Simulated annealing phase transition sampling

The resistance dependence and thermogravimetric analysis (TGA) of Er-Ba Cu O has been measured in the temperature range 2/ C to 920 C. The heat treatments and oxygen flow rates simulated actual sintering and annealing processes used in sample preparation. Evidence of a phase transition in Er Ba Cu O near 680 C is discussed, as well as the implications of the maximum oxygen uptake near 400 C. The impact of sample preparation procedures on sample features is also discussed. 

The essential feature of PC materials is the ultrafast phase transition between amorphous and crystalline structures that occurs on a nanosecond time scale. In the previous sections, we have discussed extensively the amorphous and crystalline structures of GST and their properties. These correspond to the starting and end points for the actual phase transition, which are crucial to understand the function of PC materials. We now present results for the nucleation-driven crystallization process of GST using DF calculations combined with MD [31], A sample of fl-GST with 460 atoms was studied at 500, 600, and 700 K, and a second sample of 648 atoms was simulated at 600 K. In all cases we used a fixed crystalline seed (58 atoms, 6 vacancies) in order to speed up the crystallization process. More recent experience has shown that the time scale for the crystallization is of the order of several nanoseconds for these system sizes in the absence of a fixed seed, while those here are of the order of 0.3-0.6ns. This means that we cannot discuss the onset of nucleation, but this is also true in the case of smaller systems (<200 atoms) discussed by other groups. In very small systems, periodic boundary conditions bias the process severely. Our larger samples reduce finite-size effects, and we show the effect of choosing different annealing temperatures. Simulations of this scale (up to 648 atoms over 1 ns) are near the limit of present day DF/MD calculations. 

Snapshots of the twin microstructure in the simulated sample annealed below the transition are shown in Figure 6. Only A1 atoms belonging to a single crankshaft and located at T1 sites are shown in Figure 6, and different symbols are used to represent A1 atoms at Tlo and Tim sites. The A1 atoms at T2 positions and all Si and host atoms are not shown in order to clearly distinguish the two variants of the ordered phase. At early... 

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