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We apply the trajectory formulation to analyze the anomalous dynamics of cold atoms in an optical lattice. The phase space probability density function of cold atoms, their dynamics, and the mechanism of dynamic evolution from an initial Gaussian distribution to a power-law distribution are analyzed. The results of the trajectory formulation are in good agreement with the previously reported experimental results for the exponent of position variance for a long time and the position–momentum correlation. The self-similar natures of trajectories in phase space are found for Lévy distributions. Our results unify the raw moments that can be expressed as the summation of a number of independent, identically distributed variables and the anomalous dynamics, which holds promise for an intuitive interpretation anomalous behavior and their kinetic mechanisms from initial Gaussian to anomalous distributions for a long time. [ABSTRACT FROM AUTHOR]

We describe a theoretical and computational approach to calculate the vibrational, elastic, and thermal properties of materials from the low-temperature quantum regime to the high-temperature anharmonic regime. This approach is based on anharmonic lattice dynamics and the Boltzmann transport equation. It relies on second and third-order force constant tensors estimated by fitting temperature-dependent empirical potentials from path-integral quantum simulations with a first-principles machine learning Hamiltonian. The temperature-renormalized harmonic force constants are used to calculate the elastic moduli and the phonon modes of materials. Harmonic and anharmonic force constants are combined to solve the phonon Boltzmann transport equation to compute the lattice thermal conductivity. We demonstrate the effectiveness of this approach on bulk crystalline silicon in the temperature range from 50 to 1200 K, showing substantial improvement in the prediction of the temperature dependence of the target properties compared to experiments. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is a promising ultra-wide bandgap semiconductor in high-power and high-frequency electronics. The low thermal conductivity of β-Ga2O3, which can be further suppressed by the intrinsic vacancy, has been a major bottleneck for improving the performance of β-Ga2O3 power devices. However, deep knowledge on the thermal transport mechanism of β-Ga2O3 with defect is still lacking now. In this work, the thermal transport of β-Ga2O3 with vacancy defects is investigated using the machine learning-assisted calculation method. First, the machine learning moment tensor potential (MTP), which can accurately describe the lattice dynamics behaviors of pristine β-Ga2O3 and solves the problem of low computational efficiency of existing computational models in β-Ga2O3 large-scale simulations, is developed for studying the thermal transport of the pristine β-Ga2O3. Then, the MTP is further developed for investigating the thermal transport of β-Ga2O3 with vacancy and the thermal conductivity of β-Ga2O3 with oxygen atom vacancies, which are evaluated by machine learning potential combined with molecular dynamics. The result shows that 0.52% oxygen atom vacancies can cause a 52.5% reduction in the thermal conductivity of β-Ga2O3 [100] direction, illustrating that thermal conductivity can be observably suppressed by vacancy. Finally, by analyzing the phonon group velocity, participation ratio, and spectral energy density, the oxygen atom vacancies in β-Ga2O3 are demonstrated to lead to a significant change in harmonic and anharmonic phonon activities. The findings of this study offer crucial insights into the thermal transport properties of β-Ga2O3 and are anticipated to contribute valuable knowledge to the thermal management of power devices based on β-Ga2O3. [ABSTRACT FROM AUTHOR]

We have done comprehensive first-principles calculations of He-vacancy-interstitial clusters in FCC nickel. The calculated total binding energy of V n (Ni vacancy cluster), I n (Ni interstitial cluster), He n (helium interstitial cluster), He n V (helium interstitials in one vacancy), and He n V 2 (helium interstitials in one divacancy) cluster is reported. The total binding energy of a two Ni interstitial cluster is relatively large (1.07 eV), and the binding energy between a monovacancy and a vacancy cluster containing < 20 vacancies is relatively small (< 0.8 eV). The dissociation/emission energy barrier of a Ni interstitial from a He 3 interstitial cluster (three helium interstitial clusters) and a He 8 V cluster (eight helium interstitials in one vacancy) is ≤ 1.06 and ≤ 1.32 eV, respectively. The diffusion activation energy of helium is 1.36 eV via a dissociative mechanism. Comparisons with reported experimental results of helium diffusion and helium desorption have been done to verify the calculation results. The relative stability of stacking fault tetrahedron and void has been investigated further based on quasi-harmonic phonon calculations directly to consider the temperature effects. The reported binding energy results can be used to build molecular simulation potentials or provide input parameters for the cluster dynamics or lattice Monte Carlo simulations of helium-defect cluster evolution. [ABSTRACT FROM AUTHOR]

The pyrite phase of sodium superoxide, NaO 2 , is studied using equilibrium molecular dynamics simulations and lattice dynamics calculations to understand the impacts of static disorder and dynamic disorder on its thermal conductivity. Three structural regimes are observed based on the rotational dynamics and orientations of O 2 − ions. At low temperatures, where the O 2 − ions librate and the system is fully ordered, thermal conductivity exhibits a crystal-like temperature dependence, decreasing with increasing temperature. As temperature increases, the static disorder regime emerges, where the O 2 − ions transition between different orientations on a time scale larger than the librational period. In this regime, the thermal conductivity continues to decrease and then becomes temperature independent. At higher temperatures, where the O 2 − ions freely rotate, the system is dynamically disordered and the thermal conductivity is temperature independent, as in an amorphous solid. Using instantaneous normal mode analysis and Allen–Feldman theory, 80% of the thermal conductivity in the dynamic disorder regime is attributed to diffusons, vibrational modes that are non-propagating and non-localized. When increasing the lattice constant at a constant temperature, transitions from librations to static disorder to dynamic disorder are also observed, with the thermal conductivity decreasing monotonically. The presented methodology can be applied to other crystals with rotational degrees of freedom, offering strategies for the design of thermal conductivity switches that are responsive to external stimuli. [ABSTRACT FROM AUTHOR]

The pyrite phase of sodium superoxide, NaO 2 , is studied using equilibrium molecular dynamics simulations and lattice dynamics calculations to understand the impacts of static disorder and dynamic disorder on its thermal conductivity. Three structural regimes are observed based on the rotational dynamics and orientations of O 2 − ions. At low temperatures, where the O 2 − ions librate and the system is fully ordered, thermal conductivity exhibits a crystal-like temperature dependence, decreasing with increasing temperature. As temperature increases, the static disorder regime emerges, where the O 2 − ions transition between different orientations on a time scale larger than the librational period. In this regime, the thermal conductivity continues to decrease and then becomes temperature independent. At higher temperatures, where the O 2 − ions freely rotate, the system is dynamically disordered and the thermal conductivity is temperature independent, as in an amorphous solid. Using instantaneous normal mode analysis and Allen–Feldman theory, 80% of the thermal conductivity in the dynamic disorder regime is attributed to diffusons, vibrational modes that are non-propagating and non-localized. When increasing the lattice constant at a constant temperature, transitions from librations to static disorder to dynamic disorder are also observed, with the thermal conductivity decreasing monotonically. The presented methodology can be applied to other crystals with rotational degrees of freedom, offering strategies for the design of thermal conductivity switches that are responsive to external stimuli. [ABSTRACT FROM AUTHOR]

First-principles calculations were employed to investigate the impact of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of Pm 3 ̄ AlM(M = Hf, Zr)H6 at pressures of 0.3–21.2 GPa (AlHfH6) and 4.7–39.5 GPa (AlZrH6) within the stochastic self-consistent harmonic approximation. A correction is predicted for the crystal lattice parameters, phonon spectra, and superconducting critical temperatures, previously estimated without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for lattice dynamics. The findings suggest that quantum ionic fluctuations have a significant impact on the crystal lattice parameters, phonon spectra, and superconducting critical temperatures. Based on our anharmonic phonon spectra, the structures will be dynamically stable at 0.3 GPa for AlHfH6 and 6.2 GPa for AlZrH6, ∼6 and 7 GPa lower than pressures given by the harmonic approximation, respectively. Due to the anharmonic correction of their frequencies, the electron–phonon coupling constants (λ) are suppressed by 28% at 11 GPa for AlHfH6 and 22% at 30 GPa for AlZrH6, respectively. The decrease in λ causes Tc to be overestimated by ∼12 K at 11 GPa for AlHfH6 and 30 GPa for AlZrH6. Even if the anharmonic and quantum effects are not as strong as those of Pm 3 ̄ n-AlH3, our results also indicate that metal hydrides with hydrogen atoms in interstitial sites are subject to anharmonic effects. Our results will inevitably stimulate future high-pressure experiments on synthesis, structural, and conductivity measurements. [ABSTRACT FROM AUTHOR]

Halide perovskite nanocrystals are at the forefront of materials research due to their remarkable optoelectronic properties and versatile applications. While their lattice structure and optical properties have been extensively investigated for the structure–property correlation, their lattice dynamics, the physical link between the lattice structure and optoelectronic properties, has been much less visited. We report the evolution of structural dynamics of a series of cesium lead halide perovskite nanocrystals whose size and morphology are systematically varied by synthesis temperature. Low-frequency Raman spectroscopy uncovers the nanocrystals' structural dynamics, including a relaxational spectral continuum from ligand librations and a phonon spectrum evolving with nanocrystal size. As the size of nanocrystals increases, their phonon spectrum becomes more intense, and their spectral weights redistribute with new first- and second-order modes being activated. The linewidth of the observed phonon modes generally broadens as the nanocrystal grows larger, an interesting deviation from the established phonon confinement model. We suggest that strong confinement and truncation of the lattice and ligands anchoring on the surface might lead to pinning of the lattice dynamics at nanoscale. These findings offer new insights into the bulk–nano-transition in halide perovskite soft semiconductors. [ABSTRACT FROM AUTHOR]

The Ising model, initially proposed about 100 years ago to explain ferromagnetism and phase transitions, has become a central pillar of statistical physics and a powerful tool for diverse applications in other fields including environmental studies. In this paper, we introduce continuous spin values between −1 and +1 to a two-dimensional Ising model and utilize the generalized Ising lattice to simulate the dynamics of sea ice/water transition for a large area of 1500 km by 1500 km in the Arctic region. The simulation process follows the Metropolis-Hastings algorithm and incorporates an innovative factor to account for the inertia of spin value changes. Using the sea ice concentration data collected by the National Snow and Ice Data Center, our results exhibit striking similarity between the simulated and the observed ice melting and freezing dynamics, and two numerical measures from the simulation—the ice coverage percentage and the ice extent—match closely with the data statistics. Moreover, the model's best-fit parameters demonstrate the substantial impact of the external forces, which can be further enriched and linked to the environmental factors in other climate change research. [ABSTRACT FROM AUTHOR]

The glassy state of zeolitic imidazolate frameworks (ZIFs) has shown great potential for energy-related applications, including solid electrolytes. However, their thermal conductivity (κ), an essential parameter influencing thermal dissipation, remains largely unexplored. In this work, using a combination of experiments, atomistic simulations, and lattice dynamics calculations, we investigate κ and the underlying heat conduction mechanism in ZIF glasses with varying ratios of imidazolate (Im) to benzimidazolate (bIm) linkers. The substitution of bIm for Im tunes the node–linker couplings but exhibits only a minor impact on the average diffusivity of low-frequency lattice modes. On the other hand, the linker substitution induces significant volume expansion, which, in turn, suppresses the contributions from lattice vibrations to κ, leading to decreased total heat conduction. Furthermore, spatial localization of internal high-frequency linker vibrations is promoted upon substitution, reducing their mode diffusivities. This is ascribed to structural deformations of the bIm units in the glasses. Our work unveils the detailed influences of linker substitution on the dual heat conduction characteristics of ZIF glasses and guides the κ regulation of related hybrid materials in practical applications. [ABSTRACT FROM AUTHOR]

Thermal transport in wurtzite aluminum nitride (w-AlN) significantly affects the performance and reliability of corresponding electronic devices, particularly when lattice strains inevitably impact the thermal properties of w-AlN in practical applications. To accurately model the thermal properties of w-AlN with high efficiency, we develop a machine learning interatomic potential based on the atomic cluster expansion (ACE) framework. The predictive power of the ACE potential against density functional theory (DFT) is demonstrated across a broad range of properties of w-AlN, including ground-state lattice parameters, specific heat capacity, coefficients of thermal expansion, bulk modulus, and harmonic phonon dispersions. Validation of lattice thermal conductivity is further carried out by comparing the ACE-predicted values to the DFT calculations and experiments, exhibiting the overall capability of our ACE potential in sufficiently describing anharmonic phonon interactions. As a practical application, we perform a lattice dynamics analysis using the potential to unravel the effects of biaxial strains on thermal conductivity and phonon properties of w-AlN, which is identified as a significant tuning factor for near-junction thermal design of w-AlN-based electronics. [ABSTRACT FROM AUTHOR]

The enigmatic mechanism underlying unconventional high-temperature superconductivity, especially the role of lattice dynamics, has remained a subject of debate. Theoretical insights have long been hindered due to the lack of an accurate first-principles description of the lattice dynamics of cuprates. Recently, using the r2SCAN meta-generalized gradient approximation (meta-GGA) functional, we have been able to achieve accurate phonon spectra of an insulating cuprate YBa2Cu3O6 and discover significant magnetoelastic coupling in experimentally interesting Cu–O bond stretching optical modes [Ning et al., Phys. Rev. B 107, 045126 (2023)]. We extend this work by comparing Perdew–Burke–Ernzerhof and r2SCAN performances with corrections from the on-site Hubbard U and the D4 van der Waals (vdW) methods, aiming at further understanding on both the materials science side and the density functional side. We demonstrate the importance of vdW and self-interaction corrections for accurate first-principles YBa2Cu3O6 lattice dynamics. Since r2SCAN by itself partially accounts for these effects, the good performance of r2SCAN is now more fully explained. In addition, the performances of the Tao–Mo series of meta-GGAs, which are constructed in a different way from the strongly constrained and appropriately normed (SCAN) meta-GGA and its revised version r2SCAN, are also compared and discussed. [ABSTRACT FROM AUTHOR]

The enigmatic mechanism underlying unconventional high-temperature superconductivity, especially the role of lattice dynamics, has remained a subject of debate. Theoretical insights have long been hindered due to the lack of an accurate first-principles description of the lattice dynamics of cuprates. Recently, using the r2SCAN meta-generalized gradient approximation (meta-GGA) functional, we have been able to achieve accurate phonon spectra of an insulating cuprate YBa2Cu3O6 and discover significant magnetoelastic coupling in experimentally interesting Cu–O bond stretching optical modes [Ning et al., Phys. Rev. B 107, 045126 (2023)]. We extend this work by comparing Perdew–Burke–Ernzerhof and r2SCAN performances with corrections from the on-site Hubbard U and the D4 van der Waals (vdW) methods, aiming at further understanding on both the materials science side and the density functional side. We demonstrate the importance of vdW and self-interaction corrections for accurate first-principles YBa2Cu3O6 lattice dynamics. Since r2SCAN by itself partially accounts for these effects, the good performance of r2SCAN is now more fully explained. In addition, the performances of the Tao–Mo series of meta-GGAs, which are constructed in a different way from the strongly constrained and appropriately normed (SCAN) meta-GGA and its revised version r2SCAN, are also compared and discussed. [ABSTRACT FROM AUTHOR]

The dynamical properties of nuclei, carried by the concept of phonon quasiparticles , are central to the field of condensed matter. While the harmonic approximation can reproduce a number of properties observed in real crystals, the inclusion of anharmonicity in lattice dynamics is essential to accurately predict properties such as heat transport or thermal expansion. For highly anharmonic systems, non-perturbative approaches are needed, which result in renormalized theories of lattice dynamics. In this article, we apply the Mori–Zwanzig projector formalism to derive an exact generalized Langevin equation describing the quantum dynamics of nuclei in a crystal. By projecting this equation on quasiparticles in reciprocal space, and with results from linear response theory, we obtain a formulation of vibrational spectra that fully accounts for the anharmonicity. Using a mode-coupling approach, we construct a systematic perturbative expansion in which each new order is built to minimize the following ones. With a truncation to the lowest order, we show how to obtain a set of self-consistent equations that can describe the lineshapes of quasiparticles. The only inputs needed for the resulting set of equations are the static Kubo correlation functions, which can be computed using (fully quantum) path-integral molecular dynamics or approximated with (classical or ab initio) molecular dynamics. We illustrate the theory with an application on fcc 4He, an archetypal quantum crystal with very strong anharmonicity. [ABSTRACT FROM AUTHOR]

In this work, a creation of donor-acceptor (DA) complexes (trimmers) in highly anisotropic layered crystals of CdI2 is analyzed. The impact of DA complexes on lattice dynamics is investigated. Experimental studies of the temperature dependence of the heat capacity of crystals were applied and the significant impact of impurities in the crystal matrix was revealed. Einstein temperatures for samples of CdI2, CdI2 with Cu, CdI2 with Cu, Au were calculated for the first time and the consistency of the proposed model with theory and experiment is demonstrated. [ABSTRACT FROM AUTHOR]

FFLUX is a multipolar machine‐learned force field that uses Gaussian process regression models trained on data from quantum chemical topology calculations. It offers an efficient way of predicting both lattice and free energies of polymorphs, allowing their stability to be assessed at finite temperatures. Here the Ih, II and XV phases of ice are studied, building on previous work on formamide crystals and liquid water. A Gaussian process regression model of the water monomer was trained, achieving sub‐kJ mol−1 accuracy. The model was then employed in simulations with a Lennard‐Jones potential to represent intermolecular repulsion and dispersion. Lattice constants of the FFLUX‐optimized crystal structures were comparable with those calculated by PBE+D3, with FFLUX calculations estimated to be 103–105 times faster. Lattice dynamics calculations were performed on each phase, with ices Ih and XV found to be dynamically stable through phonon dispersion curves. However, ice II was incorrectly identified as unstable due to the non‐bonded potential used, with a new phase (labelled here as II′ and to our knowledge not found experimentally) identified as more stable. This new phase was also found to be dynamically stable using density functional theory but, unlike in FFLUX calculations, II remained the more stable phase. Finally, Gibbs free energies were accessed through the quasi‐harmonic approximation for the first time using FFLUX, allowing thermodynamic stability to be assessed at different temperatures and pressures through the construction of a phase diagram. [ABSTRACT FROM AUTHOR]

The lattice Boltzmann method (LBM) is a finite element and finite volume method for studying the reaction rate, mass diffusion and concentration of species. We are used the LBM to investigate the effect of the Damköhler number (Da) and Reynolds number (Re) on the laminar flow in a channel with an open square cavity and a reactive bottom wall in two dimensions in a first-order chemical reaction. The reactant A is transported through the cavity, where it undergoes a reaction on the reactive surface, resulting in the synthesis of product B. The effect of Da < 1 on the reaction rate is negligible for all investigated Re values; the generation of product B is slower because of the effect of the momentum diffusivity on the velocity inside the cavity. For Re = 5 and 1 < Da ≤ 100, the concentration of B inside the cavity reaches the maximum for Da = 100, and A is absorbed almost entirely on the bottom of the cavity. In our simulations, we observed that for all values of Re and Da > 100, the effect of the momentum diffusivity is negligible in the cavity, and the reaction on the surface is almost instantaneous. [ABSTRACT FROM AUTHOR]

In this study, advanced first‐principles calculations combined with self‐consistent phonon theory and the Boltzmann transport equation are used to assess the thermoelectric properties of novel Full–Heusler materials X2YTe (X = Na, K, Rb, Cs; Y = Zn, Cd, Hg). These materials exhibit low formation energies, facilitating synthesis under standard conditions. This analysis showed that lattice anharmonicity increases with the atomic mass of X and decreases with the atomic mass of Y due to the rattling effect of Y atoms. Significant rattling effects are identified, prompting the inclusion of higher‐order anharmonic effects. By renormalizing the phonon spectrum and accounting for three‐phonon and four‐phonon scattering, the mechanisms behind the low lattice thermal conductivity in these compounds is uncovered. The combination of low lattice thermal conductivity and high power factors resulted in remarkable thermoelectric figure of merit values at optimal doping concentrations and temperatures. Notably, except for Na2ZnTe and Na2HgTe, all other materials surpassed the long‐standing figure of merit record of less than 3, with Rb2ZnTe achieving a figure of merit of 8.11 at 700 K with an n‐type doping concentration of 2 × 1019. The inclusion of spin‐orbit coupling led to less than a 10% reduction in the thermoelectric figure of merit, underscoring the superior thermoelectric performance of these materials. [ABSTRACT FROM AUTHOR]

Nanomaterials, heralded as the "new materials of the 21st century" for their remarkable physical and chemical properties and broad application potential, have attracted substantial attention in recent years. Among these materials, which challenge traditional physical boundaries, nanodiamonds (NDs) are widely applied across diverse industries due to their exceptional surface multifunctionality and chemical stability. Nevertheless, atomic-level manipulation of NDs presents considerable challenges, which require detailed structural analysis to thoroughly elucidate their properties. This study utilizes density functional theory (DFT), lattice dynamics, and molecular dynamics (MD) simulations to analyze the structural and property characteristics of NDs. Fine structural analysis reveals that, despite variations in particle size, surface layer thickness remains relatively constant at approximately 3 Å. DFT methods enable computation of the surface layer to capture subtle electronic characteristics, while the internal core is analyzed via MD. Further investigation into amorphous structure control indicates that ND surface amorphous structures with a packing coefficient above 0.38 are thermodynamically stable. This study offers a novel approach to nanomaterial control in practical applications by elucidating the core–shell interactions and surface structures of NDs. [ABSTRACT FROM AUTHOR]

ABSTRACT Sr2YRuO6 is a material that provides an ideal platform for studying magnetic frustration in three‐dimensional geometries. Herein, we combine Raman spectroscopy and density functional theory calculations to establish connections between the lattice dynamics and magnetic states in Sr2YRuO6 single crystals. The x‐ray diffraction profiles reveal that Sr2YRuO6 possesses an ordered double‐perovskite structure with distorted monoclinic symmetry. Three magnetic phase transitions are observed and linked to the presence of weak ferromagnetism at 135 K and short‐ and long‐range antiferromagnetic orderings at 32 and 26 K. The oxygen‐octahedron antistretching and stretching modes, observed at 570 and 766 cm−1, exhibit anomalies near the magnetic phase transition temperatures, indicating an intriguing interplay between the lattice and spin degrees of freedom. Their spin–phonon coupling constants of 0.7 cm−1 reflect the weak spin–lattice interactions in Sr2YRuO6. [ABSTRACT FROM AUTHOR]

Graphene, celebrated for its remarkable atomic structure, stands as a key material in diverse fields such as nanoelectronics and materials science. This paper presents a thorough investigation into the metric dimensions of graphene, emphasizing its unique spatial properties and their broad implications. We analyze how these metric dimensions affect graphene’s electrical, thermal, and mechanical behavior in various applications. Our study delves into the physical mechanisms behind these properties, including atomic interactions, lattice dynamics, and external factors like temperature and pressure.The metric dimension of a graph G is the minimum cardinality of a subset W⊆V(G) such that every pair of distinct vertices u,v∈V(G) is uniquely identified by their distance vectors relative to W and the resolving set, ensures for any u,v∈V(G) with u≠v, there exists a vertex w∈W such that d(u,w)≠d(v,w), where d denotes the distance function in G. In this work, we will determine the metric dimension and Resolving set. However, these findings pave the way for innovative applications in electronics, composites, and beyond. [ABSTRACT FROM AUTHOR]

In this paper, we numerically study the evolution of a classical real scalar field in (1 + 1) dimensions with initial conditions describing thermal fluctuations around a metastable vacuum. We track false vacuum decay in real time and compare several observables to the predictions of the standard Euclidean formalism. We find agreement for the shape of the critical bubble and the exponential suppression of the decay rate. However, the decay rate prefactor is almost an order of magnitude lower than the predicted value. We argue that this signals a breakdown of thermal equilibrium during the bubble nucleation. In addition, the inefficient thermalization in the system biases the properties of the statistical ensemble and leads to further decrease of the decay rate with time. We substantiate our interpretation with a suite of stochastic field simulations with controlled thermalization time. By varying this time, we find that the predictions of the standard equilibrium formalism are recovered when it is sufficiently short. We propose an upper bound on the thermalization time that must be satisfied in order to ensure the applicability of the Euclidean rate calculation. We discuss that this bound is unavoidably violated in common single-field models, irrespective of the number of space–time dimensions, implying that deviations from equilibrium in these models cannot be neglected. In theories with multiple fields, the bound may or may not hold, depending on the setup details. We investigate one more signature of nonequilibrium dynamics — coherent oscillonic precursors to the critical bubble nucleation. We show that they get suppressed in the stochastic dynamical simulations when the thermalization time is reduced. [ABSTRACT FROM AUTHOR]

The energetics of nucleation of martensite by soft mode mechanism is studied. First the soft modes of a 2D model material are identified by means of classical lattice dynamics. This soft mode is mimicked by inducing static displacement modulations in the austenitic lattice and calculating the potential energy of the whole material for various wave packages. It is found that modulations with small spatial extent cause an increase of the potential energy and are expected to yield oscillatory motion of the atoms. As the spatial extent of the modulation increases – the potential energy of unstable modes descends into local minima with increasing amplitude of the modulation. The spatial extent which thus allows the transformation is a critical modulation. [ABSTRACT FROM AUTHOR]

Transition metal ion doping presents an effective approach to enhance the magnetic properties of BiFeO3 (BFO). Herein, we explored the impact of dual transition metal ion doping in BFO by preparing BFO and BiFe0.9CoxMn0.1-xO3 (X = 0, 0.03, 0.05, 0.07, 0.1) samples using a sol-gel method. Comprehensive investigations into the substitution-driven structural, optical and magnetic transformation of Mn, Co co-doped BiFeO3 have been conducted. The dual ions doping led to the BFO's structural distortion, which was proved by the Riteveld refinement and Raman spectra. With the addition of dual ions, new energy levels might create additional absorption channels for photons, thereby, increasing photon absorption efficiency and adjusting the bandgap. Analysis of magnetic hysteresis data indicates enhanced magnetism in doped samples. Particularly noteworthy is the coercivity of the BiFe0.9Co0.03Mn0.07O3 sample, which reaches 14989.6 Oe, compared to the control sample's coercivity of only 163.9 Oe—almost 100 times greater. This study underscores the efficiency of varying the ratio of double ions in doping samples for enhancing both magnetic and optical properties. [ABSTRACT FROM AUTHOR]

Beginning with a more generalized discrete 2 × 2 matrix spectral problem and applying the Tu scheme, a mixed integrable lattice hierarchy based on the negative and positive lattice hierarchies is constructed, it includes the well-known relativistic Toda lattice hierarchy and can reduce to other new integrable lattice hierarchies. For the first nontrivial lattice equation in the mixed hierarchy, the corresponding infinite number of conservation laws and N -fold Darboux transformation are established on the base of its Lax pair. As an application of the obtained Darboux transformation, we obtain the discrete N -fold explicit solutions in determinant form, from which we get one-and two-soliton solutions with proper parameters and their dynamical properties and evolutions are illustrated graphically. Some interesting soliton structures are presented, such as kink and bell-shaped two-soliton, bell and anti-bell shaped two-soliton and anti-bell shaped two-soliton and so on. What is more, we observe that these solitary waves pass through without change of shapes, amplitudes, wave-lengths and directions, which means they are much stable during the propagation. These results and properties given in this paper may help us better understand nonlinear lattice dynamics. [ABSTRACT FROM AUTHOR]

Transition metal ion doping presents an effective approach to enhance the magnetic properties of BiFeO3 (BFO). Herein, we explored the impact of dual transition metal ion doping in BFO by preparing BFO and BiFe0.9CoxMn0.1-xO3 (X = 0, 0.03, 0.05, 0.07, 0.1) samples using a sol-gel method. Comprehensive investigations into the substitution-driven structural, optical and magnetic transformation of Mn, Co co-doped BiFeO3 have been conducted. The dual ions doping led to the BFO's structural distortion, which was proved by the Riteveld refinement and Raman spectra. With the addition of dual ions, new energy levels might create additional absorption channels for photons, thereby, increasing photon absorption efficiency and adjusting the bandgap. Analysis of magnetic hysteresis data indicates enhanced magnetism in doped samples. Particularly noteworthy is the coercivity of the BiFe0.9Co0.03Mn0.07O3 sample, which reaches 14989.6 Oe, compared to the control sample's coercivity of only 163.9 Oe—almost 100 times greater. This study underscores the efficiency of varying the ratio of double ions in doping samples for enhancing both magnetic and optical properties. [ABSTRACT FROM AUTHOR]

X-ray diffraction (XRD) has evolved significantly since its inception, becoming a crucial tool for material structure characterization. Advancements in theory, experimental techniques, diffractometers and detection technology have led to the acquisition of highly accurate diffraction patterns, surpassing previous expectations. Extracting comprehensive information from these patterns necessitates different models due to the influence of both electron density and thermal motion on diffracted beam intensity. While electron-density modelling has seen considerable progress [e.g. the Hansen–Coppens multipole model and Hirshfeld Atom Refinement (HAR)], the treatment of thermal motion has remained largely unchanged. We have developed a novel method that combines the strengths of the advanced charge-density models [Aspherical Atom Models (AAMs), such as HAR or the Transferable Aspherical Atom Model (TAAM)] and the thermal motion model (normal modes refinement, NoMoRe). We denote this approach AAM_NoMoRe, wherein instead of refining routine anisotropic displacement parameters (ADPs) against single-crystal X-ray diffraction data, we refine the frequencies obtained from periodic density functional theory (DFT) calculations. In this work, we demonstrate the effectiveness of this model by presenting its application to model compounds, such as alanine, xylitol, naphthalene and glycine polymorphs, highlighting the influence of our method on the H-atom positions and shape of their ADPs, which are comparable with neutron data. We observe a significant decrease in the similarity index for H-atom ADPs after AAM_NoMoRe in comparison to only AAM, aligning more closely with neutron data. Due to the use of aspherical form factors (AAM), our approach demonstrates better fitting performance, as indicated by consistently lower wR2 values compared to the Independent Atom Model (IAM) refinement and a significant decrease compared to the traditional NoMoRe model. Furthermore, we present the estimation of a key thermodynamic property, namely, heat capacity, and demonstrate its alignment with experimental calorimetric data.

Lattice dynamics (LD) enables the calculation of mode-resolved transmittance of phonons passing through an interface, which is essential for understanding and controlling the thermal boundary conductance (TBC). However, the original LD method may yield unphysical transmittance over 100% due to the absence of the constraint of energy conservation. Here, we present a robust LD algorithm that utilizes linear algebra transformations and projection gradient descent iterations to ensure energy conservation. Our approach demonstrates consistency with the original LD method on the atomically smooth Si/Ge interface and exhibits robustness on rough Si/Ge interfaces. The evanescent modes and localized effects at the interface are revealed. In addition, bottom-up analysis of the phonon transmittance shows that the anisotropy in the azimuth angle can be ignored, while the dependency on the frequency and polar angle can be decoupled. The decoupled expression reproduces the TBC precisely. This work provides comprehensive insights into the mode-resolved phonon transmittance across interfaces and paves the way for further research into the mechanism of TBC and its relation to atomic structures. [ABSTRACT FROM AUTHOR]

The all-temperature magnon (ATM) theory [Datta and A. Panda, J. Phys. Condens. Matter 21, 336003 (2009)] has been used to analyze the temperature dependence of magnetization and internal energy components of a mono-domain ferromagnetic solid. One impact of the ATM formulation is that calculated critical exponents are in better agreement with experiments than their counterparts from mean-field and critical phenomenon theories. These exponents can vary from one ferromagnet to another of similar symmetry and dimensionality but differing in spin and can be field-dependent. The ATM finding is that exponent β depends on spin and increases as T approaches TC, whereas the exponent γ is weakly dependent on spin and the applied field but relies on crystal symmetry. The main thrust of the present work has been to derive the thermally averaged spin-center force constants in terms of the baseline related (solid) and exchange-cum-field mediated (magnetic) components and to formulate phonon frequencies and their modifications by magnon–phonon coupling. The derived expressions are suitable for correct quantum chemical evaluation. A detailed calculation on different spin configurations at varying geometries is still hardly possible and beyond the scope of the present work that emphasizes the correctness of formulas and has the significance of explaining properties. The phonon frequency shift due to lattice expansion is always negative. It is also clarified that frequency modification by the magnon–phonon interaction is negative for certain phonon branches near TC, and the ratio of frequency modification and phonon frequency is approximately proportional to the ratio of curvatures of involved energy surfaces. [ABSTRACT FROM AUTHOR]

We investigate the existence, stability and propagation dynamics of shock waves existing in defocusing saturable media modulated by a semi-infinite photonic lattice. Such surface waves consist of modulationally stable pedestals in bulk media and decaying oscillatory tails in semi-infinite photonic lattices. Due to Bragg-type reflection, they are strongly localized at the edge of the optical lattice. The kink steepness, pedestal height and localization degree can be controlled by propagation constant, saturable degree and lattice depth. Two types of kinks, i.e., "out-of-phase" and two branches of "in-phase" shock waves are revealed. Out-of-phase shock waves are stable in a substantial part of their existence domain. While the lower-branch in-phase waves with low energy flow are stable in almost their whole existence domain, the higher-branch in-phase waves are completely unstable. We thus show the first example of stable in-phase shock waves in nonlinear optics. Our findings provide new insight into the dynamics of semi-localized nonlinear surface shock waves or kink solitons. [ABSTRACT FROM AUTHOR]

The driving force of a charge density wave (CDW) transition in quasi-two dimensional systems is still debated, while being crucial in understanding electronic correlation in such materials. Here we use femtosecond time- and angle-resolved photoemission spectroscopy combined with computational methods to investigate the coherent lattice dynamics of a prototypical CDW system. The photo-induced temporal evolution of the periodic lattice distortion associated with the amplitude mode reveals the dynamics of the free energy functional governing the order parameter. Our approach establishes that optically-induced screening rather than CDW melting at the electronic level leads to a transiently modified potential which explains the anharmonic behaviour of the amplitude mode and discloses the structural origin of the symmetry-breaking phase transition. The charge density wave (CDW) formation mechanisms in 2D and quasi-2D systems are still highly debated. Here, the authors combine time-resolved ARPES and ab initio calculations to map the free energy functional in the prototypical CDW compound 1T-TaSe2 concluding that the CDW state is driven by structural rather than electronic instabilities. [ABSTRACT FROM AUTHOR]

Strain engineering, as a non‐chemical tuning knob, can enhance the performance of semiconductor devices. Here, an efficient manipulation of light emission is revealed in thin‐layered 2D perovskite strongly correlated to layer numbers of [PbI6]4− octahedron (n) and [C6H5(CH2)2NH3]2(CH3NH3)n‐1PbnI3n+1 (N) by applying uniaxial strains (ɛ) via bending the flexible substrate. As increases from 1 to 3, an efficient light emission redshift (ɛ from −0.97% to 0.97%) is observed from bandgap shrinkage, and the shrinkage rate increases from 1.97 to 10.38 meV/%, which is attributed to the predominant uniaxial intralayer deformation due to the anisotropy of the [PbI6]4− octahedron lattice strain. Conversely, as increases from 7 to 48 for n = 3, the deformation related to bandgap shrinkage rate is more prominent in small‐N flakes ( ≈ 7, 15.2 meV/%) but is easily offset in large‐N flakes ( ≈ 48, 7.7 meV/%). This anisotropic lattice deformation, meanwhile, inevitably modulates the carrier recombination dynamics of [C6H5(CH2)2NH3]2(CH3NH3)n‐1PbnI3n+1, which is essential for the development of highly efficient photoelectronic devices. [ABSTRACT FROM AUTHOR]

Lithium niobate (LNO) and lithium tantalate (LTO) see widespread use in fundamental research and commercial technologies reaching from electronics over classical optics to integrated quantum communication. The mixed crystal system lithium niobate tantalate (LNT) allows for the dedicate engineering of material properties by combining the advantages of the two parental materials LNO and LTO. Vibrational spectroscopies such as Raman spectroscopy or (Fourier transform) infrared (IR) spectroscopy are vital techniques to provide detailed insight into the material properties, which is central to the analysis and optimization of devices. This work presents a joint experimental–theoretical approach allowing to unambiguously assign the spectral features in the LNT material family through both Raman and IR spectroscopy, as well as providing an in‐depth explanation for the observed scattering efficiencies based on first‐principles calculations. The phononic contribution to the static dielectric tensor is calculated from the experimental and theoretical data using the generalized Lyddane–Sachs–Teller relation and compared with the results of the first‐principles calculations. [ABSTRACT FROM AUTHOR]

We argue that the spatial discretization of the strongly nonlinear Lefever–Lejeune partial differential equation defines a nonlinear lattice that is physically relevant in the context of the nonlinear physics of ecosystems, modelling the dynamics of vegetation densities in dry lands. We study the system in the lattice ℤ2$$ {\mathrm{\mathbb{Z}}}&#x0005E;2 $$, which is especially relevant because of its natural dimension for the emergence of pattern formation. Theoretical results identify parametric regimes for the system that distinguish between extinction and potential convergence to nontrivial states. Importantly, we analytically identify conditions for Turing instability, detecting thresholds on the discretization parameter for the manifestation of this mechanism. Numerical simulations reveal the sharpness of the analytical conditions for instability and illustrate the rich potential for pattern formation even in the strongly discrete regime, emphasizing the importance of the interplay between higher dimensionality and discreteness. [ABSTRACT FROM AUTHOR]