Localization of deformation in shear bands is widely observed preceding intense damage and fracture in ductile or granular materials. However, mechanisms about how shear bands form are generally not known. Here, we show that nanocrystalline ceramic (NCC) nanopillars of ZrN fracture by shear banding under compression. The microstructure evolution of entire shear banding process was in situ monitored by electron imaging and diffraction. Results show that the nanopillars deform through a series of granular activities due to intermittent nucleation and propagation of dislocations. The stress drops associated with the dislocation activities are relatively small since dislocation avalanches are restricted in NCCs because of the effect of nanograin size. Localized and cooperative granular activities, as well as nanocracks, are discovered in the regions where shear bands form. A model about shear band and nanocrack formation is proposed. These findings thus demonstrate for the first time of spatial and temporal nature of intermittent granular activities, leading to shear band formation in NCCs.

Amorphous solids are known to exhibit excess heat capacity that shows a hump near 10 K and diverges from the Debye $T^3$ law at low temperature below 1 K. Here we report that these glassy features are also observed in an insulating crystalline solid. Substitutional chemical suppression of the structural phase transition temperature ($T_{\mathrm{C}}$) of the ferroelectric oxide ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Al}}_2{\mathrm{O}}_4$ results in the disappearance of the $T_{\mathrm{C}}$ at $x=0.07$. For the compositional window of $x=0.2$–0.5, the lattice heat capacity shows a large hump below 10 K and diverges from the $T^3$ law below approximately 2.5 K. Synchrotron x-ray diffraction experiments on single crystals reveal the short-range correlation in the crystal structure that survives down to low temperature; this short-range correlation is responsible for the observed glasslike features in its lattice heat capacity.

We demonstrate theoretically that the golden phase of SmS ($g$-SmS), a correlated mixed-valent system, exhibits nontrivial surface states with diverse topology. It turns out that this material is an ideal playground to investigate different band topologies in different surface terminations. We have explored surface states on three different (001), (111), and (110) surface terminations. The topological signature on the (001) surface is not apparent due to a hidden Dirac cone inside the bulk-projected bands. In contrast, the (111) surface shows a clear gapless Dirac cone in the gap region, demonstrating the unambiguous topological Kondo nature of $g$-SmS. Most interestingly, the (110) surface exhibits both topological-insulator-type and topological-crystalline-insulator (TCI)-type surface states simultaneously. Two different types of double Dirac cones, Rashba-type and TCI-type, realized on the (001) and (110) surfaces, respectively, are analyzed with the mirror eigenvalues and mirror Chern numbers obtained from the model-independent ab initio band calculations.

Current generations of 4H-SiC metal-oxide-semiconductor field-effect transistors are still challenged by the high number of defects at the ${\mathrm{SiO}}_2/\mathrm{SiC}$ interface that limit both the performance and gate reliability of these devices. One potential source of the high density of interface defect states ($D_{it}$) is the stepped morphology on commonly used off-axially grown epitaxial surfaces, favoring incomplete oxidation and the formation of defective transition layers. Here we report measurements on intentionally modified 4H-SiC surfaces exhibiting both atomically flat and stepped regions where the generation of interface defects can be directly linked to differences in surface roughness. By combining spatially resolving structural, chemical, optical, and electrical analysis techniques, a strong increase of $D_{it}$ for stepped surfaces was revealed while regions with an atomically flat SiC surface exhibited close-to-ideal interface properties.

Motivated by recent theoretical studies predicting a large thermopower anisotropy in the layered delafossite $\mathrm{PdCo}{\mathrm{O}}_2$, we have used pulsed laser deposition to synthesize thin films on (0001)-oriented and offcut $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ substrates. By combining transport measurements on films with different offcut angles, tensor rotation relations, and an iterative fit procedure for the transport parameters, we have determined the resistivity and the thermopower along the main crystallographic axes in the temperature range 300–815 K. The data reveal a small positive Seebeck coefficient along the delafossite planes and a large negative Seebeck coefficient perpendicular to the planes, in excellent agreement with density functional calculations in the presence of moderate Coulomb correlations. The methodology introduced here is generally applicable for measurements of the thermoelectric properties of materials with highly anisotropic electronic structures.

Contact electrification, or tribocharging, is pertinent to a broad range of industrial and natural processes involving dielectric materials. However, the basic mechanism by which charge is transferred between insulators is still unclear. Here, we use a simple apparatus that brings two macroscopic surfaces into repeated contact and measures the charge on the surfaces after each contact. We vary the temperature of the surfaces and find that increasing temperature leads to a decrease in the magnitude of charge transfer. When paired with a Monte Carlo simulation and thermogravimetric analysis measurements, our results support a mechanism where adsorbed surface water is crucial for charge exchange. Our setup is easily extendable to a variety of industrially relevant materials.

We report observation of a radial dependence in the magnetic anisotropy of epitaxially strained $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ nanopillars in a $\mathrm{BaTi}{\mathrm{O}}_3$ matrix. This archetypal example of a multiferroic heterostructure with a self-assembling three-dimensional architecture possesses significant out-of-plane uniaxial magnetic anisotropy. The anisotropy originates from the large magnetostriction of $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ and the state of stress within the nanocomposite. Magnetometry suggests the existence of two magnetic phases with different anisotropies. Micromagnetic simulations of a core-shell magnetic anisotropy qualitatively reproduce features of the magnetic hysteresis and elucidate the magnetization reversal mechanism: The magnetization initially reorients within the pillar core, followed by that of the shell. This is consistent with polarized small-angle neutron scattering which can be described by a $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ magnetization that is nonuniform on nanometer length scales. As the length scale of inhomogeneity of the magnetic anisotropy is similar to estimates of the relaxation of the displacement field from the $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4-\mathrm{BaTi}{\mathrm{O}}_3$ interface, stress and its influence on structure provide an important route to new functionality of vertically aligned nanopillars for applications in low-power memory, computing, and sensing.

Muon spin rotation experiments involve muons that experience zero-point vibration at their implantation sites. Quantum-mechanical calculations of the host material usually treat the muon as a point impurity, ignoring its zero-point vibrational energy that, however, plays a role in determining the stability of calculated implantation sites and estimating physical observables. As a first-order correction, the muon zero-point motion is usually described within the harmonic approximation, despite the anharmonicity of the crystal potential. Here we apply the stochastic self-consistent harmonic approximation, a quantum variational method devised to include anharmonic effects in total energy and vibrational frequency calculations, in order to overcome these limitations and provide an accurate ab initio description of the quantum nature of the muon. We applied this full quantum treatment to the calculation of the muon contact hyperfine field in textbook-case metallic systems, such as Fe, Ni, Co including MnSi and MnGe, improving agreement with experiments. Our results show that there are anharmonic contributions to the muon vibrational frequencies with the muon zero-point energies above 0.5 eV. Finally, in contrast to the harmonic approximation, we show that including quantum anharmonic fluctuations, the muon stabilizes at the octahedral site in bcc Fe.

Laterally structured materials can exhibit properties uniquely suited for applications in electronics, magnetoelectric memory, photonics, and nanoionics. Here, a patterning approach is presented that combines the precise geometric control enabled by lithography with topochemical anionic manipulation of complex oxide films. Utilizing oxidation and fluorination reactions, striped patterns of $\mathrm{SrFe}{\mathrm{O}}_{2.5}/{\mathrm{SrFeO}}_3,\mathrm{SrFe}{\mathrm{O}}_{2.5}/{\mathrm{SrFeO}}_2\mathrm{F}$, and ${\mathrm{SrFeO}}_3/{\mathrm{SrFeO}}_2\mathrm{F}$ have been prepared with lateral periodicities of 200, 20, and 4 μm. Coexistence of the distinct chemical phases is confirmed through x-ray diffraction, optical and photoemission microscopies, and optical spectroscopy. The lateral heterostructures exhibit highly anisotropic electronic transport and also enable transience and regeneration of patterns through reversible redox reactions. This approach can be broadly applied to a variety of metal-oxide systems, enabling chemically reconfigurable lateral heterostructures tailored for specific electronic, optical, ionic, thermal, or magnetic functionalities.

Herein, we report on the response of the MAX phase, ${\mathrm{Ti}}_3\mathrm{Si}{\mathrm{C}}_2$, to shock wave compression at strain rates above ${10}^4{\mathrm{s}}^{-1}$. The shock response was determined by measuring the rear, free surface, and velocity of samples—subjected to impact by high-velocity projectiles launched by a gas-gun—using interferometry. The effects of temperature and sample thickness on the dynamic yield and dynamic tensile (spall) strengths were studied. The most important result of this work is the unique dual nature, at high strain rates, of the response of ${\mathrm{Ti}}_3\mathrm{Si}{\mathrm{C}}_2$, in that it is reminiscent of both metals and ceramics. For low-energy impacts, the elastic response is reminiscent of ductile metals. However, for high-energy impacts, it performed like a hard ceramic with quite high work hardening rates. In other words, ${\mathrm{Ti}}_3\mathrm{Si}{\mathrm{C}}_2$ behaves like nothing before it and thus must reflect its nanolayered structure. This work not only provides results on the dynamic mechanical properties of ${\mathrm{Ti}}_3\mathrm{Si}{\mathrm{C}}_2$, but is a critical first step toward understanding the response of ripplocations in layered solids to high strain rates.

The interplay between strong spin-orbit coupling and electron correlations has recently been the subject of intense investigation, due to a number of theoretically predicted phases such as quantum spin liquids, unconventional superconductivity, complex magnetic orders, and correlated topological phases of matter. In particular, iridates have been proposed as a promising family of materials which could host a number of these phases. Here we report the existence of Dirac nodal lines in the binary oxide ${\mathrm{IrO}}_2$, through a combination of reactive oxide molecular beam epitaxy and angle-resolved photoemission spectroscopy. Unlike in other such materials reported to date, these Dirac nodal lines have the unique property of being simultaneously (i) robust against spin-orbit coupling, as they are protected by the nonsymmorphic symmetry of the rutile structure, and (ii) only partially occupied, since they cross the Fermi level. This should have direct implications on the low-energy physics properties tied to the band velocity such as magnetoresistance and spin Hall effect.

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.