and Cu. The experimental study suggests that doping a host material may enhance heteroatom diffusivities if (i) dopant migration occurs through the Frank-Turnbull mechanism and (ii) there is overlap between the (cationic/anionic) migration sublattices of dopant and heteroatom. Under such conditions a migration catalysis may occur at sufficiently low dopant concentrations, despite the decreased absolute concentration of point defects responsible for heteroatom diffusion. Herein, a mechanism of doping-assisted catalytic migration is presented.

A detailed magnetoresistance (MR) study of bulk and microflake samples of highly oriented pyrolytic graphite in a broad temperature $240\gtrsim T\gtrsim 1\mathrm{K}$ and magnetic field ${\mu }_{0}H\le 62\mathrm{T}$ range, reveals the existence of three independent phenomena, the contributions of which are observed at different temperatures and fields. The identification of the three phenomena was possible by studying the MR of samples with thickness of 25 $\mu \mathrm{m}$ to 23 nm. At temperatures $T\gtrsim 100\mathrm{K}$ the MR is mainly given by the semiconducting stacking order regions. At lower temperatures the contribution of the internal interfaces of graphite to its MR is clearly observable. These interfaces are the origin of the commonly observed electronic phase transitions at fields $35\lesssim {\mu }_{0}H\lesssim 55\mathrm{T}$ at $T\lesssim 10$ K as well as a background MR in the whole field range that resembles the MR measured in granular superconductors.

Nuclear magnetic resonance measurements of spin relaxation are used in studies of liquid-saturated porous media where multidimensional relaxation correlations probe interpore diffusion and other transport processes. However, the magnetic susceptibility contrast between the solid and liquid results in pore-scale field inhomogeneities that influence all measurements of transverse $T_2$ relaxation time. In a previous publication we conjectured that experimentally observed exchange phenomena can correspond to intrapore diffusion between localized volumes of coherent magnetization generated by these internal gradients, rather than interpore diffusion. Here, we use a finite-element method to explore the decay of magnetization in a single pore (interstice between spheres) for the Carr-Purcell-Meiboom-Gill and two-dimensional $T_2\text{−}{T}_2$ exchange experiments. These simulations permit direct visualization of the time-dependent distribution of magnetization in the pore. The chosen grain size and susceptibility contrast are matched to values for Bentheimer sandstone, allowing for a comparison of simulated and experimental results. Despite the simplicity of the model pore geometry, the salient features of the magnetization decay observed experimentally are reproduced in simulation. We demonstrate that intrapore magnetization transfer can explain observations of diffusive exchange in porous materials characterized by a monomodal pore size distribution and large internal gradients.

Grain-boundary (GB) structures and energies are often calculated and have revealed correlations between the GB energy and change in electron density at the GB. In this work, the plasmon peak in valence electron energy-loss spectroscopy (VEELS) was used to determine the variation in electron density across four well-characterized GBs in Al, spanning a range of known GB energies. The results show that the plasmon energy is lower at the GB than in the adjacent grains due to a decrease in electron density, and the GB energy increases proportional to the density decrease. The decrease in electron density also extends further into adjacent grains with increasing GB energy, extending beyond the geometric changes, or physical width, revealed by electron microscopy. Plasmon damping also increases with increasing GB energy, indicative of increasing disruption of the electron density with increasing GB energy. These results demonstrate that VEELS can be a valuable tool for detecting small electron density changes at GBs, and this change clearly influences, and is correlated to, the GB energy.

On the basis of first-principles DFT calculations the wave-vector and temperature dependencies of the Lindhard response function of the blue bronze ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$ have been calculated. The $k_F^I+k_F^{II}$ interband component of the response, which is responsible for the Peierls instability, has been quantitatively analyzed. It is found that (i) the electron-hole coherence length of this response determines the length scale of the experimental intrachain CDW correlations, and (ii) the intrachain $q_{\parallel }$ dependence of such a response also determines the shape of the Kohn anomaly experimentally measured. These findings provide compelling evidence that the Peierls transition of the blue bronze ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$ follows the weak electron-phonon coupling scenario in the adiabatic approximation, something that had not yet been proved on the basis of first-principles calculations for a real material. It is proposed that the CDW interchain coupling occurs through a Coulomb coupling between dipolar CDWs. The nature of the phonon mode leading to the dipolar nature of the CDWs is also discussed, and the relevance of these results to rationalize the CDW instabilities in other oxides and bronzes is pointed out. These findings are also contrasted with recent results for other CDW materials like chalcogenides and tellurides.

Thin-film solar cells consist of several layers. The interfaces between these layers can provide critical recombination paths and consequently play a vital role in the efficiency of the solar cell. One of the main challenges for polycrystalline semiconductor absorber materials is the absorber-buffer interface. The $\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right)\mathrm{S}{\mathrm{e}}_2$ system is particularly interesting in this context, since Cu-rich absorbers are dominated by recombination at the interface, while Cu-poor ones are not. This paper unveils the root cause of the challenge in the interface of Cu-rich solar cells in terms of a Se-related defect with an activation energy of $200\pm 20\mathrm{meV}$. This defect causes interface recombination and is responsible for the deficiency of open-circuit voltage in Cu-rich cells. Moreover, this paper demonstrates that the origin of this defect is due to the etching step necessary to remove secondary phases. Postdeposition surface treatments or modified buffer layers are shown to passivate this defect, to reduce interface recombination, and to increase the efficiency.

Nodal loops in two-dimensional (2D) systems are typically vulnerable against spin-orbit coupling (SOC). Here, we explore 2D systems with a type of doubly degenerate nodal loops that are robust under SOC and feature an hourglass type dispersion. We present symmetry conditions for realizing such hourglass Weyl loops, which involve nonsymmorphic lattice symmetries. Depending on the symmetry, the loops may exhibit different patterns in the Brillouin zone. Based on first-principles calculations, we identify the monolayer GaTeI-family materials as a realistic material platform to realize such loops. These materials host a single hourglass Weyl loop circling around a high-symmetry point. Interestingly, there is also a spin-orbit Dirac point enabled by an additional screw axis. We show that the hourglass Weyl loop and the Dirac point are robust under a variety of applied strains. By breaking the screw axis, the Dirac point can be transformed into a second Weyl loop. Furthermore, by breaking the glide mirror, the hourglass Weyl loop and the spin-orbit Dirac point can both be transformed into a pair of spin-orbit Weyl points. Our work offers guidance and realistic material candidates for exploring fascinating physics of several novel 2D emergent fermions.

Polar metals are rare because free carriers in metals screen electrostatic potential and eliminate internal dipoles. Degenerate doped ferroelectrics may create an approximate polar metallic phase. We use first-principles calculations to investigate $n$-doped ${\mathrm{LiNbO}}_3$-type oxides (${\mathrm{LiNbO}}_3$ as the prototype) and compare to widely studied perovskite oxides (${\mathrm{BaTiO}}_3$ as the prototype). In the rigid-band approximation, substantial polar displacements in $n$-doped ${\mathrm{LiNbO}}_3$ persist even at 0.3 $e/\mathrm{f}.\mathrm{u}.$ $\left(\simeq {10}^{21}{\mathrm{cm}}^{-3}\right)$, while polar displacements in $n$-doped ${\mathrm{BaTiO}}_3$ quickly get suppressed and completely vanish at 0.1 $e/\mathrm{f}.\mathrm{u}.$ Furthermore, in $n$-doped ${\mathrm{LiNbO}}_3$, Li-O displacements decay more slowly than Nb-O displacements, while in $n$-doped ${\mathrm{BaTiO}}_3$, Ba-O and Ti-O displacements decay approximately at the same rate. Supercell calculations that use oxygen vacancies as electron donors support the main results from the rigid-band approximation and provide more detailed charge distributions. Substantial cation displacements are observed throughout ${\mathrm{LiNbO}}_{3-\delta }(\delta =4.2\%)$, while cation displacements in ${\mathrm{BaTiO}}_{3-\delta }(\delta =4.2\%)$ are almost completely suppressed. We find that conduction electrons in ${\mathrm{LiNbO}}_{3-\delta }$ are not as uniformly distributed as in ${\mathrm{BaTiO}}_{3-\delta }$, implying that the rigid-band approximation should be used with caution in simulating electron-doped ${\mathrm{LiNbO}}_3$-type oxides. Our work shows that polar distortions and conduction can coexist in a wide range of electron concentration in $n$-doped ${\mathrm{LiNbO}}_3$, which is a practical approach to generating an approximate polar metallic phase. Combining doped ferroelectrics and doped semiconductors may create new functions for devices.

Canonical relaxors exhibit an electric-field-induced phase transition between a macroscopically nonpolar and polar phase that can be tuned from being stable at low temperature to being reversible at high temperature. The reversibility of this phase change determines the electromechanical performance and large strains can be achieved if the polar phase is intrinsically unstable. This paper is on the thermal depolarization characteristics of a BNT-BT-based multiphase relaxor ceramic observed through the transition temperature from field-induced polar to nonpolar state. It is shown that the progress of detexturization strongly depends on the crystallographic phase. In the more susceptible phase, it becomes significant about 40 °C below the macroscopically observed transition temperature. Additionally, the surface domain structure vanishes at lower temperatures than expected from both dielectric and structural measurements. The development of strong interfacial stresses aiding depolarization, and a mismatch in chemical pressure between surface and bulk, are discussed as the origins for the observed effects. Tailoring of interfacial stresses through chemical adaption of crystallographic phase fractions opens up a pathway to optimize the strain performance of actuator materials and can become a useful tool to stabilize metastable crystallographic phases as well as for property tuning in piezotronics, Mott insulators and multiferroics.

Although spin-orbit coupling (SOC) in ferroelectrics contributes to the manipulation of the electron spin by an electric field, it also breaks the spin rotation symmetry resulting in the decay of spin polarization. Here, we showed that the [110] Dresselhaus model in the two-dimensional ferroelectrics with in-plane ferroelectricity can host a persistent spin helix and render the spin lifetime infinite. Taking a ${\mathrm{WO}}_2{\mathrm{Cl}}_2$ monolayer as an example, we demonstrated from first principles that SOC leads to a sizable [110] Dresselhaus-type band splitting near the conduction-band minimum accompanied by an out-of-plane Dresselhaus-type spin texture which is decisive for a long spin lifetime. More interestingly, such a spin texture can be reversed by switching the ferroelectric polarization, suggesting the fully electrically controllable persistent spin helix. Our findings uncover the possibility of an electrically controllable persistent spin helix in ferroelectrics, offering a promising platform for novel spintronics applications.