Computer simulations of a rough sphere fluid I

A molecular dynamics simulation of a rough sphere fluid with variable roughness is presented from which the intermediate scattering function, orientational correlation functions and velocity autocorrelation functions are calculated for several values of the density, roughness and wave number. The dependence of these correlation functions on the parameters is discussed and a comparison with a neutron scattering experiment on neopentane [C(CH₃)₄] is made, which gives good agreement between the two experiments.     

Channel formation and intermediate range order in sodium silicate melts and glasses

We use inelastic neutron scattering and molecular dynamics simulation to investigate the interplay between the structure and the fast sodium ion diffusion in various sodium silicates. With increasing temperature and decreasing density the structure factors exhibit an emerging prepeak around 0.9 A(-1). We show that this prepeak has its origin in the formation of sodium rich channels in the static structure. The channels serve as preferential ion conducting pathways in the relative immobile Si-O matrix. On cooling below the glass transition this intermediate range order is frozen in.     

Short range correlations and the EMC effect

This Letter shows quantitatively that the magnitude of the EMC effect measured in electron deep inelastic scattering at intermediate x(B), 0.35≤x(B)≤0.7, is linearly related to the short range correlation (SRC) scale factor obtained from electron inclusive scattering at x(B)≥1. The observed phenomenological relationship is used to extract the ratio of the deuteron to the free pn pair cross sections and F(2)(n)/F(2)(p), the ratio of the free neutron to free proton structure functions. We speculate that the observed correlation is because both the EMC effect and SRC are dominated by the high virtuality (high momentum) nucleons in the nucleus.     

Experimental evidence for fast heterogeneous collective structural relaxation in a supercooled liquid near the glass transition

We have extended the exploration of microscopic dynamics of supercooled liquids to small wave numbers Q corresponding to the scale of intermediate range order, by developing a new experimental approach for precise data correction for multiple scattering noise in inelastic coherent neutron scattering. Our results in supercooled Ca0.4K0.6(NO3)(1.4) reveal the first direct experimental evidence, after a decade of controversy, that the so-called picosecond process around the glass transition corresponds to a predicted first, faster stage of the structural relaxation. In addition, they show that this process takes the spatial form of fast heterogeneous collective flow of correlated groups of atoms.     

Model for single-particle dynamics in supercooled water

We analyze a set of 10 M-step molecular dynamics (MD) data of low-temperature SPC/E model water with a phenomenological analytical model. The motivation is twofold: to extract various k-dependent physical parameters associated with the single-particle or the self-intermediate scattering functions (SISFs) of water at a deeply supercooled temperature and to apply this analytical model to analyses of new high resolution quasielastic neutron scattering data presented elsewhere. The SISF of the center of mass computed from the MD data show clearly time-separated two-step relaxations with a well defined plateau in between. We model the short time relaxation of the test particle as a particle trapped in a harmonical potential well with the vibrational frequency distribution function having a two-peak structure known from previous inelastic neutron scattering experiments. For the long time part of the relaxation, we take the alpha relaxation suggested by mode-coupling theory. The model fits the low-temperature SISF over the entire time range from 1 fs to 10 ns, allowing us to extract peak positions of the vibrational density of states, the structural relaxation rate 1/tau of the cage (the potential well) and the stretch exponent beta. The structural relaxation rate has a power law dependence on the magnitude of the wave vector transfer k and the stretch exponent varies from 0.55 at large k to unity at small k.     

The coupling between translational and rotational motions

The coupling between molecular reorientation and translational displacement is studied in a fluid of rough spheres. The correlation functions frequently encountered in thermal neutron scattering and laser light scattering are computed using a binary collision approximation. These are compared with the corresponding uncoupled results. In the diffusion limit coupled and uncoupled diffusion coefficients are found. The Hubbard relation is generalized. The maximum deviation between the coupled and uncoupled results occur for wavenumbers commonly found in thermal neutron scattering. The ratio of uncoupled and coupled correlation times displays regions where translation-rotation coupling is clearly important. In these regions there are important differences in the computed coupled and uncoupled correlation functions.     

Scaling behavior in the dynamics of a supercooled Lennard-Jones mixture

Summary We present the results of a large-scale molecular-dynamics computer simulation of a binary, supercooled Lennard-Jones fluid. At low temperatures and intermediate times the time dependence of the intermediate scattering function is well described by a von Schweidler law. The von Schweidler exponent is independent of temperature and depends only weakly on the type of correlator. For long times the correlation functions show a Kohlrausch behavior with an exponent β that is independent of temperature. This dynamical behavior is in accordance with the mode-coupling theory of supercooled liquids. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Investigation of the orientational correlation of the molecules in liquid H2S with reverse Monte Carlo simulation

Reverse Monte Carlo simulation of liquid H₂S has been performed on the basis of an X-ray and a neutron diffraction experiment, and the resulting structure is analysed in detail. Partial pair correlation functions, spatial distribution of the molecules around each other and the preferred relative orientation of the nearest neighbours are investigated. The orientational correlation of the molecules is discussed in terms of the coefficients of the spherical harmonic expansion of the orientational pair correlation function. It is found that H₂S is not a closely packed liquid: the average first-shell coordination number of the molecules is about 8. This is found to be a direct consequence of the molecular shape, i.e., the excluded volume of the H atoms prevents the central molecule from being surrounded by more first-shell neighbours. The relative orientation of the molecules is found to be almost completely uncorrelated. Moreover, even the existing small correlation is confined to a rather small distance range and vanishes beyond about the third nearest neighbour.     

Smectic H-smectic VI pretransitional effect by orientational ordering in TBBA

New neutron quasi-elastic scattering data on TBBA in its supercooled smectic H phase are presented and analyzed in terms of the elastic incoherent structure factor. Contrary to what was found in the normal H phase, the results support the idea of partial orientational ordering of the molecules. It is suggested that this ordering (of dynamical nature) is a precursory effect to the smectic H-smectic VI phase transition.     

Hydrostatic pressure and temperature dependence study of d-camphor and dl-borneole by neutron scattering

Abstract

Neutron scattering investigations have been performed down to helium temperatures under different hydrostatic pressures up to 330 MPa. Neutron diffraction (ND) and incoherent inelastic neutron scattering (IINS) spectra showed clearly the existence of a phase transition in d-camphor, which was not observed in dl-borneole. The ND spectra point to a possible cubic structure of the dl-borneole crystal down to helium temperatures. Pressure and temperature dependence of the IINS spectra assumed a glass-like transition from the dynamical to static orientational disorder of molecules in this crystal.     

Quasi-elastic neutron scattering studies of the slow dynamics of supercooled and glassy aspirin

Aspirin, also known as acetylsalicylic acid (ASA), is not only a wonderful drug, but also a good glass former. Therefore, it serves as an important molecular system to study the near-arrest and arrested phenomena. In this paper, a high-resolution quasi-elastic neutron scattering (QENS) technique is used to investigate the slow dynamics of supercooled liquid and glassy aspirin from 410 down to 350 K. The measured QENS spectra can be analyzed with a stretched exponential model. We find that (i) the stretched exponent β(Q) is independent of the wavevector transfer Q in the measured Q range and (ii) the structural relaxation time τ(Q) follows a power-law dependence on Q. Consequently, the Q-independent structural relaxation time τ(0) can be extracted for each temperature to characterize the slow dynamics of aspirin. The temperature dependence of τ(0) can be fitted with the mode-coupling power law, the Vogel-Fulcher-Tammann equation and a universal equation for fragile glass forming liquids recently proposed by Tokuyama in the measured temperature range. The calculated dynamic response function χ(T)(Q, t) using the experimentally determined self-intermediate scattering function of the hydrogen atoms of aspirin shows direct evidence of the enhanced dynamic fluctuations as the aspirin is increasingly supercooled, in agreement with the fixed-time mean squared displacement ?x(2)? and the non-Gaussian parameter α(2) extracted from the elastic scattering.     

Dispersion relation of low-energy excitations in Zr<Subscript>50</Subscript>Be<Subscript>50</Subscript> metallic glass

A high-resolution (1.5 meV) inelastic neutron scattering experiment is carried out to investigate in detail the low momentum (0.6–1.5 Å^?1) dynamic response in Zr₅₀Be₅₀ metallic glass. The results are obtained on the hot neutron three-axis spectrometer IN1 in the Laue-Langevin Institute (Grenoble, France). A comparison with recent neutron scattering results for the momentum transfer region above 1.4 Å^?1 shows a minimum in the dispersion relation of low energy excitations near the prepeak of the static structure factor. These excitations appear to be due to the short-range order in this amorphous system.     

Molecular motion in supercooled glycerol

Quasi-elastic Rayleigh scattering of 14·4 keV photons has been measured on supercooled liquid glycerol at -30°C and 0°C by employing the Mössbauer effect. Total scattered intensity, quasi-elastically scattered intensity I _q and energy width of I _q(k, ω) have been determined for k=0·6 to 4·2 Å^-1. The molecular motion is modelled as: random-walk diffusional motions for the centre-of-mass translation and for the orientation of independent rigid molecules, plus fast-bounded translational jitter (vibration). The model parameters are evaluated. The temperature dependence of the translational diffusion constant corresponds to an activation energy of 12 kcal/mol. Comparison is made especially with N.M.R. results for rotational motion. The effect of orientational jitter (libration) is considered and its possible influence on nuclear magnetic relaxation is pointed out.     

Atomic dynamics in liquids with competing interactions

The influence of the type of atomic interaction on the atomic dynamics is studied for liquid Na(x)Sn(1-x) (x = 0.9, 0.77, 0.57, 0.5, 0.33) alloys by cold neutron inelastic scattering. The dispersions obtained from the longitudinal current correlation function J(l)(Q,omega) show clear evidence for the dependence of the dynamics on the type of interaction (metallic, ionic, partly covalent) tuned by changing the composition of the alloy. For the first time, a second dispersion branch is observed in the total J(l)(Q,omega) around Q(p), the position of the principal peak of S(Q), for the Sn-rich compositions. The dynamic properties are discussed and compared to results of recent ab initio molecular dynamics simulations.     

Ionic self-diffusion in concentrated aqueous electrolyte solutions

A self-consistent microscopic theory is developed to understand the anomalously weak concentration dependence of ionic self-diffusion coefficient D(ion) in electrolyte solutions. The self-consistent equations are solved by using the mean spherical approximation expressions of the static pair correlation functions for unequal sizes. The results are in excellent agreement both with the known experimental results for many binary electrolytes and also with the new Brownian dynamics simulation results. The calculated velocity time correlation functions also show quantitative agreement with simulations. The theory also explains the reason for observing different D(ion) in recent NMR and neutron scattering experiments.     

Universal power law in the orientational relaxation in thermotropic liquid crystals

We observe a surprisingly general power law decay at short to intermediate times in orientational relaxation in a variety of model systems (both calamitic and discotic, and also lattice) for thermotropic liquid crystals. As all these systems transit across the isotropic-nematic phase boundary, two power law relaxation regimes, separated by a plateau, emerge, giving rise to a steplike feature (well known in glassy liquids) in the single-particle second-rank orientational time correlation function. In contrast to its probable dynamical origin in supercooled liquids, we show that the power law here can originate from the thermodynamic fluctuations of the orientational order parameter, driven by the rapid growth in the second-rank orientational correlation length.     

Experimental observation of the alpha relaxation in supercooled water

Intermediate scattering functions for density fluctuation in D2O contained in pores of a Vycor glass have been measured using an improved neutron spin-echo spectrometer at two supercooled temperatures. The measurements cover the time range from 1 to 2300 ps with the Q range spanning the first diffraction peak of water. The time correlation functions can be fitted to a stretched exponential relaxation function with a Q-dependent amplitude. Both the stretch exponent and the relaxation time peak approximately at the Q value corresponding to the first diffraction peak, confirming the validity of the mode coupling idea in supercooled water.     

Spin-state transition in LaCoO3: direct neutron spectroscopic evidence of excited magnetic states

A gradual spin-state transition occurs in LaCoO3 around T approximately 80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at approximately 0.6 meV appears, whose intensity increases with temperature, following the bulk magnetization. Within a model including crystal-field interaction and spin-orbit coupling, we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (t(2g)(5)e(g)(1), S=1) versus high-spin (t(2g)(4)e(g)(2), S=2) states. Since the g factor obtained from the field dependence of the inelastic neutron scattering is g approximately 3, the second interpretation is definitely favored.     

Chain dynamics in a hexadecane melt as seen by neutron scattering and identified by molecular dynamics simulations

Different local and global chain dynamics in a C(16)H(34) melt could be revealed by resolution resolved time-of-flight quasielastic neutron scattering and complementary molecular dynamics simulations. Thereby it has been demonstrated that the measured intermediate scattering functions can validate the simulated data on the pico- to nanosecond timescale. Remarkably the shape of the experimentally measured intermediate scattering functions can be reproduced excellently by molecular dynamics simulations. It was found that although the extracted apparent activation energy corresponds to the long-range diffusion value, the molecular dynamics in this time range are mainly due to local bond rotations and the rotation of entire molecules.     

On the origin of the heterogeneous orientation dynamics of semiflexible polymers in solutions

Collective rotational motion in nondilute isotropic solutions of semirigid chains is sensitively probed by depolarized light scattering over a broad time range. The bimodal shape and the peculiar dependence of the orientational relaxation function on the scattering angle might arise from the coupling between orientational and shear modes of molecular motion (P.G. de Gennes, Mol. Cryst. Liq. Cryst. 12, 193 (1971)). The dynamic heterogeneity, i.e. the separation of the time scales and the shape of the relaxation functions appears to be system specific.