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possible in incommensurate electronic systems
James E. Kloeppel, Physical Sciences Editor
Ill.— Researchers at the University of Illinois
at Urbana-Champaign have demonstrated that quantum coherence is possible
in electronic systems that are incommensurate, thereby removing one
obstacle in the development of quantum devices.
Electronic effects in thin films and at interfaces lie at the heart
of modern solid-state electronic technology. As device dimensions shrink
toward the nanoscale, quantum coherence and interference phenomena
become increasingly important.
“At quantum dimensions, quantum mechanics says device components will
couple together and act in a concerted manner, where everything affects everything
else,” said Tai-Chang Chiang, a professor of physics and a researcher
at the university’s Frederick Seitz Materials Research Laboratory. “Most
scientists assume that electronic layers must be commensurate, so
electrons will flow without being diverted or scattered.”
In fact, however, most material interfaces are incommensurate as
a result of differences in crystal sizes, symmetries or atomic spacing.
Random scattering of electrons was thought to destroy quantum coherence
in such systems at the nanoscale.
Now, by studying electron fringe structure in silver films on highly
doped silicon substrates, Chiang and his research group show that
even when electronic layers are incommensurate, they can still be
coherent. The researchers report their findings in the
Nov. 3 issue of the journal Science.
In work performed at the Synchrotron Radiation Center at the University
of Wisconsin at Madison, the researchers grew atomically uniform
silver films on highly doped n-type silicon substrates. Then they
used a technique called angle-resolved photoemission to examine the
fine-structured electronic fringes.
Although the silver films and silicon substrates are lattice mismatched
and incommensurate, the wave functions are compatible and can be
matched over the interface plane, Chiang said. The resulting state
is coherent throughout the entire system.
The fringes the scientists recorded correspond to electronic states
extending over the silver film as a quantum well and reaching into
the silicon substrate as a quantum slope, with the two parts coherently
coupled through an incommensurate interface structure.
“An important conclusion drawn from the present study is that coherent
wave function engineering, as is traditionally carried out in lattice-matched
epitaxial systems, is possible for incommensurate systems,” the researchers
wrote, “which can substantially broaden the selection of materials
useful for coherent device architecture.”
The other authors of the paper besides Chiang are graduate student
and lead author Nathan Speer, postdoctoral researcher Shu-Jung Tang
and research professor Thomas Miller. The work was funded by the
U.S. Department of Energy, the National Science Foundation and the
Petroleum Research Fund.
Editor’s note: To reach Tai-Chang Chiang,
call 217-333-2593; e-mail: email@example.com.