Synthesizing stoichiometric and epitaxial thin films of pyrochlore iridates is an essential step toward the experimental realization of unusual topological and magnetic states that are theoretically predicted in this unique spin-orbit coupled material system. Here, we report on the stoichiometry control and electronic and transport properties of pyrochlore iridate $\mathrm{B}{\mathrm{i}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ thin films grown by pulsed laser deposition. The as-grown films form a bilayerlike structure, in which the top surface is highly Ir deficient while the bottom layer is mainly composed of iridium metal. By postannealing the as-deposited films in $\mathrm{Ir}{\mathrm{O}}_2+{\mathrm{O}}_2$ atmosphere, we improved the stoichiometry and homogeneity through the film thickness with the lattice constant close to the bulk value. Density functional theory calculation in the bulk limit shows a fourfold degenerate Dirac node slightly below the Fermi energy at the $X$ point, along with trivial bands around the Γ point. The projected partial density of states suggests that the states in the vicinity of the Fermi energy (−3 to 0 eV) mainly consist of highly hybridized Ir $5d$ and O $2p$ with minor contributions from Bi $6s$ and $6p$, while those far below the Fermi energy (−9 to –3 eV) are contributed primarily by the O bands. Transport measurements revealed a weakly metallic behavior at higher temperatures transitioning to a weakly insulating behavior below 150 K, and a low-temperature magnetoresistance qualitatively ascribed to multicarrier and band-structural effects. The transport features are influenced by a density of states sharply peaked at the Fermi energy, and by the coexistence of trivial bands with the Dirac node, as revealed by the density functional theory calculations.

In nature, materials such as ferroelastics and multiferroics can switch their microstructure in response to external stimuli, and this reconfiguration causes a simultaneous modulation of their material properties. Rapid prototyping technologies have enabled origami and kirigami-inspired architected materials to provide a means for designing shape-shifting structures, and here we show how multistable structures inspired by kirigami provide novel design criteria for preparing mechanical metamaterials with tunable properties. By changing the geometry of kirigami unit cells, we obtain multistable kirigami lattice structures endowed with a bistable snap-through mechanism. We demonstrate the precise control of material stiffness, along with the ability to tune this property in situ by locally and reversibly switching the unit cell configurations. We anticipate these mechanical metamaterials will provide a platform to achieve in situ tunable electrical, optical, and mechanical properties for a variety of applications in multifunctional materials, two-dimensional materials, and soft robotics.

Determining the atomic configuration of an interface is one of the most important issues in materials science research. Although theoretical simulations are effective tools, an exhaustive search is computationally prohibitive due to the high degrees of freedom of the interface structure. In the interface structure search, multiple energy surfaces created by a variety of orientation angles need to be explored, and the necessary computational costs for different angles vary substantially owing to significant variations in the supercell sizes. In this paper, we introduce two machine-learning concepts, called transfer learning and cost-sensitive search, to the interface-structure search. As a case study, we demonstrate the effectiveness of our method, called cost-sensitive multitask Bayesian optimization, using the fcc-Al [110] tilt grain boundary. Four microscopic parameters, the three-dimensional rigid body translation, and the number of atomic columns, are optimized by transferring knowledge of energy surfaces among different orientation angles. We show that transferring knowledge of different energy surfaces can accelerate the structure search, and that considering the cost variations further improves the total efficiency.

Two-dimensional materials have attracted considerable research attention recently due to their extraordinary physical and chemical properties. In this paper, we systematically investigate the electronic structure of ultrathin tetragonal CoSe films with different thicknesses that are isostructural to two-dimensional high-temperature superconductor FeSe using high-resolution angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and ab initio calculation. Interestingly, the nonsymmorphic symmetry of the CoSe monolayer protects both exotic saddle band degeneracy and Dirac points, realizing two-dimensional Dirac fermions. Our temperature and film-thickness-dependent measurements reveal momentum-dependent electron-phonon coupling in CoSe films, which is enhanced with reduced film thickness. Moreover, the electronic states in the CoSe monolayer strongly couple to high-frequency phonons in the $\mathrm{SrTi}{\mathrm{O}}_3$ substrate, similar to the situation in the FeSe monolayer. Our results not only present a platform to investigate the electronic properties in close vicinity of the high-temperature superconductor FeSe monolayer, but also shed light on the understanding of the coupling between two-dimensional materials and substrates beneath.

In topological crystalline insulators the topological conducting surface states are protected by crystal symmetry, in principle making it possible to pattern nanoscale insulating and conductive motifs solely by breaking local symmetries on an otherwise homogeneous, single-phase material. We show, using scanning tunneling microscopy/spectroscopy, that defects that break local symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations. Complementary ab initio calculations show how local symmetry breaking obstructs topological surface states as shown by a threefold reduction of the spectral weight of the topological surface states. The finding highlights the potential benefits of manipulating the surface morphology to create devices that take advantage of the unique properties of topological surface states and can operate at practical temperatures.

It has been proposed [L. S McCarty and G. M Whitesides, Angew. Chem., Int. Ed. 47, 2188 (2008); Lee et al., Phys. Rev. Mater. 2, 035602 (2018)] that contact charging between insulators may be mediated by nonequilibrium transport of adsorbed ions. We remark here that if adsorbed ions transport charges between surfaces, they could equally transport charges across a single surface. We test this hypothesis by contacting initially neutral insulating spheres. We find that localized charge patterns appear very far (centimeters) from a point of contact. We visualize the charges, evaluate their spatial distributions, and discuss mechanisms and implications of this apparent action at a distance.

Tuning oxygen vacancy concentrations is a well-established approach to controlling properties of complex oxides. Recent work on perovskite cobaltites has shown that ordering of oxygen vacancies can also be tuned, via heteroepitaxial strain, presenting new opportunities. Here we demonstrate that strain-engineered vacancy ordering can control and enhance magnetic anisotropy in $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Co}{\mathrm{O}}_{3-\delta }$. In particular, in-plane oxygen vacancy order induced by compressive strain is shown to result in remarkably strong perpendicular magnetic anisotropy, with anisotropy constant up to $6\times {10}^6\mathrm{erg}/\mathrm{c}{\mathrm{m}}^3$. The anisotropy is thickness independent, ruling out surface and film/substrate interface anisotropy, but strongly correlated with lateral coherence of defect order from electron microscopy. The results are discussed in terms of the unit-cell-level superlattice induced by the oxygen vacancy order, generating intriguing analogies with metal-based multilayer systems. Generally, this Rapid Communication highlights the significant potential of strain-based manipulation of oxygen vacancy ordering to control and enhance complex oxide properties.

According to our current understanding, thermal rectification in heterojunctions derives from the different temperature dependence of the thermal conductivity of the constituent materials and from the thermal boundary resistance (TBR). This view, however, neglects the importance of the relation between these two factors, namely, how in nanoscale systems a resistance mismatch condition is needed to observe the contribution to the rectification of the TBR that can ultimately become the main driving factor. Here we determine the temperature dependence of thermal conductivity of the considered materials by means of first-principles calculations combined with an iterative solution of the Boltzmann transport equation. We then include the results in the one-dimensional stationary heat equation to show that at the nanoscale the rectification is determined by the thermal boundary resistance; it can be larger and can even change sign with respect to bulk systems where it can be neglected.

Austenite ($\gamma$-Fe, face-centered cubic) to ferrite ($\alpha$-Fe, body-centered cubic) phase transformation in steel is of great significance from the point of view of industrial applications. In this work, using classical molecular dynamics simulations, we study the atomistic mechanisms involved during the growth of the ferrite phase embedded in an austenite phase. We find that the disconnections present at the interphase boundary assist in growth of the ferrite phase. Relatively small interface velocities (1.19–4.67 m/s) confirm a phase change via massive transformation mechanism. Boundary mobilities obtained in a temperature range of 1000 to 1400 K show an Arrhenius behavior, with activation energies ranging from 30–40 kJ/mol.

Single-phase ferromagnets show deterioration of magnetic permeability when operating under a prestress due to the stress-induced anisotropy, which is a serious restriction for soft magnets. However, technologically, it is highly desired that soft magnets can maintain stable permeability under high stress. Here we report that a ferromagnetic Fe-Ga composite exhibits nearly unchanged magnetic permeability and coercivity even when subjected to compressive prestress as high as 200 MPa. Such stress-insensitive magnetic properties stem from the compensation of stress-induced anisotropies between the two ferromagnetic components with opposite magnetostriction signs (λ > 0 for the bcc phase and λ < 0 for the fcc phase). The magnetomechanical measurements and phase field simulations revealed that the stress-induced domain switching pathways are competitive between the bcc and the fcc phases, ruling out the prestress influence on magnetization. The present finding may provide a general recipe for designing soft magnets with stable magnetic properties under high stress.

We report a density functional theory study of the Pockels effect (linear electro-optical effect) in epitaxially strained $\mathrm{SrTi}{\mathrm{O}}_3$. The electro-optical response is calculated for biaxial strain values ranging from −2.0 to 2.0% relative to the theoretically optimized lattice constant. Under 1.0% tensile strain, the Pockels tensor components increase dramatically, with the largest components reaching maximum values of $r_{111}=r_{222}=505.64\mathrm{pm}/\mathrm{V}$. Under 1.2% compressive strain, the Pockels tensor exhibits a similarly large peak with a maximum value of $r_{333}=236.55\mathrm{pm}/\mathrm{V}$. These peaks in the electro-optical response originate from the softening of the phonon modes associated with ferroelectric phase transitions that result in the loss of inversion symmetry. Our results suggest that under the right circumstances $\mathrm{SrTi}{\mathrm{O}}_3$ can yield a very large electro-optical response, comparable to that of $\mathrm{BaTi}{\mathrm{O}}_3$, which has one of the largest known responses.

Mechanical breaking of covalent bonds is induced by thermal fluctuations which leads to temperature and loading rate dependent rupture forces. These forces are much smaller than the maximum slope of the bond potential at practically accessible loading rates. Calculation of force-dependent activation energies obtained with a one-dimensional constrained geometry method is shown to work for isolated weak bonds or very simple molecules only. The situation is analyzed in a two-dimensional picture taking the broken bond explicitly into account. A single force-independent universal path appears that contains full information about the force dependence of the minima and transition states. The activation energies obtained from this path are of similar accuracy as those from force-dependent nudged elastic band calculations. Strong variations of rupture forces with loading rate and temperature are found for mechanochromophores, where the rupture force is more than one order of magnitude smaller than the maximum slope in agreement with experiment.

Many applications of ferroic materials take advantage of the microstructural redistribution under an external stimulus. Despite extensive studies on the microstructural evolution during both domain switching and phase transformations of high-performance ferroelectrics near morphotropic phase boundaries (MPBs), the synergetic interactions between both leading factors remain unclear. Herein, we have illustrated and discerned the correlative response of phase transformation and domain switching in an archetypical piezoceramic of ${\mathrm{PbTiO}}_3-{\mathrm{BiScO}}_3$ using a combination of in situ high-energy synchrotron x-ray diffraction and phase-field simulations. The direct structural evidence and domain development from simulations reveal a significant reversible orientation-dependent emerging phase transformation and enhanced domain switching. Increasingly populated polarization variants aligned with the applied electric field owing to this strong synergistic interaction play an important role in enhancing the piezoelectric performances of MPB ferroelectrics. In addition to providing further insight into the microstructures of ferroelectrics, the present results have the potential to guide the design of high-performance materials.

Ferroelectric domain walls are attracting broad attention as atomic-scale switches, diodes, and mobile wires for next-generation nanoelectronics. Charged domain walls in improper ferroelectrics are particularly interesting as they offer multifunctional properties and an inherent stability not found in proper ferroelectrics. Here we study the energetics and structure of charged walls in improper ferroelectric ${\mathrm{YMnO}}_3,{\mathrm{InMnO}}_3$, and ${\mathrm{YGaO}}_3$ by first-principles calculations and phenomenological modeling. Positively and negatively charged walls are asymmetric in terms of local structure and width. The wall width scales with the amplitude of the primary structural order parameter and the coupling strength to the polarization, reflecting that polarization is not the driving force for domain formation. We introduce general rules for how to engineer $n$- and $p$-type domain wall conductivity based on the domain size, polarization, and electronic band gap. This opens the possibility of fine tuning the local transport properties and designing $p\text{−}n$-junctions for domain wall-based nanocircuitry.

Lead halide perovskite nanocrystals are promising light emitting materials with high photoluminescence quantum yields and widely tunable emission wavelengths. Their optical responses are significantly influenced by efficient nonradiative Auger recombination of trions (charged excitons) and biexcitons. Here, we report the observation of positive and negative trions in formamidinium lead bromide nanocrystals and discuss their role in the photoluminescence dynamics. It is found that the addition of copper thiocyanate causes an additional intermediate state by photochemical doping. We clarify that as-prepared nanocrystals show a two-state blinking between neutral excitons and positive trions, while the postsynthesis treated nanocrystals exhibit blinking between all three states, including negative trions. We confirmed that the biexciton Auger rate evaluated from femtosecond transient absorption measurements can be expressed as the sum of the Auger rates of positive and negative trions obtained by single-dot spectroscopy.

High quality heteroepitaxy of CdTe is challenging due to lattice mismatches of CdTe with many substrates. Herein, we demonstrate the epitaxial growth of single crystalline CdTe films, in multilevel island format, on mica using metalorganic chemical vapor deposition, regardless of large in-plane lattice mismatch (∼13%) between CdTe(111) and mica(001). X-ray and electron diffractions suggest that CdTe is epitaxially aligned with mica: out-of-plane $\mathrm{CdTe}\left(111\right)\text{//mica}\left(001\right)$ and in-plane CdTe $\left[\overline{1}2\overline{1}\right]$//mica $\left[010\right]$. Full-width-at-half-maximum (FWHM) of x-ray rocking curve and FWHM of x-ray azimuthal in-plane angular dispersion of CdTe are shown to be $0.{11}^{\circ }$ and $0.{38}^{\circ }$, respectively, better than most CdTe films reported. Electron backscattering diffraction shows that CdTe grains are tens of μm and, if twin boundaries are excluded, in excess of 250 μm in size. In contrast to the belief that overlayer growth on mica is purely through van der Waals interaction, our first-principle calculations uncover that van der Waals interaction only contributes to 20% of the total interfacial energy, and 80% of the interfacial energy comes from chemical interaction. We believe such a strong chemical interaction is accountable for the high-quality epitaxy. The demonstration of epitaxial growth of high-quality semiconductor on mica with a large lattice mismatch creates opportunities for flexible optoelectronic devices.

We report on a magnetotransport and quantum oscillations study on high quality single crystals of the transition metal di-tellurides ${\mathrm{PtTe}}_2$ and ${\mathrm{PdTe}}_2$. The de Haas-van Alphen (dHvA) oscillations in the magnetization measurements on ${\mathrm{PtTe}}_2$ reveal a complicated, anisotropic band structure characterized by low effective masses and high mobilities for the carriers. Extracted transport parameters for ${\mathrm{PtTe}}_2$ reveal a strong anisotropy which might be related to the tilted nature of the Dirac cone. Using a Landau level fan diagram analysis we find at least one Fermi surface orbit with a Berry phase of $\pi$ consistent with Dirac electrons for both ${\mathrm{PtTe}}_2$ and ${\mathrm{PdTe}}_2$. The light effective mass and high mobility are also consistent with Dirac electrons in ${\mathrm{PtTe}}_2$. Our results therefore suggest that similar to ${\mathrm{PdTe}}_2$, ${\mathrm{PtTe}}_2$ might also be a three-dimensional Dirac semimetal.