Large $(>100\mathrm{m}{\mathrm{m}}^3)$, relatively pure (type II) and low birefringence single crystal diamond can be produced by high pressure high temperature (HPHT) synthesis. In this study we examine a HPHT sample of good crystalline perfection, containing less than 1 ppb (part per billion carbon atoms) of boron impurity atoms in the (001) growth sector and only tens of ppb of nitrogen impurity atoms. It is shown that the boundaries between $\left\{111\right\}$ and $\left\{113\right\}$ growth sectors are decorated by negatively charged nitrogen vacancy centers $\left(\mathrm{N}{\mathrm{V}}^{-}\right)$: no decoration is observed at any other type of growth sector interface. This decoration can be used to calculated the relative $\left\{111\right\}$ and $\left\{113\right\}$ growth rates. The bulk (001) sector contains concentrations of luminescent point defects (excited with 488- and 532-nm wavelengths) below ${10}^{11}\mathrm{c}{\mathrm{m}}^{-3}$ $\left({10}^{-3}\mathrm{ppb}\right)$. We observe the negatively charged silicon-vacancy $\left(\mathrm{Si}{\mathrm{V}}^{-}\right)$ defect in the bulk $\left\{111\right\}$ sectors along with a zero phonon line emission associated with a nickel defect at 884 nm (1.40 eV). No preferential orientation is seen for either $\mathrm{N}{\mathrm{V}}^{-}$ or $\mathrm{Si}{\mathrm{V}}^{-}$ defects, but the nickel related defect is oriented with its trigonal axis along the $\langle 111\rangle$ sector growth direction. Since the $\mathrm{N}{\mathrm{V}}^{-}$ defect is expected to readily reorientate at HPHT diamond growth temperatures, no preferential orientation is expected for this defect but the lack of preferential orientation of $\mathrm{Si}{\mathrm{V}}^{-}$ in $\left\{111\right\}$ sectors is not explained.

Exotic metastable phases of silicon formed under high pressure are expected to have attractive semiconducting properties including narrow band gaps that open up novel technological applications. Confined microexplosions induced by powerful ultrashort laser pulses have been demonstrated as an advanced tool for the creation of new high-pressure phases that cannot be synthesized by other means. Tightly focused laser pulses are used to generate localized modifications inside the material structure, providing the possibility for precise controlled band-gap engineering. In this study, noninvasive Raman spectroscopy was used for analysis of laser-modified zones in silicon and to determine the metastable high-pressure phases contained. Low laser energies induced the formation of amorphous-only silicon, while higher energies led to crystalline silicon polymorphs within the modifications, albeit under considerable residual stress up to 4.5 GPa. The presence of the structurally similar r8-Si, bc8-Si, and bt8-Si phases is revealed, as well as other yet to be identified phases, and the stacking-related 9R Si polytype is evidenced, presumably stress-induced by the highly compressed laser-modified zone. The ab initio random structure searching approach is used to calculate the Raman signatures and to help identify different Si polymorphs. These findings by Raman spectroscopy from ultrashort laser-induced microexplosion sites may yield insights into the local structure and properties of new silicon metastable phases and the prospect of utilizing exotic phases for extending current applications.

Precipitation in Mg-Zn alloys was analyzed by means of first-principles calculations. Formation energies of symmetrically distinct hcp ${\mathrm{Mg}}_{1\text{−}x}{\mathrm{Zn}}_x\left(0<x<1\right)$ configurations were determined, and potential candidates for Guinier-Preston zones coherent with the matrix were identified from the convex hull. The most likely structures were ranked depending on the interface energy along the basal plane. In addition, the formation energy and vibrational entropic contributions of several phases reported experimentally (${\mathrm{Mg}}_4{\mathrm{Zn}}_7, \mathrm{Mg}{\mathrm{Zn}}_2$ cubic, $\mathrm{Mg}{\mathrm{Zn}}_2$ hexagonal, ${\mathrm{Mg}}_{21}{\mathrm{Zn}}_{25}$, and ${\mathrm{Mg}}_2{\mathrm{Zn}}_{11}$) were calculated. The formation energies of ${\mathrm{Mg}}_4{\mathrm{Zn}}_7, \mathrm{Mg}{\mathrm{Zn}}_2$ cubic, and $\mathrm{Mg}{\mathrm{Zn}}_2$ hexagonal Laves phases were very close because they were formed by different arrangements of rhombohedral and hexagonal lattice units. It was concluded that ${\beta }_1^{\prime }$ precipitates were formed by a mixture of all of them. Nevertheless, the differences in the geometrical arrangements led to variations in the entropic energy contributions which determined the high-temperature stability. It was found that the $\mathrm{Mg}{\mathrm{Zn}}_2$ hexagonal Laves phase is the most stable phase at high temperature and thus ${\beta }_1^{\prime }$ precipitates tend to transform into the ${\beta }_2^{\prime }$ ($\mathrm{Mg}{\mathrm{Zn}}_2$ hexagonal) precipitates with higher aging temperature or longer aging times. Finally, the equilibrium $\beta$ phase (${\mathrm{Mg}}_{21}{\mathrm{Zn}}_{25}$) was found to be a long-range order that precipitates the last one on account of the kinetic processes necessary to trigger the transformation from a short-range order phase ${\beta }_2^{\prime }$ to $\beta$.

Triuranium octoxide $\left({\mathrm{U}}_3{\mathrm{O}}_8\right)$ undergoes an orthorhombic to hexagonal structural phase transition near $T_s=305{}^{\circ }\mathrm{C}$, and a separate nonstructural phase transition at $T_c=210{}^{\circ }\mathrm{C}$. The later transition has previously been associated with temperature-induced fluctuations in the uranium oxidation state. A discontinuity in the slope of electrical conductivity versus temperature measurement at $210{}^{\circ }\mathrm{C}$ has supported this idea. The orthorhombic phase has three crystallographic sites in two distinct oxidation configurations [2 U(V) and 1 U(VI)], whereas the hexagonal phase has one distinct uranium site. High-resolution x-ray diffraction measurements eliminate the possibility of superlattice Bragg reflections to less than 0.2 ${\mathrm{e}}^{-}$ scattering power and ${\mathrm{U}}_3{\mathrm{O}}_8$ is not metallic; consequently, the presence of oxidation fluctuations is required for charge balancing. Interestingly, the order-to-disorder transition occurs at a much lower temperature than the structural transition. Using temperature-dependent x-ray diffraction and Raman spectroscopy, we show anisotropic lattice expansion in the in-plane $b$ and $c$ lattice constants. A specific discontinuity in the temperature derivatives of the $b$ and $c$ lattice constants are the first reported structural signatures of the order-to-disorder transition, suggestive of a change in local U–O coordination. Phonon frequencies of ${\mathrm{U}}_3{\mathrm{O}}_8$ measured by Raman spectroscopy show significant temperature-dependent dynamics. Redshifting of several modes between 40 and $300{}^{\circ }\mathrm{C}$ cannot be explained by unit cell expansion alone because the unit cell volume decreases in this region. Instead, we show that phonon frequencies are highly correlated with the anisotropic lattice expansion/contraction along specific crystallographic directions.

We report on the orientation and morphology of solid-state dewetting holes obtained from a kinetic Monte Carlo model with nearest neighbor and next nearest neighbor interactions on a cubic lattice. The morphologies found in simulations share strong similarities with those of diffusion-limited crystal growth: We find compact shapes, isotropic and anisotropic seaweed shapes, and dendritic shapes. Some of these shapes have been observed in solid-state dewetting experiments with various semi-conductors or metals on insulating substrates. The sides of the fingers can exhibit two types of zigzag instabilities with ${90}^{\circ }$ or ${45}^{\circ }$. Due to fourfold symmetry, dewetting hole shapes are found to exhibit two possible orientations with fingers along the (110) or (100) directions. We find that the rotation transition from one orientation to the other can be observed in our model by varying anisotropy, temperature, or dewetting strength. The two orientations correspond, respectively, to experimental observations of dewetting for silicon and germanium on insulator and to the dewetting of metal films with different reducing gas flow rate.

$\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ superlattices deposited on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates are studied using a combination of x-ray diffraction, piezoresponse force microscopy, scanning transmission electron microscopy, transport measurements, and impedance spectroscopy. The superlattices are found to have two inequivalent interfaces resulting from differences in the growth modes for $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ and $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$. X-ray diffraction measurements show that, despite being sandwiched between metallic $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ layers, the ferroelectric layers possess dense nanoscale domains. The observed domain sizes are comparable to those found in ferroelectric-dielectric systems, and they are attributed to the depolarizing field caused by the finite screening length of the $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3-\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ interface. The macroscopic capacitance of the ultrathin $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ layers was measured, and its temperature dependence was found to be consistent with permittivity enhancement due to domain wall motion.

To understand the underlying mechanism behind the precipitation of high-density face-centered cubic Al nanoparticles during the annealing or preparation of Al-based metallic glass, we used molecular dynamics simulation technique to study the atomic structure evolution of Al-rich and Al-depleted melts in the Al-Zr binary system under rapid cooling conditions. From the quantitative characterization of thermodynamic properties, kinetic properties, and atomic structure, we found that an abnormal orderly transformation occurs in the supercooled liquid of Al-rich alloy, which includes two basic stages, namely, directional diffusion and stabilization of defect clusters. The thermodynamic essence of this transformation is to reduce the Gibbs free energy of the entire system by improve the cluster packing efficiency of atomic structure, preventing the supercooled liquid from being in an unstable state. An important consequence of the abnormal orderly transformation is the formation of nanoscale Al-rich regions in the supercooled liquid. These regions can readily promote the precipitation of high-density face-centered cubic Al nanoparticles, thereby reducing the complete amorphization ability of Al-based alloy melt.

We develop a set of machine-learning interatomic potentials for elemental V, Nb, Mo, Ta, and W using the Gaussian approximation potential framework. The potentials show good accuracy and transferability for elastic, thermal, liquid, defect, and surface properties. All potentials are augmented with accurate repulsive potentials, making them applicable to radiation damage simulations involving high-energy collisions. We study melting and liquid properties in detail and use the potentials to provide melting curves up to 400 GPa for all five elements.

An advanced modeling approach is presented to shed light on the thermal transport properties of van der Waals materials (vdWMs) composed of single-layer transition metal dichalcogenides (TMDs) stacked on top of each other with a total or partial overlap only in the middle region. It relies on the calculation of dynamical matrices from first principles and on their usage in a phonon quantum transport simulator. We observe that vibrations are transferred microscopically from one layer to the other along the overlap region which acts as a filter selecting out the states that can pass through it. Our work emphasizes the possibility of engineering heat flows at the nanoscale by carefully selecting the TMD monolayers that compose vdWMs.

Magnetoelectric materials are attractive for several applications, including actuators, switches, and magnetic field sensors. Typical mechanisms for achieving a strong magnetoelectric coupling are rooted in transition metal magnetism. In sharp contrast, here we identify $\mathrm{CsEr}{\left({\mathrm{MoO}}_4\right)}_2$ as a magnetoelectric material without magnetic transition metal ions, thus ensuring that the Er ions play a key role in achieving this interesting property. Our detailed study includes measurements of the structural, magnetic, and magnetoelectric properties of this material. Bulk characterization and neutron powder diffraction show no evidence for structural phase transitions down to 0.3 K and therefore $\mathrm{CsEr}{\left({\mathrm{MoO}}_4\right)}_2$ maintains the room temperature $P2/c$ space group over a wide temperature range without external magnetic field. These same measurements also identify collinear antiferromagnetic ordering of the ${\mathrm{Er}}^{3+}$ moments below $T_N=0.87\mathrm{K}$. Complementary dielectric constant and pyroelectric current measurements reveal that a ferroelectric phase with a maximum polarization $P\sim 0.6\mathrm{nC}/{\mathrm{cm}}^2$ emerges when applying a modest external magnetic field, which indicates that this material has a strong magnetoelectric coupling. We argue that the magnetoelectric coupling in this system arises from a pseudo Jahn-Teller distortion induced by the magnetic field.

The emergence of spintronic applications has stimulated intense interest due to expectations for new functionalities and devices. However, in a I-II-V diluted magnetic semiconductor Li(Zn, Mn)As system as a corresponding material, the mechanism of the doping and ferromagnetism is not clear yet. Here, we used aberration-corrected scanning transmission electron microscopy to investigate the microstructures of the paramagnetic and ferromagnetic ${\mathrm{Li}}_{1+y}{\mathrm{Zn}}_{1–x}{\mathrm{Mn}}_x\mathrm{As}$ (nominal composition: ${\mathrm{Li}}_{1.0}{\mathrm{Zn}}_{0.9}{\mathrm{Mn}}_{0.1}{\mathrm{As}}_{1.0}$ and ${\mathrm{Li}}_{1.1}{\mathrm{Zn}}_{0.9}{\mathrm{Mn}}_{0.1}{\mathrm{As}}_{1.0}$) compounds at atomic scale. Our results provide clear evidence that superstructures exist in a diluted magnetic semiconductor. We also reveal the origin of the modulated structure of the Li(Zn, Mn)As system. With the above studies, we indicate the relationship between the structure and magnetism, and investigate the charge and spin controlling mechanism in the diluted magnetic semiconductor. Our studies may provide a way to search for diluted magnetic semiconductors with novel and excellent properties.

Single crystal $\beta \text{−}{\mathrm{V}}_2{\mathrm{PO}}_5$ was synthesized by chemical vapor transport method and characterized by transport, thermodynamic, neutron diffraction, nuclear magnetic resonance measurements and first-principles calculation. It was shown to be a semiconductor with a band gap of 0.48 eV, undergoing a charge ordering (unusual ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$) phase transition accompanied by a tetragonal to monoclinic structural distortion at 610 K and a paramagnetic to ferrimagnetic phase transition at 128 K with a propagation vector of $\mathbf{k}=0$. The easy axis is in the monoclinic $ac$ plane pointing 47(9)${}^{\circ }$ away from the monoclinic $a$ axis. This collinear ferrimagnetic structure and anisotropic isothermal magnetization measurements suggest weak magnetic anisotropy in this compound. The first-principles calculations indicate that the intrachain interactions in the face-sharing ${\mathrm{VO}}_6$ chains dominate the magnetic hamiltonian and identify the ${\mathrm{\Gamma }}_5^{+}$ normal mode of the lattice vibration to be responsible for the charge ordering and thus the structural phase transition.

The morphological and electronic properties of isolated and monolayer phases of carboxyl- and amine-functionalized carboranethiols on unreconstructed Au(111) were determined using density functional theory calculations including van der Waals interactions. The groups are effective in the assembly of pristine adlayers on gold and also offer functionality when exposed at the SAM-environment interface. Anisotropy brought by both functional groups increases tilting of carboranethiols relative to the surface normal and absolute values of the dissociative chemisorption energies. Positional isomerization and the functional groups modify the molecular dipole moments which influence the adsorption characteristics. Even though carboxylic acid and amine groups have different chemical properties, they have similar effects on the adsorption characteristics of carboranethiols. Dense packing favors intermolecular interactions which gives a stronger binding relative to isolated adsorption. The carboranethiols with the functional groups can be arranged in the same lateral direction or in a dimer conformation with molecules facing each other. Carboxyl and amine groups offer functionalization of carboranethiol SAMs and in conjunction with positional isomerization shift tunability ranges of the work function of gold to higher energies.

We report a systematic study on the growth conditions of ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ thin films by molecular beam epitaxy for maximization of superconducting transition temperature $T_{\mathrm{c}}$. Careful tuning of the flux ratios of Sn, In, and Te enables us to find an optimum condition for substituting rich In content $(x=0.66)$ into the Sn site in a single phase of ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ beyond the bulk solubility limit at ambient pressure $(x=0.5)$. $T_{\mathrm{c}}$ shows a dome-shaped dependence on In content $x$ with the highest $T_{\mathrm{c}}=4.20\mathrm{K}$ at $x=0.55$, being consistent to that reported for bulk crystals. The well-regulated ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ films can be a useful platform to study possible topological superconductivity by integrating them into the state-of-the-art junctions and/or proximity-coupled devices.

Understanding the formation of twins and new grains (NGs) in B2 austenite NiTi alloys under shock loading is of significance and importance for its science insights and engineering applications. However, the formation of $\left\{112\right\}$ twin in austenite phase under shock loading is still controversial, and the NGs' evolution under shock loading is unclear. The Electron Backscatter Diffraction characterizations and x-ray diffraction analyses of the NiTi samples recovered from the shock experiments reveal that the main deformation modes include dislocations, twins, and new grains, etc. Similar phenomena are also obtained in our nonequilibrium molecular dynamics simulations for nanocrystalline NiTi (nc-NiTi) with limited-duration-pulse shock loading. Simulations confirmed that $\left\{112\right\}$ twins in austenite phase can be formed by successively gliding a displacement of $\frac{a}{3}$[111] on ($\overline{2}11$) plane. It can occur in both shock compression and release stages, in addition, the grain boundaries triple junction and the interaction of slip bands with different slip systems can serve as the nucleation of twins. Moreover, that the nanoscale rotational deformation leads to the formation of NGs is found in our simulation of nc-NiTi, without experiencing the conventional disorder-recrystallization-grain refinement stages in the corresponding region. Related to shear stress $\tau$, the shock loading velocity $U_p$ plays the key role in the formations of twins and NGs. These early successes may hope to get some insights into the deformation mechanism of NiTi under shock loading.

Stress annealing (SA)-induced magnetic anisotropy is known in iron, nickel, and cobalt-based ferromagnetic metallic glass ribbons and it has already been used in commercial processes. Uniaxial elastic strain is introduced by SA and is quenched into the ribbons even after cooling and removing the external stress. The release of the uniaxial quenched strains is clearly observed as an anomaly in the linear thermal-expansion coefficient (LTEC) when the ribbon is reheated without stress. The rate of strain release corresponding to the LTEC anomaly reaches a maximum at the temperature at which the original SA was performed. We have observed this temperature memory effect over the whole temperature range from 280 to 400 °C, which is below the crystallization temperature $T_x$. The observed results are explained well by the existence of a localized “flow unit” embedded in an elastic matrix, which is accepted as the origin of the shear band formation and rejuvenation of metallic glasses with $T_g$ (glass transition temperature) $<T_x$. Although $T_g$ is hardly visible in calorimetry measurements in most of the ferromagnetic metallic glass ribbons ($T_g>T_x$), the results here indicate that the same important structural feature is common to metallic glasses with both $T_g<T_x$ and $T_g>T_x$. Because magnetization behavior is very sensitive to the existence of residual elastic strain which is difficult to evaluate in most of the metallic glasses, detailed studies and a revival of interest in ferromagnetic ribbons will help us to understand more about the nature of the localized flow unit, as well as nonaffine deformations.

We report the study of the thermoelectric properties of layered ternary telluride ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$. The temperature dependence of the thermoelectric power (TEP) evolves from nonlinear to linear when the thickness of the devices is reduced, consistent with the suppression of electron-phonon interaction caused by quantum confinement. The magnitude of TEP strongly depends on the hole density. It increases with decreasing hole density when the hole density is low, as observed in ionic-liquid-gated thin flakes. However, the device with the largest hole density possesses the highest TEP. Theoretical analysis suggests that the high TEP in the device with the largest hole density can be ascribed to the phonon-mediated intervalley scatterings. The highest TEP reaches $\sim 230\mu \mathrm{V}/\mathrm{K}$ at 370 K while the electrical resistivity of the device is maintained below $1.5\mathrm{m}\mathrm{\Omega }\mathrm{cm}$. Therefore, a large power factor PF $\sim 36\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ comparable to the record values reported in $p$-type materials is obtained.

The design of low dimensional materials through surface assisted self-assembly requires a better understanding of the factors that limit and control surface diffusion. We reveal how substrate surface defects hinder the mobility of submonolayer organic adsorbates on a metal surface with the model CuPc/Cu(111) system. Postdeposition annealing bonds CuPc molecules into dendritelike clusters that are often mobile at room temperature. Surface defects on Cu(111) create energetic barriers that prevent CuPc cluster motion on the metal surface. This phenomenon was unveiled by the motion of small clusters that show rigid-body diffusion solely in the available space in between defects. When clusters are sufficiently surrounded by defects, they become completely pinned in place and become immobilized.