Competition between exotic magnetic and electronic ground states underpins the properties of strongly correlated transition metal oxides and can result in electronic phase separation (EPS). The 6H-perovskite ${\mathrm{Ba}}_3{\mathrm{SrMo}}_2{\mathrm{O}}_9$ exhibits no magnetic order down to 1.6 K and EPS is observed at 230 K. The ground state of this material contains a complex mixture of spin-singlet ${\mathrm{Mo}}_2{\mathrm{O}}_9$ dimers and quasimolecular ${\mathrm{Mo}}_2{\mathrm{O}}_9$ clusters. Segregation in ${\mathrm{Ba}}_3{\mathrm{SrMo}}_2{\mathrm{O}}_9$ emerges due to competition between direct Mo–Mo bonding and Mo–O–Mo magnetic superexchange, comprising an unusual mechanism of EPS in transition-metal oxides.

Alloying element segregation often occurs under a reactive environment but its interplay with the subsequent surface oxidation of the alloy remains unclear. Using synchrotron-based ambient-pressure x-ray photoelectron spectroscopy, we dynamically monitor the surface segregation in ${\mathrm{Cu}}_3\mathrm{Pt}\left(100\right)$ in response to temperature and oxygen gas. Vacuum annealing leads to surface segregation of Cu along with the enrichment of Pt in the subsurface region. Upon switching to the ${\mathrm{O}}_2$ atmosphere, dissociative chemisorption of oxygen does not change the surface segregation profile from that under the vacuum annealing condition. A stepwise increase in the oxygen pressure results in the transformation pathway of $\mathrm{Cu}\to {\mathrm{Cu}}_2\mathrm{O}\to \mathrm{CuO}$, in which the selective oxidation of Cu gives rise to further accumulation of Pt underneath the oxide/alloy interface that hinders the supply of Cu from the bulk to the oxide/alloy interface, thereby leading to the termination of the surface oxidation after the ${\mathrm{Cu}}_2\mathrm{O}\to \mathrm{CuO}$ conversion is completed. This differs from the transformation pathway of $\mathrm{Cu}\to {\mathrm{Cu}}_2\mathrm{O}\to {\mathrm{Cu}}_2\mathrm{O}$/CuO for the oxidation of pure Cu and Cu-Au alloys, in which the oxidation of Cu continues and the ${\mathrm{Cu}}_2\mathrm{O}$/CuO bilayer growth is constantly maintained. These key differences provide useful insight into alloy design for controlling the surface properties such as corrosion resistance and catalytic performance of Cu base alloys.

In light of the forecast of recent topics of magnetic van der Waals materials [Burch et al., Nature (London) 563, 47 (2018); Gong and Zhang, Science 363, 706 (2019)], it was encouraged to explore new magnetic two-dimensional (2D) materials that contain heavy atoms so as to elevate the magnetic transition temperature. Here, relativistic density functional theory calculations, combined with ab initio molecular dynamics simulations, lattice dynamical calculations, and classical Monte Carlo (MC) simulations, were performed on transition metal trihalide (${\mathrm{TMI}}_3$; TM = La, Hf, Ta, W, Re, Os, Ir, Pt) layers, for which heavy $5d$-block and $5p$-block elemental atoms were considered owing to their strong spin-orbit coupling. We identified that these newly designed layers are energetically, dynamically, thermally, and thermomagnetically stable at room temperature. Our investigations revealed that most of them possess the intrinsic semiconducting characteristic with tunable band gap, diverse magnetic properties (ferromagnetic, antiferromagnetic, and nonferromagnetic orders), and large magnetic anisotropy energy (generally larger than 1.9 meV/f.u. and up to 29 meV/f.u.). Based on MC simulations, we proposed that ${\mathrm{TaI}}_3$, ${\mathrm{OsI}}_3$, and ${\mathrm{PtI}}_3$ layers have high Curie temperatures of 575, 225, and 170 K, respectively, which are much larger than those of most magnetic 2D materials such as the notable ${\mathrm{CrI}}_3$ (45 K), ${\mathrm{CrGeTe}}_3$ (28 K), and ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ layers (130 K). Importantly, our calculations evidenced the high stability of the ${\mathrm{TaI}}_3$ layer under ambient conditions, large magnetic moment of $2{\mu }_B$, large magnetic anisotropy energy of $-7.3$ meV/f.u., and high Curie temperature of 575 K, making it possible to experimentally study this layer by more ex situ measurements and even to utilize it in future electronic devices.

The unique response of amorphous ionic oxides to changes in oxygen stoichiometry is investigated using computationally intensive ab initio molecular dynamics simulations, comprehensive structural analysis, and hybrid density-functional calculations for the oxygen defect formation energy and electronic properties of amorphous ${\mathrm{In}}_2{\mathrm{O}}_{3-x}$ with $x=0$–0.185. In marked contrast to nonstoichiometric crystalline nanocomposites with clusters of metallic inclusions inside an insulating matrix, the lack of oxygen in amorphous indium oxide is distributed between a large fraction of undercoordinated In atoms, leading to an extended shallow state for $x<0.037$, a variety of weakly and strongly localized states for $0.074<x<0.148$, and a percolation-like network of single-atom chains of metallic In-In bonds for $x>0.185$. The calculated carrier concentration increases from $3.3\times {10}^{20}{\mathrm{cm}}^{-3}$ at $x=0.037$ to $6.6\times {10}^{20}{\mathrm{cm}}^{-3}$ at $x=0.074$ and decreases only slightly at lower oxygen content. At the same time, the density of deep defects located between 1 and 2.5 eV below the Fermi level increases from $0.4\times {10}^{21}{\mathrm{cm}}^{-3}$ at $x=0.074$ to $2.2\times {10}^{21}{\mathrm{cm}}^{-3}$ at $x=0.185$. The wide range of localized gap states associated with various spatial distributions and individual structural characteristics of undercoordinated In is passivated by hydrogen that helps enhance electron velocity from $7.6\times {10}^4$ to $9.7\times {10}^4$ m/s and restore optical transparency within the visible range; H doping is also expected to improve the material's stability under thermal and bias stress.

We use classical potential atomistic modeling to demonstrate that negative vacancy and self-interstitial formation energies are inherently present in amorphous silicon ($a$-Si). Our conclusions are based on comprehensive calculations of vacancy and self-interstitial formation energies in nine different models, ranging in size from 1000 to 2744 atoms and formed by quenching liquid Si at rates between ${10}^{10}$ and ${10}^{12}\mathrm{K}/\mathrm{s}$. We account for a range of possible reference energies corresponding to different choices of reservoirs for atoms inserted into or removed from our models. Moreover, we show that successive addition of either vacancies or self-interstitials does not deplete the number of negative formation energy sites. Instead, it further lowers defect formation energies. These findings lead us to conclude that $a$-Si is not a structure at or near a local enthalpy minimum nor can it be brought to a local enthalpy minimum through the introduction of constitutional defects.

Borophene has triggered a surge of interest due to its outstanding properties including mechanical flexibility, polymorphism, and optoelectrical anisotropy. Very recently, a novel semi-hydrogenated borophene, called ${\alpha }^{\prime }$-4H, was synthesized in large-scale freestanding samples, exhibiting excellent air-stability and semiconducting nature. Herein, using the density functional theory (DFT) and many-body perturbation theory (MBPT), we investigate the electronic and excitonic optical properties of ${\alpha }^{\prime }$-4H borophene. The DFT results reveal that hydrogenation breaks the mirror symmetry and increases the buckling height of pure ${\alpha }^{\prime }$ borophene, which results in an orbital hybridization and indirect band gap of 1.49 eV in ${\alpha }^{\prime }$-4H borophene. Moreover, the optical spectrum achieved from solving the Bethe-Salpeter equation (i.e., $\mathrm{GW}+\mathrm{BSE}$) shows an optical band gap of 2.40 eV, which corresponds to a strongly bound and stable bright exciton with a binding energy of 1.18 eV. The mean value of absorption within the visible area is 1.68 (1.13)$\times {10}^7{\mathrm{m}}^{-1}$ for $E\parallel x$ ($E\parallel y$) polarization, which shows a linear dichroism for visible light. The effective mass and Bohr radius of the ground-state exciton are 0.78 $m_0$ and 2.03 Å, respectively, which demonstrates the characteristics of Frenkel exciton. The excitonic states are robust against tension up to 10%, under which the monolayer is dynamically stable. We also study the bilayer ${\alpha }^{\prime }$-4H borophene with different stackings. For the weak van der Waals interactions between the layers, the bilayer preserves most of the structural and electronic properties of the monolayer. Our study exposes the underlying physics behind the structural, electronic, and optical properties of ${\alpha }^{\prime }$-4H borophene and suggests it as a very promising candidate for flexible optoelectronic applications.

We revisit the Slater half-occupation technique within the DFT-1/2 method to provide improved accuracy of 2D and 3D halide perovskites band gaps at a moderate computational cost. We propose an electron removal scheme from the halide states that drastically improve the predicted band gaps of 2D compounds. Concurrently, we compute the effective masses of the considered structures and show that DFT-1/2 describes them with a nice degree of accuracy when compared to available experimental data. Moreover, we assess the suitability of DFT-1/2 to compute the energy level alignments of this family of compounds. We compare the results in light of those we predict using the hybrid functional HSE06 and the many body perturbation theory within the $G0W0$ approximation. We discuss the limitations of our refined DFT-1/2 scheme, which tends to overestimate the downshift of the absolute valence energy levels of the layered perovskite systems. We anticipate that our results would be useful to initiate benchmark studies and investigate large systems such as heterostructures for which other approaches may be out of reach.

By first-principles calculations and Monte Carlo simulations, we investigate the influence of biaxial strain on the band structures, magnetic characteristics, and Curie temperature ($T_{\mathrm{C}}$) of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4 \left(\mathrm{MBT}\right)/\mathrm{Cr}{\mathrm{I}}_3$ heterojunction. Different from bilayer MBT or ${\mathrm{CrI}}_3$ with antiferromagnetic ordering, the interlayer magnetic coupling of $\mathrm{MBT}/\mathrm{Cr}{\mathrm{I}}_3$ prefers ferromagnetic ordering. By applying biaxial strain, a phase transition from semiconductor to metal can be found and a band inversion is observed under a strain of $-5\%$, suggesting the existence of nontrivial topological phase transition. Moreover, magnetic anisotropy energy (MAE) and $T_{\mathrm{C}}$ are sensitive to compressive strain rather than tensile strain. The magnitude of MAE is increased ten times in the strain range $-5$ to 5%. Meanwhile, the compressive strain enhances the ferromagnetism, leading to a boost of $T_{\mathrm{C}}$ about 13.8% up to 91.1 K. Our findings can provide beneficial guidance for designing the spintronic which hosts robust ferromagnetism and improved $T_{\mathrm{C}}$.

A polycrystalline sample of ${\mathrm{La}}_2{\mathrm{NiIrO}}_6$ has been synthesized and characterized using magnetization and neutron powder diffraction to explore the magnetic ground state. Three anomalies are evident in the bulk magnetization measurements at 28, 50, and 74 K, of which only the 74 K transition is associated with long-range ordering of ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Ir}}^{4+}$ spins. The neutron thermodiffractogram recorded between 1.5 and 90 K and Rietveld refinements of the temperature difference data 1.5–90 K, 40–90 K, and 62–90 K confirm that both the Ni and Ir moments order at the same temperature below 74 K. No change in the magnetic peak intensities or new peaks have been found further below 50 and 28 K indicating that the spin structure remains the same down to 1.5 K. The magnetic propagation vector is $k=(½, ½, 0)$ and the estimated magnetic moment values at 1.5 K are ${\mu }_{\mathrm{Ni}}=1.39\left(8\right){\mu }_{\mathrm{B}}$ and ${\mu }_{\mathrm{Ir}}=0.35\left(3\right){\mu }_{\mathrm{B}}$. Contrary to a recent publication by Ferreira et al. [Phys. Rev. Mater. 5, 064406 (2021)] on the same compound, the improved data quality allows now to ascertain the presence of an ordered moment on the iridium sites. The magnetic structure proposed by Ferreira et al. is discussed and it is shown that there are several magnetic structures compatible with the data but indistinguishable by neutron powder diffraction methods.

This paper provides an atomistic exploration of the lattice dissipation mechanisms accompanying the formation of charged point defects through a femtosecond resolved study of ${\mathrm{F}}^{+}$-center creation in NaCl. Our findings, following from a classical molecular dynamics based investigation of this model system, point to general range of properties that should be present in similar systems. Immediately after the creation of such a charged defect center, its excess energy is imparted amongst the highest energy optical modes with no clear preference based on their degree of localization. This energy is then dissipated through equilibration amongst a bath of lower energy phonon modes. The temporal behavior primarily follows exponential decay trends at all the temperatures and energies explored, with a small degree of competition between phonon population and depopulation amongst lower energy bath modes. Moreover, the dissipation timescale is found to be approximately the same amongst all phonon energies. A temperature-dependent analysis shows the expected decrease in phonon lifetimes with increasing temperature. This is accompanied by similarly more rapid dissipation of thermal energy around the defect center at lower temperatures when the phonon mean free path is increased. An intuitive phenomenological model based on Langevin dynamics is also provided to interpret the atomistically derived phonon decay characteristics in the temporal domain. More broadly, these results are expected to aid the design and experimental investigation of strongly correlated materials where charged defect centers can play an important role in technological applications.

Directly computing linear mass transport coefficients in stochastic models entails integrating over time the equilibrium correlations between atomic displacements. Here, we show how to improve the accuracy of kinetic Monte Carlo simulations via correlation splitting and conditioning, which statistically amounts to estimating the mass transport coefficients through a law of total diffusion. We illustrate the approach with kinetic path sampling simulations of atomic diffusion in a random alloy model in which percolating solute clusters trap the mediating vacancy. There, Green functions serve to generate first-passage paths escaping the traps and to propagate the long-time dynamics. When they also serve to estimate mean-squared displacements via conditioning, colossal reductions of statistical errors are achieved.

We have assessed mechanical bending as a powerful controlling tool for the electronic structure and optical properties of phosphorene nanoribbons. We use state-of-the-art density functional approximations in our work. The overall performance of the recently developed meta-generalized gradient approximation (meta-GGA) mTASK [Phys. Rev. Materials 5, 063803 (2021)] functional establishes the method as a useful alternative to the screened hybrid HSE06 for better band gaps of phosphorene nanoribbons. We present a detailed analysis to interpret the optical absorption of bent phosphorene nanoribbons using the GW-Bethe-Salpeter approximation. We demonstrate the important role of the unoccupied in-gap state and conclude that this in-gap state in armchair nanoribbons introduced by bending can significantly affect the properties of low-energy excitons and add some useful opportunities for applications in optoelectronic devices.

We present the vibrational spectra of polyacrylonitrile-based carbon fibers collected using inelastic neutron scattering. We ascertain the behavior of a broad range of vibrational spectra that are optically silent, and demonstrate a direct connection between these modes and thermomechanical properties of the fibers. We show directionally dependent coupling of hydrogen in the carbon fiber matrix, which is directly linked to mechanical properties. Further, we show hydrogen preferentially couples to the midband of the vibrational spectrum, and that there are higher overall mode populations in the traditional Raman D–G intervalley region, suggesting involvement of these modes in tensile strength reduction and transport properties.

The crystal structure analysis of ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$ reveals that it crystallizes in a bixbyite-type symmetry ($I{2}_{1}3$). Analysis of the structure suggests a randomly occupied cation sublattice with infinite correlation length associated with long-range ordered anion sublattice with half of the vacant sites of an ideal bixbyite filled, hence the name anion-excess bixbyite. Ion irradiation experiments and quantitative x-ray diffraction analysis were used to study the separate response of the anion sublattice to swift heavy ion radiation. Analysis of anion and cation correlation lengths as a function of fluence shows that the topological disorder on the anion sublattice grows faster than that on the cation sublattice. The microstructural response at increasing radiation fluences leads to a decrease of the strain after an initial increase, while the variance of the strain increases following the increase of the microdomain wall density. This particular behavior seems responsible for the exceptional radiation resistance of this system that does not display any significant amorphization, even at the highest fluence.

The discovery of atomically thin two-dimensional (2D) magnetic semiconductors has triggered enormous research interest recently. In this paper, we use first-principles many-body perturbation theory to study a prototypical 2D ferromagnetic semiconductor, monolayer chromium tribromide (${\mathrm{CrBr}}_3$). With broken time-reversal symmetry, spin-orbit coupling, and excitonic effects included through the full-spinor $GW$ and $GW$ plus Bethe-Salpeter equation ($GW$-BSE) methods, we compute the frequency-dependent layer polarizability tensor and dielectric function tensor that govern the optical and magneto-optical (MO) properties. In addition, we provide a detailed theoretical formalism for simulating magnetic circular dichroism, MO Kerr effect, and Faraday effect, demonstrating the approach with monolayer ${\mathrm{CrBr}}_3$. Due to reduced dielectric screening in 2D and the localized nature of the Cr $d$ orbitals, we find strong self-energy effects on the quasiparticle band structure of monolayer ${\mathrm{CrBr}}_3$ that give a 3.8 eV indirect bandgap. Also, excitonic effects dominate the low-energy optical and MO responses in monolayer ${\mathrm{CrBr}}_3$ where a large exciton binding energy of 2.3 eV is found for the lowest bright exciton state with excitation energy at 1.5 eV. We further find that the MO signals demonstrate strong dependence on the excitation frequency and substrate refractive index. Our theoretical framework for modeling optical and MO effects could serve as a powerful theoretical tool for future study of optoelectronic and spintronics devices consisting of van der Waals 2D magnets.

MXenes have many potential applications in electronics, energy storage, sensors, thermal management, and biomedical catalysis. As one of the most widely explored two-dimensional (2D) materials, their transport properties are of broad interest. Due to the difficulty in direct experimental measurements of their thermal conductivity, existing works mainly rely on theoretical approaches. However, only lattice contribution to thermal conductivity in MXenes was studied in previous works, while the role of electrons in thermal transport MXenes has never been elucidated. Herein, we investigate the electron and phonon contribution to the thermal conductivity of three typical metallic 2D MXenes ${[\mathrm{Ti}}_2{\mathrm{CF}}_2, {\mathrm{Ti}}_2{\mathrm{CCl}}_2$, and ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2]$ with a first-principles approach, in which the mode-level electron-phonon coupling is rigorously considered. The thermal conductivity values are predicted to be 69.1, 104.7, and 54.3 W/mK for ${\mathrm{Ti}}_2{\mathrm{CF}}_2, {\mathrm{Ti}}_2{\mathrm{CCl}}_2$, and ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$, respectively. The contribution of electron to total thermal conductivity (37.3–61.3%) is much larger than most existing 2D materials. We find that the relatively large electron density in metallic MXenes leads to the considerable electronic thermal conductivity. Moreover, due to large phonon-electron scattering phase space and matrix element, the phonon thermal conductivity is largely suppressed by the scattering with electrons. Our results clarify the role of electrons in the thermal transport in metallic MXenes and can provide a deeper understanding of the transport mechanism in metallic 2D materials.

Stanene, the two-dimensional (2D) monolayer form of tin, has been predicted to be a 2D topological insulator due to its large spin-orbit interaction. However, a clear experimental demonstration of stanene's topologically nontrivial properties has eluded observation, in part because of the difficulty of choosing a substrate on which stanene will remain topologically nontrivial. In this paper, we present first-principles density functional theory calculations of epitaxial monolayer stanene grown on the (0001) surface of alumina, ${\mathrm{Al}}_2{\mathrm{O}}_3$, as well as freestanding decorated stanene under strain. By describing the energetics and nature of how monolayer stanene binds to alumina, we show a strong energetic drive for the monolayer to be coherently strained and epitaxial to the substrate. By analyzing the electronic structure of strained stanene, we find it to be a quantum spin Hall insulator on ${\mathrm{Al}}_2{\mathrm{O}}_3$. We also describe the effect of in situ fluorine decoration on the bound stanene monolayer, including on its potential for mechanical exfoliation.

The segmental dynamics of one-component nanocomposites (OCNC) is significantly influenced by the molecular weight $\left(M_w\right)$ of the grafted polymer. Neutron backscattering shows that compared to the neat polymer the OCNCs exhibit a shift from slower segmental dynamics at low $M_w$ to faster dynamics at high $M_w$. We model the local relaxation as distribution of exponential diffusers. This approach reveals the presence of fast and slow segments in both OCNCs. However, their fractional contribution varies with $M_w$ leading to different average relaxation times. Our results present important insights into the origin of segmental mobility in OCNC and address the inconsistencies in different literature reports.

The atomic cluster expansion (ACE) provides a general, local, and complete representation of atomic energies. Here we present an efficient framework for parametrization of ACE models for elements, alloys, and molecules. To this end, we first introduce general requirements for a physically meaningful description of the atomic interaction, in addition to the usual equivariance requirements. We then demonstrate that ACE can be converged systematically with respect to two fundamental characteristics—the number and complexity of basis functions and the choice of nonlinear representation. The construction of ACE parametrizations is illustrated for several representative examples with different bond chemistries, including metallic copper, covalent carbon, and several multicomponent molecular and alloy systems. We discuss the Pareto front of optimal force to energy matching contributions in the loss function, the influence of regularization, the importance of consistent and reliable reference data, and the necessity of unbiased validation. Our ACE parametrization strategy is implemented in the freely available software package pacemaker that enables largely automated and GPU accelerated training. The resulting ACE models are shown to be superior or comparable to the best currently available ML potentials and can be readily used in large-scale atomistic simulations.

We investigated the intrinsic magnetic properties of the two-dimensional ferromagnetic candidate monolayer $1T\text{−}{\mathrm{VSe}}_2$ films using x-ray magnetic circular dichroism (XMCD), an element-specific magnetic probe. By performing high-resolution measurements, we succeeded in detecting a clear XMCD signal from the atomically thin $1T\text{−}{\mathrm{VSe}}_2$ films under an external magnetic field. Through manipulation of the x-ray incidence angle, we were able to disentangle the in-plane and out-of-plane magnetic properties and found a strong magnetic anisotropy. Moreover, magnetic-field- and temperature-dependent XMCD revealed that there is no long-range ferromagnetic ordering even at 6 K, but short-range ferromagnetic and antiferromagnetic interactions between neighboring vanadium ions exist. Such low-temperature magnetic behavior signifies that the monolayer $1T\text{−}{\mathrm{VSe}}_2$ is on the verge of ferromagnetism, and this fact accounts for the reported ferromagnetism in ${\mathrm{VSe}}_2$-based heterostructures.