We compute the structural energies, elastic constants, and stacking fault energies, and investigate the phase stability of monoborides with different compositions $\left(X_{1-x}^1{X}_x^2\right)\text{B}$ $\left(X=\text{Ti/Fe/Mo/Nb/V}\right)$ using density functional theory in order to search for Ti monoborides with improved mechanical properties. Our computed Young's modulus and Pugh's modulus ratio, which correlate with stiffness and toughness, agree well with predictions from Vegard's law with the exceptions of mixed monoborides containing Mo and Fe. Among all the monoborides considered in this paper, TiB has the smallest Pugh's ratio, which suggests that the addition of solutes can improve the toughness of a Ti matrix. When $X^1\mathrm{B}$ and $X^2\mathrm{B}$ are respectively most stable in the $B_{27}$ and $B_f$ structures, the mixed monoborides $\left(X_{1-x}^1{X}_x^2\right)\text{B}$ have a lower or similar stacking fault energy than TiB and could therefore improve the ductility of the Ti matrix. Among all $\left(X_{0.5}^1{X}_{0.5}^2\right)\text{B}$, mixed $\left({\mathrm{Ti}}_{0.5}{\mathrm{Mo}}_{0.5}\right)\text{B}$ and mixed $\left({\mathrm{Ti}}_{0.5}{\mathrm{V}}_{0.5}\right)\text{B}$ have a higher Young's modulus, a higher Pugh's ratio, and a smaller stacking fault energy than TiB. We also construct phase diagrams and find large solubility limits for solid solutions containing Ti compared to those containing Fe.

Few layer bismuth nanofilms with (111) orientation have shown striking electronic properties, especially as building blocks of novel two-dimensional heterostructures. In this paper we present state-of-the-art first principles calculations, based on both density functional theory and maximally localized Wannier functions, that encompass electronic and structural properties of free-standing Bi(111) nanofilms. We accurately evaluate both the in-plane lattice constant and, by including the van der Waals interaction between bismuth bilayers, the intra/interlayer distances. Interestingly and somehow unexpectedly, the in-plane lattice constant is predicted to shrink by about 5% going from the thickest investigated nanofilm $\left(\sim 80\AA \right)$ to single bilayer Bi(111), entailing a thickness dependent lattice mismatch in complex heterostructures involving ultrathin Bi(111). Moreover, quantum confinement effects, that would be expected to rule the electronic structure at this size range, compete with surface states that appear close to and across the Fermi level. The implication is that not only all but the thinnest films have a metallic band structure but also that such surface states might play a role in either the formation of interfaces with other materials or for sensing applications. Finally, the calculated electronic structure compares extremely well with ARPES measurements.

Soft x-ray photoemission electron microscopy with an in situ magnetic field has been used to study the relationship between ferromagnetic and antiferromagnetic spin alignment and the switching/reversal field of epitaxial micromagnetic structures. We investigated a model system consisting of a bilayer of ferromagnetic $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ and antiferromagnetic $\mathrm{LaFe}{\mathrm{O}}_3$ where the spin axes in each layer can be driven from mutually perpendicular (spin-flop) to parallel alignment by varying the temperature between 30 and 300 K. Results show that not only does this spin alignment noticeably influence the bilayer micromagnet coercivity compared to $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ single-layer micromagnets, but the coercivity within this materials system can be tuned over a wide range by careful balance of material properties.

The existence of an intermediate phase of tin oxide was first reported in 1882. However, its stoichiometry and its crystal structure have been dubious and heavily debated ever since, despite a multitude of structural investigations. We show that Raman spectroscopy combined with ab initio theory offers a viable alternative for structure determination in cases where diffraction studies are inconclusive. We unambiguously identify the intermediate phase as ${\mathrm{Sn}}_3{\mathrm{O}}_4$ and rule out the other likely candidate, ${\mathrm{Sn}}_2{\mathrm{O}}_3$. We assign the one-phonon Raman signals of ${\mathrm{Sn}}_3{\mathrm{O}}_4$ to the mode symmetries of the corresponding point group $C_{2h}$ and confirm that $P{2}_1/c$ with 14 atoms per unit cell is the space group of ${\mathrm{Sn}}_3{\mathrm{O}}_4$.

We address the issue of achieving high oxide ion conductivity at low temperatures $\left(\le 500{}^{\circ }\mathrm{C}\right)$ in stable electrolytes for solid oxide fuel cell (SOFC) techonology. We report here a high oxide ion conductivity $(\sim {10}^{-2}\mathrm{S}\mathrm{c}{\mathrm{m}}^{-1})$ in the La substituted yttrium iron garnets $\mathrm{L}{\mathrm{a}}_x{\mathrm{Y}}_{3\text{−}x}\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12+\delta }$ at low temperatures $(\sim 450{}^{\circ }\mathrm{C})$. The judicious substitution of high electropositive $\mathrm{L}{\mathrm{a}}^{3+}$ ions in the yttrium iron garnet $(x\le 0.45)$ affords continuous and easy oxide ion conduction pathways through polyhedral networks even at a low temperature of $\sim 127{}^{\circ }\mathrm{C}$. The conduction pathways have been visualized from the maximum entropy method analyzed soft bond valence sum distribution, obtained from neutron diffraction data. Our study also shows that these iron garnets have high structural stability over a broad range of temperature (at least over $27\text{–}800{}^{\circ }\mathrm{C})$, and compatible thermal expansion coefficient $(\sim 10.6\times {10}^{-6}{}^{\circ }{\mathrm{C}}^{-1})$ with that of the commonly used interconnect and electrode SOFC materials. We reveal that the oxide ion conduction in the present garnet is based on an excess oxide ion concentration mechanism. The observed high oxide-ion conductivity at low temperatures in the present trivalent substituted garnets is a remarkable finding for the development of an efficient SOFC technology.

We study an insulator-metal transition in a ternary chalcogenide glass $({\mathrm{GeSe}}_3{)}_{1-x}{\mathrm{Ag}}_x$ for $x=0.15$ and 0.25. The conducting phase of the glass is obtained by using gap sculpting [Prasai et al., Sci. Rep. 5, 15522 (2015)] and it is observed that the metallic and insulating phases have nearly identical density functional energies but have a conductivity contrast of $\sim {10}^8$. As such, we demonstrate an example of polyamorphism for which energetically close phases exhibit dramatically different optical properties. The transition from insulator to metal involves growth of an Ag-rich phase accompanied by a depletion of tetrahedrally bonded $\mathrm{Ge}{\left({\mathrm{Se}}_{1/2}\right)}_4$ in the host network. The relative fraction of the amorphous ${\mathrm{Ag}}_2\mathrm{Se}$ phase and ${\mathrm{GeSe}}_2$ phase is shown to be a critical determinant of dc conductivity.

Recently, the fabrication of CdSe nanoplatelets became an important research topic. Nanoplatelets are often described as having a similar electronic structure as two-dimensional quantum wells and are promoted as colloidal quantum wells with monolayer precision width. In this paper we show that nanoplatelets are not ideal quantum wells, but cover depending on the size: the strong confinement regime, an intermediate regime, and a Coulomb dominated regime. Thus, nanoplatelets are an ideal platform to study the physics in these regimes. Therefore, the exciton states of the nanoplatelets are numerically calculated by solving the full four-dimensional Schrödinger equation. We compare the results with approximate solutions from semiconductor quantum well and quantum dot theory. The paper can also act as review of these concepts for the colloidal nanoparticle community.

The quasi-two-dimensional electron gas at oxide interfaces provides a platform for investigating quantum phenomena in strongly correlated electronic systems. Here, we study the transport properties at the high-mobility $\left(\mathrm{L}{\mathrm{a}}_{0.3}\mathrm{S}{\mathrm{r}}_{0.7}\right)\left(\mathrm{A}{\mathrm{l}}_{0.65}\mathrm{T}{\mathrm{a}}_{0.35}\right){\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface. Before oxygen annealing, the as-grown interface exhibits a high electron density and electron occupancy of two subbands: higher-mobility electrons (${\mu }_1\approx {10}^4\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 2 K) occupy the lower-energy $3d_{\mathrm{xy}}$ subband, while lower-mobility electrons (${\mu }_1\approx {10}^3\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 2 K) propagate in the higher-energy $3d_{\mathrm{xz}/\mathrm{yz}}$-dominated subband. After removing oxygen vacancies by annealing in oxygen, only a single type of $3d_{\mathrm{xy}}$ electrons remain at the annealed interface, showing tunable Shubnikov–de Haas oscillations below 9 T at 2 K and an effective mass of $0.7m_{\mathrm{e}}$. By contrast, no oscillation is observed at the as-grown interface even when electron mobility is increased to $50000\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ by gating voltage. Our results reveal the important roles of both carrier mobility and subband occupancy in tuning the quantum transport at oxide interfaces.

We have fabricated bulk nanostructured ferrimagnetic materials with different grain sizes by sintering ball-milled ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG) nanoparticles and measured the grain-size dependence of the thermal conductivity and spin Seebeck thermopower. The nanostructuring reduces both thermal conductivity and thermopower, but the reduction of the latter was found to be considerably stronger despite the moderate difference in magnetization, which suggests that the length scales of transport of magnons and phonons contributing to the spin Seebeck effect are significantly larger than that of phonons carrying thermal current. This is consistent with the measurements of high-magnetic-field response of the spin Seebeck thermopower and low-temperature thermal conductivity, where the quenching of magnons seen in single-crystalline YIG was not observed in nanostructured YIG due to scattering of long-range low frequency magnons.

The spectral characteristics of near-field thermal emission from nanoparticle arrays are explained by comparison to the dispersions for propagating modes. Using the coupled dipole model, we analytically calculate the spectral emission from single particles, chains, planes, and three-dimensional arrays of $\mathrm{Si}{\mathrm{O}}_2$ and SiC. We show that the differences in their spectra are due to the existence or absence of propagating surface phonon polariton modes and that the emission is dominated by these modes when they are present. This work paves the way for understanding and control of near-field radiation in nanofluids, nanoparticle beds, and certain metamaterials.

This work reports a rigorous and comprehensive three-dimensional electromagnetic computation to investigate and design photoluminescence enhancement from a single silicon-vacancy center (SVC) in a nanodiamond crystal embedded in various metallic nanoantennae, each having a different geometry. The study demonstrates how each antenna design enhances the photoluminescence of SVCs in diamond. In particular, our report discusses how the 2D or 3D curvature of the nanoantenna and the control of the local environment of the SVC can lead to significant field enhancement of its optical field. Our calculated optimal photoluminescence for each design enhances the emission intensity by $15\text{–}300\times$ that of a single SVC without antenna. The enhancement mechanisms are investigated using four representative structures that can be fabricated under feasible and realistic growth conditions, i.e., spherical-, nanorod-, nanodisk-dimer, and bow-tie nanoantennae. These results demonstrate a method for rationally designing arbitrary metallic nanoantenna/emitter assemblies to achieve optimal SVC photoluminescence.

Thorough characterization of the thermomechanical properties of materials requires difficult and time-consuming experiments. This severely limits the availability of data and is one of the main obstacles for the development of effective accelerated materials design strategies. The rapid screening of new potential materials requires highly integrated, sophisticated, and robust computational approaches. We tackled the challenge by developing an automated, integrated workflow with robust error-correction within the AFLOW framework which combines the newly developed “Automatic Elasticity Library” with the previously implemented GIBBS method. The first extracts the mechanical properties from automatic self-consistent stress-strain calculations, while the latter employs those mechanical properties to evaluate the thermodynamics within the Debye model. This new thermoelastic workflow is benchmarked against a set of 74 experimentally characterized systems to pinpoint a robust computational methodology for the evaluation of bulk and shear moduli, Poisson ratios, Debye temperatures, Grüneisen parameters, and thermal conductivities of a wide variety of materials. The effect of different choices of equations of state and exchange-correlation functionals is examined and the optimum combination of properties for the Leibfried-Schlömann prediction of thermal conductivity is identified, leading to improved agreement with experimental results than the GIBBS-only approach. The framework has been applied to the AFLOW.org data repositories to compute the thermoelastic properties of over 3500 unique materials. The results are now available online by using an expanded version of the REST-API described in the Appendix.

Pyrite $\mathrm{Fe}{\mathrm{S}}_2$ has outstanding potential as an earth-abundant, low-cost, nontoxic photovoltaic, but underperforms dramatically in solar cells. While the full reasons for this are not clear, one certain factor is the inability to understand and control doping in $\mathrm{Fe}{\mathrm{S}}_2$. This is exemplified by the widely accepted but unexplained observation that unintentionally doped $\mathrm{Fe}{\mathrm{S}}_2$ single crystals are predominantly $n$ type, whereas thin films are $p$ type. Here we provide a potential resolution to this “doping puzzle,” arrived at via Hall effect, thermopower, and resistivity measurements on a large set of $\mathrm{Fe}{\mathrm{S}}_2$ single crystals and films that span five orders of magnitude in mobility. The results reveal three main findings. First, in addition to crystals, the highest mobility thin films in this study are shown to be definitively $n$ type, from both Hall effect and thermopower. Second, as mobility decreases an apparent crossover to $p$ type occurs, first in thermopower, then in Hall measurements. This can be understood, however, in terms of the crossover from diffusive to hopping transport that is clearly reflected in resistivity. Third, universal behavior is found for both crystals and films, suggesting a common $n$ dopant, possibly sulfur vacancies. We thus argue that $n$-type doping is facile in $\mathrm{Fe}{\mathrm{S}}_2$ films, that apparent $p$-type behavior in low mobility samples can be an artifact of hopping, and that the prevailing notion of predominantly $p$-type films must be revised. These conclusions have deep implications, both for interpretation of prior work on $\mathrm{Fe}{\mathrm{S}}_2$ solar cells and for the design of future studies.

We have measured in situ the progression of dewetting from a large number of holes in immiscible polymer bilayer films. Using x-ray photon correlation spectroscopy (XPCS) in grazing incidence we probe independently the evolving dewetting process both at the top surface and the buried interface of the bilayer. At an early stage, differences in the evolution of the velocities measured by XPCS between the surface and buried interface indicate that the holes do not penetrate the bottom layer. The rim velocity at late stages decays according to a wave-vector-dependent power law, which indicates inhomogeneous flows in the film. The changes in the static scattering show that observed slow-down of the dewetting velocity is correlated with the changing roughness at the buried interface of the polymer bilayer.

For soft materials with Young's moduli below 100 kPa, quantifying mechanical and interfacial properties by small scale indentation is challenging because in addition to adhesion and elasticity, surface tension plays a critical role. Until now, microscale contact of very soft materials has only been studied by static experiments under zero external loading. Here we introduce a combination of the colloidal probe technique and confocal microscopy to characterize the force-indentation and force-contact radius relationships during microindentation of soft silicones. We confirm that the widespread Johnson-Kendall-Roberts theory must be extended to predict the mechanical contact for soft materials. Typically a liquid component is found within very soft materials. With a simple analytical model, we illustrate that accounting for this liquid surface tension can capture the contact behavior. Our results highlight the importance of considering liquid that is often associated with soft materials during small scale contact.