A molecular dynamics (MD) method, along with the analytical bond-order potential, is applied to study defect production in InAs. This potential is modified to obtain a better description for point-defect properties and is extended for proper applications in radiation damage simulation. By using this modified potential, the threshold displacement energy $\left(E_d\right)$, as one of the crucial parameters in radiation damage studies, is calculated over thousands of crystallographic directions for incorporating spatial anisotropy. However, the $E_d$ dependence on directions is found to be relatively weak. The defect production, clustering, and evolution in InAs are further investigated for the energies of the primary knock-on atom (PKA) ranging from 500 eV to 40 keV. A nonlinear defect production is seen with increasing PKA energy. This nonlinearity, which is associated with the direct-impact amorphization, is very distinctive for PKA energies ranging from 1 to 20 keV. Based on the damage density evaluated from molecular dynamics simulations, a theoretical model is developed for determining nonionizing energy loss (NIEL), which can be used for quantifying the electronic device degradation in a space radiation environment. The NIELs of InAs for proton, alpha, and Xe particles are calculated from the displacement threshold up to 60 MeV in comparison with available data so as to validate our model in the current study.

Iridium manganese (IrMn) is arguably the most important antiferromagnetic material for device applications due to its metallic nature, high Néel temperature, and exceptionally high magnetocrystalline anisotropy. Despite its importance, its magnetic properties are poorly understood due to its intrinsic complexity and the interplay between structural and magnetic properties. Here we present a unifying atomistic model of ${\mathrm{Ir}}_x{\mathrm{Mn}}_{(1-x)}$ alloys which reproduces the key experimental facts of the material, while providing unprecedented understanding of the compositional and structural origins of its magnetic ground state and thermodynamic properties. We find that the Néel temperature is strongly dependent on the nature of the ground-state magnetic order which varies with $x$ from a triangular to tetrahedral spin structure, leading to different levels of geometric spin frustration. The Néel temperature increases linearly with manganese concentration for the disordered phase, while the ordered phases show a peak for ${\mathrm{Ir}}_{50}{\mathrm{Mn}}_{50}$ followed by a decrease due to increased spin frustration. The ground-state tetrahedral spin structure of the disordered phase is composition independent for manganese concentrations in the 50–95% range, while the degree of spin order varies strongly in the same range. For low manganese concentrations, we find antiferromagnetic spin-glass and ferromagnetic ground-state spin structures. The magnetic anisotropy energy exhibits a complex dependence on the lattice symmetry, presenting easy-plane, cubic, and unconventional symmetries for the principal phases, and a similarly complex variation of magnitude. The complexity of behavior represents a dual blessing and a curse in that the properties of a particular sample depend strongly on the degree of order and composition, while also providing a large state space to engineer an antiferromagnet with optimal symmetry, magnetic anisotropy, and thermal stability. Such effects are important for the future development of nanoscale sensor devices and antiferromagnetic spintronics.

We report the results of an unpolarized small-angle neutron scattering (SANS) study on Mn-Bi-based rare-earth-free permanent magnets. The magnetic SANS cross section is dominated by long-wavelength transversal magnetization fluctuations, and has been analyzed in terms of the Guinier-Porod model and the distance distribution function. This provides the radius of gyration which, in the remanent state, ranges between about $220-240\mathrm{nm}$ for the three different alloy compositions investigated. Moreover, computation of the distance distribution function, in conjunction with results for the so-called $s$ parameter obtained from the Guinier-Porod model, indicates that the magnetic scattering of a ${\mathrm{Mn}}_{45}{\mathrm{Bi}}_{55}$ sample has its origin in slightly shape-anisotropic structures.

One-dimensional silicon nanoribbons (SiNRs) have the potential for application to future electronic devices because of their compatibility with current silicon-based electronic devices and their theoretical outstanding electronic properties, such as the size-dependent band gap. Here we grew SiNRs by the deposition of Si on a Ag(111) surface. We investigated SiNRs on the Ag(111) surface using a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. The atomic resolution STM and AFM images revealed that SiNRs have the same width and align along the equivalent orientations of Ag(111). The main body of SiNRs was found to be symmetric about the long axes of the ribbons, but the terminals of SiNRs break the symmetry. The DFT calculation results revealed a buckled single-layer structure of SiNRs, which is composed of Si hexagonal and tetragonal rings.

In complex oxide heteroepitaxy, strain engineering is a powerful tool to obtain phases in thin films that may be otherwise unstable in bulk. A successful example of this approach is mixed-phase bismuth ferrite ($\mathrm{BiFe}{\mathrm{O}}_3$) epitaxial thin films. The coexistence of a tetragonal-like ($T$-like) matrix and rhombohedral-like ($R$-like) striations provides an enhanced electromechanical response, along with other attractive functional behaviors. In this paper, we compare the energetics associated with two thickness-dependent strain relaxation mechanisms in this system: domain walls arising from monoclinic distortion in the $T$-like phase, and the interphase boundary between the host $T$-like matrix and tilted $R$-like phases. Combining x-ray-diffraction measurements with scanning probe microscopy, we extract quantitative values using an empirical energy balance approach. The domain wall and phase-boundary energies are found to be 113 ± 21 and $426\pm 23\mathrm{mJ}{\mathrm{m}}^{–2}$, respectively. These numerical estimates will help us realize designer phase boundaries in multiferroics, which possess colossal responses to external stimuli, attractive for a diverse range of functional applications.

Nontrivial spin structures in itinerant magnets can give rise to the topological Hall effect (THE) due to the interacting local magnetic moments and conductive electrons. While, in series of materials, THE has mostly been observed at low temperatures far below room temperature (RT) limiting its potential applications. Here, we report the anisotropic anomalous Hall effect (AHE) near RT in $\mathrm{La}{\mathrm{Mn}}_2{\mathrm{Ge}}_2$, a noncollinear ferromagnetic compound with Curie temperature $T_{\mathrm{C}}\sim 325\mathrm{K}$. A large topological Hall resistivity of $\sim 1.0\mu \mathrm{\Omega }\mathrm{cm}$ in a broad temperature range $(190\mathrm{K}<T<300\mathrm{K})$ is realized as field ($H$) is parallel to the $ab$ plane $\left(H//ab\right)$ and current along the $c$ axis $\left(I//c\right)$, in contrast to the conventional AHE for $H//c$ and $I//ab$. The emergence of THE is attributed to the spin chirality of noncoplanar spin configurations stabilized by thermal fluctuation during spin-flop process. Moreover, the constructed temperature-field ($H-T$) phase diagrams based on the isothermal topological Hall resistivity reveal a field-induced transition from the noncoplanar spin configuration to polarized ferromagnetic state. Our experimental realization of large THE near RT highlights $\mathrm{La}{\mathrm{Mn}}_2{\mathrm{Ge}}_2$ as a promising system for functional applications in novel spintronic devices.

We study the thermodynamics of boron (B) incorporation into gallium nitride (GaN) using first-principles calculations. In the dilute limit, we have calculated the formation energies of different configurations of the B impurity in GaN and found that substitution on the cation site is favored over substitution on the anion site. Under $p$-type conditions, interstitial boron can become the more favorable configuration and will ultimately limit the $p$-type conductivity. At higher B concentrations we use the generalized quasi-chemical approximation to elucidate the thermodynamic stability of boron gallium nitride (BGaN) alloys. We also investigate the effects of strain, which will be present if BGaN alloys are grown pseudomorphically on a GaN substrate. Without strain, B incorporation at typical growth conditions is limited to about 1.4% at $800{}^{\circ }\mathrm{C}$. Pseudomorphic strain raises the limit to 3.0% at the same temperature, close to experimentally observed levels of B incorporation.

Prussian blue analogues (PBAs) are model host compounds for the intercalation of monovalent cations for electrochemical energy storage and separations. However, the interactions among interstitial species and their effects on atomic arrangements therein are understood mainly at a phenomenological level. Analyzing correlations between electronic interactions and polyatomic arrangements in hydrated Prussian blue analogues is complicated by the nonlocal hydrogen-bonding interactions between zeolitic water and framework lattices. Here, we train machine-learning (ML) models to learn DFT-calculated energy landscapes of nickel hexacyanoferrate PBA lattices with various lattice hydration degrees, oxidation states, and types of intercalated alkali cations based on various three-particle feature parameters. This ML approach is enabled by using gradient-boosted regression trees with features that are rotationally invariant geometric parameters. ML model accuracy is shown to be a cation-specific indicator of correlations between energy and polyatomic arrangements. Overlap population analysis among correlated atoms further confirms that such correlations are caused by the competition for dative bonding between Lewis-acid intercalated cations and Lewis bases (cyanide and oxygen in ${\mathrm{H}}_2\mathrm{O}$). Examination of lowest-energy structures reveals that cation hydrophilicity and bare ionic radius determine dative-bonding strength, resulting in cation-${\mathrm{H}}_2\mathrm{O}$ ordering in interstitial space. The projected energy landscapes of hydrated PBA lattices is also explored in subspaces spanned by certain many-particle feature parameters inspired by ML analysis. The downhill traces in such landscapes indicate that lattice distortion is accompanied by two kinds of collective movements: (1) rearrangements in the hydration shells around small and hydrophilic cations and (2) collective attack of ${\mathrm{H}}_2\mathrm{O}$ molecules on nickel-cyanide bonds promoted by large, hydrophobic cations.

We present a formulation for calculating the transversal flexoelectric coefficient of nanostructures at finite deformations from first principles. Specifically, we introduce the concept of radial polarization to make the coefficient a well-defined quantity for uniform bending deformations. We use the framework to calculate the flexoelectric coefficient for group IV atomic monolayers using density functional theory. We find that graphene's coefficient is significantly larger than previously reported, with a charge transfer mechanism that differs from other members of its group.

Density functional theory calculations using well-established semilocal and hybrid functionals are typically accurate for structural properties of semiconductors, but they often fail to quantitatively describe electronic-structure and optical properties of these materials. An improvement to conventional hybrid functionals is the class of screened range-separated hybrid (SRSH) functionals. In the SRSH approach, the range-separation parameter is used to empirically fit the band gap of the semiconductor, which was shown to lead to highly accurate predictions of electronic-structure and optical properties. Here we assess the accuracy of the SRSH approach for computing other important bulk properties of seven prototypical semiconductors, including lattice constants and phonon dispersion relations. Our SRSH results are compared to data from semilocal (PBE) and hybrid (HSE) functional calculations as well as to experimental data from the literature. We find that SRSH can compete with the high accuracy provided by the two well-established functionals PBE and HSE for computing bulk properties of semiconductors. Furthermore, similarly to the case of the HSE functional, the SRSH method yields phonon dispersion relations of semiconductors that tend to be more accurate than those calculated with PBE. The SRSH approach thus provides a consistently accurate framework for calculations of semiconductor bulk properties.

Despite the importance of the electron correlation in the first-principles description of the Li-ion cathode materials, the Coulomb interaction parameter $U$ is often treated as an ad hoc value. In practice, one usually relies on empirical ways of parametric treatment of $U$ to optimally match the experimentally observed physical properties such as band gap or reaction energy. Here, using the constrained random phase approximation (cRPA) method, we self-consistently evaluate the Coulomb $U$ and Hund $J$ values for representative layered cathode materials including not only Li compounds but also Na compounds $\mathrm{Li}\mathrm{Co}{\mathrm{O}}_2, \mathrm{Li}\mathrm{Ni}{\mathrm{O}}_2, \mathrm{Li}\mathrm{Mn}{\mathrm{O}}_2, \mathrm{Na}\mathrm{Co}{\mathrm{O}}_2, \mathrm{Na}\mathrm{Ni}{\mathrm{O}}_2$, and $\mathrm{NaMn}{\mathrm{O}}_2$. We found that the Coulomb interaction parameters for Li and Na compounds and their polymorphs with different layer stacking types do not deviate much, which shows the dominant role of the local environment rather than of global structural features. We have analyzed the origin of variable Coulomb parameters, which is mainly due to the competition between the localization and screening. We provided cRPA Coulomb parameters for battery cathode materials and validate the values by observing systematic improvement in describing the experimentally observed average intercalation voltage and lattice parameters. These results can be applied for the first-principles calculations as well as model-based simulations for the theoretical investigation of cathode systems.

We present a theory of viscoelasticity of amorphous media, which takes into account the effects of confinement along one of three spatial dimensions. The framework is based on the nonaffine extension of lattice dynamics to amorphous systems, or nonaffine response theory. The size effects due to the confinement are taken into account via the nonaffine part of the shear storage modulus $G^{\prime }$. The nonaffine contribution is written as a sum over modes in $k$-space. With a rigorous argument based on the analysis of the $k$-space integral over modes, it is shown that the confinement size $L$ in one spatial dimension, e.g., the $z$ axis, leads to a infrared cutoff for the modes contributing to the nonaffine (softening) correction to the modulus that scales as $L^{-3}$. Corrections for finite sample size $D$ in the two perpendicular dimensions scale as $\sim {(L/D)}^4$, and are negligible for $L\ll D$. For liquids it is predicted that $G^{\prime }\sim L^{-3}$ is in agreement with a previous more approximate analysis, whereas for amorphous materials $G^{\prime }\sim G_{\text{bulk}}^{\prime }+\beta L^{-3}$. For the case of liquids, four different experimental systems are shown to be very well described by the $L^{-3}$ law. The theory can also explain previous simulation data of confined jammed granular packings.

Structural and magnetic properties of rare-earth Tb metal have been studied by means of neutron powder diffraction at pressures up to 9 GPa in the temperature range 7–290 K. A structural phase transition from the initial hexagonal close-packed (hcp) to the Sm-type rhombohedral phase develops gradually at high pressures above 4 GPa. The initial ferromagnetic state in the hcp phase is suppressed and an antiferromagnetic state is developed in the pressure-induced phase. In the Sm-type structure and the temperature range below $T_{\mathrm{MO}}=110\mathrm{K}$ (at 9 GPa) down to 50 K, long-range order of Tb magnetic moments located in the layers resembling hexagonal close-packing type is formed with a propagation vector $k_{\mathrm{AF}1}=\left(00\frac{1}{2}\right)$, while the layers resembling cubic close-packing type remain disordered. This partial disorder disappears at temperatures below 50 K when magnetic order, including the moments in the latter layers, develops with a propagation vector $k_{\mathrm{AF}2}=\left(\frac{1}{2}0\frac{1}{2}\right)$. The relative stability of the hcp and Sm-type structures under pressure was examined by density functional theory calculations, providing significant support to the experimental findings. The calculated bulk moduli of the hcp and Sm-type phases are close to the experimentally determined ones and the estimate $P_0\approx 4\mathrm{GPa}$ obtained for the equilibrium transition pressure is close to the onset pressure found in real material. The volume collapse at the hcp to Sm-type transition was evaluated to amount to $0.4{\AA }^3$ per atom.

Defect physics of europium (Eu) doped GaN is investigated using first-principles hybrid density-functional defect calculations. This includes the interaction between the rare-earth dopant and native point defects (Ga and N vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present or intentionally incorporated into the host material. While the trivalent ${\mathrm{Eu}}^{3+}$ ion is often found to be predominant when Eu is incorporated at the Ga site in wurtzite GaN, the divalent ${\mathrm{Eu}}^{2+}$ is also stable and found to be predominant in a small range of Fermi-level values in the band-gap region. The ${\mathrm{Eu}}^{2+}$/${\mathrm{Eu}}^{3+}$ ratio can be tuned by tuning the position of the Fermi level and through defect association. We find co-doping with oxygen can facilitate the incorporation of Eu into the lattice. The unassociated ${\mathrm{Eu}}_{\mathrm{Ga}}$ is an electrically and optically active defect center and its behavior is profoundly impacted by local defect-defect interaction. Defect complexes such as ${\mathrm{Eu}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{N}}$, ${\mathrm{Eu}}_{\mathrm{Ga}}\text{−}{\mathrm{Si}}_{\mathrm{Ga}}$, ${\mathrm{Eu}}_{\mathrm{Ga}}\text{−}{\mathrm{H}}_i$, ${\mathrm{Eu}}_{\mathrm{Ga}}\text{−}{\mathrm{Mg}}_{\mathrm{Ga}}$, and ${\mathrm{Eu}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{N}}\text{−}{\mathrm{Mg}}_{\mathrm{Ga}}$ can efficiently act as deep carrier traps and mediate energy transfer from the host into the ${\mathrm{Eu}}^{3+}4f$-electron core which then leads to sharp red intra-$f$ luminescence. Eu-related defects can also give rise to defect-to-band luminescence. The unassociated ${\mathrm{Eu}}_{\mathrm{Ga}}$, for example, is identified as a possible source of the broad blue emission observed in $n$-type, ${\mathrm{Eu}}^{2+}$-containing GaN. This work calls for a re-assessment of certain assumptions regarding specific defect configurations previously made for Eu-doped GaN and further investigation into the origin of the photoluminescence hysteresis observed in (Eu,Mg)-doped samples.

We focus on the role of dynamical heterogeneities on the weak mechanical nonlinearities of amorphous polymers near and above the glass transition temperature Tg by combining experiments and numerical coarse-grained simulations. The acceleration of the macroscopic nonlinear modulus relaxation resulting from the applied stress is measured below yielding. As a result of dynamic disorder, the macroscopic acceleration differs from the local acceleration. We obtain a good agreement of experimental measurements with simulations computed by using an exponential function of the square stress for the local acceleration. Further, the length scale of dynamical heterogeneities is deduced.

${\mathrm{Zr}}_{65}{\mathrm{Cu}}_{27.5}{\mathrm{Al}}_{7.5}$ metallic glass thin films with widely varying kinetic stability as a function of deposition rate were synthesized by single-target direct current magnetron sputtering. Fluctuation electron microscopy and angular correlations in coherent electron nanodiffraction show that glasses with increased stability have increased nanoscale structural order, particularly of icosahedral character. The most kinetically stable film's reduced modulus was 22% higher than a glass of the same composition rapidly quenched from the liquid, consistent with increased density and improved thermodynamic stability. These results suggest that enhanced nanoscale icosahedral order contributes both to the kinetic stability of the glass and its resistance to mechanical deformation.

The magnetic properties of the two isostructural molecule-based magnets—$\mathrm{Ni}{\left(\mathrm{NCS}\right)}_2{\left(\mathrm{thiourea}\right)}_2, S=1 [\mathrm{thiourea}=\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2]$ and $\mathrm{Co}{\left(\mathrm{NCS}\right)}_2{\left(\mathrm{thiourea}\right)}_2, S=3/2$—are characterized using several techniques in order to rationalize their relationship with structural parameters and to ascertain magnetic changes caused by substitution of the spin. Zero-field heat capacity and muon-spin relaxation measurements reveal low-temperature long-range ordering in both compounds, in addition to Ising-like ($D<0$) single-ion anisotropy ($D_{\mathrm{Co}}\sim -100$ K, $D_{\mathrm{Ni}}\sim -10$ K). Crystal and electronic structure, combined with dc-field magnetometry, affirm highly quasi-one-dimensional behavior, with ferromagnetic intrachain exchange interactions $J_{\mathrm{Co}}\approx +4$ K and $J_{\mathrm{Ni}}\sim +100$ K and weak antiferromagnetic interchain exchange, on the order of $J^{\prime }\sim -0.1$ K. Electron charge- and spin-density mapping reveals through-space exchange as a mechanism to explain the large discrepancy in $J$-values despite, from a structural perspective, the highly similar exchange pathways in both materials. Both species can be compared to the similar compounds $M{\mathrm{Cl}}_2{\left(\mathrm{thiourea}\right)}_4, M$ = Ni(II) (DTN) and Co(II) (DTC), where DTN is known to harbor two magnetic-field-induced quantum critical points. Direct comparison of DTN and DTC with the compounds studied here shows that substituting the halide ${\mathrm{Cl}}^{-}$ ion for the ${\mathrm{NCS}}^{-}$ ion results in a dramatic change in both the structural and magnetic properties.

Cadmium telluride is the most commercially important second generation thin-film photovoltaic, with a record solar cell conversion efficiency of 22.1%. However as-deposited cells are <5% efficient and require a cell activation treatment with ${\mathrm{CdCl}}_2$ at about 400 ${}^{\circ }\mathrm{C}$ to reach commercially viable efficiencies. Such a treatment is a routine process during CdTe module manufacturing. However, the precise mechanisms at work for this remarkable efficiency enhancement are not well understood. In this paper, atomistic modeling techniques are used to improve the fundamental understanding of the structural and electronic properties of CdTe by modeling the effects of chlorine and other elements with their interaction with extended defects and grain boundaries (GBs). Studies at high spatial resolution with nanoscale secondary ion mass spectrometry, transmission electron microscopy (TEM), and energy dispersive x-ray analysis show that chlorine atoms are concentrated at grain boundaries in CdTe after the ${\mathrm{CdCl}}_2$ treatment. Density functional theory calculations show that both ${\mathrm{Cl}}_{\mathrm{Te}}$ and ${\mathrm{Cl}}_i$ are stabilized at the grain boundaries compared to bulk CdTe. Similar defect formation energies of these defects suggest both will be present at the grain boundaries. As expected, four single-particle levels are present in the $\mathrm{\Sigma }3$ (112) GB band gap, which explains the low efficiencies prior to treatment. ${\mathrm{Cl}}_{\mathrm{Te}}$ substitutions passivate one of these levels and partially passivate another two. Remarkably, further addition of ${\mathrm{Cl}}_i$ fully passivates the remaining single-particle levels. This passivation of single-particle levels is most likely to be the primary cause of the efficiency enhancement on chlorine treatment. Further to this, alternative halogens were then trialed as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends.

Amorphous silicon contains tunneling two-level systems, which are the dominant energy loss mechanisms for amorphous solids at low temperatures. These two-level systems affect both mechanical and electromagnetic oscillators and are believed to produce thermal and electromagnetic noise and energy loss. However, it is unclear whether the two-level systems that dominate mechanical and dielectric losses are the same; the former relies on the coupling between phonons and two-level systems, with an elastic field coupling constant $\gamma$ while the latter depends on a two-level systems dipole moment $p_0$, which couples to the electromagnetic field. Mechanical and dielectric loss measurements as well as structural characterization were performed on amorphous silicon thin films grown by electron beam deposition with a range of growth parameters. Samples grown at 425 ${}^{\circ }\mathrm{C}$ show a large reduction of mechanical loss (34 times) and a far smaller reduction of dielectric loss (2.3 times) compared to those grown at room temperature. Additionally, mechanical loss shows lower loss for thicker films, while dielectric loss shows lower loss for thinner films. Analysis of these results indicate that mechanical loss correlates with atomic density, while dielectric loss correlates with dangling-bond density, suggesting a different origin for these two energy dissipation processes in amorphous silicon.