Ion irradiation is a powerful tool to tune the properties of two-dimensional (2D) materials by creating defects and introducing impurities. At the same time, efficient defect production and especially ion implantation into 2D materials require a careful choice of ion energies, as when energies are too low or too high, production of defects and implantation rate will be unsatisfactorily low. As for the bulk systems, various approaches have been employed to predict optimum ion energies for specific tasks, but they cannot always be directly applied to 2D materials. Here, we carry out ab initio molecular dynamics (MD) and analytical potential (AP) MD simulations and compare the results to those obtained with a simple binary collision approximation (BCA) model. We show that when chemical interactions between the ions and target atoms are essential, as in the case of B and N ion implantation into graphene, as compared to inert gas ions, the AP MD and the BCA model are inadequate. We further suggest a modified-BCA approach with the corrected displacement threshold energies, which account for chemical interactions between the ion and target atoms. The threshold energy can be obtained from first-principles calculations, and the modified-BCA model gives qualitatively and, for some ions, even quantitatively correct results for the energies corresponding to the onset of defect production and substitution probabilities, while being at the same time many orders of magnitude computationally less expensive than the first-principles MD. We show that in any case the BCA and modified-BCA calculations provide the upper and lower bounds on the optimum ion energy.

Strong circular dichroism (CD) has been reported in triply periodic, co-continuous gyroid thin films for certain orientations and surface terminations. However, processing of gyroid thin films introduces distortions experimentally, creating a mismatch between the structures created practically and those explored computationally. This work explores the impact of compression normal to the substrate ($z$-compression) with conserved volume in (110)-oriented plasmonic silver double gyroid thin films on CD using finite-difference time-domain (FDTD) simulations. As compression reaches 15% and above, new features emerge including termination-dependent opposite-handed CD responses and, at larger compressions, shorter wavelength responses that span many surface terminations. The longest wavelength responses of the system redshift with increasing compression. The top surface structure contributes strongly to the emerging opposite-handed features and redshifting of wavelengths. However, the less surface termination-dependent features arise from a mixture of contributions from the top surface and interior of the films. Interplay of these leads to CD-switching phenomena as a function of compression for certain terminations and wavelengths. When alternative methods are utilized to compress the system, such as compression with a Poisson's ratio of 0.33 (comparable to polystyrene) or the generation of compressed equilibrium structures with nonaffine strut changes via self-consistent field theory, similar optical responses persist. Overall, this study highlights the significant impact experimentally relevant distortions (especially compression and some nonaffine structural shifts) can have on the CD response of block copolymer templated plasmonic double gyroid thin films, and it provides mechanistic insight into the film interior versus surface contributions to the CD response during compression.

We performed scanning tunneling microscopy/spectroscopy (STM/STS) measurements to examine the Kondo interaction in the monolayer FeTe grown on ${\mathrm{SrTiO}}_3\left(001\right)-(\sqrt{13}\times \sqrt{13})$. Topographic images showed that the $C_4$ symmetry of the FeTe lattice is broken due to the substrate surface. Furthermore, the electron doping variation from the substrate surface not only leads to two types of Fano-line $\mathrm{d}I/\mathrm{d}V$ spectra representing the Kondo resonance, but also suppresses the hybridization gap observed in bulk FeTe. This finding demonstrates the importance of considering the reduced dimensionality and the substrate surface effect in discussing the complex ground state of the iron chalcogenides.

We develop a technique for predicting the Curie temperature of magnetic materials using density functional theory calculations suitable to include in high-throughput frameworks. We apply four different models, including physically relevant observables, and assess numerical constants by studying 32 ferro- and ferrimagnets. With the best-performing model, the Curie temperature can be predicted with a mean absolute error of approximately 126 K. As predictive factors, the models consider either the energy differences between the magnetic ground state and a magnetically disordered paramagnetic state, or the average constraining fields acting on magnetic moments in a disordered local moments calculation. Additionally, the energy differences are refined by incorporating the magnetic entropy of the paramagnetic state and the number of nearest magnetic neighbors of the magnetic atoms. The most advanced model is found to extend well into ${\mathrm{Fe}}_{1\text{−}x}{\mathrm{Co}}_x$ alloys, indicating the potential efficacy of utilizing our model in designing materials with tailored Curie temperatures by altering alloy compositions. This examination can illuminate the factors influencing magnetic transition temperatures in magnetic materials and provide insights into how they can be employed to make quantitative predictions of Curie temperatures. Our approach is not restricted to specific crystal structures or chemical compositions. It offers a more cost-effective alternative, in terms of human time and need for hands-on oversight, to other density functional theory methods for predicting the Curie temperature. As a result, it provides a practical strategy for conducting high-throughput screening for new technologically applicable magnetic materials. Alternatively, it can complement ML-based screening of magnetic materials by integrating physical principles into such approaches, thereby enhancing their prediction accuracy.

This study employs molecular dynamics (MD) simulations and lattice dynamics (LD) calculations to investigate the superionic transition in ${\mathrm{UO}}_2$, as well as the characteristics of the superionic phase. The superionic transition is found to be a second-order phase transition associated with an inflection point in enthalpy and lattice parameter at 2600 K. This resembles displacive phase transitions and is associated with an oxygen vibration mode becoming imaginary. The superionic state shows a combination of different local environments and dynamic features similar to glass-forming liquids. The analysis of its dynamics reveals seemingly contradictory characteristics: properties follow those of the lower-temperature crystalline phase in some respects, while exhibiting liquidlike behavior in others. The study highlights the complexity of the superionic phase, including heterogeneous mobility and diffusion mechanisms involving stringlike anion clusters.

Recent research has highlighted a remarkably large thermal conductivity and high ambipolar mobility in cubic boron arsenide (c-BAs), positioning it as a promising heat control material substituting diamond for future semiconductor technologies with intensified power and elevated temperatures. However, many thermodynamic properties of c-BAs under high temperature and pressure remain unexplored, one of which is its poorly understood melting behavior. While a few early experiments have reported thermal instability of c-BAs above 1200 K ($920{}^{\circ }\mathrm{C}$), a much higher melting temperature ($>2000\mathrm{K}$) was predicted by recent theoretical estimates via an empirical formula relying on the elastic constants of c-BAs calculated at 0 K. In this work, we investigate the phonon stability, Born mechanical stability, deformation stability under large strains of c-BAs at high temperature, together with its pressure-dependent melting behavior, using ab initio molecular dynamics (AIMD) simulations. The results reveal that the equilibrium structure of c-BAs is dynamically and mechanically stable up to 1900 K. Although the mechanical strength is found to reduce approximately by half as the temperature increases from 300 to 1900 K, c-BAs remains stable at 1900 K when the (tensile, compression, and shear) deformation strains are within 0.1. Moreover, the ultralarge elasticity observed in nanosized c-BAs under large deformation strains at room temperature can mostly preserve up to a high working temperature around $1000{}^{\circ }\mathrm{C}$. The melting temperature of c-BAs determined by our AIMD solid-liquid coexistence tests is $1900\pm 100\mathrm{K}$, close to those predicted by the empirical formula. Under applied pressures, the melting temperature of c-BAs reveals a marked reentrant behavior, which initially increases with pressure but then decreases when the pressure exceeds 12.5 GPa, similar to the observed unique pressure dependence of its thermal conductivity. However, detailed analyses unveil that the reentrant melting behavior of c-BAs comes from the special liquid state of BAs where boron atoms form dense clusters so that the specific volume of liquid BAs quickly becomes smaller than that of solid BAs under a moderate pressure of 12.5 GPa, while a pressure as high as 450 GPa is needed to observe the reentrant melting behavior of diamond. These findings provide more deepened understandings in the mechanical stability and melting dynamics of c-BAs under extreme conditions, which are essential for its potential applications in advanced semiconductor chips and as efficient thermal conductive substrates.

Thin films of short-range-ordered $\mathrm{F}{\mathrm{e}}_{60}{\mathrm{V}}_{40}$ alloys undergo a transition to bcc structure caused by the penetration of energetic $\mathrm{N}{\mathrm{e}}^{+}$ ions. The magnetic ordering correlates to the lattice order, where the short-range-ordered alloy is paramagnetic and the bcc structure becomes ferromagnetic. We show that during reordering, a process of point defect agglomeration along with the formation of vacancy clusters occurs. Depending on the thin-film growth conditions, reordering occurs at different rates, thus the structural disorder in the final bcc structure correlates with both the initial monovacancy concentration and the initial degree of short-range order of the as-grown films. Furthermore, we show that the response towards $\mathrm{N}{\mathrm{e}}^{+}$ irradiation is highly element-specific, revealing higher structural disorder and an overall lower irradiation response for V as compared to Fe. The fraction of the crystalline, ferromagnetic bcc phase, however, increases at the expense of the short-range ordered paramagnetic phase with increasing ion fluence applied.

Transport of spin-polarized electrons injected into light metals is central to the function of numerous spintronic devices and is understood to be limited by Elliott-Yafet spin relaxation. Exploration and quantification of Elliott-Yafet spin relaxation in light metals has not advanced beyond a handful of elements, however, despite the exciting potential for long-range spin transport in alloys. Here, we present a detailed study of spin transport in nonlocal spin valves based on the promising intermetallic alloy ${\mathrm{Al}}_2\mathrm{Cu}$. X-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy, atomic force microscopy, and electronic transport confirm single-phase, textured, polycrystalline $\theta \text{−}{\mathrm{Al}}_2\mathrm{Cu}$ thin films, particularly after moderate annealing. Measurements on $\mathrm{Co}\text{−}\mathrm{Fe}/{\mathrm{Al}}_2\mathrm{Cu}\text{−}\mathrm{based}$ nonlocal spin valves (including the Hanle effect) then enable full quantification of spin relaxation parameters. The spin diffusion length in ${\mathrm{Al}}_2\mathrm{Cu}$ films is found to exceed 100 nm in the low-temperature limit, with Elliott-Yafet constants for phonon and defect scattering that significantly exceed those of Cu. These results are interpreted in light of density functional theory calculations considering spin relaxation hot spots, which also highlight other Al-Cu alloys of high potential interest. This work thus expands the quantitative study of spin transport and relaxation to alloys, laying the foundation for full exploration of light-metal alloys for long-range metallic spin transport.

We report the study of the cocktail effect on superconductivity in high-entropy alloys (HEAs), using hexagonal close-packed HEAs as a prototype system. Compared with the compositional averages of the constituent elements, the superconducting transition temperature $T_{\mathrm{c}}$ is enhanced by from a factor of about $2$ to over one order of magnitude. This $T_{\mathrm{c}}$ enhancement correlates with the reduction in the Debye temperature, underlining the importance of phonon softening in triggering the cocktail effect. Furthermore, we show that the $T_{\mathrm{c}}$ in these HEAs is governed by the average phonon frequency and electron-phonon coupling strength, the latter of which scales linearly with the inverse HEA molecular weight and is progressively weakened with increasing mixing entropy. Our study paves the way toward quantitative understanding of the superconductivity in HEAs.

Aluminum's light weight and high strength make it highly desirable as a structural material in the transportation sector. However, its relatively high corrosion susceptibility poses challenges for its wide application under exposure to the elements. Traditionally, hexavalent chromium (VI) oxide (${\mathrm{CrO}}_3$) coatings have been employed to provide corrosion protection, but their toxicity and environmental impact have motivated research to seek less harmful options. Trivalent chromium (III) oxide (${\mathrm{Cr}}_2{\mathrm{O}}_3$) suggests itself as a natural alternative due to its similar chemistry and lower toxicity. However, the stability of its bonding to pure Al metal substrates has not been thoroughly investigated. This study focuses on calculations of the adhesion work ($W_{\mathrm{ad}}$) and interface energy (${\gamma }_{ij}$) of $\mathrm{Al}/{\mathrm{CrO}}_3$ and $\mathrm{Al}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ interface structures with different Al crystallographic orientations ((100), (110), and (111)) using density functional theory calculations. Significant variations in the adhesion work $W_{\mathrm{ad}}$ and interface energy ${\gamma }_{ij}$ were observed for $\mathrm{Al}/\mathrm{CrO}{}_3$ interface structures across different Al planes, primarily attributed to the formation of interpenetrating Al-Cr oxide substructures and the formation of Al-O-Al bonds within the interface region. In contrast, the $\mathrm{Al}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ interface energies exhibited a relatively weak dependence on the metal substrate orientation, with well-defined oxide and metallic layers and no oxygen penetration into the aluminum. Notably, $\mathrm{Al}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ interfaces generally displayed higher adhesion work $W_{\mathrm{ad}}$ and lower interface energy ${\gamma }_{ij}$ compared to $\mathrm{Al}/{\mathrm{CrO}}_3$, suggesting that ${\mathrm{Cr}}_2{\mathrm{O}}_3$ forms more stable interfaces and adheres more strongly to Al. These findings contribute to the development of sustainable corrosion protection solutions in the aerospace and marine industries by highlighting the potential of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ coatings as a viable environmentally friendly alternative to ${\mathrm{CrO}}_3$.

Combining the excellent thermal and electrical properties of Cu with the high abrasion resistance and thermal stability of W, Cu-W nanoparticle-reinforced metal matrix composites and nano-multilayers are finding applications as brazing fillers and shielding material for plasma and radiation. Due to the large lattice mismatch between fcc Cu and bcc W, these systems have complex interfaces that are beyond the scales suitable for ab initio methods, thus motivating the development of chemically accurate interatomic potentials. Here, a neural network potential (NNP) for Cu-W is developed within the Behler-Parrinello framework using a curated training dataset that captures metallurgically relevant local atomic environments. The Cu-W NNP accurately predicts (i) the metallurgical properties (elasticity, stacking faults, dislocations, thermodynamic behavior) in elemental Cu and W, (ii) energies and structures of Cu-W intermetallics and solid solutions, and (iii) a range of fcc Cu/bcc W interfaces, and exhibits physically reasonable behavior for solid W/liquid Cu systems. As will be demonstrated in forthcoming work, this near ab initio accurate NNP can be applied to understand complex phenomena involving interface-driven processes and properties in Cu-W composites.

The present work reports the study of spin-phonon coupling and site-disorder in polycrystalline ${\mathrm{YBaCuFeO}}_5$ with complimentary experimental methods. Temperature-dependent magnetization measurements indicate the development of incommensurate spiral antiferromagnetic (AFM) transition (${\mathrm{T}}_{N2}$) at about 220 K. Temperature-dependent x-ray diffraction (XRD) measurements indicate small discontinuity in lattice parameters across ${\mathrm{T}}_{N2}$, which are further confirmed by the principal component analysis (PCA) of the XRD data. Analysis of temperature-dependent Raman spectroscopy data with PCA clearly indicate the presence of spin-phonon coupling across both the magnetic transitions, i.e., ${\mathrm{T}}_{N1}$, which is expected to be around 440 K corresponding to paramagnetic to commensurate AFM structure and ${\mathrm{T}}_{N2}$. ${}^{57}\mathrm{Fe}$ Mössbauer measurements carried out at low temperatures clearly indicate the presence of single Fe site in YBCFO. Further the in-field Mössbauer data at the lowest temperature imply the development of weak ferromagnetism corroborating the bulk magnetic measurements. Temperature-dependent x-ray absorption spectroscopy (XAS) measurements indicate that ${\mathrm{T}}_{N2}$ transition predominantly affects the angles between ${\mathrm{FeO}}_n$ coordination polyhedra, while the polyhedra themselves remain largely unaltered.

Recent advances in the characterization of hexagonal-diamond silicon (2H-Si) have shown that this material possesses remarkably different structural, electronic and optical properties as compared to the common cubic-diamond (3C) polytype. Interestingly, despite the wide range of physical properties analyzed, to date no study has investigated impurity energy levels in 2H-Si. Here, we present the results of ab initio DFT simulations to describe the effect of p- and n-type substitutional doping on the structural and electronic properties of hexagonal-diamond Si (2H-Si). We first provide a detailed analysis of how a given impurity can assume a different local symmetry depending on the host crystal phase. Then, by studying neutral and charged dopants, we carefully estimate donors' and acceptors' transition energy levels in 2H-Si and compare them with the cubic-diamond (3C) case. In the case of acceptors, the formation energy is always lower in 2H-Si and is associated with a shallower charge transition level with respect to 3C-Si. On the other hand, donors prefer the cubic phase and have transition energies smaller with respect to 2H-Si. Finally, by employing a simple model based on the 2H/3C band offset diagram, we prove the physical validity of our findings and we show how holes can be used to stabilize the 2H-Si phase. Overall, the described doping properties represent a robust starting point for further theoretical and experimental investigations.

Atoms within moiré bilayers relax in plane to minimize elastic energy [e.g., Cazeaux et al., J. Elast. 154, 443 (2023)]; such relaxation brings their space group symmetries down to P1. Here, the ab initio second harmonic generation (SHG) of twisted and atomistically optimized hBN bilayers was determined at four twist angles ($\theta =38.{21}^{\circ }, 60.{00}^{\circ }, 73.{17}^{\circ }$, and $98.{21}^{\circ }$) and for three displacements $\mathit{\tau }$ measured away from the ground state $A{A}^{\prime }$ configuration. All moiré bilayers have a P1 space symmetry after structural optimization. This situation is quite different to monolayers with hexagonal lattices, which retain a threefold symmetry. We point out that the actual symmetries of the SHG reported for hBN bilayers on two experimental works do not coincide with the sixfold symmetric theoretical profiles they provide [either ${sin}^2\left(3\varphi \right)$ or ${cos}^2\left(3\varphi \right)$], and show that the intrinsic low structural symmetry of (atomically optimized) hBN bilayer moirés can in fact be read out from experimental SHG intensity profiles—which are tunable by $\theta$ and by the frequency of light $\omega$: The SHG is most definitely not sixfold symmetric because moirés do not retain a threefold symmetry. Furthermore, an extrinsic twofold symmetry of the SHG emission is realized by tilting the pump by an angle $\alpha$ away from the 2D material's normal, regardless of $\theta$ and $\omega$. The design of in-plane and ultrathin sources of SHG with low symmetry could be useful for the eventual creation of entanglement sources from 2D materials.

Polycrystalline samples of ${\mathit{R}}_2{\mathrm{O}}_2{\mathrm{CO}}_3$ ($R$ = Pr, Nd, Gd, Tb, Dy, Ho, Er, and Yb) with a unique rare-earth bilayer of triangular lattice were synthesized and studied by DC and AC magnetic susceptibility. Data reveals various magnetic ground states including (i) a nonmagnetic ground state for the Pr sample; (ii) long-range magnetic ordering for the Nd, Gd, Tb, Dy, Ho, and Er samples. Besides the Gd sample, they also exhibit field-induced spin state transitions. More interestingly, a series of spin state transitions in the Nd and Dy samples could be attributed to the field-induced up-up-down spin state and the 2:1 canted spin state. Neutron powder diffraction measurements of the Er sample suggest a spiral spin structure below its $T_{\text{N}}$; and (iii) a short-range ordering for the Yb sample. The disrupted interlayer interaction due to the shift of ${\mathrm{Yb}}^{3+}$ ions within the bilayer prevents long-range magnetic ordering down to 30 mK and makes it another Yb-related triangular lattice antiferromagnet that has the potential to realize a quantum spin liquid state.

We employ ab initio modeling to investigate the possibility of attaining high-temperature conventional superconductivity in ambient-pressure materials based on the known ${\mathrm{MgB}}_2{\mathrm{C}}_2$ and recently proposed thermodynamically stable NaBC ternary compounds. The constructed $(T,P_{\text{M}})$ phase diagrams (M = Mg or Na) indicate that these layered metal borocarbides can be hole doped via thermal deintercalation that has been successfully used in previous experiments to produce ${\mathrm{Li}}_{1>x\gtrsim 0.5}\mathrm{BC}$ samples. The relatively low temperature threshold required to trigger NaBC desodiation may help prevent the formation of defects shown recently to be detrimental to the electron-phonon coupling in the delithiated LiBC analog. According to our numerical solutions of the anisotropic full-bandwidth Migdal-Eliashberg equations, the proposed ${\mathrm{Mg}}_x{\mathrm{B}}_2{\mathrm{C}}_2$ and ${\mathrm{Na}}_x\mathrm{BC}$ materials exhibit superconducting critical temperatures between 43 K and 84 K. At the same time, we demonstrate that buckling of defect-free honeycomb BC layers, favored in heavily doped ${\mathrm{Na}}_x\mathrm{BC}$ compounds, can substantially reduce or effectively suppress the materials' potential for ${\mathrm{MgB}}_2$-type superconductivity.

With their exceptional efficiency, ease of processing, and compatibility with large-scale deposition methods, hybrid halide perovskite solar cells have emerged as promising candidates for advanced photovoltaic devices in next-generation technologies. Halide hybrid perovskite materials are known to undergo degradation when exposed to light in the presence of oxygen, leading to the decomposition and volatilization of degradation products. In this study, we propose a novel characterization method based on layer-by-layer stripping of perovskite using high-energy ultraviolet radiation under an ambient environment through simultaneous monitoring of the transmittance spectrum of the sample in the visible range. This approach enables the spatial characterization of the intrinsic degradation rate throughout the thickness of the sample in approximately one hour. Furthermore, it allows monitoring the spatial homogeneity of the film by a developed model that interprets the evolution of the spectrum. Thin films of ${\mathrm{MAPbI}}_3$ containing a stoichiometric excess of ${\mathrm{PbI}}_2$ were examined using this method, verifying the existence of a surface layer associated with partial segregation of ${\mathrm{PbI}}_2$ and establishing that it has greater degradation resistance. As a powerful feature, our method allows us to determine whether the ${\mathrm{PbI}}_2$ excess is beneficial or detrimental to the stability of ${\mathrm{MAPbI}}_3$ films. In summary, the proposed characterization technique represents a simple and efficient method for monitoring the homogeneity and susceptibility to degradation of perovskite materials in photovoltaic applications.

We have successfully synthesized single crystals of the site-ordered cubic (C15b) Laves phase compound ${\mathrm{LuInCo}}_4$ with the Co-pyrochlore sublattice. ${\mathrm{LuInCo}}_4$ undergoes a ferromagnetic transition at 306 K and has a saturation moment of $3.43{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ at 5 K. The strongly ferromagnetic nature was verified by density functional theory calculations, suggesting that $\mathrm{Co}\text{−}3d$ flat bands near the Fermi level induce the spin polarization. The magnetization is isotropic above $\approx 100\mathrm{K}$, and saturates most easily in the [100] direction at low temperatures. In this anisotropic ferromagnetic state, the magnetization undergoes a metamagnetic transition in the [111] direction. Our results suggest that ${\mathrm{LuInCo}}_4$ is a strong but unusual itinerant electron ferromagnet, which deserves further study as a pyrochlore metal.