On the frequency spectrum of an artificial amorphous solid

Summary The frequency spectrum of an amorphous Lennard-Jones solid, as determined by molecular-dynamics methods, is entirely different from that of a crystalline solid of the same nature, although its radial distribution function shows the environment of each atom to be similar in both cases. Such a disparity is interpreted in terms of a preponderance of the effects of the local forces (i.e. those between direct neighbours) which, in the case of the amorphous solid, is supposed to take place because of the absence of long-range co-ordination among individual atomic motions. To give the observed spectrum, this feature combines with a structure in which atomic clusters of different radii seem to coexist.

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Riassunto Lo spettro di frequenza di un solido amorfo costituito da atomi che interagiscono secondo un potenziale di Lennard-Jones risulta, secondo ricerche di dinamica molecolare, del tutto diverso da quello di un solido cristallino della stessa natura. D'altra parte la funzione di distribuzione radiale non mostra, nella regione del picco principale, cioè quella dei vicini diretti, nessuna significativa deviazione dalla normale struttura del solido cristallino. Questa disparità degli effetti di quelle che sono essenzialmente le stesse forze è interpretabile in termini di una quasi indipendenza delle oscillazioni locali che si unisce, per dare il caratteristico spettro osservato, ad una peculiare struttura nella quale coesistono gruppi sferici di atomi vicini aventi raggi, entro certi limiti, diversi.

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Резюме Частотный спектр аморфного твердого тела Леннарда-Джонсона, определяемЫй методами молекулярной динамики, оказЫвается полностью отличнЫм от частотного спектра кристаллического твердого тела той же природЫ; хотя радиальные функции распределения показывают, что окружение каждого атома является сходным в обоих случаях. Таксе несоответствие интерпретируется в терминах преобладания эффектов локальных сил (t.e. сил между соседними атомами), которое, как предполагается, имеет место в случае аморфного твердого тела, t.e. отсутствуют длиннодействуюшие корреляции между движениями отдельных атомов. Чтобы получить наблюдаемый спектр, эта особенность комбинируется со структурой, в которой сосуществуют атомные кластеры различных радиусов.

     

Surface crystallization of amorphous solid water

We demonstrate that the crystallization of thin, supported amorphous solid water layers is initiated at the water surface. This is concluded from the observation of sequential crystallization of amorphous water at the surface, in the bulk, and at the water-support interface. A surface nucleation model quantitatively reproduces the observed transformation kinetics at the three sites.     

New coupling mechanism of high frequency phonons at interfaces

Experiments with solids coated by oxide, metal or He films — or simply by not specified dirt — show diffuse and inelastic scattering at such interfaces for phonons above 10¹⁰ Hz. A new interaction mechanism is proposed enhancing the diffuse scattering at coated surfaces. This mechanism is due to charges in the coating pinned to the interface, so that their relative motion under impinging phonons generates rf fields, which then act back on the charges.     

Relation between the high density phase and the very-high density phase of amorphous solid water

It has been suggested that high-density amorphous (HDA) ice is a structurally arrested form of high-density liquid (HDL) water, while low-density amorphous ice is a structurally arrested form of low-density liquid (LDL) water. Recent experiments and simulations have been interpreted to support the possibility of a second distinct high-density structural state, named very high-density amorphous (VHDA) ice, questioning the LDL-HDL hypothesis. We test this interpretation using extensive computer simulations and find that VHDA is a more stable form of HDA and that, in fact, VHDA should be considered as the amorphous ice of the quenched HDL.     

More on the frequency spectrum of an artificial amorphous solid

Summary In a previous paper on the same subject an analysis of the frequency spectrum of an amorphous Lennard-Jones solid, as determined by methods of molecular dynamics by Rahman and collaborators, was given. There the general form of the spectrum was considered. Here we show how the analysis of the spectrum can be refined so as to allow a separation of the contribution of the propagating waves from the contribution of that fraction of atomic oscillations that do not possess propagational character. It is thus possible to bring indirectly into evidence bands of transverse and longitudinal waves, contributing a nonnegligible fraction of atomic motion.     

Generation and detection of very high frequency acoustic phonons in diamond

Generation and detection of very high frequency acoustic phonons in diamond is reported. We generate phonons at a frequency of 28 THz by defect-induced one-phonon absorption of CO₂ laser radiation and observe, after pulsed excitation, phonon decay products in the frequency range from 1 THz to 7 THz. For detection vibronic sideband spectroscopy is used. We find strongly frequency dependent lifetimes for frequencies above 4 THz which we attribute to spontaneous phonon decay.Dedicated to K. Dransfeld on occasion of his 60th birthday     

Electrons and phonons in amorphous pyramidally bonded solids

Electronic and phonon state densities of amorphous pyramidally bonded solids are calculated with As as a prototype. Using a Bethe-lattice, the Greaves-Davis random network model and the Cluster-Bethe-lattice method, the results show: (1) the phonon states are isomorphic to, and $\text{obtained}$ from, the p-like electron states, (2) the photoemission and infrared measurements are easily interpreted, and (3) the electronic and phonon state densities are $\text{extremely}$ sensitive to the local topology.     

Poiseuille flow of phonons in solid hydrogen

The thermal conductivity of solid parahydrogen is investigated using the stationary method with a plane sample in the temperature range 1.5–6.0 K in order to reveal a Poiseuille flow in solid hydrogen. It is established that the thermal conductivity at temperatures below the low-temperature maximum decreases very rapidly in accordance with the law K ～ T ⁿ (3 < n < 8). This finding is a direct indication that the possibility exists of observing a Poiseuille flow in solid hydrogen. The results obtained are compared with those for solid helium, in which the Poiseuille flow was observed for the first time in dielectric solids. According to the estimates, the mean free path of phonons at a temperature of approximately 3 K exceeds the radius of a cylindrical sample (3 mm). The thermal conductivity in the vicinity of the low-temperature maximum is found to be two times higher than the value available in the literature.     

Observation of the onset of strong scattering on high frequency acoustic phonons in densified silica glass

The linewidth of longitudinal acoustic waves in densified silica glass is obtained by inelastic x-ray scattering. It increases with a high power alpha of the frequency up to a crossover where the waves experience strong scattering. We find that alpha is at least 4, and probably larger. Resonance and hybridization of acoustic waves with the boson-peak modes seems to be a more likely explanation for these findings than Rayleigh scattering from disorder.     

Interactions between glycine and amorphous solid water nanoscale films

The interactions of glycine (Gly) with amorphous solid water (ASW) nanolayers (≤ 100 ML), vapor-deposited on single crystalline AlO_x surfaces at 100 K, have been investigated by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K-edge, temperature-programmed thermal desorption (TPD), X-ray photoelectron spectroscopy (XPS), and temperature-dependent work function measurements. Gly-on-ASW, ASW-on-Gly, and Gly on top of ASW-on-Gly ultrathin films have been fabricated. In contrast to the uniform ASW films grown directly on the hydrophilic AlO_x, water molecules adsorb on the hydrophobic Gly films in the form of 3D ASW clusters. This leads to significant differences in the NEXAFS and work function data obtained from ASW-on-AlO_x and ASW-on-Gly films, respectively. Furthermore, these structural differences influence the chemical state of Gly molecules (neutral vs. zwitterionic) adsorbed on top of ASW films. N1s XPS measurements revealed an increased amount of neutral Gly molecules in the film top-deposited on the ASW-on-Gly structure in comparison to the neutral Gly in the films directly condensed on AlO_x or grown on the ASW substrate. H₂O TPD spectra demonstrate that the crystallization and desorption processes of ASW are affected in a different way by the Gly layers, top-deposited on to ASW-on-AlO_x and ASW-on-Gly films. At the same time, Gly adlayers sink into the ASW film during crystallization/desorption of the latter and land softly on the alumina surface in the form of zwitterionic clusters.     

Interaction of ultrasonic phonons with defects in hydrogenated amorphous germanium

The attenuation of acoustic surface waves of 300 MHz has been measured in sputtered films of amorphous Ge and a-GeH_x (0x0.25) at temperatures between 0.5 K and 475 K. A strong absorption maximum due to structural relaxation is found at 135 K in pure a-Ge. This absorption maximum is shifted by annealing and is suppressed by hydrogen incorporation. Below 20 K the attentuation varies linearly with temperatures in all films investigated so far.On leave from: Department of Physics, Maharshi Dayanand University, Rohtak, 124001, India     

Theoretical study of intrinsic surface phonons in hydrogenated amorphous silicon

The extra-mode at 214 cm^-1 which is observed in the infrared spectrum of hydrogenated amorphous silicon is interpreted as being due to the presence of small (? 7 atoms) internal surfaces in the samples. Calculations of the phonon density of states at internal surfaces in bulk Si Bethe lattices show a pronounced peak at the edge of the TA band (≈ 210 cm^-1. It is shown that when hydrogen is present the mode is infrared active through a dynamical charge transfer mechanism.     

Dispersion of low frequency phonons in the deuterated naphthalene crystal

The dispersion curves of the external and the four lowest internal phonon modes in d₈-naphthalene (C₁₀D₈) crystal have been measured by coherent inelastic neutron scattering for the [010] and [100] directions at 98K and partially at 5K. The results are compared with calculations based on model atom-atom “6-exp” potential with the Kitaigorodski set of parameters for C-C, C-H and H-H interactions.     

Evidence for two different solid phases of two-dimensional electrons in high magnetic fields

We have observed two different rf resonances in the frequency dependent real diagonal conductivity of very high quality two-dimensional electron systems in the high magnetic field insulating phase and interpret them as coming from two different pinned electron solid phases (labeled as "A" and "B"). The "A" resonance is observable for Landau level filling nu<2/9 [reentrant around the nu=1/5 fractional quantum Hall effect (FQHE)] and then crosses over to the different "B" resonance which dominates at sufficiently low nu. Moreover, the "A" resonance is found to show dispersion with respect to the size of the transmission line, indicating that the "A" phase has a large correlation length. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.     

Finite size effect on the Raman frequency of phonons in nano-semiconductors

A comprehensive study on Raman spectroscopy with different excitation wavelengths, sample sizes, and sample shapes for optic phonons (OPs) and acoustic phonons (APs) in polar and non-polar nano-semiconductors has been performed. The study affirms that the finite size effect does not appear in the OPs of polar nano-semiconductors, while it exists in all other types of phonons. The absence of the FSE is confirmed to originate from the long-range Fr¨ohlich interaction and the breaking of translation symmetry. The result indicates that the Raman spectra of OPs cannot be used as a method to characterize the scale and crystalline property of polar nano-semiconductors.     

Repulsive effect of low frequency phonons on superconductivity

The Eliashberg integral equations are investigated to determine the effect of low frequency phonons on the superconducting transition temperature T_c. It is found that phonons of frequency less than T_c are repulsive (diminish T_c) while phonons of higher frequency have a diminished attraction unless the frequency is substantially above T_c. Various implications are discussed concerning observed values of T_c and predicted mechanisms for raising T_c.     

New phases in the phase transition in amorphous InSb films

In this paper the structures in the process of the phase transition in amorphous InSb film condensed at low temperature are studied by X-ray diffraction experiments. The highest T_c = 4.18 K is as a result of hexagonal phase. The results of the conductance and the critical temperature are explained by means of the experiments of X-ray diffraction in the process of phase transition.