Dynamic disorders, which possess a finite charge delocalization, play a critical role in the charge transport properties of high-mobility molecular organic semiconductors. The use of two-dimensional (2D) charge transport in crystalline organic semiconductors can effectively facilitate reducing the sensitivity of charge carriers to thermal energetic disorders existing in even single crystals to enhance the carrier mobility. An isotropic transfer integral among adjacent molecules enables a dimensional transition from quasi-one-dimensional to 2D for charge transport among molecules. Herein, a tuned molecular packing, especially molecular rotation, was achieved in highly crystalline organic thin films via a brush-coating method. This tuned molecular packing was favorable for approaching isotropic transfer integrals. Consequently, high-performance organic transistors with a carrier mobility up to $21.5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and low angle dependence were obtained. This work presents a unique modulation of molecular packing at the molecular scale to enable less sensitivity of the charge transport to dynamic disorders, providing an alternative route for enhancing the electrical performance of organic electronic devices.

The surface reconstruction on the (110) surface of $Ln\mathrm{Sc}{\mathrm{O}}_3$ ($Ln=\mathrm{Gd}$, Tb, Dy) has been investigated using x-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron diffraction coupled with higher-level density-functional theory methods. The experimental techniques generate constraints on the surface chemistry and structure that are used to inform the theoretical comparison of a number of potential surface structures. The resulting structure is then compared to experimental aberration-corrected transmission electron microscopy results using multislice simulation. The surfaces of both single-crystalline substrates and hydrosauna synthesized nanoparticles exhibit a Sc-rich double layer with a $\mathrm{S}{\mathrm{c}}_3{\mathrm{O}}_4$ termination.

To design the mechanical strength of body-centered-cubic (bcc) iron, clarifying the dislocation dynamics is very important. Using systematically constructed reference data based on density functional theory (DFT) calculations, we construct an atomic artificial neural network (ANN) potential to investigate the dislocation dynamics in bcc iron with the accuracy of DFT calculations. The bulk properties and defect formation energies predicted by the constructed ANN potential are in good agreement with the reference DFT calculations. The $a_0/2\langle 111\rangle \left\{110\right\}$ screw dislocation core structure predicted by the ANN potential is compact and nondegenerate. The Peierls barrier predicted by the ANN potential is 35.3 meV per length of the Burgers vector. These results are consistent with the DFT results. Furthermore, not only the Peierls barrier, but also the two-dimensional energy profile of the screw dislocation core position predicted by the ANN potential are in excellent agreement with the DFT results. These results clearly demonstrate the reproducibility and transferability of the constructed ANN potential for investigating dislocation dynamics with the accuracy of the DFT. Combined with advanced atomistic techniques, the ANN potential will be highly useful for investigating the dislocation dynamics in bcc iron at finite temperatures.

We present a detailed characterization of the structure and transport properties of aluminum–gallium solid–liquid interfaces using classical molecular-dynamics simulation and an embedded-atom model. In addition, ab initio molecular-dynamics simulations are performed to confirm the structural results from the classical force field. For the (100), (110), and (111) Al crystal orientations, we calculate density, potential energy, stress, and diffusion constant profiles as well as a two–dimensional Fourier analysis of the interfacial layers. We find that, for each crystal orientation, the first one to two layers of the Ga liquid have a lateral structure that is commensurate with the underlying Al substrate. Ab initio molecular-dynamics simulations of a smaller system size for the (100) and (111) orientations were also performed. The results support the liquid absorption behaviors from the larger scale classical simulations.

Room-temperature tunable bipolar magnetization switching and magnetically switchable magnetocaloric phenomena having numerous application potentials are reported in $\mathrm{GdM}{\mathrm{n}}_{1-x}\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_3$ ($x=0.55$ and 0.60) polycrystalline samples. Substitution of Fe in antiferromagnetic $\mathrm{GdMn}{\mathrm{O}}_3$ induces a first-order spin-reorientation transition $\left(T_{SR}\right)$, which along with antiferromagnetic ordering transition $\left(T_{\mathrm{N}}\right)$ gives rise to anomalies in dielectric spectra, a signature of magnetodielectric effect. Temperature-dependent Raman spectra confirm the spin-phonon coupling which could be the origin of the magnetodielectric effect in the system. Notably, low-field magnetically tunable magnetization reversal is found to appear between $T_{\mathrm{SR}}$ and $T_{\mathrm{N}}$ in these compounds owing to the competition between single-ion magnetic anisotropy and antisymmetric Dzyaloshinsky-Moriya interaction. Additionally, the $\mathrm{GdM}{\mathrm{n}}_{0.40}\mathrm{F}{\mathrm{e}}_{0.60}{\mathrm{O}}_3$ sample reveals the coexistence of magnetically switchable conventional and inverse magnetocaloric effect at 250 and 310 K. The tailoring of these coexisting room-temperature magnetization reversal and magnetocaloric phenomena in a single-phase system suggests a route to design the material suitable for potential applications in electromagnetic and memory devices.

Wurtzite AlN is an ultrawide bandgap semiconductor that has been developed for applications including power electronics and optoelectronics. Thermal management of these applications is the key for stable device performance and allowing for long lifetimes. However, the intrinsic high thermal conductivity of bulk AlN predicted by theoretical calculations has not been experimentally observed because of the difficulty in producing high-quality materials. This work reports the growth of thick (>15 μm) AlN layers by metal-organic chemical vapor deposition and experimental observation of intrinsic thermal conductivity from 130 to 480 K that matches density-functional-theory calculations for single crystal AlN, producing some of the highest values ever measured. Detailed material characterizations confirm the high quality of these AlN samples with one or two orders of magnitude lower impurity concentrations than commercially available bulk AlN. The thermal conductivity of these commercially available bulk AlN substrates are also measured as comparison. To interpret the reduced thermal conductivity, a simple Callaway model is built. This work demonstrates the possibility of obtaining theoretically high values of thermal conductivity in AlN and will impact the thermal management and reliability of future electronic and optoelectronics devices.

Despite the decades of development and successful implementations of organic light-emitting diodes (OLEDs) in various devices, the poor reliability mainly incurred by chemical degradations has remained a crucial issue. The overall mechanism of degradation has been outlined as an effect of excited carriers, and its microscopic details have yet to be understood. Here, using occupation-constrained density functional theory calculations, we investigate the role of excited carriers on the stability of OLED host materials. Unlike the electronic ground state, in which the chemical stability is mainly determined by the local atomic structure, the stability in the excited state is largely affected by the entire molecular structure, resulting in a nonlocal effect in the chemical degradation. Our results suggest the importance of this nonlocal effect in the excited state, represented by the excited carrier energy and potential energy landscape, to improve the reliability of the OLED device.

$\mathrm{Sm}{\left(\mathrm{CoFeCuZr}\right)}_z$ permanent magnets are of great technological interest due to their good magnetic performance and excellent thermal stability. The Fe content plays a key role for magnetic properties, determining the maximum energy product and the highest working temperature. Here we investigated the Fe site preferences in $\mathrm{Sm}{\left(\mathrm{CoFeCuZr}\right)}_z$ magnets with Fe content up to 26 wt. %, the solubility limit in sintered magnets by first-principles calculations. It is shown that Fe dissolves preferably in the rhombohedral $\mathrm{T}{\mathrm{h}}_2\mathrm{Z}{\mathrm{n}}_{17}$-type (2:17 $R$) phases, with a strong preference for the dumbbell ($6c$) sites. After $6c$ sites are fully occupied, Fe distributes in $18f$ sites as scattered as possible. The crystal structures of 2:17 $R$ type $\mathrm{S}{\mathrm{m}}_2{\left(\mathrm{Co},\mathrm{Fe}\right)}_{17}$ lattice were presented with varying Fe content. The calculated structure and magnetic properties were analyzed comparing with experimental results of 2:17 $R$ phases in multicomponent alloys. Also, the gradually increased substitution energy with continuous doping explained the difficulty in preparation of $\mathrm{Sm}{\left(\mathrm{CoFeCuZr}\right)}_z$ magnets with much Fe.

The ionic materials ${\mathrm{Li}}_4{\mathrm{P}}_2{\mathrm{S}}_6$ and ${\mathrm{Na}}_4{\mathrm{P}}_2{\mathrm{S}}_6$ are both based on the same building blocks of the dimer ions (${\mathrm{P}}_2{\mathrm{S}}_6{)}^{4-}$. Motivated by new experimental structural and ion conductivity studies, we computationally examine this family of materials, finding ${\mathrm{Na}}_4{\mathrm{P}}_2{\mathrm{S}}_6$ and its modification ${\mathrm{Li}}_2{\mathrm{Na}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ to be promising Na ion electrolytes. Using first-principles calculations based on density functional theory and density functional perturbation theory within the harmonic phonon approximation, we show that vibrational effects provide nontrivial contributions to the structural stabilization of these materials. Computed nonresonant Raman phonon spectra and temperature-dependent ionic conductivity for ${\mathrm{Na}}_4{\mathrm{P}}_2{\mathrm{S}}_6$ are both found to be in reasonable agreement with experiment. First-principles analysis of ionic conductivity in both ${\mathrm{Na}}_4{\mathrm{P}}_2{\mathrm{S}}_6$ and ${\mathrm{Li}}_2{\mathrm{Na}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ indicates that Na ions move primarily within the interlayer region between the (${\mathrm{P}}_2{\mathrm{S}}_6{)}^{4-}$ layers, efficiently proceeding via direct or indirect hops between vacancy sites, with indirect processes involving intermediate interstitial sites.

The interplanar bond strength in graphite has been identified to be very low, owing to the contribution of the van der Waals interaction. However, in this study, we use microscopic picosecond ultrasound to demonstrate that the elastic constant, $C_{33}$, along the $c$ axis of defect-free monocrystalline graphite exceeds 45 GPa, which is higher than reported values by 20%. Based on the experimental finding, we find that the $\mathrm{LDA}+U+\mathrm{RPA}$ method, including both random phase approximation correlation and short-range correlation in $2p$ Wannier orbitals, can be a promising solution. The agreement of thus calculated stiffness with the observation indicates non-negligible electron correlation effects with respect to both the short-range and long-range interactions.

Binary Mg-Zn and Al-Zn alloys have been investigated theoretically under static isotropic pressure. The stable phases of these binaries on both initially hexagonal-close-packed (hcp) and face-centered-cubic (fcc) lattices have been determined by utilizing an iterative approach that uses a configurational cluster expansion method, Monte Carlo search algorithm, and density functional theory (DFT) calculations. Based on 64-atom models, it is shown that the most stable phases of the Mg-Zn binary alloy under ambient condition are ${\mathrm{MgZn}}_3$, ${\mathrm{Mg}}_{19}{\mathrm{Zn}}_{45}$, $\mathrm{MgZn}$, and ${\mathrm{Mg}}_{34}{\mathrm{Zn}}_{30}$ for the hcp lattice, and ${\mathrm{MgZn}}_3$ and $\mathrm{MgZn}$ for the fcc lattice, whereas the Al-Zn binary is energetically unfavorable throughout the entire composition range for both the hcp and fcc lattice symmetries under all pressure conditions. By applying an isotropic pressure in the hcp lattice, ${\mathrm{Mg}}_{19}{\mathrm{Zn}}_{45}$ turns into an unstable phase at $P\approx 10\mathrm{GPa}$, a new stable phase ${\mathrm{Mg}}_3\mathrm{Zn}$ appears at $P\gtrsim 20\mathrm{GPa}$, and ${\mathrm{Mg}}_{34}{\mathrm{Zn}}_{30}$ becomes unstable for $P\gtrsim 30\mathrm{GPa}$. For the fcc lattice, the ${\mathrm{Mg}}_3\mathrm{Zn}$ phase weakly touches the convex hull at $P\gtrsim 20\mathrm{GPa}$ while the other stable phases remain intact up to $\approx 120\mathrm{GPa}$. Furthermore, making use of the obtained DFT results, the bulk modulus has been computed for several compositions up to pressure values on the order of $\approx 120\mathrm{GPa}$. The findings suggest that one can switch between $\mathrm{Mg}$-rich and $\mathrm{Zn}$-rich early-stage clusters simply by applying external pressure. $\mathrm{Zn}$-rich alloys and precipitates are more favorable in terms of stiffness and stability against external deformation.

We study colloidal gels formed by competing electrostatic repulsion and short-range attraction by means of extensive numerical simulations under external shear. We show that, upon varying the repulsion strength, the gel structure and its viscoelastic properties can be largely tuned. In particular, the gel fractal dimension can be either increased or decreased with respect to mechanical equilibrium conditions. Unexpectedly, gels with stronger repulsion, despite being mechanically stiffer, are found to be less viscous with respect to purely attractive ones. We provide a microscopic explanation of these findings in terms of the influence of an underlying phase separation. Our results allow for the design of colloidal gels with desired structure and viscoelastic response by means of additional electrostatic interactions, easily controllable in experiments.

Suppressing electron scattering is essential to achieve high-mobility two-dimensional electron systems (2DESs) that are clean enough to probe exotic interaction-driven phenomena. In heterostructures it is common practice to utilize modulation doping, where the ionized dopants are physically separated from the 2DES channel. The doping-well structure augments modulation doping by providing additional screening for all types of charged impurities in the vicinity of the 2DES, which is necessary to achieve record-breaking samples. Despite its prevalence in the design of ultrahigh-mobility 2DESs, the working principles of the doping-well structure have not been reported. Here we elaborate on the mechanics of electron transfer from doping wells to the 2DES, focusing on GaAs/AlGaAs samples grown by molecular beam epitaxy. Based on this understanding we demonstrate how structural parameters in the doping well can be varied to tune the properties of the 2DES.

The incidence of topology on the band structure and physical properties of layered compounds has been extensively studied in semimetals. How those evolve in the presence of electronic correlations has been less investigated so far. In the sodium cobaltates $\mathrm{N}{\mathrm{a}}_x\mathrm{Co}{\mathrm{O}}_2$ considered here, unexpected magnetic properties associated with correlations on the Co sites were disclosed about 15 years ago. It has also been found that various orderings of the Na atoms occur in between the $\mathrm{Co}{\mathrm{O}}_2$ layers in the stable phases of these cobaltates. The distinct Na orders of these phases have been shown to induce specific Co charge disproportionation with large-size unit cells in the $\mathrm{Co}{\mathrm{O}}_2$ planes, linked with the diverse magnetic behaviors. This provides an original playground in the studies of interplays between topology and correlations in these layered materials. We present here transport measurements on a series of single crystals and demonstrate that we do synthetize pure phases with quite reproducible transport properties. We show that above room temperature those display a similar behavior whatever the Na content. On the contrary, we provide evidence for a great diversity in Hall effect low-temperature dependences which underscores the specificities of the Fermi-surface reconstructions induced by the Na order. We study in some detail the difference between two metallic phases, one $\left(x=0.77\right)$ antiferromagnetic below $T_N=22\mathrm{K}$ and the second $\left(x=2/3\right)$ paramagnetic down to $T=0$. Both show a sign change in the Hall effect with decreasing $T$. We demonstrate that this can be attributed to quite distinct physical effects. For the $x=0.77$ phase the negative Hall effect has a nonlinear field dependence and occurs in similar conditions to the anomalous Hall effect found in various magnetic metals. In the $x=2/3$ phase in which the Co sites are disproportionated in a kagome substructure, the negative sign of the Hall effect and its two-step $T$ variation have to be assigned to specificities of its Fermi surface. In both cases the anomalies detected in the Hall effect are certainly associated with the topology of their Co electronic bands.

The transition metal oxides MnO, FeO, NiO, and CoO are essential materials systems for catalysis applications and energy technologies. These materials exhibit a magnetic phase transition from paramagnetism to antiferromagnetism upon cooling. In this work, we show that an accurate treatment of the surfaces requires a description of the disordered local magnetic moments of the paramagnetic phase. We determine how the magnetic phase transition and the presence of solvent affect the surface energies using density-functional theory and a solvation model. To accurately account for the correlations in the $d$-electron system, and to match the observed magnetic order and band gaps of the room-temperature phases, we include the Hubbard-$U$ correction. In the case of MnO, FeO, and CoO, which are paramagnetic semiconductors or insulators at room temperature, we demonstrate the importance of the local magnetic moments and model the materials using special quasirandom structures. To determine the equilibrium shape of MnO, FeO, NiO, and CoO nanocrystals, we calculate the surface energies of their low-energy (100), (110), and (111) facets and perform the Wulff construction. For the (111) facet, we consider various reconstructions that remove the polar nature of the unreconstructed surface. The processing conditions of these oxide nanoparticles, in most cases, involve a solvent. To analyze the influence of the solvent environment on the surface energies of different facets and thereby the crystal shape, we calculate the surface energies of these oxides in water using the continuum solvation model VASPsol. We find that the surface energies decrease due to the dielectric screening. However, as the ratio of surface energies remains sufficiently similar, we predict that the equilibrium crystal shape is only weakly affected by the presence of the solvent.

Strengthening by nanotwins in metals has been widely reported to have a limit, i.e., softening occurs when the twin thickness is reduced below a critical value, similar to the Hall-Petch breakdown in nanocrystalline materials. Here we report based on atomistic simulations that such a limit can be eliminated in high-entropy alloys (HEAs) by tuning their composition and the corresponding stacking fault energies. By using nanotwinned ${\left(\mathrm{CoCrFeMn}\right)}_{1\text{−}x}\mathrm{N}{\mathrm{i}}_x$ HEA as a model system, our results indicate that continuous strengthening can be enabled in this HEA at Ni concentrations below 44% by martensite phase transformation even when the twin thickness approaches the theoretical minimum (i.e., three-atomic-layers thick). In contrast, at Ni concentration above 44%, a strengthening-softening transition appears in the HEA due to detwinning when the twin thickness decreases, which is similar to that in conventional fcc metals with nanoscale twins. A simple parameter based on the ratio between the energy barriers for detwinning and martensite transformation has been proposed to explain these findings, which should shed some light on a generalized deformation map in nanotwinned metals.

Molybdenum mononitrides (MoN) exhibit superior strength and hardness among the large class of transition-metal light-element compounds, but the underlying atomistic mechanisms for their outstanding mechanical properties and the variations of those properties among various MoN phases adopting different crystal structures remain largely unexplored and require further investigation. Here we report first-principles calculations that examine the stress-strain relations of these materials, and systematically compare results under pure and indentation shear deformations. In particular, we examine the distinct bonding structures and the associated mechanical properties in four different crystal phases of MoN that have been experimentally synthesized and stabilized under various physical conditions. Our results reveal evolution patterns of bonding configurations and the resulting structural deformation modes in these MoN phases, which produce diverse stress responses and unexpected strength variations. These findings elucidate the structural and bonding characters that are responsible for the rich and distinct mechanical properties in various MoN structures, providing insights for understanding the experimentally observed phenomena and further exploring advanced superhard materials among the promising transition-metal nitrides, borides, and carbides.

Despite similar chemical compositions, ${\mathrm{LiOsO}}_3$ and ${\mathrm{NaOsO}}_3$ exhibit remarkably distinct structural, electronic, magnetic, and spectroscopic properties. At low temperature, ${\mathrm{LiOsO}}_3$ is a polar bad metal with a rhombohedral $R3c$ structure without the presence of long-range magnetic order, whereas ${\mathrm{NaOsO}}_3$ is a $G$-type antiferromagnetic insulator with an orthorhombic $Pnma$ structure. By means of comparative first-principles $\text{DFT}+U$ calculations with the inclusion of spin-orbit coupling, we (i) identify the origin of the different structural $(R3c$ vs $Pnma)$ properties using a symmetry-adapted soft-mode analysis, (ii) provide evidence that all considered exchange-correlation functionals (local-density approximation, the Perdew-Burke-Ernzerhof functional as well as its improved version for solids, the strongly constrained appropriately normed functional, and the hybrid functional HSE06) and the spin disordered polymorphous descriptions are unsatisfactory to accurately describe the electronic and magnetic properties of both systems simultaneously, and (iii) clarify that the distinct electronic (metallic vs insulating) properties originate mainly from a cooperative steric and magnetic effect. Finally, we find that although at ambient pressure ${\mathrm{LiOsO}}_3$ with a $Pnma$ symmetry and ${\mathrm{NaOsO}}_3$ with a $R\overline{3}c$ symmetry are energetically unfavorable, they do not show soft phonons and therefore are dynamically stable. A pressure-induced structural phase transition from $R3c$ to $Pnma$ for ${\mathrm{LiOsO}}_3$ is predicted, whereas for ${\mathrm{NaOsO}}_3$ no symmetry change is discerned in the considered pressure range.