1,721,062 research outputs found
Universal relation between viscous flow and fast dynamics in glass-forming materials
No full textThe connection between viscous flow and vibrational properties in glass-forming materials is scrutinized examining the fragility of a wide set of liquids and the nonergodicity factor of the corresponding glasses. Building on the same line of reasoning which allows us to extend the connection between viscosity and thermodynamics in complex systems, we show here how the two quantities are strongly correlated once the effect of those secondary relaxation processes due to internal degrees of freedom is correctly accounted for. This result provides a missing thermodynamic rationale for the recently debated universality of the correlation between fast and slow degrees of freedom
Theory of vibrational anomalies in glasses
No full textThe theory of elasticity with spatially fluctuating elastic constants (heterogeneous-elasticity theory) is reviewed.
It is shown that the vibrational anomalies associated with the boson peak can be qualitatively and quantitatively explained in terms of this theory. Two versions of a mean-field theory for solving the stochastic equation of motion are presented: the coherent-potential approximation (CPA) and the self-consistent Born approximation (SCBA). It is shown that the latter is included in the former in the Gaussian and weak-disorder limit. We are able to discuss and explain cases in which the change of the vibrational spectrum by varying an external parameter can be accounted for by changing the Debye frequency (elastic transformation) and cases in which this is not possible. In the latter case a change in the distribution of the elastic moduli has occurre
Microscopic dynamics in liquid metals: the experimental point of view.
No full textThe experimental results relevant for the understanding of the microscopic dynamics in liquid metals are reviewed, with special regard to the ones achieved in the last two decades. Inelastic neutron scattering played a major role since the development of neutron facilities in the 1960s. The last ten years, however, saw the development of third generation radiation sources, which opened the possibility of performing inelastic scattering with x rays, thus disclosing previously unaccessible energy-momentum regions. The purely coherent response of x rays, moreover, combined with the mixed coherent or incoherent response typical of neutron scattering, provides enormous potentialities to disentangle aspects related to the collectivity of motion from the single-particle dynamics. If the last 20 years saw major experimental developments, on the theoretical side fresh ideas came up to the side of the most traditional and established theories. Beside the raw experimental results therefore models and theoretical approaches are reviewed for the description of microscopic dynamics over different length scales, from the hydrodynamic region down to the single-particle regime, walking the perilous and sometimes uncharted path of the generalized hydrodynamics extension. Approaches peculiar of conductive systems, based on the ionic plasma theory, are also considered, as well as kinetic and mode coupling theory applied to hard-sphere systems, which turn out to mimic with remarkable detail the atomic dynamics of liquid metals. Finally, cutting edge issues and open problems, such as the ultimate origin of the anomalous acoustic dispersion or the relevance of transport properties of a conductive system in ruling the ionic dynamic structure factor, are discussed
comment on "Collective dynamics in liquid lithium, sodium and aluminum" by S. Singh and K. Tankeshwar.
No full textLandau-Placzek ratio for heat density dynamics and its application to heat capacity of liquids
No full textExact relation for contributions to heat capacity of liquids is obtained from hydrodynamic theory. It is shown from analysis of the long-wavelength limit of heat density autocorrelation functions that the heat capacity of simple liquids is represented as a sum of two contributions due to "phonon-like" collective excitations and heat relaxation. The ratio of both contributions being the analogy of Landau-Placzek ratio for heat processes depends on the specific heats ratio. The theory of heat density autocorrelation functions in liquids is verified by computer simulations. Molecular dynamics simulations for six liquids having the ratio of specific heats. in the range 1.1-2.3, were used for evaluation of the heat density autocorrelation functions and predicted Landau-Placzek ratio for heat processes. The dependence of contributions from collective excitations and heat relaxation process to specific heat on. is shown to be in excellent agreement with the theory. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4774406
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