Range Of Wave Vectors Research Articles

Changing character of electronic transitions in graphene: From single-particle excitations to plasmons

In this paper we clarify the nature of $\pi$ and $\pi+\sigma$ electron excitations in pristine graphene. We clearly demonstrate the continuous transition from single particle to collective character of such excitations and how screening modifies their dispersion relations. We prove that $\pi$ and $\pi+\sigma$ plasmons do exist in graphene, though occurring only for a particular range of wavevectors and with finite damping rate. The particular attention is paid to compare the theoretical results with available EELS measurements in optical ($\mathrm{Q\approx 0}$) and other ($\mathrm{Q\neq 0}$) limits. The conclusions, based on microscopic numerical results, are confirmed in an approximate analytical approach.

Positive sound dispersion in vitreousGeO2at high pressure

We determined the density fluctuations spectrum $[S(Q,\ensuremath{\omega})]$ of vitreous and polycrystalline ${\text{GeO}}_{2}$ by inelastic x-ray scattering in the wave vector ($Q$) range between 3 and $13\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$ at pressures ($P$) between ambient and 7.1 GPa. The polycrystalline sample displays the expected $P$ evolution of the $Q$-dispersion relation of longitudinal acoustic (LA) modes. On the contrary, the dispersion relation of the glassy sample for $P\ensuremath{\ge}1$ GPa shows an excess energy of the LA modes, while at ambient pressure follows the trend expected from ultrasonic and light scattering data. These findings point towards a pressure-induced positive sound dispersion, with an associated high-frequency sound velocity comparable to that of the polycrystalline sample at the same $P$.

Observing backfolded and unfolded acoustic phonons by broadband optical light scattering

We use broadband time domain Brillouin scattering to observe coherently generated phonon modes in bulk and nanolayered samples. We transform the measured transients into a frequency-wavevector diagram and compare the resulting dispersion relations to calculations. The detected oscillation amplitude depends on the occupation of phonon modes induced by the pump pulse. For nanolayered samples with an appropriately large period, the whole wavevector range of the Brillouin zone becomes observable by broadband optical light scattering. The backfolded modes vanish, when the excitation has passed the nanolayers and propagates through the substrate underneath.

High resolution electron energy loss spectroscopy of spin waves in ultra-thin film — The return of the adiabatic approximation?

The paper reports on recent considerable improvements in electron energy loss spectroscopy (EELS) of spin waves in ultra-thin films. Spin wave spectra with 4meV resolution are shown. The high energy resolution enables the observation of standing modes in ultra-thin films in the wave vector range of 0.15Å−1<q||<0.3Å−1. In this range, Landau damping is comparatively small and standing spin wave modes are well-defined Lorentzians for which the adiabatic approximation is well suited, an approximation which was rightly dismissed by Mills and collaborators for spin waves near the Brillouin zone boundary. With the help of published exchange coupling constants, the Heisenberg model, and a simple model for the spectral response function, experimental spectra for Co-films on Cu(100) as well as for Co films capped with further copper layers are successfully simulated. It is shown that, depending on the wave vector and film thickness, the most prominent contribution to the spin wave spectrum may come from the first standing mode, not from the so-called surface mode. In general, the peak position of a low-resolution spin wave spectrum does not correspond to a single mode. A discussion of spin waves based on the “dispersion” of the peak positions in low resolution spectra is therefore subject to errors.

Inflationary freedom and cosmological neutrino constraints

The most stringent bounds on the absolute neutrino mass scale come from cosmological data. These bounds are made possible because massive relic neutrinos affect the expansion history of the universe and lead to a suppression of matter clustering on scales smaller than the associated free streaming length. However, the resulting effect on cosmological perturbations is relative to the primordial power spectrum of density perturbations from inflation, so freedom in the primordial power spectrum affects neutrino mass constraints. Using measurements of the cosmic microwave background, the galaxy power spectrum and the Hubble constant, we constrain neutrino mass and number of species for a model independent primordial power spectrum. Describing the primordial power spectrum by a 20-node spline, we find that the neutrino mass upper limit is a factor three weaker than when a power law form is imposed, if only CMB data are used. The primordial power spectrum itself is constrained to better than 10 % in the wave vector range $k \approx 0.01 - 0.25$ Mpc$^{-1}$. Galaxy clustering data and a determination of the Hubble constant play a key role in reining in the effects of inflationary freedom on neutrino constraints. The inclusion of both eliminates the inflationary freedom degradation of the neutrino mass bound, giving for the sum of neutrino masses $\Sigma m_\nu < 0.18$ eV (at 95 % confidence level, Planck+BOSS+$H_0$), approximately independent of the assumed primordial power spectrum model. When allowing for a free effective number of species, $N_{eff}$, the joint constraints on $\Sigma m_\nu$ and $N_{eff}$ are loosened by a factor 1.7 when the power law form of the primordial power spectrum is abandoned in favor of the spline parametrization.

Nonreciprocal dispersion of spin waves in ferromagnetic thin films covered with a finite-conductivity metal

We study the effect of one-side metallization of a uniform ferromagnetic thin film on its spin-wave dispersion relation in the Damon–Eshbach geometry. Due to the finite conductivity of the metallic cover layer on the ferromagnetic film, the spin-wave dispersion relation may be nonreciprocal only in a limited wave-vector range. We provide an approximate analytical solution for the spin-wave frequency, discuss its validity, and compare it with numerical results. The dispersion is analyzed systematically by varying the parameters of the ferromagnetic film, the metal cover layer and the value of the external magnetic field. The conclusions drawn from this analysis allow us to define a structure based on a 30 nm thick CoFeB film with an experimentally accessible nonreciprocal dispersion relation in a relatively wide wave-vector range.

Localized parallel parametric generation of spin waves in a Ni81Fe19 waveguide by spatial variation of the pumping field

We present the experimental observation of localized parallel parametric generation of spin waves in a transversally in-plane magnetized Ni81Fe19 magnonic waveguide. The localization is realized by combining the threshold character of parametric generation with a spatially confined enhancement of the amplifying microwave field. The latter is achieved by modulating the width of the microstrip transmission line which is used to provide the pumping field. By employing microfocussed Brillouin light scattering spectroscopy, we analyze the spatial distribution of the generated spin waves and compare it with numerical calculations of the field distribution along the Ni81Fe19 waveguide. This provides a local spin-wave excitation in transversally in-plane magnetized waveguides for a wide wave-vector range which is not restricted by the size of the generation area.

Broadband evolution of phononic-crystal-waveguide eigenstates in real- and k-spaces

Control of sound in phononic band-gap structures promises novel control and guiding mechanisms. Designs in photonic systems were quickly matched in phononics, and rows of defects in phononic crystals were shown to guide sound waves effectively. The vast majority of work in such phononic guiding has been in the frequency domain, because of the importance of the phononic dispersion relation in governing acoustic confinement in waveguides. However, frequency-domain studies miss vital information concerning the phase of the acoustic field and eigenstate coupling. Using a wide range of wavevectors k, we implement an ultrafast technique to probe the wave field evolution in straight and L-shaped phononic crystal surface-phonon waveguides in real- and k-space in two spatial dimensions, thus revealing the eigenstate-energy redistribution processes and the coupling between different frequency-degenerate eigenstates. Such use of k-t space is a first in acoustics, and should have other interesting applications such as acoustic-metamaterial characterization.

Impact of eddy currents on the dispersion relation of surface spin waves in thin conducting magnetic films

We propose a rigorous solution to a long-standing problem of the impact of eddy currents on the dispersion relation of surface spin waves propagating in thin conducting magnetic films. Our results confirm the prediction of the Almeida–Mill's exchange-free theory that the inclusion of the eddy-current contribution results in a deviation of the dispersion curve for the fundamental mode from the Damon–Eshbach law and a substantial linewidth broadening in a large wave vector range. We show that the decrease in the spin-wave frequency is due to an increase in the in-plane component of the dynamic magnetic field within the conducting film. The decrease in the frequency is accompanied by a drastic change in the asymmetry of the modal profiles for the waves. This effect is not observable in magneto-insulating films and therefore it is unambiguously attributed to eddy currents that appear in conducting films only. We also show that the wave vector range in which eddy currents affect the dispersion curve is strongly correlated with the value of the film conductivity. This result holds for conducting films with the thickness 10–100 nm, which are considered promising for future magnonic and spintronic applications.

Dynamics of liquid Au from neutron Brillouin scattering andab initiosimulations: Analogies in the behavior of metallic and insulating liquids

We performed a neutron Brillouin scattering determination of the dynamic structure factor of liquid gold in the wave-vector range $6&lt;Q&lt;16$ nm${}^{\ensuremath{-}1}$. Despite the experimental difficulties due to the considerable neutron absorption and the high melting temperature of the sample, a non-negligible coherent signal could successfully be extracted and revealed the presence of underdamped ion density-fluctuation modes in the whole $Q$ range of the experiment. The quality of the data further enabled a significant comparison with the results of ab initio simulations for liquid gold. The agreement found between neutron and simulation data not only provides a necessary test of ab initio methods still limited to the use of few hundreds atoms, but also reasonably justifies a thorough analysis of the simulated dynamics in the extended $Q$ range that can be more comfortably accessed by calculations. A viscoelastic modeling of the simulated spectra proves to be appropriate and, surprisingly, the so-derived salient dynamic features of liquid gold show an overall $Q$ behavior globally similar to that already found for insulating liquids. The present and recent results on the collective properties of simple liquids thus lead us to embrace the concept that simple liquid metals and insulators, at the nanometer length scale, are dynamically much less different than expected.

Equilibrium theory of the hard sphere fluid and glasses in the metastable regime up to jamming. II. Structure and application to hopping dynamics

Building on the equation-of-state theory of Paper I, we construct a new thermodynamically consistent integral equation theory for the equilibrium pair structure of 3-dimensional monodisperse hard spheres applicable up to the jamming transition. The approach is built on a two Yukawa generalized mean spherical approximation closure for the direct correlation function (DCF) beyond contact that reproduces the exact contact value of the pair correlation function and isothermal compressibility. The detailed construction of the DCF is guided by the desire to capture its distinctive features as jamming is approached. Comparison of the theory with jamming limit simulations reveals good agreement for many, but not all, of the key features of the pair correlation function. The theory is more accurate in Fourier space where predictions for the structure factor and DCF are accurate over a wide range of wavevectors from significantly below the first cage peak to very high wavevectors. New features of the equilibrium pair structure are predicted for packing fractions below jamming but well above crystallization. For example, the oscillatory DCF decays very slowly at large wavevectors for high packing fractions as a consequence of the unusual structure of the radial distribution function at small separations. The structural theory is used as input to the nonlinear Langevin equation theory of activated dynamics, and calculations of the alpha relaxation time based on single particle hopping are compared to recent colloid experiments and simulations at very high volume fractions.

Fibrillar Structure of Methylcellulose Hydrogels

It is well established that aqueous solutions of methylcellulose (MC) can form hydrogels on heating, with the rheological gel point closely correlated to the appearance of optical turbidity. However, the detailed gelation mechanism and the resulting gel structure remain poorly understood. Herein the fibrillar structure of aqueous MC gels was precisely quantified with a powerful combination of (real space) cryogenic transmission electron microscopy (cryo-TEM) and (reciprocal space) small-angle neutron scattering (SANS) techniques. The cryo-TEM images reveal that MC chains with a molecular weight of 300 000 g/mol associate into fibrils upon heating, with a remarkably uniform diameter of 15 ± 2 nm over a range of concentrations. Vitrified gels also exhibit heterogeneity in the fibril density on the length scale of hundreds of nanometers, consistent with the observed optical turbidity of MC hydrogels. The SANS curves of gels exhibit no characteristic peaks or plateaus over a broad range of wavevector, q, from 0.001-0.2 Å(-1). The major feature is a change in slope from I ∼ q(-1.7) in the intermediate q range (0.001 - 0.01 Å(-1)) to I ∼ q(-4) above q ≈ 0.015 Å(-1). The fibrillar nature of the gel structure was confirmed by fitting the SANS data consistently with a model based on the form factor for flexible cylinders with a polydisperse radius. This model was found to capture the scattering features quantitatively for MC gels varying in concentration from 0.09-1.3 wt %. In agreement with the microscopy results, the flexible cylinder model indicated fibril diameters of 14 ± 1 nm for samples at elevated temperatures. This combination of complementary experimental techniques provides a comprehensive nanoscale depiction of fibrillar morphology for MC gels, which correlates very well with macro-scale rheological behavior and optical turbidity previously observed for such systems.

Elastic incoherent neutron scatterings wavevector and thermal analysis on glass-forming homologous disaccharides

A wavelet analysis of Elastic Incoherent Neutron Scattering (EINS) intensity data collected, as a function of wavevector, on three glass forming systems, i.e. on the homologous disaccharides trehalose, maltose and sucrose, is presented. The wavelet transform allows to explore the wavevector behavior of the scattered intensity at different scales highlighting the correlation between the signal and the set of the scaled and translated mother functions. The analysis performed in the wavevector range of Q=0.28Å−1÷4.28Å−1 reveals the existence of different kinds of protons dynamics which cover different spatial scales. It emerges that, differently from previous analyses, for trehalose the intensity data at all the investigated temperature values are constantly lower and sharper in respect to those of maltose and sucrose, giving rise to a global spectral density along the wavevector range markedly less extended. Furthermore, the application of an interpretative model for the partial and global EINS intensity temperature behavior allows to put into evidence both the different wavevector dependence of the system relaxation times, extracted from the intensity vs. temperature inflection point, and the higher thermal restrain of trehalose in respect to the other two homologous disaccharides.

An effective fitting scheme for the dynamic structure of pure liquids

A scheme of analysis for the dynamic structure functions in pure liquids is presented which can be implemented with both experimental and simulation data. Expressions for contributions of relaxing and propagating modes proposed earlier in the framework of the generalized collective modes approach are optimized in order to strictly fulfil three among the required sum-rules. The method is applied to simulation data for liquid cesium, the description of which appears to only require one relaxing and one propagating mode in the investigated wavevector range. These expressions are able to account for the dynamics in both the hydrodynamic and the kinetic regimes, being quantitatively accurate up to the onset of the first peak of the static structure factor and qualitatively beyond. Features of the modes can thus be obtained easily, without resorting to heavy formalism. The scheme of analysis can be straightforwardly extended to account for a higher number of relaxing and propagating modes.

Spin Wave Dispersion in Permalloy Antidot Array With Alternating Holes Diameter

The dispersion of spin waves in a Permalloy binary antidot array constituted by two alternated antidot sublattices with different diameters of the circular holes has been investigated. The diameters of the holes are D1 = 300 nm and D2 = 150 nm with a holes center-to-center spacing of 425 nm. The spin wave frequency has been measured with Brillouin light scattering applying a magnetic field along the edge of the square unit cell of the lattice and sweeping the spin-wave wave vector along the perpendicular direction. The measured dispersion has been compared with calculations performed by the plane wave method, achieving a good agreement with the experimental results over the whole wave vector range.

Tight-binding description of graphyne and its two-dimensional derivatives

We investigated band structures of α-graphyne and its derivative two-dimensional carbon compounds (2DCCs) via tight-binding approximations with "two-site" and "all-atom" models. The renormalized "two-site" model captures the band-gap features of α-graphyne and 2DCCs. This model suggests ways of tuning the band gaps of graphynes, namely, by adding adatoms or substituting the vertex sp(2) carbons with heteroatom. Because the "two-site" model cannot accurately reproduce first-principles results over a large range of wave vectors, we derived an "all-atom" model, which includes all pz orbitals in a unit cell. All atom tight-binding calculations show improved performances in describing the DFT band structures, and reveal that the flat bands in DFT band structures are mainly ascribed to the pz orbitals of the edge carbons. The results will help to uncover the underlying mechanisms of the band features of graphyne and 2DCCs and to design other graphyne- or graphdiyne-based 2DCCs for applications in the future.

The Plasma Oscillations in a Two-Dimensional Electron–Hole Liquid Under Influence Rashba Spin-Orbit Coupling

The plasma oscillations of the two-dimensional electron-hole system in the presence of a Rashba spin-orbit coupling are studied. Only the intra-Landau level excitations are taken into account when the electrons and holes are situated on their lowest Landau levels, the filling factor $v^{2}$ being less than 1. The ground state of the two-dimensional electron-hole system is supposed to be the electron-hole liquid. The dispersion relations for the optical and acoustical plasmon modes were obtained. The acoustical plasmon branch has a linear dispersion law in the range of small wave vectors and monotonically increases with saturation at higher values of wave vectors. The optical plasmon branch has a quadratic dependence in the range of long wavelength and a monotonic increasing with saturation as in the case of acoustical branch.

Investigations of homologous disaccharides by elastic incoherent neutron scattering and wavelet multiresolution analysis

In the present paper the results of a wavevector and thermal analysis of Elastic Incoherent Neutron Scattering (EINS) data collected on water mixtures of three homologous disaccharides through a wavelet approach are reported. The wavelet analysis allows to compare both the spatial properties of the three systems in the wavevector range of Q=0.27Å−1÷4.27Å−1. It emerges that, differently from previous analyses, for trehalose the scalograms are constantly lower and sharper in respect to maltose and sucrose, giving rise to a global spectral density along the wavevector range markedly less extended. As far as the thermal analysis is concerned, the global scattered intensity profiles suggest a higher thermal restrain of trehalose in respect to the other two homologous disaccharides.

Bioprotectant Effectiveness of Homologous Disaccharides by Elastic Incoherent Neutron Scattering

A new wavevector analysis performed by wavelet transform of Elastic Incoherent Neutron Scattering (EINS) data on three bioprotectant systems, i.e. on the homologous disaccharides trehalose, maltose and sucrose, is presented. The analysis allows to compare the spatial properties of the three systems in the wavevector range of , revealing the existence of different kinds of protons dynamics. It emerges that, differently from previous wavevector analyses, both the low and high wavevector contributions for trehalose, at all the investigated temperature values, are constantly lower and sharper, giving rise to a global energy distribution along the wavevector range markedly less extended. Furthermore an innovative model for the global EINS intensity temperature behavior is applied to put into evidence the higher thermal restrain for trehalose in respect to the other two homologous disaccharides. These findings give an account for the bioprotectant effectiveness of trehalose in the medical, pharmaceutical and cosmetic fields.

Thermal Properties Investigation on Systems of Biophysical Interest by EINS and Wavelet Analysis

In this work anew wavevector analysis of Elastic Incoherent Neutron Scattering (EINS) data on bioprotectant systems based on a wavelet approach is presented. The wavevector analysis allows to compare the spatial properties of the three systems such as three glass-forming homologous disaccharides (trehalose, maltose and sucrose),in the wavevector range of Q=0.28-4.27 Co-1 and the wavelet transform reveals the existence of different kinds of protons dynamics. The comparison among the EINS spectra of the investigated systems points out systematically lower and sharper contributions for trehalose than for maltose and sucrose both at low and high wavevector, so highlighting a less extended global energy distribution along the wavevector range for trehalose.