| GENERAL RESEARCH INTERESTS:
- THEORETICAL CONDENSED MATTER PHYSICS
My Ph.D research was to study the properties and characteristics
of semiconductor materials, specifically surface features and corresponding
deposition growth sites in Silicon and GaAs. I employed ab initio
DFT methods, highly realistic potentials and specialized electron
correlation treatments to also examine band structures of novel
materials. The models that we use to predict the transport properties
of new materials suffers from under-predicting the experimentally
determined energy gap. Some work I did was aimed at attempting to
correct this problem, by treating the electronic interactions more
realistically. (These models are highly simplified because computing
time can increase with the cube of the number of particles you want
to simulate.) The model is called the "Coulomb hole" and
it tries to model the way electrons repel each other. It's based
on the fact that one single electron repels all the other electrons
in the material, but if you pick a different single electron, the
same thing is happening. So the charge can't go too far -- in fact,
it goes away by some characteristic distance (the radius of the
"hole") and then piles up on the other side. Imagine digging
a hole and throwing the dirt out. You'd have a hole next to a big
pile of dirt, whose volume must be the same as that of the hole.
(This is how I incorporated charge conservation into the model.)
If you're interested (or incredibly bored), see more in the first
paper listed below.
I also did some beginning investigation into the conductive properties
of spin-dependent half-metallic transition metal compounds. This
field is now combining with electronics to be known as "spintronics".
How can something be "half metallic" you ask? As with
many scientific words, the word "metal" actually means
something very specific and precise. It means a material whose Fermi
level (the energy that divides conduction electrons from valence
electrons) is located inside a partially filled band. Thus, there
is NO energy gap separating the valence bands and the conduction
bands. Therefore, many electrons can be thermally excited to higher
energy levels and becoming conducting with great ease. In the case
of a "half metal", the conducting states and valence states
depend on the spin of the electrons. Electrons have only two spin
possibilities: up or down. Suppose the spin-up electrons have a
Fermi level that lies within a partially filled spin-up band, while
the spin-down electrons have a Fermi level that lies within a gap
separating the filled (valence) states from the unfilled (conduction)
states. Then you'd have a material that conducts easily, but only
with spin-up electrons. Thus, this is a "half metal" because
only half (or some fraction) of the electrons are able to act like
a metal and conduct heat or electricity easily.
These materials will become highly useful in the future, because
we can use them to create spin polarized currents, or as spin polarized
current switches. All that needs to happen for us to create a spin
polarized current is to pass electrons of both spin types through
a half metal. This can prevent the spin-down electrons from passing,
but allow the spin-up electrons to speed through. Then, by varying
the applied voltage on the half metal, we can push the Fermi levels
up and down, which means we can pass the spin-down electrons while
blocking the spin-up electrons. Thus, if a spin-up current is being
used, we've just shut it off. Pretty neat!!
Publications:
- M.D. Watson, C.Y. Fong, Solid State Communications,
124, 12, pp. 457-461, 2002. Full
text (pdf).
- C.Y. Fong, M.D. Watson, L.H. Yang, S. Ciraci,
Modelling and Simulation in Material Science and Engineering,
10, 5, pp. R61-R77, 2002.
- C.D. Consorte, C.Y. Fong, M.D. Watson, L.H. Yang,
S. Ciraci, Materials Science & Engineering B, 96, pp.
141-144, 2002.
- C.D. Consorte, C.Y. Fong, M.D. Watson, L.H. Yang,
S. Ciraci, Physical Review B, 63, pp. 041301R, 2001. Full
text (pdf).
- Ph.D Dissertation Title: Investigations of
the Reconstruction and Growth on the Si (100) Surface, and Studies
of an Interelectronic Correlation Function (2001).
- AEROSOL & PHOTOCHEMICAL AIR QUALITY MODELING
Studies of air quality have shown that the gas-phase pollution
cycles are well-understood. Currently, the major focus is on aerosol
particulate matter (PM). PM can range from a collection of a few
molecules to 10 microns in diameter (for comparison, human hair
has a diameter of around 60 microns). During inhalation, the lung
has problems not due to more mass of PM, but rather more number
concentrations of PM. By far the highest number concentration of
PM is located in the sub-micron size range. PM is emitted from (for
example) hot diesel exhaust, and these small particles can form
nucleation sites for condensation growth to larger size ranges.
I modeled regional emissions and investigated the growth of nuclei
and establishment of secondary PM, and how different sources in
the region contribute to this secondary aerosol PM in the atmosphere.
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Charge density plot of bulk Silicon
Graph of the Coulomb hole function
Charge density plot of layered GaAs.
A half-metal schematic band structure.
This photograph was taken by a former student during her
trip to Japan. We had just discussed the principles of reflection and
she saw this!
Question: why do we see the underneath of the bridge?
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