Number 212, January 30, 1995 by Phillip F. Schewe and Ben Stein
A NEW EXPERIMENTAL STUDY OF BUCKYBALL SUPERCONDUCTIVITY furnishes further
evidence for the idea that interactions between electrons and phonons (vibrational
modes of the crystal lattice) are responsible for superconductivity in
fullerides. The fulleride compounds, crystals of carbon-60 molecules doped
with alkali atoms such as potassium or rubidium, can become superconducting
at temperatures as high as 30 K and above, higher than for any other materials
except the cuprate superconductors. Scientists at the Institute for Solid
State Research in Julich, Germany and at the Max Planck Institute in Stuttgart
have examined the photoemission spectra of negatively charged C-60 molecules.
They use laser light to detach single electrons from the buckyballs; in
the process, the departing electrons sometimes lose energy by exciting
phonons in the cagelike C-60 molecules. The existence of the phonons and
their energies can be deduced from the observed spectrum of the electrons.
The German scientists feel justified in relying on the more easily interpretable
spectra of gaseous C-60 (rather than that of crystalline C-60) and in asserting
that these spectra support the notion of an electron- phonon mechanism
as an explanation for C-60 superconductivity, because previous experimental
and theoretical research had shown that the intra-molecular phonons (those
thought to be responsible for superconductivity) are only slightly different
for the solid and gaseous phases. (O. Gunnarsson et al., upcoming article
in Physical Review Letters.)
THE TOP U.S. PHYSICS DEPARTMENTS , from the standpoint of reference citations
per paper in the scientific literature, are as follows, in descending order:
Harvard, Princeton, UC Santa Barbara, Chicago, Penn, Caltech, Yale, Stanford,
Stony Brook, and MIT. The citation period covered in this report is 1981-93.
(Results from the Nov/Dec 1994 issue of Science Watch; mentioned in Science,
6 January 1995.)
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