Precipitation hardening, which relies on a high density of intermetallic precipitates, is a commonly utilized technique for strengthening structural alloys. At high temperatures, however, the precipitates often coarsen to reduce the excess energy of the interface, resulting in a significant reduction in the strengthening achieved. In certain ternary alloys, secondary solute segregation to the interface has been observed to result in the formation of a high density of nanosized precipitates that provide enhanced strength and are resistant to coarsening. To understand the chemical effects involved, and to identify such segregating systems, we develop a thermodynamic model using the framework of the regular nanocrystalline solution model. For various global compositions, temperatures, and thermodynamic parameters, we evaluate equilibrium configurations of a Mg-Sn-Zn alloy by minimizing the Gibbs free energy function with respect to region-specific (bulk solid solution, interface, and precipitate) concentrations and sizes. The results show that ${\mathrm{Mg}}_2\mathrm{Sn}$ precipitates can be stabilized to nanoscale sizes through Zn segregation to the $\mathrm{Mg}/{\mathrm{Mg}}_2\mathrm{Sn}$ interface, and the precipitates can be stabilized against coarsening at high temperatures through strong Mg-Zn interface interaction. Together with the inclusion of elastic strain energy effects, kinetic contributions, and the input of computationally informed interface parameters in the future, the model is expected to provide a more realistic prediction of segregation and precipitate stabilization in ternary alloys of structural importance.

The demagnetizing factor $N$ is of both conceptual interest and practical importance. Considering localized magnetic moments on a lattice, we show that for nonellipsoidal samples, $N$ depends on the spin dimensionality (Ising, XY, or Heisenberg) and orientation, as well as the sample shape and susceptibility. The generality of this result is demonstrated by means of a recursive analytic calculation as well as detailed Monte Carlo simulations of realistic model spin Hamiltonians. As an important check and application, we also make an accurate experimental determination of $N$ for a representative collective paramagnet (i.e., the ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ spin ice compound) and show that the temperature dependence of the experimentally determined $N$ agrees closely with our theoretical calculations. Our conclusion is that the well-established practice of approximating the true sample shape with “corresponding ellipsoids” for systems with long-range interactions will in many cases overlook important effects stemming from the microscopic aspects of the system under consideration.

We measure the surface of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ single crystals by making use of two-photon photoemission spectroscopy. Our method monitors the electronic distribution of photoexcited electrons, explicitly discriminating the initial thermalization from slower dynamical processes. The reported results disclose the fast-dissipation channels of hot carriers (0.25 ps), set an upper bound to the surface-induced recombination velocity $(<4000$ cm/s), and reveal the dramatic effect of shallow traps on the electrons dynamics. The picosecond localization of excited electrons in degraded ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ samples is consistent with the progressive reduction of photoconversion efficiency in operating devices. Minimizing the density of shallow traps and solving the aging problem may boost the macroscopic efficiency of solar cells to the theoretical limit.

Magnetism in Weyl semimetals is desired to investigate the interaction between the magnetic moments and Weyl fermions, e.g., to explore anomalous quantum Hall phenomena. Here we demonstrate that proton irradiation is an effective tool to induce ferromagnetism in the Weyl semimetal TaAs. The intrinsic magnetism is observed with a transition temperature above room temperature. The magnetic moments from $d$ states are found to be localized around Ta atoms. Further, the first-principles calculations indicate that the $d$ states localized on the nearest-neighbor Ta atoms of As vacancy sites are responsible for the observed magnetic moments and the long-ranged magnetic order. The results show the feasibility of inducing ferromagnetism in Weyl semimetals so that they may facilitate the applications of this material in spintronics.

We discuss, based on first-principles calculations, the possibility of tuning the magnetism of oxygen vacancies at the (001) surface of strontium titanate $\left({\mathrm{SrTiO}}_3\right)$. The magnetic moment of single and clustered vacancies stemming from $\mathrm{Ti}-\mathrm{O}$ broken bonds can be both quenched and stabilized controllably by chemical potential adjustment associated with doping the system with electrons or holes. We discuss to what extent this route to magnetization state control is robust against other external influences such as chemical doping, mechanical action, and electric field. Such control of the vacancy state and magnetization can conceivably be achieved experimentally by using local probe tips.

High critical current density $\left(J_c\right)$ values in superconducting wires/tapes are desirable for high magnetic field applications. Recently developed pnictide wires/tapes exhibit exceptional superconducting properties such as high critical temperature $\left(T_c\right)$, upper critical field $\left(H_{c2}\right)$, and almost field-independent $J_c$. Despite the great fabrication efforts, however, the newly discovered pnictide wires/tapes are still not able to replace low-temperature superconductors such as $\mathrm{N}{\mathrm{b}}_3\mathrm{Sn}$, due to their inferior $J_c$ values. Ag-clad $\mathrm{S}{\mathrm{r}}_{0.6}{\mathrm{K}}_{0.4}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ tapes have demonstrated significant superconducting performance, although their low $J_c$ in comparison to $\mathrm{N}{\mathrm{b}}_3\mathrm{Sn}$ is still a major challenge. By successfully employing hydrostatic pressure, a remarkably significant enhancement of $J_c$ by an order of magnitude can be achieved in $\mathrm{S}{\mathrm{r}}_{0.6}{\mathrm{K}}_{0.4}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ tapes in both low and high fields. This is a promising technological step forward towards high-field applications, as the record high $J_c$ values $(\sim 2\times 10^5\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ at 4.2 K and 13 T, $P=1.1\mathrm{GPa})$ obtained for $\mathrm{S}{\mathrm{r}}_{0.6}{\mathrm{K}}_{0.4}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ tape are superior to those of $\mathrm{N}{\mathrm{b}}_3\mathrm{Sn}$ and other pnictide wires/tapes. Here, we used magnetic $J_c$ data for comparison to the other reported transport $J_c$ data, due to the lack of transport measurement facility under hydrostatic pressure. Our systematic analysis shows that pressure-induced pinning centers are the main source of $J_c$ enhancement, along with a fractional contribution from geometric changes around the grain boundaries under pressure. We expect that utilization of an appropriate pressure approach will be a way to significantly enhance $J_c$ to beyond the cutoff (maximum) values in various superconductors produced using other existing methods for $J_c$ enhancement.

In polar insulators where longitudinal and transverse optical phonon modes differ substantially, the electron-phonon coupling affects the energy-band structure primarily through the long-range Fröhlich contribution to the Fan term. This diagram has the same structure as the $GW$ self-energy where $W$ originates from the electron part of the screened Coulomb interaction. The two can be conveniently combined by combining electron and lattice contributions to the polarizability. Both contributions are nonanalytic at the origin, and diverge as $1/q^2$ so that the predominant contribution comes from a small region around $\mathbf{q}=0$. Here we adopt a simple estimate for the Fröhlich contribution by assuming that the entire phonon part can be attributed to a small volume of $\mathbf{q}$ near $\mathbf{q}=0$. We estimate the magnitude for $\mathbf{q}\to 0$ from a generalized Lyddane-Sachs-Teller relation, and the radius from the inverse of the polaron length scale. The gap correction is shown to agree with Fröhlich's simple estimate $-{\alpha }_P{\omega }_{\mathrm{LO}}/2$ of the polaron effect with ${\alpha }_P$ the polaron coupling factor.

Multilayer graphene with rhombohedral stacking is a promising carbon phase possibly displaying correlated states like magnetism or superconductivity due to the occurrence of a flat surface band at the Fermi level. Recently, flakes of thickness up to 17 layers were tentatively attributed to ABC sequences although the Raman fingerprint of rhombohedral multilayer graphene is currently unknown and the 2D resonant Raman spectrum of Bernal graphite is not understood. We provide a first principles description of the 2D Raman peak in three and four layers graphene (all stackings) as well as in Bernal, rhombohedral, and an alternation of Bernal and rhombohedral graphite. We give practical prescriptions to identify long range sequences of ABC multilayer graphene. Our work is a prerequisite to experimental nondestructive identification and synthesis of rhombohedral graphite.

Molecular dynamics is used in combination with density functional theory to determine the thermal transport properties of the single-layer hexagonal boron nitride (SL $h$-BN) from ab initio calculations. Within this approach, the possible anisotropy in the thermal conductivity of SL $h$-BN was studied. For samples with finite length (of the order of 20 nm), we find a significant dependence of the conductivity on the transport direction. We make a direct comparison of the results obtained for two-dimensional (2D) layers and for nanoribbons with similar size, and show that, as a consequence of edge scattering, the ribbon geometry induces a significant decrease in the conductivity, and produces a strong change in the anisotropy. For the zigzag and armchair transport directions, the dependence of the thermal conductivity on the system length was also obtained, as well as its value in the 2D bulk limit case. A very small anisotropy was found for the limit of long samples, in contrast with the finite length ones. This is explained analyzing the dependence of the average square group velocities on the transport direction and the phonon frequency.

Anomalously large dielectric aging is found in the high-susceptibility plateau ferroelectric regime of nominally $\sim 95\%$ deuterated potassium dihydrogen phosphate (DKDP). Much less aging is found in nondeuterated KDP throughout most of its plateau region. Optical images of the DKDP domain structure show no dramatic change during aging. Small changes in electric field restore the preaged susceptibility, but the previous aging almost recovers after returning to a respective aging field. Susceptibility vs field can show memory of at least two prior aging fields. Aging effects are not fully erased even by brief heating above the Curie point, indicating a role for diffusion of hydrogen to the domain walls, leaving changes in disorder that can survive temporary absence of domain walls. Asymmetrical nonlinear susceptibility develops for fields slightly above or below a prior aging field and during field sweeps, with the dependence of the second-harmonic magnitude on sweep rate giving a characteristic time comparable to the time for hydrogen to diffuse a domain-wall width.

Boron nitride structures have excellent thermal and chemical stabilities. Based on state-of-art theoretical calculations, we propose a wide-gap semiconducting BN crystal with a three-dimensional hyperhoneycomb structure (Hp-BN), which is both mechanically and thermodynamically stable. Our calculated results show that Hp-BN has a higher bulk modulus and a smaller energy gap as compared to c-BN. Moreover, due to the unique bonding structure, Hp-BN exhibits anisotropic electronic and optical properties. It has great adsorption in the ultraviolet region, but it is highly transparent in the visible and infrared region, suggesting that the Hp-BN crystal could have potential applications in electronic and optical devices.

In this work, we present a highly accurate spectral neighbor analysis potential (SNAP) model for molybdenum (Mo) developed through the rigorous application of machine learning techniques on large materials data sets. Despite Mo's importance as a structural metal, existing force fields for Mo based on the embedded atom and modified embedded atom methods do not provide satisfactory accuracy on many properties. We will show that by fitting to the energies, forces, and stress tensors of a large density functional theory (DFT)-computed dataset on a diverse set of Mo structures, a Mo SNAP model can be developed that achieves close to DFT accuracy in the prediction of a broad range of properties, including elastic constants, melting point, phonon spectra, surface energies, grain boundary energies, etc. We will outline a systematic model development process, which includes a rigorous approach to structural selection based on principal component analysis, as well as a differential evolution algorithm for optimizing the hyperparameters in the model fitting so that both the model error and the property prediction error can be simultaneously lowered. We expect that this newly developed Mo SNAP model will find broad applications in large and long-time scale simulations.

Centimeter-sized single crystals of ${\mathrm{Sr}}_{1.5}{\mathrm{La}}_{0.5}{\mathrm{RhO}}_4$ were grown by the floating zone method at oxygen pressures of 20 bar. The quality of our single crystals was confirmed by x-ray Laue, powder and single crystal x-ray diffraction, neutron and x-ray absorbtion spectroscopy measurements. At $\sim 50\%$ electron doping we observe ${\mathrm{RhO}}_3$ octahedral rotations of $\sim 8.2^{\circ }$ within the octahedral basal plane which are incompatible with space group $I4/mmm$. Our single crystal was further characterized by susceptibility, electrical transport, and, finally, specific heat measurements showing a temperature-dependent Debye temperature.

We provide experimental evidence for confinement of water molecules in the pores of hexagonal structure of ${\mathrm{YPO}}_4$ at elevated temperatures up to 600 K using powder neutron diffraction. In order to avoid the large incoherent scattering from the hydrogen, deuterated samples of doped ${\mathrm{YPO}}_4$:Ce-Eu were used for diffraction measurements. The presence of water molecules in the triangular and hexagonal pores in the hexagonal structure was established by detailed simulation of the diffraction pattern and Rietveld refinement of the experimental data. It was observed that the presence of water leads specifically to suppression of the intensity of a peak around $Q=1.04{\AA }^{-1}$ while the intensity of peaks around $Q=1.83{\AA }^{-1}$ is enhanced in the neutron-diffraction pattern. We estimate the number of water molecules as 2.36 (6) per formula units at 300 K and the sizes of the hexagonal and triangular pores as 7.2 (1) and 4.5 (1) Å, respectively. With an increase in temperature, the water content in both pores decreases above 450 K and vanishes around 600 K. Analysis of the powder-diffraction data reveals that the hexagonal structure with the pores persist up to 1273 K, and transforms to another structure at 1323 K. The high-temperature phase is not found to have the zircon- or monazite-type structure, but a monoclinic structure (space group $P2/m)$ with lattice parameters $a_m=6.826\left(4\right)\AA ,b_m=6.645\left(4\right)\AA ,c_m=10.435\left(9\right)\AA$, and $\beta =107.21{\left(6\right)}^{\circ }$. The monoclinic structure has about 14% smaller volume than the hexagonal structure which essentially reflects the collapse of the pores. The phase transition and the change in the volume are also confirmed by x-ray-diffraction measurements. The hexagonal-to-monoclinic phase transition is found to be irreversible on cooling to room temperature.

We investigate the effect of surface tunneling on charge distributions of two-dimensional hole gases (2DHGs) in undoped Ge/GeSi heterostructures. As in the electron channel case, the 2DHG density saturates at a high gate voltage. As the channel depth of 2DHGs increases, a crossover of charge distributions in the system from equilibrium to nonequilibrium is observed at a depth of $\sim 50$ nm. A surface tunneling model is proposed to explain the density crossover. Magnetotransport analysis is performed to investigate the limiting scattering mechanisms. The power law dependence of mobility on density suggests that the dominant scattering mechanisms for the shallow- and deep-channel 2DHGs are remote impurity and background impurity scattering, respectively. Clear quantum Hall plateaus and vanishing longitudinal magnetoresistance are observed in the 2DHG device of channels as shallow as 9 nm.

Half-metallic ferromagnetism (HMFM) occurs rarely in materials and yet offers great potential for spintronic devices. Recent experiments suggest a class of compounds with the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ (122) structure (isostructural and containing elements common with Fe pnictide-based superconductors) can exhibit HMFM. Here, we use ab initio density functional theory calculations to understand the onset of half-metallicity in this family of materials and explain the appearance of ferromagnetism at a quantum critical point. We also predict new candidate materials with HMFM and potentially high Curie temperatures through $A$-site alloying.