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Shear modulus of simulated glass-forming model systems: Effects of boundary condition, temperature, and sampling time
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Image of FIG. 1.
FIG. 1.

Setup, notations, and ensembles investigated numerically: (a) Schematic experimental setup for probing the shear modulus G. (b) Expected shear modulus G(t) as a function of measurement time t at TT g (top curve), T slightly below T g (middle curve) and in the liquid limit for TT g (bottom curve). (c) We use periodic simulation boxes with deformable box shapes where we impose τ = 0. The shear modulus G may be computed using Eqs. (6) or (9) from the instantaneous strains and stresses . (d) Using Eq. (3) G may also be computed using the affine shear elasticity μA and the shear stress fluctuations μF in ensembles of fixed strain (γ = 0). (e) Combined volumetric and shear strain fluctuations allowing the computation of both compression modulus K and shear modulus G from the respective strain fluctuations.

Image of FIG. 2.
FIG. 2.

Density ρ(T) and determination of glass transition temperature T g. Inset: Plotting log (1/ρ) vs. T for the KA model as suggested in Ref. 8 reveals a linear low-T (solid line) and a linear high-T regime which confirms T g ≈ 0.41. 49 Main panel: Number density ρ for pLJ systems at constant normal pressure P = 2 as a function of temperature T for different quench rates R in units of MCS. The crosses refer to a quench in the NPτT-ensemble where the box shape is allowed to fluctuate at τ = 0. All other data refer to the NPγT-ensemble with γ = 0. The solid line and the dashed line are linear fits to the best density estimates (stars) obtained for, respectively, low and high temperatures. By matching both lines one determines T g ≈ 0.26. A similar value is obtained if log (1/ρ) is plotted as a function of T.

Image of FIG. 3.
FIG. 3.

Energy per particle e (main panel) and acceptance rate A (inset) obtained for pLJ systems quenched at constant pressure P = 2 at a given quench rate R as indicated. The crosses refer to a quench at imposed τ = 0. The solid line and the dashed line are linear fits to the energy for, respectively, low and high temperatures confirming the value T g ≈ 0.26 for the pLJ model. Only local MC moves with one fixed (non-adaptive) maximum particle displacement δr max for all temperatures T have been used for the MC data shown in the main panel. The acceptance rate decreases thus with decreasing T as shown in the inset for δr max = 0.01 (upper data) and δr max = 0.1.

Image of FIG. 4.
FIG. 4.

Rescaled shear modulus G(t)/μA vs. sampling time t in the high-T liquid limit of the pLJ model comparing various ensembles and observables. The filled squares show the bare shear modulus obtained using Eq. (3) for the NVγT-ensemble without the impulsive correction ΔμB ≈ 0.2 (horizontal line). The corrected modulus for the NVγT-ensemble (open squares) and the NPγT-ensemble (open diamonds) vanishes as it should. Note that G ττ decreases slightly faster than G γτG γγ. As shown by the dashed line, all operational definitions of G decay inversely with t. The dashed-dotted line indicates the expected power-law exponent −1/2 for the correlation coefficient c γτ (stars).

Image of FIG. 5.
FIG. 5.

Rescaled shear modulus G(t)/μA vs. sampling time t for systems of permanent linear springs corresponding to the dynamical matrix of pLJ systems at P = 2, τ = 0, and T = 0.001. As indicated by the bold line, we obtain GA ≈ 0.67 in the long-time limit. In agreement with Eq. (19) G ττ vanishes with time if it is computed in an ensemble where γ fluctuates as shown for the NVτT-ensemble (filled squares).

Image of FIG. 6.
FIG. 6.

Shear stress fluctuations μF(t)/μA with μA = 33.9 for the same systems as in Fig. 5 . We observe μF(t)/μA → 1 (dash-dotted line) for imposed τ, while μF(t)/μA → 1 − GA ≈ 0.32 (dashed line) for imposed γ. Note the broad time-dependence of μF(t) in the latter cases.

Image of FIG. 7.
FIG. 7.

G γτ(t)/μA, G γγ(t)/μA and G ττ(t)/μA for pLJ beads in the low-temperature limit for P = 2, τ = 0 and T = 0.001 where μA ≈ 33.9. Qualitatively, the data compares well with the various shear moduli presented in Fig. 5 . The filled triangles show G ττ for the NPτT-ensemble which is seen to vanish rapidly with sampling time t. The dashed line indicates a 1/t-decay.

Image of FIG. 8.
FIG. 8.

Reduced shear stress fluctuation μF(t)/μA for pLJ beads using the same representation as in Fig. 6 . The small filled squares refer to data obtained for T = 0.010 using the NVγT-ensemble, all other data to T = 0.001. The reduced Born coefficient μB(t)/μA (thin line) and the affine shear elasticity μA(t)/μA (dashed-dotted line) are essentially time independent while μF(t)/μA approaches its large-t limit (dashed line) from below.

Image of FIG. 9.
FIG. 9.

Stress fluctuation μF (open symbols) and affine shear elasticity μA (small filled symbols) vs. temperature T for both models. The indicated values have been obtained from the longest simulation runs available for a given temperature. μA decays roughly linearly with T as shown by bold solid lines for the low-T and by dashed line for the high-T regime. For constant-τ systems and for all systems with TT g we confirm that μF ≈ μA, while μF|γ (open spheres and squares) is seen to be non-monotonous with a maximum at T g.

Image of FIG. 10.
FIG. 10.

Unscaled shear modulus G as a function of temperature T for both models. For the pLJ model the values G ττ obtained in the NVγT ensemble (squares) are essentially identical to the moduli G γτ (crosses) and G γγ (triangles) obtained for constant τ. As shown by the small filled triangles, G ττ vanishes in the NPτT-ensemble for all T in agreement with Eq. (19) . The solid and the dashed line indicate the cusp-singularity, Eq. (65) , for, respectively, the KA model and the pLJ beads. 30,31

Image of FIG. 11.
FIG. 11.

Rescaled shear modulus y = GA vs. reduced temperature x = T/T g for the KA model at P = 1 (NVγT-ensemble) and the pLJ model at P = 2 (NVτT-, NPτT-, NVγT-, and NPγT-ensembles). The bold line indicates again a continuous cusp-singularity. Inset: Squared correlation coefficient for NVτT- and NPτT-ensembles.

Image of FIG. 12.
FIG. 12.

Unscaled shear modulus G γτ as a function of sampling time t in MCS obtained for the pLJ model in the NPτT-ensemble for different temperatures as indicated. The thin line indicates the power-law slope −1 for large times in the liquid regime. A clear shoulder (on the logarithmic scales used) can only be seen below T ≈ 0.2 and only below T = 0.01 one observes a t-independent plateau over two orders of magnitude. We emphasize that we present here a sampling time effect and not a time correlation function. The same sampling time effect is observed after additional tempering.

Image of FIG. 13.
FIG. 13.

Reduced shear modulus G(T)/μA(T) as a function of reduced temperature x = T/T g for different sampling times t as indicated: (a) G ττ(T) for the KA model (NVγT-ensemble), (b) G γτ(T) for the pLJ model (NPτT-ensemble). For both models the glass transition is seen to become sharper with increasing t. The continuous cusp-singularity is indicated by the bold line. More points in the vicinity of x ≈ 1 and much larger sampling times are needed in the future to decide whether the shear modulus is continuous or discontinuous at the glass transition.

Image of FIG. 14.
FIG. 14.

Affine dilatational elasticity ηA(T) and compression modulus K(T) for both models. Inset: ηA vs. reduced temperature x = T/T g. The small triangles indicate ηF for the pLJ model using the NPτT-ensemble confirming that Eq. (A7) holds. Main panel: The Rowlinson formula K pp , Eq. (A14) , for the NVγT-ensemble is given for both the KA model (spheres) and the pLJ model (squares). For the pLJ model we indicate in addition K vp (crosses), K vv (large triangles) and the rescaled correlation coefficient (stars) which collapses on the other pLJ data as suggested by Eq. (A8) .


Generic image for table
Table I.

Some properties for the KA model at normal pressure P = 1 and shear stress τ = 0: the mean density ρ, the mean energy per volume eρ, the impulsive correction term ΔμB (Sec. II D ), the (corrected) Born-Lamé coefficient μB, the affine shear elasticity μA = μBP ex, the shear modulus G ττ obtained in the NVγT-ensemble, the hypervirial ηB and the affine dilatation elasticity ηA = 2P id + ηB + P ex discussed in the Appendix and the compression modulus K pp obtained for the NVγT-ensemble. The affine elasticities μA and ηA are the natural scales for, respectively, the shear modulus G and the compression modulus K.

Generic image for table
Table II.

Some properties of the pLJ model at P = 2 and τ = 0: the mean density ρ, the mean energy per volume eρ, the impulsive correction term ΔμB, the affine shear elasticity μA, the shear moduli G γτ and G γγ obtained in the NPτT-ensemble and G ττ obtained in the NVγT-ensemble, the affine dilatation elasticity ηA, the compression modulus K vp obtained in the NPτT-ensemble, and the Rowlinson formula K pp for the NVγT-ensemble. All data have been obtained for a sampling time t = 107 MCS.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Shear modulus of simulated glass-forming model systems: Effects of boundary condition, temperature, and sampling time