superconductor makes better circuits
Technology Research News
note: this research has been withdrawn by the scientists.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
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,"
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
Timeline: 5 years
TRN Categories: Superconductors
Story Type: News
Related Elements: Technical paper, "Josephson Junctions with
Tunable Weak Links," Science, April 13, 2001.
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