Dynamic Anharmonicity Research Articles

Long-Range Anion Correlations Mediating Dynamic Anharmonicity and Contributing to Glassy Thermal Conductivity in Well-Ordered K2Ag4Se3.

Herein an extremely low (0.32‒0.25 Wm-1K-1) and glassy temperature-dependence (300-600K) of lattice thermal conductivity (κlat)in a monoclinic K2Ag4Se3 is reported. It is found that the effective carrier delocalization, contributed by the perfect p-d* hybridization paradigm, can efficiently facilitate the spatial transfer of electron cloud perturbations induced by the anisotropic thermal vibrations of Ag4 atoms, thereby favoring long-range Se‒Se correlations. The localized rattling-like vibration of Ag4 atoms induce short phonon lifetimes, large scattering phase space, and then a low particle-like propagation. While the correlated interactions mediated competitive expressions between bubble diagrams and loop diagrams can suppress the generation of wavelike phonons from off-diagonal coupling. Ultimately, the AgSe3 structural units can enable the dual confinement of both the particle-like propagation of phonons and wavelike tunneling of coherence. The study highlights that the correlated AgSe3 coordination units can simultaneously target particle-like and wavelike phonons and then reduce their contribution to the κlat by mediating long-range transfer of charge polarization. These fundamental advances will advance the design of crystalline materials with tailored thermal properties.

Coherent Transfer of Lattice Entropy via Extreme Nonlinear Phononics in Metal Halide Perovskites

Entropy transfer in metal halide perovskites, characterized by significant lattice anharmonicity and low stiffness, underlies the remarkable properties observed in their optoelectronic applications, ranging from solar cells to lasers. The conventional view of this transfer involves stochastic processes occurring within a thermal bath of phonons, where the lattice arrangement and energy flow from higher- to lower-frequency modes. Here, we unveil a comprehensive chronological sequence detailing a conceptually distinct coherent transfer of entropy in a prototypical perovskite CH3NH3Pbl3. The terahertz periodic modulation imposes vibrational coherence into electronic states, leading to the emergence of mixed (vibronic) quantum beat between approximately 3 and 0.3 THz. We highlight a well-structured bidirectional time-frequency transfer of these diverse phonon modes, each developing at different times and transitioning from high to low frequencies from 3 to 0.3 THz, before reversing direction and ascending to around 0.8 THz. First-principles molecular dynamics simulations disentangle a complex web of coherent-phononic coupling pathways and identify the salient roles of the initial modes in shaping entropy evolution at later stages. Capitalizing on coherent entropy transfer and dynamic anharmonicity presents a compelling opportunity to exceed the fundamental thermodynamic (Shockley-Queisser) limit of photoconversion efficiency and to pioneer novel optoelectronic functionalities. Published by the American Physical Society 2024

Lead-free Zr-doped ceria ceramics with low permittivity displaying giant electrostriction

Electrostrictors, materials developing mechanical strain proportional to the square of the applied electric field, present many advantages for mechanical actuation as they convert electrical energy into mechanical, but not vice versa. Both high relative permittivity and reliance on Pb as the key component in commercial electrostrictors pose serious practical and health problems. Here we describe a low relative permittivity (<250) ceramic, ZrxCe1-xO2 (x < 0.2), that displays electromechanical properties rivaling those of the best performing electrostrictors: longitudinal electrostriction strain coefficient ~10−16 m2/V2; relaxation frequency ≈ a few kHz; and strain ≥0.02%. Combining X-ray absorption spectroscopy, atomic-level modeling and electromechanical measurements, here we show that electrostriction in ZrxCe1-xO2 is enabled by elastic dipoles produced by anharmonic motion of the smaller isovalent dopant (Zr). Unlike the elastic dipoles in aliovalent doped ceria, which are present even in the absence of an applied elastic or electric field, the elastic dipoles in ZrxCe1-xO2 are formed only under applied anisotropic field. The local descriptors of electrostrictive strain, namely, the cation size mismatch and dynamic anharmonicity, are sufficiently versatile to guide future searches in other polycrystalline solids.

Molecular dynamics simulations of the dynamics of ions in single and mixed alkali glasses

Molecular dynamics simulations were carried out at 700K in single alkali and mixed alkali metasilicate glasses to reproduce the dramatic mixed alkali effect occurring in the dilute foreign alkali region. Previous experimental work showed that one foreign alkali appears to immobilize a large number of host ions. This large number can now be rationalized with the help of the simulations as being due to blockage by the less mobile foreign ions of the co-operative ion dynamics that originate from ion–ion interaction and correlation. Molecular dynamics simulations of Li metasilicate glass at 500K showed that the ubiquitous nearly constant loss (NCL) comes from the motions of caged Li ions that experience ‘dynamic anharmonicity’ caused by the motions of the caging matrix atoms, particularly the oxygen atoms. The length scales of the Li ion motions are distributed according to a Lévy distribution that has a long tail, which is responsible for the peculiar frequency dependence of the NCL.

Dynamics of caged ions in glassy ionic conductors.

At sufficiently high frequency and low temperature, the dielectric responses of glassy, crystalline, and molten ionic conductors all invariably exhibit nearly constant loss. This ubiquitous characteristic occurs in the short-time regime when the ions are still caged, indicating that it could be a determining factor of the mobility of the ions in conduction at longer times. An improved understanding of its origin should benefit the research of ion conducting materials for portable energy source as well as the resolution of the fundamental problem of the dynamics of ions. We perform molecular dynamics simulations of glassy lithium metasilicate (Li2SiO3) and find that the length scales of the caged Li+ ions motions are distributed according to a Levy distribution that has a long tail. These results suggest that the nearly constant loss originates from "dynamic anharmonicity" experienced by the moving but caged Li+ ions and provided by the surrounding matrix atoms executing correlated movements. The results pave the way for rigorous treatments of caged ion dynamics by nonlinear Hamiltonian dynamics.

Philosophy of scaling the quantum mechanical molecular force field versus philosophy of solving the inverse vibrational problem

The peculiarities characterising the traditional approach used in calculational vibrational spectroscopy and the approach based on using scaled quantum mechanical force fields are considered. Some results on the determination of the equilibrium geometry of benzene in both the harmonic approximation and in the approximation taking into account the kinematic and dynamic anharmonicity corrections by solving the inverse vibrational problem are discussed. Using the quantum mechanical force fields of the C 2F 6 molecule, calculated at three different theoretical levels as an example, the results of the determination of scale factors by different mathematical techniques are compared.

Fifth-order raman spectrum of an atomic liquid: simulation and instantaneous-normal-mode calculation

Experimental artifacts and technical difficulties in carrying out theoretical calculations have consistently frustrated attempts to obtain the two-dimensional (5th-order) Raman spectrum of a liquid. We report here a new theoretical development: the first microscopic numerical simulation of the 5th-order Raman signal in a liquid. Comparison with an instantaneous-normal-mode treatment, a fully microscopic model which interprets liquid dynamics as arising from coherent harmonic modes, shows that the 5th-order spectrum reveals profound effects stemming from dynamical anharmonicity.

Equilibrium Structure of Beryllium Dibromide from Combined Gas Electron Diffraction and Vibrational Spectroscopy Analysis

The molecular structure of BeBr2 has been investigated by gas-phase electron diffraction at the temperature 800(10) K. The conventional analysis yielded the following values: rg(Be–Br) = 1.944(6)A, l(Be–Br) = 0.068(4)A, rg(Br–Br) = 3.848(8)A, l(Br–Br) = 0.109(3)A, k(Be–Br) = 1.1(1.1) × 10−5 A3, κ(Br–Br) = 2.1(1.0) × 10−5 A3. Three models of nuclear dynamics were used to simulate the conventional analysis values—infinitesimal vibrations and two models, which take into account the kinematic and dynamic anharmonicity of the bending vibration. All models give similar values of bond angle, amplitudes, and shrinkage, excluding the harmonic model, which yields too low value l(Br–Br). The equilibrium bond distance re(Be–Br) = 1.932(11) A was estimated, taking into account the anharmonicity corrections for stretching and bending vibrations and centrifugal distortion.

A classical molecular dynamics study of the vibrational dynamics of the H/C(111)-(1×1) system

Vibrational dynamics of H atoms on C(111) surface was studied by classical molecular dynamics method with the use of a harmonic force field for the diamond lattice and of both harmonic and anharmonic potential for the C–H bond. Results of the calculations show a strong coupling of the C–C–H bending vibration to the the lattice modes. In addition to the experimentally observed band of the bending vibration at 1331 cm −1 the calculations predict a band at ca. 1000 cm −1 in the infrared spectrum of the system with the surface-parallel polarization. Results of the study show a strong dynamic anharmonicity of the large amplitude vibrations of hydrogen atoms. Simulations of the C–H stretching energy relaxation lead to values of the energy relaxation time close to those measured. The agreement is improved in the anharmonic model for the C–H bond potential. Results of the calculations point to the interaction between the C–H stretching vibration and the overtone of the C–C–H bending vibration as the main channel for the energy relaxation.

Coupling effect between quadrupole phonon and pairing vibration in odd-mass nuclei

Abstract The dynamical anharmonicity effect due to the coupling between a quadrupole-phonon mode and a pairing-vibrational mode is analyzed by applying the Dyson mapping theory to odd-mass nuclei. The original fermion (quasiparticle) space consisting of a product of multi-quadrupole-phonon states, multi-pairing-vibration states, and single-quasiparticle states is transformed into an ideal boson-fermion space by the Dyson mapping. In this ideal boson-fermion space, numerical analyses are carried out for some odd-mass nuclei, by putting special emphasis on 93 Nb. The results show that the effect of many-phonon states including both of the quadrupole phonon and the pairing vibration is very important and the Dyson mapping theory is quite useful for such analyses.

Microscopic Description of Anharmonic Gamma-Vibrations by Means of the Selfconsistent-Collective-Coordinate Method. III

Properties of the anharmonic gamma-vibrations and their (N, Z)-dependence are investigated in a wide region of the rare-earth deformed nuclei. The effective Hamiltonian which contains, in addition to the monopole-pairing plus (doubly-stretched) quadrupole-quadrupole interactions, the quadrupole-pairing interaction and the effective three-body force is used in the microscopic calculation. Dynamical anharmonicity effects arising from the mode-mode couplings are analysed in detail in relation to these effective interactions. The couplings between the gamma-vibrations and the pairing rotations are also taken into account in order to recover the nucleon-number consevation. The numerical calculation indicates that the properties of the anharmonic gamma-vibrations significantly change from one nucleus to another reflecting the subshell structure near the Fermi surface. A characteristic (N, Z)-dependence of the double gamma-vibrational states is predicted for some Dy and Er isotopes.

Semirigid model of the bending-rotational Hamiltonian in electron diffraction analysis of triatomic molecules

In order to investigate the structure and to determine the parameters of the potential functions for triatomic molecules of the XY/sub 2/ type using the gas electron diffraction method, the authors have proposed a semirigid model for the bending-rotational Hamiltonian, which has been previously applied in the analysis of vibrational-rotational spectra. The authors have obtained relationships for the statistical probability density distribution functions for the internal coordinates and the electron scattering intensities, taking into account the effects of vibrational-rotational coupling, kinematic and dynamic anharmonicity of the vibrations. The proposed approach does not require a priori knowledge of the symmetry of the equilibrium configuration, and may be used in electron diffraction studies of arbitrary molecules of the XY/sub 2/ type.

Dynamical Anharmonicity Effects in Low-Lying Negative-Parity States of Odd-Mass Sn Isotopes

Dynamical Anharmonicity Effects in Low-Lying Negative-Parity States of Odd-Mass Sn Isotopes