Transverse Acoustic Phonons Research Articles

Functionalization of single-layer TaS2 and formation of ultrathin Janus structures

Abstract

Two-colour Three-pulse Spectrally Resolved Nonlinear Spectroscopy for Studies of Dynamics in Semiconductor Si Quantum Dots

We demonstrate that a two-colour three-pulsenonlinear spectroscopy can be used to study the dynamics of excited carriers inSi quantum dot structures embedded in SiN. Decays of the transverse optical phonon population and the transverse acoustic phonon population are measured and discussed. A simple theoretical modelis also used to support interpretation of our experimental observations.

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline \u03b1-MgAgSb

Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound’s intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.

Optical property of nature source: UV–visible emissions from calcined oyster shells

Calcination of oyster shells produces vacancies of Ca and O acting as color centers. Light emissions take place at 20 K and 300 K with intensity much greater than value observed in ZnO nanocrystals. Optical transitions proceed through three-body mechanisms where non-radiative processes involve transverse acoustical and optical phonons. Oxides purchased from suppliers are nearly optical silent, attributed to water absorption that compromises radiative processes.

Novel elastic evolution of carbide Mo2Ga2C under pressure: Ab initio theoretical investigation

The pressure dependence of elastic properties of Mo2Ga2C is studied via first-principles calculation. The present investigation shows that differing from other MAX phases, in Mo2Ga2C the [Formula: see text] is larger than [Formula: see text], because of the strong Ga–Ga interlayer bonds along [Formula: see text]-axis. Moreover, under pressure, the [Formula: see text] increases more rapidly, originating from the faster strengthening of Ga–Ga bonds. Interestingly, elastic constants [Formula: see text] soften under high pressure (more than 20 GPa). Especially, the calculated phonon structure demonstrates that transverse acoustic (TA) phonon mode also softens under pressure, implying possible phase transition. The reduction of [Formula: see text] and softening of phonon mode are attributed to significantly weakened Mo–Mo interaction in contrast to the strengthening of Ga–Ga bonds under high pressure. Our present results further indicate that Mo2Ga2C is more ductile under pressure.

A systematic study of the negative thermal expansion in zinc-blende and diamond-like semiconductors

Upon heating, almost all zinc-blende (ZB) and diamond-like semiconductors undergo volume contraction at low temperature, i.e. negative thermal expansion (NTE), instead of commonly expected expansion. Specifically, CuCl has the largest NTE among these semiconductors with a coefficient comparable with the record value of ZrW2O8. So far, underlying physical mechanism remains ambiguous. Here, we present a systematic and quantitative study of the NTE in ZB and diamond-like semiconductors using first-principles calculations. We clarified that the material ionicity, which renders the softening of the bond-angle-bending and thus, the enhancement of excitation of the transverse acoustic (TA) phonon, is responsible for the NTE of ZB and diamond-like semiconductors. With the increase in the ionicity from the groups IV, III-V, IIB-VI to IB-VII ZB semiconductors, the coefficient of the maximum NTE increases due to the weakness in bond-rotation effect, which makes the relative motion between cation and anion transverse to the direction of the bond more feasible and the mode Grüneisen parameters of the TA modes more negative. Since CuCl has the highest ionicity among all ZB and diamond-like semiconductors, it is expected to have the largest NTE, in good agreement with the experimental observation. This understanding would be beneficial for tetrahedral materials with specific applications.

Extremely space and time restricted thermal transport in the high temperature Cmcm phase of thermoelectric SnSe

Phonon dispersions and linewidths of layered thermoelectric SnSe in the high temperature Cmcm phase have been mapped by inelastic neutron scattering measurements. Downturns in the phonon dispersion reveal soft phonon energies at larger wave vectors for phonon modes propagating in the crystallographic basal a-c plane. The downturns can be described by a q2-dependency, indicative of the appearance of strong electron-phonon scattering in the Cmcm phase. A q3-dependency is also needed for a satisfactory description of the dispersions, revealing the existence of a huge lattice anharmonicity. Group velocities of the transverse acoustic phonons are reduced by as much as 37% through the structural phase transition at 798 K. The scattering of phonons is so strong that they propagation only over a few unit cells in length, with the lifetime as short as ~0.3 ps. The very short phonon lifetimes and very limited phonon propagation length, together with negative phonon group velocity at large wavevectors restricts heat transport at high temperatures. Our results reveal the origin of the extremely low thermal conductivity of SnSe in the Cmcm phase.

Mode-dependent phonon transmission in a T-shaped three-terminal graphene nanojunction

The multi-terminal systems can be used as basic building blocks of nanoelectronic devices. However, the understanding of mode-dependent phonon transmission in these systems remains poorly developed. Hence, we examine the modal transmission processes for a T-shaped three-terminal structure, which has one zigzag graphene nanoribbon (GNR) vertically joined together with an armchair GNR as the side terminal. The atomistic Green’s function method is improved to be suitable for exploring the polarized phonon transmittance between terminals in the multi-terminal system, by treating the terminals in a uniform and standard way. Initially propagating along a zigzag GNR, the phonons turn 90° to the side terminal, or go straight ahead. The change of transport direction to the side terminal leads to a relatively low transmittance in most cases. However, the acoustic phonon transmittance spectrum has alternating prominent dips and sharp peaks, and can have a higher value even with the abrupt direction changing in some frequency ranges. Along the zigzag GNR, most of acoustic phonons do not change polarization mode after scattering. However, most of low frequency longitudinal (LA), transverse (TA), flexural (ZA) and rotational (ZA4) acoustic phonons convert into TA, LA, ZA4 and ZA phonons in the side terminal, respectively.

Temperature behavior of long-wave acoustic phonons in relaxor ferroelectric Na1/2Bi1/2TiO3

This work presents the results of study of the behavior of longitudinal (LA), quasilongitudinal (QLA), and transverse (TA) acoustic phonons in single crystals of Na1/2Bi1/2TiO3 (NBT) within a temperature range from 110 K to 850 K. Measurements were carried out by means of Brillouin light scattering on phonons with a wave vector qph || [100], qph || [010], and qph || [110], regarding the cubic phase of NBT. Temperature dependences of sound velocity, attenuation, and intensity of LA, QLA, and TA phonons have been plotted. Anomalies in the behavior of sound velocity and attenuation of LA phonons in the vicinity of antiferrodistortive structural phase transition from the cubic to the tetragonal phases have been found. It is shown that broad anomalies in the sound velocity and attenuation of acoustic phonons typical to diffuse phase transition are not connected directly with the anomaly in the dielectric permittivity of the crystal. Elastic C11 and C44 modules in the cubic phase of NBT have been defined.

Magnetophonon spectroscopy of Dirac fermion scattering by transverse and longitudinal acoustic phonons in graphene

Recently observed magnetophonon resonances in the magnetoresistance of graphene are investigated using the Kubo formalism. This analysis provides a quantitative fit to the experimental data over a wide range of carrier densities. It demonstrates the predominance of carrier scattering by low energy transverse acoustic (TA) mode phonons: the magnetophonon resonance amplitude is significantly stronger for the TA modes than for the longitudinal acoustic (LA) modes. We demonstrate that the LA and TA phonon speeds and the electron-phonon coupling strengths determined from the magnetophonon resonance measurements also provide an excellent fit to the measured dependence of the resistivity at zero magnetic field over a temperature range of 4-150 K. A semiclassical description of magnetophonon resonance in graphene is shown to provide a simple physical explanation for the dependence of the magneto-oscillation period on carrier density. The correspondence between the quantum calculation and the semiclassical model is discussed.

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.

Mechanical properties of amorphous PEI, PES, and PVC up to 11 GPa studied by Brillouin light scattering

We applied high-pressure Brillouin light spectroscopy to study the mechanical properties of representative amorphous polymers: polyetherimide (PEI), polyethersulfone (PES), and unplasticized polyvinylchloride (PVC). The pressure dependence of optical and elastic properties such as refractive indices and acoustic phonon velocities was obtained by employing both a forward, symmetric scattering and a backscattering geometry up to ~11 GPa. While PEI and PES showed longitudinal and transverse acoustic phonon modes simultaneously at above a certain pressure, PVC showed only one longitudinal acoustic phonon mode, which limited the access to further information. For PEI, PES, and PVC films, we successfully calculated pressure-dependent Poisson's ratios, bulk moduli, shear moduli, Young's moduli, and density-pressure isotherms. Furthermore, by using the Lorentz-Lorenz equation, we corroborated the conventional assumption that the mean polarizability of polymeric materials is nearly constant even when pressure is applied.

Magnetoelastic excitation spectrum in the rare-earth pyrochlore Tb2Ti2O7

Tb2Ti2O7 presents an ongoing conundrum in the study of rare-earth pyrochlores. Despite the expectation that it should be the prototypical unfrustrated noncollinear Ising antiferromagnet on the pyrochlore lattice, it presents a puzzling correlated state that persists to the lowest temperatures. Effects which can reintroduce frustration or fluctuations are therefore sought, and quadrupolar operators have been implicated. One consequence of strong quadrupolar effects is the possible coupling of magnetic and lattice degrees of freedom, and it has previously been shown that a hybrid magnetoelastic mode with both magnetic and phononic character is formed in Tb2Ti2O7 by the interaction of a crystal field excitation with a transverse-acoustic phonon. Here, using polarized and unpolarized inelastic neutron scattering, we present a detailed characterization of the magnetic and phononic branches of this magnetoelastic mode, particularly with respect to their composition, the anisotropy of any magnetic fluctuations, and also the temperature dependence of the different types of fluctuation that are involved. We also examine the dispersion relations of the exciton branches that develop from the crystal field excitation in the same temperature regime that the coupled mode appears, and find three quasidispersionless branches where four are expected, each with a distinctive structure factor indicating that they are nonetheless cooperative excitations. We interpret the overall structure of the spectrum as containing four branches, one hybridized with the phonons and gaining a strong dispersion, and three remaining dispersionless.

Influence of pH and Fe doping on structural and physical properties of Mg0.95Mn0.05-xFexO (x = 0, 0.04) nanoparticles

We investigate the effects of pH and Fe doping on the size, structural and physical properties of Mg0.95Mn0.05-xFexO (x = 0, 0.04) nanoparticles. All the nanoparticles were synthesized by the sol-gel autocombustion method by varying the pH of the solution. Furthermore, the phase purity, size, and size distribution of the nanoparticles were studied by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Irrespective of the pH, Rietveld refinement analysis of the XRD data reveals single crystallographic cubic phases with MgO-type wurtzite structure (space group Fm3¯m) along with a lattice parameter of 4.2121(8) Å for x = 0.0 and 4.2083(5) Å for x = 0.04. This change in lattice parameter with Fe doping is due to the smaller size of Fe2+ (78 p.m.) compared with Mn2+ (83 p.m.). The size of the nanoparticles increases monotonically from approximately 10 nm to approximately 16 nm for x = 0.0 and from approximately 18 nm to approximately 23 nm for x = 0.04 with increasing pH. SEM micrographs of the powders show highly agglomerated nanoparticles with porous and irregular morphology. The TEM findings support the XRD results, showing that the nanoparticles increase in size with increase in pH. Because Raman modes should be absent in bulk MgO and present in MgO nanoparticles, the strong intensity of the three Raman modes at approximately 380 cm−1, 470 cm−1, and 608 cm−1 further supports the formation of small nanoparticles of Mg0.95Mn0.05-xFexO (x = 0, 0.04). The decrease in peak intensity with increase in pH suggests an increase in the size of the nanoparticles. The peak at 470 cm−1 corresponds to a transverse optical phonon, while the peak at 380 cm−1 corresponds to a transverse acoustic phonon.

First-Principles Analysis of Vibrational Properties of Type II SiGe Alloy Clathrates.

We have mostly performed vibrational studies of Type-II silicon-germanium clathrate alloys, namely, Si136-xGex (0 < x ≤ 128), using periodic density functional theory (DFT). Our computed lattice constant for various stoichiometric amount, namely, x, of Ge agrees to some extent with the observed X-ray diffraction (XRD) data, along with monotonically increasing dependence on x. According to our bandgap energy calculation via Vienna ab initio simulation package (VASP), Si128Ge8 has a “nearly-direct” bandgap of approximately 1.27 eV, which agrees well with the previously calculated result (~1.23 eV), which was obtained using the Cambridge sequential simulation total energy package (CASTEP). Most of our first-principles calculations focus on exploring the low-energy transverse acoustic (TA) phonons that contribute dominantly to the induction of negative thermal expansion (NTE) behavior. Moreover, our work has predicted that the Si104Ge32 framework exhibits NTE in the temperature range of 3–80 K, compared to the temperature regime (10–140 K) of NTE observed in such pure Si136. It is posited that the increased number of Ge–Ge bonds may weaken the NTE effect substantially, as the composition, which is denoted as x, in Si136-xGex is elevated from 32 (or 40) to 96 (or 104).

Temperature-dependent carrier injection routes under optical excitation in high-purity diamond crystals

We have investigated carrier injection routes in high-purity diamond crystals in a wide temperature range using the time-resolved cyclotron resonance method. By analysing the excitation spectra, we find that carriers are injected under optical excitation above the energy threshold for exciton generation with emission of a transverse acoustic phonon at 10 K and 80 K, whereas with absorption of a transverse optical phonon at 300 K. The excitation power dependence clearly shows that the carriers are generated by thermal dissociation of excitons at 300 K.

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.

Response to comment on "Giant electromechanical coupling of relaxor ferroelectrics controlled by polar nanoregion vibrations".

Gehring et al. argue that a splitting observed by us in the transverse acoustic (TA) phonon in the relaxor ferroelectric Pb[(Mg1/3Nb2/3)1-x Ti x ]O3 with x = 0.30 (PMN-30PT) is caused by a combination of inelastic-elastic multiple scattering processes called ghostons. Their argument is motivated by differences observed between their measurements made on a triple-axis spectrometer and our measurements on a time-of-flight spectrometer. We show that the differences can be explained by differences in the instrument resolution functions. We demonstrate that the multiple scattering conditions proposed by Gehring et al. do not work for our scattering geometry. We also show that, when a ghoston is present, it is too weak to detect and therefore cannot explain the splitting. Last, this phonon splitting is just one part of the argument, and the overall conclusion of the original paper is supported by other results.

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.