Some of the strongest polymer-based fibers are obtained by processing a nematic solution of poly-$p$-phenylene terephthalamide (PPTA) in concentrated acid. Molecular dynamics simulations reveal that the lateral density of PPTA chains in the acid solution is inhomogeneous on scales shorter than the persistence length. Hairpin turns, insertions, and other defects are stabilized by chain-chain contacts. These results suggest a mechanism of fiber assembly through coalescence of nanofibrils, and an explanation for the limited strength of PPTA fibers even at high crystallinity.

A singly oriented, single layer of tungsten disulfide (${\mathrm{WS}}_2$) was epitaxially grown on Au(111) and characterized at the nanoscale by combining photoelectron spectroscopy, photoelectron diffraction, and low-energy electron microscopy. Fast x-ray photoelectron spectroscopy revealed that the growth of a single crystalline orientation is triggered by choosing a low W evaporation rate and performing the process with a high temperature of the substrate. Information about the single orientation of the layer was obtained by acquiring x-ray photoelectron diffraction patterns, revealing a 1H polytype for the ${\mathrm{WS}}_2$ layer and, moreover, determining the structural parameters and registry with the substrate. The distribution, size, and orientation of the ${\mathrm{WS}}_2$ layer were further ascertained by low-energy electron microscopy.

Formation and migration energies, elastic dipole, and relaxation volume tensors of nanodefects are the parameters determining the rates of evolution of microstructure under irradiation as well as macroscopic elastic stresses and strains resulting from the accumulation of defects in materials. To find the accurate values of these parameters, we have performed density functional theory simulations of self-interstitial and vacancy defects in all the body-centred cubic metals, including alkaline metals (Li, Na, K, Rb and Cs), alkaline-earth metal (Ba), nonmagnetic transition metals (V, Nb, Mo, Ta and W), and magnetic transition metals (Cr and Fe), correcting the computed values for the effect of finite cell size and periodic boundary conditions. The lowest energy structure of a self-interstitial atom defect is universal to all the nonmagnetic bcc metals, including metals of groups 1 and 2 of the periodic table, and has the $\langle 111\rangle$ symmetry. The only exceptions are the $\langle 110\rangle$ self-interstitial defect configuration in Fe, and a $\langle 11\xi \rangle$ configuration in Cr. We have also computed elastic dipole tensors and relaxation volumes of self-interstitial and vacancy defects in all the bcc metals and explored how elastic relaxation parameters vary along the defect migration pathways.

${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ ($x+y+z=20$) compounds crystallizing in the chiral $\beta$-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, ${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ compounds have also been found to host metastable nonequlibrium skyrmion lattices in a broad temperature and field range, including down to zero field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and dc and ac magnetic measurements to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site preference for the diamondoid $8c$ site in the crystal structure, while Mn tends to share the geometrically frustrated $12d$ site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich $8c$ sublattice and the Mn on the $12d$ sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 and 470 K) with a moment around $1{\mu }_B/\mathrm{atom}$ and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about $3{\mu }_B$) which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

We report a facile mechanochemical approach to achieve precisely controlled van der Waals spacing in molybdenum disulfide $\left(\mathrm{Mo}{\mathrm{S}}_2\right)$ nanosheets. This involves mechanochemical, solid-state grinding of bulk $\mathrm{Mo}{\mathrm{S}}_2$ with solid sodium chloride (NaCl), resulting in intercalation of $\mathrm{N}{\mathrm{a}}^{+}$ ions within the $\mathrm{Mo}{\mathrm{S}}_2$ sheets. Consequently, distinct changes to the van der Waals spacing of $\mathrm{Mo}{\mathrm{S}}_2$ is observed and attributed to coordinative adsorption followed by a diffusion-assisted intercalation. In contrast to Li-ion intercalation, the native 2H structure of $\mathrm{Mo}{\mathrm{S}}_2$ is stabilized and completely preserved. Thus the resulting 2H-$\mathrm{Mo}{\mathrm{S}}_2$ layers exhibit expanded van der Waals spacing. Spectroscopic investigations, through time-dependent Raman and X-ray photoelectron spectroscopy, confirm the kinetics of intercalation resulting in precise tunability of van der Waals spacing from 0.61 to 1.25 nm. Such a scalable approach for tunable intercalation of nanosheets presents a route for controlling defect density in $\mathrm{Mo}{\mathrm{S}}_2$, thereby providing directions in defect engineering and heterogeneous catalysis.

Layered solids are ubiquitous in nature: from subnanometer graphene and mica layers, to wood, laminated composites, and paperboard at the centimeter scale, to geologic formations at the kilometer range. And while the similarities between the latter two have been recognized [Budd et al., Philos. Trans. R. Soc. A 370, 1723 (2012)], what has not is that the same physics applies at the atomic-layer scale of crystalline solids. Herein, using a combination of atomistic simulations of graphite and simple instrumented cylindrical indentation experiments on various layered solids—plastic cards, thin steel, and Al sheets—we show that in all cases, confined buckling results in an instability that leads to the nucleation of multiple, oppositely signed ripplocation boundaries that rapidly propagate away from under the indenter in a wavelike manner. Crucially, upon unloading, they disappear, after dissipating considerable amounts of frictional energy. Understanding ripplocation nucleation, self-assembly, and propagation is fundamental to understanding the deformation of most layered solids.

We describe a lattice-based crystallographic approximation for the analysis of distorted crystal structures via electron backscatter diffraction (EBSD) in the scanning electron microscope. EBSD patterns are closely linked to local lattice parameter ratios via Kikuchi bands that indicate geometrical lattice plane projections. Based on the transformation properties of points and lines in the real projective plane, we can obtain continuous estimations of the local lattice distortion based on projectively transformed Kikuchi diffraction simulations for a reference structure. By quantitative image matching to a projective transformation model of the lattice distortion in the full solid angle of possible scattering directions, we enforce a crystallographically consistent approximation in the fitting procedure of distorted simulations to the experimentally observed diffraction patterns. As an application example, we map the locally varying tetragonality in martensite grains of steel.

The emergent ferroelectricity in ${\mathrm{HfO}}_2$-based systems has attracted significant attention as this simple binary high-$k$ dielectric now offers the possibility of nonvolatile function. In this work we employ zero-field and field-dependent pyroelectricity to show that thin films of silicon-doped ${\mathrm{HfO}}_2$ do exhibit a broken-inversion symmetry and are indeed ferroelectric. In addition, the pyroelectric response is found to exhibit a wake-up behavior akin to the wake-up phenomenon observed in the ferroelectric polarization with electric-field cycling. Using polarization-electric field hysteresis measurements, this wake-up phenomenon is attributed to the presence of defect dipoles which explains the measured pyroelectric response. Finally, direct electrocaloric measurements are performed on these silicon-doped ${\mathrm{HfO}}_2$ thin films, revealing an electrocaloric coefficient $\sim 4$ times larger in magnitude than that expected for the measured pyroelectric coefficient. This enhancement is explained using the plausible role played by defect dipoles that contribute to additional configurational or dipolar entropy.

Metal-polymer nanocomposites are hybrid materials combining the superior plasmonic, electrical, and thermal properties of metals with the good elasticity and manufacturability of polymers. This renders metal-polymer nanocomposites promising candidates for conductive filler and coating applications, where mechanical properties are optothermally coupled. Here, we study the interplay of nanostructure, thermoplasmonics, and elastic mechanical properties of silver-polystyrene nanocomposites (AgPS) by transmission electron microscopy, small-angle x-ray scattering, Brillouin light scattering (BLS), and other supplemental techniques. We utilize the well-known particle-brush architecture to ensure a homogeneous and isotropic nanoparticle distribution throughout the hybrid material. The effective longitudinal modulus of the as-prepared samples is found to decrease from 5.7 to 4.8 GPa with increasing Ag content from 0 to 4.4 vol.%. Temperature-dependent BLS measurements reveal the unique contribution of local thermoplasmonic heating that depends on the Ag nanoparticle composition. This thermoplasmonic effect results in a lower apparent glass transition temperature $\left(T_{\mathrm{g}}\right)$ and a stronger laser power dependence of the speed of sound. Exceeding moderate thermal annealing temperatures $\left(>150{}^{\circ }\mathrm{C}\right)$ leads to a strong structural rearrangement within the homogeneous nanocomposite material with a peculiar clustering-redispersion effect, which also translates into altered mechanical properties. The annealing-induced Ag nanoparticle aggregation results in an even stronger thermoplasmonic effect. We validate our experimental findings with complementary thermographic measurements and finite-element modeling. Overall, this work demonstrates the combined effects of composition and (reversible) aggregation on the mechanical and thermoplasmonic properties of metal-polymer nanocomposites. It not only deepens our understanding of the interaction between light, temperature, and mechanical properties in metal-polymer nanocomposites but also provides a guide for customizing AgPS nanocomposites for potential applications.

Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of x-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute ($1\mathrm{at}.\%$) Mn-doped lithium titanate ($\mathrm{L}{\mathrm{i}}_{4/3}\mathrm{T}{\mathrm{i}}_{5/3}{\mathrm{O}}_4$, or LTO) the dopant $\mathrm{M}{\mathrm{n}}^{2+}$ ions reside on tetrahedral ($8a$) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the $1s\to 4p$ transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intensity of this peak accurately. This dopant location can explain the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result, the observed remarkable $20\%$ decrease in electrochemical capacity of the $1\%$ Mn-doped LTO compared to pristine LTO.

The influence of surface treatments on the transport properties of epitaxial films of the three-dimensional Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ is investigated. We show that exposure of the surface to a low-energy nitrogen plasma improves carrier mobilities, their temperature dependence, and allows for the observation of the quantum Hall effect in scaled thin films. The results provide insights into surface band bending as a function of the surface chemistry and the relative contributions of surface and bulk states to the measured transport properties.

Crystal plasticity causes structural fluctuations that can be scale-free and therefore admit power-law distributions. Numerous experiments, modeling, and theory have reported a scaling exponent that is in good agreement with mean-field predictions or a jamming-unjamming scenario. Via experiments on pure single crystals, we show here that the scaling exponent of a stress-integrated distribution for dislocation-avalanche sizes is nontrivial and can be in agreement with both models by admitting values between 1.0 and 2.3. This range is dictated by the structure and orientation of the deforming crystal, as long as the applied rate is below a critical value. For the highest symmetry tested, plastic strain can drive a change from truncated power-law scaling to pure exponential scaling. These findings show how the same crystal may yield different scaling exponents depending on intrinsic and extrinsic factors.

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single layer's native properties. This dilemma can potentially be solved by decoupling the single layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer ${\mathrm{WS}}_2$ epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer ${\mathrm{WS}}_2$-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer ${\mathrm{WS}}_2$.

Rotational twins are fundamental defects in III-V epitaxy, in particular for the growth on nonpolar (111) surfaces. Based on density functional theory (DFT) calculations, we develop a general model for III-V nucleation on vicinal nonpolar (111)-oriented substrates and focus on the important differences in the atomic step configuration of different miscut directions. We verify this model by a relevant materials system when growing GaP epilayers on As-terminated Si(111): Scanning tunneling microscopy measurements reveal the formation of straight double bilayer steps after As passivation of the Si(111) surface, which persist after III-V growth, as we display when measuring the buried heterointerface with cross-sectional high-resolution transmission electron microscopy. A twin amount in the GaP epilayers is observed in dependence on the misorientation and our nucleation model explains the underlying mechanisms: The number of back bonds at the step edges determines the nucleation site. Accordingly, the substrate misorientation towards $\left[11\overline{2}\right]$ yields twin suppression, which is in full agreement with experiment. Finally, we use DFT input for kinetic Monte Carlo calculations to explain the formation of GaP rotational twins on Si(111):As in order to explain their volume fraction observed by high-resolution x-ray diffraction measurements. We thus derive a complete picture of the formation and suppression of rotational twins relevant for low-defect III-V–on–Si integration.

We present a first-principles approach to compute the transport properties of 2D materials in an accurate and automated framework. We use density-functional perturbation theory in the appropriate bidimensional setup with open-boundary conditions in the third direction. The materials are charged by field effect via planar countercharges. In this approach, we obtain electron-phonon matrix elements in which dimensionality and doping effects are inherently accounted for, without the need for post-processing corrections. This treatment highlights some unexpected consequences, such as an increase of electron-phonon coupling with doping in transition-metal dichalcogenides. We use symmetries extensively and identify pockets of relevant electronic states to minimize the number of electron-phonon interactions to compute; the integrodifferential Boltzmann transport equation is then linearized and solved beyond the relaxation-time approximation. We apply the entire protocol to a set of much studied materials with diverse electronic and vibrational band structures: electron-doped ${\mathrm{MoS}}_2,{\mathrm{WS}}_2,{\mathrm{WSe}}_2$, phosphorene, arsenene, and hole-doped phosphorene. Among these, hole-doped phosphorene is found to have the highest mobility, with a room temperature value around $600{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Last, we identify the factors that affect most phonon-limited mobilities, such as the number and the anisotropy of electron and hole pockets, to provide a broader understanding of the driving forces behind high mobilities in two-dimensional materials.

We investigate metastable and thermodynamically stable phases that can be expected to occur in electroformed filaments in resistively switching hafnia, and discuss their relevance for the switching process. To this end, we conduct a study, based on density functional theory combined with an evolutionary algorithm determining the composition-dependent (meta)stable phases in ${\mathrm{HfO}}_x$, focusing on the region $0<x<2$. We find that oxygen vacancies in hafnia tend to form regular patterns, which leads to periodic metastable structures featuring one-dimensional open channels, thus favoring ionic conductivity in the host material, i.e., oxygen migration. The band gap of such structures is systematically lowered with increasing oxygen deficiency, resulting in metallic behavior when oxygen migrates out of the channels. Moreover, we find that the solubility of oxygen in metallic Hf is very high, up to one oxygen per six metallic atoms, the concentration corresponding to a thermodynamically stable and ordered metallic compound, ${\mathrm{Hf}}_6\mathrm{O}$. Therefore, thick enough metallic capping of Hf could play the role of an active electrode for hosting oxygen which migrates out of ${\mathrm{HfO}}_2$. In combination with reversible oxygen migration in predicted suboxide phases, this should lead to robust resistive memory cells with high endurance and long retention.

Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically as observed in semiconductors and ferroelectric materials through screening charges and/or domain wall formation. In the case of prototypical polar complex oxides such as the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ system the nature of screening charges for different interface terminations is not symmetric. Electron accumulation is observed near the ${\mathrm{LaAlO}}_3/{\mathrm{TiO}}_2\text{−}{\mathrm{SrTiO}}_3$ interface, while the ${\mathrm{LaAlO}}_3$/SrO-${\mathrm{SrTiO}}_3$ stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric ${\mathrm{LaAlO}}_3$ films in contact with the two different surface layers of ${\mathrm{SrTiO}}_3$ crystals, ${\mathrm{TiO}}_2$ and SrO. Using several element-specific probes, we find that the surface termination of ${\mathrm{LaAlO}}_3$ remains ${\mathrm{AlO}}_2$ irrespective of the starting termination of ${\mathrm{SrTiO}}_3$ substrate surface. We use a combination of cross-plane tunneling measurements and first-principles calculations to understand the effects of this unexpected termination on band alignments and polarity compensation of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. An asymmetry in ${\mathrm{LaAlO}}_3$ polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures.

We have studied the physical properties of ${\mathrm{Nd}}_2{\mathrm{O}}_3$ with neutron diffraction, inelastic neutron scattering, heat-capacity, and magnetic susceptibility measurements. ${\mathrm{Nd}}_2{\mathrm{O}}_3$ crystallizes in a trigonal structure with ${\mathrm{Nd}}^{3+}$ ions surrounded by cages of seven oxygen anions. The crystal-field spectrum consists of four excitations spanning the energy range of 3–60 meV. The refined eigenfunctions indicate XY spins in the $ab$ plane. The Curie-Weiss temperature of ${\theta }_{CW}=-23.7\left(1\right)\mathrm{K}$ was determined from magnetic susceptibility measurements. Heat capacity measurements show a sharp peak at 550 mK and a broader feature centered near 1.2 K. Neutron diffraction measurements show that the 550 mK transition corresponds to long-range antiferromagnetic order implying a frustration index of ${\theta }_{CW}/T_N\approx 43$. These results indicate that ${\mathrm{Nd}}_2{\mathrm{O}}_3$ is a structurally and chemically simple model system for frustration caused by competing interactions with moments with predominant XY anisotropy.

We perform experiments and numerical simulations to investigate the low energy states of a triangular colloidal ice, realized by confining repulsive paramagnetic colloids to a lattice of topographic double wells. We find that the collective interactions between the particles lead to a unique ground state characterized by vertices with three colloids pointing inward and three outwards, similar to what was predicted, but never observed, for artificial spin ice. We show that a bias force that magnetizes the system allows us to easily access the elusive GS via a structural martensiticlike transition characterized by the coherent sliding of one particle at each vertex.

Recently published discoveries of acoustic- and optical-mode inversion in the phonon spectrum of certain metals became the first realistic example of noninteracting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this paper, we provide strong theoretical and numerical evidence that for a class of metallic compounds (known as triple-point topological metals), the points in the phonon spectrum, at which three (two optical and one acoustic) phonon modes (bands) cross, represent a well-defined topological material phase, in which the hosting metals have very strong thermoelectric response. The triple-point bosonic collective excitations appearing due to these topological phonon band-crossing points significantly suppress the lattice thermal conductivity, making such metals phonon glasslike. At the same time, the topological triple-point and Weyl fermionic quasiparticle excitations present in these metals yield good electrical transport (electron crystal) and cause a local enhancement in the electronic density of states near the Fermi level, which considerably improves the thermopower. This combination of phonon glass and electron crystal is the key for high thermoelectric performance in metals. We call these materials topological thermoelectric metals and propose several compounds for this phase (TaSb and TaBi). We hope that this work will lead researchers in physics and materials science to the detailed study of topological phonon phases in electronic materials, and the possibility of these phases to introduce more efficient use of thermoelectric materials in many everyday technological applications.