Thin films of the electron-doped infinite-layer ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$, with $R={\mathrm{La}}^{3+}$, ${\mathrm{Nd}}^{3+}$, and ${\mathrm{Ce}}^{4+}$, have been synthesized using molecular beam epitaxy. The solubility limits of $R$ in ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ are $0.060\pm 0.002$, $0.080\pm 0.010$, and $<0.01$ for $R={\mathrm{Nd}}^{3+}$, ${\mathrm{La}}^{3+}$, and ${\mathrm{Ce}}^{4+}$, respectively. Using high-resolution reciprocal space mapping we show that the in-plane lattice constants of ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ follow the same trend as is observed for other cuprates with square-planar coordinated copper, i.e., elongation of the Cu-O bond length upon electron doping. We measured the temperature dependencies of the resistivity and the Hall coefficient to trace the influence of the doped charge carriers on the electronic response. We show that the Hall coefficient is negative below 300 K and that the $R$ substitution level is insufficient to achieve a positive Hall coefficient, a necessary prerequisite for superconductivity.

In this work we explore the potential of a data-driven approach to the design of exchange-correlation (xc) functionals. The approach, inspired by convolutional filters in computer vision and surrogate functions from optimization, utilizes convolutions of the electron density to form a feature space to represent local electronic environments and neural networks to map the features to the exchange-correlation energy density. These features are orbital free, and provide a systematic route to including information at various length scales. This work shows that convolutional descriptors are theoretically capable of an exact representation of the electron density, and proposes Maxwell-Cartesian spherical harmonic kernels as a class of rotationally invariant descriptors for the construction of machine learned functionals. The approach is demonstrated using data from the B3LYP functional on a number of small molecules containing C, H, O, and N along with a neural network regression model. The machine learned functionals are compared to standard physical approximations and the accuracy is assessed for the absolute energy of each molecular system as well as formation energies. The results indicate that it is possible to reproduce the exchange-correlation portion of B3LYP formation energies to within chemical accuracy using orbital-free descriptors with a spatial extent of 0.2 Å. The findings provide empirical insight into the spatial range of electron exchange, and suggest that the combination of convolutional descriptors and machine learning regression models is a promising framework for xc functional design, although challenges remain in obtaining training data and generating models consistent with pseudopotentials.

The process of surface degradation and material removal due to adhesive force between rubbing surfaces (i.e., adhesive wear) involves an apparent contradiction. The material exchange between sliding surfaces requires a strong adhesion between surface asperities, whereas the detachment of final wear particles demands a weak adhesion between worn fragments and sliding surfaces. Here, using a coarse-grained numerical technique, we study the complete process of wear particle formation (i.e., nucleation, evolution, and detachment) during adhesive sliding contact. We show that discrepant experimental and theoretical wear observations can be attributed to different stages of the wear particle formation. In particular, we address the opposite contribution of adhesion into the formation and detachment processes of wear particles. Our simulations reveal that reducing adhesion diminishes the adhesive wear in three ways: (i) reducing the probability of wear particle formation, (ii) increasing the required energy per unit volume of removed materials (i.e., decreasing the energy efficiency for particle formation), and (iii) alleviating the growth of formed wear fragments.

Hexagonal boron nitride (hBN) has been grown on sapphire substrates by ultrahigh-temperature molecular beam epitaxy (MBE). A wide range of substrate temperatures and boron fluxes have been explored, revealing that high crystalline quality hBN layers are grown at high substrate temperatures, $>1600{}^{\circ }\mathrm{C}$, and low boron fluxes, $\sim 1\times {10}^{-8}$ Torr beam equivalent pressure. In situ reflection high-energy electron diffraction revealed the growth of hBN layers with ${60}^{\circ }$ rotational symmetry and the $\left[11\overline{2}0\right]$ axis of hBN parallel to the $\left[1\overline{1}00\right]$ axis of the sapphire substrate. Unlike the rough, polycrystalline films previously reported, atomic force microscopy and transmission electron microscopy characterization of these films demonstrate smooth, layered, few-nanometer hBN films on a nitridated sapphire substrate. This demonstration of high-quality hBN growth by MBE is a step toward its integration into existing epitaxial growth platforms, applications, and technologies.

Origami is a promising method for creating various structures from filmlike materials via local deconstruction rather than elastic bending. Transition-metal dichalcogenides (TMDCs) have high bending stiffness making the formation of highly curved nanostructures, such as nanotube or nanocages, via bending difficult. Here, we propose the use of two-dimensional (2D) material origami to build stable TMDC nanostructures. Various nanostructures, such as polygonal nanotubes or polyhedral nanocages, can be created by introducing line defects, which incurs only a very small energy penalty. Through first-principles calculations and high-resolution transmission electron microscopy imaging, we confirmed their stability and the possibility of synthesis experimentally via line defect formation. As an example, the widely observed TMDC nanowires are produced with this approach, and many experimentally observed nanostructures agree with these origami creases/line defects. This work opens a door to synthesize nanostructures of few-atomic-thick 2D materials for various potential applications.

Engineering wear models are generally empirical and lack connections to the physical processes of debris generation at the nanoscale to microscale. Here, we thus analyze wear particle formation for sliding interfaces in dry contact with full and reduced adhesion. Depending on the material and interface properties and the local slopes of the surfaces, we find that colliding surface asperities can either deform plastically, form wear particles, or slip along the contact junction surface without significant damage. We propose a mechanism map as a function of material properties and local geometry, and confirm it using quasi-two-dimensional and three-dimensional molecular dynamics and finite-element simulations on an amorphous, siliconlike model material. The framework developed in the present paper conceptually ties the regimes of weak and strong interfacial adhesion together and can explain that even unlubricated sliding contacts do not necessarily lead to catastrophic wear rates. A salient result of the present paper is an analytical expression of a critical length scale, which incorporates interface properties and roughness parameters. Therefore, our findings provide a theoretical framework and a quantitative map to predict deformation mechanisms at individual contacts. In particular, contact junctions of sizes above the critical length scale contribute to the debris formation.

We analyze the various morphing structures obtained by actuating Janus filaments comprising driven and passive sectors and textiles incorporating driven and passive filaments. Transitions between alternative shapes and coexistence of absolutely stable and metastable states within a certain range of relative extension upon actuation are detected both in Janus rings and textiles. Both single filaments and textiles can be reverse designed to bend into desired shapes by controlling both the size and orientation of driven sectors.

The thermodynamic state of lipid interfaces was observed during shock wave induced cavitation in water with submicrosecond resolution, using the emission spectra of hydration-sensitive fluorescent probes colocalized at the interface. The experiments show that the cavitation threshold is lowest near a phase transition of the lipid interface. The cavitation collapse time and the maximum state change during cavitation are found to be a function of both the driving pressure and the initial state of the lipid interface. The experiments show dehydration and crystallization of lipids during the expansion phase of cavitation, suggesting that the heat of vaporization is absorbed from within the interface, which is adiabatically uncoupled from the free water. The study underlines the critical role of the thermodynamic state of the interface in cavitation dynamics, which has mechanistic implications for ultrasound-mediated drug delivery, acoustic nerve stimulation, ultrasound contrast agents, and the nucleation of ice during cavitation.

We report on a study of charge transport in ${\mathrm{EuTiO}}_{3-\delta }$ single crystals with carrier density tuned across several orders of magnitude. Comparing this system with other quasicubic perovskites, in particular strontium titanate, we draw a comprehensive picture of metal-insulator transition and dilute metallicity in this $AB{\mathrm{O}}_3$ family. Because of a lower electric permittivity, the metal-insulator transition in ${\mathrm{EuTiO}}_{3-\delta }$ occurs at higher carrier densities compared to ${\mathrm{SrTiO}}_3$. At low temperature, a distinct $T^2$ resistivity is visible. Its prefactor $A$ smoothly decreases with increasing carrier concentration in a similar manner in three different perovskites. Our results draw a comprehensive picture of charge transport in doped quantum paraelectrics.

Nanoscale complex-oxide thin films prepared by well-established growth techniques, such as pulsed-laser deposition or molecular-beam epitaxy, often exhibit compositions that deviate from the ideal stoichiometry. Even small variations in composition can lead to substantial changes in the technologically relevant electronic, magnetic, and optical properties of these materials. To assess the reasons behind this variability, and ultimately to allow tuning the properties of oxide films with precise control of the deposition parameters, high-resolution detection of the nonstoichiometry introduced during growth is needed. The resolution of current techniques, such as x-ray diffraction, fluorescence, or spectroscopy, is limited to estimating composition differences in the percent level, which is often insufficient for electronic-device quality. We develop an unconventional approach based on scanning tunneling microscopy for enabling the determination of cation imbalance introduced in thin films with exceptionally small detection limit. We take advantage of the well-controlled surface reconstructions on ${\mathrm{SrTiO}}_3$(110), and use the established relation between those reconstructions and the surface composition to assess the cation excess deposited in pulsed-laser grown ${\mathrm{SrTiO}}_3$(110) films. We demonstrate that a <0.1% change in cation nonstoichiometry is detectable by our approach. Furthermore, we show that, for thin films that accommodate all the nonstoichiometry at the surface, this method has no fundamental detection limit.

For several decades, the striking contradiction between the Huang diffuse scattering experiments, resistivity recovery data, and predictions derived from density functional theory (DFT) remained one of the mysteries of defect physics in molybdenum. Since the 1970s, observations of Huang x-ray diffuse scattering appeared to indicate that a self-interstitial atom (SIA) defect in Mo adopts a $\langle 110\rangle$ dumbbell configuration. However, the low temperature defect diffusion data supported the DFT prediction of a different, highly mobile $\langle 111\rangle$ SIA defect structure in the same metal. Using DFT simulations, we show that an SIA adopts a symmetry-broken configuration in all the group 6 metals: chromium, molybdenum, and tungsten. The symmetry-broken defect structure, a $\langle 11\xi \rangle$ dumbbell, where $\xi$ is an irrational number, agrees with nudged elastic band analyses of $\langle 110\rangle$ to $\langle 111\rangle$ transformations. Direct simulations of Huang diffuse scattering by symmetry-broken defect configurations predicted by DFT explain why no zero intensity lines were observed in experiment and resolve the long outstanding question about the structure of defects in Mo and similar metals. A $\langle 11\xi \rangle$ defect migrates on average one dimensionally through a sequence of three-dimensional nonplanar $\left[11\xi \right]$ to $\left[\xi 11\right]$ or $\left[1\xi 1\right]$ transitions. Barriers for defect migration in nonmagnetic Cr, antiferromagnetic Cr, Mo, and W derived from DFT calculations, 0.052, 0.075, 0.064, and 0.040 eV are well correlated with the onset of defect migration temperatures observed experimentally.

Cellulose nanocrystals (CNCs) are biosourced chiral nanorods that can form stable colloidal suspensions able to spontaneously assemble above a critical concentration into a cholesteric liquid crystal, with a cholesteric pitch usually in the micron range. When these suspensions are dried on a substrate, solid films with a pitch of the order of few hundreds of nanometers can be produced, leading to intense reflection in the visible range. However, the resulting cholesteric nanostructure is usually not homogeneous within a sample and comports important variations of the cholesteric domain orientation and pitch, which affect the photonic properties. In this work, we first propose a model accounting for the formation of the photonic structure from the vertical compression of the cholesteric suspension upon solvent evaporation, starting at the onset of the kinetic arrest of the drying suspension and ending when solvent evaporation is complete. From that assumption, various structural features of the films can be derived, such as the variation of the cholesteric pitch with the domain tilt, the orientation distribution density of the final cholesteric domains and the distortion of the helix from the unperturbed cholesteric case. The angular-resolved optical response of such films is then derived, including the iridescence and the generation of higher-order reflection bands, and a simulation of the angular optical response is provided, including its tailoring under external magnetic fields. Second, we conducted an experimental investigation of CNC films covering a structural and optical analysis of the films. The macroscopic appearance of the films is discussed and complemented with angular-resolved optical spectroscopy, optical and electron microscopy, and our quantitative analysis shows an excellent agreement with the proposed model. This allows us to access the precise composition and the pitch of the suspension when it transited into a kinetically arrested phase directly from the optical analysis of the film. This work highlights the key role that the anisotropic compression of the kinetically arrested state plays in the formation of CNC films and is relevant to the broader case of structure formation in cast dispersions and colloidal self-assembly upon solvent evaporation.

We have studied the barocaloric properties associated with the martensitic transition of a shape memory Heusler alloy ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5}$ which is composed of all-$d$-metal elements. The composition of the sample has been tailored to avoid long-range ferromagnetic order in both austenite and martensite. The lack of ferromagnetism results in a weak magnetic contribution to the total entropy change, thereby leading to a large transition entropy change. The combination of such a large entropy change and a relatively large volume change at the martensitic transition gives rise to giant barocaloric properties in this alloy. When compared to other shape memory Heusler alloys, our material exhibits values for adiabatic temperature and isothermal entropy changes significantly larger than values reported so far for this class of materials. Furthermore, our ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5}$ also compares favorably to the best state-of-the-art magnetic barocaloric materials.

We study the electronic structure of delafossite ${\mathrm{PtCoO}}_2$ to elucidate its extremely small resistivity and high mobility. The band exhibits steep dispersion near the Fermi level despite the fact that it is formed mainly by Pt $d$ orbitals that are typically localized. We propose a picture based on two hidden kagome-lattice-like electronic structures: one originating from Pt $s+p_x/p_y$ orbitals, and the other from Pt $d_{3z^2-r^2}+d_{xy}/d_{x^2-y^2}$ orbitals, each placed on the bonds of the triangular lattice. In particular, we find that the underlying Pt $s+p_x/p_y$ bands actually determine the steepness of the original dispersion, so that the large Fermi velocity can be attributed to the large width of the Pt $s+p_x/p_y$ band. In addition, the kagome-like electronic structure gives rise to “orbital-momentum locking” on the Fermi surface, which reduces the electron scattering by impurities. We conclude that the combination of the large Fermi velocity and the orbital-momentum locking is likely to be the origin of the extremely small resistivity in ${\mathrm{PtCoO}}_2$.

We derive surface polarity at and near ferroelastic domain boundaries from molecular dynamics simulations based on an ionic spring model. Interatomic gradient forces lead to flexoelectricity which, in turn, generates polarity at the surface and in twin boundaries. We then derive generic properties of electron scattering spectra equivalent to those observed in low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) experiments. Negatively (positively) charged surfaces reflect (attract) incident electrons with low kinetic energy. The electron images reveal the valley and ridge surface structures near the intersection of the twin boundary and the surface. Polarity in surface layers is predicted to be visible in LEEM and MEM spectra at neutral surfaces, but much less when surfaces are charged. Inward polarity reflects electrons similar to negative surface charges, and outward polarity backscatters electrons like positive surface charges. Both the polarity in the twin boundary and the physical topography scatter electrons, consistent with experimental LEEM and MEM experiments on $\mathrm{CaTi}{\mathrm{O}}_3$ with (001) and (111) surface terminations.

Electron-defect ($e-\mathrm{d}$) interactions govern charge carrier dynamics at low temperature, where they limit the carrier mobility and give rise to phenomena of broad relevance in condensed matter physics. Ab initio calculations of $e-\mathrm{d}$ interactions are still in their infancy, mainly because they require large supercells and computationally expensive workflows. Here we develop an efficient ab initio approach for computing elastic $e-\mathrm{d}$ interactions, their associated $e-\mathrm{d}$ relaxation times (RTs), and the low-temperature defect-limited carrier mobility. The method is applied to silicon with simple neutral defects, such as vacancies and interstitials. Contrary to conventional wisdom, the computed $e-\mathrm{d}$ RTs depend strongly on carrier energy and defect type, and the defect-limited mobility is temperature dependent. These results highlight the shortcomings of widely employed heuristic models of $e-\mathrm{d}$ interactions in materials. Our method opens avenues for studying $e-\mathrm{d}$ scattering and low-temperature charge transport from first principles.

Edge dislocations are frequently found in epitaxial ${\mathrm{BiFeO}}_3$ multiferroic thin films and are expected to exhibit distinctive and localized magnetoelectric properties. However, an exhaustive characterization of these dislocations at the atomic level has to date been largely overlooked. Here, we use a combination of scanning transmission electron microscopy techniques, atomistic simulations obtained from classical molecular dynamics calculations, and real-space multiple-scattering theory to explore the chemical properties and the bonding characteristics of the atoms located at and near the dislocation cores. We find that in addition to Bi, small amounts of Fe atoms are present in the ${\mathrm{BiFeO}}_3$ dislocation cores which result in uncompensated Fe spins along the dislocations and give rise to a magnetic signal. Our results suggest that edge dislocations in ${\mathrm{BiFeO}}_3$ films could be efficiently used for realizing ${\mathrm{BiFeO}}_3$-based magnetic devices.

(001)-oriented thin films of the three-dimensional Dirac semimetal cadmium arsenide can realize a quantum spin Hall insulator and other kinds of topological physics, all within the flexible architecture of epitaxial heterostructures. Here, we report a method for growing (001) cadmium arsenide films using molecular beam epitaxy. The introduction of a thin indium arsenide wetting layer improves surface morphology and structural characteristics, as measured by x-ray diffraction and reflectivity, atomic force microscopy, and scanning transmission electron microscopy. The electron mobility of 50-nm-thick films is found to be $9300\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$ at 2 K, comparable to the highest-quality films grown in the conventional (112) orientation. This work demonstrates a simple experimental framework for exploring topological phases that are predicted to exist in proximity to the three-dimensional Dirac semimetal phase.

We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ thin films. For a precursor film, which was co-evaporated at $320{}^{\circ }\mathrm{C}$, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at $500{}^{\circ }\mathrm{C}$, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition.

X-ray absorption spectroscopy is a premier element-specific technique for materials characterization. Specifically, the x-ray absorption near-edge structure (XANES) encodes important information about the local chemical environment of an absorbing atom, including coordination number, symmetry, and oxidation state. Interpreting XANES spectra is a key step towards understanding the structural and electronic properties of materials, and as such, extracting structural and electronic descriptors from XANES spectra is akin to solving a challenging inverse problem. Existing methods rely on empirical fingerprints, which are often qualitative or semiquantitative and not transferable. In this paper, we present a machine learning-based approach, which is capable of classifying the local coordination environments of the absorbing atom from simulated $K$-edge XANES spectra. The machine learning classifiers can learn important spectral features in a broad energy range without human bias and once trained, can make predictions on the fly. The robustness and fidelity of the machine learning method are demonstrated by an average 86% accuracy across the wide chemical space of oxides in eight $3d$ transition-metal families. We found that spectral features beyond the preedge region play an important role in the local structure classification problem especially for the late $3d$ transition-metal elements.