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Superfluididty of a Fermi gas
1 September 2004
It has already been reported in
(Phys. Usp. 174 531 (2004)) on the possible observation of
a superfluid fermion gas in an experiment by J Thomas and
colleagues on the vibrations of a magneto-optically trapped gas
cloud. An independent, methodologically different experiment by R.Grimm
and his colleagues of the University of Innsbruck, Austria,
also discovered a superfluid fermion gas. The gas under study was
that of 6Li atoms, cooled to a temperature of 500 nK in a magnetic
field. The atoms first formed Cooper pairs and made a transition
to the BEC state. Varying the magnetic field strength caused the
condensate to transform into a strongly interacting Fermi gas -
until the Cooper pairs ultimately broke up under the action of a
radio frequency radiation. Measuring the frequency of the radio
waves absorbed by the gas provided the magnitude of the energy
gap (i.e., the pair binding energy) and made it possible to trace
its growth with temperature. The presence of a gap is important
evidence for the superfluidity of the fermion gas. The results of
the experiments are in good agreement with the theoretical
calculations.
Source: http://physicsweb.org
Glowing nanotubes
1 September 2004
In 2003, P.Avouris and his colleagues at the IBM Research Center
in New York saw carbon nanotubes glowing when an electric current
was passed through them. The reason was that the
counterpropagating flows of electrons and holes (electron
vacancies) recombined in the narrow transverse layer. Now, in
their new experiment, the same team used a microscope and an
infrared camera to monitor the position of the bright spot, and a
control electrode, to move the spot along the tube. The current
through the tube (about 50 mkm in length) was passed by means of
two electrodes attached to its ends. The third, control (or gate)
electrode was separated from the nanotube by a thin insulating
layer, making the entire device similar to a field transistor. To
the gate electrode, a voltage of 0 to 40V was applied. Because
electrons and holes propagate diffusively from nanotube ends, the
gate voltage turned out to affect exactly where the charges meet
to recombine and thus to produce glow. Varying the potential
allowed the bright spot to be moved along the entire length of
the nanotube connecting the electrodes. With this technique, T.Hertel
suggests, the surface defects of nanotubes can be explored
by observing the way the bright spot passes through the defects.
Source: Phys. Rev. Lett. 93 076803 (2004)
The origin of peaks in laser spectra
1 September 2004
Dye lasers or random lasers are terms referring to devices that
use ground liquids or suspensions instead of a single crystal
with reflecting faces. Such lasers emit light in all directions
and over a wide range of frequencies. A moot question, until
recently, was why do high intensity peaks of unpredicted
frequencies appear in the absorption spectra of such lasers. D.Wiersma
of the European Laboratory for Nonlinear Spectroscopy in
Florence and his colleagues were able to find an answer after a
number of experiments and computer simulations. Their laser had
as its active medium an alcohol solution of dye to which the
light-scattering powder of zinc oxide was added. To excite the
laser, short pulses from the pumping flashlamp were used. Of
those photons forming the radiation continuum, most underwent
only a few scattering in the medium. As it turned out, a small
part of the photons underwent 100 to 1000 scattering before
escaping from the medium. It is precisely those photons which
stimulated powerful peaks at certain frequencies.
Source: Phys. Rev. Lett. 93 052903 (2004)
Controlling the state of individual atoms
1 September 2004
Researchers from Switzerland and Sweden have developed a
technique with which single electrons can be added to or removed
from individual gold atoms. The manipulations were performed with
a tip of a scanning tunneling microscope at temperatures of 5 to
60 K. Gold atoms were absorbed on the surface of insulator sodium
chloride deposited only three atomic layers thick on a copper
crystal. When the STM tip potential of +0.6V gave rise to a
tunnelling current, a gold atom gained an additional electron and
thus became an ion - as the STM image clearly showed. Under a
potential of -1V, an atom lost an electron and returned to its
original state.
Source: http://physicsweb.org
Determining the mass of a single star
1 September 2004
Ground-based and Hubble space telescope observations have
provided the first high-precision measurement of the mass of a
single star (other than the Sun). The year 1993 witnessed a
microlensing event in which a star in our Galaxy gravitationally
focused light from a star in the Large Magellanic Cloud. At that
moment, however, the source and the lens were both along the line
of sight and impossible to resolve spatially. After several years
of its travel through the Galaxy, the lens star moved enough
angular distance from the source star for both objects to be seen
as separate stars by a telescope. Besides, it proved possible to
measure the parallax of the lens star - and so to place it at
about 1,800 light years from Earth. Also, from the way the event
took place, it was established that the lens is a single star,
rather than a member of a multiple star system. By combining the
microlensing and parallax data, the lens star was calculated to
have a mass of 0.097+-0.016 times the mass of the Sun, making it
a class M red dwarf star.
Source: http://hubblesite.org,
astro-ph/0405124
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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources.
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