It has been long hypothesized that the structure of a material bombarded by energetic particles might approach a certain asymptotic steady state in the limit of high exposure to irradiation. There is still no definitive verdict regarding the validity of this hypothesis or the conditions where it applies. To clarify this, we explore a highly simplified model for microstructural evolution that retains full atomic detail of the underlying crystal structure and involves random events of generation and relaxation of defects. We explore the dynamics of evolution of the model in the limit $T=0$, where the defect and dislocation microstructure is driven purely by the spatially fluctuating stress field accumulating as a result of stochastic generation of point defects. Using body-centred cubic iron and tungsten as examples, we show that their microstructure exhibits a structural transition and then approaches a limiting asymptotic state at doses of order $O\left(0.1\right)$ and $O\left(1\right)$ canonical defects per atom, respectively, and analyze the microscopic and macroscopic parameters characterizing both the transition and the asymptotic microstructural state.

When exposed to air, metallic yttrium dihydride ${\mathrm{YH}}_2$ films turn into insulating and transparent yttrium oxyhydride (YHO). The incorporation of oxygen causes the lattice expansion of ${\mathrm{YH}}_2$ and the emergence of photochromic properties, i.e., YHO darkens reversibly when illuminated with light of adequate energy and intensity. However, the adequate bleaching of the photodarkened samples once the illumination has stopped is much faster in air than in inert atmosphere. According to this experimental evidence, the photochromic mechanism has to be related to an oxygen diffusion and exchange process. Since this process is accompanied by a lattice expansion/contraction, it can be said that YHO “breathes” when subjected to illumination/darkness cycling. Another interesting side effect of the breathing is the unexpected enhancement of the hydrophobicity of the YHO samples under illumination. A theoretical model able to explain the breathing in YHO is presented, together with the discussion of other alternative explanations.

Perovskite oxides used in heterogeneous catalysis and electrocatalysis are tuned by substitutional doping. Sr-doped $\mathrm{LaMn}{\mathrm{O}}_3$ is a popular choice of electrode material for electrochemical energy conversion. At elevated temperatures, relevant to solid oxide fuel or electrolysis cells, Sr enriches at the surface, which leads to fast degradation of the oxygen exchange activity at the surface. In this work, we investigate the effect of oxygen partial pressure $p\left(O_2\right)$ and temperature on Sr segregation at the $\mathrm{L}{\mathrm{a}}_{0.75}\mathrm{S}{\mathrm{r}}_{0.25}\mathrm{Mn}{\mathrm{O}}_3\left(001\right)$ (LSM25) surface by ab initio thermodynamics calculations, taking into account different terminations and point defects ${\mathrm{Sr}}_{\mathrm{La}}^{‴}$, ${\mathrm{V}}_{\mathrm{O}}^{··}$, ${\mathrm{V}}_{\mathrm{Mn}}^{‴}$, ${\mathrm{V}}_{\mathrm{La}}^{‴}$, ${\mathrm{O}}_{\mathrm{ad}}^{}$, as well as associated defects of the type $[{\mathrm{V}}_{\mathrm{O}}^{··}-{\mathrm{V}}_{\mathrm{cation}}^{‴}]$. This model of the LSM25(001) surface addresses the connection between point defects and surface stability and makes quantitative predictions about the surface termination and Sr enrichment. Our results indicate that the $\mathrm{Mn}{\mathrm{O}}_2$ termination is stable under oxidizing conditions, while a partially covering SrO termination on $\mathrm{Mn}{\mathrm{O}}_2$ is stable for reaction conditions between 1000 and 1400 K and effective oxygen partial pressures between ${10}^{-11}\mathrm{bar}$ and 0.1 bar relevant to solid oxide fuel cell (SOFC) cathodes. Under more reducing conditions, we find that Sr is enriched at the surface regardless of surface termination. This trend is a direct consequence of the increased formation of positively charged oxygen vacancies at the surface, and this conclusion holds regardless of which surface termination is assumed. The partial SrO termination is exceptionally stable under SOFC cathode conditions because it minimizes the surface dipole density, while all the defect-free terminations of LSM25(001) are unstable due to surface polarity. This newly proposed termination leaves both Sr, as well as Mn and O sites of the $\mathrm{Mn}{\mathrm{O}}_2$ layer exposed at the surface and available for interaction with the gas phase. We propose that the LSM25(001) surface may consist of coexisting (La,Sr)O and $\mathrm{Mn}{\mathrm{O}}_2$ domains under SOFC cathode conditions, and we propose to include such a micropatterned surface in the future discussion of the active site in the oxygen reduction reaction.

Ferrimagnetic iron garnets are promising materials for spintronics applications, characterized by ultralow damping and zero current shunting. It has recently been found that few nm-thick garnet films interfaced with a heavy metal can also exhibit sizable interfacial spin-orbit interactions, leading to the emergence, and efficient electrical control, of one-dimensional chiral domain walls. Two-dimensional bubbles, by contrast, have so far only been confirmed in micrometer-thick films. Here, we show by high resolution scanning transmission x-ray microscopy and photoemission electron microscopy that submicrometer bubbles can be nucleated and stabilized in $\sim 25$-nm-thick thulium iron garnet films via short heat pulses generated by electric current in an adjacent Pt strip, or by ultrafast laser illumination. We also find that quasistatic processes do not lead to the formation of a bubble state, suggesting that the thermodynamic path to reaching that state requires transient dynamics. X-ray imaging reveals that the bubbles have Bloch-type walls with random chirality and topology, indicating negligible chiral interactions at the garnet film thickness studied here. The robustness of thermal nucleation and the feasibility demonstrated here to image garnet-based devices by x-rays both in transmission geometry and with sensitivity to the domain wall chirality are critical steps to enabling the study of small spin textures and dynamics in perpendicularly magnetized thin-film garnets.

Recently, remarkable experimental progress has been made toward understanding the metastable phases in Mg-Sn alloys, a promising candidate for rare-earth free Mg alloys. However, the detailed structures, stability, and formation mechanisms of these metastable phases at early stages of precipitation remain unclear. Here, we report a first-principles study of the ground-state and finite-temperature phase stability of hexagonal closest packed (HCP)- and face-centered cubic (FCC)-based phases in Mg-Sn alloys. The ground-state phases are searched by the cluster expansion approach coupled with energy input from first-principles calculations. The Monte Carlo simulations with Hamiltonian from the cluster expansion are performed to calculate the configurational free energies and the corresponding metastable phase diagrams are constructed. Our calculations reveal rich varieties of orderings on HCP and FCC lattices, metastable ${\mathrm{Mg}}_3\mathrm{Sn}$ phases with $\mathrm{L}{1}_2$ and $\mathrm{D}{0}_{19}$ structures, and possible precursors for Guinier-Preston (G.P.) zones. Further analysis of the vibrational free energy indicates that $\mathrm{L}{1}_2$ ${\mathrm{Mg}}_3\mathrm{Sn}$ is stabilized by vibrational entropy at high temperatures. A new precipitation sequence for Mg-Sn alloys is suggested, i.e., supersaturated solid solution → G.P. zone → $\mathrm{D}{0}_{19}$ ${\mathrm{Mg}}_3\mathrm{Sn}\to \mathrm{L}{1}_2$ ${\mathrm{Mg}}_3\mathrm{Sn}\to \beta {\mathrm{Mg}}_2\mathrm{Sn}$.

Materials with increased functionality are often based on crystalline structures with significant local disorder. Typical examples are ferroic oxides exhibiting large spontaneous polarization that can be rotated by an applied electric field, finding use in many important applications. Despite years of investigation, the exact structural origin of the increased piezoelectric response of oxide ferroics is still unclear. Frequently evoked models attribute it to emerging polar nanoregions inside a nonpolar matrix, the existence of a morphotropic boundary separating polar phases with different crystallographic symmetry, low-symmetry bridging phases facilitating polarization rotation, and displacive and order-disorder structural phase transitions. Here we use both conventional and resonant high-energy x-ray diffraction coupled to atomic pair distribution function analysis and three-dimensional computer simulations to examine the relationship between the local structure and piezoelectric properties of exemplary sodium-potassium niobate ferroics. We show that their increased piezoelectric response is primarily due to a geometrical frustration in the underlying perovskite lattice induced by local fluctuations in the tilt pattern of the constituent niobium-oxygen octahedra, and not to a crystal-crystal phase transition or distinct nanodomains. The fluctuations peak when the sodium to potassium ratio approaches 1, leading to a softening of the perovskite lattice and easing of polarization rotation under an electric field. Based on the experimental data and model calculations involving Goldschmidt's tolerance factor for the stability of perovskites, we also show that the fluctuations are driven by the mismatch between the radii of sodium and potassium atoms, and the increased piezoelectric response of sodium-potassium niobates indeed scales with the variance in the distribution of these radii about the average value. Thus, we settle important aspects of the debate over the structure-piezoelectric property relationship for oxide ferroics, thereby providing a different perspective on the ongoing effort to improve their useful properties.

This work provides a direct route to measure the degree of hybridization of $f$ states in rare earths. The interference between electric dipole and octupole transitions is measured at the $L_1$ edge of Gd in ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ using x-ray natural linear dichroism (XNLD) and high energy resolution fluorescence detection. The Gd $4f\text{−}6p$ admixture is quantified through the integral of the dipole-octupole XNLD using a sum rule easily applicable to experimental data. The mixing of the Gd valence states with the O ligand orbitals, calculated from first principles, reveals that despite their localized character, the Gd $4f$ orbitals mix with the O $2p$ and $2s$ orbitals with an antibonding and bonding character, respectively.

Silicene, a silicon analog of graphene, has the potential to become a candidate next-generation material by virtue of its novel physical and chemical properties. Although a rich variety of rotationally nonequivalent silicene structures have been observed with silicene grown on Ag(111) surfaces, the $T$ phase, which has been considered a precursor phase, has a less-ordered structure, and thus, it is difficult to clarify its atomic structure. In this paper, we report the atomic structures of the $T$ phase observed by high-resolution atomic force microscopy (AFM). While scanning tunneling microscopy images of the $T$ phase show characteristic round dots, AFM reveals that the $T$ phase has a continuous Si honeycomb arrangement, thus forming silicene. We identify two types of $T$ phases with different silicene rotation angles with respect to the Ag substrate: the ($\sqrt{13}\times \sqrt{13}$) type-I $T$ phase and the tiling-pattern $T$ phase. The former has a unit cell corresponding to the conventionally proposed ($\sqrt{13}\times \sqrt{13}$) type I with less periodicity of Si buckling, while the latter is identified by the tessellation with four different rhombuses. We also investigate the $T$ phase by site-specific force spectroscopy and find that some Si atoms at the $T$ phase have slightly different chemical reactivities.

We report on extensive investigations of magnetism at the $n$-type $\mathrm{LaAl}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ interface performed by utilizing a spectrum of local and integrative analytical techniques: Scanning superconducting quantum interference device microscopy, polar Kerr magnetometry, ferromagnetic resonance, and magnetic torque magnetometry. The samples originated from the same wafers. For nominally fully oxidized samples, we find that the mere presence of the conducting interface does not induce magnetism to values exceeding already present magnetic signals originating from the substrates, irrespective of the measurement technique. With the controlled introduction of oxygen vacancies, however, the different analytical techniques with their inherently different sensitivities and potential interactions with possible magnetic moments in the samples yield different results concerning the existence of magnetism. These unexpected differences obtained with the various measurement techniques are a possible source of the disagreement in the literature about the existence of ferromagnetism at $\mathrm{LaAl}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ interfaces.

Realizing quantum materials in few atomic layer morphologies is a key to both observing and controlling a wide variety of exotic quantum phenomena. This includes topological electronic materials, where the tunability and dimensionality of few layer materials have enabled the detection of $Z_2$, Chern, and Majorana phases. Here we report the development of a platform for thin film correlated, topological states in the magnetic rare-earth monopnictide ($RX$) system GdBi synthesized by molecular beam epitaxy. This material is known from bulk single crystal studies to be semimetallic antiferromagnets with Neel temperature $T_{\mathrm{N}}=28\mathrm{K}$ and is the magnetic analog of the non-$f$-electron containing system LaBi proposed to have topological surface states. Our transport and magnetization studies of thin films grown epitaxially on ${\mathrm{BaF}}_2$ reveal that semimetallicity is lifted below approximately eight crystallographic unit cells while magnetic order is maintained down to our minimum thickness of five crystallographic unit cells. First-principles calculations show that the nontrivial topology is preserved down to the monolayer limit, where quantum confinement and the lattice symmetry give rise to a  $C=2$ Chern insulator phase. We further demonstrate the stabilization of these films against atmospheric degradation using a combination of air-free buffer and capping procedures. These results together identify thin-film $RX$ materials as potential platforms for engineering topological electronic bands in correlated magnetic materials.

The rapid degradation of the functional properties of many Ti-based alloys is due to the precipitation of the $\omega$ phase. In the conventional high-temperature shape memory alloy Ti-Ta, the formation of this phase compromises completely the shape memory effect, and high ($>100{}^{\circ }\text{C}$) transformation temperatures cannot be maintained during cycling. A solution to this problem is the addition of other elements to form Ti-Ta-$X$ alloys, which often modifies the transformation temperatures; due to the largely unexplored space of possible compositions, very few elements are known to stabilize the shape memory effect without decreasing the transformation temperatures below $100{}^{\circ }\text{C}$. In this study, we use transparent descriptors derived from first-principles calculations to search for new ternary Ti-Ta-$X$ alloys that combine stability and high temperatures. We suggest four alloys with these properties, namely Ti-Ta-Sb, Ti-Ta-Bi, Ti-Ta-In, and Ti-Ta-Sc. Our predictions for the most promising of these alloys, Ti-Ta-Sc, are subsequently fully validated by experimental investigations, the alloy Ti-Ta-Sc showing no traces of $\omega$ phase after cycling. Our computational strategy is transferable to other materials and may contribute to suppress $\omega$ phase formation in a large class of alloys.

The magnetic properties of a material are determined by a subtle balance between the various interactions at play, a fact that makes the design of new magnets a daunting task. High-throughput electronic structure theory may help to explore the vast chemical space available and offers a design tool to the experimental synthesis. This method efficiently predicts the elementary magnetic properties of a compound and its thermodynamical stability, but it is blind to information concerning the magnetic critical temperature. Here we introduce a range of machine-learning models to predict the Curie temperature $T_{\mathrm{C}}$ of ferromagnets. The models are constructed by using experimental data for about 2500 known magnets and consider the chemical composition of a compound as the only feature determining $T_{\mathrm{C}}$. Thus we are able to establish a one-to-one relation between the chemical composition and the critical temperature. We show that the best model can predict $T_{\mathrm{C}}$'s with an accuracy of about 50 K. Most importantly our model is able to make predictions in regions of the chemical space, where only a small fraction of the data was considered for training. Furthermore, it is able to assess the uncertainty of such predictions. This is demonstrated by tracing the $T_{\mathrm{C}}$ of binary intermetallic alloys along their composition space and for the Al-Co-Fe ternary system.

Computational design of more efficient rare earth/transition metal (RE-TM) permanent magnets requires accurately calculating the magnetocrystalline anisotropy (MCA) at finite temperature, since this property places an upper bound on the coercivity. Here, we present a first-principles methodology to calculate the MCA of RE-TM magnets which fully accounts for the effects of temperature on the underlying electrons. The itinerant electron TM magnetism is described within the disordered local moment picture, and the localized RE-$4f$ magnetism is described within crystal field theory. We use our model, which is free of adjustable parameters, to calculate the MCA of the $R{\mathrm{Co}}_5 (R=\mathrm{Y}, \text{La-Gd})$ magnet family for temperatures 0–600 K. We correctly find a huge uniaxial anisotropy for ${\mathrm{SmCo}}_5$ ($21.3\text{MJ}{\text{m}}^{-3}$ at 300 K) and two finite temperature spin reorientation transitions for ${\mathrm{NdCo}}_5$. The calculations also demonstrate dramatic valency effects in ${\mathrm{CeCo}}_5$ and ${\mathrm{PrCo}}_5$. Our calculations provide quantitative, first-principles insight into several decades of RE-TM experimental studies.

The extraordinary ability of shape-memory alloys to recover after large imposed deformation motivates efforts to transpose these properties onto ceramics, which would enable practical shape-memory properties at high temperatures and in harsh environments. The theory of mechanical compatibility was utilized to predict promising ceramic candidates in the system ${\left({\mathrm{Y}}_{0.5}{\mathrm{Ta}}_{0.5}{\mathrm{O}}_2\right)}_{1-x}\text{−}{\left({\mathrm{Zr}}_{0.5}{\mathrm{Hf}}_{0.5}{\mathrm{O}}_2\right)}_x$, $0.6<x<0.85$. When these compatibility conditions are met, a reduction in thermal hysteresis by a factor of 2.5, a tripling of deformability, and a 75% enhancement in strain recovery within the shape-memory effect was found. These findings reveal that predicting and optimizing the chemical composition of ceramics to attain improved crystallographic compatibility is a powerful tool for enabling and enhancing their deformability that could ultimately lead to a highly reversible oxide ceramic shape-memory material.

Mechanical and phononic metamaterials exhibiting negative elastic moduli, gapped vibrational spectra, or topologically protected modes enable precise control of structural and acoustic functionalities. While much progress has been made in their experimental and theoretical characterization, the inverse design of mechanical metamaterials with arbitrarily programmable spectral properties and mode localization remains an unsolved problem. Here, we present a flexible computational inverse-design framework that allows the efficient tuning of one or more gaps at nearly arbitrary positions in the spectrum of discrete phononic metamaterial structures. The underlying algorithm optimizes the linear response of elastic networks directly, is applicable to ordered and disordered structures, scales efficiently in two and three dimensions, and can be combined with a wide range of numerical optimization schemes. We illustrate the broad practical potential of this approach by designing mechanical band-gap switches that open and close preprogrammed spectral gaps in response to an externally applied stimulus such as shear or compression. We further show that the designed structures can host topologically protected edge modes, and validate the numerical predictions through explicit three-dimensional finite-element simulations of continuum elastica with experimentally relevant material parameters. Generally, this network-based inverse design paradigm offers a direct pathway toward manufacturing phononic metamaterials, DNA origami structures, and topoelectric circuits that can realize a wide range of static and dynamic target functionalities.

Determination of the adsorption structure of water molecules on metal surfaces is an imperative challenge to understanding the mechanisms of the wetting process and water-related heterogeneous catalysis. We identify water monolayers formed on Ni(111) via low-temperature atomic force microscopy, which enables the visualization of individual water molecules in monolayers with higher spatial resolution than scanning tunneling microscopy. On the terraces of Ni(111) at 150 K, water forms monolayers comprising fused pentagonal, hexagonal, and heptagonal rings. Water adsorbates on step sites assemble in a different manner, forming a hydrogen-bonding network with fused pentagonal and octagonal rings aligned in the step direction. Because similar water networks with pentagonal rings have been proposed in monolayers or their defect sites on other metal surfaces, our structural characterization of ${\mathrm{H}}_2\mathrm{O}/\mathrm{Ni}\left(111\right)$ provides an insight into water adsorption structures on metals.

It is known that ionic liquids (ILs) assemble differently at an electrode surface compared to traditional electrolytes; however, the impact of this on electrochemical processes is not yet clear. In this work, we investigate interfacial behavior of two ILs, 1-ethyl-3-methylimidazolium dicyanamide ($\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$) and N-butyl-N-methylpyrrolidinium dicyanamide ($\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$) in the context of a rechargeable zinc battery. In previous experiments, $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ outperformed $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ in several ways (e.g., lower overpotential required to initiate electrodeposition of zinc, higher zinc deposition peak current density, and greater number of supported charge-discharge cycles). Here, using molecular-dynamics simulations, we investigate a set of mixtures containing zinc in those ILs, confined by graphene electrodes. Our results reveal the presence of layers and show how the two systems differ with respect to those layers: $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ exhibits a tendency for high overscreening and undergoes a transition into the crowding regime, whereas $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ exhibits low overscreening and shows no signs of crowding. As a result, the interfacial concentration of zinc and anions is higher in $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$, strongly suggesting a large impact on the electrochemical performance of a battery.

The electron mobility value of $320{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ observed from La-doped ${\mathrm{BaSnO}}_3$ (LBSO) single crystals has a great potential in optoelectronic applications, but LBSO films exhibit much lower mobilities. Threading dislocations from the film/substrate mismatch are believed to be the main source of this phenomenon, but previous experiments suggest that they do not fully explain the mobility suppression. In this paper, we examined the thickness dependence of electrical and thermal transport properties of LBSO films fabricated in different oxidation environments. The results show that oxygen deficiency also affects the electron mobility of LBSO films, and the mobility suppression in LBSO films is dominated by different mechanisms depending on the thicknesses. LBSO films with different oxygen vacancy contents were fabricated using the pulsed laser deposition technique. The films deposited under higher oxidative conditions exhibited lower oxygen deficiency levels and superior transport properties whereas the threading dislocation densities remained unchanged. The highest mobility value observed in this paper was $120{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, which is comparable to LBSO films fabricated on thick buffer layers. Our paper provides a broader perspective on understanding the mobility of LBSO films and confirms that threading dislocations are not the only factor.

The bismuth (Bi) substitutional donor in silicon (Si) is an attractive qubit candidate for quantum computing proposals due to its large Hilbert space, clock transitions, and potential to couple to superconducting flux qubits. Single-qubit control, coupling, and readout by surface nanocircuitry requires a Bi depth of $\sim 20$ nm in Si. This can be achieved using ion implantation of $\sim 25$ keV Bi. This work explores the activation properties of Bi implanted at 26 keV with fluences of $1\times {10}^{14}$ and $6\times {10}^{12}{\mathrm{cm}}^{-2}$ into both crystalline and preamorphized Si. The Bi electrical activation yield was measured over a broad range of annealing conditions using resistivity and Hall effect measurements, enabling optimal annealing strategies to be proposed for the different implant parameters. For the high and low fluences, the maximum Bi activation yields achieved were 64% and 46%, respectively. Above a critical thermal budget, a substantial fraction of Bi forms electrically inactive complexes in the high fluence sample only. The substitutional fraction and diffusion of high fluence Bi was quantified, with diffusion coefficients $D_0=4.0\pm 0.5$ and $7.5\pm 0.5{\mathrm{cm}}^2{\mathrm{s}}^{-1}$ found for implantation into crystalline and preamorphized Si, respectively, using Rutherford backscattering spectrometry. To demonstrate the successful activation and quantum control of near-surface implanted Bi, the full hyperfine spectrum of these donors is obtained using continuous-wave electron spin resonance at 25 K, supporting the suitability for Bi donor qubits.

We report on the selective-area chemical beam epitaxial growth of InAs in-plane, one-dimensional (1D) channels using patterned ${\mathrm{SiO}}_2$-coated InP(001), InP(111)B, and InP(011) substrates to establish a scalable platform for topological superconductor networks. Top-view scanning electron micrographs show excellent surface selectivity and dependence of major facet planes on the substrate orientations and ridge directions, and the ratios of the surface energies of the major facet planes were estimated. Detailed structural properties and defects in the InAs nanowires (NWs) were characterized by transmission electron microscopic analysis of cross-sections perpendicular to the NW ridge direction and along the NW ridge direction. Electrical transport properties of the InAs NWs were investigated using Hall bars, a field effect mobility device, a quantum dot, and an Aharonov-Bohm loop device, which reflect the strong spin-orbit interaction and phase-coherent transport characteristic present in the selectively grown InAs systems. This study demonstrates that selective-area chemical beam epitaxy is a scalable approach to realize semiconductor 1D channel networks with the excellent surface selectivity and this material system is suitable for quantum transport studies.