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.

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

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

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

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

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

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

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

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

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

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

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