We present a general scheme of range-separated hybrid functionals in which the mixing parameters of Fock exchange are fully nonempirical and determined solely from the dielectric function. We show that the full dielectric dependence leads to an unscreened Fock exchange in the short range, while in the long range the Fock exchange is correctly screened by the macroscopic dielectric constant. The range separation is obtained by fitting to the calculated static dielectric function in the long-wavelength limit. The resulting hybrid functional accurately accounts for electronic and structural properties of various semiconductors and insulators spanning a wide range of band gaps.

The layered $\beta \text{−}\mathrm{NaMn}{\mathrm{O}}_2$, a promising Na-ion energy-storage material, has been investigated for its triangular lattice capability to promote complex magnetic configurations that may release symmetry restrictions for the coexistence of ferroelectric and magnetic orders. The complexity of the neutron powder diffraction patterns underlines that the routinely adopted commensurate structural models are inadequate. Instead, a single-phase superspace symmetry description is necessary, demonstrating that the material crystallizes in a compositionally modulated $\mathbf{q}=\left(0.077\right(1)$, 0, 0) structure. Here $\mathrm{M}{\mathrm{n}}^{3+}$ Jahn-Teller distorted $\mathrm{Mn}{\mathrm{O}}_6$ octahedra form corrugated layer stacking sequences of the $\beta \text{−}\mathrm{NaMn}{\mathrm{O}}_2$ type, which are interrupted by flat sheets of the α-like oxygen topology. Spontaneous long-range collinear antiferromagnetic order, defined by the propagation vector $\mathbf{k}=(1/2,1/2,1/2)$, appears below $T_{\mathrm{N}1}=200\mathrm{K}$. Moreover, a second transition into a spatially modulated proper-screw magnetic state (k ± q) is established at $T_{\mathrm{N}2}=95\mathrm{K}$, with an antiferromagnetic order parameter resembling that of a two-dimensional (2D) system. The evolution of ${}^{23}\mathrm{Na}$ NMR spin-lattice relaxation identifies a magnetically inhomogeneous state in the intermediate $T$ region $\left(T_{\mathrm{N}2}<T<T_{\mathrm{N}1}\right)$, while its strong suppression below $T_{\mathrm{N}2}$ indicates that a spin gap opens in the excitation spectrum. High-resolution neutron inelastic scattering confirms that the magnetic dynamics are indeed gapped (Δ ∼ 5 meV) in the low-temperature magnetic phase, while simulations on the basis of the single-mode approximation suggest that Mn spins residing on adjacent antiferromagnetic chains, establish sizable 2D correlations. Our analysis points out that novel structural degrees of freedom promote cooperative magnetism and emerging dielectric properties in this nonperovskite type of manganite.

The copper zinc tin sulfide (CZTS) compound is a promising candidate as an alternative absorber material for thin-film solar cells. In this study, we investigate the direct formation of $\mathrm{C}{\mathrm{u}}_{1.92}\mathrm{ZnS}{\mathrm{n}}_x\left(\mathrm{S}{\mathrm{b}}_{1-x}\right){\mathrm{S}}_4$ compounds $\text{[CZT}\left(A\right)\text{S]}$, with $x=1$, 0.85, 0.70, and 0.50, via a mechanochemical synthesis (MCS) approach, starting from powders of the corresponding metals, zinc sulfide, and sulfur. The thermal stability of the $\text{CZT}\left(A\right)\text{S}$ compounds was evaluated in detail by in situ synchrotron high-energy x-ray diffraction measurements up to 700 °C. The $\text{CZT}\left(A\right)\text{S}$ compounds prepared via MCS revealed a sphalerite-type crystal structure with strong structural stability over the studied temperature range. The contribution of the MCS to the formation of such a structure at room temperature is analyzed in detail. Additionally, this study provides insights into the MCS of CZTS-based compounds: the possibility of a large-scale substitution of Sn by Sb and the production of single phase $\text{CZT}\left(A\right)\text{S}$ with a Cu-poor/Zn-poor composition. A slight increase in the band gap from 1.45 to 1.49–1.51 eV was observed with the incorporation of Sb, indicating that these novel compounds can be further explored for thin-film solar cells.

In crystalline solids, grain boundaries (GBs) play a significant role in determining a large number of material properties. The design and synthesis of special GBs has been a long-standing challenge in materials science and engineering. Here we demonstrate a mechanism to produce special GBs. Unique multigrain structures can be obtained through cyclic, diffusionless phase transformations under external fields, with all GBs being coherent special GBs. The crystallographic character of the GBs produced in this way is dictated by the broken symmetry during the phase transformations, while the topology of the GB network is determined by the geometric compatibility and self-organization of the multigrain structures. Such a mechanism not only suggests an alternative method of GB engineering, but also reveals a fundamental relationship between special GBs and phase transformations from a crystallographic point of view.

Hybridization of semiconducting and magnetic materials into a single heterostructure is believed to be potentially applicable to the design of functional spintronic devices. In the present work we report epitaxial stabilization of four magnetically ordered iron oxide phases (${\mathrm{Fe}}_3{\mathrm{O}}_4, \gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3, \alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and most exotic metastable $\varepsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$) in the form of nanometer-sized single crystalline films on GaN(0001) surface. The epitaxial growth of four distinctly different iron oxide phases is achieved by a single-target pulsed laser deposition technology on a GaN semiconductor substrate widely used for electronic device fabrication. The discussed iron oxides belong to a family of simple formula magnetic materials exhibiting a rich variety of outstanding magnetic properties including peculiar Verwey and Morin phase transitions in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ and $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and multiferroic behavior in metastable magnetically hard $\varepsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ ferrite. The physical reasons standing behind the nucleation of a particular phase in an epitaxial growth process is discussed in the present paper deserving interest from the fundamental point of view. The practical side of the presented study is to exploit the tunable polymorphism of iron oxides in order to create the ferroic-on-semiconductor heterostructures usable in spintronic devices. By application of a wide range of experimental techniques the surface morphology, crystalline structure and electronic and magnetic properties of the single phase iron oxide epitaxial films on GaN have been studied. A comprehensive comparison has been made to the properties of the same ferrite materials in the bulk and nanostructured form reported by other research groups.

The growth of large area single-layer graphene (1-LG) is studied using ambient pressure chemical vapor deposition on single-crystal Ni(111), Ni(110), and Ni(100). By varying both the furnace temperature in the range of 800–1100 °C and the gas flow through the growth chamber, uniform, high-quality 1-LG is obtained for Ni(111) and Ni(110) single crystals and for Ni(100) thin films. Surprisingly, only multilayer graphene growth could be obtained for single-crystal Ni(100). The experimental results are analyzed to determine the optimum combination of temperature and gas flow. Characterization with optical microscopy, Raman spectroscopy, and optical transmission support our findings. Density-functional theory calculations are performed to determine the energy barriers for diffusion, segregation, and adsorption, and model the kinetic pathways for formation of different carbon structures on the low-index surfaces of Ni.

We use alchemical first-order perturbations for the rapid yet robust prediction of band structures. The power of the approach is demonstrated for the design challenge of finding ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ semiconductor alloys with large direct band gap using computational alchemy within a genetic algorithm. Dozens of crystal polymorphs are identified for $x>\frac{2}{3}$ with direct band gaps larger than 2 eV according to HSE approximated density functional theory. Based on a single generalized gradient approximated density functional theory band-structure calculation of pure GaAs we observe convergence after visiting only $\sim 800$ crystal candidates. The general applicability of alchemical gradients is demonstrated for band-structure estimates in III-V and IV-IV crystals as well as for ${\mathrm{H}}_2$ uptake in Sr and Ca-alanate crystals.

The permeation of water between neighboring graphene oxide (GO) flakes, i.e., 2D nanochannels, are investigated using a simple model for the GO membrane. We simulate the hydrophilic behavior of nanocapillaries and study the effect of surface charge on the dynamical properties of water flow and the influence of ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions on water permeation. Our approach is based on extensive equilibrium molecular dynamics simulations to obtain a better understanding of water permeation through charged nanochannels in the presence of ions. We found significant change in the slippage dynamics of confined water such as a profound increase in viscosity/slip length with increasing charges over the surface. The slip length decreases one order of magnitude (i.e., 1/30) with increasing density of surface charge, while it increases by a factor of 2 with ion concentration. We found that commensurability induced by nanoconfinement plays an important role on the intrinsic dynamical properties of water.

X-ray photoelectron spectroscopy of graphene is important both for its characterization and as a model for other carbon materials. Despite great recent interest, the intrinsic photoemission of its single layer has not been unambiguously measured, nor is the layer dependence in free-standing multilayers accurately determined. We combine scanning transmission electron microscopy and Raman spectroscopy with synchrotron-based scanning photoelectron microscopy to characterize the same areas of suspended graphene samples down to the atomic level. This allows us to assign spectral signals to regions of precisely known layer number and purity. The core level binding energy of the monolayer is measured at 284.70 eV, thus 0.28 eV higher than that of graphite, with intermediate values found for few layers. This trend is reproduced by density functional theory with or without explicit van der Waals interactions, indicating that intralayer charge rearrangement dominates, but in our model of static screening the magnitudes of the shifts are underestimated by half.

The dc spin-to-charge conversions of tantalum (Ta) in $\mathrm{Ta}/\mathrm{C}{\mathrm{o}}_{40}\mathrm{F}{\mathrm{e}}_{40}{\mathrm{B}}_{20}$ bilayer structures are investigated utilizing spin pumping and inverse spin Hall effects (ISHE). From Ta thickness $\left(t_{\mathrm{Ta}}\right)$-dependent resistivity and x-ray diffraction measurements, we found that Ta films, below 30 nm in thickness, are $\beta$-phase dominated. The damping enhancement shows a fast increase with $t_{\mathrm{Ta}}$ when $t_{\mathrm{Ta}}<1\mathrm{nm}$ and reaches a saturation value at $\sim 1.5\mathrm{nm}$. The ISHE induced charge voltages have opposite signs for Ta and Pt. From $t_{\mathrm{Ta}}$-dependent spin pumping produced ISHE voltage and precession angle measurements, the normalized spin-charge conversion signal is found to increase with $t_{\mathrm{Ta}}$ and saturate at $\sim 15\mathrm{nm}$. Our findings can be understood with a recently developed theory [Phys. Rev. Lett. 114, 126602 (2015)], which includes spin backflow and a spin loss at the interface. With a fitted spin loss factor of $0.02\pm 0.02$, we extract the spin Hall angle and spin diffusion length of high resistivity Ta to be ${\theta }_{\mathrm{SH}}=-0.0062\pm 0.001$ and ${\lambda }_{\mathrm{sd}}=5.1\pm 0.6\mathrm{nm}$, respectively.

We demonstrate an extreme variability and tunability of the molecular gyromagnetic coupling $g$-tensor with respect to the geometric and electronic structure in a much studied class of organic semiconductors (OSCs). This class of OSCs is composed of a structural theme of phenyl- and chalcogenophene (group XVI element containing, five-membered) rings and alkyl functional groups, and it forms the basis of several intensely studied high-mobility polymers and molecular OSCs. We show how in this class the $g$-tensor shifts, $\mathrm{\Delta }g$, are determined by the effective molecular spin-orbit coupling (SOC), defined by the overlap of the atomic spin density and the heavy atoms in the polymers. We explain the dramatic variations in SOC with molecular geometry, chemical composition, functionalization, and charge dwell time, by using a first-principles theoretical model based on atomic spin populations. Our approach guides the tuning of the magnetic response of these and other OSCs by chemical synthesis.

Cation ordering is expected to influence the properties of oxides in the $\mathrm{Zr}{\mathrm{O}}_2\text{−}\mathrm{YTa}{\mathrm{O}}_4$ system, but the ordering patterns and their relative energies are not well understood. This work investigates the stability of zirconium-yttrium-tantalum orderings over the parent $t\text{−}\mathrm{Zr}{\mathrm{O}}_2$ and $m\text{−}\mathrm{Zr}{\mathrm{O}}_2$ crystal structures from first principles. The calculations predict a strong tendency for yttrium and tantalum atoms to cluster into checkerboard layers throughout the entire quasibinary. Such checkerboard layers are in fact found to be the building blocks for the lowest-energy $\mathrm{YTa}{\mathrm{O}}_4$ structures, e.g., $M^{\prime }$, $M$, and $T\text{−}\mathrm{YTa}{\mathrm{O}}_4$. The stability of these orderings is related to the propensity of each cation to achieve its favored oxygen coordination. The preference to cluster in the alloyed materials suggests a tendency for short-range ordering. It is hypothesized that checkerboard-type short-range order is a global trend in all $\mathrm{Zr}{\mathrm{O}}_2$ systems with charge-compensating stabilizers, notably rare-earth tantalates and niobates, and has important implications for ferroelastic toughening mechanisms in these materials. A study of the transformation pathways connecting $M$ and $T\text{−}\mathrm{YTa}{\mathrm{O}}_4$ predicts that unlike $t\text{−}\mathrm{Zr}{\mathrm{O}}_2,T\text{−}\mathrm{YTa}{\mathrm{O}}_4$ is dynamically unstable, suggesting that anharmonic vibrations stabilize this phase and must be introduced to rigorously account for temperature effects.

Dibenzoterrylene (DBT) has garnered interest as a potential single-photon source. To have a better grasp of any possible limitations of using DBT for this application, a better understanding of its optical properties is needed. We use a configuration interaction method to calculate the many-body wave functions of DBT and we use these wave functions to calculate its optical properties. We calculate the linear absorption spectrum and the spatial distributions of electrons involved in several bright transitions. We also calculate the two-photon absorption spectrum of DBT and show that there are several excited states that are bright due to two-photon absorption. Except at high photon energies, we predict that there are no competing optical processes regarding the use of DBT as a single-photon source. Our calculations provide details of the optical properties of DBT that are interesting in general and useful for considering optical applications of DBT.

Large-scale quantum molecular dynamics simulations unveil eight friction regimes of water-lubricated low-index diamond surfaces. Four of these friction regimes are universal, i.e., they occur on diamond (111), (001), as well as (110). Dry sliding leads to immediate cold welding accompanied by amorphization (regime I). Small amounts of water (less than $8{\mathrm{H}}_2\mathrm{O}$ per $\mathrm{n}{\mathrm{m}}^2)$ can preserve crystallinity and lower friction by localizing shear to interfacial ether groups (regime II). A further increase in water surface density results in passivating hydrogen/hydroxyl layers (regime IV) and finally (for more than $20{\mathrm{H}}_2\mathrm{O}$ per $\mathrm{n}{\mathrm{m}}^2)$ in free water layers between hydrogen/hydroxyl passivated diamond surfaces (regime V). The other four friction regimes are special, i.e., they occur only on certain surfaces. An ultralow friction regime is established by aromatic Pandey surface passivation on diamond (111) surfaces (regime III). On diamond (110) surfaces, regime II coexists with three other regimes: while partial cold welding via C–C bonds (regime VI) or C–O–C bonds (regime VII) leads to frictional shear stresses that are in-between the cold-welding regimes (I and II) and the non-cold-welding regimes (IV and V), the formation of an oxidized carbon monolayer consisting of keto and ether groups results in ultralow friction (regime VIII). Regime VIII is also observed for diamond (001) surfaces. These findings are rationalized by the structural and energetic peculiarities of the different low-index surfaces. Our study provides guidelines for nanoscale control and manipulation of oxygen functional groups on carbon surfaces in boundary lubrication with water or other oxygen-containing lubricants.

A two-dimensional electron gas (2DEG) can be formed at an interface between two insulators such as ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ without any additional doping. Nevertheless, structural imperfections or defects at the interface, which may arise during the growth process or be of another origin, can definitely affect 2DEG properties either increasing or reducing the 2DEG density. In this work, we study 2DEG formation at both perfect and imperfect interfaces of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures in the framework of the density functional theory. Using a first-principles Green function method within a coherent potential approximation, we investigate consistently the development of the 2DEG density starting from a defectless interface and introducing sequentially various kinds of long-range disorder such as cation intermixing and oxygen vacancies. Finally, we evaluate the 2DEG carrier density from the Fermi surface cross sections and effective masses of the carriers, which may be directly related to the multiple transport phenomena in the system.

According to theoretical predictions [Phys. Rev. Lett. 114, 136601 (2015)], Sb substitution in $\mathrm{F}{\mathrm{e}}_2\mathrm{TiSn}$ full-Heusler compounds is thought to enhance the $n$-type thermoelectric power factors. We prepare $\mathrm{F}{\mathrm{e}}_2\mathrm{TiSn}$ samples with 0%, 10%, and 20% Sb substitution on the Sn site and study the effects of the heat treatment protocol and of Sb substitution by structural characterization, electrical, thermoelectrical, and thermal transport measurements from room temperature down to 10 K, and by comparison with ab initio calculations. For the undoped $\mathrm{F}{\mathrm{e}}_2\mathrm{TiSn}$ phase, we find a bad metallic gapless transport behavior in the as-cast sample and a small gap semiconducting behavior for the annealed samples. These observations, together with the $p$-type character emerging from the Hall effect and Seebeck effect, are explained by ab initio calculations, provided that antisite disorder and vacancy defects are included. In these undoped $\mathrm{F}{\mathrm{e}}_2\mathrm{TiSn}$ samples, we find that increasing thermal annealing temperature up to 800 °C slightly decreases the carrier concentration, enhances the Seebeck coefficient in the low-temperature regime (below 100–200 K), induces magnetic ordering at low temperature $\left(<150\mathrm{K}\right)$, and improves the thermoelectric figure of merit and power factor in the low-temperature regime (below ∼200 K). As for Sb substitution, we find that 10% substitution increases hole carrier concentration and induces a weakly metallic behavior as compared to the semiconducting behavior of the corresponding undoped annealed samples. On the other hand, 20% substitution lowers the carrier density and increases the resistivity by a factor ∼50, with semiconducting behavior measured in the whole temperature range. We interpret the observed effect of Sb doping as a complement to disorder. Indeed, using first-principles techniques we unearth the experimental signature related to density of states features induced by native impurities in the realized samples. The Seebeck coefficient exhibits a maximum at around 200–250 K for the undoped and 20%-doped samples, as typical of semiconducting compounds, while it increases monotonically with temperature for the metallic 10%-doped sample. The latter behavior, joined with the good electrical properties of the 10% sample, determines the room-temperature power factor of $\sim 1.3\times {10}^{-4}\mathrm{W}{\mathrm{K}}^{-2}{\mathrm{m}}^{-1}$ in the 10% Sb-doped sample, more than twice as much as to the value of the undoped sample.

The sensitivity of material properties to the atomic and nanoscale morphology is most clearly demonstrated in small gas-phase clusters. In particular, magnetism serves as an extremely sensitive probe of the smallest modifications of atomic environment. This Rapid Communication demonstrates the drastic changes in both the exchange mechanism and the atomic moments in iron oxide clusters as compared to the bulk. In particular, the exchange is essentially non-Heisenberg and the exchange interactions are increased by an order of magnitude compared to bulk hematite. In addition, very large atomic magnetic moments are observed on oxygen sites due to strong spin polarization. As ${\mathrm{Fe}}_3{\mathrm{O}}_4^{+}$ and ${\mathrm{Fe}}_5{\mathrm{O}}_7^{+}$ are pure trivalent, double exchange is excluded and super exchange is the dominant exchange mechanism in these iron oxide clusters. Therefore, the non-Heisenberg behavior is attributed to covalent magnetism as the hybridization between $\mathrm{Fe}3d$ and $\mathrm{O}2p$ orbitals for clusters is strongly enhanced compared to bulk hematite and becomes magnetic state dependent.

Determining the finite-temperature structure of the hybrid perovskite ${\mathrm{MAPbI}}_3$ is a challenge for both experimental and theoretical methods. A very powerful computational method that can resolve the atomic structure is molecular dynamics (MD). The resulting structure depends on the density functional approximation (DFA) in the case of ab initio MD and the force field in classical MD. We compare the structure between 250 and 400 K obtained with different DFAs and force fields in one consistent manner. The symmetry of the ${\mathrm{PbI}}_3$ framework is analyzed as well as the relative ordering of the neighboring organic molecules inside the framework. The distribution function of the molecules is used to map out an effective energy surface for the rotation of a single molecule. This surface is accurately modeled by a pair of cubic harmonics. Available experimental data in literature are discussed and compared to the structure obtained with the different methods. The spread in these data is still too large to uniquely determine the method that “best” describes the perovskite, however, promising candidates and outliers have been identified.

This work investigates the long-term evolution of dilute solute atoms in face-centered-cubic (fcc) alloys driven by sustained vacancy fluxes, as for instance in the case of materials subjected to energetic particle irradiation. Employing the five-jump frequency framework, we provide compact analytic expressions of the Onsager matrix for dilute vacancy-solute systems in form of a ratio of polynomial functions of jump frequencies. The drag ratio is found to be a function of only two independent variables, which enables a systematic study of both the flux coupling and trapping behavior between solutes and point defects. Using an existing diffusivity database for a total of 182 solutes in 5 fcc solvents, we show that, while most vacancy-binding solutes have indeed a positive drag ratio, there are some solutes with positive, but small, or even negative drag ratio. This previously unnoticed feature is interesting as it would ensure that the trapping solute remains in the matrix despite the accumulation of irradiation dose. In the case where the drag ratio is positive, we propose a kinetic model of solute depletion over time due to flux coupling with vacancies and apply it to a Cu-1 at. % Sb alloy. This study reveals that solutes are not simply dragged to sinks by point defects, but affect the flux of point defects to sinks by modifying the transport coefficients and the driving force for point-defect elimination.