The phase transition and stability as well as corresponding electrical and optical properties of $\mathrm{C}{\mathrm{d}}_x\mathrm{Z}{\mathrm{n}}_{1-x}\mathrm{O}$ alloys over the entire composition range synthesized under different sputtering conditions, namely, substrate temperature and sputtering environment, were studied. We found that the composition at which the phase transition from the wurtzite (WZ) ZnO-like to rocksalt (RS) CdO-like phase occurs critically depends on the growth temperature and environment during sputter deposition. When deposited at room temperature in pure Ar, the composition window for the coexistence of both the WZ and RS phases is rather narrow ($0.62<x<0.7$). This mixed phase window widens considerably to $0.4<x<0.7$ when alloys were synthesized at $230{}^{\circ }\mathrm{C}$. When grown in an O-rich environment, pure WZ alloys are stable up to a much higher Cd composition of $x\sim 0.72$ with a narrow mixed phase region $\left(0.72<x<0.8\right)$, resulting in oxide alloys with band gap in the visible range ($\sim 1.8\mathrm{eV}$ for $x\sim 0.7$). The enhanced WZ phase stability agrees with density functional theory calculations by considering the incorporation of extra O as interstitial defects. Since the optical and electrical transport properties of CdZnO alloys in the WZ and RS phases differ drastically, by adjusting the growth conditions, alloys with desired properties (e.g., band gap and conductivity) can be achieved for different optoelectronic applications.

Many experiments exploit curvature-driven, surface-diffusion-mediated coarsening for tuning the characteristic structure size of metal network structures made by dealloying, such as nanoporous gold. Here we study this process by kinetic Monte Carlo simulation. The initial microstructures are leveled Gaussian random fields, approximating spinodally decomposed mixtures, of different solid fraction $\phi$. Earlier work establishes these structures as valid representations of the nanoporous gold microstructure. We find that the coarsening law for the characteristic spacing between the ligaments of the network is universal, whereas the time evolution of the characteristic ligament diameter is not. The expected time exponent 1/4 is confirmed by our simulation. Contrary to what may be expected based on continuum models, the degree of surface faceting or roughness has no apparent effect on the coarsening kinetics. In the time interval of our study, the network connectivity—as measured by a scaled density of topological genus—remains sensibly invariant for networks with $\phi \ge 0.3$, consistent with previous reports of a self-similar evolution of the microstructure during coarsening. Yet, networks with lesser $\phi$ lose their connectivity on coarsening and can even undergo a percolation-to-cluster transition. This process is slow for $\phi$ only little below 0.3 and it accelerates in networks with lesser $\phi$. The dependency of the connectivity evolution on $\phi$ may explain controversial findings on the microstructure evolution of nanoporous gold in experimental studies.

Antiferromagnetic materials are of great interest for spintronics. Here we present a comprehensive study of the growth, structural characterization, and resulting magnetic properties of thin films of the noncollinear antiferromagnet $\mathrm{M}{\mathrm{n}}_3\mathrm{Ir}$. Using epitaxial engineering on MgO (001) and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ (0001) single-crystal substrates, we control the growth of cubic γ-$\mathrm{M}{\mathrm{n}}_3\mathrm{Ir}$ in both (001) and (111) crystal orientations, and discuss the optimization of growth conditions to achieve high-quality crystal structures with low surface roughness. Exchange bias is studied in bilayers, with exchange bias fields as large as −29 mT (equivalent to a unidirectional anisotropy constant of $0.115\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}$ or $11.5\mathrm{nJ}\mathrm{c}{\mathrm{m}}^{-2}$) measured in $\mathrm{M}{\mathrm{n}}_3\mathrm{Ir}$ (111)/Permalloy heterostructures at room temperature. In addition, a distinct dependence of blocking temperature on in-plane crystallographic direction in $\mathrm{M}{\mathrm{n}}_3\mathrm{Ir}$ (001)/Permalloy bilayers is observed. These findings are discussed in the context of antiferromagnetic domain structures, and will inform progress towards chiral antiferromagnetic spintronic devices.

The structural perfection and defect microstructure of epitaxial (001) $\mathrm{SrTi}{\mathrm{O}}_3$ films grown on (001) Si was assessed by a combination of x-ray diffraction and scanning transmission electron microscopy. Conditions were identified that yield 002 $\mathrm{SrTi}{\mathrm{O}}_3$ rocking curves with full width at half-maximum below 0.03° for films ranging from 2 to 300 nm thick, but this is because this particular peak is insensitive to the $\sim 8\times {10}^{11}\mathrm{c}{\mathrm{m}}^{–2}$ density of threading dislocations with pure edge character and extended defects containing dislocations and out-of-phase boundaries. Our results show that one narrow rocking curve peak is insufficient to characterize the structural perfection of epitaxial films.

Phase diagrams for multicomponent systems represent crucial information for understanding and designing materials but are very time consuming to assess experimentally. Computational modeling plays an increasingly important role in this regard but has been largely focused on systems with matching lattice structures and/or stable boundary phases. Here, using a combination of density functional theory calculations, alloy cluster expansions, free energy integration, and Monte Carlo simulations, we obtain the phase diagram of W-Ti, a system that features metastable boundary phases on both sides of the phase diagram. We find that the mixing energy on the body-centered cubic (BCC) lattice is asymmetric and negative with a minimum of about $-120$ meV/atom, whereas for the hexagonal closed packed (HCP) lattice the mixing energy is positive. By combining these data with a model for the vibrational free energy, we propose a revision of the W-rich end of the phase diagram with a much larger solubility of Ti in BCC-W than previous assessments. Finally, by comparison with the W-V and W-Re systems, we demonstrate how strongly asymmetric phase diagrams can arise from a subtle energy balance of stable and metastable lattice structures.

The structural and chemical degradations of phosphorene severely limit its practical applications despite the enormous promise. In this regard, we investigate a cohort of microscopic kinetic mechanisms and develop a degradation phase diagram using first-principles calculations. At 400 K, the degradation and the competing self-annealing proceeds through the merger and annihilation of vacancies, respectively, which are triggered via itinerant vacancies and adatoms. A further increase in temperature beyond 650 K, the structural degradation results through the emission of the undercoordinated atoms from the defect and the concurrent pairwise sublimation. The role of interlayer vacancy diffusion is discarded in the context of structural degradation. The chemical degradation proceeds through the dissociation of an oxygen molecule that is activated at the room temperature on the pristine surface or spontaneous at the single-vacancy site. The present results are in agreement with the few available experimental conjectures and will motivate further efforts.

First-principles studies of two-dimensional transition metal dichalcogenides have contributed considerably to the understanding of their dielectric, optical, elastic, and vibrational properties. The majority of works to date focus on a single material or physical property. Here we use a single first-principles methodology on the whole family of systems to investigate in depth the relationships between different physical properties, the underlying symmetry, and the composition of these materials, and observe trends. We compare to bulk counterparts to show strong interlayer effects in triclinic compounds, and relationships between these monolayer compounds become apparent. These trends can then be exploited by the materials science, nanoscience, and chemistry communities to better design devices and heterostructures for specific functionalities.

Using hybrid density-functional calculations we investigate the effects of native point defects on the electrical and optical properties of ${\mathrm{In}}_2{\mathrm{O}}_3$. We analyze formation energies, transition levels, and local lattice relaxations for all native point defects. We find that donor defects are in general more energetically favorable than acceptor defects, except near O-rich conditions, where oxygen interstitials and indium vacancies have low formation energy in $n$-type ${\mathrm{In}}_2{\mathrm{O}}_3$. The oxygen vacancy is the lowest-energy donor defect with transition level $(2+/+)$ slightly below and $(+/0)$ slightly above the conduction-band minimum (CBM), with a predicted luminescence peak at 2.3 eV associated with the transition $V_{\mathrm{O}}^0\to V_{\mathrm{O}}^{+}$. Despite being a shallow donor, the oxygen vacancy becomes electrically inactive for Fermi levels at or higher than $\sim 0.1$ eV above the CBM. This indicates that conductivity due to oxygen vacancies will saturate at rather low carrier concentrations when compared to typical carrier concentrations required for transparent conducting oxides in many device applications.

Detailed magnetization, specific heat, and ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ are reported. At high temperatures, anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions, in agreement with the combination of a ferromagnetic Kitaev interaction ($K$) and a negative off-diagonal anisotropy ($\mathrm{\Gamma }$) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along $a$ or $c$ have only a weak effect on the system and reduce the Néel temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along $b$ causes a drastic reduction in the $T_N$ that vanishes around $H_c=2.8\mathrm{T}$, giving way to a crossover toward a quantum paramagnetic state. ${}^7\mathrm{Li}$ NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature.

We report that the single interatomic potential, developed using Gaussian regression of data from density functional theory calculations, has high accuracy and flexibility to describe phonon transport with ab initio accuracy in two different atomistic configurations: perfect crystalline Si and crystalline Si with vacancies. The high accuracies of second- and third-order force constants from the Gaussian approximation potential (GAP) are demonstrated with phonon dispersion, Grüneisen parameter, three-phonon scattering rate, phonon-vacancy scattering rate, and thermal conductivity, all of which are very close to the results from density functional theory calculations. We also show that the widely used empirical potentials (Stillinger-Weber and Tersoff) produce much larger errors compared to the GAP. The computational cost of GAP is higher than the two empirical potentials, but five orders of magnitude lower than density functional theory calculations. Our work shows that GAP can provide a new opportunity for studying phonon transport in partially disordered crystalline phases with the high predictive power of ab initio calculation but at a feasible computational cost.

The mechanical properties of heterophase interfaces are critically important for the behavior of graphene-reinforced composites. In this work, the structure, adhesion, cleavage, and sliding of heterophase interfaces formed between a ${\mathrm{ZrB}}_2$ matrix and graphene nanosheets are systematically investigated by density functional theory and compared to available experimental data. We demonstrate that the surface chemistry of the ${\mathrm{ZrB}}_2$ matrix material largely shapes the interface structures and the nature of the interfacial interaction. Zr-C interfaces present strong chemical bonding and their response to mechanical stress is significantly influenced by graphene corrugation. In contrast B-C interfaces, interacting through relatively weak $\pi -\pi$ stacking, show attributes similar to those of two-dimensional materials heterostructures. Our theoretical results provide insights into the interface bonding mechanisms in graphene/ceramic composites, and highlight the prospects for their design via interface engineering enabled by surface contacts.

In view of their immensely intriguing properties, two-dimensional (2D) materials are being intensely researched in search of novel phenomena and diverse application interests; however, studies on the realization of 2D/2D nanocomposites in the application-worthy thin-film platform are rare. Here we have grown $\mathrm{Mo}{\mathrm{S}}_2$-hexagonal boron nitride (hBN) 2D/2D composite thin films on different substrates by the pulsed laser deposition technique and made comparative studies with the pristine $\mathrm{Mo}{\mathrm{S}}_2$ and hBN films. The Raman and x-ray photoelectron spectroscopy techniques as well as high-resolution transmission electron microscopy confirm the concomitant presence of both the 1T (conducting) and 2H (semiconducting) polymorphs of $\mathrm{Mo}{\mathrm{S}}_2$ in the composite film. Interestingly, a peculiar reentrant semiconductor-metal-insulator transition is seen in the $\mathrm{Mo}{\mathrm{S}}_2-\mathrm{hBN}$ 2D/2D composite film which is absent in the $\mathrm{Mo}{\mathrm{S}}_2$ film, and it correlates well with the signatures of phonon softening seen in temperature-dependent Raman spectroscopy. Furthermore, electrostatic force microscopy reveals the presence of three distinct regions (metallic, semiconducting, and insulating) in the $\mathrm{Mo}{\mathrm{S}}_2-\mathrm{hBN}$ composite film with differing contact potentials and enhanced propensity for charge transfer with respect to pristine $\mathrm{Mo}{\mathrm{S}}_2$. A triboelectric nanogenerator device containing biphasic $\mathrm{Mo}{\mathrm{S}}_2-\mathrm{hBN}$ composite film as an electron acceptor exhibits more than twofold (sixfold) enhancement in peak-to-peak output voltage as compared to the pristine $\mathrm{Mo}{\mathrm{S}}_2$ (hBN) film. These observations bring out the potential of 2D/2D nanocomposite thin films for unfolding emergent phenomena and technological applications.

Cubic hafnia (${\mathrm{HfO}}_2$) is of great interest for a number of applications in electronics because of its high dielectric constant. However, common defects in such applications degrade the properties of hafina. We have investigated the electronic properties of oxygen vacancies and nitrogen substitution in cubic ${\mathrm{HfO}}_2$ using first-principles calculations based on density functional theory (DFT) and many-body diffusion Monte Carlo (DMC) methods. We investigate five different charge defect states of oxygen vacancies, as well as substitutional N defects which can lead to local magnetic moments. Both DMC and DFT calculations shows that an oxygen vacancy induces strong lattice relaxations around the defect. Finally, we compare defect formation energies, charge and spin densities obtained from DMC with results obtained using DFT. While the obtained formation energies from DMC are 0.6–1.5 eV larger than those from $\mathrm{GGA}+U$, the agreement for the most important defects, neutral and positively charged oxygen vacancies, and nitrogen substitutional defect, under oxygen-poor conditions are in reasonably good agreement. Our work confirms that nitrogen can act to passivate cubic hafnia for applications in electronics.

Topological phases of electronic systems often coexist in a material, well-known examples being systems which are both strong and weak topological insulators. More recently, a number of materials have been found to have the topological structure of both a weak topological phase and a mirror-protected topological crystalline phase. In this work, we first focus on the naturally occurring mineral called jacutingaite, ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$, and show based on density-functional calculations that it realizes this dual topological phase and that the same conclusion holds for ${\mathrm{Pd}}_2{\mathrm{HgSe}}_3$. Second, we introduce tight-binding models that capture the essential topological properties of this dual topological phase in materials with threefold rotation symmetry and use these models to describe the main features of the surface spectral density of different materials in the class.

Activator impurities and their distribution in the host lattice play a key role in scintillation phenomena. Here a combination of cross-sectional noncontact atomic force microscopy, x-ray photoelectron spectroscopy, and density-functional theory were used to study the distribution of ${\mathrm{Eu}}^{2+}$ dopants in a NaI scintillator activated by 3% ${\mathrm{EuI}}_2$. A variety of Eu-based structures were identified in crystals subjected to different postgrowth treatments. Transparent crystals with good scintillation properties contained mainly small precipitates with a cubic crystal structure and a size below 4 nm. Upon annealing, Eu segregated toward the surface, resulting in the formation of an ordered hexagonal overlayer with a ${\mathrm{EuI}}_2$ composition and a pronounced, unidirectional moiré pattern. Crystals with poor optical transparency showed a significant degree of mosaicity and the presence of precipitates. All investigated crystals contained a very low concentration of Eu dopants present as isolated point defects; most of the europium was incorporated in larger structures.

Upon chemical substitution of oxygen with fluor, LaMnAsO has been electron-doped in experiments, resulting in samples with remarkably high Seebeck coefficients of around $-300$ $\mu \mathrm{V}{\mathrm{K}}^{-1}$ at room temperature and 3% doping. Within the framework of density functional theory plus dynamical mean-field theory (DFT + DFMT) we not only are able to reproduce these experimental observations, but also can provide a thorough investigation of the underlying mechanisms. By considering electronic correlations in the half-filled Mn-$3d$ shells, we trace the high Seebeck coefficient back to an asymmetry in the spectral function, which is due to the emergence of an incoherent spectral weight under doping and a strong renormalization of the unoccupied states. This is only possible in correlated systems and cannot be explained by DFT-based band structure calculations.

Here, we report the structural and electronic properties of ${\mathrm{CeAu}}_2\mathrm{Bi}$, a new heavy-fermion compound crystallizing in a nonsymmorphic hexagonal structure $\left(P63/mmc\right)$. The ${\mathrm{Ce}}^{3+}$ ions form a triangular lattice which orders antiferromagnetically below $T_N=3.1\mathrm{K}$ with a magnetic hard axis along the $c$ axis. Under applied pressure, $T_N$ increases linearly at a rate of $0.07\mathrm{K}/\mathrm{kbar}$, indicating that the Ce $f$ electrons are fairly localized. In fact, heat capacity measurements provide an estimate of 150(10) $\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ for the Sommerfeld coefficient. The crystal-field scheme obtained from our thermodynamic data points to a ground state with a dominantly $|j_z=\pm 1/2\rangle$ character, which commonly occurs in systems with a hard $c$ axis. Finally, electronic band structure calculations and symmetry analysis in $k$ space reveal that ${\mathrm{CeAu}}_2\mathrm{Bi}$ hosts symmetry-protected crossings at $k_z=\pi$ in the paramagnetic state.

Body-centered cubic metals and alloys irradiated by energetic particles form highly mobile prismatic dislocation loops with $a/2〈111〉$-type Burgers vectors. We show how to simulate thermal diffusion of prismatic loops using a discrete dislocation dynamics approach that explicitly includes the stochastic forces associated with ambient thermal fluctuations. We find that the interplay between stochastic thermal forces and internal degrees of freedom of loops, in particular the reorientation of the loop habit planes, strongly influences the observed loop dynamics. The loops exhibit three fundamental types of reactions: coalescence, repulsion, and confinement by elastic forces. The confinement reactions are highly sensitive to the internal degrees of freedom of the loops. Depending on the orientation of the loop habit planes, the barrier to enter an elastically confined bound state is lowered substantially, whereas the lifetime of the bound state increases by many orders of magnitude.

When a shock wave enters a heterogeneous material, local differences in density and compressibility drive pressure and velocity gradients which can be nearly as sharp as the incident shock. Subsequent momentum equilibration is comparatively slow, relying on shock reverberation, particle collisions, and shear flow. The latter phenomenon depends on constitutive behavior that is often poorly known and so the process can be problematic to simulate. This is compounded by the fact that conventional diagnostics, such as velocimetry, blend the response of both phases and provide only a homogenized comparison for modeling efforts. For these reasons, we have collected spatially resolved and phase-specific measurements on a model particulate composite using in situ synchrotron-based radiography. This revealed the post-shock internal motion, including nanosecond resolution measurement of the metal particles’ trajectories and snapshots of the flow field within the polymer. Comparing this data to analytical and numerical predictions allowed the polymer's shear response to be inferred. The resulting constitutive model was then used in conjunction with direct numerical simulations to demonstrate the physical origins of the observed bulk composite response.

Two-dimensional (2D) materials often feature defects and strain, due to their atomic layered character, which can cause problems for applications. Antimonene, a monolayer material derived from layered bulk Sb, undergoes a semiconductor-to-topological-insulator transition under large strain. However, it is unclear whether the structure and properties of antimonene are retained under strain once defects are generated. Here, we used ab initio calculations to explore a series of the most probable defects in these materials, including the Stone-Wales (SW) defects, single vacancies, double vacancies (DV), and adatoms. Interestingly, the influence of defects can be categorized into two types: for defects involving the loss/addition of an odd number of atoms, the material becomes ferromagnetic and exhibits half-metal properties under a certain strain; for defects involving an even number of missing atoms, the material remains a nonmagnetic semiconductor. Moreover, the topological phase transitions are robust for Sb monolayers with SW defects, but the critical transition strain decreases. Conversely, topological phase transitions might vanish for DV (555|777) and DV (5|8|5) defects. Our calculations suggest that different types of defects and strain might transform antimonene into a semiconductor, half-metal, or topological insulator. Therefore, defects and strain effects in antimonene should be carefully controlled for its applications.