amor lj
I've been working on the amorphous lifetimes. I have tried many different quenching processes, including:
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varying melt temperatures (300 and 600 K).
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varying the melt volume (I typically keep the volume the same as the crystal at 0K, but I have tried expanding it)
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Different quench rates (infinite vs finite).
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Different procedures for relaxing the melted sample, incuding annealing at different temperatures in NPT before energy minimizing.
All samples display the same general characteristics which are laid out below.
The amorphous phase appears to be, at least at temperatures > 1K, meta-stable. That is, there are many many different time-avgeraged equil. structures, which are basically the same in energy. These structures fluctuate in time, which you can observe by viewing the below movies.
##movies
each structrue (frame) is time-averaged over 100000 time steps (200 LJ units)
amorphous T=1K
https://github.com/jasonlarkin/disorder/blob/master/lj/amor/amor_1K.mp4
amorphous T=10K
https://github.com/jasonlarkin/disorder/blob/master/lj/amor/amor_10K.mp4
perfect T=10K
https://github.com/jasonlarkin/disorder/blob/master/lj/amor/per_10K.mp4
###urms
Next I predicted the <u^2>^{1/2} from the MD and also from the harmonic prediction, Eq. (30) from turney_prb09_aLD.pdf:
The harmonic prediction works well for the perfect crystal, and also the amorphous phase at 1 K. However, for 5K and above the amorphous phase is more anharmonic. Even at T=1K, the amorphous phase has meta-stable states, which contribute to larger fluctuations in the instantaneous values of <u^2>^{1/2}. You could say that the RMS of u_rms is much larger in the amorphous phase than the crystal, and the amorphous phase is more anharmonic at low T than the crstal. This is to be expected, afterall, since the re-crystallization tempterature is around 20K, which is a highly-anharmonic process.
###normal mode energy autocorrelations
The meta-stability of the amorphous phase causes there to be negative correlation in the normal mode energies for the low frequency modes:
This is the reason that the lifetimes at low freq appear to be so low. I do not know a solution to this, and actually I think it is just a natural property of the LJ amorphous phase. I'm working on a-Si next, but I suspect there will not be such an issue due to the stiffness.
###lifetimes
