Longitudinal Acoustic Phonons Research Articles

Size effects in near-ultraviolet Raman spectra of few-nanometer-thick silicon-on-insulator nanofilms

We have fabricated Si-on-insulator (SOI) layers with a thickness h1 of a few nanometers and examined them by Raman spectroscopy with 363.8 nm excitation. We have found that phonon and electron confinement play important roles in SOI with h1 < 10 nm. We have confirmed that the first-order longitudinal optical phonon Raman band displays size-induced major homogeneous broadening due to phonon lifetime reduction as well as minor inhomogeneous broadening due to wave vector relaxation (WVR), both kinds of broadening being independent of temperature. Due to WVR, transverse acoustic (TA) phonons become Raman-active and give rise to a broad band in the range of 100–200 cm−1. Another broad band appeared at 200–400 cm−1 in the spectrum of SOI is attributed to the superposition of 1st order Raman scattering on longitudinal acoustic phonons and 2nd order scattering on TA phonons. Suppression of resonance-assisted 2-nd order Raman bands in SOI spectra is explained by the electron-confinement-induced direct band gap enlargement compared to bulk Si, which is confirmed by SOI reflection spectra.

Electron-Phonon Coupling and Energy Flow in a Simple Metal beyond the Two-Temperature Approximation

The electron-phonon coupling and the corresponding energy exchange was investigated experimentally and by ab initio theory in non-equilibrium states of the free-electron metal aluminium. The temporal evolution of the atomic mean squared displacement in laser-excited thin free-standing films was monitored by femtosecond electron diffraction. The electron-phonon coupling strength was obtained for a range of electronic and lattice temperatures from density functional theory molecular dynamics (DFT-MD) simulations. The electron-phonon coupling parameter extracted from the experimental data in the framework of a two-temperature model (TTM) deviates significantly from the ab initio values. We introduce a non-thermal lattice model (NLM) for describing non-thermal phonon distributions as a sum of thermal distributions of the three phonon branches. The contributions of individual phonon branches to the electron-phonon coupling are considered independently and found to be dominated by longitudinal acoustic phonons. Using all material parameters from first-principle calculations besides the phonon-phonon coupling strength, the prediction of the energy transfer from electrons to phonons by the NLM is in excellent agreement with time-resolved diffraction data. Our results suggest that the TTM is insufficient for describing the microscopic energy flow even for simple metals like aluminium and that the determination of the electron-phonon coupling constant from time-resolved experiments by means of the TTM leads to incorrect values. In contrast, the NLM describing transient phonon populations by three parameters appears to be a sufficient model for quantitatively describing electron-lattice equilibration in aluminium. We discuss the general applicability of the NLM and provide a criterion for the suitability of the two-temperature approximation for other metals.

High pressure Raman study of layered Mo0.5W0.5S2 ternary compound

Ternary two-dimensional (2D) transition metal dichalcogenide compounds exhibit a tunable electronic structure allowing for control of the interlayer and the intralayer atomic displacement to efficiently tune their physical and electronic properties. Using a diamond anvil cell, hydrostatic pressure was applied to Mo0.5W0.5S2 up to 40 GPa in order to study the optical phonon vibrational modes. Analysis of the high-pressure Raman spectra shows that the two in-plane E2g modes resembling that of pristine MoS2 and WS2, as well as disorder-activated longitudinal acoustic phonon mode, are hardened and suppressed as pressure increases. The two A1g modes of the ternary compound that resemble the A1g modes of pristine MoS2 and WS2, displayed similar Raman shifts to the pristine compounds as pressure increases. A Raman peak at 470 cm−1 that is close to A1g peaks emerges at ∼8 GPa, which represents a disorder-activated pressure-induced out-of-plane Raman mode observed only in the ternary compound under high pressure. At pressures above ∼30 GPa, a Raman peak at approximately 340 cm−1 is observed, signifying additional disorder-activated vibration mode. Our results reveal the enhanced interactions in the structural and vibrational behavior of the MoS2 and WS2 domains in the Mo0.5W0.5S2 compound under hydrostatic pressure. These results could have implications in understanding the electronic, optical, and structural properties of the new 2D ternary compound materials under extreme mechanical conditions.

Phonon Scattering Dynamics of Thermophoretic Motion in Carbon Nanotube Oscillators.

Using phonon wave packet molecular dynamics simulations, we find that anomalous longitudinal acoustic (LA) mode phonon scattering in low to moderate energy ranges is responsible for initiating thermophoretic motion in carbon nanotube oscillators. The repeated scattering of a single mode LA phonon wave packet near the ends of the inner nanotube provides a net unbalanced force that, if large enough, initiates thermophoresis. By applying a coherent phonon pulse on the outer tube, which generalizes the single mode phonon wave packet, we are able to achieve thermophoresis in a carbon nanotube oscillator. We also find the nature of the unbalanced force on end-atoms to be qualitatively similar to that under an imposed thermal gradient. The thermodiffusion coefficient obtained for a range of thermal gradients and core lengths suggest that LA phonon scattering is the dominant mechanism for thermophoresis in longer cores, whereas for shorter cores, it is the highly diffusive mechanism that provides the effective force.

Classical ferroelectriclike behavior of highly orderedPb(Sc1/2Nb1/2)O3studied by dielectric and Brillouin scattering spectroscopy

The ferroelectric phase transition behavior in the highly ordered $\mathrm{Pb}(\mathrm{S}{\mathrm{c}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ has been investigated by the dielectric and Brillouin spectroscopy. The dielectric permittivity ${\ensuremath{\varepsilon}}_{r}$ exhibits a sharp maximum without any frequency dispersion at its Curie temperature ${T}_{c}$. In the temperature range far above ${T}_{c}$, it was noted that the dielectric permittivity exhibits a noticeable deviation from the Curie-Weiss law below the characteristic intermediate temperature ${T}^{*}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}500\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is common to most of the Pb-based oxide perovskite relaxors. Upon cooling, the frequency of the longitudinal acoustic phonon mode exhibits a remarkable softening towards ${T}_{c}$. The relaxation time of the order parameter calculated using the Landau-Khalatnikov approach was determined to be more than one order of magnitude lower than that of the disordered $\mathrm{Pb}(\mathrm{S}{\mathrm{c}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ and is very close to that observed in the paraelectric phase of the classical ferroelectric, $\mathrm{BaTi}{\mathrm{O}}_{3}$. The observed dielectric and relaxation features are qualitatively discussed in terms of the difference in the strength of the random electric fields.

Inelastic x-ray scattering measurements of phonon dynamics inURu2Si2

We report high-resolution inelastic x ray scattering measurements of the acoustic phonons of URu2Si2. We observe minimal change in the phonon structure upon entering the Hidden Order phase. At all temperatures, the longitudinal acoustic phonons are anomalously broad at low q. There is no evidence to suggest that the increase in thermal conductivity in the Hidden Order phase is driven by a change in phonon dispersions or lifetimes. A more likely explanation is that the magnetic excitations in the Hidden Order state contribute to thermal transport.

Structural and optical properties of CdTe thin film: A detailed investigation using optical absorption, XRD, and Raman spectroscopies

Physical vapor deposition (PVD) is used to grow CdTe thin films of different thicknesses on glass substrates. The characterization of the samples is then investigated optically and structurally. Three experimental techniques were used in this study; optical absorption (ABS), Raman, and X‐ray diffraction (XRD) spectroscopies. In optical ABS spectroscopy, the Urbach energy related to the width of long‐wavelength tail decreases from 827 to 585 meV as the thickness increases from 100 to 500 nm, respectively. The thickness‐dependent particle size was calculated using the value of the size‐dependent optical absorption edge energy. A shift of 710 meV in the ABS edge energy takes place as the thickness increases from 100 to 500 nm. Thickness‐dependent shifting and widening of XRD line is presumably due to the size effect and strain. Relative to the phonon frequency of bulk CdTe crystal, a thickness‐dependent blueshift was observed in Raman spectra for the Longitudinal Optical (LO) phonon frequency, while a redshift for the longitudinal acoustic (LA) and transverse optical (TO) phonon frequencies is observed. In the present work, the results of optical absorption, XRD, and Raman analyses are combined in order to study the effect of particle‐size evolution, thickness‐dependent strain, and structural disorder.

Controlling the thermal conductance ofgraphene/h−BNlateral interface with strain and structure engineering

Although phonon-mediated thermal conduction in pristine graphene and hexagonal boron nitride is well understood, less is known about phonon transport in single-sheet graphene-hexagonal boron nitride ($\mathrm{Gr}/h\ensuremath{-}\mathrm{BN}$) lateral heterostructures, where the thermal resistance of the interfaces plays an important role in the overall thermal conductivity. We apply the newly developed extended atomistic Green's function method to analyze with detail the effect of strain and structure engineering on the thermal conductance ${G}_{\mathrm{int}}$ of the $\mathrm{Gr}/h\ensuremath{-}\mathrm{BN}$ interface. Our calculations show that longitudinal tensile strain leads to significant ${G}_{\mathrm{int}}$ enhancement (up to $25%$ at 300 K) primarily through the improved alignment of the flexural acoustic phonon bands, despite the reduction in the longitudinal acoustic (LA) and transverse acoustic phonon velocities. In addition, we find that alternating C-N zigzag bonds along the zigzag interface lead to a greater ${G}_{\mathrm{int}}$ than C-B bonds through more effective transmission of high-frequency LA and transverse optical phonons, especially at high strain levels. We also demonstrate how the interfacial structure dramatically affects the orientation of the transmitted optical phonons, a phenomenon that is neither seen for acoustic phonons nor predictable from conventional acoustic wave scattering theory. Insights from our paper can provide the basis for manipulating the interfacial thermal conductance in other two-dimensional heterostructures.

Influence of electronic structure, plasmon-phonon and plasmon-polariton excitations on anomalously low heat conductivity in TiAlN/Ag nanoscale multilayer coatings

TiAlN/Ag multilayer coatings with different number of bilayers and thicknesses of individual layer were fabricated by DC magnetron sputtering. Thermal conductivity and electronic structure features were measured on a set of samples with different number of bilayers and individual layer thickness from a 5–150 nm for total coating thickness of ∼0.5 or 1 μm. It was found that thermal conductivity of the multilayer insulator/metal structure becomes lower than for monolithic TiAlN coatings with the same thickness when Ag layers are thinner than 25 nm. The physical nature of thermal barrier properties of the nanolaminate TiAlN/Ag films is studied using X-ray photoelectrons (XPS) and high-resolution electron-energy loss (HREELS) spectroscopies. Based on these results, a physical model for the anomalous decrease in heat conductivity for nanoscale multilayers was developed on base of analysis of plasmon and lattice vibrations inside the layers and at the interfaces. Anomalously low heat transfer is attributed to attenuation of longitudinal acoustic phonons vibrations, decreasing of plasmon-polariton penetration on interfaces, and plasmon states splitting in Ag nanolayers due to confinement effects at multilayer coatings nanostructuring.

Ultrasensitive Characterization of Mechanical Oscillations and Plasmon Energy Shift in Gold Nanorods.

Mechanical vibrational resonances in metal nanoparticles are intensively studied because they provide insight into nanoscale elasticity and for their potential application to ultrasensitive mass detection. In this paper, we use broadband femtosecond pump-probe spectroscopy to study the longitudinal acoustic phonons of arrays of gold nanorods with different aspect ratios, fabricated by electron beam lithography with very high size uniformity. We follow in real time the impulsively excited extensional oscillations of the nanorods by measuring the transient shift of the localized surface plasmon band. Broadband and high-sensitivity detection of the time-dependent extinction spectra enables one to develop a model that quantitatively describes the periodic variation of the plasmon extinction coefficient starting from the steady-state spectrum with only one additional free parameter. This model allows us to retrieve the time-dependent elongation of the nanorods with an ultrahigh sensitivity and to measure oscillation amplitudes of just a few picometers and plasmon energy shifts on the order of 10(-2) meV.

Geometric tuning of thermal conductivity in three-dimensional anisotropic phononic crystals.

Molecular dynamics simulations are performed to investigate the thermal transport properties of a three-dimensional (3D) anisotropic phononic crystal consisting of silicon nanowires and films. The calculation shows that the in-plane thermal conductivity is negatively correlated with the out-of-plane thermal conductivity upon making geometric changes, whether varying the nanowire diameter or the film thickness. This enables the anisotropy ratio of thermal conductivity to be tailored over a wide range, in some cases by more than a factor of 20. Similar trends in thermal conductivity are also observed from an independent phonon ray tracing simulation considering only diffuse boundary scattering effects, though the range of anisotropy ratios is smaller than that obtained in MD simulation. By analyzing the phonon dispersion relation with varied geometric parameters, it is found that increasing the nanowire diameter increases the out-of-plane acoustic phonon group velocities, but reduces the in-plane longitudinal and fast transverse acoustic phonon group velocities. The calculated phonon irradiation further verified the negative correlation between the in-plane and the out-of-plane thermal conductivity. The proposed 3D phononic crystal may find potential application in thermoelectrics, energy storage, catalysis and sensing applications owing to its widely tailorable thermal conductivity.

Observation of resonance fluorescence and the Mollow triplet from a coherently driven site-controlled quantum dot

Resonant excitation of solid state quantum emitters has the potential to deterministically excite a localized exciton while ensuring a maximally coherent emission. In this work, we demonstrate the coherent coupling of an exciton localized in a lithographically positioned, site-controlled semiconductor quantum dot to an external resonant laser field. For strong continuous-wave driving we observe the characteristic Mollow triplet and analyze the Rabi splitting and sideband widths as a function of driving strength and temperature. The sideband widths increase linearly with temperature and the square of the driving strength, which we explain via coupling of the exciton to longitudinal acoustic phonons. We also find an increase of the Rabi splitting with temperature, which indicates a temperature induced delocalization of the excitonic wave function resulting in an increase of the oscillator strength. Finally, we demonstrate coherent control of the exciton excited state population via pulsed resonant excitation and observe a damping of the Rabi oscillations with increasing pulse area, which is consistent with our exciton-photon coupling model. We believe that our work outlines the possibility to implement fully scalable platforms of solid state quantum emitters. The latter is one of the key prerequisites for more advanced, integrated nanophotonic quantum circuits.

Compensation of phonon-induced renormalization of vacuum Rabi splitting in large quantum dots: Towards temperature-stable strong coupling in the solid state with quantum dot-micropillars

We study experimentally the influence of temperature on the emission characteristics of quantum dot-micropillars in the strong coupling regime of cavity quantum electrodynamics (cQED). In particular, we investigate its impact on the vacuum Rabi splitting (VRS) and we address the important question of the temperature stability of the coherent coupling regime in a semiconductor system, which is relevant in view of both fundamental study and future applications. To study the temperature dependence we investigate an unprecedentedly large number of strong coupling cases (89) in a wide temperature range from 10 up to 50 K, which constitutes a good basis for statistical analysis. The experiment indicates a statistically significant increase of the VRS with temperature in contrast to an expected decrease of the VRS due to the dephasing induced by acoustic phonons. From the theoretical point of view, the phonon-induced renormalization of the VRS is calculated using a real-time path-integral approach for strongly confined quantum dots (QDs), which allows for a numerical exact treatment of the coupling between the QD and a continuum of longitudinal acoustic phonons. The absence of the expected decrease of the VRS with temperature in our experimental data can be attributed to a unique optical property of laterally extended $\mathrm{I}{\mathrm{n}}_{0.4}\mathrm{G}{\mathrm{a}}_{0.6}\mathrm{As}$ QDs used in this study. Their electronic structure facilitates an effective temperature-driven increase of the oscillator strength of the excitonic state by up to 40% in the given temperature range. This leads to enhanced light-matter interaction and overcompensates the phonon-related decrease of the VRS. The observed persistence of strong coupling in the presence of phonon-induced decoherence demonstrates the appealing possibility to counteract detrimental phonon effects in the cQED regime via engineering the electronic structure of QDs.

Influence of the lattice mismatch on the lattice vibration modes for InAs/GaSb superlattices

Raman scattering study on a group of InAs/GaSb superlattice (SL) samples where the strain is systematically changed from tensile to compressive regime is presented. The effect of the lattice mismatch between the substrate and the epitaxially grown SL layers on particularly the InSb-like interface phonon frequencies is revealed in the backscattering geometry. The higher order folded longitudinal acoustic (FLA) phonon modes are also observed for samples having different superlattice periodicity. An ideality factor is incorporated into the model used for predicting the FLA phonon frequencies to simply express the deviation in the average acoustic velocity in the SL from the one in the homogeneous medium with abrupt transition in the interfaces.

Strained phonon–phonon scattering in carbon nanotubes

Focusing on three-phonon processes, the effect of uniaxial tensile strain on the phonon–phonon scattering rates is studied for (10,10) single-walled carbon nanotube at room temperature. Longitudinal optical (LO) and longitudinal acoustic (LA) phonon scattering rates are reported at 0%, 2.5%, 5%, 10%, 15% and 20% strains. Results show that the presence of strain can change the phonon–phonon scattering rates by as much as three orders of magnitude. For LO phonons, the mean scattering rate increases monotonically with the increasing axial strain. For LA phonons, the mean scattering rate also increases with the increasing strain. However, it decreases significantly at 10% strain. The anomaly observed in the mean scattering rate of LA at 10% strain is explained in light of the effect of strain on the density of states. It is also shown that pre-straining a CNT at 5% or less can improve the decay rate of hot phonons into lower energy phonons.

Acoustic phonon spectrum and thermal transport in nanoporous alumina arrays

We report results of a combined investigation of thermal conductivity and acoustic phonon spectra in nanoporous alumina membranes with the pore diameter decreasing from D = 180 nm to 25 nm. The samples with the hexagonally arranged pores were selected to have the same porosity ϕ ≈ 13%. The Brillouin-Mandelstam spectroscopy measurements revealed bulk-like phonon spectrum in the samples with D = 180-nm pores and spectral features, which were attributed to spatial confinement, in the samples with 25-nm and 40-nm pores. The velocity of the longitudinal acoustic phonons was reduced in the samples with smaller pores. Analysis of the experimental data and calculated phonon dispersion suggests that both phonon-boundary scattering and phonon spatial confinement affect heat conduction in membranes with the feature sizes D < 40 nm.

Phonon Heat Conduction in Multidimensional Heterostructures: Predictions Using the Boltzmann Transport Equation

In this article, two models for phonon transmission across semiconductor interfaces are investigated and demonstrated in the context of large-scale spatially three-dimensional calculations of the phonon Boltzmann transport equation (BTE). These include two modified forms of the classical diffuse mismatch model (DMM): one, in which dispersion is accounted for and another, in which energy transfer between longitudinal acoustic (LA) and transverse acoustic (TA) phonons is disallowed. As opposed to the vast majority of the previous studies in which the interface is treated in isolation, and the thermal boundary conductance is calculated using closed-form analytical formulations, the present study also considers the interplay between the interface and intrinsic (volumetric) scattering of phonons. This is accomplished by incorporating the interface models into a parallel solver for the full seven-dimensional BTE for phonons. A verification study is conducted in which the thermal boundary resistance of a silicon/germanium interface is compared against the previously reported results of molecular dynamics (MD) calculations. The BTE solutions overpredicted the interfacial resistance, and the reasons for this discrepancy are discussed. It is found that due to the interplay between intrinsic and interface scattering, the interfacial thermal resistance across a Si(hot)/Ge(cold) bilayer is different from that of a Si(cold)/Ge(hot) bilayer. Finally, the phonon BTE is solved for a nanoscale three-dimensional heterostructure, comprised of multiple blocks of silicon and germanium, and the time evolution of the temperature distribution is predicted and compared against predictions using the Fourier law of heat conduction.

Ab-initio study of dynamical properties of two dimensional MoS2 under strain

In this paper, we report the biaxial strain induced modifications in the phonon dispersion curves of monolayer MoS2 using first principles calculations in the framework of density functional perturbation theory. We have used the ultrasoft psedopotentials and the exchange correlation energies have been approximated by the local density approximation. There are significant changes in the phonon dispersion curves under strain. A systematic decrease in the optical phonon modes is observed with strain percentage. The longitudinal and transverse acoustic phonon modes of monolayer MoS2 show linear dependency for both without and with strain cases. However, the out of plane acoustic mode (ZA) which behaves quadratically with wave vector k- turns linear under strain. We have also linked ripple in single layer (SL) MoS2 by ZA mode with strain using shell elasticity theory. The strain induced softening of linear behaviour of ZA mode with strain indicates the absence of rippling under strain. At a very high strain there is a possibility of structural phase transition as the ZA mode at zone centre turns imaginary above 15% strain. Our results indicate a close relationship between the morphology and properties enabling the device tailoring and bandgap engineering in SL-MoS2 by manipulating the strain.

Study of confined coherent acoustic phonon modes in a free-standing cubic GaN membrane by femtosecond spectroscopy

Confined longitudinal coherent acoustic phonon modes are excited and detected in a sub-μm-thick free-standing cubic GaN membrane by femtosecond pump-probe spectroscopy. After fs laser excitation, Brillouin oscillation and thickness oscillation with frequencies up to 100 GHz are observed and studied in the time domain. We found an initial expansion of the membrane upon optical excitation at 400 nm. Our experimental results confirmed earlier existing theoretical predictions and experimental observations that the hydrostatic deformation potential of cubic GaN is positive.

Nanostructured TTT(TCNQ)2 Organic Crystals as Promising Thermoelectric n-Type Materials: 3D Modeling

Thermoelectric properties of quasi-one-dimensional TTT(TCNQ)2 organic crystals are investigated in order to appreciate the prospect of using this compound as n–type thermoelectric material. A more complete three-dimensional (3D) physical model is elaborated. It takes into account two the most important interactions of conduction electrons with longitudinal acoustic phonons, the electrons’ scattering on neighbor molecular chains, as well as the scattering by impurities and defects. The electrical conductivity, thermopower, the power factor, electronic thermal conductivity and the thermoelectric figure of merit in the direction along conductive molecular chains are calculated numerically for different degrees of crystal purity. It is shown that in stoichiometric compounds the thermoelectric figure of merit ZT remains small even after the increase of crystal perfection degree. The thermoelectric properties may be significantly enhanced by simultaneous increase of crystal perfection and of electron concentration. The latter can be achieved by additional doping with donors. For less pure crystals, the interaction with impurities predominates over the weak interchain interaction and the simpler one-dimensional (1D) physical model is applicable. When the impurity scattering is reduced, the interchain interaction begins to limit the carrier mobility and the application of the 3D physical model is required. The optimal parameters permitting to predict ZT~ 1 are determined.