Phonon Eigenvectors Research Articles

Unraveling Hidden Charge Density Wave Phases in 1T-TiSe_{2}.

The unexpected chiral order observed in 1T-TiSe_{2} represents an exciting area to explore chirality in condensed matter, while its microscopic mechanism remains elusive. Here, we have identified three metastable collective modes-the so-called single-q modes-in single layer TiSe_{2}, which originate from the unstable phonon eigenvectors at the zone boundary and break the threefold rotational symmetry. We show that polarized laser pulse is a unique and efficient tool to reconstruct the transient potential energy surface, so as to drive phase transitions between these states. By designing sequent layers with chiral stacking order, we propose a practical means to realize chiral charge density waves in 1T-TiSe_{2}. Further, the constructed chiral structure is predicted to exhibit circular dichroism as observed in recent experiments. These facts strongly indicate the chirality transfer from photons to the electron subsystem, meanwhile being strongly coupled to the lattice degree of freedom. Our work provides new insights into understanding and modulating chirality in quantum materials that we hope will spark further experimental investigation.

Pressure-induced evolution of the lattice dynamics for selected UO3 polymorphs

Uranium trioxide (UO3) is a stable chemical form of uranium oxide with multiple polymorphs found throughout the nuclear fuel cycle. The pressure-induced changes in the structure and lattice dynamics of four of these polymorphs are simulated with density functional perturbation theory and analyzed. Two phases, α- and δ-UO3 are found to exhibit an isotropic response to pressure and do not undergo any changes in coordination geometry up to ∼40 GPa. In contrast, the other two phases investigated, β- and γ-UO3, exhibit an anisotropic response to pressure. Decomposition of the phonon eigenvectors allows us to assign specific pressure-induced structural changes to individual phonon modes. This analysis has been performed on a per atom basis for the relatively simple α- and δ-UO3 structures, which have one symmetrically unique uranium site, and on a per coordination environment basis for β- and γ-UO3, which have multiple U sites.

Anisotropy in the dielectric function of Bi2Te3 from first principles: From the UV-visible to the infrared range

The dielectric properties of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, a layered compound crystallizing in a rhombohedral structure, are investigated by means of first-principles calculations at the random phase approximation level. A special attention is devoted to the anisotropy in the dielectric function and to the local field effects that strongly renormalize the optical properties in the UV-visible range when the electric field is polarized along the stacking axis. Furthermore, both the Born effective charges for each atom and the zone center phonon frequencies and eigenvectors needed to describe the dielectric response in the infrared range are computed. Our theoretical near-normal incidence reflectivity spectra in both the UV-visible and infrared range are in fairly good agreement with the experimental spectra, provided that the free carriers Drude contribution arising from defects is included in the infrared response. The anisotropic plasmon frequencies entering the Drude model are computed within the rigid band approximation, suggesting that a measurement of the reflectivity in the infrared range for both polarizations might allow one to infer not only the type of doping but also the level of doping.

Possible structural quantum criticality tuned by rare-earth ion substitution in infinite-layer nickelates

Materials with competing phases that have similar ground state energies often exhibit a complex phase diagram. Cuprates are a paradigmatic example of such a system that show competition between charge, magnetic, and superconducting orders. The infinite-layer nickelates have recently been revealed to feature similar characteristics. In this paper, I show that these nickelates are additionally near a structural quantum critical point by mapping the energetics of their structural instabilities using first-principles calculations. I first confirm previous results that show a phonon instability in the $P4/mmm$ phase leading to the $I4/mcm$ structure for $R\mathrm{Ni}{\mathrm{O}}_{2}$ with $R$ = Sm--Lu. I then study the non-spin-polarized phonon dispersions of the $I4/mcm$ phase and find that they exhibit rare-earth size-dependent instabilities at the $X$ and $M$ points for materials with $R$ = Eu--Lu. Group-theoretical analysis was used to enumerate all the isotropy subgroups due to these instabilities, and the distorted structures corresponding to their order parameters were generated using the eigenvectors of the unstable phonons. These structures were then fully relaxed by minimizing both the atomic forces and lattice stresses. I was able to stabilize only five out of the twelve possible distortions. The $Pbcn$ isotropy subgroup with the ${M}_{5}^{+}(a,a)$ order parameter shows noticeable energy gain relative to other distortions for the compounds with late rare-earth ions. However, the order parameter of the lowest-energy phase switches first to ${X}_{2}^{\ensuremath{-}}(0,a)+{M}_{5}^{+}(b,0)$ and then to ${X}_{2}^{\ensuremath{-}}(0,a)$ as the size of the rare-earth ion is progressively increased. Additionally, several distorted structures lie close in energy for the early members of this series. These features of the structural energetics persist even when antiferromagnetism is allowed. Such a competition between different order parameters that can be tuned by rare-earth ion substitution suggests that any structural transition that could arise from the phonon instabilities present in these materials can be suppressed to 0 K.

Lattice dynamics and Raman spectrum of supertetragonal PbVO3

Lead vanadate PbVO3 is a polar crystal with a P4mm space group under ambient conditions. PbVO3 is isostructural with the model soft mode-driven ferroelectric PbTiO3, but it differs due to the so-called “supertetragonal” elongation of its unit cell. In this study, we investigated the lattice dynamics of PbVO3 based on Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical phonon modes were identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374, and 508 cm−1; A1 modes at 188, 429, and 874 cm−1, and B1 mode at 319 cm−1. The calculations confirmed the experimental symmetry assignment and allowed us to obtain the longitudinal optical phonon wavenumbers. In addition, we analyzed the mode eigenvectors in detail in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. Despite differences in chemistry and strain, the phonon eigenvectors were found to be highly comparable in both compounds. We investigated the position of the ferroelectric soft mode in PbVO3 compared with PbTiO3. Sizeable splitting of the B1+E modes appeared as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V–O bond frequency in PbVO3 was also addressed.

Mutual spin-phonon driving effects and phonon eigenvector renormalization in nickel (II) oxide

The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interests for decades. Despite its pivotal role in transport processes, the effect of spin-phonon coupling on the phonon system, especially acoustic phonon properties, has so far been elusive. By means of inelastic neutron scattering and first-principles calculations, anomalous scattering spectral intensity from acoustic phonons was identified in the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong spin-lattice correlations that renormalize the polarization of acoustic phonon. In particular, a clear magnetic scattering signature of the measured neutron scattering intensity from acoustic phonons is demonstrated by its momentum transfer and temperature dependences. The anomalous scattering intensity is successfully modeled with a modified magneto-vibrational scattering cross-section, suggesting the presence of spin precession driven by phonon. The renormalization of phonon eigenvector is indicated by the observed "geometry-forbidden" neutron scattering intensity from transverse acoustic phonon. Importantly, the eigenvector renormalization cannot be explained by magnetostriction but instead, it could result from the coupling between phonon and local magnetization of ions.

Many Body Interactions on Lattice Dynamical Properties of Stanene, 2D Material

The study of the lattice dynamical properties of materials, phenomenological models describe a complete and straight forward description of the phonon dispersion and phonon eigenvectors in whole Brillouin Zone (BZ) and can be easily applied to the calculation of phonon density of states, elastic constants , dielectric permittivity and other properties of solid .Adiabatic Bond Charge Model (ABCM) was originally developed by W. Weber for studying the lattice dynamics of tetrahedrally bonded bulk group IV Semiconductors such as Si, Ge,Sn and diamond. The result obtained from this model is good agreement with the experimental data for Stanene. We, at present find the lattice dynamical matrix and secular equations using Adiabatic Bond Charge Model. We hope that lattice dynamical properties of Stanene as a 2D material will be good fitted with experimental data.

Davis Computational Spectroscopy Workflow-From Structure to Spectra.

We describe an automated workflow that connects a series of atomic simulation tools to investigate the relationship between atomic structure, lattice dynamics, materials properties, and inelastic neutron scattering (INS) spectra. Starting from the atomic simulation environment (ASE) as an interface, we demonstrate the use of a selection of calculators, including density functional theory (DFT) and density functional tight binding (DFTB), to optimize the structures and calculate interatomic force constants. We present the use of our workflow to compute the phonon frequencies and eigenvectors, which are required to accurately simulate the INS spectra in crystalline solids like diamond and graphite as well as molecular solids like rubrene. We have also implemented a machine-learning force field based on Chebyshev polynomials called the Chebyshev interaction model for efficient simulation (ChIMES) to improve the accuracy of the DFTB simulations. We then explore the transferability of our DFTB/ChIMES models by comparing simulations derived from different training sets. We show that DFTB/ChIMES demonstrates ∼100× reduction in computational expense while retaining most of the accuracy of DFT as well as yielding high accuracy for different materials outside of our training sets. The DFTB/ChIMES method within the workflow expands the possibilities to use simulations to accurately predict materials properties of increasingly complex structures that would be unfeasible with ab initio methods.

Evaluating the roles of temperature-dependent eigenvectors in predicting phonon transport properties of anharmonic crystals using normal mode analysis methods

Theoretical modeling of phonon transport process in strongly anharmonic materials at a finite temperature needs to accurately capture the effects of lattice anharmonicity. The anharmonicity of potential energy surface would result in not only strong phonon scatterings but also shifts of phonon frequencies and eigenvectors. In this work, we evaluated the roles of anharmonicity-renormalized phonon eigenvectors in predicting phonon transport properties of anharmonic crystals at high temperatures using molecular dynamics-based normal mode analysis (NMA) methods in both time domain and frequency domain. Using PbTe as a model of strongly anharmonic crystal, we analyzed the numerical challenges to extract phonon lifetimes using NMA methods when phonon eigenvectors deviate from their harmonic values at high temperatures. To solve these issues, we proposed and verified a better fitting strategy, Sum-up Spectrum Fitting Method (SSFM) than the original frequency-domain NMA method. SSFM is to project the total spectrum energy density data of all phonon modes onto an inaccurate (harmonic or quasi-harmonic) eigenvector base and then manually sum up the peaks that belong to the same phonon mode (at the same frequency). The SSFM relaxes the requirement for accurate temperature-dependent eigenvectors, making it robust for analyzing strongly anharmonic crystals at high temperatures.

Physical insights on the low lattice thermal conductivity of AgInSe2

Uncovering the microscopic mechanism of low lattice thermal conductivity is essential for exploration and design of high-performance thermoelectrics. AgInSe2 exhibits high thermoelectric performance mainly due to its low thermal conductivity. Here, the origin of its intrinsic low lattice thermal conductivity is studied by temperature-dependent inelastic neutron scattering (INS), X-ray absorption fine structure (XAFS) spectra measurements, and first-principles calculations. A prominent “avoided crossing” feature and low-lying optical modes in the phonon dispersion of AgInSe2 are observed experimentally. These lattice dynamical features cause a local reduction of the phonon group velocity and strongly scatter heat-carrying acoustic phonons, contributing to its intrinsic low lattice thermal conductivity. In addition, both temperature-dependent phonon dispersions and phonon density-of-states measurements reveal strong anharmonicity or phonon-phonon interactions in AgInSe2. XAFS and phonon eigenvector analysis demonstrate the dominant role of Ag vibrations, which is closely associated with the “avoided crossing”, low-lying optical modes and large structural distortion, and thus dominates the reduction of lattice thermal conductivity of AgInSe2.

Automating Analysis of Neutron Scattering Time-of-Flight Single Crystal Phonon Data

This article introduces software called Phonon Explorer that implements a data mining workflow for large datasets of the neutron scattering function, S(Q, ω), measured on time-of-flight neutron spectrometers. This systematic approach takes advantage of all useful data contained in the dataset. It includes finding Brillouin zones where specific phonons have the highest scattering intensity, background subtraction, combining statistics in multiple Brillouin zones, and separating closely spaced phonon peaks. Using the software reduces the time needed to determine phonon dispersions, linewidths, and eigenvectors by more than an order of magnitude.

Anharmonic Eigenvectors and Acoustic Phonon Disappearance in Quantum Paraelectric SrTiO_{3}.

Pronounced anomalies in the SrTiO_{3} dynamical structure factor, S(Q,E), including the disappearance of acoustic phonon branches at low temperatures, were uncovered with inelastic neutron scattering (INS) and simulations. The striking effect reflects anharmonic couplings between acoustic and optic phonons and the incipient ferroelectric instability near the quantum critical point. It is rationalized using a first-principles renormalized anharmonic phonon approach, pointing to nonlinear Ti-O hybridization causing unusual changes in real-space phonon eigenvectors, frequencies, group velocities, and scattering phase space. Our method is general and establishes how T dependences beyond the harmonic regime, assessed by INS mapping of large reciprocal-space volumes, provide real-space insights into anharmonic atomic dynamics near phase transitions.

Theoretical and Experimental Study on Dielectric Constant of Cadmium Telluride in Terahertz Band

Abstract The phonon dispersion spectrum, eigenvector and lattice vibration frequency of zincblende cadmium telluride (CdTe) are studied by density functional theory (DFT). The vertical or parallel relationship between the phonon propagating wave vector atomic vibration direction is analyzed, and the frequency of the CdTe transverse (TO)/longitudinal (LO) optical phonon is obtained. The Boltzmann transport equation clearly expresses the phonon scattering process, which is combined with the Huang Kun equation to obtain the three-phonon collision frequency (γ). The terahertz time-domain spectroscopy system is used to measure the reflectivity and dielectric constant of CdTe single crystal. The experimental results verify that the theoretical calculation based on DFT can give the correct terahertz band dielectric constant with frequency dependence characteristics completely independent of the experiment.

Negative thermal expansion behavior in orthorhombic Sc2(MoO4)3 and Sc2(WO4)3

In contrast to cubic ZrW2O8 and ZrMo2O8, which have isotropic negative thermal expansion, the compounds Sc2(WO4)3 and Sc2(MoO4)3 with the orthorhombic structure show anisotropic thermal expansion that is negative along the b and c axes. Using ab initio density functional theory calculations of lattice dynamics, we identify that the anisotropic expansion is not due to anisotropic elasticity or negative linear compressibility, which is the case in several compounds; instead, it is due to anisotropic Grüneisen parameters of specific phonons, mostly of energy around 6 meV. Such a behavior not involving anomalous elasticity is unusual. The phonon eigenvectors show that these phonons have a rather complex vibrational character of MoO4/WO4 and ScO6 polyhedral units that involve translational, librational, and internal distortion dynamics.

How to resolve a phonon-associated property into contributions of basic phonon modes

Many properties of materials are associated with phonons. To better understand the phonon-property relation, it is a common practice to decompose the phonon-associated property into the contributions of basic phonon/vibration modes (e.g. longitudinal/transverse acoustic/optical mode), and identify the mode(s) that dominate(s) the property. The existing methods rely on labelling the phonon into one of the basic modes (BMs), however, the vibration characteristics of many phonons are different from the definitions of BMs, indicating these methods may give wrong decomposition results. Here we present a new method based on treating the phonon as a mixture of the BMs. By aligning the wave vectors and then projecting the phonon eigenvector onto the eigenvectors of the BMs, we can obtain the weights of the BMs on the given phonon, which can be used to quantify the contribution of each BM to the property. As an example, we apply this method to unravel the phonon modes that dominate the scattering of the electrons at the conduction band edge in two-dimensional antimony, an emerging semiconductor that has attracted great interest for electronics, but its mobility-limiting factors remain unclear. We find that the electron scattering is dominated by the out-of-plane acoustic phonon mode followed by the longitudinal acoustic mode, which are different from the results of other methods. Our method is generally applicable to different kinds of phonon-related properties and to all crystal materials.

Exciton-Phonon Coupling in the Ultraviolet Absorption and Emission Spectra of Bulk Hexagonal Boron Nitride.

We present an abinitio method to calculate phonon-assisted absorption and emission spectra in the presence of strong excitonic effects. We apply the method to bulk hexagonal BN, which has an indirect band gap and is known for its strong luminescence in the UV range. We first analyze the excitons at the wave vector q[over ¯] of the indirect gap. The coupling of these excitons with the various phonon modes at q[over ¯] is expressed in terms of a product of the mean square displacement of the atoms and the second derivative of the optical response function with respect to atomic displacement along the phonon eigenvectors. The derivatives are calculated numerically with a finite difference scheme in a supercell commensurate with q[over ¯]. We use detailed balance arguments to obtain the intensity ratio between emission and absorption processes. Our results explain recent luminescence experiments and reveal the exciton-phonon coupling channels responsible for the emission lines.

Electron—Phonon Interaction, Phonon and Electronic Structures of Layered Electride Ca2N

The phonon and electronic properties, the Eliashberg function and the temperature dependence of resistance of electride Ca2N are investigated by the DFT-LDA (density functional theory in local density approximation) plane-wave method. The phonon dispersion, the partial phonon density of states and the atomic eigenvectors of zero-center phonons are studied. The electronic band dispersion and partial density of states conclude that Ca2N is a metal and the Ca 3p, 4s and N 2p orbitals are hybridized. For the analysis of an electron-phonon interaction and its contribution of the Eliashberg function to resistance was calculated and a temperature dependence of resistance due to electron-phonon interaction was found.

Phonon properties and thermal conductivity from first principles, lattice dynamics, and the Boltzmann transport equation

A computational framework for predicting phonon frequencies, group velocities, scattering rates, and the resulting lattice thermal conductivity is described. The underlying theory and implementation suggestions are also provided. By using input from first principles calculations and taking advantage of advances in computational power, this framework has enabled thermal conductivity predictions that agree with experimental measurements for diverse crystalline materials over a wide range of temperatures. Density functional theory and density functional perturbation theory calculations are first used to obtain the harmonic and cubic force constants. The harmonic force constants are the input to harmonic lattice dynamics calculations, which provide the phonon frequencies and eigenvectors. The harmonic properties and the cubic force constants are then used with perturbation theory and/or phenomenological models to determine intrinsic and extrinsic scattering rates. The full set of phonon properties is then used to solve the Boltzmann transport equation for the mode populations and thermal conductivity. The extension of the framework to include higher-order processes, capture finite temperature effects, and model alloys is described. A case study on silicon is presented that provides benchmarking and convergence data. Available packages that implement the framework are compared.

Theoretical Study of Phonon Dispersion of Lanthanum Aluminate in Terahertz Frequency

The phonon dispersion spectrum, eigenvector and lattice vibration frequency of cubic crystalline lanthanum aluminate (LaAlO3) are studied by density functional theory (DFT). The effects of lattice constant, K-point selection and kinetic-energy cutoff (Ecut) on the calculation accuracy are discussed. The vertical parallel relationship between the phonon propagating wave vector and the atomic vibration direction is analyzed to obtain the frequency of the LaAlO3 transverse (TO)/longitudinal (LO) optical phonon. The comparison results of the first-principles calculation software Materials Studio, the open source component package ABINIT, and he open source component package Quantum Espresso show that the calculation accuracy depends on the convergence analysis and the approximation method adopted in the calculation process.

Far-infrared reflectivity spectra of nanotwinned GaV4Se8

ABSTRACT GaV Se is one of the lacunar spinel materials able to host magnetic skyrmionic textures. Earlier investigations at temperatures below its Jahn–Teller ferroelectric phase transition disclosed a nanoscale ferroelectric domain separated by 109 domain walls. Here, we have investigated the impact of this nanoscale twinning on polar phonon modes. We report dielectric and reflectivity spectra of GaV Se crystal in the phonon frequency region, derived from the ab initio predicted phonon frequencies, eigenvectors and Born-effective charges. Reflectivity spectra of the nanotwinned crystal, calculated using exact expressions for the effective macroscopic dielectric response of a perfect regular twin, are compared with the predicted reflectivity spectra of single-domain crystal. A characteristic spectral feature caused by the nanoscale twinning has been identified. While this feature is not a well-defined geometric resonance, it may still be accessible in experiments.