Dynamical properties of solutions of LiNO3 in (1,3)-diaminopropane in the liquid and glassy states

Quasielastic neutron scattering measurements have been made for concentrated solutions of LiNO₃ in (1,3)-diaminopropane in the liquid and glassy states. The results indicate that a harmonic lattice vibration-like motion of the atoms in solid glasses changes around the glass transition temperature to an intermediate one prior to a translational diffusive motion at higher temperatures. This picture is consistent with the previous results obtaiend from NMR relaxation mearsuremts.     

The two-step scenario of the protein dynamical transition

The “protein dynamical transition” (PDT) characterizes the abrupt loss of structural flexibility at a particular temperature and time scale in response to the glass transition of protein hydration water. The water-coupled structural degrees of freedom interact with the protein via hydrogen bonds, causing fluctuations, which can be probed by dynamic neutron scattering experiments. To emphasize the properties of hydration water a perdeuterated protein C-PC hydrated with H₂O is investigated together with native myoglobin. The respective intermediate scattering function of hydration water displays a two-step decay involving fast local re-orientational fluctuations and a slow collective relaxation. The anharmonic onset in the mean squared displacements, which is generally used to identify the PDT, is derived from the properties of the intermediate scattering function at the time given by the resolution of the spectrometer. It is shown that the onset temperature depends on the shape of the relaxation time spectrum. A shape-independent transition temperature T_Δ is defined, associated with the main structural relaxation, which decreases with increasing resolution. A second onset is identified near the glass temperature T_g, which is related to the initial decay of the intermediate scattering function. This onset is independent of the instrumental resolution and causes a change in molecular elasticity and thermal expansion. With this approach a more precise definition of the PDT is given, providing answers to the critical questions about the nature and the mechanism of the effect.     

Water-network percolation transition in hydrated blue-green algae in vivo

We found that dc conductivity percolation process typical for low hydrated porous materials shows up in bulk viable blue-green algae, Arthrospira (Spirulina) platensis (strain Laporte 1963/M-132/2b) at unusually low hydrations, more than an order of magnitude lower than in, e.g., hydrated yeast [D. Sokolowska, A. Krol-Otwinowska, J.K. Moscicki, Phys. Rev. E 70 (2004) 052901]. The critical exponent is characteristic for two-dimensional network. Comparison with results for yeast and other similar materials shows that the hydration percolation threshold is a sensitive indicator of wettability of water accessible surface in porous bio- and abiotic materials.     

Quasi-elastic neutron scattering of cyanobiphenyl compounds with different terminal chains

Quasi-elastic neutron scattering measurements were performed for five cyanobiphenyl compounds with different terminal chains (5CB, 5*CB, 8OCB, 8*OCB and 6O2OCB) in their liquid and liquid-crystalline phases to investigate the molecular dynamics. The spectra were successfully decomposed into two components, indicating that two types of molecular motions are involved. For the broad (or fast) component, the temperature dependence as well as the momentum transfer dependence did not differ very much among those compounds. For the narrow (or slow) component, on the other hand, the compounds with a branched alkyl-chain (5*CB and 8*OCB) gave significantly smaller diffusion constants than the others. A molecular dynamics simulation for 5*CB and 5CB suggests that the reorientational motion of the whole molecule around the long molecular axis is responsible for the broad component, whereas a diffusive motion of the terminal chain is responsible for the narrow one.     

Analyse de la Raie Zéro-Phonon et des Processus de Relaxation Excitoniques dans le Spectre d'Absorption Singulet-Triplet du Cristal de 4-4′-Dichlorobenzophenone

Abstract

In this paper we discuss the dynamic scattering information which can be obtained from an analysis, at high resolution, of the triplet exciton absorption band in the molecular crystal 4-4′-dichlorobenzophenone. The zero phonon lineshapes, widths and positions are investigated as a function of temperature. At temperatures below 30 K the broadening and the shift are analyzed in terms of exchange theory involving a torsional mode of the molecule. The lifetime in the phonon promoted state is 0.75 ps over the range 5 K-30 K. The difference between the excited and ground state energies of the torsional mode is δω = -8.45 cm^?1. Above 30 K the change on linewidth is discussed in a model which implies a momentarily localization of the exciton by lattice phonons. During the localization time a weak coupling between the trapped excitation and phonons is assumed. At very low temperatures the lineshape is asymmetric, gaussian on the high energy side, lorentzian on the low energy side. This experimental result may be due to the fact that the k = 0 level is at the bottom of the exciton band.     

Electrons,dislocations, and low-temperature plastic deformation

The influence of electron drag processes on the plasticity of normal state metals is demonstrated in the case of zinc crystals. These results show that dislocations move as underdamped oscillators at low temperature and that dislocations flutters as they move. This flutter motion, in contrast to kink motion which is expected to have no long range stress, is the dominant drag process at temperatures below ≈ 4.2 K.     

The effect of changes in relative humidity on the hydration rate of Pachuca obsidian

The effect of relative humidity on the hydration rate of obsidian and other glasses has been debated since the early work of [I. Friedman, R. Smith, Am. Antiquity 25 (1960) 476]. While more recent work has been in general agreement that a relative humidity dependence does exist, hydration profiles as a function of relative humidity have not been obtained. In this paper we present the results of a study in which samples of Pachuca obsidian were hydrated for approximately 5 days at 150 °C at relative humidities ranging from 21% to 100%, and the resultant profiles were measured by secondary ion mass spectrometry (SIMS). The results suggest that the hydration rate is, indeed, a function of relative humidity, but for the relative humidity levels commonly observed in most soils the effects on hydration dating are expected to be relatively small. In addition, analysis of the surface values as sorption isotherms and comparisons with nitrogen sorption isotherms suggests that water is relatively strongly bound to the obsidian surface. By assuming a situation in which the ‘surface’ refers to active centers within the glass we have shown that an adsorption model provides a useful approach to modeling the diffusive process.     

Formation and stability of amorphous molecular systems: As the prototype of functional amorphous organic materials

Structural changes in vapor-deposited amorphous states of simple organic compounds were studied by Raman scattering, X-ray diffraction, and light interference in the sample. Two types of processes, namely direct crystallization and structural relaxation leading to glass transition, were observed depending on the flexibility of the molecule. Gradual relaxations were also found to occur in amorphous states of butyronitrile and 1,2-dichloroethane at temperatures much lower than their T_g or crystallization temperature. The observed structural changes are discussed by referring to the high enthalpy of the vapor-deposited amorphous systems.     

High Electric Field Transport In (TMTSF)2 Pf6 [Bis-Tetramethyltetraselenafulvalene Hexafluorophosphate] at Low Temperatures

We obtain electron and hole mobilities from the measured Hall mobility of (TMTSF)₂ PF₆. At 4K these are greater than 10⁶cm²/Vsec and agree with the mobility calculated for acoustic phonon scattering. The initial increase of conductivity in dc fields found at low temperatures in this material can be accounted for by single particle effects, specifically heating of the carriers by the field. To explain further increases we suggest that the gap is spatially nonuniform. This could be accounted for by dislocations.     

The growth and physics of ultra-high-mobility two-dimensional hole gas on (311)A GaAs surface

We report on the molecular beam epitaxy growth of modulation-doped GaAs-(Ga,Al)As heterostructures on the (311)A GaAs surface using silicon as the acceptor. Two-dimensional hole gases (2DHGs) with low-temperature hole mobility exceeding 1.2×10⁶ cm² V⁻¹ s⁻¹ with carrier concentrations as low as 0.8×10¹¹ cm⁻² have been obtained. This hole mobility is the highest ever observed at such low densities by any growth technique. We also report the first observation of persistent photoconductivity in a 2DHG. An analysis of the number density and temperature dependence of the mobility leads us to conclude that the mobility is limited by phonon scattering above 4 K and interface scattering at lower temperatures.     

Specific heat of ethanol at low temperatures

We present here new specific heat measurements at low temperatures (2–20 K) of the different phases of ethanol, characterized by the same calorimetric set-up at higher temperatures. We have extended and improved earlier measurements by implementing higher-accuracy calorimetric methods at low temperatures (using two complementary versions of the thermal relaxation method), as well as at higher temperatures (using a quasi-adiabatic, continuous method). The quantitatively very similar low temperature properties and glass-transition features of both structural glass and orientationally-disordered crystal of ethanol provide clear evidence that the lack of long-range crystalline order typical of amorphous solids is an unimportant factor regarding the universal properties of glasses. We have also employed these new measuring methods to study the possible effect of water impurities on the specific heat of the different solid phases of ethanol, and to study possible variations in the specific heat between different found phases of the monoclinic crystal of ethanol.     

Water diffusion in silicate glasses under natural weathering conditions: evidence from buried medieval stained glasses

In this study, ion-microprobe analyses of four samples of buried medieval stained-glasses are used to demonstrate that water penetrates into the matrix of pristine glasses at low temperatures, thereby showing that glass alteration is not only a surface process. The diffusion coefficients of water determined from concentration profiles of hydrogen are found to be correlated with the bulk polymerization state of the glass. This observation is discussed with respect to glass structure and implies that ionic exchange between hydrogen and network modifying metal cations is the major process responsible for glass hydration.     

Identifying nucleation temperatures for lysozyme via differential scanning calorimetry

Production of diffraction-quality protein crystals mandates nucleation of a moderate number of crystals. Excessive nucleation can lead to many small crystals of poor morphology, making them useless for structural determination via X-ray crystallography. In this work, differential scanning calorimetry (DSC) and light scattering are utilized to delineate the liquid–liquid binodal in lysozyme solutions at concentrations typically used for crystal growth (10–50 mg/ml). Such cloud point temperatures are determined over a range of protein concentrations and compared to visually observed nucleation temperatures. In all cases, nucleation occurs at temperatures above the cloud point temperature. Thus, measurement of the cloud point temperature gives the crystal grower a bound on the protein crystal growth problem. Nucleation is reproducibly induced within 2°C of the cloud point temperature; while prior work indicates that growth, when carried out near the saturation temperature, yields high quality crystals. The ease of measuring the cloud point via turbidity measurements allows for the rational selection of an initial temperature to induce protein nucleation.     

Carrier Scattering Mechanisms in GaS0.5Se0.5 Layered Crystals

Systematic dark electrical resistivity and Hall mobility measurements have been carried out in the temperature range 150‐400 K on n‐type GaS_0.5Se_0.5 layered crystals. The analysis of temperature dependent electrical resistivity and carrier concentration reveals the extrinsic type of conduction with a donor impurity level located at 0.44 eV, donor and acceptor concentrations of 3.4 ×10¹⁷ and 4.1×10¹⁶ cm^‐3, respectively, and an electron effective mass of 0.41 m₀. The Hall mobility is limited by the electron‐phonon short‐range interactions scattering at high temperatures combined with the ionized impurity scattering at low temperatures. The electron‐phonon short‐range interactions scattering mobility analysis reveals an electron‐phonon coupling constant of 0.25 and conduction band deformation potential of 5.57 eV/Å.     

Sol–gel simulation—I: Scattering response

Scattering of sol–gel structures is investigated computationally. Sol-gels are recreated through an aggregation algorithm incorporating Brownian motion and chemical reactions. Using the fractal character of sol–gels, the concept of recursion is introduced as a tool to perform multi scale computation of the response of sol–gels through the different scales from the molecular level to the macro scale. The concept is illustrated with the prediction of scattering intensity. The relationship between scattering intensity and functionality is investigated, noting that the latter is a function of the Brownian motion and chemical reactivity. Computational simulation tools are developed to predict scattering intensity as a function of density and reactivity, the former represented by the number of particles, or clusters, in the simulation box. Then, the results are correlated to an analytical model that reveals the critical wave number, or critical scale, at which percolation occurs.     

Change of bonding system in liquid SexTe1−1 alloys as shown by density measurements

Density measurements have been performed on melts in the binary chalcogenide system Se_xTe_1?x up to a temperature of 950°C. Anomalous behaviour of the density was observed. At higher temperatures the density increased on raising the temperature up to a maximum whose position shifts to lower temperatures with increasing concentration of tellurium. This anomalous density behaviour clearly shows an increase of the average coordination number of atoms in the melt. This increase of CN is caused by a change of the bonding system because of the tendency of Te to form pσ bondings. It was assumed that there exist two distinct structures in thermodynamical equilibrium at each temperature according to a law of mass action, for which cooperation effects have to be taken in account. At low temperatures Te atoms easily accept a loosely packed structure I with the aid of selenium. At high temperatures even Se atoms develop a more densely packed structure II with the aid of Te.     

Physical vapor transport growth of mercurous chloride crystals

We have carried out experiments to derive a quantitative understanding of the physical vapor transport (PVT) process and to identify convective effects on the crystal growth process. The experimental growth velocity was several orders of magnitude lower than the theoretically predicted value. The effusion holes were used to disturb the impurity boundary layers. We observed a change of 18% (not an order of magnitude) in growth velocities. The Arrhenius behavior of growth rate with temperature was used to derive the sticking coefficient. Experimental result on growth velocity as the aspect ratio was varied showed that with increasing convection, the growth rate increased up to a certain value and then dropped to a constant value. This indicated that a bifurcation had occured with a resulting change in transport behavior.

Growth velocity measurements for the PVT process as a function of orientation of the g-vector were also made. The experimental results clearly showed that the growth velocity varied with g-vector for a particular temperature profile. The effects of convection on crystal quality were studied by varying the thermal conditions (source and crystal temperatures) which affects thermal convection during PVT. The results showed that crystals grown at low Rayleight numbers had better homogeneity. While no microgravity experiments were conducted, computation of mass flux for the horizontal orientation for various gravitational levels showed two distinct regions; above 10^-3 g where the flow was convective and strong circulating cells appeared, and also below 10^-3 g, where the flow was purely diffusive and no circulating cells were predicted. Therefore it is postulated that for the conditions of growth considered, space flight experiments with acceleration less than 10^-3 g could yield crystals grown under diffusive transport.     

Microplasticity of CsI crystals

The study of the mechanisms of plastic deformation of CsI crystals has found the participation of not only the main {110} 〈100〉 slip system but also of the secondary one {110} 〈110〉. Besides that, production and motion of point defects (or small prismatic loops) take place. The gliding on secondary slip system and the deformation accounted for by the generation and motion of point defects is facilitated at low temperatures and high deformation rates. The character of the motion and multiplication of dislocations in the main slip system is investigated. From the temperature and stress dependence of the mobility of isolated dislocations quantitative data on the thermally activated motion of edge dislocations on the main slip system have been obtained. It is shown that, as in the case of other alkali halides, the thermally activated motion of edge dislocations in CsI crystals on {110} 〈100〉 system is limited by their interaction with local obstacles.     

Dynamics of Intercalated Water Molecules in Synthetic Layered Silicates

Abstract

Diffusional motions of water molecules in a synthetic fluorinated layered silicate have been studied by neutron scattering. Motions in a one-water-layer hydration state are slower than in those in a two-water-layer hydration state. Good fits to the data were obtained with the Kohlrausch-Williams-Watt lineshape that is often applied to disordered systems.     

Molecular dynamics of ion exchangers SG-1M and SGK-7 determined from neutron spectroscopy data

The inelastic neutron scattering spectra of samples of ion exchangers SG-1M and SGK-7, both “dry” and hydrated in heavy water, have been recorded in the temperature range 220–363 K on the time-of-flight spectrometers of the Kurchatov Institute and the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research. With an increase in the degree of hydration and sample temperature, the inelastic scattering spectra exhibit changes in the region of low-frequency vibrations, whereas the region of torsional vibrations of methyl groups remains relatively stable. Generalized frequency distributions are obtained for the samples studied. The effect of hydration and temperature on the noted microdynamic characteristics is discussed.