Longitudinal Acoustic Phonons Research Articles

Collective dynamics of liquid acetone investigated by inelastic X-ray scattering

The dynamic structure factors S(Q,ω) of liquid acetone were measured in a wide Q range 1.9–41.2 nm−1 with a high-resolution inelastic X-ray scattering (IXS) spectrometer installed at BL35XU of the SPring-8. Highly damped collective excitations of the longitudinal acoustic (LA) phonons are observed. The IXS spectra were analyzed by using a generalized Langevin formalism with the help of a sparse modeling to deduce reasonable Q dependences of the fitting parameters. The dispersion relation of the LA phonon excitations shows a highly positive deviation by about 65% from the adiabatic velocity of sound, which is larger than that in non-polar liquids such as CCl4 and benzene with van der Waals interactions, due probably that the acetone is a polar liquid. Microscopic dynamic parameters, such as viscoelastic relaxation rates and viscosity, are discussed by comparing with existing macroscopic dynamic properties. The fast and slow viscoelastic relaxations correspond to the vibration dynamics and reorientation correlations, respectively.

Resonant enhancement of magnetic damping driven by coherent acoustic phonons in thin Co2FeAl film epitaxied on GaAs

The magnetic dynamics of a thin Co2FeAl film epitaxially grown on GaAs substrate was investigated using the time- resolved magneto-optical Kerr measurement under an out-of-plane external field. The intrinsic magnetic damping constant, which should do not vary with the external magnetic field, exhibits an abnormal huge increase when the precession frequency is tuned to be resonant with that of the coherent longitudinal acoustic phonon in the Co2FeAl/GaAs heterostructure. The experimental finding is suggested to result from the strong coherent energy transfer from spins to acoustic phonons via magnetoelastic effect under a resonant coupling condition, which leads to a huge energy dissipation of spins and a greatly enhanced magnetic damping in Co2FeAl. Our experimental findings provide an experimental evidence of spin pumping-like effect driven by propagating acoustic phonons via magnetoelastic effect, suggesting an alternative approach to the possible long-range spin manipulation via coherent acoustic waves.

Thermal conductivity and inelastic X-ray scattering measurements on SiGeSn polycrystalline alloy

The impact of Sn on Si1-xGex through its thermal conductivity and phonon properties was investigated to develop an attractive material for thermoelectric devices. The Si-rich polycrystalline Si1-x-yGexSny alloy was synthesized by ball-milling technique. The as-synthesized alloy had substitutional Sn content of 1.60%, which it maintained as high as 0.54% after sintering at 1000 °C. The thermal conductivity was found to be reduced by the introduction of this tiny amount of Sn. Inelastic X-ray scattering measurements were carried out and we observed the longitudinal acoustic phonon mode superimposed along all crystallographic directions. In addition, the signal that originated from a local vibration at the Ge-related local structure was also successfully detected. It was found that the introduction of Sn affects the local vibration mode in the alloy. It is proposed that the incorporation of Sn atom into the local structure to make Ge–Sn pairs be considered to reduce the material’s thermal conductivity in addition to the mass difference effect.

Equation of State of TiN at High Pressures and Temperatures: A Possible Host for Nitrogen in Planetary Mantles

AbstractNitrogen, the most abundant element in Earth's atmosphere, is also a primary component of solid nitride minerals found in meteorites and on Earth's surface. If they remain stable to high pressures and temperatures, these nitrides may also be important reservoirs of nitrogen in planetary interiors. We used synchrotron X‐ray diffraction to measure the thermal equation of state and phase stability of titanium nitride (TiN) in a laser‐heated diamond anvil cell at pressures up to ∼70 GPa and temperatures up to ∼2,500 K. TiN maintains the cubic B1 (NaCl‐type) crystal structure over the entire pressure and temperature range explored. It has K0 = 274 (4) GPa, K0′ = 3.9 (2), and γ0 = 1.39 (4) for a fixed V0 = 76.516 (30) Å3 (based on experimental measurements), q = 1, and θ0 = 579 K. Additionally, we collected Raman spectra of TiN up to ∼60 GPa, where we find that the transverse acoustic (TA), longitudinal acoustic (LA), and transverse optical phonon modes exhibit mode Grüneisen parameters of 1.66(17), 0.54(15), and 0.93 (4), respectively. Based on our equation of state, TiN has a density of ∼5.6–6.4 g/cm3 at Earth's lower mantle conditions, significantly more dense than both the mantle of the Earth and the estimated densities of the mantles of other terrestrial planets, but less dense than planetary cores. We find that TiN remains stable against physical decomposition at the pressures and temperatures found within Earth's mantle, making it a plausible reservoir for deep planetary nitrogen if chemical conditions allow its formation.

What factors reduce carrier mobility of organic-inorganic hybrid perovskite for [formula omitted] and [formula omitted

To overcome the poor environmental stability and toxicity caused by lead, germanium/tin-based perovskite becomes a promising material for solar cells and optoelectronics. An important parameter for semiconductors is mobility, which determines the response speed of carriers to electric field. Here we elaborate on those factors that restrict carrier mobility for (BA)2MAGe2I7 and (BA)2MASn2I7. The mobility is calculated by deformation potential theory (DPT) and polar optical phonon (POP) models. Compared with inorganic materials, they exhibit low elastic modulus, which limits the mobility calculated from longitudinal acoustic phonon (LAP) model. Optical phonons (OP) with low energy play an important role in mobility. POP model reveals that the mobility reduces with the decrease of phonon energy and the increase of dielectric constant difference (Δ∊). In these materials, OP are distributed over a wide energy range, some of them are in the low energy part. Atomic structure is easy to produce distortion in electric field, which leads to a large Δ∊. Furthermore, the tensile strain inevitably exists in two-dimensional materials, and effective masses for electrons and holes increase sharply with the biaxial tensile strain, which is also an external factor leading to the decrease of mobility.

Indirect electron-phonon interaction leading to significant reduction of thermal conductivity in graphene

We investigate the effect of electron-phonon interaction (EPI) on the lattice thermal conductivity κph of graphene from first-principles calculations. By employing an iterative solution of Boltzmann transport equation (BTE), we highlight the marked effect of the indirect coupling between the flexural acoustic (ZA) phonons and electrons on the thermal conductivity in graphene. Although the ZA phonons, the dominant carriers of κph, do not interact with electrons directly due to the reflection symmetry with respect to the basal plane, their anharmonic interactions with the in-plane transverse acoustic (TA) and longitudinal acoustic (LA) phonons that can be effectively scattered by electrons have a significant effect on κph. This is originated from the dominance of normal processes over Umklapp processes in graphene. Specifically, this indirect effect can result in up to 21% reduction of κph even at room temperature, and 32% reduction in κph at 200 K. Moreover, κph does not decrease monotonically with increasing charge carrier density n. Instead, κph is minimized at ∼4.9×1014cm−2. This unusual finding is found to be strongly correlated with the electron density of states at corresponding Fermi levels. This indirect effect should also exist widely in other materials, whose intrinsic lattice thermal conductivities are dominated by normal processes. On the other hand, we also explore the electronic thermal conductivity κe varying with n. Intriguingly, at room temperature κe starts increasing dramatically at n=4.9×1014cm−2, where the corresponding Bloch-Grüneisen transition temperature ΘBG exceeds the room temperature.

Anharmonic phonon damping enhances theTcof BCS-type superconductors

A theory of superconductivity is presented where the effect of anharmonicity, as entailed in the acoustic, or optical, phonon damping, is explicitly considered in the pairing mechanism. The gap equation is solved including diffusive Akhiezer damping for longitudinal acoustic phonons or Klemens damping for optical phonons, with a damping coefficient which, in either case, can be directly related to the Gruneisen parameter and hence to the anharmonic coefficients in the interatomic potential. The results show that the increase of anharmonicity has a strikingly non-monotonic effect on the critical temperature $T_{c}$. The optimal damping coefficient yielding maximum $T_c$ is set by the velocity of the bosonic mediator. This theory may open up unprecedented opportunities for material design where $T_{c}$ may be tuned via the anharmonicity of the interatomic potential, and presents implications for the superconductivity in the recently discovered hydrides, where anharmonicity is very strong and for which the anharmonic damping is especially relevant.

Temperature-dependent phonon lifetimes and thermal conductivity of silicon by inelastic neutron scattering and ab initio calculations

Inelastic neutron scattering on a single crystal of silicon was performed at temperatures from 100 to 1500 K. These experimental data were reduced to obtain phonon spectral intensity at all wave vectors $\stackrel{P\vec}{Q}$ and frequencies $\ensuremath{\omega}$ in the first Brillouin zone. Thermal broadenings of the phonon peaks were obtained by fitting and by calculating with an iterative ab initio method that uses thermal atom displacements on an ensemble of superlattices. Agreement between the calculated and experimental broadenings was good, with possible discrepancies at the highest temperatures. Distributions of phonon widths versus phonon energy had similar shapes for computation and experiment. These distributions grew with temperature but maintained similar shapes. Parameters from the ab initio calculations were used to obtain the thermal conductivity from the Boltzmann transport equation, which was in good agreement with experimental data. Despite the high group velocities of longitudinal acoustic phonons, their shorter lifetimes reduced their contribution to the thermal conductivity, which was dominated by transverse acoustic modes.

Non-relaxor behaviour of low-frequency vibration spectra of relaxor ferroelectric PbCo1/3Nb2/3O3: evidences from Brillouin and Raman scattering measurements

This paper presents the results of studies of a low-frequency vibration spectrum of PbCo1/3Nb2/3O3 (PCN) relaxor ferroelectric crystal using the Brillouin and Raman light scattering in the temperature range from 80 to 750 K. The analysis of the temperature behaviour of the longitudinal acoustic phonon in Brillouin scattering spectra showed no anomalies in the vicinity of ‘diffuse phase transition’ (T m = 250 K) in PCN. Polarized Raman light scattering spectra were obtained in PCN over the entire temperature range studied. Analysis of low-frequency optical mode behaviour in PCN during temperature change also revealed no correlations with dielectric permeability anomaly in the vicinity of T m: softening of optical phonon at 43 cm−1 frequency in VV polarization is observed at 170 K. In the same temperature range, there are anomalies (a ‘narrow’ and weak component) in quasi-elastic light scattering (QELS) obtained in temperature behaviour with VH polarization in Raman spectra in PCN. A ‘wide’ and intense QELS component, obtained in Raman spectra with VV polarization, shows anomalies in the vicinity of T m. We associate the anomalies of optical phonons and QELS with structure distortions in the formation of phase stratification and the dynamics of polar nano-regions.

Picosecond laser ultrasonic measurements of interlayer elastic properties of 2H-MoSe2 and 2H-WSe2

We have used the ultrafast pump-probe technique known as picosecond ultrasonics to generate and detect coherent acoustic phonon pulses with frequencies reaching 40 GHz in exfoliated crystals of MoSe2 and WSe2 on Si and sapphire substrates. We report picosecond time-resolved reflectivity data from samples ranging from 180 nm to 920 nm in thickness and compare our results to a 1D simulation of strain-induced changes in the optical reflectivity. We find the longitudinal sound velocity along the c-axis (the interlayer direction) to be 2800 m/s ± 40 m/s for MoSe2 and 2510 m/s ± 60 m/s for WSe2. We also report the measured lifetime of longitudinal acoustic phonons approaching 40 GHz to be 0.85 ± 0.2 ns and compare this value with predictions of relaxation damping and 3-phonon models, as well as discuss its relationship to the predicted thermal conductivity of MoSe2.

Numerical Study of Thermal Dissipation Processes in Silicon

Abstract Heat dissipation in nanoelectronics has become a major bottleneck to further scaling in next-generation integrated circuits. In order to address this problem and develop more energy-efficient nanoelectronic transistor, sensor, and storage devices, we must understand thermal processes at the atomic scale, which requires numerical simulation of the interaction between electrons and heat, carried by quantized lattice vibrations called phonons. Here we examine in detail the phonon emission and absorption spectra in silicon at several elevated values for the electron temperature. The effect of electric field on the electron distribution and equivalent electron temperature is obtained from full-band Monte Carlo simulation for bulk silicon. The electron distributions are used to numerically compute the phonon emission and absorption spectra and discover trends in their behavior at high electron temperatures. The concept of electron temperature is used to understand the relationship between field and heat emission, and it is found that longitudinal acoustic (LA) phonon emission increases at high electron temperatures. It is also found that emission of slower zone-edge phonons increases for all phonon branches at high electron temperatures. These conclusions at high electric fields can be used to enable heat-conscious design of future silicon devices.

Electronic band structure of phosphorus-doped single crystal diamond: Dynamic Jahn-Teller distortion of the tetrahedral donor ground state

The electronic band structure of the phosphorus-doped high-quality high-pressure and high-temperature--grown single crystal diamond was studied by infrared absorption, magneto- and electron paramagnetic resonance spectroscopy, and first-principles calculations. The complete picture of the $1s\ensuremath{\rightarrow}n{p}_{\ifmmode\pm\else\textpm\fi{}}$ and $1s\ensuremath{\rightarrow}n{p}_{0}$ ($n=2$, 3, 4) donor transitions with new $1s\ensuremath{\rightarrow}3p{}_{0},4p{}_{0}$ and $1s\ensuremath{\rightarrow}4p{}_{\ifmmode\pm\else\textpm\fi{}}$ transitions allowed us to refine the energy levels of ground and exited states with those obtained from effective mass approximation calculation. For the first time, as far as we know, we have observed the fine doublet structures of the $1s$(${\mathrm{B}}_{2},\mathrm{E})\ensuremath{\rightarrow}2{p}_{\ifmmode\pm\else\textpm\fi{}}, 1s$(${\mathrm{B}}_{2},\mathrm{E})\ensuremath{\rightarrow}4p{}_{0}, 1s$(${\mathrm{B}}_{2},\mathrm{E})\ensuremath{\rightarrow}3p{}_{\ifmmode\pm\else\textpm\fi{}}$, and $1s$(${\mathrm{B}}_{2},\mathrm{E})\ensuremath{\rightarrow}4p{}_{\ifmmode\pm\else\textpm\fi{}}$ donor transitions with the splitting value of 1.05 meV. These doublet structures arise from the dynamic Jahn-Teller (JT) distortion which splits the energy level of the $1s$(${\mathrm{T}}_{2}$) state in the ${\mathrm{T}}_{d}$ configuration into those of the $1s$(${\mathrm{B}}_{2}$) ground state and the $1s$(E) state in the ${\mathrm{D}}_{2d}$ configuration. The dynamic JT distortion of the tetrahedral donor ground state has been proved by following facts: the observation of vibronic energy levels and temperature behavior of doublet bands in the IR spectrum, the absence of P-donor-related electron paramagnetic resonance signals, and first-principles calculations. We believe that the dramatic broadening of the $2p{}_{0}$ band attributed to the $1s$(${\mathrm{B}}_{2},\mathrm{E})\ensuremath{\rightarrow}2{p}_{0}$ donor transition at 524 meV is caused by the resonant interaction of this 524 meV energy level with that of \ensuremath{\sim}517 meV formed due to the dynamic JT coupling of the $1s$(E) energy level equal to 387 meV with the longitudinal acoustic phonon of $\ensuremath{\sim}130$ meV.

On-demand generation of higher-order Fock states in quantum-dot–cavity systems

The on-demand preparation of higher-order Fock states is of fundamental importance in quantum information sciences. We propose and compare different protocols to generate higher-order Fock states in solid state quantum-dot--cavity systems. The protocols make use of a series of laser pulses to excite the quantum dot exciton and off-resonant pulses to control the detuning between dot and cavity. Our theoretical studies include dot and cavity loss processes as well as the pure-dephasing type coupling to longitudinal acoustic phonons in a numerically complete fashion. By going beyond the two-level approximation for quantum dots, we study the impact of a finite exchange splitting, the impact of a higher energetic exciton state, and an excitation with linearly polarized laser pulses leading to detrimental occupations of the biexciton state. We predict that under realistic conditions, a protocol which keeps the cavity at resonance with the quantum dot until the desired target state is reached is able to deliver fidelities to the Fock state $| 5\rangle$ well above $40\,\%$.

Phonon Angular Momentum Hall Effect.

The spin Hall effect is the transverse flow of the electron spin in response to an external electric field. Similarly, the temperature gradient in magnets can drive a transverse flow of the magnon spin, which provides a thermal alternative for spin manipulation. Recently, phonon angular momentum (PAM), the angular momentum of atoms resulting from their orbital motion around their equilibrium positions, has garnered attention as a quantity analogous to the magnon spin. Here, we report that the temperature gradient generally induces a transverse flow of PAM, which we term the phonon angular momentum Hall effect (PAMHE). The PAMHE arises whenever there are transverse and longitudinal acoustic phonons, and it is therefore ubiquitous in condensed matter systems. As a consequence of the PAMHE, PAM accumulates at the crystal edges. When the atoms in the crystal carry a non-zero Born effective charge, the edge PAM induces edge magnetization, which can be observed through optical measurement.

Phononic bandgap and phonon anomalies in HfN and HfN/ScN metal/semiconductor superlattices measured with inelastic x-ray scattering

Epitaxial metal/semiconductor superlattice heterostructures with lattice-matched abrupt interfaces and suitable Schottky barrier heights are attractive for thermionic energy conversion, hot electron-based solar energy conversion, and optical hyperbolic metamaterials. HfN/ScN is one of the earliest demonstrations of epitaxial single-crystalline metal/semiconductor heterostructures and has attracted significant interest in recent years to harness its excellent properties in device applications. Although the understanding of the mechanism of thermal transport in HfN/ScN superlattices is extremely important for their practical applications, not much attention has been devoted to measuring their phonon dispersion and related properties. In this Letter, we employ non-resonant meV-resolution inelastic x-ray scattering to determine the momentum-dependent phonon modes in epitaxial metallic HfN and lattice-matched HfN/ScN metal/semiconductor superlattices. HfN exhibits a large phononic bandgap (∼40 meV) and Kohn anomaly in the longitudinal and transverse acoustic phonon modes at q ∼ 0.73 along the [100] and [110] directions of the Brillouin zone due to the nesting of the Fermi surface by the wave vector (q). The in-plane [100] acoustic phonon dispersion of the HfN/ScN superlattices is found to be dominated by the HfN phonons, while the optical phonons exhibit both ScN and HfN characteristics. First-principles density functional perturbation theory modeling is performed to explain the experimental phonon spectra, and temperature-dependent thermal conductivity is measured using a pump-probe spectroscopic technique. These results will help understand the phonons in HfN and HfN/ScN metal/semiconductor superlattices for thermionic energy conversion.

First-principles calculation of phonon-limited mobility in planar [formula omitted] graphene

We present the phonon-limited electron mobility in planar T graphene and find that the mobility ∝T−5.53 at low temperature T. With the increase of temperature, the power exponent 5.53 decreases towards 1. Excitingly, the room-temperature mobility of planar T graphene is 2.78×106 cm2/Vs which is about 20.59 times as large as that of n-type graphene at the same carrier density. Different from graphene with longitudinal acoustic phonon dominating the mobility, an in-plane optical phonon mode contributes the most to the mobility of planar T graphene.

Outstanding Thermal Conductivity of Single Atomic Layer Isotope-Modified Boron Nitride.

Materials with high thermal conductivities (κ) are valuable to solve the challenge of waste heat dissipation in highly integrated and miniaturized modern devices. Herein, we report the first synthesis of atomically thin isotopically pure hexagonal boron nitride (BN) and its one of the highest κ among all semiconductors and electric insulators. Single atomic layer (1L) BN enriched with ^{11}B has a κ up to 1009 W/mK at room temperature. We find that the isotope engineering mainly suppresses the out-of-plane optical (ZO) phonon scatterings in BN, which subsequently reduces acoustic-optical scatterings between ZO and transverse acoustic (TA) and longitudinal acoustic phonons. On the other hand, reducing the thickness to a single atomic layer diminishes the interlayer interactions and hence umklapp scatterings of the out-of-plane acoustic (ZA) phonons, though this thickness-induced κ enhancement is not as dramatic as that in naturally occurring BN. With many of its unique properties, atomically thin monoisotopic BN is promising on heat management in van der Waals devices and future flexible electronics. The isotope engineering of atomically thin BN may also open up other appealing applications and opportunities in 2D materials yet to be explored.

Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide.

Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light emission, and detection applications. A hot-phonon bottleneck effect significantly extends the cooling time of hot charge carriers, which thermalize through carrier-optic phonon scattering, followed by optic phonon decay to acoustic phonons and finally thermal conduction. To understand these processes, we adjust the lattice dynamics independently of electronics by changing isotopes. We show that doubling the mass of hydrogen in methylammonium lead iodide by replacing protons with deuterons causes a large 20 to 50% softening of the longitudinal acoustic phonons near zone boundaries, reduces thermal conductivity by ~50%, and slows carrier relaxation kinetics. Phonon softening is attributed to anticrossing with the slowed libration modes of the deuterated molecules and the reduced thermal conductivity to lowered phonon velocities. Our results reveal how tuning the organic molecule dynamics enables control of phonons important to thermal conductivity and the hot-phonon bottleneck.

Phonon-limited mobility for novel two-dimensional semiconductors of [formula omitted] and [formula omitted]: First-principles calculation

BC3 and C3N are all novel two-dimensional (2D) semiconductors with excellent thermodynamic stability and high lattice thermal conductivity. To obtain accurate results, phonon-limited carrier mobility is investigated by an iterative Boltzmann transport equation (IBTE) including electron–phonon interaction (EPI) matrix elements for each scattering process. The piezoelectricity (PE) and polar optical phonon (POP) are not the main scattering sources. The mobility is mainly dominated by the short-range interaction between electrons and phonons. At 300 K, the electron mobility for BC3 and C3N is predicted to be 6.56×102 and 8.54×102cm2V-1s-1, while the hole mobility is estimated to be 9.60×101 and 3.26×103cm2V-1s-1. High mobility suggests that C3N is a promising material in nanoelectronic device. Furthermore, the deformation potential theory (DPT) based on longitudinal acoustic phonon (LAP) and optical phonon (OP) scattering is used to investigate mobility. The contribution of three valleys, light and heavy electrons or holes to mobility is considered in the calculation, which can effectively avoid overestimation of mobility. The carrier mobility calculated from DPT is lower than that from IBTE. Too many assumptions make the semiempirical models unable to correctly describe the complex EPI.

Low-frequency Raman spectrum of 2D layered perovskites: Local atomistic motion or superlattice modes?

We report the low-frequency Raman spectrum (ω = 10 cm-1-150 cm-1) of a wide variety of alkylammonium iodide based 2D lead halide perovskites (2D LHPs) as a function of A-site cation (MA = methylammonium and FA = formamidinium), octahedral layer thickness (n = 2-4), organic spacer chain length (butyl-, pentyl-, hexyl-), and sample temperature (T = 77 K-293 K). Using density functional theory calculations under the harmonic approximation for n = 2 BA:MAPbI, we assign several longitudinal/transverse optical phonon modes between 30 cm-1 and 100 cm-1, the eigendisplacements of which are analogous to that observed previously for octahedral twists/distortions in bulk MAPbI. Additionally, we propose an alternative assignment for low-frequency modes below this band (<30 cm-1) as zone-folded longitudinal acoustic phonons corresponding to the periodicity of the entire layered structure. We compare measured spectra to predictions of the Rytov elastic continuum model for zone-folded dispersion in layered structures. Our results are consistent across the various 2D LHPs studied herein, with energetic shifts of optical phonons corresponding to microscopic structural differences between materials and energetic shifts of acoustic phonons according to changes in the periodicity and elastic properties of the perovskite/organic subphases. This study highlights the importance of both the local atomic order and the superlattice structure on the vibrational properties of layered 2D materials.