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       SEMINAR OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS
        Institute for Computational Sciences and Informatics
  CSI 929  (http://science.gmu.edu/physics/seminar/schedule.html)
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 Direct Simulation of Next Generation Quartz Crystal Oscillators

		     Jeremy Q.Broughton
		Complex Systems Theory Branch
	    Naval Research Laboratory, Washington D.C.

    Present quartz crystal oscillators (QCO) can be manufactured in the
100 micron length scale regime; they involve megahertz frequencies. Next
generation oscillators will hit the sub-micron regime where they will
afford gigahertz frequencies and  much higher sensitivities (eg force,
mass, etc). It is this size regime that may be directly accessed by
modern atomistic simulation; that is the multimillion atom system size.
    Modern parallel processors such as those accessible at NSF, DoE and
DoD sites are capable, now, of handling 1-10 million atom systems for
elapsed times of nanoseconds - precisely the time regime of interest.
Current computer technology doubles performance every year - enabling
easier and easier access to direct device modelling as time progresses.
    We have adopted a two pronged approach to the simulation process.
The first is the brute-force simulation of the device. Non-linearities
may be investigated this way - such as the effect of local and/or
extended defects and the way their internal motion couples to the
device vibrational modes. The second is medium scale (100,000 atom)
modelling of the material comprising the device in order to determine
such properties as elastic constants, defect concentrations, thermal
expansivities and phase transitions. The latter may be run in bulk-3D
mode or in slab mode (ie 2D) or in bar mode (ie 1D) to determine the
effects of edge/surface/volume ratios (ie device size); and of surface
mass deposition and of surface reconstruction on device frequency. Our
simulations probe these properties as a function of temperature and
applied stress.
     We have performed many of the medium scale simulations - numerical 
results will be shown as well as the procedures whereby they were obtained. 
The large, multi-million particle, system codes have been written for the 
IBM SP2s at Maui and at Cornell. A description of the parallel algorithms 
will be presented. We are currently running these. The interatomic potential 
used for SiO2 (quartz)will also be described - it gets the alpha-beta 
transition right and also the density and elastic constants of the alpha 
phase. It also does a reasonable job at the short wavelength phonons, 
as well as the structure of the amorphous phase. The system size cross-over 
from a valid continuum description to the point at which atomisticis are 
necessary, will be discussed.

                          Monday , April 1 1996
                                  5:00 pm
                          Room 206, Science & Tech. I
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