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In the glassy state, all materials undergo a process of structural recovery as they age towards equilibrium. The resultant increase of relaxation times t(α) is frequently described with a sublinear power of the wait time t(w)(μ) with an apparent aging exponent μ. We show with molecular dynamics simulations of a Lennard-Jones glass former at various temperatures that the observed aging exponent can be strongly influenced by crossover effects from the freshly quenched state at short t(w) and into the equilibrated state at long t(w). The aging behavior on the molecular level is quantitatively reproduced by a coarse-grained continuous time random walk description over the entire range of temperatures and wait times. Our model glass always shows normal aging, t(α)∼t(w), when the observation time window is no longer affected by crossover effects, in agreement with the well-known trap model of aging.

Dielectric spectroscopy studies of hydrated protein demonstrate smooth temperature variations of conductivity. This observation suggests no cusplike fragile-to-strong crossover (FSC) in the protein's hydration water. The FSC at T approximately 220 K was postulated recently on the basis of neutron scattering studies [Chen, Proc. Natl. Acad. Sci. U.S.A. 103, 9012 (2006)] and was proposed to be the main cause for the dynamic transition in hydrated proteins. Following Swenson et al. , we ascribe the neutron results to a secondary relaxation. We emphasize that no cusplike solvent behavior is required for the protein's dynamic transition.

The origin of the fragile-to-strong crossover in liquid silica is characterized in terms of properties of the potential energy landscape (PEL). Using the standard BKS model of silica we observe a low-energy cutoff of the PEL. It is shown that this feature of the PEL is responsible for the occurrence of the fragile-to-strong crossover and may also explain the avoidance of the Kauzmann paradox. The number of defects, i.e. deviations from the ideal tetrahedral structure, vanishes for configurations with energies close to this cutoff. This suggests a structural reason for this cutoff., Comment: 5 pages, 4 figures; added reference

Two-dimensional criticality is studied in the Klauder and double-Gaussian O(1) models which interpolate from a Gaussian model at y=0 to the S=1/2 Ising model at y=1. Despite strong crossover effects for 0 0 and yield a correction exponent θ=1.35± 0.25. The conjecture θ=4/3 in the absence of a related critical operator, and the observation γ eff ≃2.0 in the Klauder, double-Gaussian, and λϕ 4 models, are discussed.

Many results of the mean-field theory of spin-glasses violate simple scaling laws, including the magnetic field dependence of the transition lines, crossover effects of random anisotropy, and critical behavior in the ordered phase. These violations arise from two dangerously irrelevant variables. Below eight dimensions some mean-field exponents change and at $d=6$ scaling is restored. Below $d=6$ all scaling laws should be valid implying that the critical properties of real spin-glasses should be simpler than in mean-field theory. Experimental consequences are discussed.