Density functional theory (DFT) calculations show that self-interstitial atom (SIA) defects in nonmagnetic body-centered-cubic (bcc) metals adopt strongly anisotropic configurations, elongated in the $\langle 111\rangle$ direction [S. Han et al., Phys. Rev. B 66, 220101 (2002); D. Nguyen-Manh et al., Phys. Rev. B 73, 020101 (2006); P. M. Derlet et al., Phys. Rev. B 76, 054107 (2007); S. L. Dudarev, Annu. Rev. Mater. Res. 43, 35 (2013)]. Elastic distortions, associated with such anisotropic atomic structures, appear similar to distortions around small prismatic dislocation loops, although the extent of this similarity has never been quantified. We derive analytical formulas for the dipole tensors of SIA defects, which show that, in addition to the prismatic dislocation looplike character, the elastic field of a SIA defect also has a significant isotropic dilatation component. Using empirical potentials and DFT calculations, we parametrize dipole tensors of $\langle 111\rangle$ defects for all the nonmagnetic bcc transition metals. This enables a quantitative evaluation of the energy of elastic interaction between the defects, which also shows that in a periodic three-dimensional simple cubic arrangement of crowdions, long-range elastic interactions between a defect and all its images favor a $\langle 111\rangle$ orientation of the defect.

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors—chemical, electronic, and structural—that govern strong multiexciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (${\mathrm{PEA})}_2{\mathrm{PbI}}_4$ (PEA = phenylethylammonium). We determine the binding energy of biexcitons—correlated two-electron, two-hole quasiparticles—to be $44\pm 5$ meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalcogenides. Importantly, we show that this binding energy increases by $\sim 25\%$ upon cooling to 5 K. Our work highlights the importance of multiexciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder.

Point defects play a critical role in the structural, physical, and interfacial properties of perovskite oxide superlattices. However, understanding of the fundamental properties of point defects in superlattices, especially their transport properties, is rather limited. Here, we report predictions of the stability and dynamics of oxygen vacancies in $\mathrm{SrTi}{\mathrm{O}}_3\text{/}\mathrm{PbTi}{\mathrm{O}}_3$ oxide superlattices using first-principles calculations in combination with the kinetic Monte Carlo method. By varying the stacking period, i.e., changing of $n$ in $n\mathrm{STO}\text{/}n\mathrm{PTO}$, we discover a crossover from three-dimensional diffusion to primarily two-dimensional planar diffusion. Such planar diffusion may lead to novel designs of ionic conductors. We show that the dominant vacancy position may vary in the superlattices, depending on the superlattice structure and stacking period, contradicting the common assumption that point defects reside at interfaces. Moreover, we predict a significant increase in room-temperature ionic conductivity for 3STO/3PTO relative to the bulk phases. Considering the variety of cations that can be accommodated in perovskite superlattices and the potential mismatch of spin, charge, and orbitals at the interfaces, this paper identifies a pathway to control defect dynamics for technological applications.

It is generally understood that the resistivity of metal thin films scales with film thickness mainly due to grain boundary and boundary surface scattering. Recently, several experiments and ab initio simulations have demonstrated the impact of crystal orientation on resistivity scaling. The crystal orientation cannot be captured by the commonly used resistivity scaling models and a qualitative understanding of its impact is currently lacking. In this work, we derive a resistivity scaling model that captures grain boundary and boundary surface scattering as well as the anisotropy of the band structure. The model is applied to Cu and Ru thin films, whose conduction bands are (quasi-) isotropic and anisotropic, respectively. After calibrating the anisotropy with ab initio simulations, the resistivity scaling models are compared to experimental resistivity data and a renormalization of the fitted grain boundary reflection coefficient can be identified for textured Ru.

The atomic structure of $\mathrm{N}{\mathrm{i}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ metallic glass has been investigated by molecular dynamics (MD) simulations. The calculated structure factors from the MD glassy sample at room temperature agree well with the x-ray diffraction (XRD) and neutron diffraction (ND) experimental data. Using the pairwise cluster alignment and clique analysis methods, we show that there are three types of dominant short-range order (SRO) motifs around Ni atoms in the glass sample of $\mathrm{N}{\mathrm{i}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$, i.e., mixed-icosahedron(ICO)-cube, intertwined-cube, and icosahedronlike clusters. Furthermore, chemical order and medium-range order (MRO) analysis show that the mixed-ICO-cube and intertwined-cube clusters exhibit the characteristics of the crystalline B2 phase. Our simulation results suggest that the weak glass-forming ability (GFA) of $\mathrm{N}{\mathrm{i}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ can be attributed to the competition between the glass forming ICO SRO and the crystalline mixed-ICO-cube and intertwined-cube motifs.

Recent advances in colloidal synthesis methods have led to an increased research focus on halide perovskites. Due to the highly ionic crystal structure of perovskite materials, a stability issue pops up, especially against polar solvents such as water. In this study, we investigate water-driven structural evolution of ${\mathrm{CsPbBr}}_3$ by performing experiments and state-of-the-art first-principles calculations. It is seen that while an optical image shows the gradual degradation of the yellowish ${\mathrm{CsPbBr}}_3$ structure under daylight, UV illumination reveals that the degradation of crystals takes place in two steps: transition from a blue-emitting to green-emitting structure and and then a transition from a green-emitting phase to complete degradation. We found that as-synthesized ${\mathrm{CsPbBr}}_3$ nanowires (NWs) emit blue light under a 254 nm UV source. Before the degradation, first, ${\mathrm{CsPbBr}}_3$ NWs undergo a water-driven structural transition to form large bundles. It is also seen that formation of such bundles provides longer-term environmental stability. In addition theoretical calculations revealed the strength of the interaction of water molecules with ligands and surfaces of ${\mathrm{CsPbBr}}_3$ and provide an atomistic-level explanation to a transition from ligand-covered NWs to bundle formation. Further interaction of green-light–emitting bundles with water causes complete degradation of ${\mathrm{CsPbBr}}_3$ and the photoluminescence signal is entirely quenched. Moreover, Raman and x-ray-diffraction measurements revealed that completely degraded regions are decomposed to ${\mathrm{PbBr}}_2$ and CsBr precursors. We believe that the findings of this study may provide further insight into the degradation mechanism of ${\mathrm{CsPbBr}}_3$ perovskite by water.

Layer-by-layer oxide molecular-beam epitaxy has been used to synthesize cuprate-nickelate multilayer structures of composition ${\left({\mathrm{La}}_2{\mathrm{CuO}}_4\right)}_m/\mathrm{LaO}/{\left({\mathrm{LaNiO}}_3\right)}_n$. In a combined experimental and theoretical study, we show that these structures allow a clean separation of dopant and doped layers. Specifically, the LaO layer separating cuprate and nickelate blocks provides an additional charge that, according to density-functional theory calculations, is predominantly accommodated in the interfacial nickelate layers. This is reflected in an elongation of bond distances and changes in valence state, as observed by scanning transmission electron microscopy and x-ray absorption spectroscopy. Moreover, the predicted charge disproportionation in the nickelate interface layers leads to a metal-to-insulator transition when the thickness is reduced to $n=2$, as observed in electrical transport measurements. The results exemplify the perspectives of charge transfer in metal-oxide multilayers to induce doping without introducing chemical and structural disorder.

The temperature dependence of spontaneous polarization in ferroelectrics generates a pyroelectric response, which can be exploited in sensor and thermal energy harvesting applications. To achieve the pyroelectric effect in ferroelectric ceramics, an electric field should be applied to the materials to align the spontaneous polarization. However, here we show that in paraelectric $\mathrm{N}{\mathrm{a}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$-based ceramics, a strong temperature-dependent electric current, which is similar to a pyroelectric response, can be observed under bending deformation. Our results reveal that the observed thermal current cannot be explained by the temperature dependence of the flexoelectric effect, but originates from the flexoelectric effect induced conduction. This work provides an approach to obtain a strong thermal-electrical response in originally nonpyroelectric materials.

High-throughput density functional theory calculations are conducted to search through 1572 $AB{\mathrm{O}}_3$ compounds to find a potential replacement material for lead zirconate titanate (PZT) that exhibits the same excellent piezoelectric properties as PZT and lacks both its use of the toxic element lead (Pb) and the formation of secondary alloy phases with platinum (Pt) electrodes. The first screening criterion employed a search through the Materials Project database to find $A-B$ combinations that do not form ternary compounds with Pt. The second screening criterion aimed to eliminate potential candidates through first-principles calculations of their electronic structure, in which compounds with a band gap of 0.25 eV or higher were retained. Third, thermodynamic stability calculations were used to compare the candidates in a Pt environment to compounds already calculated to be stable within the Materials Project. Formation energies below or equal to 100 meV/atom were considered to be thermodynamically stable. The fourth screening criterion employed lattice misfit to identify those candidate perovskites that have low misfit with the Pt electrode and high misfit of potential secondary phases that can be formed when Pt alloys with the different $A$ and $B$ components. To aid in the final analysis, dynamic stability calculations were used to determine those perovskites that have dynamic instabilities that favor the ferroelectric distortion. Analysis of the data finds three perovskites warranting further investigation: $\mathrm{CsNb}{\mathrm{O}}_3, \mathrm{RbNb}{\mathrm{O}}_3$, and $\mathrm{CsTa}{\mathrm{O}}_3$.

It is known that microcrack initiation in metallic alloys containing second-phase particles may be caused by either an interfacial or an intraprecipitate fracture. So far, the dependence of these features on properties of the precipitate and the interface is not clearly known. The present study aims to determine the key properties of carbide-metal interfaces controlling the energy and critical stress of fracture, based on density functional theory (DFT) calculations. We address coherent interfaces between a fcc iron or nickel matrix and a frequently observed carbide, the $M_{23}{\mathrm{C}}_6$, for which a simplified chemical composition ${\mathrm{Cr}}_{23}{\mathrm{C}}_6$ is assumed. The interfacial properties such as the formation and Griffith energies, and the effective Young's modulus are analyzed as functions of the magnetic state of the metal lattice, including the paramagnetic phase of iron. Interestingly, a simpler antiferromagnetic phase is found to exhibit similar interfacial mechanical behavior to the paramagnetic phase. A linear dependence is determined between the surface (and interface) energy and the variation of the number of chemical bonds weighted by the respective bond strength, which can be used to predict the relative formation energy for the surface and interface with various chemical terminations. Finally, the critical stresses of both intraprecipitate and interfacial fractures due to a tensile loading are estimated via the universal binding energy relation (UBER) model, parametrized on the DFT data. The validity of this model is verified in the case of intraprecipitate fracture, against results from DFT tensile test simulations. In agreement with experimental evidences, we predict a much stronger tendency for an interfacial fracture for this carbide. In addition, the calculated interfacial critical stresses are fully compatible with available experimental data in steels, where the interfacial carbide-matrix fracture is only observed at incoherent interfaces.

It has been a great challenge to develop a high-rate anode material with high-capacity, fast Li-ions diffusion and long cycling life going beyond the commercially used graphite in Li-ion battery. Here for the first time we propose a strategy combined high-pressure synthesis method with the global structure search to find a topological semimetal porous carbon as the desired anode. Our crystal-structure searching shows that we can obtain the ground state of an orthorhombic phase $\mathrm{Li}{\mathrm{C}}_6$ with regular pores at 30 GPa, and when the Li atoms are removed, the resulting carbon structure is the recently predicted interlocked graphene network (IGN) that is a topological semimetal with an intrinsic high electronic conductivity. Based on the state-of-the-art first-principles calculations, we further find that the Li-ion migration energy barrier in the IGN is extremely low and the estimated diffusion coefficient can reach a magnitude of ${10}^{-4}\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$ at both low and high Li concentrations, which is three orders of magnitude larger than that of graphite anode. Moreover, the volume changes during the Li insertion and deinsertion are smaller than $3.2\%$, while the theoretical specific capacity is the same as that of graphite anode. Our studies not only suggest a practical way of synthesizing the topological semimetal carbon but also propose a new anode material for Li-ion battery.

Dirac semimetals, which host Dirac fermions and represent a new state of quantum matter, have been studied intensively in condensed-matter physics. The exploration of new materials with topological states is important in both physics and materials science. We report the synthesis and the transport properties of high-quality single crystals of ${\mathrm{YbMnSb}}_2$. ${\mathrm{YbMnSb}}_2$ is a new compound with metallic behavior. Quantum oscillations, including Shubnikov–de Haas (SdH) oscillation and de Haas–van Alphen-type oscillation, have been observed at low temperature and high magnetic field. Small effective masses and nontrivial Berry phase are extracted from the analyses of quantum oscillations, which provide the transport evidence for the possible existence of Dirac fermions in ${\mathrm{YbMnSb}}_2$. The measurements of angular-dependent interlayer magnetoresistance indicate that the interlayer transport is coherent. The Fermi surface of ${\mathrm{YbMnSb}}_2$ possesses a quasi-two-dimensional characteristic as determined by the angular dependence of SdH oscillation frequency. These findings suggest that ${\mathrm{YbMnSb}}_2$ is a new candidate of topological Dirac semimetals.

We have observed an extremely large magnetoresistance exceeding 1.9 million at 1.7 K at 40 T for a single crystal of the binary semimetal ${\mathrm{NbAs}}_2$. The magnetoresistive behavior for this compound is quantitatively reproduced by a semiclassical two-carrier model in which the significant enhancement of magnetoresistance is attributed to the almost full compensation of the hole and electron densities $\left(0.994<n_{\mathrm{h}}/n_{\mathrm{e}}\le 0.999\right)$ as well as the high mobility $(>6\times {10}^5{\mathrm{cm}}^2/\mathrm{V}·\mathrm{s})$. Our results indicate that binary semimetals with higher carrier densities have a great potential for exhibiting a further divergent increase in magnetoresistance merely through an improvement in crystal quality.

Nitrogen-vacancy magnetic microscopy is employed in the quenching mode as a noninvasive, high-resolution tool to investigate the morphology of isolated skyrmions in ultrathin magnetic films. The skyrmion size and shape are found to be strongly affected by local pinning effects and magnetic field history. Micromagnetic simulations including a static disorder, based on the physical model of grain-to-grain thickness variations, reproduce all experimental observations and reveal the key role of disorder and magnetic history in the stabilization of skyrmions in ultrathin magnetic films. This work opens the way to an in-depth understanding of skyrmion dynamics in real, disordered media.

X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism measurements were performed at the Ni and Fe $L_{2,3}$ absorption edges for $\mathrm{N}{\mathrm{i}}_x\mathrm{F}{\mathrm{e}}_{4-x}\mathrm{N}(x=1$ and 3) epitaxial films. Spectral line-shape analysis and element-specific magnetic moment evaluations are presented. Shoulders at approximately 2 eV above the Ni $L_{2,3}$ main peaks in the XAS spectrum of $\mathrm{N}{\mathrm{i}}_3\mathrm{FeN}$ were interpreted to originate from hybridization of orbitals between Ni $3d$ at face-centered (II) sites and N $2p$ at body-centered sites, while such features were missing in $\mathrm{NiF}{\mathrm{e}}_3\mathrm{N}$ film. Similar shoulders were observed at Fe $L_{2,3}$ edges in both films. These results indicate that the orbitals of Ni atoms did not hybridize with those of N atoms in the $\mathrm{NiF}{\mathrm{e}}_3\mathrm{N}$ film. Hence, Ni atoms preferentially occupied corner (I) sites, where the hybridization was weak because of the relatively long distance between Ni at I sites and N atoms. The relatively large magnetic moment deduced from sum-rule analysis of $\mathrm{NiF}{\mathrm{e}}_3\mathrm{N}$ also showed a good agreement with the presence of Ni atoms at I sites.

Magnetoresistive random-access memory (MRAM) is poised to become a next-generation information storage device. Yet, many materials challenges remain unsolved before it can become a widely used memory storage solution. Among them, an urgent need is to identify a material system that is suitable for downscaling and is compatible with low-power logic applications. Self-assembled, vertically aligned $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3:\mathrm{ZnO}$ nanocomposites, in which $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) matrix and ZnO nanopillars form an intertwined structure with coincident-site-matched growth occurring between the LSMO and ZnO vertical interfaces, may offer new MRAM applications by combining their superior electric, magnetic ($B$), and optical properties. In this Rapid Communication, we show the results of electrical current induced magnetic hysteresis in magnetoresistance measurements in these nanopillar composites. We observe that when the current level is low, for example, 1 µA, the magnetoresistance displays a linear, negative, nonhysteretic $B$ field dependence. Surprisingly, when a large current is used, $I$ > 10 µA, a hysteretic behavior is observed when the $B$ field is swept in the up and down directions. This hysteresis weakens as the sample temperature is increased. A possible spin-valve mechanism related to this electrical current induced magnetic hysteresis is proposed and discussed.