1,721,193 research outputs found
RELATIONSHIP BETWEEN A NONLINEAR RESPONSE AND RELAXATION INDUCED BY COLORED NOISE
No full textThe nonlinear response to external disturbances of a system of interest A (the ''atom'') in contact with a thermal bath is studied. The central issue is the relationship between the nonlinear susceptibility and the equilibrium relaxation spectra of observables of A. Paralleling the results of the linear-response theory, the nth-order nonlinear susceptibility is factorized by quantum-mechanical arguments into two terms pertaining to the disturbances and the spin-bath system, respectively. The role of slow atom states, i.e., states relaxing on times T longer than the bath correlation time tau(c), is pointed out, by recurring to recent findings on their general features [Phys. Rev, A 46, 6222 (1992)]. In the frame of a stochastic picture of the bath and a multilevel scheme of the system A, the conditions under which the nonlinear susceptibility provides the spectrum of the slow atom states are clarified. Illustrations are drawn from magnetic resonance to demonstrate that techniques based on the nonlinear response of a spin system to multiple continuous waves compete favorably with pulsed techniques to provide information on longitudinal spin relaxation
Comment on "Hyperquenched Glassy Water and Hyperquenched Glassy Ethanol Probed by Single Molecule Spectroscopy"
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Excluded-volume corrections to the single-chain static properties of a polymer melt: Temperature, density and potential effects
No full textExcluded-volume effects on single-chain statics are introduced by analytic corrections to the Rouse results. The final expressions do not depend on free parameters. They are compared with numerical simulations of a polymer melt for different values of the temperature, the density and the interaction potential. Density and interactions control the energy landscape of the system, whereas the temperature selects the accessible regions. The agreement between the theory and the Rouse modes does not depend markedly on the temperature with some improvement for the first modes (large length scales). Differently, increasing the packing and the stiffness of the monomer-monomer interaction reduces the deviations for the first modes, but it leaves the magnitude of the deviations for the high-indexes modes (short length scales) nearly unaffected or with some tendency to increase. The scaling properties of the corrections are briefly discussed. (C) 2007 Elsevier B.V. All rights reserved
Universal scaling between structural relaxation and caged dynamics in glass-forming systems: Free volume and time scales
No full textIt is shown that the Debye-Waller factor (DW), a measure of the cage dynamics, is contributed by free-volume in o-terphenyl (OTP) and glycerol. An elementary ansatz provides an alternative way to get the reduced OW from Positron Annihilation Lifetime Spectroscopy (PALS). The ansatz supports the scaling of the slow relaxation with the fast caged dynamics over about ten decades in relaxation times in OTP and glycerol. Both PALS and neutron scattering experiments show that, in order to evidence the scaling, the observation times must be shorter than the time scales of the relaxation processes. (C) 2010 Elsevier B.V. All rights reserved
Features of the structural arrest in a glass-forming polymer melt
No full textThe segmental motion and the reorientation of a model polymeric glassformer is investigated by molecular-dynamics simulations under isobaric conditions. At lower temperatures and short times cage effects are apparent. At longer times the segmental anomalous diffusion and the reorientation of the monomers are evidenced. (C) 2002 Elsevier Science B.V. All rights reserved
Anisotropy of the monomer random walk in a polymer melt: Local-order and connectivity effects
Full text linkThe random walk of a bonded monomer in a polymer melt is anisotropic due to local order and bond connectivity. We investigate both effects by molecular-dynamics simulations on melts of fully-flexible linear chains ranging from dimers (M = 2) up to entangled polymers (M = 200). The corresponding atomic liquid is also considered a reference system. To disentangle the influence of the local geometry and the bond arrangements, and to reveal their interplay, we define suitable measures of the anisotropy emphasising either the former or the latter aspect. Connectivity anisotropy, as measured by the correlation between the initial bond orientation and the direction of the subsequent monomer displacement, shows a slight enhancement due to the local order at times shorter than the structural relaxation time. At intermediate times - when the monomer displacement is comparable to the bond length - a pronounced peak and then decays slowly as t -1/2, becoming negligible when the displacement is as large as about five bond lengths, i.e. about four monomer diameters or three Kuhn lengths. Local-geometry anisotropy, as measured by the correlation between the initial orientation of a characteristic axis of the Voronoi cell and the subsequent monomer dynamics, is affected at shorter times than the structural relaxation time by the cage shape with antagonistic disturbance by the connectivity. Differently, at longer times, the connectivity favours the persistence of the local-geometry anisotropy, which vanishes when the monomer displacement exceeds the bond length. Our results strongly suggest that the sole consideration of the local order is not enough to understand the microscopic origin of the rattling amplitude of the trapped monomer in the cage of the neighbours
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