Transverse Acoustic Phonon Branch Research Articles

Anharmonic lattice dynamics and the origin of intrinsic ultralow thermal conductivity in AgI materials

Ionic conductors such as AgI with ultralow thermal conductivities $({\ensuremath{\kappa}}_{l})$ are of increasing interest because of their excellent thermoelectric properties. However, the origin of their intrinsic low ${\ensuremath{\kappa}}_{l}$ values remain elusive. In this study, comprehensive theoretical calculations of the lattice dynamics and the thermal transport properties of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$ (zinc-blende structure) and $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{AgI}$ (wurtzite structure) as functions of temperature were carried out based on many-body perturbation theory and phonon Boltzmann transport theory. First, the mean-squared displacements (MSDs) of ${\mathrm{Ag}}^{+}$ were significantly larger than those of ${\mathrm{I}}^{\ensuremath{-}}$ in both $\ensuremath{\gamma}\text{\ensuremath{-}}$ and \ensuremath{\beta}-phases below the order-disorder phase transition temperature $({T}_{\mathrm{c}})$, which led to a characteristic ``rattling'' feature and low-frequency, nearly flat local phonon vibrations. According to our previous work [Xie et al., Phys. Rev. Lett. 125, 245901 (2020)], such nondispersive flat phonon band structures are expected to give rise to four-phonon resonance and result in a dramatic increase in the four-phonon scattering over the conventional three-phonon scattering. For $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$, similar four-phonon resonance behavior was also discovered for the low-lying transverse acoustic phonon branches, and it was found that their four-phonon scattering rates were an order of magnitude larger than the corresponding three-phonon scattering rates. Considering the four-phonon scattering, the theoretical ${\ensuremath{\kappa}}_{l}$ of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$ was predicted to be $\ensuremath{\sim}0.32$ W/m K at 300 K, which was in good agreement with the value deduced from our experiments ($\ensuremath{\sim}0.36$ W/m K at 300 K). Compared to $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$, the acoustic phonons in $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{AgI}$ were more dispersive, and they intertwined with low-energy optical phonons at the zone boundaries. It was found that three-phonon resonance became as important as four-phonon resonance for the nearly flat longitudinal phonon band. The theoretical ${\ensuremath{\kappa}}_{l}$ for $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{AgI}$ was determined to be around $\ensuremath{\sim}0.32$ W/m K at room temperature, closely reproducing our measurement value $\ensuremath{\sim}0.29$ W/m K. Our results for AgI demonstrate the strong quartic anharmonicity in materials characterized by the rattling of weak bonding atoms as well as dispersionless phonon band structures. It is believed that this intimate relationship between the low-${\ensuremath{\kappa}}_{l}$ and flat phonon dispersion can be employed as a good indicator when searching for material systems with ultralow ${\ensuremath{\kappa}}_{l}$ values, e.g., cagelike rattling structures, quasi-two-dimensional structures, and chainlike structures.

Ultrafast Suppression of the Ferroelectric Instability in KTaO_{3}.

We use an x-ray free-electron laser to study the lattice dynamics following photoexcitation with ultrafast near-UV light (wavelength 266nm, 50fs pulse duration) of the incipient ferroelectric potassium tantalate, KTaO_{3}. By probing the lattice dynamics corresponding to multiple Brillouin zones through the x-ray diffuse scattering with pulses from the Linac Coherent Light Source (LCLS) (wavelength 1.3Å and <10 fs pulse duration), we observe changes in the diffuse intensity associated with a hardening of the transverse acoustic phonon branches along Γ to X and Γ to M. Using force constants from density functional theory, we fit the quasiequilibrium intensity and obtain the instantaneous lattice temperature and density of photoexcited charge carriers. The density functional theory calculations demonstrate that photoexcitation transfers charge from oxygen 2p derived π-bonding orbitals to Ta 5d derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure.

Optical phonon contribution to the thermal conductivity of a quantum paraelectric

Motivated by recent experimental findings, we study the contribution of a quantum critical optical phonon branch to the thermal conductivity of a paraelectric system. We consider the proximity of the optical phonon branch to transverse acoustic phonon branch and calculate its contribution to the thermal conductivity within the Kubo formalism. We find a low temperature power law dependence of the thermal conductivity as T α , with 1 < α < 2, (lower than T 3 behavior) due to optical phonons near the quantum critical point. This result is in accord with the experimental findings and indicates the importance of quantum fluctuations in the thermal conduction in these materials.

Raman scattering from highly-stressed anvil diamond* *Project support by the National Natural Science Foundation of China (Grant No. 11774247).

The high-frequency edge of the first-order Raman mode of diamond reflects the stress state at the culet of anvil, and is often used for the pressure calibration in diamond anvil cell (DAC) experiments. Here we point out that the high-frequency edge of the diamond Raman phonon corresponds to the Brillouin zone (BZ) center Γ point as a function of pressure. The diamond Raman pressure gauge relies on the stability of crystal lattice of diamond under high stress. Upon the diamond anvil occurs failure under the uniaxial stress (197 GPa), the loss of intensity of the first-order Raman phonon and a stress-dependent broad Raman band centered at 600 cm−1 are observed, which is associated with a strain-induced local mode corresponding to the BZ edge phonon of the L 1 transverse acoustic phonon branch.

Body-centered-cubic structure and weak anharmonic phonon scattering in tungsten

It was recently found that the anharmonic phonon–phonon scattering in tungsten is extremely weak at high frequencies, leading to a predominance of electron–phonon scattering and consequently anomalous phonon transport behaviors. In this work, we calculate the phonon linewidths of W along high-symmetry directions from first principles. We find that the weak phonon–phonon scattering can be traced back to two factors. The first is the triple degeneracy of the phonon branches at the P and H points, a universal property of elemental body-centered-cubic (bcc) structures. The second is a relatively isotropic character of the phonon dispersions. When both are met, phonon–phonon scattering rates must vanish at the P and H points. The weak phonon–phonon scattering feature is also applicable to Mo and Cr. However, in other elemental bcc substances like Na, the isotropy condition is violated due to the unusually soft character of the lower transverse acoustic phonon branch along the Γ-N direction, opening emission channels and leading to much stronger phonon–phonon scattering. We also look into the distributions of electron mean-free paths (MFPs) at room temperature in tungsten, which can help engineer the resistivity of nanostructured W for applications such as interconnects.

Probing the acoustic phonon dispersion and sound velocity of graphene by Raman spectroscopy

The extraordinary thermal and elastic properties of graphene, mainly originating from its unique acoustic phonon branches near Γ point in Brillouin zone, have attracted great attention in its fundamental researches and practical applications. Here, we introduce an optical technique to accurately probe longitudinal acoustic (LA) and transverse acoustic (TA) phonon branches of graphene near Γ point by double resonant Raman scattering of the combination phonon modes in the range of 1650−2150 cm−1 along with the overtone 2D' mode at ∼3200 cm−1. The corresponding sound velocities (νTA=12.9km/s,νLA=19.9km/s) of graphene have been accessed, which are about 10% smaller than those of graphite. Based on νTA and νLA, the two-dimensional (2d) elastic stiffness (tension) coefficients c11 and c66, Young's modulus Y2d and Poisson's ratio σ2d can be estimated. The results demonstrate again that double resonant Raman spectroscopy is a powerful tool to probe the fundamental properties of graphene.

Comment on "Giant electromechanical coupling of relaxor ferroelectrics controlled by polar nanoregion vibrations".

Manley et al. (Science Advances, 16 September 2016, p. e1501814) report the splitting of a transverse acoustic phonon branch below T C in the relaxor ferroelectric Pb[(Mg1/3Nb2/3)1-x Ti x ]O3 with x = 0.30 using neutron scattering methods. Manley et al. argue that this splitting occurs because these phonons hybridize with local, harmonic lattice vibrations associated with polar nanoregions. We show that splitting is absent when the measurement is made using a different neutron wavelength, and we suggest an alternative interpretation.

Remarkable thermoelectric performance in BaPdS2 via pudding-mold band structure, band convergence, and ultralow lattice thermal conductivity

Efficient thermoelectric materials require a rare and contraindicated combination of materials properties: large electrical conductivity, large Seebeck coefficient, and low thermal conductivity. One strategy to achieve the first two properties is via low-energy electronic bands containing both flat and dispersive parts in different regions of crystal momentum space, known as a pudding-mold band structure. Here, we illustrate that ${\mathrm{BaPdS}}_{2}$ successfully achieves the pudding-mold band structure for the valence band, contributing to a large $p$-type thermoelectric power factor, due to its anisotropic crystal structure containing zigzag chains of edge-sharing square planar ${\mathrm{PdS}}_{4}$ units; large power factor is achieved for $n$-type doping as well due to band convergence. In addition, ${\mathrm{BaPdS}}_{2}$ exhibits ultralow lattice thermal conductivity, and thus also achieves the third property, due to extremely soft and anharmonic interactions in its transverse acoustic phonon branch. We predict a remarkably large thermoelectric figure of merit, with peak values between 2 and 3 for two of the three crystallographic directions, suggesting ${\mathrm{BaPdS}}_{2}$ warrants experimental investigation.

Structural properties of barium stannate

BaSnO3 has attracted attention as a transparent conducting oxide with high room temperature carrier mobility. We report a series of measurements that were carried out to assess the structure of BaSnO3 over a variety of length scales. Measurements included single crystal neutron and x-ray diffraction, Rietveld and pair distribution analysis of neutron powder diffraction, Raman scattering, and high-pressure x-ray diffraction. Results from the various diffraction probes indicate that both the long-range and local structures are consistent with the cubic symmetry. The diffraction data under pressure was consistent with a robustly cubic phase up to 48.9 GPa, which is supported by density functional calculations. Additionally, transverse phonon velocities were determined from measured dispersion of the transverse acoustic phonon branches, the results of which are in good agreement with previous theoretical estimates and ultrasound measurements.

Pre-transitional evolution of central peaks and transverse acoustic phonon branch in single crystal lead zirconate titanate with Ti concentration 0.7%

Inelastic X-ray scattering measurements have been done to study the lattice dynamics in lead zirconate titanate solid solution with 0.7% of PbTiO3. The temperature evolution of central peak and low-energy transverse phonon branches has been traced. Temperature dependent in-plane transverse polarized acoustic phonon branch in <1 1 0> direction has been revealed. The central peaks of two types have been found. The central peak at small wave vectors can be attributed to the relaxational-type soft ferroelectric mode, while the latter at Q = (1.5 -0.5 0) could be linked to the formation of M-superstructure in the intermediate ferroelectric phase.

Ab initio investigation of photoinduced non-thermal phase transition in β-cristobalite**Project support by the National Natural Science Foundation of China (Grant Nos. 11374217 and 11547158).

Using the linear-response method, we investigate the phonon properties of β-cristobalite crystal under electronic excitation effect. We find that the transverse-acoustic phonon frequency becomes imaginary as the electron temperature is increased, which means that the lattice of β-cristobalite becomes unstable under intense laser irradiation. In addition, for the optic phonon mode, the LO(H)–TO(H) splitting disappears when the electronic temperature reaches a certain value, corresponding to the whole transverse-acoustic phonon branches becoming negative. It means that the electronic excitation destroys the macroscopic electric field of β-cristobalite. Based on the calculated phonon band structures, some thermodynamic properties are calculated as a function of temperature at different electronic temperatures. These investigations provide evidence that non-thermal melting takes place during a femtosecond pulse laser interaction with β-cristobalite.

Absence of spin-orbit coupling induced effects on the lattice dynamics inCePt3Si

Motivated by model calculations, for the heavy fermion superconductor $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$ predicting phonon anomalies because of antisymmetric spin-orbit coupling, we performed a detailed experimental study of the lattice dynamical properties of $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$. In particular, we investigated the dispersion of transverse acoustic and low energy optic phonon branches along the [110] direction using inelastic neutron scattering. In these branches we found deviations from our ab initio lattice dynamical calculations, which overall give a good description of the phonon dispersion in $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$. However, the agreement for the [110] transverse modes can be improved if we neglect the Ce $4f$ states, done in an additional calculation. We conclude that the lattice dynamics of $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$ are conventional and that the observed deviations are not related to effects of antisymmetric spin-orbit coupling. More likely, ab initio calculations overestimate the exchange between different phonon branches, particularly in the presence of $4f$ electron states. Our results imply that the ASOC plays less a role in noncentrosymmetric superconductors than commonly believed.

Structural, dynamical and thermodynamics properties of YBi

We have performed first-principles calculations using the full-potential linear augmented plane-wave, plane wave pseudopotential methods and density linear-response theory to study the structural, electronic, dynamical and thermodynamic properties of the yttrium bismuth YBi. Total energy calculations of the zinc-blende, rocksalt, and cesium chloride phases are investigated. The NaCl structure is found to be the ground state phase of YBi. Bismuth atom induces strong relativistic effects, creating a large spin–orbit splitting of ∼2000meV. Moreover, the phonon band structure is characterized by an appreciable overlap between longitudinal and transverse optical modes. Furthermore we found that the transverse and longitudinal acoustic phonon branches show respectively small variation along the X→W and W→L directions, while the longitudinal optical modes show flatness along a large part of the Brillouin zone. Bismuth atoms show low vibrational amplitudes as compared with yttrium atoms. Moreover we found a strong contribution to heat capacity and entropy from optical (acoustic)-modes of Y (Bi) atoms, with relatively small contribution of Y-acoustic modes and a vanishing contribution of optical modes of Bi.

Giant magnetoelastic interaction in UFe2

Using the unique coverage of (Q, ω) space available on the BRISP spectrometer, we have investigated the low Q dynamical response of a single crystal of UFe2. A strong inelastic signal was found with a dispersion corresponding to the transverse acoustic phonon branch, forbidden in the first Brillouin zone. A detailed analysis of the neutron intensities as a function of temperature reveals a possible magnetic contribution superposed to this signal, which could originate from the magnon-phonon interaction, explaining the anomalous magnetoelastic behavior in this compound.

Response of Acoustic Phonons to Charge and Orbital Order in the 50% Doped Bilayer ManganiteLaSr2Mn2O7

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr(2)Mn(2)O(7). For excitation energies less than 15 meV, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at q = (h, h, 0), h ≤ 0.3, upon entering the low temperature charge and orbital ordered state (T(COO) = 225 K). This indicates a reduced electron-phonon coupling due to a decrease of electronic states at the Fermi level leading to a partial removal of the Fermi surface below T(COO) and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.

Soft-phonon instability in zincblende HgSe and HgTe under moderate pressure:Ab initiopseudopotential calculations

The mercury chalcogenides HgSe and HgTe show peculiar behavior under moderate compression such as the unusual observation of a ``hidden'' transition from their ambient-pressure zincblende phase to an orthorhombic metastable phase with space group $C{222}_{1}$. We present here the results of an ab initio pseudopotential study within the framework of the density-functional theory of the stability of the low-pressure zincblende phases of HgSe and HgTe and the transition to the $\text{C}{222}_{1}$ structure. We relate the observation of the $\text{C}{222}_{1}$ phase to an instability in the transverse-acoustic phonon branches of the zincblende phases of these materials. We have studied in detail the mechanism of the $\text{zincblende}\ensuremath{\rightarrow}\text{C}{222}_{1}$ transition as well as the structural evolution of the $\text{C}{222}_{1}$ phase under pressure.

Giant Kohn Anomaly and the Phase Transition in Charge Density WaveZrTe3

A strong Kohn anomaly in ZrTe3 is identified in the mostly transverse acoustic phonon branch along the modulation vector q_{P} with polarization along the a;{*} direction. This soft mode freezes to zero frequency at the transition temperature T_{P}, and the temperature dependence of the frequency is strongly affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate superstructure shows a power-law scaling of the intensity and the correlation length that is compatible with an order parameter of dimension n=2.

Synthesis and Raman spectroscopic characterisation of carbon nanoscrolls

Raman characteristics of nanoscrolls including the G-peak, the D-peak and new bands in the low-wavenumber region are discussed. Distinct peaks at ∼450 cm −1, ∼600 cm −1, ∼690 cm −1 and ∼870 cm −1 are identified to originate from transverse optic and transverse acoustic phonon branches at the K-point, the M-point and the Γ-point. The Raman spectrum of nanoscrolls is compared with that of highly oriented pyrolytic graphite (HOPG) and the elastic strain in the scrolls due to curvature is estimated analytically to account for the broadening of the G-peak. Synthesis of carbon nanoscrolls by splitting and rupturing graphene planes is also described in detail. The structure and composition of graphene were verified using X-ray diffraction and X-ray photoelectron spectroscopy. The structure of the nanoscrolls was confirmed by electron microscopy.

Structural, elastic, and electronic properties of Fe3C from first principles

Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite (Fe3C). Its nine single-crystal elastic constants were obtained by computing total energies or stresses as a function of applied strain. Furthermore, six of them were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010], and [001] directions. The three methods agree well with each other; the calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.

Pressure-induced amorphization ofCuFeS2studied byFe57nuclear resonant inelastic scattering

Static and dynamic structural properties of $\mathrm{Cu}\mathrm{Fe}{\mathrm{S}}_{2}$ under high pressure were investigated by x-ray diffraction and $^{57}\mathrm{Fe}$ nuclear resonant inelastic scattering. Using x-ray diffraction, we obtained evidence that a pressure-induced amorphization occurs at about $6.3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which represents a metal-insulator transition. Although partial phonon densities of states extracted from $^{57}\mathrm{Fe}$ nuclear resonant inelastic scattering spectra are similar in the two phases, the derived thermodynamical parameters show anomaly at the transition. The results of extracted partial phonon densities of states and ab initio phonon calculations indicate that this pressure-induced amorphization is caused by anomalous pressure dependences of the lower optical phonon branches and the initial slopes of the transverse acoustic phonon branches.