Intermediate Frequency Modes Research Articles

Unified Quantification of Quantum Defects in Small-Diameter Single-Walled Carbon Nanotubes by Raman Spectroscopy.

The covalent functionalization of single-walled carbon nanotubes (SWCNTs) with luminescent quantum defects enables their application as near-infrared single-photon sources, as optical sensors, and for in vivo tissue imaging. Tuning the emission wavelength and defect density is crucial for these applications. While the former can be controlled by different synthetic protocols and is easily measured, defect densities are still determined as relative rather than absolute values, limiting the comparability between different nanotube batches and chiralities. Here, we present an absolute and unified quantification metric for the defect density in SWCNT samples based on Raman spectroscopy. It is applicable to a range of small-diameter semiconducting nanotubes and for arbitrary laser wavelengths. We observe a clear inverse correlation of the D/G+ ratio increase with nanotube diameter, indicating that curvature effects contribute significantly to the defect activation of Raman modes. Correlation of intermediate frequency modes with defect densities further corroborates their activation by defects and provides additional quantitative metrics for the characterization of functionalized SWCNTs.

Structure-Resolved Monitoring of Single-Wall Carbon Nanotube Functionalization from Raman Intermediate Frequency Modes.

Single-wall carbon nanotubes (SWCNTs) can be covalently modified to generate useful changes in their spectroscopic and photophysical properties. We report here a new method to monitor the extent of such functionalization reactions for different nanotube structures. Raman spectra are analyzed to find the intensities of structure-specific intermediate frequency mode (IFM) features in the range of ca. 350 to 650 cm-1, which are induced by introduction of sp3 defects. The IFM frequencies are found to depend on both the nanotube diameter and Raman excitation wavelength. The growth of IFM features is accompanied by a decrease in RBM intensities, so the IFM to RBM intensity ratio can provide a sensitive, structure-specific measure of nanotube functionalization.

On the Co-existence of Transonic Buffet and Separation-Bubble Modes for the OALT25 Laminar-Flow Wing Section

Transonic buffet is an unsteady flow phenomenon that limits the safe flight envelope of modern aircraft. Scale-resolving simulations with span-periodic boundary conditions are capable of providing new insights into its flow physics. The present contribution shows the co-existence of multiple modes of flow unsteadiness over an unswept laminar-flow wing section, appearing in the following order of increasing frequency: (a) a low-frequency transonic buffet mode, (b) an intermediate-frequency separation bubble mode, and (c) high-frequency wake modes associated with vortex shedding. Simulations are run over a range of Reynolds and Mach numbers to connect the lower frequency modes from moderate to high Reynolds numbers and from pre-buffet to established buffet conditions. The intermediate frequency mode is found to be more sensitive to Reynolds-number effects compared to those of Mach number, which is the opposite trend to that observed for transonic buffet. Spectral proper orthogonal decomposition is used to extract the spatial structure of the modes. The buffet mode involves coherent oscillations of the suction-side shock structure, consistent with previous studies including global mode analysis. The laminar separation-bubble mode at intermediate frequency is fundamentally different, with a phase relationship between separation and reattachment that does not correspond to a simple ‘breathing’ mode and is not at the same Strouhal number observed for shock-induced separation bubbles. Instead, a Strouhal number based on separation bubble length and reverse flow magnitude is found to be independent of Reynolds number within the range of cases studied.

Radial-tangential mode of single-wall carbon nanotubes manifested by Landau regulation: reinterpretation of low- and intermediate-frequency Raman signals

The low-frequency Raman signals of single-wall carbon nanotubes (SWNTs), appearing in the range of 100–300 cm−1, have been interpreted as radial-breathing mode (RBM) comprising pure radial Eigenvectors. Here, we report that most of the low-frequency and intermediate-frequency signals of SWNTs are radial-tangential modes (RTMs) coexisting radial and tangential Eigenvectors, while only the first peak at the low-frequency side is the RBM. Density functional theory simulation for SWNTs of ~ 2 nm in diameter shows that dozens of RTMs exhibit following the RBM (~ 150 cm−1) up to G-mode (~ 1592 cm−1) in order with Landau regulation. We specify the RBM and the RTM on Raman spectra obtained from SWNTs, where both appear as prominent peaks between 149 and 170 cm−1 and ripple-like peaks between 166 and 1440 cm−1, respectively. We report that the RTMs have been regarded as RBM (~ 300 cm−1) and ambiguously named as intermediate-frequency mode (300–1300 cm−1) without assignment. The RTMs gradually interlink the RBM and the G-mode resulting in the symmetric Raman spectra in intensity. We reveal high-resolution transmission microscope evidence for a helical structure of SWNTs, informing the typical diameter of commercial SWNTs to be 1.4–2 nm.

Analysis of Source-Load Coupling Characteristics and Stability in Battery Energy Storage System

A quasi-precise modeling method based on the accurate source-load coupling model and the average model of battery energy storage system with pulsed load (BESS-PL), which retained its unique pulse characteristics and reasonably simplified the difficulty of theoretical analysis, was proposed to achieve the stability analysis of the source-load coupling and pulse characteristics of BESS-PL. Based on the quasi-precise model exchange work of the BESS-PL framework was calculated, and the influence of pulse frequency and duty cycle ratio on system stability was examined using the Bode diagram. The strength of the key parameters of the BESS-PL system, as well as the instantaneous power of source and load, were analyzed and discussed based on the waveform analysis and eigenvalue calculation of the actual test platform. This analysis and discussion further validate the stability analysis based on the quasi-precise model. The findings demonstrate that the BESS-PL system exhibits substantial coupling between the source and the pulse load, with stability being lowest at a particular frequency point in the intermediate frequency mode and the pulse duty ratio showed 50%.

Experimental and theoretical study of anomalous temperature dependence of phonons in Y2Ti2O7 pyrochlore

We present Raman spectroscopic as well as first-principles theoretical studies of the temperature dependence of phonon spectra in the pyrochlore ${\mathrm{Y}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. Our experimental results show that the frequencies of several phonons display anomalous softening upon cooling. Our theoretical calculations of the temperature dependence, via phonon self-energy corrections arising from cubic anharmonic interactions, lead to such anomalies but only for a couple of high-frequency phonon modes. Our work strongly supports the suggestion that anharmonic phonon-phonon interactions are strong in the pyrochlore titanates and that they are responsible for the phonon anomalies observed. The strong anharmonic effects are likely due to the presence of the vacant Wyckoff sites and the shifts of the oxygen atoms from their actual positions in the pyrochlore lattice. However, the theoretical calculations fail to explain the anomalous behavior of low and intermediate frequency modes observed in our experiments. It seems likely that a much more sophisticated, perhaps nonperturbative, treatment of the anharmonic interactions will be required for a full explanation of the anomalies.

Multiscale Analysis of Runoff Complexity in the Yanhe Watershed

Runoff complexity is an important indicator reflecting the sustainability of a watershed ecosystem. In order to explore the multiscale characteristics of runoff complexity and analyze its variation and influencing factors in the Yanhe watershed in China during the period 1991–2020, we established a new analysis method for watershed runoff complexity based on the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) method for the decomposition of multiscale characteristics and the refined composite multiscale entropy (RCMSE) method for the quantification of the system complexity. The results show that runoff and its components all present multiscale complexity characteristics that are different from random signals, and the intermediate frequency modes contribute the most to runoff complexity. The runoff complexity of the Yanhe watershed has decreased gradually since 1991, and 2010 was a turning point of runoff complexity, when it changed from a decline to an increase, indicating that the ecological sustainability of this basin has improved since 2010, which was mainly related to the ecological restoration measures of the Grain for Green Project. This study expands the research perspective for analyzing the variation characteristics of runoff at the multiscale, and provides a reference for the study of watershed ecological sustainability and ecological management.

Mode localization and suppressed heat transport in amorphous alloys

Glasses usually represent the lower limit for the thermal conductivity of solids, but a fundamental understanding of lattice heat transport in amorphous materials can provide design rules to beat such a limit. Here we investigate the role of mass disorder in glasses by studying amorphous silicon-germanium alloy (a-Si$_{1-x}$Ge$_x$) over the full range of atomic concentration from $x=0$ to $x=1$, using molecular dynamics and the quasi-harmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-Si$_{1-x}$Ge$_x$ as a function of $x$ exhibits a smoother U-shape than in crystalline mass-disordered alloys. The main contribution to the initial drop of thermal conductivity at low Ge concentration stems from the localization of otherwise extended modes that make up the lowest 8\% of the population by frequency. Contributions from intermediate frequency modes are decreased more gradually with increasing Ge to reach a broad minimum thermal conductivity between concentrations of Ge from $x=0.25$ to $0.75$.

Exploring the risk spillover effects between carbon market and electricity market: A bidimensional empirical mode decomposition based conditional value at risk approach

This study proposes a bidimensional empirical mode decomposition based conditional value at risk approach to explore the risk spillover effects between carbon market and electricity market. Firstly, bidimensional empirical mode decomposition is applied to decompose the pair of carbon and electricity prices into a series of simple modes with different frequencies. Secondly, conditional value at risk is used to capture the risk spillover effects between carbon market and electricity market at each frequency. The empirical results show that the risks of high frequency modes are higher than those of intermediate and low frequency modes. Carbon market has a positive risk spillover on electricity market, and electricity market has a negative risk spillover on carbon market. For high and low frequency modes, the risk spillover effects from carbon market to electricity market are positive, however those from electricity market to carbon market are negative. For intermediate frequency modes, there are bidirectional negative risk spillover effects between carbon market and electricity market.

Investigation of asymmetric flow past a slender body at high angles of attack

This paper presents an investigation of flow asymmetry around a slender body at high angles of attack. The paper investigated the numerical aspect of simulating such flows. The impact of three simulation parameters, including grid resolution, discretization scheme, and turbulent flow modeling, was assessed. It was shown that insufficient grid density resulted in highly dissipated solution. At high angles, where flow asymmetry is expected to develop around the body, the dissipation from poor grid resolution prevented the flow asymmetry. At higher grid resolution, the solution demonstrated a switch between two bistable states. Two spatial discretization schemes, namely central and bounded, were tested in the course of this study. The results illustrated the necessity to use non-dissipative unbiased discretization schemes. Large eddy simulation was performed using two sub-grid-scale models in addition to a run without a model. The sub-grid-scale models generated similar results except for switching of asymmetry direction and the axial location of separation foci. The study shows that grid resolution and solution scheme have a profound effect on the validity of the simulation of flow around slender bodies at high angles of attack. The study also showed that stringent grid requirements marginalized the effect of the sub-grid-scale model. Computations were then carried out at seven angles of attack $$\alpha = 30^{\circ }$$ , $$40^\circ $$ , $$50^\circ $$ , $$52.5^\circ $$ , $$55^\circ $$ , $$57.5^\circ $$ , and $$60^\circ $$ . Analysis was performed on mean and unsteady flow fields. The total normal force increased with increasing angle of attack. On the other hand, the total side force started to increase rapidly for angles of attack $$\alpha > 50^{\circ }$$ and reached a maximum at $$\alpha =57.5^{\circ }$$ before decreasing at $$\alpha =60^{\circ }$$ . A bistable mode was observed for $$\alpha > 50^{\circ }$$ in which the orientation of resultant forces switches with angle of attack. Comparison of computed dominant frequencies with experiment showed an acceptable agreement. Several dominant modes were identified: very low-frequency mode, low-frequency mode, intermediate-frequency mode, and high-frequency mode. The modes were shown to develop with increasing angle of attack. Surface flow pathlines revealed the existence of separation foci at $$\alpha =57.5^{\circ }$$ and $$60^{\circ }$$ , and a high-frequency tonal mode was observed to accompany the formation of separation foci.

New results from exact solution of the dispersion relation for nonextensive electron-ion plasma in the framework of collisionless kinetic theory

The electrostatic (ES) oscillations, spatio-temporal Landau damping and growth in an electron-ion (EI) plasma is investigated by taking into account the effect of ion (electron) dynamics on the electron (ion) oscillations in the context of nonextensive particle distributions. The dispersion relation is exactly solved without employing approximations on the phase velocity, in contrast to the procedure developed by Landau where some approximations are applied on the phase velocity. This enables us to obtain some results which are not shown previously duo to approximate numerical and analytical studies. It is remarked that four kinds of ES modes; a high frequency mode (well-known Langmuir wave), two low and high intermediate frequency (LIF and HIF) modes and a very low frequency mode (well-known IA wave) may propagate in desired plasma system. In particular, these ES modes represent very different behavior when moving to left and right directions, from the point of view of spatio-temporal damping and growth. In addition, it is shown that exact solution of the dispersion relation gives rise to some interesting phenomena; such as the existence of non-acoustic electron and ion modes, the frequency and wavelength cutoffs and the existence of backward waves.

Chirality dependence of electron-phonon matrix elements in semiconducting single-walled carbon nanotubes

Evaluations of quantum coupling between electrons and phonons in well-defined nanostructure will be necessary when applications based on the vibrations of various materials move into the quantum regime. Raman scattering, in which changes in polarization within a material are probed by light, is an excellent means of analyzing electron-phonon coupling. In this study, the Raman intensities of individually suspended single-walled carbon nanotubes were determined in order to examine variations in electron-phonon interactions in response to changes in the arrangement of carbon atoms (i.e., chirality). Unambiguous assignment of nanotube chirality was achieved by photoluminescence spectroscopy and similar variations in the radial breathing mode and intermediate frequency mode peak intensities with changes in chirality were found. These phenomena were explained based on prior theoretical studies. The D-mode and G-mode peaks were also observed to respond in the same manner, based on which we assigned the longitudinal optical phonon branch to the D-mode. The results of this work demonstrate that the Raman intensity analysis can provide useful information regarding electron-phonon coupling in nanomaterials.

First‐principles study of vibrational properties of TiSiO4 clusters

AbstractFirst‐principles calculations were performed to investigate the vibrational properties of monomers and dimers of titania, silica, and titania‐silica hybrid clusters. Density functional theory‐based formulism was employed to optimize the geometry at the B3LYP level and calculate the infrared and Raman spectra of the clusters by using the GGA‐PBE exchange‐correlation functional. It was found that the vibrational spectra of Ti2O4, Si2O4, and TiSiO4 hybrid clusters provide fingerprint information about structures and structural transitions during the formation of cluster structures. In the case of Si2O4 the mode at 410 cm−1 exhibited the largest vibration of Si atoms, whereas in the case of Ti2O4 the mode at 442 cm−1 exhibited the largest vibration of Ti atoms. The hybrid cluster TiSiO4 was structured using two different methods to explore the effects of starting geometry on the structures and vibrational modes of the clusters. The structural properties of the clusters remained unchanged but vibrational modes were found to be different. It is found that Si shows notable vibrations, but the metal atom Ti merely shows any vibration in the case of TiSiO4 hybrid clusters. The low and intermediate frequency modes were stiffened, whereas the three highest frequency modes were softened when the starting geometry of the hybrid clusters was changed from Si2O4 to Ti2O4.

Theoretical study of stability and superconductivity ofScHn(n=4–8) at high pressure

The synthesis of hydrogen sulfides, with the potential of high-temperature superconductivity, was recently proposed at high ${T}_{c}$ = 203 K. It motivated us to employ an ab initio approach for the predictions of crystal structures to find the stable scandium hydrides. In addition to the earlier predicted three stoichiometries of ScH, ${\mathrm{ScH}}_{2}$, and ${\mathrm{ScH}}_{3}$, we identify three other metallic stoichiometries of ${\mathrm{ScH}}_{4}, {\mathrm{ScH}}_{6}$, and ${\mathrm{ScH}}_{8}$, which show superconductivity at significantly higher temperatures. The phases of ${\mathrm{ScH}}_{4}$ and ${\mathrm{ScH}}_{6}$, whose stability does not require extremely high pressures ($l150$ GPa with ZPE), are primarily ionic compounds containing exotic quasimolecular ${\mathrm{H}}_{2}$ arrangements. The present electron-phonon calculations revealed the superconductive potential of ${\mathrm{ScH}}_{4}$ and ${\mathrm{ScH}}_{6}$ with estimated ${T}_{c}$ of 98 K and 129 K at 200 GPa and 130 GPa, respectively. The superconductivity of ${\mathrm{ScH}}_{n}$ stems from the large electron-phonon coupling associated with the wagging, bending, and intermediate-frequency modes attributed mainly to the hydrogen atoms.

Superconductivity and unexpected chemistry of germanium hydrides under pressure

Following the idea that hydrogen-rich compounds might be high-T$_c$ superconductors at high pressures, and the very recent breakthrough in predicting and synthesizing hydrogen sulfide with record-high T$_c$ = 203 K, ab initio evolutionary algorithm for crystal structure prediction was employed to find stable germanium hydrides. In addition to the earlier structure of germane with space group Ama2, we propose a new C2/m structure, which is energetically more favorable at pressures above 278 GPa (with inclusion of zero point energy). Our calculations indicate metallicity of the new C2/m phase of germane with T$_c$ = 67 K at 280 GPa. Germane is found to exhibit thermodynamic instability to decomposition to hydrogen and the new compound Ge$_3$H$_{11}$ at pressures above 300 GPa. Ge$_3$H$_{11}$ with space group I$\bar{4}$m2 is found to become stable at above 285 GPa with T$_c$ = 43 K. We find that the pressure-induced phase stability of germanium hydrides is distinct from its analogous isoelectronic systems, e.g., Si-hydrides and Sn-hydrides. Superconductivity stems from large electron-phonon coupling associated with the wagging, bending and stretching intermediate-frequency modes derived mainly from hydrogen.

ZA-derived phonons in the Raman spectra of single-walled carbon nanotubes

We report the observation of four Raman modes in individual, aligned single-walled carbon nanotubes, with frequencies between the RBM and D modes, and above the G± modes. By studying the diameter and excitation-energy dependence, we find that these strongly dispersive and comparatively intense modes share the lowest-frequency helical phonon branch as the origin, which can be derived from the out-of-plane acoustic (ZA) phonon branch of graphene. This ZA phonon is observed due to a defect-assisted, double-resonant process, similar to the D mode; its two-phonon overtones and combinations with transverse optical (TO) phonons do not require defects. Our assignment is supported by a theoretical model based on sixth-nearest neighbour tight-binding and a force-constant approach. We present a complete theoretical evaluation of the diameter and excitation-energy dependence of the ZA, 2ZA, and TO±ZA modes, which fits very well to our experimental results and previous studies of the so-called region of intermediate frequency modes.

Resonance Raman Optical Activity of Single Walled Chiral Carbon Nanotubes.

Resonance (vibrational) Raman Optical Activity (ROA) spectra of six chiral single-walled carbon nanotubes (SWCNTs) are studied by theoretical means. Calculations are performed imposing line group symmetry. Polarizability tensors, computed at the π-electron level, are differentiated with respect to DFT normal modes to generate spectral intensities. This computational protocol yields a ROA spectrum in good agreement with the only experiment on SWCNT, available at present. In addition to the conventional periodic electric dipole operator we introduce magnetic dipole and electric quadrupole operators, suitable for conventional k-space calculations. Consequences of the complex nature of the wave function on the scattering cross section are discussed in detail. The resonance phenomenon is accounted for by the short time approximation. Involvement of fundamental vibrations in the region of the intermediate frequency modes is found to be more notable in ROA than in Raman spectra. Calculations indicate exceptionally strong resonance enhancement of SWCNT ROA signals. Resonance ROA profile of the (6,5) tube shows an interesting sign change that may be exploited experimentally for SWCNT identification.

Raman spectroscopy of nondispersive intermediate frequency modes and their overtones in carbon nanotubes

We performed resonance Raman spectroscopy on HiPCO-produced single-walled carbon nanotubes in the spectral region of 846 and , using laser excitation energies between and . We observe a characteristic peak around and its overtones around 1692. Especially the latter was controversially discussed in the past and has been assigned to a double-resonant Raman process involving two oTO phonons. We demonstrate that both peaks slightly depend on the excitation laser energy by means of their shape, intensity, and peak position. Following the model proposed by Dobardžić et al., we relate both peaks to oTO -point phonons. As we analyze both, the first-order band and its overtone simultaneously, we are able to provide a unified explanation of these low-intensity Raman bands in carbon nanotubes. Raman spectrum of an HiPCO-produced buckypaper carbon nanotube sample at 2.33 eV laser energy. Both insets show a magnification of the oTO-related first- and second-order Raman bands, which we discuss in this article.

Modal Analysis of Inclined Film Cooling Jet Flow

Thermal and hydrodynamic flow field over a flat surface cooled with a single round inclined film cooling jet and fed by a plenum chamber is numerically investigated using large eddy simulation (LES) and validated with published measurements. The calculations are done for a freestream Reynolds number Re = 16,000, density ratio of coolant to freestream fluid ρj/ρ∞=2.0, and blowing ratio BR=ρjV/ρ∞V=1.0. A short delivery tube with aspect ratio l/D=1.75 and 35 deg inclination is considered. The evolution of the Kelvin–Helmholtz (K-H), hairpin and counterrotating vortex pair (CVP) vortical structures are discussed to identify their origins. Modal analysis of the complete 3D flow and temperature field is carried out using a dynamic mode decomposition (DMD) technique. The modal frequencies are identified, and the specific modal contribution toward the cooling wall temperature fluctuation is estimated on the film cooling wall. The low and intermediate frequency modes associated with streamwise and hairpin flow structures are found to have the largest contribution (in-excess of 28%) toward the wall temperature (or cooling effectiveness) fluctuations. The high frequency Kelvin–Helmholtz mode contributes toward initial mixing in the region of film cooling hole away from the wall. The individual modal temperature fluctuations on the wall and their corresponding hydrodynamic flow structures are presented and discussed.

Low- and High-Frequency Intermediate Modes with Step-Like Dispersion in Resonance Raman Scattering of Carbon Nanotubes

Resonance Raman spectra of mixture of single-wall carbon nanotubes were investigated in details. The diameter distribution of the investigated nanotubes was estimated from the experimental frequencies of radial breathing modes. Two series of two-phonon lines revealing step-like behavior with excitation energy as well as non-dispersive single-phonon lines were registered in the intermediate frequency range 200-1200 cm(-1). Observed Raman lines were analyzed and their assignment to particular phonons was carried out. Step-like dispersive high intermediate-frequency modes in the range of 720-1000 cm(-1) are attributed to resonance two-phonon processes with combinations of optical and acoustical modes Low intermediate-frequency modes in the range of 300-650 cm(-1), also revealing step-like behavior, are attributed to resonance two-phonon processes with combinations of flexural optical and acoustical modes.