Chiral magnetic $\mathrm{M}{\mathrm{n}}_x\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{Ge}$ compounds have an antisymmetric exchange interaction that is tunable with the manganese stoichiometric fraction, $x$. Although millimeter-scale, polycrystalline bulk samples of this family of compounds have been produced, thin-film versions of these materials will be necessary for devices. In this study, we demonstrate the growth of epitaxial $\mathrm{M}{\mathrm{n}}_x\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{Ge}$ thin films on Si(111) substrates with a pure B20 crystal structure in the stoichiometric fraction range $x$ from 0 to 0.81. Following systematic structural analysis and magnetic characterization consisting of dc magnetization measurements and microwave absorption spectroscopy, we quantify the antisymmetric exchange interaction and helical period as a function of $x$, which ranges from 200 nm to 8 nm. Our results demonstrate an approach to engineering the size of magnetic skyrmions in epitaxial films that are grown using scalable techniques.

We investigate reported ferromagnetism in a new form of amorphous carbon. We use spin constrained first-principles simulations to obtain amorphous carbon structures with the desired magnetization. We show the existence of $s{p}^2$-like threefold coordinated carbon atoms plays an important role in causing magnetism in amorphous carbon. We predict detailed geometries of threefold carbon atoms that induce the magnetic order in amorphous carbon.

Recently, a group of MAX phases, ${\left({\mathrm{Mo}}_{2/3}{\mathrm{Y}}_{1/3}\right)}_2\mathrm{AlC},{\left({\mathrm{Mo}}_{2/3}{\mathrm{Sc}}_{1/3}\right)}_2\mathrm{AlC},{\left({\mathrm{W}}_{2/3}{\mathrm{Sc}}_{1/3}\right)}_2\mathrm{AlC},{\left({\mathrm{W}}_{2/3}{\mathrm{Y}}_{1/3}\right)}_2\mathrm{AlC}$, and ${\left({\mathrm{V}}_{2/3}{\mathrm{Zr}}_{1/3}\right)}_2\mathrm{AlC}$, with in-plane ordered double transition metals, named iMAX phases, have been synthesized. Experimentally, some of these MAX phases can be chemically exfoliated into two-dimensional (2D) single- or multilayered transition metal carbides, so-called MXenes. Accordingly, the 2D nanostructures derived from iMAX phases are named iMXenes. Here we investigate the structural stabilities and electronic structures of the experimentally discovered iMAX phases and their possible iMXene derivatives. We show that the iMAX phases and their pristine, F, or OH-terminated iMXenes are metallic. However, upon O termination, ${\left({\mathrm{Mo}}_{2/3}{\mathrm{Y}}_{1/3}\right)}_2\mathrm{C},{\left({\mathrm{Mo}}_{2/3}{\mathrm{Sc}}_{1/3}\right)}_2\mathrm{C},{\left({\mathrm{W}}_{2/3}{\mathrm{Y}}_{1/3}\right)}_2\mathrm{C}$, and ${\left({\mathrm{W}}_{2/3}{\mathrm{Sc}}_{1/3}\right)}_2\mathrm{C}$ iMXenes turn into semiconductors. Owing to the absence of centrosymmetry, the semiconducting iMXenes may find applications in piezoelectricity. Our calculations reveal that the semiconducting iMXenes possess giant piezoelectric coefficients as large as $45\times {10}^{-10}$ C/m.

A complete knowledge of absolute surface energies with any arbitrary crystal orientation is important for the improvements of semiconductor devices because it determines the equilibrium and nonequilibrium crystal shapes of thin films and nanostructures. It is also crucial in the control of thin film crystal growth and surface effect studies in broad research fields. However, obtaining accurate absolute formation energies is still a huge challenge for the semipolar surfaces of compound semiconductors. It mainly results from the asymmetry nature of crystal structures and the complicated step morphologies and related reconstructions of these surface configurations. Here we propose a general approach to calculate the absolute formation energies of wurtzite semipolar surfaces by first-principles calculations, taking GaN as an example. We mainly focused on two commonly seen sets of semipolar surfaces: a-family $\left(11\overline{2}\mathrm{X}\right)$ and m-family $\left(10\overline{1}\mathrm{X}\right)$. For all the semipolar surfaces that we have calculated in this paper, the self-consistent accuracy is within $1.5\mathrm{meV}/{\AA }^2$. Our work fills the last technical gap to fully investigate and understand the shape and morphology of compound semiconductors.

Topological semimetals exhibit band crossings near the Fermi energy, which are protected by the nontrivial topological character of the wave functions. In many cases, these topological band degeneracies give rise to exotic surface states and unusual magnetotransport properties. In this paper, we present a complete classification of all possible nonsymmorphic band degeneracies in hexagonal materials with strong spin-orbit coupling. This includes (i) band crossings protected by conventional nonsymmorphic symmetries, whose partial translation is within the invariant space of the mirror/rotation symmetry; and (ii) band crossings protected by off-centered mirror/rotation symmetries, whose partial translation is orthogonal to the invariant space. Our analysis is based on (i) the algebraic relations obeyed by the symmetry operators and (ii) the compatibility relations between irreducible representations at different high-symmetry points of the Brillouin zone. We identify a number of existing materials where these nonsymmorphic nodal lines are realized. Based on these example materials, we examine the surface states that are associated with the topological band crossings. Implications for experiments and device applications are briefly discussed.

We present studies of the magnetic domain structure of ${\mathrm{UMn}}_2{\mathrm{Ge}}_2$ single crystals using a home-built low temperature magnetic force microscope. The material has two distinct magnetic ordering temperatures, originating from the Mn and U moments. At room temperature, where the Mn moments dominate, there are flowerlike domain patterns similar to those observed in uniaxial ferromagnets. After exposing the sample to a 1 T magnetic field near 40 K, the evolution of the magnetic domains are imaged through zero-field warming up to 210 K. Near the ordering temperature of the uranium moments a clear change in the domain wall motion is observed. The domain size analysis of the flowerlike pattern reveals that the domain structure is consistent with a model of branching domains.

Zirconia (zirconium dioxide) and hafnia (hafnium dioxide) are binary oxides used in a range of applications. Because zirconium and hafnium are chemically equivalent, they have three similar polymorphs, and it is important to understand the properties and energetics of these polymorphs. However, while density functional theory calculations can get the correct energetic ordering, the energy differences between polymorphs depend very much on the specific density functional theory approach, as do other quantities such as lattice constants and bulk modulus. We have used highly accurate quantum Monte Carlo simulations to model the three zirconia and hafnia polymorphs. We compare our results for structural parameters, bulk modulus, and cohesive energy with results obtained from density functional theory calculations. We also discuss comparisons of our results with existing experimental data, in particular for structural parameters where extrapolation to zero temperature can be attempted. We hope our results of structural parameters as well as for cohesive energy and bulk modulus can serve as benchmarks for density-functional theory based calculations and as a guidance for future experiments.

Nearly all intermetallic compounds in the Mg-Al phase diagram have been characterized using Kohn-Sham density functional theory (KSDFT), providing insight into their properties. Two compounds with a common chemical formula of $\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$, known as the β and β′ Samson phases, remain challenging for KSDFT to interrogate due to their large unit cells containing partially occupied atomic sites and many hundreds of atoms. The much less expensive orbital-free DFT (OFDFT) is as accurate as KSDFT for Al-Mg alloys, provided one employs a nonlocal kinetic energy density functional. Here, we use nonlocal OFDFT to evaluate properties of the relatively simpler ${\beta }^{\prime }\text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$ phase with a unit cell of 879 atoms, as a first step toward full DFT characterization of these Samson phases. We employ the virtual crystal approximation to treat the potentials associated with the partially occupied atomic sites in the ${\beta }^{\prime }\text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$ unit cell. OFDFT lattice and elastic constants, with pair distribution functions (PDFs) of ${\beta }^{\prime }\text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$ computed via OFDFT molecular-dynamics simulations, are consistent with available experimental data; the PDF is a prediction, as it has not yet been measured. The predicted OFDFT formation energy of ${\beta }^{\prime }\text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$ (derived from the phonon density of states) increases with temperature, consistent with the experimental observation that this phase becomes increasingly unstable at higher temperatures. Using two common metrics of ductility (Pugh's ratio and brittleness), OFDFT-derived ideal tensile and shear strains/stresses of ${\beta }^{\prime }\text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$ and $\mathrm{M}{\mathrm{g}}_{17}\mathrm{A}{\mathrm{l}}_{12}$ (precipitates present at Al/Mg interfaces) reveal that ${\beta }^{\prime }\text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$ should be more ductile than $\mathrm{M}{\mathrm{g}}_{17}\mathrm{A}{\mathrm{l}}_{12}$. The superior ductility of ${\beta }^{\prime }\text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_3$ suggests that welding of Al to Mg should be done at temperatures where this alloy remains stable and under Al-rich conditions to favor its formation.

We study the phase stability and martensitic transformation of orthorhombic and monoclinic polyethylene by means of density functional theory using the nonempirical consistent-exchange vdW-DF-cx functional [Phys. Rev. B 89, 035412 (2014)]. The results show that the orthorhombic phase is the most stable of the two. Owing to the occurrence of soft librational phonon modes, the monoclinic phase is predicted not to be stable at zero pressure and temperature, but becomes stable when subjected to compressive transverse deformations that pin the chains and prevent them from wiggling freely. This theoretical characterization, or prediction, is consistent with the fact that the monoclinic phase is only observed experimentally when the material is subjected to mechanical loading. Also, the estimated threshold energy for the combination of lattice deformation associated with the T1 and T2 transformation paths (between the orthorhombic and monoclinic phases) and chain shuffling is found to be sufficiently low for thermally activated back transformations to occur. Thus, our prediction is that the crystalline part can transform back from the monoclinic to the orthorhombic phase upon unloading and/or annealing, which is consistent with experimental observations. Finally, we observe how a combination of such phase transformations can lead to a fold-plane reorientation from {110} to {100} type in a single orthorhombic crystal.

The formation of precipitates in metallic alloys is determined by a two-way chemomechanical coupling. While the dependence on solute diffusion is apparent, the opposite effect, namely the impact of nanoprecipitates on the diffusion, is highlighted in the present paper. Using severe plastic deformation and post-deformation annealing of an Al-based alloy, different microstructures and sizes of ${\mathrm{Al}}_3\mathrm{Sc}$-based nanoprecipitates are produced. The enhanced diffusion along grain boundaries in the so-called C-type kinetic regime is used as a probe to ensure a well-defined distance of the diffusion path from the precipitates and to prohibit their evolution during measurements. Tracer measurements with the radioisotope ${}^{57}\mathrm{Co}$ reveal a remarkable nonmonotonic dependence of the diffusion rates on the annealing temperature. It has been fully explained by an ab initio informed phenomenological model that considers the elastic stress around the coherent and noncoherent precipitates.

Solution-processed metal halide perovskites are attracting significant attention as high-performance photovoltaic materials. The electron-phonon interactions in lead halide perovskites are currently in the focus of interest because these interactions play an important role to the observed superior material properties and high device performance. Here, we investigated the electron-phonon coupling in lead-free $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Sn}{\mathrm{I}}_3$ thin films by analyzing the temperature-dependence of the photoluminescence (PL) spectra. The PL linewidth shows a superlinear dependence on the temperature, which suggests that the electron–longitudinal-optical (LO) phonon interaction is the dominating broadening mechanism of the optical transitions in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Sn}{\mathrm{I}}_3$ near room temperature. Furthermore, from the analysis of the low-energy tail of the PL spectra by using the van Roosbroeck-Shockley relation, we determined the Urbach energy below the band gap and the optical-phonon energy that contributes to the optical absorption. This phonon energy coincides with the LO phonon energy determined from the PL width analysis. Our work quantitatively verifies that the electrons in tin halide perovskites efficiently couple with optical phonons, and demonstrates that their intrinsic optical properties at room temperature are comparable to those of lead halide perovskites implemented in high-efficiency solar cells.

Necking instabilities, in which tensile (extensional) deformation localizes into a small spatial region, are generic failure modes in elastoviscoplastic materials. Materials in this very broad class—including amorphous, crystalline, polycrystalline, and other materials—feature a predominantly elastic response at small stresses, plasticity onset at a rather well-defined yield stress, and rate dependence. Necking instabilities involve a unique coupling between the system's geometry and its constitutive behavior. We consider generic elastoviscoplastic constitutive relations involving an internal-state field, which represents the structural evolution of the material during plastic deformation, and study necking in the long-wavelength approximation (sometimes termed the lubrication or slender-bar approximation). We derive a general expression for the largest time-dependent eigenvalue in an approximate WKB-like linear stability analysis, highlighting various basic physical effects involved in necking. This expression is then used to propose criteria for the onset of necking and more importantly for the emergence of strong localization. Applications to strain-softening amorphous plasticity, in the framework of the shear-transformation-zone model, and to strain-hardening crystalline/polycrystalline plasticity, in the framework of the Kocks-Mecking model, are presented. These quantitative analyses of widely different material models support the theoretical predictions, most notably for the strong localization during necking.

We have studied NpPdSn by means of the heat capacity, electrical resistivity, Seebeck and Hall effect, ${}^{237}\mathrm{Np}$ Mössbauer spectroscopy, and neutron diffraction measurements in the temperature range 2–300 K and under magnetic fields up to 14 T. NpPdSn orders antiferromagnetically below the Néel temperature $T_N=19\mathrm{K}$ and shows localized magnetism of ${\mathrm{Np}}^{3+}$ ion with a doubly degenerate ground state. In the magnetic state the electrical resistivity and heat capacity are characterized by electron-magnon scattering with spin-waves spectrum typical of anisotropic antiferromagnets. An enhanced Sommerfeld coefficient and typical behavior of magnetorestistivity, Seebeck and Hall coefficients are all characteristic of systems with strong electronic correlations. The low temperature antiferromagnetic state of NpPdSn is verified by neutron diffraction and ${}^{237}\mathrm{Np}$ Mössbauer spectroscopy and possible magnetic structures are discussed.

Finding a new derivative structure of FeSe-based superconductors with a high superconducting transition temperature ($T_c$) is of great significance. Here, in this work, an organic-ion-[cetyltrimethyl ammonium (${\mathrm{CTA}}^{+}$)]-intercalated FeSe-based superconductor ${\left(\mathrm{CTA}\right)}_{0.3}\mathrm{FeSe}$ with $T_c$ as high as 45 K is synthesized by an electrochemical intercalation method. The as-prepared ${\left(\mathrm{CTA}\right)}_{0.3}\mathrm{FeSe}$ has a hybrid crystal structure consisting of double layers of ${\mathrm{CTA}}^{+}$ and one layer of FeSe, which is consistent with a lateral-bilayer model. The superconductivity of ${\left(\mathrm{CTA}\right)}_{0.3}\mathrm{FeSe}$ is confirmed by magnetic susceptibility. Furthermore, a negative pressure effect on superconductivity $d{T}_c/dP=-5$ K/GPa is observed. This is the first pure organic-ion-intercalated FeSe-based superconductor with a high $T_c$ and high crystallinity. Our work offers a new method to modify the interface interaction between the FeSe layer and the intercalator, and sheds new light on finding new FeSe-based superconductors with higher $T_c$.

We study localization of waves in a one-dimensional disordered metamaterial of bilayers composed of thin fixed length scatterers placed randomly along a homogenous medium. As an interplay between order and disorder, we identify a new regime of strong disorder where the localization length becomes independent of the amount of disorder but depends on the frequency of the wave excitation and on the properties of the fixed length scatterer. As an example of a naturally occurring nearly one-dimensional disordered bilayer, we calculate the wavelength-dependent reflection spectrum for Koi fish using the experimentally measured parameters and find that the main mechanisms for the emergence of their silver structural coloration can be explained through the phenomenon of localization of light in the regime of strong disorder discussed above. Finally, we show that, by tuning the thickness of the fixed length scatterer, the above design principles could be used to engineer disordered metamaterials which selectively allow harmonics of a fundamental frequency to be transmitted in an effect which is similar to the insertion of a half-wave cavity in a quarter-wavelength stack. However, in contrast to the Lorentzian resonant peak of a half-wave cavity, we find that our disordered layer has a Gaussian line shape whose width becomes narrower as the number of disordered layers is increased.

Degenerate conduction-band minima, or “valleys”, in materials such as Si, AlAs, graphene, and ${\mathrm{MoS}}_2$ allow them to host two-dimensional electron systems (2DESs) that can access a valley degree of freedom. These multivalley 2DESs present exciting opportunities for both pragmatic and fundamental research alike because not only are they a platform for valleytronic devices, but they also provide a tool to tune and investigate the properties of complex many-body ground states. Here, we report ultra-high-quality, modulation-doped AlAs quantum wells containing 2DESs that occupy two anisotropic valleys and have electron mobilities peaking at $2.4\times {10}^6{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at a density of $2.2\times {10}^{11}{\mathrm{cm}}^{-2}$. This is more than an order of magnitude improvement in mobility over previous results. The unprecedented quality of our samples is demonstrated by magneto-transport data that show high-order fractional quantum Hall minima up to the Landau level filling $\nu =8/17$, and even the elusive $\nu =1/5$ quantum Hall state.

A theoretical examination of the thermal expansion behavior of a variety of metals is conducted using a combination of nonlinear elasticity theory and first-principles calculations that is suitable for high throughput computation. Results of this method show good agreement with experimental values. This method is then used to better understand the low thermal expansion behavior of gum metal by comparing the thermal expansion tensor of ${\mathrm{Ti}}_3\mathrm{Nb}$ austenitic ($\beta$) and martensitic (${\alpha }^{″}$) gum metal approximants. The thermal expansion coefficient of $\beta$ is found to be in agreement with experimental results for that of annealed gum metal. The thermal expansion tensor of the ${\alpha }^{″}$ phase is shown to be highly anisotropic and exhibit negative thermal expansion along ${\langle 110\rangle }_{\beta }$. It is demonstrated that the thermal expansion of the two-phase system, $\beta +{\alpha }^{″}$, can be estimated using the rule of mixing. By applying this averaging scheme and allowing a texturing along $\langle 110\rangle {\left\{001\right\}}_{\beta }$ in tandem with the growth of the ${\alpha }^{″}$ phase, values for the thermal expansion similar to that seen in cold-rolled gum metal are calculated.

We report on changes in the thermal conductivity of solid-state synthesized $\mathrm{M}{\mathrm{n}}_{x}M\mathrm{Ge}$ ($M=\mathrm{Co}$, Ni, $0.98<x<1.02$) alloys through their temperature-induced martensitic structural transition. The thermal conductivity is measured by time-domain thermoreflectance. $\mathrm{M}{\mathrm{n}}_{1.014}\mathrm{NiGe}$ exhibits an increase in thermal conductivity from 11 to $15.5\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ from approximately 575 to 625 K, and $\mathrm{M}{\mathrm{n}}_{1.007}\mathrm{CoGe}$ exhibits an increase in thermal conductivity from 7 to $8.5\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ from 500 to 550 K. In $\mathrm{M}{\mathrm{n}}_x\mathrm{NiGe}$, the transition temperature and the magnitude of the change in thermal conductivity are strongly dependent on the alloy composition. Our study advances the fundamental understanding of the thermal transport properties in the $\mathrm{M}{\mathrm{n}}_{x}M\mathrm{Ge}\left(M=\mathrm{Co},\mathrm{Ni}\right)$ family of alloys and opens a new direction in the search for solid-state phase transition materials with potential applications as thermal regulators.