Reply to the Comment of Muessel and Rieger on ``Aging Effects in a Lennard-Jones Glass''

This is a Reply to the Comment of Muessel and Rieger cond-mat/9804063 to out paper on ``Aging Effects in a Lennerd-Jones Glass''. We show that the scaling function proposed by Muessel and Rieger does not lead to a satisfactory scaling of our aging data.Comment: 1 page of Revtex, 1 eps figur

How does the relaxation of a supercooled liquid depend on its microscopic dynamics?

Using molecular dynamics computer simulations we investigate how the relaxation dynamics of a simple supercooled liquid with Newtonian dynamics differs from the one with a stochastic dynamics. We find that, apart from the early beta-relaxation regime, the two dynamics give rise to the same relaxation behavior. The increase of the relaxation times of the system upon cooling, the details of the alpha-relaxation, as well as the wave vector dependence of the Edwards-Anderson-parameters are independent of the microscopic dynamics.Comment: 6 pages of Latex, 4 figure

A quantitative test of the mode-coupling theory of the ideal glass transition for a binary Lennard-Jones system

Using a molecular dynamics computer simulation we determine the temperature dependence of the partial structure factors for a binary Lennard-Jones system. These structure factors are used as input data to solve numerically the wave-vector dependent mode-coupling equations in the long time limit. Using the so determined solutions, we compare the predictions of mode-coupling theory (MCT) with the results of a previously done molecular dynamics computer simulation [Phys. Rev. E 51, 4626 (1995), ibid. 52, 4134 (1995)]. From this comparison we conclude that MCT gives a fair estimate of the critical coupling constant, a good estimate of the exponent parameter, predicts the wave-vector dependence of the various nonergodicity parameters very well, except for very large wave-vectors, and gives also a very good description of the space dependence of the various critical amplitudes. In an attempt to correct for some of the remaining discrepancies between the theory and the results of the simulation, we investigate two small (ad hoc) modifications of the theory. We find that one modification gives a worse agreement between theory and simulation, whereas the second one leads to an improved agreement.Comment: Figures available from W. Ko

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

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 $\beta$ that is independent of temperature. This dynamical behavior is in accordance with the mode-coupling theory of supercooled liquids.Comment: 6 pages, RevTex, three postscript figures available on request, MZ-Physics-10

Fluctuation-dissipation relation in a sheared fluid

In a fluid out of equilibrium, the fluctuation dissipation theorem (FDT) is usually violated. Using molecular dynamics simulations, we study in detail the relationship between correlation and response functions in a fluid driven into a stationary non-equilibrium state. Both the high temperature fluid state and the low temperature glassy state are investigated. In the glassy state, the violation of the FDT is quantitatively identical to the one observed previously in an aging system in the absence of external drive. In the fluid state, violations of the FDT appear only when the fluid is driven beyond the linear response regime, and are then similar to those observed in the glassy state. These results are consistent with the picture obtained earlier from theoretical studies of driven mean-field disordered models, confirming the similarity between these models and real glasses.Comment: 4 pages, latex, 3 ps figure

Shearing a Glassy Material: Numerical Tests of Nonequilibrium Mode-Coupling Approaches and Experimental Proposals

The predictions of a nonequilibrium schematic mode-coupling theory developed to describe the nonlinear rheology of soft glassy materials have been numerically challenged in a sheared binary Lennard-Jones mixture. The theory gives an excellent description of the stress/temperature `jamming phase diagram' of the system. In the present paper, we focus on the issue of an effective temperature Teff for the slow modes of the fluid, as defined from a generalized fluctuation-dissipation theorem. As predicted theoretically, many different observables are found to lead to the same value of Teff, suggesting several experimental procedures to measure Teff. New, simple experimental protocols to access Teff from a generalized equipartition theorem are also proposed, and one such experiment is numerically performed. These results give strong support to the thermodynamic interpretation of Teff and make it experimentally accessible in a very direct way.Comment: Version accepted for publication - Physical Review Letter

Relaxation in a glassy binary mixture: Comparison of the mode-coupling theory to a Brownian dynamics simulation

We solved the mode-coupling equations for the Kob-Andersen binary mixture using the structure factors calculated from Brownian dynamics simulations of the same system. We found, as was previously observed, that the mode-coupling temperature, Tc, inferred from simulations is about two times greater than that predicted by the theory. However, we find that many time dependent quantities agree reasonably well with the predictions of the mode-coupling theory if they are compared at the same reduced temperature epsilon = (T-Tc)/Tc, and if epsilon is not too small. Specifically, the simulation results for the incoherent intermediate scattering function, the mean square displacement, the relaxation time and the self-diffusion coefficient agree reasonably well with the predictions of the mode-coupling theory. We find that there are substantial differences for the non-Gaussian parameter. At small reduced temperatures the probabilities of the logarithm of single particle displacements demonstrate that there is hopping-like motion present in the simulations, and this motion is not predicted by the mode-coupling theory. The wave vector dependent relaxation time is shown to be qualitatively different than the predictions of the mode-coupling theory for temperatures where hopping-like motion is present.Comment: To be published in Physical Review