Science is poised to take a quantum leap as more mysteries of how atoms behave and interact with each other are unlocked.
The field of quantum physics, with its complex mathematical equations
for predicting the interactions and energy levels of atoms and
electrons, already has made possible many technologies we rely on every
day from computers and smartphones, to lasers and magnetic resonance
imaging. And experts say revolutionary advancements are destined to
come.
But to take a giant leap, you have to be physically fit, and
researchers at the University of Delaware have found an area of quantum
physics that could use some more calisthenics, you might say. The
research, performed by doctoral student Muhammed Shahbaz with his
adviser, Prof. Krzysztof Szalewicz in the UD Department of Physics and Astronomy, was published recently in Physical Review Letters, the journal of the American Physical Society.
Just like people, atoms can be attracted to each other, or, well, be
repulsed. Take argon the third most abundant gas in Earths
atmosphere. This non-reactive gas has a variety of uses, from protecting
historical documents to preventing the tungsten filament from corroding
in fluorescent lights. When two argon atoms are far away from one
another, they will be attracted to each other until they get down to
about 3.5 angstroms and then they will repel each other. Its as though
once theyve gotten a really good look at each other, theyre ready to
move on.
But thats not what physicists found about two decades ago when they
tested the density-functional theory (DFT), which is now widely used to
model and predict the electronic structure of atoms. Most versions of
DFT were either predicting no attraction or only a very weak one. Where
did the failure lie? The attraction between argon atoms originates from
dispersion interactions between electrons, as the motions of the
electrons of one atom influence the motions of the electrons of its
partner. DFT cant accurately account for these correlated motions at
long range.
And thats a problem, especially in a field like materials science,
where physicists may design and predict the properties of a new material
from its strength to its magnetism to its ability to conduct heat
without ever going into a lab to do an experiment.
So physicists began developing fudge factors in the early 2000s to
account for this dispersion energy. Some of these methods turned out to
give reasonably good results and became an extremely popular tool in
computational physics, chemistry and materials science. The scientific
papers proposing such methods have been cited tens of thousands of
times.
What Shahbaz and Szalewicz have shown, after more than a year of
intense analyses, is that all of these fudged methods are actually based
on a faulty assumption. DFT can describe how the motion of one electron
both affects, and gets affected by, the motion of another electron when
the distance between them is on the order of one angstrom. At
separations above one angstrom to about seven angstroms, the correction
methods assume that DFT recovers a fraction of these effects. Shahbaz
and Szalewicz have found that this quantity does not have the
characteristic properties of dispersion energy and actually originates
from errors in the theory that are unrelated to dispersion. Thus, the
researchers say, the correction methods may get good results, but for
the wrong reasons.
We are telling the physics community that you have to go further,
toward a universal method of prediction that works for the right
reasons, Shahbaz says. We are not here to criticize, but to help
improve, he humbly adds.
Currently, Szalewicz and Shahbaz are on a team of theorists and
experimentalists from universities across the United States who are
using quantum physics to predict the structures and energies of
crystals, the stuff of which snowflakes, ice, most rocks and minerals,
some plastics, pharmaceuticals, energetic material and other products
are made. Their complex calculations predict, for example, how much
energy can be packed into a given volume of rocket fuel.
Shahbaz, who is the first author on the journal article, says he
never would have guessed as a child in his small village in Pakistan
that he would someday become a physics professor. He grew up helping his
father, who is a farmer, grow reed, rice, chilis, tomatoes, eggplants,
radishes and okra. Now he is the first in his family to be awarded a
college diploma not to mention academias highest degree, which is now
in plain sight.
When he was applying to graduate school, he received offers from
universities in the U.S. and Canada, but says he ultimately decided on
UD because of the Universitys reputation and the flexibility to work on
a masters degree first. He says that helped him decide what he really
wanted to focus his research on.
When he completes his doctorate in the next few months, he already
has a job lined up, as an assistant professor of physics at the
University of the Punjab in Lahore, where he is destined to hook
students on how light and gravity work, just as he was enthralled as a
youngster.
So why does he like physics so much?
Physics tells you about the laws of nature, Shahbaz says. It also
demands reasoning. You dont have to memorize anything just absorb
life.
This work was supported by the U.S. Army Research Laboratory, the Army Research Office and the National Science Foundation.
Article by Tracey Bryant; photo by Evan Krape
