3 Search Results

The extended principle of superposition has been a touchstone of spin-glass dynamics for almost 30 years. Here the Uppsala group has demonstrated its validity for the metallic spin glass, CuMn, for magnetic fields H up to 10 Oe at the reduced temperature T_r = T/T_g = 0.95, where T_g is the spin-glass condensation temperature. For H > 10 Oe, they observe a departure from linear response which they ascribe to the development of nonlinear dynamics. The thrust of this paper is to develop a microscopic origin for this behavior by focusing on the time development of the spin-glass correlation length,more » ξ(t, t_w; H). Here, t is the time after H changes, and t_w is the time from the quench for T > T_g to the working temperature T until H changes. We connect the growth of ξ(t, t_w; H) to the barrier heights Δ(t_w) that set the dynamics. The effect of H on the magnitude of Δ (t_w) is responsible for affecting differently the two dynamical protocols associated with turning H off (TRM, or thermoremanent magnetization) or on (ZFC, or zero-field-cooled magnetization). This difference is a consequence of nonlinearity based on the effect of H on Δ (t_w). Superposition is preserved if Δ(t_w) is linear in the Hamming distance Hd (proportional to the difference between the self-overlap q_EA and the overlap q[Δ(t_w)]). However, superposition is violated if Δ(t_w) increases faster than linear in Hd. We have previously shown, through experiment and simulation, that the barriers Δ(t_w) do increase more rapidly than linearly with Hd through the observation that the growth of ξ(t,t_w;H) slows down as ξ(t,t_w; H) increases. In this paper, we display the difference between the zero-field-cooled ξ_ZFC(t, t_w; H) and the thermoremanent magnetization ξ_TRM(t, t_w; H) correlation lengths as H increases, both experimentally and through numerical simulations, corresponding to the violation of the extended principle of superposition in line with the finding of the Uppsala Group.« less

Metal–Organic Frameworks (MOFs) have the potential to change the landscape of molecular separations in chemical processes owing to their ability of selectively binding molecules. Their molecular sorting properties generally rely on the micro- and meso-pore structure, as well as on the presence of coordinatively unsaturated sites that interact with the different chemical species present in the feed. In this work, we show a first-of-its-kind tomographic imaging of the crystal morphology of a metal– organic framework by means of transmission X-ray microscopy (TXM), including a detailed data reconstruction and processing approach. Corroboration with Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) images showsmore » the potential of this strategy for further (non-destructively) assessing the inner architecture of MOF crystals. By doing this, we have unraveled the presence of large voids in the internal structure of a MIL-47(V) crystal, which are typically thought of as rather homogeneous lattices. This challenges the established opinion that hydrothermal syntheses yield relatively defect-free material and sheds further light on the internal morphology of crystals.« less