We calculate using the Boltzmann transport theory the density-dependent mobility of two-dimensional (2D) electrons in GaAs, SiGe, and AlAs quantum wells as well as of 2D holes in GaAs quantum wells. The goal is to precisely understand the recently reported breakthrough in achieving a record 2D mobility for electrons confined in a GaAs quantum well. Comparing our theory with the experimentally reported electron mobility in GaAs quantum wells, we conclude that the mobility is limited by unintentional background random charged impurities at an unprecedented low concentration of $\sim {10}^{13}{\mathrm{cm}}^{-3}$. We find that this same low level of background disorder should lead to 2D GaAs hole and 2D AlAs electron mobilities of $\sim {10}^7$ and $\sim 4\times {10}^7{\mathrm{cm}}^2/\text{V}\text{s}$, respectively, which are much higher theoretical limits than the currently achieved experimental values in these systems. We therefore conclude that the current GaAs hole and AlAs electron systems are much dirtier than the state-of-the-art 2D GaAs electron systems. We present theoretical results for 2D mobility as a function of density, effective mass, quantum-well width, and valley degeneracy, comparing with experimental data.

It is known that the temperature-dependent thermal conductivity of conventional crystals follows the classical 1/$T$ trend due to the dominant umklapp phonon-phonon scattering at high enough temperatures. However, the thermal conductivity of many crystalline metal-organic frameworks is very low and shows a weak temperature dependence when all the vibrational modes are occupied. By studying two metal-organic frameworks, i.e., zeolitic imidazolate framework-4 (ZIF-4) and crystal zeolitic imidazolate framework-62 (ZIF-62), we computationally prove that the ultralow thermal conductivity in ZIF-4 and ZIF-62 is resulting from the strong scattering among the vibrations due to the large mass difference between the metal atom and the organic sites and the large diffusion of organic sites. Unlike only propagating vibrational modes, i.e., phonons, existing in the conventional crystalline Si, our mean free path spectrum analysis uncovers that both propagating and nonpropagating anharmonic vibrational modes exist and contribute largely to thermal conductivity in ZIF-4 and ZIF-62. The obtained weak temperature dependence of thermal conductivity is found to stem from the temperature dependencies of those two kinds of vibrations. Our study provides a fundamental understanding of thermal transport in metal-organic frameworks and will guide the design of thermal-related applications, e.g., inflammable gas storage, chemical catalysis, and solar thermal conversion.

The P2-layered oxide ${\mathrm{NaMnO}}_2$ is known as a cheap and high-capacity material for secondary batteries, but has been limited in application due to the large Jahn-Teller lattice distortion. Through the highly accurate hybrid functional method (HSE06), we computationally evaluate the doping influences on the lattice distortions, stabilities, electronic structures, redox potentials, and diffusion mechanisms. Our calculations indicate that dopants not only reduce the lattice distortion degree, especially for Li, Mg, Ti, and V cases, but also increase the stability of the structure, implying the dopants would alleviate the Jahn-Teller lattice distortion. At low Na concentrations, the Li dopant preferably diffuses out of the ${\mathrm{MnO}}_2$ layer, but hardly moves to the Na layer, suggesting that P2-layered oxides can prevent the dopant's migrations during the Na extractions. At full Na concentrations, all the considered dopants, except for Ti and V, have a small effect on the redox potential. The effect on the diffusion mechanism is described through the diffusion of a Na ion–polaron complex near the dopant's environments. Fe, Mg, Ti, and Cr dopants can hinder the Na ion-polaron complex diffusion with significantly higher activation energies, respectively, while the Al dopant almost remains the activation energy as well as the perfect structure. However, Li, V, Co, and Ni dopants benefit from such complex diffusion with much lower activation energy so the ion diffusivities increase significantly. It is found that the doping influence on the activation energy for Na ion diffusion is associated with the $M$–O bond change and charges of the neighboring dopants.

Scattering of carriers with ionized impurities governs charge transport in doped semiconductors. However, electron interactions with ionized impurities cannot be fully described with quantitative first-principles calculations, so their understanding relies primarily on simplified models. Here we show an ab initio approach to compute the interactions between electrons and ionized impurities or other charged defects. It includes the short- and long-range electron-defect ($e$-d) interactions on equal footing and allows for efficient interpolation of the $e$-d matrix elements. We combine the $e$-d and electron-phonon interactions in the Boltzmann transport equation to compute the carrier mobilities in doped silicon over a wide range of temperature and doping concentrations, seamlessly spanning the defect- and phonon-limited transport regimes. The individual contributions of the defect- and phonon-scattering mechanisms to the carrier relaxation times and mean-free paths are analyzed. Our method provides a powerful tool to study electronic interactions in doped materials. It broadens the scope of first-principles transport calculations, enabling studies of a wide range of doped semiconductors and oxides with application to electronics, energy, and quantum technologies.

The zero K formation energies of metal and nitrogen vacancies in several (Ti,Al)N alloys and at the (001) (Ti,Al)N/AlN interface are obtained in ab initio supercell calculations. The dependence of the formation energies of metal vacancies on their local environment and type are analyzed and explained in terms of effective cluster interactions for unrelaxed calculations. The common trend for all investigated types of vacancies is that their formation energy increases with the number of Al nearest neighbors if local lattice relaxations are not allowed. However, local lattice relaxations produce a dramatic effect especially in the case of metal vacancies leading to a complicated nonlinear dependence on the local environment indicating the existence of strong multisite strain-induced interactions.

We investigate the electrical and thermal transport coefficients of an emergent transition metal tri-chalcogenide, namely, ${\mathrm{TiS}}_3$. We implement an accurate description of the electron-phonon coupling and thus of the energy dependence of the relaxation time, which goes beyond the commonly used approximations. We find that standard calculation methods fail in accurately describing the thermoelectric transport properties of ${\mathrm{TiS}}_3$ and one needs to go beyond standard approximations to correctly investigate the thermoelectric performance of ${\mathrm{TiS}}_3$ and other 2D materials. By applying state-of-the-art ab initio methods, we conclude that ${\mathrm{TiS}}_3$ stands as a potential candidate for thermoelectric applications.

We present a first-principles computational study of the thermodynamics of carbon defects in hexagonal boron nitride (hBN). The defects considered are carbon monomers, dimers, trimers, and larger carbon clusters, as well as complexes of carbon with vacancies, antisites, and substitutional oxygen. Our calculations show that monomers $({\text{C}}_{\text{B}},$ ${\text{C}}_{\text{N}})$, dimers, trimers, and ${\text{C}}_{\text{N}}{\text{O}}_{\text{N}}$ pairs are the most prevalent species under most growth conditions. Compared with these defects, larger carbon clusters, as well as complexes of carbon with vacancies and antisites, occur at much smaller concentrations. Our results are discussed in view of the relevance of carbon defects in single-photon emission in hBN.

Prior to the superconducting transition at $T_{\mathrm{c}}\approx 1.5\mathrm{K}$, the pyrite-type ${\mathrm{CuS}}_2$ undergoes a charge-density-wave (CDW) transition at $T_{\mathrm{s}}\approx 150\mathrm{K}$ at ambient pressure. Here we studied the interplay between CDW and superconductivity (SC) in ${\mathrm{CuS}}_2$ by tracking their evolutions with pressure via detailed measurements of resistivity and magnetic susceptibility under hydrostatic pressures up to 6.6 GPa. The high-quality ${\mathrm{CuS}}_2$ samples used in the present study were synthesized under high-pressure and high-temperature conditions, and the formation of the $2\times 2\times 2$ supercell associated with the commensurate CDW order below $T_{\mathrm{s}}$ was confirmed by low-temperature transmission electron microscope. Our high-pressure results uncover a pressure-driven superconducting dome with an enhanced ${T_{\mathrm{c}}}^{max}\sim 2.3\mathrm{K}$ at $\sim 1.8\mathrm{GPa}$ accompanying the gradual suppression of the CDW order. This work thus demonstrates the competing nature between CDW and SC in the pyrite-type superconductor ${\mathrm{CuS}}_2$.

The electronic structures of semiconducting heterojunctions are critically dependent on composition including the presence and concentrations of dopants, both intended and unintended. Dopant profiles in the interfacial region can have major effects on band energies which in turn drive transport properties. Here we use core-level photoelectron line shapes excited with hard x rays to extract information about electric fields resulting from internal charge transfer in epitaxial ${\mathrm{La}}_{0.03}{\mathrm{Sr}}_{0.97}{\mathrm{Zr}}_x{\mathrm{Ti}}_{1\text{–}x}{\mathrm{O}}_3/\mathrm{Ge}\left(001\right)$ $\left(0.1\le x\le 0.7\right)$ heterostructures. Experiments were carried out for heterojunctions involving both $n$- and $p$-type Ge substrates. These heterojunctions were not amenable to electronic characterization of all regions by transport measurements because the doped substrates act as electrical shunts, precluding probing the more resistive films and masking interface conductivity. However, the core-level line shapes were found to be a rich source of information on built-in potentials that exist throughout the heterostructure, and yielded valuable insight into the impact of band bending on band alignment at the buried interfaces. The electronic effects expected for Ge with uniform $n$- and $p$-type doping are eclipsed by those of unintended oxygen dopants in the Ge near the interface. This study illustrates the power of hard x-ray photoemission spectroscopy and related modeling to determine electronic structure in material systems for which insight from traditional transport measurements is limited.

In this work, we propose a set of nonconventional graphyne lattices inspired by previously studied $s{p}^2$ nanocarbon sheets, known as haeckelites, and we study their electronic properties by density functional theory methods. Two of these systems are solely composed of five- and seven-membered rings in rectangular lattices, while a third system features a mixture of five-, six-, and seven-polygonal rings in a hexagonal network. We show that these systems exhibit dynamical, thermal, and mechanical stability as demonstrated by phonon band structures, molecular dynamics simulations, and by the computation of their elastic constants. The three lattices exhibit a metallic nature similar to their full-$s{p}^2$ counterparts. However, the graphynes' electronic properties are strongly different from those of the original haeckelites, as the largest contribution to their frontier states comes from the $sp$ hybridized atoms. Furthermore, the three proposed systems feature different electronic signatures from each other, as their frontier states feature different energy ranges for forbidden electronic states, and one of them features a set of Dirac-like cones.

First-order metal-insulator transition (MIT) observed in strongly correlated systems such as vanadium dioxide $\left({\mathrm{VO}}_2\right)$ holds potential in electronics, energy, to optical applications. Starting from a vanadium diselenide $\left({\mathrm{VSe}}_2\right)$ bulk crystal, we demonstrated a direct surface conversion from ${\mathrm{VSe}}_2$ to ${\mathrm{VO}}_2$ via laser exposure in ambient condition. The process generates defects, and the heat from the laser promotes oxidation forming ${\mathrm{VO}}_x$. Raman spectra at room temperature suggest the resulting oxide formed is monoclinic (M1) ${\mathrm{VO}}_2$. Above the transition temperature $\left(T_C\right)$, all the phonon modes are damped indicating formation of the rutile phase (metallic). Photoluminescence (PL) intensity enhancement and peak shifts observed at $T_C$ suggest correlation to the band structure transformation. In addition, we observed electrically induced MIT in our lateral ${\mathrm{VSe}}_2\text{−}{\mathrm{VO}}_x$ heterojunction device.

Motivated by the recently synthesized insulating nickelate ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, which has been reported to have an unusual noncollinear magnetic order of ${\mathrm{Ni}}^{2+}S=1$ moments with a nontrivial angle between adjacent spins, we construct an effective spin-1 model on the honeycomb lattice, with the exchange parameters determined with the help of first-principles electronic-structure calculations. The resulting bilinear-biquadratic model, supplemented with the realistic crystal-field induced anisotropy, favors the collinear Néel state. We find that the crucial key to explaining the observed noncollinear spin structure is the inclusion of the Dzyaloshinskii–Moriya (DM) interaction between the neighboring spins. By performing variational mean-field and linear spin-wave theory (LSWT) calculations, we determine that a realistic value of the DM interaction $D\approx 2.78$ meV is sufficient to quantitatively explain the observed angle between the neighboring spins. We furthermore compute the spectrum of magnetic excitations within the LSWT and random-phase approximation, which should be compared to future inelastic neutron measurements.

We study the emergence of magnetism and its interplay with structural properties in a two-dimensional molecular crystal of cyclacenes, using density functional theory (DFT). Isolated cyclacenes with an even number of fused benzenes host two unpaired electrons in two topological protected zero modes, at the top and bottom carbon rings that form the molecule. We show that, in the gas phase, electron repulsion promotes an open-shell singlet with strong intramolecular antiferromagnetic exchange. We consider a closed packing triangular lattice crystal phase and we find a strong dependence of the band structure and magnetic interactions on the rotation angle of the cyclacenes with respect to the crystal lattice vectors. The orientational ground state maximizes the intermolecular hybridization, yet local moments survive. Intermolecular exchange is computed to be antiferromagnetic, and DFT predicts a broken symmetry ${120}^{\circ }$ spin phase reflecting the frustration of the intermolecular spin coupling. Thus, the cyclacene crystal realizes a bilayer of two antiferromagnetically coupled $S=1/2$ triangular lattices. Our results provide a bottom-up route towards carbon based strongly correlated platforms in two dimensions.

In steels, the interaction between screw dislocations and carbon solutes has a great influence on the yield strength. Fe-C potentials used in molecular dynamics (MD) simulations yield a poor description of screw dislocation properties—their core structure and Peierls barrier—compared to ab initio calculations. Here we combine two EAM potentials from the literature, which greatly improves dislocation property accuracy in FeC alloys. Using this hybrid potential, MD simulations of dislocation glide in random solid solutions confirm a powerful solute strengthening, caused by complex interaction processes. We analyze these processes in a model geometry, where a row of carbon atoms is inserted in the dislocation core with varying separations. We use a combination of MD simulations, minimum-energy path calculations, and a statistical model based on the harmonic transition state theory to explain the strengthening induced by carbon. We unveil that carbon disrupts the glide process, as unpinning requires the successive nucleation of two kink pairs. When solute separation is below about 100 Burgers vectors, the activation enthalpy of both kink pairs are markedly increased compared to pure iron, resulting in a strong dependence of the unpinning stress on solute spacing. Our simulations also suggest an effect of carbon spacing on the kink-pair activation entropy. This work provides elementary processes and parameters that will be useful for larger-scale models and, in particular, kinetic Monte Carlo simulations.

The microstructure of advanced high-strength steels often shows a sensitive dependence on alloying. For example, adding Cr to improve the corrosion resistance of medium-Mn steels also enhances the precipitation of carbides. The current study focuses on the behavior of H in such complex multicomponent carbides by employing different methodological strategies. We systematically analyze the impact of Cr, Mn, and Fe using density functional theory (DFT) for two prototype precipitate phases, ${\mathrm{M}}_3\mathrm{C}$ and ${\mathrm{M}}_{23}{\mathrm{C}}_6$, where M represents the metal sublattice. Our results show that the addition of these alloying elements yields strong nonmonotonic chemical trends for the H solubility. We identify magnetovolume effects as the origin for this behavior, which depend on the considered system, the sites occupied by H, and short- vs long-range interactions between H and the alloying elements. We further show that the H solubility is directly correlated with the occupation of its nearest-neighbor shells by Cr and Mn. Based on these insights, DFT data from H containing binary-metal carbides are used to design a ridge regression based model that predicts the solubility of H in the ternary-metal carbides (Fe-Cr-Mn-C).

We present a combined experimental and theoretical analysis of the evolution of the near-band-gap electronic and optical properties of ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x-y}{\mathrm{Sn}}_y$ alloys lattice-matched to Ge and GaAs substrates. We perform photoreflectance (PR) and photoluminescence (PL) measurements on ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x-y}{\mathrm{Sn}}_y$ epitaxial layers grown via chemical vapor deposition for Si (Sn) compositions up to $x=9.6$% ($y=2.5$%). Our measurements indicate the presence of an indirect fundamental band gap, with PL observed $\approx 200\text{–}250$ meV lower in energy than the direct $E_0$ transition identified by PR measurements. The measured PL is Ge-like, suggesting that the alloy conduction band (CB) edge is primarily derived from the Ge $L$-point CB minimum. Interpretation of the PR and PL measurements is supported by atomistic electronic structure calculations. Effective alloy band structures calculated via density functional theory confirm the presence of an indirect fundamental band gap, and reveal the origin of the observed inhomogeneous broadening of the measured optical spectra as being alloy-induced band hybridization occurring close in energy to the CB edge. To analyze the evolution of the band gap, semiempirical tight-binding (TB) calculations are employed to enable calculations for large supercell sizes. TB calculations reveal that the alloy CB edge is hybridized in nature, consisting at low Si and Sn compositions of an admixture of Ge $L$-, $\mathrm{\Gamma }$-, and $X$-point CB edge states, and confirm that the alloy CB edge retains primarily Ge $L$-point CB edge character. Our experimental measurements and theoretical calculations confirm a direct transition energy close to 1 eV in magnitude for Si and Sn compositions $x=6.8\%$–9.6% and $y=1.6\%$–2.2%.

The Raman spectra of boron carbide presented by Roma et al. are severely impaired by surface excitations. The reason is the inappropriate energy of the laser used in their spectrometer. Thus, the claimed correlation between point defects in the structure and the resonance peaks in their Raman spectra is highly questionable.

In response to Werheit's Comment, we first discuss the following three points: (i) Attribution of the Raman bands with the help of calculations based on the density functional perturbation theory (DFPT), (ii) similarity of the Raman spectrum of boron carbide to the Raman spectrum of $\alpha$ rhombohedral boron, and (iii) dependence of the experimental Raman spectrum on the frequency of the excitation laser. Central to the point at stake, we also present a Raman spectrum computed for the first time on a 405-atom unit-cell of boron carbide, where the simulation cell accounts for the substitutional disorder of the polar carbon atom in the six atomic positions that are equivalent in the trigonal symmetry. We thus obtain a theoretical spectrum whose agreement with experiment is unprecedented in boron carbide, all of the features being present, with however negligible intensities for the doublet at $270/320{\mathrm{cm}}^{-1}$. We argue that the doublet intensity seen in experiments is not purely vibrational—as computed at mechanical equilibrium—and that FT-Raman spectroscopy taken at $1.06\mu \mathrm{m}$ lacks the accuracy that is necessary to study vibrational modes of boron carbides.

Using Labusch-type solid solution strengthening models parameterized with DFT-computed solute-dislocation interaction energies, we perform a computational search for 63 solutes across the periodic table to find those that lower anisotropy ratios (non-basal to basal CRSS) of magnesium potentially increasing its ductility per the von Mises criterion. For this purpose, we compute changes in strength for solutes as a function of composition and temperature, and compute anisotropy ratios for solutes that include both rare earth and non-rare earth elements. We specifically focus on solute-dislocation interaction energies in the following DFT-optimized dislocations as representative of three non-basal plastic deformation modes: $\langle c+a\rangle$ edge, $\left(10\overline{1}2\right)$ tension twinning edge, and the $\left(10\overline{1}1\right)$ compression twinning edge. We find that solute-induced changes in non-basal deformation modes can be approximated using a second-order polynomial in the size misfit of the solutes, which permits rapid screening of solutes. Our approach to identify solutes known to improve strengthening incorporates solute solubility, and suggests other solutes that not have been previously explored for strengthening. The 8 rare-earth solutes that our method suggests as the best, ordered by increasing anisotropy ratios at their optimal concentrations, are: Gd, Tb, Dy, Nd, Ho, Er, Tm, and Yb. The 12 non-rare-earth solutes that our method suggests as the best, ordered by increasing anisotropy ratios, are: Y, Mn, Sc, Pb, Ca, Ag, Bi, Tl, Zn, Li, Ga, and Al. Of these, Gd, Nd, Er, Yb, Y, Mn, Ca, Zn, Li, and Al are used in commercial Mg alloys.