Static and dynamical properties of a supercooled liquid confined in a pore

We present the results of a Molecular Dynamics computer simulation of a binary Lennard-Jones liquid confined in a narrow pore. The surface of the pore has an amorphous structure similar to that of the confined liquid. We find that the static properties of the liquid are not affected by the confinement, while the dynamics changes dramatically. By investigating the time and temperature dependence of the intermediate scattering function we show that the dynamics of the particles close to the center of the tube is similar to the one in the bulk, whereas the characteristic relaxation time tau_q(T,rho) of the intermediate scattering function at wavevector q and distance rho from the axis of the pore increases continuously when approaching the wall, leading to an apparent divergence in the vicinity of the wall. This effect is seen for intermediate temperatures down to temperatures close to the glass transition. The rho-dependence of tau_q(T,rho) can be described by an empirical law of the form tau_q(T,\rho)=f_q(T) exp [Delta_q/(rho_p-rho)], where Delta_q and \rho_q are constants, and f_q(T) is the only parameter which shows a significant temperature dependence.Comment: 4 pages of Latex, 4 figures Pari

The relaxation dynamics of a supercooled liquid confined by rough walls

We present the results of molecular dynamics computer simulations of a binary Lennard-Jones liquid confined between two parallel rough walls. These walls are realized by frozen amorphous configurations of the same liquid and therefore the structural properties of the confined fluid are identical to the ones of the bulk system. Hence this setup allows us to study how the relaxation dynamics is affected by the pure effect of confinement, i.e. if structural changes are completely avoided. We find that the local relaxation dynamics is a strong function of z, the distance of the particles from the wall, and that close to the surface the typical relaxation times are orders of magnitude larger than the ones in the bulk. Because of the cooperative nature of the particle dynamics, the slow dynamics also affects the dynamics of the particles for large values of z. Using various empirical laws, we are able to parameterize accurately the z-dependence of the generalized incoherent intermediate scattering function F_s(q,z,t) and also the spatial dependence of structural relaxation times. These laws allow us to determine various dynamical length scales and we find that their temperature dependence is compatible with an Arrhenius law. Furthermore, we find that at low temperatures time and space dependent correlation function fulfill a generalized factorization property similar to the one predicted by mode-coupling theory for bulk systems. For thin films and/or at sufficiently low temperatures, we find that the relaxation dynamics is influenced by the two walls in a strongly non-linear way in that the slowing down is much stronger than the one expected from the presence of only one confining wall. ....Comment: 22 pages of Late

Frequency Dependent Specific Heat of Amorphous Silica: A Molecular Dynamics Computer Simulation

We use molecular dynamics computer simulations to calculate the frequency dependence of the specific heat of a SiO_2 melt. The ions interact with the BKS potential and the simulations are done in the NVE ensemble. We find that the frequency dependence of the specific heat shows qualitatively the same behavior as the one of structural quantities, in that at high frequencies a microscopic peak is observed and at low frequencies an alpha-peak, the location of which quickly moves to lower frequencies when the temperature is decreased.Comment: 6 pages of Latex, 2 figures, uses aipproc.sty; to appear in proceedings of "Neutrons and Numerical Methods" Grenoble, Dec. 1998, Ed. H.G. Buttner et a

Elastic constants from microscopic strain fluctuations

Fluctuations of the instantaneous local Lagrangian strain $\epsilon_{ij}(\bf{r},t)$, measured with respect to a static ``reference'' lattice, are used to obtain accurate estimates of the elastic constants of model solids from atomistic computer simulations. The measured strains are systematically coarse- grained by averaging them within subsystems (of size $L_b$) of a system (of total size $L$) in the canonical ensemble. Using a simple finite size scaling theory we predict the behaviour of the fluctuations $$ as a function of $L_b/L$ and extract elastic constants of the system {\em in the thermodynamic limit} at nonzero temperature. Our method is simple to implement, efficient and general enough to be able to handle a wide class of model systems including those with singular potentials without any essential modification. We illustrate the technique by computing isothermal elastic constants of the ``soft'' and the hard disk triangular solids in two dimensions from molecular dynamics and Monte Carlo simulations. We compare our results with those from earlier simulations and density functional theory.Comment: 24 pages REVTEX, 10 .ps figures, version accepted for publication in Physical Review

Growing length scales in a supercooled liquid close to an interface

We present the results of molecular dynamics computer simulations of a simple glass former close to an interface between the liquid and the frozen amorphous phase of the same material. By investigating F_s(q,z,t), the incoherent intermediate scattering function for particles that have a distance z from the wall, we show that the relaxation dynamics of the particles close to the wall is much slower than the one for particles far away from the wall. For small z the typical relaxation time for F_s(q,z,t) increases like exp(Delta/(z-z_p)), where Delta and z_p are constants. We use the location of the crossover from this law to the bulk behavior to define a first length scale tilde{z}. A different length scale is defined by considering the Ansatz F_s(q,z,t) = F_s^{bulk}(q,t) +a(t) exp[-(z/xi(t))^{beta(t)}], where a(t), xi(t), and beta(t) are fit parameters. We show that this Ansatz gives a very good description of the data for all times and all values of z. The length xi(t) increases for short and intermediate times and decreases again on the time scale of the alpha-relaxation of the system. The maximum value of xi(t) can thus be defined as a new length scale xi_max. We find that tilde{z} as well as xi_max increase with decreasing temperature. The temperature dependence of this increase is compatible with a divergence of the length scale at the Kauzmann temperature of the bulk system.Comment: 9 pages of Latex, 4 figure