Harnessing nonlinear interactions between phonon modes in condensed matter presents a powerful avenue for tailoring material properties for innovative applications in future information devices. Here we report a study of nonlinear coupling between infrared (IR) active and Raman-active phonon modes in rare-earth orthoferrites $\mathrm{ErFe}{\mathrm{O}}_3, \mathrm{TmFe}{\mathrm{O}}_3$, and $\mathrm{TbFe}{\mathrm{O}}_3$. To reveal the nonlinear phonon interactions, we resonantly excite IR-active phonons with an ultrashort midinfrared pulse, with its central wavelength tunable in the 3–18 µm (16.5–100 THz) range, and trace the phonon modes in the time domain after the excitation. We identify two main coupling mechanisms: quadratic linear and trilinear, which stand for a nonlinear coupling of the Raman-active phonon with either one or two distinct IR-active modes, respectively. The origin of driving each Raman-active phonon mode is identified and analyzed with polarization-dependent pump-probe spectroscopy. Our results show a strong dependence of the trilinear coupling mechanism on the crystallographic orientation of the sample, thus revealing that only specific phonon modes can exhibit this coupling.

The native surface oxide of plutonium plays a critical role in ensuring the stability and safe storage of the underlying metal; consequently, understanding the role of defects and impurities in determining the properties of the oxide layer is critical. Here, we use hybrid density-functional theory calculations to evaluate the electronic structure and defect chemistry of ${\mathrm{PuO}}_2$, the most stable of the native oxide phases, including both native and extrinsic defects. We find that oxygen vacancies ($V_{\mathrm{O}}$) form readily in ${\mathrm{PuO}}_2$, as do polarons. Electron polarons (${\eta }^{-}$) are the lowest-energy acceptor species in ${\mathrm{PuO}}_2$, while the charge compensating donor species will shift from $V_{\mathrm{O}}$ under O-poor conditions to hole polarons (${\eta }^{+}$) under O-rich conditions. Nitrogen and fluorine can substitute readily for oxygen atoms under O-poor conditions, while fluorine can also incorporate in an interstitial configuration (${\mathrm{F}}_i^{-}$) under more O-rich conditions. Carbon and chlorine incorporation in ${\mathrm{PuO}}_2$ will be very limited. We also evaluate the kinetic barriers for oxygen-related defects, which we find to diffuse readily when present. Our results provide valuable insights into the critical role and variable chemistry of point defects and impurities in ${\mathrm{PuO}}_2$, which in turn have important implications for the safe storage of the underlying metal layer. In short, exposure of freshly prepared plutonium to reactive nitrogen- and fluorine-containing contaminants should be avoided, while carbon- or chlorine-containing contaminants are less likely to incorporate readily into the oxide.

In this work, we simulate the spectral emissivity of various stoichiometric crystal phases of ${\mathrm{Mo}}_x{\mathrm{Si}}_y$ compounds using density functional perturbation theory. The dielectric function, including electronic and ionic contributions, is calculated for each phase. We use the bulk properties obtained to simulate the optical absorption spectrum originating from the compound in thin film ($\sim 20\mathrm{nm}$) form. We find that most thin films of ${\mathrm{Mo}}_x{\mathrm{Si}}_y$ are metallic, however, our results indicate that their emissivity is not simply correlated with the Mo content. For hot metallic films at around 900 K, we predict a maximal emissivity between 5–10 nm thickness. Our results are in good qualitative agreement with experiments, confirming that the emissivity of hexagonal ${\mathrm{MoSi}}_2$ is much lower than in the tetragonal phase. This is related to the small band gap (hexagonal ${\mathrm{MoSi}}_2$) and low density of states at the Fermi level (tetragonal ${\mathrm{MoSi}}_2$). Furthermore, test calculations on defected ${\mathrm{MoSi}}_2$ demonstrate that the infrared emissivity of ${\mathrm{MoSi}}_2$ thin films can be substantially increased by introducing defects.

We present an in-depth study of electronic transport in atomic-sized gold contacts using Break-Junction (BJ) techniques under cryogenic and ambient conditions. Our experimental results, supported by classical molecular dynamics (CMD) simulations and ab initio calculations, provide compelling evidence for the formation of three-atom-thick structures in gold nanocontacts under tensile stress. These findings extend previous studies that confirmed the existence of one- and two-atom-thick chains. Beyond identifying these novel atomic configurations, we introduce a fast and robust calibration method for Break-Junction systems, leveraging the characteristic length of these structures to convert piezo-distance into absolute distance in angstroms. Our approach presents a novel and robust method for calibrating atomic distances in atomic conductor systems at both cryogenic and room temperatures. The results also enable the assessment of electrode sharpness, even under ambient conditions.

Accurately predicting the crack behavior of magnesium-lithium (Mg-Li) alloys through atomic-scale simulations is essential for understanding crack-defect interactions and designing high-plasticity alloys. However, existing molecular dynamics potentials for Mg-Li alloys are inadequate for reliably modeling crack-related phenomena, such as stacking fault energies and dislocation core structures. In this work, by combining 1722 density functional theory (DFT) calculated data points with machine learning method, we develop a broadly applicable machine learning potential (MLP) for Mg-Li alloys. This MLP accurately reproduces lattice constants, elastic constants, surface energies, and generalized stacking fault energy curves, showing reasonable agreement with DFT data. Furthermore, MLP simulations reveal that either the continuous formation of basal ${\mathrm{I}}_2$ stacking faults or the formation of basal ${\mathrm{I}}_1$ stacking faults trigged by the cross-slip of pyramidal dislocation effectively relieves stress concentration at the crack tip, resulting in crack blunting. These findings help clarify the experimental debate regarding the dual role of basal slip: basal slip bands can also delay crack propagation, while discontinuous basal slip can initiate fatigue cracks.

We present a study of low-temperature electric and thermal transport in ${\mathrm{RuO}}_2$, a metallic oxide which has attracted much recent attention. Careful scrutiny of electric resistivity reveals a quadratic temperature dependence below $\sim 20$ K undetected in previous studies of electronic transport in this material. The prefactor of this $T^2$ resistivity, given the electronic specific heat, corresponds to what is expected by the Kadowaki-Woods scaling. The variation of its amplitude across four different samples is negligible despite an eightfold variation of residual resistivity. There is also a $T^5$ resistivity due to scattering by phonons. By measuring thermal conductivity $\kappa$ at zero field and at 12 T, we separate its electronic and phononic components and find that the former respects the Wiedemann-Franz law at zero temperature and deviates downward at finite temperature. The latter corresponds to a threefold discrepancy between the prefactors of the two (thermal and electric) $T$-square resistivities. Our results, establishing ${\mathrm{RuO}}_2$ as a weakly correlated Fermi liquid, provide input for the ongoing theoretical attempt to give a quantitative account of electron-electron scattering in metallic oxides starting from first principles.

Bulk boron phases are typically insulating due to sufficiently localized valence electrons, so that stable metallic boron phases at ambient conditions are rare. This situation is further exacerbated by the limited understanding of mechanism governing electronic properties of boron phases. Here, we report a series of stable metallic and insulating boron phases at ambient pressure, identified by an extensive structural search combining graph theory, group theory-based random strategies, and global optimization algorithms. Among these, an R3c-symmetry phase (${\mathrm{B}}_{\text{R}3\mathrm{c}}$), composed of six ${\mathrm{B}}_9$ structural units, exhibits exceptional thermal stability up to 1200 K, a Vickers hardness exceeding 40 GPa, and higher thermodynamic stability than the experimentally known ε-B phase by 0.11 eV/atom. First-principles analysis reveals that the ${\mathrm{B}}_{\text{R}3\mathrm{c}}$ metallicity stems from delocalized electronic states by uniform B-B bond lengths and high-coordinate number. Motivated by this, we develop a physically interpretable one-dimensional descriptor based on the bond topology and electron localization parameters, which can correctly predict 95.1% of all reported metallic or insulating boron phases. These results constitute a predictive framework for designing boron and boron-based functional materials.

We report the development of ferromagnetic nematic colloids using a two-step protocol tailored for investigating emergent magnetoelastic phenomena. First, we prepared a ferromagnetic nematic liquid crystal by dispersing well-defined magnetic nanoparticles into a conventional nematic host, ensuring magnetic ordering while preserving the characteristic director field. Subsequently, microparticles were suspended within this ferromagnetic nematic matrix that generates elastic dipoles and quadrupoles in the director field. We show that a slowly rotating magnetic field induces surface domain $\pi$ walls that quasistatically reorient the defects and linear colloidal assemblies—an effect that is absent in ordinary nematic liquid crystals lacking spontaneous magnetization. The results are interpreted using the Landau–de Gennes Q-tensor model, extended to capture the coupling between nematic and magnetic characteristics of ferromagnetic nematic liquid crystals. This composite system enables the exploration of novel elastic and magneto-optic interactions at the interface of liquid crystalline order and nanoscale magnetism, broadening the experimental landscape for both fundamental studies and potential applications in adaptive materials that involve colloidal composites.

While ultrasonication is a well established method to disperse nanorods, insufficient attention has been given to the impact of the particle mass fraction $W_s$ at which sonication takes place. Its importance goes far beyond viscosity tuning, since a dilute isotropic nanorod suspension transitions to a liquid crystal (LC) phase if $W_s$ surpasses a threshold, via a wide biphasic window of isotropic–LC equilibrium phase coexistence. By systematically varying $W_s$ of suspensions of cellulose nanocrystals (CNCs) across and beyond the biphasic range we show that $W_s$ strongly influences the sonication process. Near the original suspension's stability limit of the isotropic phase, $W_0^{*}$, the sonication shear flow induces a temporarily ordered paranematic state which assists particle individualization and minimizes damage. Our atomic force microscopy study supports this, finding increasing aspect ratio nanorods as $W_s$ is increased from the fully isotropic range past the onset of the equilibrium biphasic window. $W_s$ impacts also the macroscopic properties like the phase diagram and the pitch of the cholesteric LC helix into which CNCs self-assemble. A further key finding is that CNC suspensions concentrated beyond the threshold for gelation should be sonicated to remove end-to-end chains formed in the gel, since otherwise these remain even after dilution. Our conclusions apply also to many other nanorod, nanotube and nanoplatelet suspensions, where reproducibility problems may be due to the lack of control of $W_s$ and to agglomerates remaining from high-concentration precursor suspensions.

Here, we propose a comprehensive first-principles atomistic approach to predict the Wulff-Kaischew equilibrium shape of crystals heterogeneously integrated on a dissimilar material. This method uses both reconstructed surface and interface absolute energies, as determined by density functional theory, to infer the morphology and wetting properties of Volmer-Weber islands over the whole range of accessible chemical potentials. The predicted equilibrium shapes of GaP crystals heterogeneously grown on Si are found to be in good agreement with experimental observations performed by transmission electron microscopy. This method provides a tool for optimization of heterostructured, multifunctional, and smart materials and devices.

We propose and theoretically demonstrate a platform for realizing both the axion insulator and quantum anomalous Hall (QAH) phases. This platform is a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$-based topological insulator film that is confined between two out-of-plane magnetized films of ${\mathrm{MnBi}}_2{\mathrm{Se}}_4$ and ${\mathrm{CrBi}}_2{\mathrm{Se}}_4$. Our first-principles calculations show that this design induces substantial splitting of the topological surface states which facilitates the realization of axion and QAH phases at elevated temperatures. This work establishes a tunable material platform for probing the interplay between distinct topological magnetic phases and for exploiting their exotic properties in next-generation devices.

We report the anisotropic magnetization and the anomalous Hall effect (AHE) in single-crystalline ${\mathrm{AlFe}}_2{\mathrm{B}}_2$, which undergoes a ferromagnetic (FM) transition at 275 K. The intrinsic mechanism is essential for the AHE in the FM state, resulting in a significant anomalous Hall conductivity of over 200 ${\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ at low temperatures. Due to its moderate susceptibility as a soft magnet, this material exhibits a high resistivity sensitivity across a wide temperature range, suggesting its potential as an anomalous Hall sensor for detecting weak magnetic fields.

The role of environmental water in graphite lubrication has generated conflicting observations and considerable confusion. Early hypotheses suggested water intercalation between graphene layers, but x-ray diffraction studies revealed no change in interlayer spacing with changing humidity. Conversely, recent Raman spectroscopy studies indicated water intercalation in bilayer graphene under humid conditions. To reconcile these discrepancies, we report density functional theory (DFT) and molecular dynamics (MD) simulations coupled with a reinterpretation of experimental measurements. While DFT calculations reveal softer surface interlayers suggesting potential for water intercalation, MD simulations indicate that such intercalation requires a significant reduction in van der Waals interactions, beyond what surface-induced softening can achieve. Alternatively, if water does intercalate, it likely occurs between the surface layers where graphene interactions are over 80% weaker than in bulk graphite, making such intercalation plausible. Instead, we demonstrate that water forms clusters on the graphene surface, maintaining a stable two-atomic layer structure across a wide temperature range, emphasizing the importance of surface water adsorption. We therefore propose that the lubrication effect is primarily related to the graphene-water interface rather than the graphene-graphene interface. While we do not completely rule out water intercalation, our findings suggest that graphene-water interactions dominate lubrication. These results resolve previous discrepancies by highlighting the role of surface or near-surface water layers over bulk intercalation, underscoring the significance of molecular interactions at the graphene-water interface in confined nanoscale spaces.

The efficacy of single-site defect engineering in graphitic carbon nitride ($\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$) is often compromised by the inevitable creation of charge recombination centers. To overcome this limitation, we propose a synergistic dual-site defect strategy and develop a high-pressure and high-temperature (HPHT) method to concurrently introduce oxygen dopants and nitrogen vacancies into $\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$. During HPHT treatment, adsorbed oxygen atoms are embedded into the lattice, while adjacent nitrogen atoms are expelled to form vacancies. Such dual-defects induce slight band-gap narrowing of $\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$ while significantly promote charge transfer and separation, thereby boosting photocatalytic hydrogen evolution. Theoretical calculations confirm that the oxygen dopants (p-type doping) and nitrogen vacancies (n-type doping) act as donor-acceptor pairs. This charge compensation effect prevents the formation of localized impurity states in the gap but instead modifies the band edges to enhance the oxidation capability and facilitate the spatial separation of electron and hole carriers. Benefiting from these advantages, the optimized $\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4$ exhibits a twofold enhancement in photocatalytic activity, accompanied by a marked increase in carrier concentration. This work provides a novel strategy and fundamental insights for synthesizing highly efficient defect-engineered photocatalysts.

Borophene, a unique two-dimensional boron-based material with a graphenelike structure, has attracted growing interest due to its special configurations and remarkable physical and chemical properties. This study focuses on four different borophene phases: $\alpha , \beta 12, \gamma 3$, and trigonal, and systematically investigates their hydrogen evolution reaction (HER) performance. By using density functional theory (DFT), we evaluate the intrinsic catalytic activity of freestanding borophene monolayers as well as the influence of an Ag(111) substrate on their HER behavior. The results indicate that the freestanding $\alpha , \beta 12$, and $\gamma 3$ phases exhibit excellent HER activity, characterized by optimal Gibbs free energies for hydrogen adsorption. Especially, the on-top site with the lowest coordination number is identified as the most active site. However, interaction with the Ag(111) support significantly modifies the hydrogen binding, leading to suppressed HER performance in these phases. In contrast, the trigonal phase displays relatively poor HER performance, attributed to its fully saturated bonding environment, while the silver substrate significantly modifies its geometric configuration and slightly enhances the HER performance. This work highlights both the promising intrinsic HER potential of borophene monolayers and the critical impact of substrate interactions, guiding the design of efficient borophene-based electrocatalysts.

A quantitative measure of symmetry breaking is introduced that allows the quantification of which symmetries are most strongly broken due to the introduction of some kind of defect in a perfect structure. The method uses a statistical approach based on the Jensen-Shannon divergence. The measure is calculated by comparing the transformed atomic density function with its original. Software code is presented that carries the calculations out numerically using Monte Carlo methods. The behavior of this symmetry breaking measure is tested for various cases including finite size crystallites (where the surfaces break the crystallographic symmetry), atomic displacements from high symmetry positions, and collective motions of atoms due to rotations of rigid octahedra. The approach provides a powerful tool for assessing local symmetry breaking and offers new insights that can help researchers understand how different structural distortions affect different symmetry operations.