Tunable superconductor makes better circuits

By Kimberly Patch, Technology Research News

Editor's note: this research has been withdrawn by the scientists.

In order to make practical use of any type of electricity conduit, including superconductors, you have to be able to control how the electricity flows.

Researchers from Lucent Technologies' Bell Labs have made a tunable circuit that allows them to modulate the flow of electricity through organic superconductors like polymers and fullerenes. Fullerenes, or buckyballs, are made entirely of carbon and resemble molecular-scale soccer balls. Polymers are long strings of molecules.

The circuit is a type of Josephson junction, which is a superconductor ring with a gap in it. The gap is spanned by a thinner piece of superconductor that causes electricity to flow more slowly than if the ring were unbroken. The researchers tuned the device by changing the voltage of current flowing through an electrode at the gap.

This voltage change affected the electrical field surrounding the connecting piece of organic material, which in turn changed its properties from insulating to superconducting. Insulators slow or block electricity flow and superconductors allow electricity to flow with perfect efficiency.

The tunable junctions "take advantage of the fact that it is possible to switch between an insulator and a superconductor just by changing a voltage," said Jan Hendrik Schön, a Bell Labs researcher. The junctions "can be tuned over more than five orders of magnitude," he added.

The electrical field changes the material's properties by changing the concentration of the negatively-charged electrons or positively-charged holes that reside around a molecule's periphery and allow it to conduct electricity. At high carrier concentrations electricity flows more quickly, while low concentrations change the material to an insulator.

The researchers have demonstrated "an exciting novel effect which is scientifically significant and potentially useful in applications," said J. T. Chen, a Professor of Physics at Wayne State University.

The scientific significance is that "the strength of superconductivity can... be varied by an electric field controlled by a bias voltage [making it] possible to study superconducting properties such as critical current and energy gap as a function of carrier density without changing temperature," said Chen.

In addition to scientific measurement tools the junctions could eventually be used to make superconducting circuits, which could be faster than conventional electronic circuits, said Schön. It is well known that Josephson junctions can be used as circuits. One example is superconducting quantum interference devices (SQUIDs), which contain two Josephson junctions. More complex superconducting structures can be used for logic circuits, Schön said.

In the 1970's, IBM attempted to develop computers by using Josephson junctions that were switched using a magnetic field controlled by current, said Chen. The Bell Labs researchers' electrically-switched Josephson junctions show more promise as circuits, said Chen. "Since [the] electric field is more localized, voltage-controlled Josephson junctions may be easier [to use in] designing a circuit," he said.

The researchers are looking for additional materials to use for the junctions. They're also working on making more complicated circuits of two or more links, said Schön.

"In the present experiments, we used one gate to control the junction. By using separate gates for each superconducting region... a better control of the properties could be achieved," he said.

These tunable, superconducting junctions could be used as quantum bits, or qubits in quantum computers in the far future, Schon said.

Schön's research colleagues were Christian Kloc and Harold Y. Hwang Of Bell Laboratories and Bertram Batlogg of Bell Laboratories and the Swiss Federal Institute of Technology (ETH). They published the research in the April 13, 2001 issue of the journal Science. The research was funded by Lucent Technologies.

Timeline:   5 years
Funding:   Corporate
TRN Categories:   Superconductors
Story Type:   News
Related Elements:  Technical paper, "Josephson Junctions with Tunable Weak Links," Science, April 13, 2001.


May 16, 2001

Page One

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Tunable superconductor makes better circuits

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