Dr. Marco Buongiorno-Nardelli leans over his computer in Cox Hall, scanning the results flashing across the screen from a state away. He’s exploring the feasibility of using carbon nanotubes in nanoscale electronic devices, and is using the Oak Ridge supercomputer to run his own suite of codes simulating electron transport in nanotubes. Although carbon nanotubes are very small objects on the human scale, simulation of their behavior is possible only on a very large-capacity supercomputer.

The biggest supercomputer in the Research Triangle area, located at MCNC, has one- teraflop capacity, handling one trillion floating point calculations per second. But the reigning supercomputer at ORNL is an IBM Power 4, nicknamed “Cheetah,” which has six teraflops of computing power. ORNL has recently acquired a test Cray X1 system, which could be expanded and made even faster in the next few years, bringing ORNL’s capacity to ten teraflops—Christmas for a certain young theoretical physicist.

Buongiorno-Nardelli came to the U.S. from Italy as a post-doc in 1995. He became an assistant professor in the College of Physical and Mathematical Sciences at NC State in 2001, having already begun his interaction with ORNL the year before. He now holds one of the two new positions shared between NC State and ORNL.

With a young family, Buongiorno-Nardelli was concerned about working for organizations in two different states. But a new high-speed fiber-optic link 10,000 times faster than today’s fastest networks has been set up to connect ORNL, the “Atlanta gigapop,” and the Research Triangle. “Now, there is no difference between my sitting in a control room in Oak Ridge and my lab in Raleigh,” says Buongiorno-Nardelli. He splits his time by teaching one semester at NC State, then spending a week a month at Oak Ridge in the following semester, and more time there in summer. “There is no substitute for the face-to-face interaction with the scientists at Oak Ridge,” he maintains, “so I visit as often as I can.”

Using the ORNL supercomputer, Buongiorno-Nardelli predicted that it would be possible to build a nano-rheostat, similar to a dimmer light switch. In such a device, a carbon nanotube—a cylinder resembling rolled-up chicken wire because its carbon atoms are arranged in a hexagonal configuration—is placed on a sheet of graphite whose carbon atoms also have a hexagonal arrangement (see illustration on top left).

Computational simulations verified that the interface between a carbon nanotube and graphite gives tunable resistance. “If you place the carbon cylinder on the graphite sheet so that the carbon atoms of both are aligned, a current will flow at the interface,” Buongiorno-Nardelli says. “As you rotate the carbon cylinder on the graphite sheet, changing the angle between the atoms in the system, you get increased electrical resistance and reduced current flow. As the atoms become aligned, you get low resistance and high current flow.” His theoretical predictions agreed with experimental results at the University of North Carolina at Chapel Hill, and were published in Science magazine in 2000.

Buongiorno-Nardelli and his NC State colleagues are also computationally modeling a proposed molecular memory cell that would allow laptop computer batteries to last 100 times longer than today’s batteries. “We’re not actually making things,” explains Buongiorno-Nardelli. “We’re simulating nanoscopic pieces for experimentalists to use in fabricating devices. This is such a great opportunity to be a part of the interplay between theory and experiment.”

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