Low-frequency Modes Research Articles

Micromagnetic simulation of Nd–Fe–B grain with a dysprosium-containing shell

The effects of Dy-doped shell thickness on the magnetic properties and magnetization reversal behavior of a Nd–Fe–B grain have been systematically investigated using micromagnetic simulations based on a core–shell model. The Dy-containing shell can significantly enhance the coercivity of the model. Notably, when the shell thickness reaches 12 nm, the coercivity reaches its maximum value, and the nucleation sites shift from the core–shell interface to the grain corners. Ferromagnetic resonance spectra indicate that the low-frequency modes are associated with the nucleation sites, and the increased coercivity is related to the shift of the core–shell simultaneous resonance mode to higher frequency. In addition, we propose a multi-shell model to further enhance the coercivity by shifting the nucleation points. This approach of adjusting the nucleation sites can also be applied to improve the magnetic properties of other magnets.

Modified ADRC based on optimized repetitive control for low-frequency vibration and harmonic disturbance suppression: Design and application to inertially stabilized platforms

Mechanical vibrations in low-frequency resonance and harmonic modes severely affect the closed-loop performance of inertially stabilized platforms (ISPs). This article proposes a new vibration control scheme capable of suppressing low-frequency vibration with large amplitude and harmonic disturbances in application to an ISP. Concretely, we first design an optimized repetitive control (ORC) framework with a plug-in optimization function, where the ORC retains the suppression effect of vibration in tip-tilt modes and mitigates the waterbed effect. To enhance vibration suppression, by introducing the ORC-based inner calibration network (ICN), a modified ADRC framework with the reduced-order extended state observer (ESO) design is established. Subsequently, substantial comparison experiments have been verified on an ISP to evaluate the superiorities of the proposed method, and the experiment results verify the effectiveness of the hybrid vibration control method, attenuating vibration in resonance mode to −51.94 dB.

Role of Superlattice Phonons in Charge Localization Across Quantum Dot Arrays.

Understanding charge transport in semiconductor quantum dot (QD) assemblies is important for developing the next generation of solar cells and light-harvesting devices based on QD technology. One of the key factors that governs the transport in such systems is related to the hybridization between the QDs. Recent experiments have successfully synthesized QD molecules, arrays, and assemblies by directly fusing the QDs, with enhanced hybridization leading to high carrier mobilities and coherent band-like electronic transport. In this work, we theoretically investigate the electron transfer dynamics across a finite CdSe-CdS core-shell QD array, considering up to seven interconnected QDs in one dimension. We find that, even in the absence of structural and size disorder, electron transfer can become localized by the emergent low-frequency superlattice vibrational modes when the connecting neck between QDs is narrow. On the other hand, we also identify a regime where the same vibrational modes facilitate coherent electron transport when the connecting necks are wide. Overall, we elucidate the crucial effects of electronic and superlattice symmetries and their couplings when designing high-mobility devices based on QD superlattices.

Observation of Radio-Frequency Mie Resonances in High-Permittivity Dielectric Spheres

Abstract We investigate Mie resonances in a high-permittivity dielectric (ceramic) sphere within the radio-frequency (RF) range, exploring its potential application in magnetic resonance imaging (MRI). Using a transmitting and a receiving RF coil, we observe Mie resonance signatures in both the transmission frequency spectrum and the magnetic field distribution surrounding a 40 mm-diameter sphere. Through full-wave numerical simulations and a standard fitting technique, we identify and characterize the lowest-frequency transverse-electric resonance modes of the sphere at 141.6, 201.4, 259.6, and 282.6 MHz, each exhibiting high-quality factors (300). Remarkably, we experimentally map the magnetic field distributions corresponding to each Mie resonance. Our results provide valuable insights for developing innovative approaches to manipulate RF waves, particularly by shaping and controlling the near-field outside the dielectric resonator through Mie resonances, an area largely unexplored in MRI technology.

Abnormal chirality in antiferromagnetic resonance modes of van der Waals 2D magnets

Two-dimensional van der Waals (2D vdW) materials have attracted widespread research interest due to their unique physical properties and potential application prospects. In this study, an atomistic-level dynamical simulation method is employed to investigate the chirality of antiferromagnetic resonance modes in CrI3 bilayer. Beyond the typical observations of a linear increase in high-frequency resonance mode and a linear decrease in low-frequency resonance mode, we have identified a distinct magnetization precession chirality in the CrI3 bilayer at low magnetic fields: Spins in different layers exhibit opposite precession chirality. This unusual chirality phenomenon is attributed to the weak interlayer coupling inherent in vdW materials, which can be adjusted by tuning their interlayer coupling and perpendicular magnetic anisotropy. These findings provide valuable insights into the intrinsic antiferromagnetic resonance characteristics of atomically thin vdW materials and their potential implications for the development of spintronic devices.

Deciphering the abnormal IR spectral density of phthalic acid dimer crystals: Unveiling the role of the dynamical effects of the Davydov coupling and the mechanisms of relaxation.

To consistently determine the anomalous characteristics of phthalic acid crystal (PAC) derivatives, we performed quantum dynamics simulations of the infrared spectral density of the h6-PAC and d6-PAC isotopomers that show up in the H/D isotopic frequency domain at two different temperatures viz. 77 and 298 K. A theoretical framework explaining the dynamical cooperative interactions within the hydrogen bonds (HBs) in the PAC crystals across a simulation of IR spectral density of the stretching band was developed. The model presupposes HBs from the nearby (COOH)2 cycles to have exceptionally strong dynamical cooperative contacts. The approach precisely hinges upon on a model that consider the dimer as centrosymmetric in which the low-frequency intermonomer mode (that is, O⋯O) stretches anharmonically and committing the O-H vibrational mode, associated with the high-frequency mode, to stretch harmonically. The approach considers also the following effects: Davydov coupling arising from the interference between the two HBs in the dimer; strong anharmonic coupling between the two stretching modes within the same HB; direct relaxation mechanism of the high stretches in the HB; and indirect relaxation mechanism that is devoted to the high stretches via the slow stretches. Using this approach, the principal characteristics of the O-H(D) experimental bands of the isotopomers studied herein with their peculiarities are simulated by selecting physically sound parameters that uniquely identify the onset of the specific features of the IR spectral densities. The impacts of the important parameters, characterizing the different leading mechanisms within this approach on the spectra, are revealed. The contrast of the emerging simulated spectra with the experimental ones reveals decent agreement for the studied isotopomers. By congregating the different mechanisms employed herein within the pioneering context of the linear response theory of Kubo, the obtained results strongly demonstrate and identify the main effects responsible for the generation of the experimental O-H and O-D bands. Through an examination of the effect of each mechanism involved within the h 6-PAC and d4-PAC isotopomers, it is possible to relate the emergence of IR spectral density to an interplay between the Davydov coupling and the relaxation dynamics.

2D Raman-THz spectroscopy of imidazolium-based ionic liquids.

An investigation of the low-frequency (i.e., less than 5THz), inter-molecular dynamics of three imidazolium-based ionic liquids-1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]), 1-butyl-3-methylimidazolium dicyanamide ([C4mim][DCA]), and 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][DCA])-is presented using two-dimensional (2D) Raman-THz spectroscopy combined with molecular dynamics (MD) simulations. By observing an echo in the 2D Raman-THz response, the experimental results indicate that the substitution of a small [DCA]- anion with a larger [NTf2]- one leads to a substantial increase in the structural inhomogeneity of the low-frequency modes of the system. These findings are corroborated by MD simulations, comparing the experimentally observed echo decay times to those of a computed velocity echo. The comparison suggests that the echo decay time reflects the instantaneous amount of structural order related to the charge alternation network, which is enhanced for the ionic liquid with the larger anion.

Symmetry-Breaking Charge-Separation in a Subphthalocyanine Dimer Resolved by Two-Dimensional Electronic Spectroscopy.

Understanding the role of structural and environmental dynamics in the excited state properties of strongly coupled chromophores is of paramount importance in molecular photonics. Ultrafast, coherent, and multidimensional spectroscopies have been utilized to investigate such dynamics in the simplest model system, the molecular dimer. Here, we present a half-broadband two-dimensional electronic spectroscopy (HB2DES) study of the previously reported ultrafast symmetry-breaking charge separation (SB-CS) in the subphthalocyanine oxo-bridged homodimer μ-OSubPc2. Electronic structure calculations and 2D cross-peaks reveal the dimer's excitonic structure, while ultrafast evolution of the multidimensional spectra unveils subtle features of structural relaxation, solvation dynamics, and inhomogeneous broadening in the SB-CS. Analysis of coherently excited vibrational motions reveals dimer-specific low-frequency Raman active modes coupled to higher-frequency vibrations localized on the SubPc cores. Finally, beatmap amplitude distributions characteristic of excitonic dimers with multiple bright states are reported and analyzed.

Engineering Protein Dynamics through Mutational Energy Landscape Traps.

Protein dynamics is essential for various biological processes, influencing functions such as enzyme activity, molecular recognition, and signal transduction. However, traditional protein engineering methods often focus on static structures, lacking tools to precisely manipulate dynamic behaviors. Here, we developed Mutational Energy Landscape Trap (MELT), a novel method designed to control protein dynamics by combining Normal Mode Analysis (NMA) and in silico mutagenesis. MELT works by displacing protein structures along low-frequency normal modes and introducing mutations to either lock proteins in these conformations or increase dynamics along the chosen normal modes. We tested MELT using hen-egg lysozyme as a model system. The method was validated by monitoring relevant collective coordinates during molecular dynamics simulations and evaluation of the collective movements of each construct. Our experiments showed that MELT was able to consistently create new protein sequences with the desired dynamical behavior in simulations. It demonstrates its potential for applications in the field of protein engineering, being an unprecedented way of manipulating protein features.

Sulfur isotope engineering in heterostructures of transition metal dichalcogenides.

Heterostructuring of two-dimensional materials offers a robust platform to precisely tune optoelectronic properties through interlayer interactions. Here we achieved a strong interlayer coupling in a double-layered heterostructure of sulfur isotope-modified adjacent MoS2 monolayers via two-step chemical vapor deposition growth. The strong interlayer coupling in the MoS2(34S)/MoS2(32S) was affirmed by low-frequency shear and breathing modes in the Raman spectra. The photoluminescence emission spectra showed that isotope-induced changes in the electronic structure and strong interlayer coupling led to the suppression of intralayer excitons, resulting in dominant emission from the MoS2(32S) layer. Time-resolved photoluminescence experiments indicated faster lifetimes in the MoS2(34S)/MoS2(32S) heterostructure compared to the conventional bilayers with the natural isotopic abundance, highlighting nuanced interlayer exciton dynamics due to the isotopic modification. This study underscores the great potential of isotope engineering in van der Waals heterostructures, as it enables tailoring the band structure and exciton dynamics at the nuclear level without the need of chemical modification.

Zero damping conditions of magnetic bilayer in microwave cavity

Experimental and theoretical studies have demonstrated that single magnon mode and cavity photon can be coupled coherently and dissipatively, in which the interference between two types of coupling will give rise to zero damping condition. In magnetic bilayers or multilayers, there exist more than one magnon modes which could be directly coupled by interface exchange interaction. In this work, we extend single magnon mode to two magnon modes and study the effect of two magnon modes on zero damping condition. Using eigenfrequency analysis and microwave transmission spectra, we derive analytical expressions of zero damping condition and the frequency detuning. By comparing analytical results to numerical results, we obtain the dependence of zero damping condition on system parameters. In the absence of direct interface exchange magnon-magnon coupling, the zero damping condition occurs for dissipative coupling or hybrid coupling. As the coupling strength increases, the distance between two zero damping conditions increases. For hybrid coupling, the curves become asymmetric around the point of zero detuning, which is different from pure coupling. Moreover, we study the effect of interface exchange magnon-magnon interaction on zero damping condition. The interface exchange coupling results in the splitting of microwave transmission spectra, but the zero damping condition occurs for low-frequency mode only. As the interface exchange coupling strength increases, the frequency at which the zero damping condition happens will shift to lower frequency. Due to extremely narrow line-width of microwave transmission dip at the zero damping condition, our work is expected to be useful for the design of magnon-based quantum sensing devices.

Superconductivity in o-MAX phases.

In recent years, MAX phases and their two-dimensional counterparts, MXenes, have emerged as significant subjects of interest in the fields of science and engineering, owing to their varied geometries, compositions, and extensive range of applications. This research employs first-principles calculations to explore the geometrical structures, electronic characteristics, phonon dispersions, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of 27 out-of-plane ordered double transition metal carbides, referred to as o-MAX phases, characterized by the general formula M2M'AlC2 (where M = Nb, Mo, W and M' = Sc, Ti, Zr, Hf, V, Nb, Ta, Mo, W). We have identified 16 superconducting o-MAX phases, with four specific compounds W2VAlC2, W2NbAlC2, W2TaAlC2, and Mo2NbAlC2 exhibiting a critical temperature (Tc) that surpasses 10 K, representing the highest Tc reported experimentally for MAX phases thus far. The calculated EPC constants for these materials are 0.98, 0.99, 1.02, and 0.74, correlating with Tc values of 17.9, 14.8, 14.5, and 11 K, respectively. Remarkably, the predicted transition temperature of 17.9 K stands as the highest Tc theoretically anticipated for any MAX phase to date. We conduct a thorough analysis of the specific mechanisms that facilitate superconductivity in these o-MAX systems. Our findings suggest that the presence of Kohn anomalies in low-frequency modes enhances electron-phonon interactions, resulting in increased superconducting transition temperatures (Tc). Additionally, our results indicate that Nb2M'AlC2 compounds do not display superconducting behavior.

Nonvolatile Ferroic and Topological Phase Control under Nonresonant Light.

Light-matter interaction is a long-standing promising topic that can be dated back to a few centuries ago and has witnessed the long-term debate between the particle and wave nature of light. In modern condensed matter physics and materials science, light usually serves as a detection tool to effectively characterize the physical and chemical features of samples. The light modulation on intrinsic properties of materials, such as atomic geometries, electronic bands, and magnetic behaviors, is more intriguing for information control and storage. This corresponds to a light-induced order parameter switch in the phase space. Most prior works focus on the situation when photon energy is larger than the material band gap, in which the photon is absorbed by the electron subsystem and then transfers its energy into other subsystems such as phonon and spin. This can be described by the imaginary part of the dielectric function. In contrast, recent theoretical predictions and experimental advances have suggested that the real part of dielectric function could also vary the energy landscape in phase space, so that it triggers phase transition in an athermic approach (without direct photon absorption). In this Perspective, we review some recent theoretical, computational, and experimental developments of such a low-frequency light-induced phase transition, focusing on ferroic and topological order parameters. We also elucidate its fundamental mechanisms by comparing it with the optical tweezers technique, and light irradiation could trigger impulsive stimulated Raman phonon excitation. Finally, we propose some further developments and challenges in such a nonresonant light-matter interaction.

The V30 benchmark set for anharmonic vibrational frequencies of molecular dimers.

Intermolecular vibrations are extremely challenging to describe but are the most crucial part for determining entropy and hence free energies and enable, for instance, the distinction between different crystal-packing arrangements of the same molecule via THz spectroscopy. Herein, we introduce a benchmark dataset-V30-containing 30 small molecular dimers with intermolecular interactions ranging from exclusively van der Waals dispersion to systems with hydrogen bonds. All the calculations are performed with the gold standard of quantum chemistry CCSD(T). We discuss vibrational frequencies obtained via different models starting with the harmonic approximation over independent Morse oscillators up to second-order vibrational perturbation theory (VPT2), which allows a proper anharmonic treatment including coupling of vibrational modes. However, large amplitude motions present in many low-frequency intermolecular modes are problematic for VPT2. In analogy to the often used treatment for internal rotations, we replace such problematic modes by a simple one-dimensional hindered rotor model. We compare selected dimers to the available experimental data or high-level calculations of potential energy surfaces and show that VPT2 in combination with hindered rotors can yield a very good description of fundamental frequencies for the discussed subset of dimers involving small and semi-rigid molecules.

Monomer size effect in inelastic collisional dynamics of non-equilibrium soot nucleation.

Molecular dynamics (MD) simulations of the collisional dynamics of the coronene-acepyrene and coronene radical-acepyrene pairs have been carried out to investigate the size effect of monomers of polycyclic aromatic hydrocarbons (PAH) on their non-equilibrium dimerization. The results compared to the previous MD simulations of the smaller pyrene-acepyrene and pyrenyl-acepyrene systems corroborate the non-equilibrium hypothesis of crosslinking PAH dimerization enhanced by physical interaction between the monomers. The phenomenon of inelastic collisional dynamics responsible for non-equilibrium van der Waals dimerization, which fosters a covalent bond formation between the monomers, amplifies with increasing PAH size. The increase in the size of the colliding monomers enhances the non-equilibrium effects as the growing pool of low-frequency modes provides a larger sink for the energy of the colliding PAH monomers. Based on the direct count of the crosslinking reaction events observed in the MD simulations, the forward rate constant for the coronene radical-acepyrene association is estimated at ∼10-11cm3molecule-1s-1, showing a 15-fold increase with respect to the value from the statistical Rice-Ramsperger-Kassel-Marcus calculations. A comparison with the eightfold increase reported previously for the pyrenyl-acepyrene system shows that the statistical (equilibrium-based) calculations increasingly underestimate the reaction rate with the increasing size of the interacting PAHs from pyrene to coronene. The total increase of the MD-assessed rate constant for the coronene radical-acepyrene dimerization reaction as compared to pyrenyl-acepyrene is a factor of 2.4, with the overall collision efficiency to produce dimerized products growing by 30%.

Bandlike charge transport and electron-phonon coupling in organic molecular crystals.

Charge transport is important in organic molecular crystals (OMCs), where high carrier mobilities are desirable for a range of applications. However, modeling and predicting the mobility is chal- lenging in OMCs due to their complex crystal and electronic structures and electron-phonon (e-ph) interactions. Here we show accurate first-principles calculations of electron and hole carrier mobility in several OMCs: benzene, anthracene, tetracene, pentacene, and biphenyl. Our calculations use the Boltzmann transport equation (BTE) formalism with e-ph interactions computed from first principles. These calculations describe transport in the bandlike, weak e-ph coupling regime, and include all phonon modes and electronic bands on equal footing. In all systems studied, we predict the mobility and its temperature dependence in very good agreement with experiments between 100-400 K, where transport is phonon-limited. We show that e-ph scattering from low-frequency (LF) phonons with energy below 150 cm-1 primarily limits the mobility, even though these modes are not the ones with the strongest e-ph coupling. These LF modes are shown to consist mainly of intermolecular vibrations, with admixed long-range intramolecular character in OMCs with larger molecules. Furthermore, we find that the LF-mode scattering rates vary significantly with strain, suggesting that strain engineering can effectively modulate e-ph coupling and enhance the mobility. This work sheds light on bandlike transport mechanisms in OMCs and advances the rational design of high-mobility organic semiconductors.

Linear and Two-Dimensional Infrared Spectroscopy of the Multifunctional Vibrational Probe, 3-(4-Azidophenyl) Propiolonitrile. Deperturbing a Fermi Triad by Isotopic Substitution.

Infrared probes are chemical moieties whose vibrational modes are used to obtain spectroscopic information about structural dynamics of complex systems; in particular, of biomacromolecules. Here, we explore the vibrational spectroscopy and dynamics of a reagent, 3-(4-azidophenyl)propiolonitrile (AzPPN), for selectively tagging thiols in protein environments with a multifunctional infrared probe containing both, an azide and a nitrile chromophore. The linear infrared spectrum of AzPPN is heavily perturbed in the antisymmetric azide stretching region as a result of accidental Fermi resonances. Isotopically labeling the azide group at the β-position deperturbs the spectrum considerably and reveals two combination tones that mix with the antisymmetric stretching fundamental into a Fermi triad of hybrid vibrational excitations. Moreover, two-dimensional infrared (2DIR) spectra were recorded for 15Nβ-labeled AzPPN, which reveal waiting-time-dependent spectral shifts of diagonal peaks and dynamic buildups of cross peaks. The 2DIR-spectral evolution is indicative of intramolecular distribution of the pump-induced excess vibrational energy into low-frequency modes of the molecule that are coupled to either the azide or the nitrile stretching transition dipoles. Finally, IR-pump/IR-probe spectra with selective narrowband excitation reveal a time constant of 2.3 ps for intramolecular vibrational redistribution (IVR) and 18 ps for the final energy dissipation into the solvent. The cross-peak dynamics corroborate a notion in which IVR within the AzPPN-molecule is an irreversible process.

Is process damping effective in the stability of robotic milling?

Chatter stability is a major constraint in milling, where low and high cutting speeds are used. At low cutting speed regime, process damping leads to increased stability, whereas at high cutting speeds lobing effect is beneficial. Excitation frequency depends on spindle speed and the number of cutting flutes on the milling tool. Hence the vibration mode governing chatter stability varies for multi-mode milling systems. In CNC milling, low frequency structural modes are stiffer than cutting spindle-holder-tool (SHT) assembly. However, robotic milling demonstrates a distinct behavior as low frequency modes are significantly more flexible. This study investigates the effect of robot structure induced low frequency vibration modes on stability limits at low cutting speeds, where process damping is expected to increase stability limits. Time domain simulations are used to explain the variation of the dominant mode from high frequency to low frequency with the decreasing spindle speed. Simulated stability diagram for the multi-mode robotic milling system is verified by experiments. It was shown that especially the vibration modes in the range of 15 to 20 Hz do not generate enough process damping force due to long vibration waves, i.e. cutting speed – to – chatter frequency ratio, when low frequency modes govern chatter stability. Simulation of stability diagrams showed that there is a spindle speed region where the stability lobes governed by the robot structure crosscut the stability lobes governed by the THS assembly. Due to the inherent effect of tool diameter (D) and number of cutting flutes (Z) on cutting speed and excitation frequency, this region shifts according to the D/Z ratio. It was shown through simulations that D/Z ratio is a critical metric to benefit from process damping without the interference of the low frequency excitation of the robotic structure. The simulation results are used to provide suggestions for milling tool selection in robotic milling, where the main conclusion is to use lower D/Z ratio, which means that using high number of cutting flutes.

A Robust ALOHA and Sequential clustering-based mode estimator for low frequency oscillations in power system using synchrophasors

Accurate estimation of low frequency modes in power system are very much important for improving small signal stability. The parametric model parameters estimator known as Total least square estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT) works effectively even in noisy conditions. However, this model parameter estimator requires prior information about numbers of modes of the signal. There are different Model order (MO) estimation techniques discussed in the recent past, which consider the significant eigenvalues of auto-correlation matrix (ACM) for its estimation. As the eigenvalues of ACM highly affected by bad measurements and Outliers, making these techniques inefficient and harder to automate in real time. So, to overcome aforementioned limitations, this paper proposes a robust mode estimation technique that can precisely detect the signal low frequency modes even in the presence of high variance noise and outliers. So, in this proposed work, an annihilating filter-based low-rank Hankel matrix (ALOHA) technique is implemented to obtain the rank deficient Hankel matrix to nullify the existence of noise and outliers in Phasor measurement unit (PMU) signal, thereafter approximated low rank Hankel matrix is considered for estimation of signals dominant modes. Wherein a sequential clustering technique (Sequential K-Mean++) is implemented for detection of numbers of prominent low frequency modes by segregating eigenvalues of ACM into two opponents i.e the signal and noise subspace. Thereafter the estimated MO is considered for estimation of modes through TLS-ESPRIT. The robustness of the proposed technique is validated by scheduling comparative study with recently developed techniques for synthetic signal, two area data, real PMU data of Western Electricity Coordinating Council (WECC) and oscillatory power data obtained from Western System Coordinating Council (WSCC) 9 bus system and IEEE68 bus system simulated through Real Time Digital Simulator (RTDS).

Dilated convolution neural operator for multiscale partial differential equations

This paper introduces a data-driven operator learning method for multiscale partial differential equations, with a particular emphasis on preserving high-frequency information. Drawing inspiration from the representation of multiscale parameterized solutions as a combination of low-rank global bases (such as low-frequency Fourier modes) and localized bases over coarse patches (analogous to dilated convolution), we propose the Dilated Convolutional Neural Operator (DCNO). The DCNO architecture effectively captures both high-frequency and low-frequency features while maintaining a low computational cost through a combination of convolution and Fourier layers. We conduct experiments to evaluate the performance of DCNO on various datasets, including the multiscale elliptic equation, its inverse problem, Navier–Stokes equation, and Helmholtz equation. Our results demonstrate that DCNO achieves an optimal balance between accuracy and computational efficiency, making it a promising approach for multiscale operator learning.