In this study, thermal unpinning of edge dislocations from voids in $\alpha$-Fe is investigated by means of molecular dynamics simulations. The activation energy as a function of shear stress and temperature is systematically determined. Simulations with a constant applied stress are compared with dynamic simulations with a constant strain rate. We found that a constant applied stress results in a temperature-dependent activation energy. The temperature dependence is attributed to the elastic softening of iron. If the stress is normalized with the softening of the specific shear modulus, the activation energy is shown to be temperature-independent. From the dynamic simulations, the activation energy as a function of critical shear stress was determined using previously developed methods. The results from the dynamic simulations are in good agreement with the constant stress simulations, after the normalization. This indicates that the computationally more efficient dynamic method can be used to obtain the activation energy as a function of stress and temperature. The obtained relation between stress, temperature, and activation energy can be used to introduce a stochastic unpinning event in larger-scale simulation methods, such as discrete dislocation dynamics.

The 6000 series Al alloys, which include a few percent of Mg and Si, are important in automotive and aviation industries because of their low weight, as compared to steels, and the fact their strength can be greatly improved through engineered precipitation. To enable atomistic-level simulations of both the processing and performance of this important alloy system, a neural network (NN) potential for the ternary Al-Mg-Si has been created. Training of the NN uses an extensive database of properties computed using first-principles density functional theory, including complex precipitate phases in this alloy. The NN potential accurately reproduces most of the pure Al properties relevant to the mechanical behavior as well as heat of solution, solute-solute, and solute-vacancy interaction energies, and formation energies of small solute clusters and precipitates that are required for modeling the early stage of precipitation and mechanical strengthening. This success not only enables future detailed studies of Al-Mg-Si but also highlights the ability of NN methods to generate useful potentials in complex alloy systems.

The existence of several two-dimensional (2D) materials with heavy atoms has recently been demonstrated. The electronic and optical properties of these materials can be accurately computed with numerically intensive density-functional theory methods. However, it is desirable to have simple effective models that can accurately describe these properties at low energies. Here we present an effective model for stanene that is reliable for electronic and optical properties for photon energies up to 1.1 eV. For this material, we find that a quadratic model with respect to the lattice momentum is the best suited for calculations based on the band structure, even with respect to band warping. We also find that splitting the two spin-$\widehat{\mathbf{z}}$ subsectors is a good approximation, which indicates that the lattice buckling can be neglected in calculations based on the band structure. We illustrate the applicability of the model by computing the linear optical injection rates of carrier and spin densities in stanene. Our calculations indicate that an incident circularly polarized optical field only excites electrons with spin that matches its helicity.

Large-area thin films of topological Dirac semimetal ${\mathrm{Na}}_3\mathrm{Bi}$ are grown on amorphous ${\mathrm{SiO}}_2$:Si substrates to realize a field-effect transistor with the doped Si acting as a back gate. As-grown films show charge carrier mobilities exceeding 7 $000{\mathrm{cm}}^2$/V s and carrier densities below $3\times {10}^{18}{\mathrm{cm}}^{-3}$, comparable to the best thin-film ${\mathrm{Na}}_3\mathrm{Bi}$. An ambipolar field effect and minimum conductivity are observed, characteristic of Dirac electronic systems. The results are quantitatively understood within a model of disorder-induced charge inhomogeneity in topological Dirac semimetals. The hole mobility is significantly larger than the electron mobility in ${\mathrm{Na}}_3\mathrm{Bi}$ which we ascribe to the inverted band structure. When present, these holes dominate the transport properties.

Thermoresponsive poly($N$-isopropylacrylamide) (PNIPAM) particles of a nearly constant swelling ratio and with polydispersity indices (PDIs) varying over a wide range (7.4%–48.9%) are synthesized to study the effects of polydispersity on the dynamics of suspensions of soft PNIPAM colloidal particles. The PNIPAM particles are characterized using dynamic light scattering (DLS) and scanning electron microscopy (SEM). The zero shear viscosity $\left({\eta }_0\right)$ data of these colloidal suspensions, estimated from rheometric experiments as a function of the effective volume fraction ${\varphi }_{\text{eff}}$ of the suspensions, increases with an increase in ${\varphi }_{\text{eff}}$ and shows a dramatic increase at ${\varphi }_{\text{eff}}={\varphi }_0$. The data for ${\eta }_0$ as a function of ${\varphi }_{\text{eff}}$ fit well to the Vogel-Fulcher-Tammann (VFT) equation. It is observed that increasing PDIs results in increasingly fragile supercooled liquidlike behavior, with the parameter ${\varphi }_0$, extracted from the fits to the VFT equation, shifting towards higher ${\varphi }_{\text{eff}}$. The observed increase in fragility is attributed to the prevalence of dynamical heterogeneities (DHs) in these polydisperse suspensions, while the simultaneous shift in ${\varphi }_0$ is ascribed to the decoupling of the dynamics of the smallest and largest particles. Finally, it is observed that the intrinsic nonlinearity of these suspensions, estimated at the third harmonic near ${\varphi }_0$ in Fourier transform oscillatory rheological experiments, increases with an increase in PDIs. Our results are in agreement with theoretical predictions and simulation results for polydisperse hard sphere colloidal glasses and clearly demonstrate that jammed suspensions of polydisperse colloidal particles can be effectively fluidized with increasing PDIs. Suspensions of these particles are therefore excellent candidates for detailed experimental studies of the effects of polydispersity on the dynamics of glass formation.

Organic pi-conjugated polymers are deemed to be soft materials with strong electron-phonon coupling, which results in the formation of polarons, i.e., charge carriers dressed by self-localized distortion of the nuclei. Universal signatures for polarons are optical resonances below the band gap and intense vibrational modes (IVMs), both found in the infrared (IR) spectral region. Here, we study $p$-doped conjugated homo- and copolymers by combining first-principles modelling and optical spectroscopy from the far-IR to the visible. Polaronic IVMs are found to feature absorption intensities comparable to purely electronic transitions and, most remarkably, show only loose resemblance to the Raman or IR-active modes of the neutral polymer. The IVM frequency is dramatically scaled down (up to 50%) compared to the backbone carbon-stretching modes in the pristine polymers. The very large intensity of IVMs is associated with displacement of the excess positive charge along the backbone driven by specific vibrational modes. We propose a quantitative picture for the identification of these polaron shifting modes that solely based on structural information, directly correlates with their IR intensity. This finding finally discloses the elusive microscopic mechanism behind the huge IR intensity of IVMs in doped polymeric semiconductors.

Using low-energy electron microscopy, we image in real time the intercalation of a cobalt monolayer between graphene and the (111) surface of iridium. Our measurements reveal that the edges of a graphene flake represent an energy barrier to intercalation. Based on a simple description of the growth kinetics, we estimate this energy barrier and find small, but substantial, local variations. These local variations suggest a possible influence of the graphene orientation with respect to its substrate and of the graphene edge termination on the energy value of the barrier height. Besides, our measurements show that intercalated cobalt is energetically more favorable than cobalt on bare iridium, indicating a surfactant role of graphene.

A density functional study is presented of the interstitial ${\mathrm{Zn}}_i, {\mathrm{Ge}}_i$, and ${\mathrm{N}}_i$ in ${\mathrm{ZnGeN}}_2$. Corrections to the band gap are included by means of the LDA+U method. The Zn and Ge interstitials are both found to strongly prefer the larger octahedral site compared to the two types of tetrahedral sites. The Zn interstitial is found to be a shallow double donor, but it has higher energy than previously studied antisite defects. It has a resonance in the conduction band that is Zn-$s$ like. The Ge interstitial is an even higher energy of formation defect and also behaves as a shallow double donor, but it also has a deep level in the gap corresponding to a Ge-$s$ orbital character while the Ge-$p$ forms a resonance in the conduction band. The nitrogen interstitial forms a split-interstitial configuration, as also occurs in GaN. Its electronic levels can be related to that of a ${\mathrm{N}}_2$ molecule. The defect levels in the gap correspond to the ${\pi }_g$-like lowest unoccupied molecular orbital of the molecule, which here becomes filled with three electrons in the defect's neutral charge state. They are found to prefer a high-spin configuration in the $q=+1$ state. The corresponding transition levels are obtained and show that this is an amphoteric trap level occurring in $+2, +1$, 0, and $-1$ charge states. The two possible sites for this split interstitial, on top of Zn or on top of Ge, differ slightly in ${\mathrm{N}}_2$ bond length. While the ${\mathrm{N}}_i$ defects have the lowest formation energy among the interstitials, it is still higher than that of the antisites. Hence they are not expected to occur in sufficient concentration to affect the intrinsic Fermi level position. In particular, they do not contribute to the unintentional $n$-type background doping.

Using molecular dynamics simulations to explicitly model fluid molecules, we study the effect of solvent wetting on the behavior of polyhedral nanoparticles at a fluid-fluid interface. First, we quantify the positional and orientational free-energy characteristics of an isolated nanoparticle. Our results suggest that the structure of the interface can introduce nontrivial effects on the preferential particle orientations. A continuum model is proposed to account for the finite-interfacial mixing region, and a qualitative comparison with the molecular simulation results is presented. We examine the effect on the free energy of the system of changes in the particle's solvation preference towards one fluid, and the degree of miscibility between the two fluids. By tuning these interaction parameters, we can potentially access and favor different orientations for the particle shapes examined. Further, we extend the insights gained from single-particle behavior to the attachment of two particles. Our results reveal conditions that could drive the assembly of cuboctahedra into either two-dimensional puckered honeycomb lattices or linear rodlike structures.

Using ab initio molecular dynamics based on density functional theory, we study the atomic and electronic structure, and transport properties of copper-doped germanium-based chalcogenide glasses. These mixed ionic-electronic conductor materials exhibit resistance or threshold switching under external electric field depending on slight variations of chemical composition. Understanding the origin of the transport character is essential for the functionalization of glassy chalcogenides for nanoelectronics applications. To this end, we generated atomic structures for ${\mathrm{GeX}}_3$ and ${\mathrm{GeX}}_6$ (X = S, Se, Te) at different copper concentrations and characterized the atomic origin of electronic states responsible for transport and the tendency of copper clustering as a function of metal concentration. Our results show that copper dissolution energies explain the tendency of copper to agglomerate in telluride glasses, consistent with filamentary conduction. In contrast, copper is less prone to cluster in sulfides and selenides leading to hysteresisless threshold switching where the nature of transport is dominated by electronic midgap defects derived from polar chalcogen bonds and copper atoms. Simulated $I-V$ curves show that at least 35% by weight of copper is required to achieve the current demands of threshold-based devices for memory applications.

Noncrystalline solids can be classified into glassy and amorphous, wherein glasses and amorphous solids relax toward the supercooled liquid and crystalline states upon heating, respectively. However, the structural origin of such distinction remains unknown. Herein, based on molecular-dynamics simulations of irradiation-induced disordering of $\alpha$-quartz, we demonstrate the existence of an amorphous-to-glassy transition. We show that the transition to the glassy state originates from the appearance of structural defects within the medium-range order of the atomic network. Such defects arise from the percolation of short-range defects and kinetically prevent crystallization. Overall, this suggests that the propensity of a disordered system for crystallization is controlled by the similarity between its medium-range order and that of the isochemical crystal.

The introduction of manganite buffer layers, $\mathrm{L}{\mathrm{a}}_{7/8}\mathrm{S}{\mathrm{r}}_{1/8}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) in particular, at the metallic interface between $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) and another band insulator suppresses the carrier density of the interfacial two-dimensional electron gas (2DEG) and improves significantly the electron mobility. However, the mechanisms underlying the extreme mobility enhancement remain elusive. Herein, we used ${}^{18}\mathrm{O}$ isotope exchanged $\mathrm{SrT}{\mathrm{i}}^{18}{\mathrm{O}}_3$ as substrates to create 2DEG at room temperature with and without the LSMO buffer layer. By mapping the oxygen profile across the interface between $\mathrm{ST}{\mathrm{O}}^{18}$ and disordered $\mathrm{LaAl}{\mathrm{O}}_3$ or yttria-stabilized zirconia (YSZ), we provide unambiguous evidence that redox reactions occur at oxide interfaces even grown at room temperature. Moreover, the manganite buffer layer not only suppresses the carrier density but also strongly suppresses the oxygen exchange dynamics of the STO substrate, which likely prevents the reduction of STO during the formation of the 2DEG. The underlying mechanism on the enhanced electron mobility at buffered oxide interfaces is also discussed.

X-ray magnetic circular dichroism (XMCD) reveals the magnetic properties of intercalated europium metal under graphene on SiC(0001). The intercalation of Eu nanoclusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu ${\mathrm{M}}_{4,5}$ edges at $T=15$ K, our samples show paramagnetic-like behavior with distinct anomaly at $T\approx 90$ K, which may be related to the Nèel transition, $T_N=91$ K, of bulk metal Eu. We find no evidence of ferromagnetism due to EuO or antiferromagnetism due to ${\mathrm{Eu}}_2{\mathrm{O}}_3$, indicating that the graphene layer protects the intercalated metallic Eu against oxidation over months of exposure to atmospheric environment.

We perform ab initio many-body Green's function calculations to investigate the quasiparticle energies and optical properties of diamond polytypes that have been predicted to be producible via a pressure-induced structural phase transition from carbon nanotube solids. We find, through quasiparticle band-structure calculations within the $GW$ approximation, that the band gaps of two hexagonal $(2H$- and $4H$-type) polytypes of diamond differ significantly from that of cubic diamond as well as from that of the crystalline $s{p}^3$ carbon phase with a body-centered-tetragonal structure, called bct ${\mathrm{C}}_4$. We also examine the dielectric functions of three polytypes of diamond (cubic, $2H$, and $4H)$ by employing the $GW$ plus Bethe-Salpeter equation approach. The calculated optical absorption spectra are found to be distinct from each other. The lattice mismatches of carbon layers of these diamond polytypes are very small and the total-energy differences are also small. Our work opens up the possibility of fabricating diamond superlattices with various electronic and optoelectronic properties by utilizing and controlling the different stacking sequences of carbon layers.

The influence of collapsed shape on the thermal transport of carbon nanotubes is studied by nonequilibrium molecular dynamics. Nanotubes of different lengths, diameters, chiralities, and degrees of twist are simulated in the regime in which the thermal transport extends from ballistic to diffusive. In contrast with graphene nanoribbons, which are known to exhibit substantial rough-edge and cross-plain phonon scatterings, the collapsed tubes preserve the quasiballistic phononic transport encountered in cylindrical nanotubes. Stacked-collapsed nanotube architectures, closely related with the strain-induced aligned tubes occurring in stretched nanotube sheets, are shown to inherit the ultrahigh thermal conductivities of individual tubes, and are therefore proposed to form highways for efficient heat transport in lightweight composite materials.

The two-dimensional (2D) allotrope of tin with low buckled honeycomb structure named stanene is proposed to be an ideal 2D topological insulator with a nontrivial gap larger than 0.1 eV. Theoretical works also pointed out the topological property of stanene amenability to strain tuning. In this paper we report the successful realization of high quality, monolayer stanene film as well as monolayer stanene nanoribbons on Sb(111) surface by molecular-beam epitaxy, providing an ideal platform to the study of stanene. More importantly, we observed a continuous evolution of the electronic bands of stanene across the nanoribbon, related to the strain field gradient in stanene. Our work experimentally confirmed that strain is an effective method for band engineering in stanene, which is important for fundamental research and application of stanene.

We investigate the stability of electrodeposition at solid-solid interfaces for materials exhibiting an anisotropic mechanical response. The stability of electrodeposition or resistance to the formation of dendrites is studied within a linear stability analysis. The deformation and stress equations are solved using the Stroh formalism and faithfully recover the boundary conditions at the interface. The stability parameter is used to quantify the stability of different solid-solid interfaces incorporating the full anisotropy of the elastic tensor of the two materials. Results show a high degree of variability in the stability parameter depending on the crystallographic orientation of the solids in contact, and point to opportunities for exploiting this effect in developing Li metal anodes.

The geometry and electronic structure of self-assembled atomic scale Au wires on Si(775) are investigated within density functional theory. The calculated surface diagram indicates the existence of two stable configurations with Au coverages of 0.32 and 0.96 monolayers, respectively. The low-coverage structure is predicted to host an antiferromagnetic spin chain localized at the Si rest atom dangling bonds, while the high-coverage structure is characterized by a Au-induced $\beta \text{−}\sqrt{3}$-like structure on the terrace. These structural models are supported by the comparison of measured and calculated surface optical anisotropies.