Number 227, May 22, 1995 by Phillip F. Schewe and Ben Stein
ULTRANARROW LUMINESCENCE LINES FROM SINGLE QUANTUM DOTS. In general,
the miniaturization of electronic devices not only saves space. If carried
far enough it can also bring into play useful quantum phenomena (which
lead to greater efficiency and control in the switching of currents). In
a quantum well, for example, electrons are confined between two thin semiconductor
layers in a two-dimensional zone. The electrons trapped there do not possess
a continuum of energies but only a set of discrete energies. If one plotted
the density of allowed electron states versus electron energy, the graph
would be not a smooth curve but a series of steps. Nanofabrication techniques
can be used to restrict further the electron's freedom; in a quantum wire
electrons are restricted to one dimension and the density-of-states-vs-energy
plot becomes more spiky than for the quantum well. Finally, when electrons
are confined in all three dimensions, in a structure called a quantum dot,
the allowable energies are completely quantized: that is, the spectrum
of electron states corresponds to a set of specific energies. Nanometer-size
dots can be produced and have been studied for several years. It has been
difficult, however, to measure the discrete nature of electron energies
in dots because often they are studied in ensembles whose properties, averaged
over many dissimilar dots, are not precisely the same as for any one dot.
Now a Berlin-Halle-St. Petersburg collaboration has observed luminescence
(excited by a Ne-He laser) from single dots. The light from a 12-nm InAs
dot appears, as expected, as ultranarrow lines, monoenergetic to less than
0.15 milli-electron-volts. The researchers claim that this represents the
first direct evidence of the discrete nature of electron states in single
nm-scale quantum dots. (M. Grundmannn et al., Physical Review Letters,
15 May.)
A CURRENT DENSITY OF A MILLION AMPS PER SQ CM has been achieved in a
YBaCuO superconductor at a temperature of 77 K. Representing a team of
Los Alamos scientists, Stephen Folytyn reported at last month's Materials
Research Society meeting that their material even remains superconducting
while carrying currents of 500,000 A/cm**2 in a magnetic field of 8 Tesla.
The Los Alamos sample consisted of thin layers of YBCO deposited on a flexible
metal tape. For applications in magnet windings, superconductors must not
only carry high currents in the presence of strong fields, but must also
be ductile enough to be worked into flexible wires. Scientists at Los Alamos
and other labs are striving to find the right process (the fewer production
steps the better) that will result in spools of wire kilometers long. (Science
News, 29 Apr.; Science, 5 May; Nature, 11 May.)
PAPERS SUBMITTED TO PHYSICAL REVIEW BY FOREIGN AUTHORS now exceed those
from U.S. authors by a ratio of two to one. A task force of The American
Physical Society, studying the growth of its journals, finds that the increase
of submissions (currently about 8% annually) to Physical Review and Physical
Review Letters in recent years has been mostly from foreign authors. This
reflects what the task force chairman Eugen Merzbacher of the University
of North Carolina calls "the globalization of physics research."
Correspondingly, the number of manuscripts published has gone up. But since
production and distribution charges generally go up along with the number
of pages published, the APS feels that future growth will have to be held
in check, such as by lowering the article acceptance rates. (APS News,
May 1995.)
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