Marvin Cohen holds a model of carbon atoms, 1997 Credit,
Howard Ford, UC Berkeley
Cohen and the art of predicting the existence of new materials
10 May 2002
By Robert Sanders, Media Relations
Berkeley - Marvin Cohen has been thinking about physics since
he was a child in Montreal, before he realized it was called
physics. In fact, every day for the past 50 years physics has
occupied his mind.
When he's not trying to predict what kind of material you'd
get by mixing different atoms, he's devising problems for his
students to solve, or trying to understand a new discovery in
a totally different area of physics. He may jump from considering
the effects of rolling a sheet of atoms into a cylinder, to
wondering whether you can turn a tangled clump of nanotubes
into a computer.
And since theoreticians need only pen and paper, he can do
his thinking anywhere; a café in Paris or a Honolulu
beach as easily as his fifth-floor office in Birge Hall, overlooking
spooky that you can actually use these equations to predict
new materials that never existed in the laboratory."
"My favorite spots to do physics are Paris and Hawaii,"
This relentless quest to understand the physics of the world,
in particular the solid stuff - metals, semiconductors, insulators
and superconductors - this week has garnered the 67-year-old
Cohen the National Medal of Science, an award from the President
of the United States to the nation's premier researchers.
Cohen's ties to UC Berkeley go back to his enrollment as an
undergraduate in 1953, the same year he became a U.S citizen.
Back then, before the free speech movement, his life revolved
around fraternity dances and playing the clarinet and jazz saxophone.
He didn't lose sight of his plan to become a physicist, however,
and finally began to apply himself in 1957, when he enrolled
as a graduate student at the University of Chicago.
"I got serious at that point," he said, hooking up
with a mentor, Jim Phillips, who steered him into theoretical
solid state physics. Choosing his own thesis topic, he showed
theoretically that semiconductors could be superconductors and
successfully predicted the first superconducting oxide, the
precursor to the high temperature superconductors discovered
23 years later.
After a year with Bell Laboratories in New Jersey, Cohen migrated
back to UC Berkeley, joining the physics faculty in 1964 and
the research staff at Lawrence Berkeley National Laboratory
in 1965. He continued to think about a problem proposed by Phillips
when he was at Chicago, the solution to which has had ramifications
throughout the world, ranging from the design of new semiconductors
for the electronics industry to the search for zero-resistance
superconductors and new forms of matter, such as nanotubes.
In the 1960s, solid state physicists were trying to understand
why solids with different atomic structures have different properties.
Some mixtures of atoms turn out to be metals and conduct electricity,
other mixtures are insulators, and some, semiconductors have
properties in between. Throwing together still other chemicals
creates materials that, at low temperatures, conduct electricity
without any resistance. A theory to explain these superconductors
had been proposed in 1957 by Bardeen, Cooper and Schrieffer,
and in 1972 won its originators a Nobel Prize.
Years later, in 1987, Cohen worked with the university's dance
program to illustrate the pairing motion of the electrons in
a superconductor for a NOVA television show on superconductivity,
which aired the following year and won an Emmy Award.
To explain material properties, theoretical physicists began
with the theory of quantum mechanics and fundamental principles
of physics, but because each solid contains billions upon billions
of atoms, and each atom carries dozens of electrons, the mathematical
challenge was too great. The only solutions obtainable were
for very idealized structures which only vaguely resembled real
Phillips and others had suggested that simplifying the picture
might help by using an idea introduced by Enrico Fermi in 1934.
Assume that the only electrons you need worry about are those
floating relatively freely on the surface of the atom. Ignore
the inner electrons, which are so tightly bound to the nucleus
that they don't get involved in chemical reactions anyway.
Cohen took the hint and developed a "pseudopotential theory"
that treats the inner electrons and nucleus as one entity creating
an electrical field in which only the surface or valence electrons
roam. With the advent of more and more advanced computers, he
and his students created generations of computer programs to
calculate the physical properties of almost any solid based
on the movement of valence electrons in a pseudopotential field.
The model he created works so well that, in cases where theory
and experiment disagree, Cohen and others have come to trust
"It's spooky that you can actually use these equations
to predict new materials that never existed in the laboratory,"
he said. "Quantum mechanics is spooky anyway, but this
The computer programs were quickly picked up by researchers
in academia and industry - Cohen makes them available free of
charge - and now "band-gap" engineers use them to
design custom electronic components. Today, they are used around
"Using the programs we developed, scientists and engineers
are able to tell how the properties of a material will change
when they put it under pressure, or add other alloys,"
he said. "The applications of the science are very important."
Cohen and his group also used the programs to predict successfully
that silicon under pressure would change from a semiconductor
to a superconductor and new superstrong solids and fibers.
The model even works at the scale of a nanometer - the size
of a mere 10 hydrogen atoms laid end to end - which has steered
Cohen into the burgeoning field of nanoscience to work alongside
his former student and now UC Berkeley professor of physics,
Cohen came up with one successful prediction while flying cross-country
thinking about buckminsterfullerenes - molecules formed from
60 carbon atoms arranged in the shape of a soccer ball, reminiscent
of the geodesic domes designed by the late architect Buckminster
Fuller. Carbon does this easily, and these molecules can be
isolated out of the soot from an electrical arc. But what about
other atoms? He thought about substituting boron and nitrogen,
but while they fit nicely in a hexagonal arrangement, they wouldn't
normally form the pentagons also needed to fully cover the surface
of a soccer ball. If you arranged them on a sheet of hexagons,
like chicken wire, and rolled them up, he thought, perhaps they
would form a stronger and easier-to-make structure than the
It took six months to convince his graduate students that the
idea wasn't off the wall, and when they ran the structure through
their modeling program, it predicted not only that it would
be a semiconductor but also that it would have a strength greater
than that of steel. When UC Berkeley colleague Alex Zettl, professor
of physics, finally synthesized boron nitride nanotubes in 1995,
they had all the predicted properties.
He and Zettl later teamed up to found a company, now called
Nanomix, to develop nanotubes as sensors and hydrogen storage
systems for use in fuel cells.
"I have known and worked with Marv for 20 years, and I
still am inspired every time I talk with him," Zettl said.
"He's a delight to work with, he gets the best graduate
students, and he is full of interesting ideas. He's an inspiration
to us all."
His pseudopotential theory has led his and Zettl's groups to
many new nanotube configurations, including an oxygen sensor,
a diode and other electronic circuit devices.
"I joke that I can make a radio with just 50 atoms, but
it operates in the X-ray," Cohen said. "That's not
very useful at present, but it shows we can make components
atom by atom."
Cohen now is investigating materials called nanocrystals that
contain a mere 100 to several thousand atoms, and which exhibit
properties that larger clumps made of the same material do not.
These accomplishments have earned Cohen many honors, including
election to the National Academy of Sciences and elevation to
University Professor, which essentially makes him a faculty
member of the whole 10-campus University of California system.
According to formal surveys of citations, Cohen is one of the
most cited condensed matter physicist of the past 30 years.
Now, as one of UC Berkeley's 25 National Medal of Science winners,
he adds another laurel, one he will receive at the White House
on June 13 in the presence of his wife, Suzy Locke Cohen, his
son, Mark, and daughter and son-in-law, Susie and Jerry Crumpler.
"During the past 35 years, there has been a revolution
in the use of quantum theory to predict the existence of new
materials and properties, and Dr. Cohen is the individual most
responsible for this advance," praised the citation from
the White House.
"This is really an award for Berkeley - the campus and
the lab," Cohen said. "I've had numerous students
and post-docs, and they certainly share in this award too.".