Wafer-scale two-dimensional (2D) materials grown directly on substrates via epitaxy methods are desired for building high-performance electronic devices. Up to now, the selection of the appropriate substrates has been dominated by trial and error, which has greatly hindered the mass production of 2D materials for device applications. In this paper, based on the evolutionary trend of the formation energy during growth, we propose that the epitaxy of monoelemental 2D materials on metal substrates can be classified into three types, and only the third type with a significant energy benefit has the opportunity to grow into large-scale 2D monolayers. By extensive first-principles calculations, we find that this concept can coincide well with experimental reports when the energy threshold is set to 0.6 eV/atom, which provides a straightforward way to evaluate interlayer interactions between substrates and adsorbates. Furthermore, taking the growth of blue phosphorene (blue-P) on Ag(111) as an example, a heterogeneous epitaxial strategy to achieve a transition in growth type by surface alloying is proposed. We verify the feasibility of this strategy and investigate the nucleation process of blue-P on the ${\mathrm{Ag}}_2\mathrm{Sb}$ surface alloy. Our findings provide a valid path for substrate selection and tuning, and we believe that this general strategy will stimulate the development of the large-scale synthesis of 2D materials.

We introduce a computational database with calculated structural, thermodynamic, electronic, magnetic, and optical properties of 820 one-dimensional materials. The materials are systematically selected and exfoliated from experimental databases of crystal structures based on a dimensionality scoring parameter. The database is furthermore expanded by chemical element substitution in the materials. The materials are investigated in both their bulk form and as isolated one-dimensional components. We discuss the methodology behind the database, give an overview of some of the calculated properties, and look at patterns and correlations in the data. The database is furthermore applied in computational screening to identify materials, which could exhibit Majorana bound states.

In this paper we report an experimental study of structural, magnetic, and mechanical properties of quaternary Heusler alloys ${\mathrm{Co}}_{2-x}{Y}_x\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, Ti, or Sc, $0\le x\le 1$) and the experimental findings are supported by ab initio electronic structure calculations. The alloys were synthesized using an arc-melting technique. Single phase microstructures are observed for all alloys substituted with low-valence transition metals $Y$ except Sc. X-ray powder diffraction patterns at room temperature show the presence of Heusler-like face-centered cubic crystal structure in all single phase specimens. The low-temperature saturation magnetic moments, as determined from magnetization measurements, agree fairly well with our theoretical results and also follow the Slater-Pauling rule of thumb for half-metals, a prerequisite for half-metallicity. The alloys are predicted to exhibit half-metallic ferromagnetism by ab initio electronic structure calculations using the $\mathrm{GGA}+U$ approach. All stable compounds are observed to have high Curie temperatures with linear dependence with the valence electrons concentration in the alloys. Relatively high hardness values are also measured, approaching 15.7 GPa for Ti-substituted material, highest among the values reported for Heuslers so far. All these properties strongly suggest the alloys are promising for the spintronic applications at room temperature and above.

The manner in which metallic glasses fail under external loading is known to correlate well with those glasses' Poisson's ratio $\nu$: Low-$\nu$ (compressible) glasses typically feature brittle failure patterns with scarce plastic deformation, while high-$\nu$ (incompressible) glasses typically fail in a ductile manner, accompanied by a high degree of plastic deformation and extensive liquidlike flow. Since the technological utility of metallic glasses depends on their ductility, materials scientists have been concerned with fabricating high-$\nu$ glassy alloys. To shed light on the underlying micromechanical origin of high-$\nu$ metallic glasses, we employ computer simulations of a simple glass-forming model with a single tunable parameter that controls the interparticle potential's stiffness. We show that the presented model gives rise to ultrahigh-$\nu$ glasses, reaching $\nu =0.45$ and thus exceeding the most incompressible laboratory metallic glass. We discuss the possible role of the so-called unjamming transition in controlling the elasticity of ultrahigh-$\nu$ glasses. To this aim, we show that our higher-$\nu$ computer glasses host relatively softer quasilocalized glassy excitations, and establish relations between their associated characteristic frequency, macroscopic elasticity, and mechanical disorder.

Historically, finding two-dimensional (2D) magnets is well known to be a difficult task due to the instability against thermal spin fluctuations. Metals are also normally considered poor thermoelectric (TE) materials. Combining intrinsic magnetism in two dimensions with conducting properties, one may expect to get the worst for thermoelectrics. However, we will show this is not always the case. Here, we investigate the spin-dependent TE properties of monolayer chromium pnictides ($\mathrm{Cr}X$, where $X=\text{P}$, As, Sb, and Bi) using first-principles calculations of electron- and phonon-energy dispersion, along with the Boltzmann transport formalism under an energy-dependent relaxation time approximation. All the $\mathrm{Cr}X$ monolayers are dynamically stable and they also exhibit half metallicity with ferromagnetic ordering. Using the spin-valve setup with an antiparallel spin configuration, the half metallicity and ferromagnetism in monolayer $\mathrm{Cr}X$ enable the manipulation of spin degrees of freedom to tune the TE figure of merit $\left(ZT\right)$. At an optimized chemical potential and operating temperature of 500 K, the maximum $ZT$ values ($\approx 0.22$, 0.12, and 0.09) with the antiparallel spin-valve setup in CrAs, CrSb, and CrBi improve up to almost twice the original values ($ZT\approx 0.12$, 0.08, and 0.05) without the spin-valve configuration. Only in CrP, which is the lightest species and less spin polarized among $\mathrm{Cr}X$, the maximum $ZT$ $(\approx 0.34)$ without the spin-valve configuration is larger than that $(\approx 0.19)$ with the spin-valve one. We also find that, at 500 K, all the $\mathrm{Cr}X$ monolayers possess exceptional TE power factors of about 0.02–0.08 W/m ${\mathrm{K}}^2$, which could be one of the best values among 2D conductors.

It has long been suggested that the familiar intrinsic point defects (vacancies and self-interstitials) encountered in crystals at low temperatures ($T$) transform into extended domains characterized by a missing or excess atom compared with the same-sized region in the perfect crystal so that such extended defects may be viewed as dropletlike regions of enhanced or diminished density. However, the implications of such a transformation, or whether it even occurs in crystalline Si, remain uncertain. To address this fundamental problem, we consider a comprehensive thermodynamic analysis of the thermodynamics of vacancy and self-interstitial formation over a broad $T$ range based on thermodynamic integration with a focus on entropic contributions. In cooled liquids, it is well known that the form of the intermolecular potential can greatly influence the configurational entropy $S_c$, and correspondingly, we analyze several empirical Si potentials to determine how the potential influences both the $T$ dependence of $S_c$ and the enthalpy and entropy of defect formation. We indeed find that the $S_c$ associated with point defects increases significantly upon heating, consistent with the existence of extended defects. Moreover, each type of defect species gives a significantly different contribution to $S_c$ at elevated $T$ and to a qualitive difference in the $T$ dependence of the entropy of defect formation in the extended defect regime. We discuss some potential consequences of these thermodynamic changes of defect formation on the $T$ dependence of diffusion in heated crystals.

${\mathrm{MoBe}}_{22}$ and ${\mathrm{WBe}}_{22}$ compounds belong to the binary $X{\mathrm{Be}}_{22}$ $(X=4d$ or 5$d$ metal) family of superconductors, whose critical temperature depends strongly on $X$. Despite the multiphase nature of these samples, it is possible to investigate the superconducting properties of ${\mathrm{MoBe}}_{22}$ and ${\mathrm{WBe}}_{22}$ at the macro- and microscopic level. A concurrent analysis by means of magnetization and heat-capacity measurements, as well as muon-spin spectroscopy $\left(\mu \mathrm{SR}\right)$ was implemented. At ambient pressure, both compounds enter the superconducting state below $2.6\pm 0.1$ K $\left({\mathrm{MoBe}}_{22}\right)$ and $4.1\pm 0.10$ K $\left({\mathrm{WBe}}_{22}\right)$ and show modest upper critical fields [$({\mu }_0{H}_{\text{c2}}\left(0\right)=48\pm 1$ mT and ${\mu }_0{H}_{\text{c2}}\left(0\right)=58\pm 1$ mT, respectively]. In ${\mathrm{WBe}}_{22}$, the temperature-dependent superfluid density suggests a fully gapped superconducting state, well-described by an $s$-wave model with a single energy gap. Heat-capacity data confirm that such a model applies to both compounds. Finally, ac magnetic susceptibility measurements under applied pressures up to 2.1 GPa reveal a linear suppression of the superconducting temperature, typical of conventional superconducting compounds.

We investigate the complex relationship between the growth conditions and the structural and transport properties of ${\mathrm{Ti}}_x{\mathrm{O}}_y$ thin films grown by molecular beam epitaxy. Transport properties ranging from metallicity to superconductivity and insulating states are stabilized by effectively tuning the O/Ti ratio via the Ti flux rate and the O partial pressure $P_{\mathrm{O}x}$ for films grown on $\left(0001\right)\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ substrates at ${850}^{\circ }$ C. A cubic $c\text{−}{\mathrm{TiO}}_{1\pm \delta }$ buffer layer is formed for low O/Ti ratios, while a corundum $\mathrm{cr}\text{−}{\mathrm{Ti}}_2{\mathrm{O}}_3$ layer is formed under higher-oxidizing conditions. Metallicity is observed for $c\text{−}{\mathrm{TiO}}_{1-\delta }$ buffer layers. The superconducting $\gamma \text{−}{\mathrm{Ti}}_3{\mathrm{O}}_5$ Magnéli phase is found to nucleate on a $c\text{−}{\mathrm{TiO}}_{1-\delta }$ buffer for intermediate $P_{\mathrm{O}x}$ conditions, and an insulator-superconducting transition is observed at 4.5 K $({\mathrm{T}}_C^{\mathrm{onset}}=6K)$ for 85 nm thick films. Strain relaxation of $\gamma \text{−}{\mathrm{Ti}}_3{\mathrm{O}}_5$ occurs with increasing film thickness and correlates with a thickness-dependent increase in $T_C$ observed for ${\mathrm{Ti}}_x{\mathrm{O}}_y$ thin films.

Hypothetical metals having optical absorption losses as low as those of the transparent insulators, if found, could revolutionize optoelectronics. We perform a high-throughput search for lossless metals among all known inorganic materials in the databases of over 100 000 entries. The 381 candidates are identified—having well-isolated partially filled bands—and are analyzed by defining the figures of merit and classifying their real-space conductive connectivity. The superconductor lithium titanate is shown to satisfy the lossless criteria but have a limited bandwidth for low loss. The existing experimental evidence of most candidates being insulating, instead of conducting, is due to the limitation of current density functional theory in predicting narrow-band metals that are unstable against magnetism, structural distortion, or electron-electron interactions. We propose future research directions including conductive oxides, intercalating layered materials, and compressing these false-metal candidates under high pressures into eventual lossless metals.

In the realm of organic-inorganic-hybrid metal-halide perovskites, ${\mathrm{FAPbI}}_3$ is seeing increasing attention as a potentially more stable alternative to ${\mathrm{MAPbI}}_3$. To add to our previous paper, where we studied the reaction kinetics of the thermal decomposition of ${\mathrm{MAPbI}}_3$, here we analyze the compositional change and crystal phase evolution during the thermal decomposition of ${\mathrm{FAPbI}}_3$ thin films. To this end, we prepare the perovskite using thermal coevaporation and monitor the growth and thermal decomposition in vacuum with an in situ x-ray diffraction setup. The experimental procedure has been carried out via three approaches: producing a partially decomposed sample with the help of a graded temperature profile, using a temperature ramp and a set of isothermal decomposition experiments. From this data we analyze and calculate the stoichiometry and phase changes, the activation energy $E$ and the frequency factor $A$ of the thermal decomposition process, in addition to the thermal expansion coefficient during heating. We compare our results to the ones obtained for ${\mathrm{MAPbI}}_3$ thin films by the same experimental method, confirming the enhanced thermal stability of ${\mathrm{FAPbI}}_3$.

Hydrogenation of amorphous silicon $(a\text{−}\mathrm{Si} : \mathrm{H})$ is critical for reducing defect densities, passivating midgap states and surfaces, and improving photoconductivity in silicon-based electro-optical devices. Modeling the atomic-scale structure of this material is critical to understanding these processes, which in turn is needed to describe $c\text{−}\mathrm{Si}/a\text{−}\mathrm{Si} : \mathrm{H}$ heterojunctions that are at the heart of modern solar cells with world-record efficiency. Density functional theory (DFT) studies achieve the required high accuracy but are limited to moderate system sizes of 100 atoms or so by their high computational cost. Simulations of amorphous materials have been hindered by this high cost because large structural models are required to capture the medium-range order that is characteristic of such materials. Empirical potential models are much faster, but their accuracy is not sufficient to correctly describe the frustrated local structure. Data-driven, machine-learned interatomic potentials have broken this impasse and have been highly successful in describing a variety of amorphous materials in their elemental phase. Here, we extend the Gaussian approximation potential (GAP) for silicon by incorporating the interaction with hydrogen, thereby significantly improving the degree of realism with which amorphous silicon can be modeled. We show that our Si : H GAP enables the simulation of hydrogenated silicon with an accuracy very close to DFT but with computational expense and run times reduced by several orders of magnitude for large structures. We demonstrate the capabilities of the Si : H GAP by creating models of hydrogenated liquid and amorphous silicon and showing that their energies, forces, and stresses are in excellent agreement with DFT results, and their structure as captured by bond and angle distributions are in agreement with both DFT and experiments.

Tripentaphenes (TPHs) are theoretically proposed two-dimensional nanocarbons resulting from the assembly of acepentalene-like units. They exhibit a metallic behavior, as well as chemical resonance mechanisms governing their structural properties. Here we computationally and theoretically investigate the electronic properties emerging from quantum confinement when TPHs are cut into nanoribbons. We use density functional theory to investigate the influence of edge chirality and width on the electronic properties of tripentaphene nanoribbons that are fully edge passivated with hydrogen. We show that these systems are usually metallic with frontier states spread over the inner region of the structure. We also find a number of semiconducting and half-metallic systems related to the presence of spin-polarized states. These findings are meaningful to understand the relationship between physical properties and details of the atomic structure on TPH nanoribbons that can be tuned for targeted applications in nanoelectronics and spintronics.

The seminal experimental discovery of the remarkably stable $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayer has led to a handful of predicted magnetic two-dimensional (2D) materials in the ${\mathrm{MA}}_2{\mathrm{Z}}_4$ family (M = transition metals, A = Si, Ge, and Z = N, P, As). These magnetic monolayers were predicted to be dynamically stable, but none of them has been synthesized to date. In this Research Letter, from first-principles thermodynamic stability analysis, we demonstrate that only the nitrides are thermodynamically stable and this occurs under N-rich conditions. Based on this finding, we propose two ferromagnetic, semiconducting Janus monolayers in the family: ${\mathrm{VSiGeN}}_4$ and ${\mathrm{VSiSnN}}_4$. They are both dynamically and thermally stable, but only the former is thermodynamically stable. Intriguingly, Janus ${\mathrm{VSiGeN}}_4$ and ${\mathrm{VSiSnN}}_4$ monolayers show weak in-plane anisotropy compared with the $\mathrm{V}{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayer. These two emerging Janus magnetic semiconductors offer opportunities for studying 2D magnetism and spin control for spintronics applications.

We have developed an atomistic kinetic Monte Carlo (kMC) model describing carbon diffusion, trapping and detrapping in the stress field generated by a 1/2[111] screw dislocation in bcc iron. By performing long time scale carbon diffusion simulations we study the formation of carbon Cottrell atmospheres around screw dislocations at different temperatures and background carbon concentrations. The kMC simulations allow us to predict the rate of formation and strength of carbon atmospheres which control the dislocation's behavior resulting in dynamic strain aging in steel. From the kMC simulations of carbon diffusivity in a block of iron crystal containing a screw dislocation we extract the effective diffusion constants. Our results show that except at low carbon concentrations the diffusivity resulting from carbon transport inside the dislocation core dominates the effective diffusivity, which indicates that pipe diffusion in a screw dislocation is likely. The kMC approach, which explicitly accounts for the behavior of individual carbon atoms, offers an atomistic view of the carbon drag mechanism by which mobile dislocations can collect and transport carbon within their cores. We simulate the carbon drag mechanism at different temperatures, background carbon concentrations, and dislocation velocities and estimate the maximal dislocation velocity at which the atmosphere of carbon atoms can follow a moving screw dislocation.

The mechanical performance—including deformation, fracture and radiation damage—of zirconium is determined at the atomic scale. With Zr and its alloys extensively used in the nuclear industry, understanding that atomic scale behavior is crucial. The defects controlling that performance are at size scales far larger than accessible by first principles methods, necessitating the use of semiempirical interatomic potentials. Existing potentials for Zr are not sufficiently quantitative, nor easily extendable to alloys, oxides, or hydrides. To overcome these issues, a neural network machine learning potential (NNP) is developed here within the Behler-Parrinello framework for Zr. With a careful choice of descriptors of the atomic environments and the creation of a first-principles training dataset that includes a wide spectrum of configurations of metallurgical relevance, a very accurate NNP is demonstrated. Specifically, the Zr NNP yields a good description of dislocation structures and their relative energies and fracture behavior, along with bulk, surface, and point-defect properties and structures, and significantly outperforms the best available traditional potentials. Results here will enable large-scale simulations of complex processes and provide the basis for future extensions to alloys, oxides, and hydrides.

Strain engineering is a powerful mean for tuning the various functionalities of $AB{\mathrm{O}}_3$ perovskite oxide thin films. Rare-earth orthoferrite $R{\mathrm{FeO}}_3$ materials such as ${\mathrm{NdFeO}}_3$ (NFO) are of prime interest because of their intriguing magnetic properties as well as their technological potential applications especially as thin films. Here, using a large set of complementary and advanced techniques, we show that NFO epitaxial thin films, successfully grown by pulsed laser deposition on (001)-${\mathrm{SrTiO}}_3$, show a strong magnetic anisotropy below a critical thickness $t_{\mathrm{c}}$ of ∼54 nm, associated with the occurrence of structural modifications related to symmetry and domain pattern changes. By varying the tensile misfit strain through the decrease of film thickness below $t_c$, the amplitudes of in- and out-of-plane magnetization can be continuously tuned while their ratio stays constant. Furthermore, different low-temperature magnetic behaviors are evidenced for strained and relaxed films, suggesting that the strain-induced structural state impacts the magnetic phase stability.

The second- and third-order nonlinear susceptibilities of the ferroelectric oxides ${\mathrm{LiNbO}}_3$, ${\mathrm{LiTaO}}_3$, and ${\mathrm{KNbO}}_3$ are calculated from first principles. Two distinct methodologies are compared: one approach is based on a perturbative approach within the frequency domain, another on the time evolution of the electric polarization. The frequency dependence of the second harmonic coefficients of the ferroelectric phase of ${\mathrm{LiNbO}}_3$ calculated within the two approaches is in excellent agreement. This is further validated by experimental data for ${\mathrm{LiNbO}}_3$ and ${\mathrm{LiTaO}}_3$, measured for an incident range of photon energies between 0.78 and 1.6 eV. The real-time-based approach is furthermore employed to estimate the third-order nonlinear susceptibilities of all investigated ferroelectric oxides. We show that the quasiparticle effects, considered by means of a scissors shift in combination with the computationally efficient independent particle approximation, result in a shift of all spectral features towards higher energies, and decrease the magnitude of the optical nonlinearities. The energy of the main resonances in the hyperpolarizabilities suggests that the spectra can be understood by multiphoton absorption within the fundamental band gap for all investigated materials.

Motivated by recent breakthroughs in molecular beam epitaxy of GaAs/AlGaAs quantum wells [Y. J. Chung et al., Nat. Mater. 20, 632 (2021)], we examine contributions to mobility and quantum mobility from various scattering mechanisms and their dependencies on the electron density. We find that at lower electron densities, $n_e\lesssim 1\times {10}^{11}{\mathrm{cm}}^{-2}$, both transport and quantum mobility are limited by unintentional background impurities and follow a power-law dependence, $\propto n_e^{\alpha }$, with $\alpha \approx 0.85$. Our predictions for quantum mobility are in reasonable agreement with an estimate obtained from the resistivity at filling factor $\nu =1/2$ in a sample of Y. J. Chung et al. with $n_e=1\times {10}^{11}{\mathrm{cm}}^{-2}$. Consideration of other scattering mechanisms indicates that interface roughness (remote donors) is likely a limiting factor of transport (quantum) mobility at higher electron densities. Future measurements of quantum mobility should yield information on the distribution of background impurities in GaAs and AlGaAs.

The superconductivity in bulk FeSe depends sensitively on the stoichiometry, ditto for the monolayer FeSe films grown on $\mathrm{SrTi}{\mathrm{O}}_{3-\delta }$ and other substrates. To tune the stoichiometry, we deposit Fe onto monolayer FeSe films after epitaxial growth. Using in situ low-temperature scanning tunneling microscopy/spectroscopy, we find that the postdeposited Fe atoms incorporate into monolayer films and combine with excess Se to form FeSe, with an upper coverage limit of ∼0.4 unit-cell. Consequently, the superconducting gap concentrates in 10 to 12 meV. The work demonstrates a simple method for stoichiometry tuning and improving the spatial uniformity of monolayer FeSe films. Compared with the gap of ∼15 to 20 meV for the annealed films, the smaller superconducting gap indicates weakened FeSe-${\mathrm{TiO}}_2$ coupling due to interfacial Se atom substitution.

Crystallographic disorder is known to be crucial for the understanding of transport properties of intermetallic compounds. ZrSiS-type phases with the general formula ZnXSb, where X is a transition metal, are recognized for their extraordinarily large amount of structural defects. Here, we explain the origin of the disorder and investigate its influence on the electronic behavior of ZnCrSb, ZnMnSb, and ZnFeSb. Synchrotron diffraction data revealed substantial amounts of vacancies on the transition-metal sites for all three compounds. Based on bonding analysis and calculations of crystal orbital Hamilton population (COHP), we claim that the defects originate from antibonding interactions on a two-dimensional square sublattice formed by X atoms. To test our thesis, we compare the ZrSiS-type materials with the chemically similar phase, ZnNiSb, which crystallizes in the MgAgAs-type cell. In this structure, the closest X-X distance is almost doubled with respect to the ZrSiS-type counterparts. In concert with expectations, the COHP calculation for ZnNiSb does not show an X-X antibonding interaction, and virtually no defects are detectable via the experimental diffraction analysis. From the perspective of electron transport, ZrSiS-type compounds were characterized by a residual resistivity ratio (RRR) that was significantly lower than that of ZnNiSb, which indicates stronger point-defect scattering of electrons in our disordered compounds. At the lowest temperatures, we also observed, for ZrSiS-type compounds, signatures of another phenomenon known exclusively for disordered metallic conductors: the Altshuler-Aronov (AA) correction to Coulomb interaction. The effect is not present for the defect-free compound ZnNiSb. ZnFeSb shows the lowest RRR and the highest onset temperature of the AA effect. Furthermore, heat-capacity studies revealed order of magnitude enhancement of Sommerfeld coefficient for ZnFeSb with respect to the value obtained from ab initio density of states calculations. This finding might be correlated to the structural instability. The presented analysis can be useful for understanding materials in which structural disorder significantly affects the electronic properties.