We study the impact of dislocations on the bcc-to-hcp-to-bcc phase transition cycle using density-functional theory. The transformation is studied under two external constraints: first under pressure, and second under uniaxial shear. In both cases, we find that the elastic strain created by $\pm \frac{1}{2}\left[111\right]$ screw dislocations induces a shear deformation which initiates the bcc-to-hcp transformation through the Burgers mechanism, as suggested by the location of the phases and their orientations. For the pressure-induced transformation, a hysteresis appears in the P-V curve and the analysis of structures reveals that only the three hcp variants topologically compatible with the screw dislocations emerge. Our calculations thus capture characteristics of microstructures containing grains (variant hcp) and defects (triple junction and grain boundaries). Interestingly, a small bcc inclusion is present in the parent bcc after reversion. A careful analysis of the underlying deformation reveals that its origin is explained by three reversion transformations which are self-accommodating and thus stable. Under shear, the strain field induced by the dislocation decreases the energetic barrier considerably.

Polycrystalline samples of $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ were synthesized at ambient pressure (AP) and high pressure (HP) conditions, respectively. Both samples are electrically semiconducting. The AP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ crystallizes in the $6H$ perovskite structure with space group $P{6}_3/mmc$, consisting of face-sharing $\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_9$ dimer units and corner-sharing $\mathrm{Ni}{\mathrm{O}}_6$ octahedra. Magnetic measurements indicated that AP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ is antiferromagnetically ordered below 130 K. HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ crystallizes in the $6H$ perovskite structure too, but the face-sharing octahedral sites appear to be occupied by both $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{5+}$ ions, whereas the corner-sharing site is occupied exclusively by $\mathrm{O}{\mathrm{s}}^{5+}$. HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ undergoes a high-temperature ($\approx 400\mathrm{K}$) weak ferromagnetic transition, which is much different from the antiferromagnetism of the AP phase. The long-range magnetic order of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ was confirmed by neutron powder diffraction. X-ray magnetic circular dichroism analysis supported ferromagnetic coupling between Os and Ni moments which leads to a spin arrangement, where the ferromagnetic moments mainly arise from $\mathrm{N}{\mathrm{i}}^{2+}$ ions.

Graphene is considered an enabling material for next generation sensors, due to the remarkable properties derived from its atomic thickness. However, these promises come with acute sensitivity to parasite adsorbates, that may hinder reaching the full potential of commercial devices. Herein we report a detailed study of the dramatic effect of adsorbed carbon chains on the effective mechanical properties of graphene monolayers supported by different liquids. We first extract the equivalent bending stiffness of graphene/alcohol bilayers by observing the spontaneous wrinkling of graphene sheets floating on solutions of increasing alcohol contents. We find gains in flexibilities of more than three orders of magnitude compared to expected and previously reported values. We then implement the role of frustrated adsorption in classical membrane models and predict quantitatively the wrinkling transition with no fitting parameters. We also predict a simultaneous transition in alcohol adsorbed on the wrinkled graphene monolayer. We finally characterize this dual transition by detailed combined optical analysis and confocal Raman spectroscopy, and confirm the predictions of the presented model as well as the claims of superflexibility.

In this paper, we describe the manifestation of localized states of a diffuse elastic wavefield inside a two-dimensional metamaterial made of a collection of vertical long beams glued to a thin plate. Through mesoscopic physics, we demonstrate that localized states arise due to multiwave interactions at the beam-plate attachment when the beams act as coupled resonators for both compressional and flexural resonances on the metasurface. In practice, when the compressional resonance of the beams clamps the plate on a large frequency band gap, thus preventing wave propagation inside the metamaterial area, the flexural resonance of the same beams permits evanescent waves to be diffused on a narrow frequency band between clusters of neighboring beams. This experiment physically highlights a tight-binding-like coupling in the localized regime for this two-dimensional metamaterial.

Al-Mg-Si alloys with total solute contents ranging from 0.8 to 1.4 wt % were solutionized, quenched, and then artificially aged (AA) at $180{}^{\circ }\mathrm{C}$, after which positron annihilation lifetime spectroscopy was applied to obtain information about precipitation and vacancy evolution during the preceding ageing step. Hardness and electrical resistivity measurements were carried out to complement these measurements. AA was carried out in four different heating media, which allowed for varying the average heating rate from 2.4 to $170\mathrm{K}{\mathrm{s}}^{-1}$. The main result of the study is that there is a competition between vacancy losses and precipitation. Any precipitation taking place during quenching or during heating to the AA temperature helps to prevent vacancies from going to sinks and allows them to assist in solute clustering. Higher solute content, slower heating to $180{}^{\circ }\mathrm{C}$, and natural preageing before AA were found to have a comparable effect.

Under sharp contact loading, glass deforms elastically and then plastically in the form of densification, shear flow, and network structure changes, which interplay with each other and lead to stress/residual stress buildup and cracking. Vickers indentation is often used to study the deformation and cracking behavior of glass; however, it is not easy to delineate the individual contribution of each deformation mode under indentation due to experimental difficulties associated with in situ investigations at a local scale (tens of microns) under nonuniform stresses. Given the stress field under an indenter is largely compressive, hydrostatic compression and decompression in a diamond anvil cell (DAC) were used in this work to help understand the response of glass to indentation during the loading and unloading process. To this end, an optical microscopy technique was developed to measure the volume of glass under pressure in the DAC by using argon as a pressure transmitting medium. This provided the densification and recovery of glass under hydrostatic compression and decompression. In situ Brillouin light scattering experiments were carried out at the same time to measure the elastic response of glass to pressure. A few multicomponent glasses with vastly different indentation cracking behaviors were selected for study in this work. Our experiments reveal that glass with a high ability to undergo reversible structure changes in response to hydrostatic compression and decompression shows a high cracking resistance under sharp contact loading.

Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. Here we report the observation of structural distortion and noncollinear magnetism in layered pnictide ${\mathrm{EuAg}}_4{\mathrm{As}}_2$ via transport, magnetization, single crystal x-ray, and neutron diffraction data. ${\mathrm{EuAg}}_4{\mathrm{As}}_2$ single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of $q_1=\pm$(0, 0.25, 0.5) and $q_2=\pm$(0.25, 0, 1) emerge. Between 9–15 K, the hexagonal ${\mathrm{Eu}}^{2+}$ sublattice enters an unpinned incommensurate magnetic state, with magnetic Bragg reflections pictured as circular sectors. Below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12) and a very rare magnetic structure, which is helical along the $c$ axis and cycloidal along the $b$ axis with a moment of 6.4 ${\mu }_B/{\mathrm{Eu}}^{2+}$. Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in ${\mathrm{EuAg}}_4{\mathrm{As}}_2$ are revealed.

The development of energy efficient techniques for the generation of spin waves (magnons) is important for the implementation of low-dissipation spin-wave-based logic circuits and memory elements. A promising approach to achieve this goal is based on the injection of short strain pulses into ferromagnetic films with a strong magnetoelastic coupling between the spins and strains. Here, we report micromagnetoelastic simulations of the magnetization and strain dynamics excited in ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ films by picosecond and nanosecond acoustic pulses created in a GaAs substrate by a transducer subjected to an optical or electrical impulse. The simulations performed via the numerical solution of the coupled Landau-Lifshitz-Gilbert and elastodynamic equations show that the injected strain pulse induces an inhomogeneous magnetization precession in the ferromagnetic film. The precession lasts up to 1 ns and can be treated as a superposition of magnon modes having the form of standing spin waves. For ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ films with a nanoscale thickness, up to seven (six) distinct modes have been revealed under free-surface (pinning) magnetic boundary conditions. Remarkably, magnon modes with frequencies over 1 THz can be excited by acoustic pulses with an appropriate shape and duration in films subjected to a moderate external magnetic field. This finding shows that short strain pulses represent a promising tool for the generation of THz spin waves necessary for the implementation of high-speed magnonic devices.

The properties of electronic defects and their relation to structural defects are of high relevance for $\mathrm{CuInS}{\mathrm{e}}_2$ photovoltaic absorbers. Here, we use Raman scattering and steady-state photoluminescence to study the intrinsic optoelectronic properties of near-stoichiometric $\mathrm{CuInS}{\mathrm{e}}_2$ samples with a lateral composition gradient around the Cu saturation point. Apart from a well-known shallow defect band at 0.97 eV, we also observe a deep defect band at 0.8 eV, which is not discernable in photoluminescence spectra at lower temperatures. The preparation of a laterally graded sample with a very precise relative composition range by in situ process control allows for a measurement of a significant decrease of the photoluminescence emission yield at the Cu-poor/Cu-rich transition on a very narrow composition scale. Possible assignments of the bands to structural point defects are discussed.

We present measurements of in-plane thermal and electrical conductivity in thermally evaporated gold thin-film samples ranging in thickness from $\approx 20$ to $>300\mathrm{nm}$, performed using a micromachined silicon-nitride membrane thermal isolation platform. In both $\approx 300$-nm-thick films grown in a single Au deposition and a sample built up to $>300\mathrm{nm}$ by many sequential depositions of thinner layers, we observe strong “violations” of the Wiedemann-Franz law that relates electrical and thermal conductivities. While electrical conductivity behaves essentially as expected, thermal conductivity first rises with growing total film thickness, and then surprisingly drops as the film becomes thicker. The sharp reduction of thermal conductivity decreases the Lorenz number $L$ for $\approx 300\text{−}\mathrm{nm}$-thick samples to less than half the Sommerfeld value over the entire 78–300-K temperature range studied. Such violation near room temperature, in a metal film where electron transport should be well described by Fermi-liquid theory, is previously unreported, even in the presence of disorder introduced by grain boundaries and rough surfaces. This indicates an inelastic-scattering process that we argue, based on detailed characterization of grain size in these films, is likely driven by a combination of modified phonon density of states and structural anisotropy introduced from the strongly columnar grain structure in thicker films. This highly unusual reduction of thermal conductivity while maintaining high electrical conductivity is potentially promising for increasing thermoelectric performance of nanoscale systems.

Vacancies in iron are hydrogen traps, important in the understanding of hydrogen embrittlement of steel. We present a grand canonical approach to computing the trap occupancy as a function of both temperature and hydrogen concentration from practically zero to supersaturation. Our method couples a purpose-made machine-learned H-Fe potential, which enables rapid sampling with near-density-functional-theory accuracy, with a statistical mechanical calculation of the trap occupancy using the technique of nested sampling. In contrast to the conventional assumption (based on Oriani theory) that at industrially relevant hydrogen concentrations and ambient conditions vacancy traps are are fully occupied, we find that vacancy traps are less than fully occupied under these conditions, necessitating a reevaluation of how we think about “mobile hydrogen” in iron and steel.

Local strain-dependent spin-polarized electronic structure of a two-dimensional (2D) magnetic layer is an exciting property for practical applications. For example, it holds the promise for advanced ultrathin spintronic nanodevices with customized electronic and magnetic properties by local strain engineering. Here, we demonstrate that the spin-polarized electronic structure of a 2D manganese gallium nitride (MnGaN-2D) magnetic monolayer is sensitive to intrinsic local lattice strain, as proven by first-principles calculations and indicated by scanning tunneling spectroscopy measurements. Atomic resolution images reveal a highly non-Gaussian lattice spacing/strain distribution, while the spectroscopy reveals variations in the electronic density of states. Simulations of the MnGaN-2D monolayer based on first-principles calculations, including both isotropic and anisotropic strains, confirm a highly strain-dependent manganese partial density of states. Spin-orbit coupling is included which indicates either out-of-plane perpendicular magnetic anisotropy (PMA) or in-plane magnetic anisotropy, dependent on the type of strain whether compressive or tensile, suggesting that MnGaN-2D is magnetoelastic. The MnGaN-2D PMA is further supported by superconducting quantum interference device magnetometry measurements which reveal a high spin polarization of $\sim 79$% at room temperature.

Microscopically revealing the interactions between interfacial water and the quantum states of matter is an important task from both the materials science and the physics points of view. Here we report a low-temperature scanning tunneling microscopy (STM) and spectroscopy study of water adsorption on the charge density wave compound $1T\text{−}{\mathrm{TaS}}_2$, which has a Mott-insulating ground state. Interfacial water forms monolayer islands with $6\times 6$ superstructures on the surface of $1T\text{−}{\mathrm{TaS}}_2$, and the charge order under water islands can be directly imaged in STM topographies taken with negative bias voltages. Compared with the original $\sqrt{13}\times \sqrt{13}$ charge order in $1T\text{−}{\mathrm{TaS}}_2$, the charge order under water islands becomes significantly disordered and denser. A $\mathsf{V}$-shaped gaplike feature emerges in water-covered $1T\text{−}{\mathrm{TaS}}_2$, which may be due to the enhanced dielectric constant of interfacial water, which reduces short-range Coulomb repulsion and induces Mott gap collapse. Our observations open the way to microscopically understanding the interactions between interfacial water and the correlated quantum states of matter.

We report on the epitaxial growth of ${\mathrm{PrVO}}_3$ and ${\mathrm{LaVO}}_3$ by molecular beam epitaxy on (001)-oriented ${\mathrm{SrTiO}}_3$ substrates. We show that a high control of the deposition is achieved and leads to a highly reproducible layer-by-layer growth mode. We evidence also the effect of epitaxial strain on the magnetic properties of the epitaxial films. High-resolution transmission electron microscopy and scanning transmission electron microscopy observations reveal that an antipolar motion of the rare-earth atoms exists in both ${\mathrm{LaVO}}_3$ and ${\mathrm{PrVO}}_3$ thin films. The investigation of the structural distortions and atomic displacements in the vanadate thin films may be generalized to other orthorhombic perovskite oxides and contributes to the search of ferroelectricity by design in symmetry-breaking magnetic oxide superlattices.

During crystallization a continuum of patterns could emerge due to the interplay of growth kinetics, material or solution chemistry, and crystallographic defects. The coherent twin boundary is widely known to catalyze growth in pristine crystals including polycrystalline Si. Much remains unknown about the impact of changing the chemical environment of the crystallization process through the deliberate addition of trace metallic species—termed chemical modification. Pristine Si has been reported to grow through the classical model of two parallel twin planes acting in concert to enable steady-state propagation of the solid-liquid interfaces. Here, we achieve a vision on the growth process via in situ synchrotron x-ray microtomography and further corroborated by ex situ crystallographic investigation. We find that steady-state growth is impossible in chemically modified alloys that consist of trace (0.1 wt.%) Sr. This is because the Sr modifier poisons the concave re-entrant grooves, thereby deactivating the advantage of the twin-plane re-entrant edge mechanism and leading to a singly-twinned interface. This study may serve as a proxy to chemically modified crystallization pathways of eutectic Si in Al-Si alloys and, more broadly, as a framework for the crystallization-mediated synthesis of materials.

Oxide glasses continue to be among the most important materials for a sustainable society owing to their unique advantages, e.g., transparency, formability, low cost, and tailorable properties and functionalities. However, their high brittleness has severely been hindering the use of oxide glasses for many engineering and functional applications. Previous approaches to overcome this limitation by improving the fracture toughness have largely failed, but herein we report the use of a simple post-treatment, namely surface aging in a humid atmosphere, to improve not the resistance to crack growth, but rather the resistance to crack initiation. The effect of such aging on the mechanical performance is found to be highly composition dependent, and in stark contrast to most previous work, we thus find that water incorporation is not always a foe, but can also be a friend for glass mechanics. Specifically, we demonstrate crack-free ultrasharp cube corner indents in an aged caesium aluminoborate glass at loads above 25 N, while other aged oxide glasses studied in this work easily crack at loads below 1 N. The aging effect is found to be due to a combination of stress and hydrolysis assisted structural changes in the flexible aluminoborate glass network.

A perpendicularly magnetized ferromagnetic layer is an important building block for recent/future high-density spintronic memory applications. This paper reports the fabrication of perpendicularly magnetized Ni/Pt superlattices and the characterization of their structures and magnetic properties. The optimization of film growth conditions allowed us to grow epitaxial Ni/Pt (001) superlattices on $\mathrm{SrTi}{\mathrm{O}}_3$ (001) single-crystal substrates. We investigated their structural parameters and magnetic properties as a function of the Ni layer thickness and obtained a high uniaxial magnetic anisotropy energy of $1.9\times {10}^6\mathrm{erg}/\mathrm{c}{\mathrm{m}}^3$ for a [Ni (4.0 nm)/Pt (1.0 nm)] superlattice. In order to elucidate the detailed mechanism on perpendicular magnetic anisotropy for the Ni/Pt (001) superlattices, the experimental results were compared with the first-principles calculations. It has been found that the strain effect is a prime source of the emergence of perpendicular magnetic anisotropy.

Actinide and lanthanide-based materials display exotic properties that originate from the presence of itinerant or localized $f$ electrons and include unconventional superconductivity and magnetism, hidden order, and heavy-fermion behavior. Due to the strongly correlated nature of the $5f$ electrons, magnetic properties of these compounds depend sensitively on applied magnetic field and pressure, as well as on chemical doping. However, precise connection between the structure and magnetism in actinide-based materials is currently unclear. In this investigation, we established such structure-property links by assembling and mining two datasets that aggregate, respectively, the results of high-throughput density functional theory simulations and experimental measurements for the families of uranium- and neptunium-based binary compounds. Various regression algorithms were utilized to identify correlations among accessible attributes (features or descriptors) of the material systems and predict their cation magnetic moments and general forms of magnetic ordering. Descriptors representing compound structural parameters and cation $f$-subshell occupation numbers were identified as most important for accurate predictions. The best machine learning model developed employs the random forest regression algorithm. It can predict both spin and orbit moment size with root-mean-square error of $0.17{\mu }_{\mathrm{B}}$ and $0.19{\mu }_{\mathrm{B}}$, respectively. The random forest classification algorithm is used to predict the ordering (paramagnetic, ferromagnetic, and antiferromagnetic) of such systems with 76% accuracy.