Wide Range Of Wave Vectors Research Articles

Role of the single-particle dynamics in the transverse current autocorrelation function of a liquid metal.

A recent simulation study of the transverse current autocorrelation of the Lennard-Jones fluid [Guarini et al., Phys. Rev. E 107, 014139 (2023)] revealed that this function can be perfectly described within the exponential expansion theory [Barocchi et al., Phys. Rev. E 85, 022102 (2012)]. However, above a certain wavevector Q, not only transverse collective excitations were found to propagate in the fluid, but a second oscillatory component of unclear origin (therefore called X) must be considered to fully account for the time dependence of the correlation function. Here, we present an extended investigation of the transverse current autocorrelation of liquid Au as obtained by abinitio molecular dynamics in the very wide range of wavevectors 5.7 ≤ Q ≤ 32.8 nm-1 in order to also follow the behavior of the X component, if present, at large Q values. A joint analysis of the transverse current spectrum and its self-portion indicates that the second oscillatory component arises from the longitudinal dynamics, as suggested by its close resemblance with the previously determined component accounting for the longitudinal part of the density of states. We conclude that such a mode, albeit featuring a merely transverse property, fingerprints the effect of longitudinal collective excitations on single-particle dynamics, rather than arising from a possible coupling between transverse and longitudinal acoustic waves.

Ultra-flat and long-lived plasmons in a strongly correlated oxide

Plasmons in strongly correlated systems are attracting considerable attention due to their unconventional behavior caused by electronic correlation effects. Recently, flat plasmons with nearly dispersionless frequency-wave vector relations have drawn significant interest because of their intriguing physical origin and promising applications. However, these flat plasmons exist primarily in low-dimensional materials with limited wave vector magnitudes (q < ~0.7 Å−1). Here, we show that long-lived flat plasmons can propagate up to ~1.2 Å−1 in α-Ti2O3, a strongly correlated three-dimensional Mott-insulator, with an ultra-small energy fluctuation (<40 meV). The strong correlation effect renormalizes the electronic bands near Fermi level with a small bandwidth, which is responsible for the flat plasmons in α-Ti2O3. Moreover, these flat plasmons are not affected by Landau damping over a wide range of wave vectors (q < ~1.2 Å−1) due to symmetry constrains on the electron wavefunctions. Our work provides a strategy for exploring flat plasmons in strongly correlated systems, which in turn may give rise to novel plasmonic devices in which flat and long-lived plasmons are desirable.

Collective dynamics of liquid sulfur scrutinized over three decades in frequency.

Liquid sulfur consists mainly of eight-membered rings and hence can be regarded as a model of a molecular liquid. A liquid, which is built from different molecular structures, will demonstrate a wide range in relaxation processes and excitation modes. Three inelastic neutron scattering experiments have been performed to study the collective dynamics of liquid sulfur over three decades in frequencies. A wide range of wave vectors was studied to reveal the response of density fluctuations over different lengthscales. A viscoelastic model with a two-times memory function was applied to the data. The analysis revealed a slow relaxation mode, an acoustic-type excitation, and a high-frequency mode, which resembles an optic-type excitation. The wave-vector dependence of the slow relaxation mode width exhibits the signs of a de Gennes narrowing around the wave vector where the structure factor has a shoulder. This slow relaxation process could be related to diffusive particle movements. The acoustic-type modes evidence a viscoelastic reaction with a 50% enhancement of the sound velocity. This enhancement of the sound velocity and the spectral line shape is qualitatively similar to spectra of molecular liquids. The two relaxation times of the memory function are separated by about two orders of magnitude and underpin the need for a wide frequency range investigation of this complex liquid. The high-frequency response can be interpreted as optic-type modes in the liquid.

Electrical spectroscopy of the spin-wave dispersion and bistability in gallium-doped yttrium iron garnet

Yttrium iron garnet (YIG) is a magnetic insulator with record-low damping, allowing spin-wave transport over macroscopic distances. Doping YIG with gallium ions greatly reduces the demagnetizing field and introduces a perpendicular magnetic anisotropy, which leads to an isotropic spin-wave dispersion that facilitates spin-wave optics and spin-wave steering. Here, we characterize the dispersion of a gallium-doped YIG (Ga:YIG) thin film using electrical spectroscopy. We determine the magnetic anisotropy parameters and Gilbert damping from the frequency and linewidth of the ferromagnetic resonance (FMR). Next, we use propagating spin wave spectroscopy in the Damon–Eshbach configuration to detect the small spin-wave magnetic fields of this ultrathin weak magnet over a wide range of wavevectors, enabling the extraction of the exchange constant αex=1.32×10−12 J/m. We observe foldover of the FMR with increasing drive power, leading to frequency shifts of the spin-wave modes and a bistable region in the spin-wave spectra. Our results shed light on isotropic spin-wave transport in Ga:YIG and highlight the potential of electrical spectroscopy to map out the dispersion and bistability of propagating spin waves in magnets with a low saturation magnetization.

Surface light scattering in reflection geometry: capabilities and limitations.

In the present study, the capabilities and limitations of surface light scattering (SLS) experiments in reflection geometry are investigated. Based on the study of the transparent reference fluid toluene at 303.15 K over a wide range of wave vectors between (0.3and6.6)×105m-1, the performance of two different detection schemes analyzing light scattered from the vapor-liquid interface in a perpendicular and non-perpendicular direction is assessed. Considering various aspects such as the quality of the heterodyne correlation functions, the input information for data evaluation, and the line-broadening effects, both detection schemes show comparable overall efficiency. For wave vectors larger than 4.5×105m-1, where line-broadening effects are suppressed, the results obtained for liquid viscosity and surface tension agree with measurements in transmission geometry, validating the capability of the apparatus. For wave vectors smaller than 1.5×105m-1, the SLS signals are distinctly affected by line-broadening effects, which will result in erroneous values for surface tension and in particular viscosity, even if empirical fitting approaches commonly used in literature are applied. The modeling of the influence of line broadening on the measurements results by a simple Gaussian-weighted sum of individual damped oscillations reveals the increasing complexity of the underlying wave vector distribution toward smaller wave vectors chosen for the scattering geometry.

Laser-driven anisotropic and nonlinear Rashba spin splitting in GaAs monolayer

We investigate the electronic properties of the buckled honeycomb GaAs monolayer under a periodically circularly polarized laser (CPL) by first-principles calculations and Floquet theory. Under the irradiation of CPL with propagation direction in the $xy$ plane, a remarkable Rashba spin splitting (RSS) is induced in the valence and conduction bands at $\mathrm{\ensuremath{\Gamma}}$ point. The laser-driven RSS deviates from the classical linear Rashba model in a wide range of wave vectors, and can be interpreted by a nonlinear model involving the third-order terms. The phase diagram of Rashba effect with laser amplitude and propagation direction is worked out. The anisotropy of the nonlinear RSS is mainly determined by the laser propagation direction, providing the possibility of controlling the Rashba anisotropy characteristics. By carefully choosing the laser frequency and amplitude, giant anisotropic RSS can be obtained, which allows the generation of spin-polarized electrons via doping and facilitates the flexibility of spintronic devices.

Deep learning surrogate model for kinetic Landau-fluid closure with collision

In this work, the kinetic Landau-fluid (LF) closure with collision and periodic boundary condition is used in the development of the deep learning (DL) surrogate model. A classical neural network, namely, feedforward neural network or sometimes termed multilayer perceptron, is constructed and trained to learn the kinetic LF closure in the static limit and arbitrary mean free path in configuration space. The preliminary relation between best hyperparameters and critical parameters for data generation is found. Compared with the numerical approach (non-Fourier method) of the LF closure, the deep learning surrogate model shows an order of magnitude of improvement in terms of accuracy. Perhaps most importantly, the surrogate model closure has been integrated for the first time with fluid simulations. Our DL-enabled fluid simulations, for the first time, give the correct Landau damping rate for a wide range of wave vectors, while the Hammett–Perkins closure cannot produce the correct damping rate. We correctly connect the collisionless Hammett–Perkins closure and collisional Braginskii closure to reproduce the intrinsic nonlocal feature of the heat flux with DL techniques. We address the most concerning error accumulation problem and find that simulations with the deep learning surrogate model are as good as, if not better than, simulations with the analytic closure in terms of long-term numerical stability in the linear Landau damping test.

Dynamics of resonantly excited exciton-polariton fluids in GaAs microcavities in the absence of an exciton reservoir

We address dynamics of a low polariton (LP) system excited resonantly in a wide range of wave vectors by converging 2.5-ps-long Gaussian pulses. The spatial coherence in an LP system excited by incoherent light is found to form very slowly in the absence of an exciton reservoir, the coherence length at the delay time of 250 ps and T=2 K being less than 2.5 μm. The LP fluid excited by coherent linearly polarized pulses does not lose the inherited high spatial coherence and polarization and demonstrates dynamic compression to a condensate state at the LP branch bottom. In the LP fluid excited by elliptically polarized pulses the components with opposite circular polarizations are compressed almost independently of each other.

Calculation of self-diffusion coefficient and surface tension of liquid alkali metals using square-well fluid

The self-diffusion coefficient (D) and the surface tension (γ) of the liquid alkali metals such as cesium (Cs) and rubidium (Rb) were calculated based on the Einstein's relation in a wide range of densities and temperatures. A developed mean spherical approximation (MSA) theory was applied to compute the direct correlation function using the effective square-well potential model for pair interaction in each temperature and density. The static structure factor, S(k), of the studied liquid species was evaluated by the proposed MSA theory in a wide range of wave vectors. The linear isothermal regularity equation of state was employed to determine the radial distribution function at a contact point at any thermodynamic state. A good agreement was obtained between the numerical results for both the D and γ values for the titled species and their available experimental data over the whole liquid range. For both liquids Cs and Rb, the temperature and density dependencies of the self-diffusion coefficient were investigated. Moreover, a correlation relation between the liquid surface tension and density and the temperature was found, in which the order of magnitude and sign of the coefficients were in good agreement with those obtained by experiment.

Dynamic Compression of Spinor Exciton-Polariton Systems in Semiconductor Microcavities

The dynamically compressed coherent exciton-polariton condensate state was implemented at the low polariton (LP) band bottom in GaAs microcavity resonantly excited in a wide range of wavevectors by the convergent 2.5-ps long Gaussian linearly polarized beam when the active region of the cavity was ahead of the waist. Formation of the dynamically compressed condensate state at the LP band bottom and its further dynamics are shown to be well described by the Gross-Pitaevskii equation. The state persists for several picosecond until LP-LP repulsion causes its expansion in both spatial and momentum spaces. Dynamic compression of an elliptically polarized LP system is found to occur without preserving its initial polarization: the dynamic compression of weakly interacting components with σ+ and σ− polarization occurs almost independently.

Self-shaping of cavity-polariton systems created by resonant broadband excitation

The dynamics of a pure low polariton (LP) system created by resonant broadband excitation in a wide range of wave vectors was investigated in a high-$Q$ GaAs-based microcavity. The LP system is shown to inherit the high spatial coherence from the laser pulse and does not lose it during decay. As a result, its dynamics is well controlled by the spatial and momentum distributions of photons in the exciting pulse and described by the Gross-Pitaevskii equations. In particular, the purely dynamic formation of the highly populated coherent LP state was implemented at the LP band bottom in the cavity excited in a large spot by converging ps-long Gaussian laser pulses when the active region of the cavity is in front of its waist. The formed state is found to persist for several picoseconds until the LP-LP repulsion leads to the creation of high-energy LPs dissipating from the ground state with high velocities.

Multiple PT symmetry and tunable scattering behaviors in a heterojunction cavity

We study a heterojunction cavity with one vacuum gap inserted between two dielectric slabs with conjugate complex refractive indices, i.e., of balanced gain and loss. This cavity is found to exhibit a multiple parity-time (PT) symmetry characterized by alternating symmetric and broken phases, when examined in terms of both of the eigenvalues of the scattering matrix. Scattering behaviors, including reflection and transmission, however, should be classified into three distinct types because a transition region may exist between the symmetric and broken phases. It is of particular interest that vanishing reflectance could occur for light waves input from both the left and right sides, though of different wave vectors, in the symmetric phases. Moreover, it is viable to attain unidirectional invisibility for light waves only input from the left side and simultaneously realize coherent perfect absorption and lasing for a given wave vector in the transition regions. These unique scattering behaviors can be easily tuned to suit a wide range of wave vectors by adjusting the gap width.

Effect of a dc magnetic field on the anomalous dispersion of localized Josephson plasma modes in layered superconductors

We study theoretically the propagation of Josephson plasma waves (JPWs) localized on a slab of layered superconductor in the presence of an external dc magnetic field. The slab is sandwiched between two dielectric half-spaces and the wave modes propagate across the layers. We derive analytic expressions for the dispersion relations of the localized JPWs and present the numerical simulation for the effect of the external dc magnetic field on the dispersion. The anomalous dispersion of localized JPWs is predicted for a wide range of frequencies, wave vectors, and dc fields. Also, we discuss the possibility of the internal reflection of the localized modes in the inhomogeneous dc magnetic field. This phenomenon can find application in the terahertz electronics for the control of the localized mode propagation.

Nonlocal and Nonadiabatic Effects in the Charge-Density Response of Solids: A Time-Dependent Density-Functional Approach.

The charge-density response of extended materials is usually dominated by the collective oscillation of electrons, the plasmons. Beyond this feature, however, intriguing many-body effects are observed. They cannot be described by one of the most widely used approaches for the calculation of dielectric functions, which is time-dependent density functional theory (TDDFT) in the adiabatic local density approximation (ALDA). Here, we propose an approximation to the TDDFT exchange-correlation kernel which is nonadiabatic and nonlocal. It is extracted from correlated calculations in the homogeneous electron gas, where we have tabulated it for a wide range of wave vectors and frequencies. A simple mean density approximation allows one to use it in inhomogeneous materials where the density varies on a scale of 1.6 r_{s} or faster. This kernel contains effects that are completely absent in the ALDA; in particular, it correctly describes the double plasmon in the dynamic structure factor of sodium, and it shows the characteristic low-energy peak that appears in systems with low electronic density. It also leads to an overall quantitative improvement of spectra.

Measurements of long-wavelength spin waves for the magnetic field in the Damon-Eshbach, backward-volume and forward-volume geometries of an yttrium iron garnet film

We report systematic measurements of the dispersion of long wavelength spin waves for a wide range of wave vectors for the magnetic field along the three principal directions defining the forward volume, backward volume and Damon-Eshbach modes of a 9.72 μm thick film of an yttrium iron garnet obtained using lithographically patterned, multi-element, spatially resonant, antennas. Overall good agreement is found between the experimental data for the backward volume and Damon-Eshbach modes and the magnetostatic theory of Damon and Eshbach. Also, good agreement is found between the experimental data for the forward volume mode and the theory of Damon and van de Vaart.

Wave vector dependent damping of THz collective modes in a liquid metal

Well-defined damped collective modes have been observed in liquid metals over a wide range of wave vectors. Hydrodynamics predicts that viscosity and thermal conductivity are the cause for the damping of the collective modes. Here we present experimental data from neutron spectroscopy on the damping of collective modes of liquid rubidium over a wide range of wave vectors. We propose a phenomenological model derived from generalized hydrodynamics to describe the damping of the modes and the evolution with increasing wave vector based on the viscoelastic picture of liquid response. As necessary ingredients a wave vector dependent high frequency shear modulus and shear relaxation time appear. We obtain a remarkable good agreement on a quantitative basis between experiment and calculation over a wide range of wave vectors. The emergent picture is that the lifetime of the collective modes in the THz regime is mainly limited through the diffusion of momentum. The proposed methodology might be applicable to a wide range of liquids.

Determination of structure factor and effective intermolecular parameters of liquid alkali metals using mean spherical approximation theory

The structural properties of liquid cesium (Cs) were investigated in the temperature range of 400–2000 K by application of the effective hard-core attractive Yukawa (H-Y) potential function to the mean spherical approximation (MSA) theory. An analytical equation of state was applied to calculate the molecular parameters of a potential function at any thermodynamic state. The proposed approach was used to predict the behavior of the structure factor, S(k), at a wide range of wave vectors (k). The results obtained were compared with the available experimental measurements, which revealed a good agreement over the whole liquid range for Cs. The structure factor at a long wavelength limit, S(0), was also calculated and compared with the experimental data, which showed the accuracy of the Ornstein-Zernike (OZ) behavior of S(k) at the thermodynamic states below the boiling temperature (Tb). For the case of Cs, the presented intermolecular potential model was very much responsive to the thermodynamic states and led to the indication of the two distinct isotherm ranges T <Tb and T >Tb. Two analytical expressions were obtained for the temperature dependency of the potential well-depth (Keff) over the temperature ranges of T <Tb and T >Tb. Furthermore, the proposed approach showed several metal-nonmetal transitions for Cs at about 500, 1100, 1350, and 1650 K. Using the Keff relations for the other liquid alkali metals Na, K, and Rb provided the S(k) results that were more accurate in comparison with the experimental data.

Longitudinal, transverse, and single-particle dynamics in liquid Zn:Ab initiostudy and theoretical analysis

We perform ab initio molecular dynamics simulations of liquid Zn near the melting point in order to study the longitudinal and transverse dynamic properties of the system. We find two propagating excitations in both of them in a wide range of wave vectors. This is in agreement with some experimental observations of the dynamic structure factor in the region around half the position of the main peak. Moreover, the two-mode structure in the transverse and longitudinal current correlation functions had also been previously observed in high pressure liquid metallic systems. We perform a theoretical analysis in order to investigate the possible origin of such two components by resorting to mode-coupling theories. They are found to describe qualitatively the appearance of two modes in the dynamics, but their relative intensities are not quantitatively reproduced. We suggest some possible improvements of the theory through the analysis of the structure of the memory functions. We also analyze the single-particle dynamics embedded in the velocity autocorrelation function, and explain its characteristics through mode-coupling concepts.

Tunable optical bound states in the continuum beyond in-plane symmetry protection

The formation of tunable bound states in the continuum (BICs) within photonic crystal (PC) slabs has been investigated by using a semianalytical coupled-wave theory framework. An analytic expression of the radiative wave has been derived in order to depict the condition of BICs. As a result, in addition to well-known symmetry-protected BICs, a novel type of vertical-cancellation BIC can be realized through continuously varying a given parameter to eliminate radiative waves at the boundaries. We investigated one-dimensional and two-dimensional (2D) periodic structures, and found that such tunable BICs can occur for a wide range of wave vectors by the selection of appropriate slab thicknesses. For a 2D PC slab, a ring of high-$Q$ modes is predicted and confirmed by numerical simulation.

A neutron spectrometer concept implementing RENS for studies in life sciences

BackgroundResolution Elastic Neutron Scattering (RENS) method involves performing elastic scattering intensity scans as a function of the instrumental energy resolution and as a function of temperature. MethodsIn the framework of RENS, numerical simulation and experimental data show that in the measured elastic scattering law against the logarithm of the instrumental energy resolution an inflection point occurs when the resolution time intersects the system relaxation time; conversely, in the measured elastic scattering law against temperature an inflection point turns up when the system relaxation time intersects the resolution time. ResultsFor practical implementation of the RENS technique, a dedicated neutron spectrometer would be needed. Here we propose a concept of such a spectrometer that utilizes mechanical velocity selection of both incident and scattered neutrons over a wide angular range. The instrument is able to collect intensity scans vs energy resolution where the instrumental resolution time changes crisscrossing the system relaxation time, and intensity scans vs temperature where the system relaxation time changes intersecting the instrumental resolution time. ConclusionsWe propose a RENS spectrometer concept based on velocity selection of incident neutrons and wide-angle velocity selection of scattered neutrons achieved by the same rotating collimator-type mechanical device with the optimized shape of blades. General SignificanceRENS spectrometer is strongly appealing and innovative because of the simultaneous data collection as a function of energy resolution, wide wavevector range and temperature. Such a spectrometer would be the first practical implementation of RENS concept with a broad range of applications in Life Sciences.This article is part of a Special Issue entitled “Science for Life” Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.