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The five-quark baryon
1 April 2004
In 2003, physicists discovered two baryons consisting of four
quarks and a five-quark
(pentaquark) baryon containing two u-quarks, two d-quarks,
and s-quark. Now, the H1 collaboration at the DESY
laboratory in Germany has found evidence for the existence
of a new charmed five-quark baryon (and its corresponding
antiparticle) which has a c-quark and two pairs of u- and
d-quarks in its composition. The collaboration used the
HERA accelerator to study inelastic electron-proton collisions
and detected events which produced protons and
excited D-mesons containing d- and c-quarks (or their
antiparticles). The pentaquark is identified as a narrow peak
in the distribution of such events as a function of the collision
energy. The pentaquark mass corresponding to the position
of the peak turned out to be 3099MeV. The detection and
study of new baryons are important for a better under-
standing of the nature of strong interactions. The H1 team
includes Russian scientists from the Institute of Theoretical
and Experimental Physics (Moscow), P N Lebedev Physics
Institute of the RAS (Moscow), and the Joint Institute for
Nuclear Research (Dubna).
Source: http://arxiv.org/abs/hep-ex/0403017
The magnetism of carbon nanotubes
1 April 2004
Michael Coey and his colleagues at Trinity College in Dublin,
Republic of Ireland, have shown evidence that carbon
nanotubes can acquire magnetic properties when brought
into a contact with a ferromagnetic material. That carbon
nanotubes should exhibit this property was earlier predicted
theoretically by M Ferreira and S Sanvito. The basis of the
effect is the spin-polarized charge transfer at the interface
between the ferromagnetic metal substrate and the multi-
walled carbon nanotube. The principal difficulty of the
experiment was in detecting the weak magnetic moment of
the nanotubes against the background of a strong magnetic
moment of the ferromagnetic sample. The sample used in the
experiment was a smooth ferromagnetic thin film of cobalt or
magnetite uniformly magnetized strictly in one direction.
Examination under a magnetic force microscope revealed
weak perturbing magnetic fields, which the nanotubes create
on the surface of the sample. According to the measurements
taken, the graphite magnetization
corresponds to a spin transfer of about
0.1 Bohr magneton per carbon atom in contact with the film.
When in contact with nonmagnetic substances such as silicon,
copper or gold, carbon nanotubes do not possess magnetism,
according to control experiments.
Source: J. Phys.: Condens. Matter 16 L155
(2004)
Doping an isolated fullerene molecule
1 April 2004
A technique for introducing potassium atoms into isolated
soccerball-shaped C60 fullerene molecules (`buckyballs') has
been developed by M Crommie and his team of researchers
from the Lawrence Berkeley National Laboratory and the
University of California. Earlier, extended monolayers and
bulk crystals of fullerenes could only be doped by introducing
metal atoms, when part of the C60 molecules absorbed alkali
metal atoms by chance. In the new technique, C60 molecules
and K atoms were placed on a very smooth silver surface,
where their positions were mapped with high precision using a
scanning tunneling microscope. The team then used the tip of
the microscope to move a buckyball over a potassium atom,
with the result that the former caught the latter inside itself. In
the same way, ensuing atoms could be placed within the C60
molecule. In this manner, a buckyball could reliably be made
to pick up from one to four K atoms. The researchers could
also extract atoms from the doped molecule. To do this, a C60
molecule was moved to an impurity (most likely, an oxygen
atom) located on the silver surface. The attraction from the
oxygen atom caused a K atom to escape from the C60
molecule. To study doped molecules, the tunneling current
through the tip of the microscope was measured. As already
established in previous experiments, doping strongly affects
the electronic properties of fullerene molecules.
Source: http://www.lbl.gov/
A new cooling technique
1 April 2004
In conventional free-space laser techniques used for cooling
atoms, in particular for creating a Bose-Einstein
condensate, the atoms absorb a directed laser light and
then spontaneously emit isotropic radiation when making
electronic transitions back to their ground states. The
initial quantum state of an atom is destroyed in this
approach. Now G Rempe and his colleagues at the Max
Planck Institute for Quantum Optics in Garching, Germany
have developed a new atom cooling technique which
does not change the quantum state of the atoms and, in
addition to that, is five times faster than the conventional
technique. The experiment involved placing individual
rubidium atoms in a microcavity between two mirrors. In
this cavity, a standing electromagnetic wave was excited
using a laser. While the electromagnetic field shifted the
energy levels of the atom when interacting with it, it caused
no transitions between the energy levels. The energies of
the photons escaping the cavity were somewhat larger than
their initial energy. The additional energy was extracted
from the kinetic energy of the atomic motion in the cavity,
with the result that the atom was cooled without its inner
quantum state being changed. The new cavity cooling
technique has possible applications in quantum information devices.
Source: Nature 428 50 (2004)
Dynamics of cosmological expansion
1 April 2004
While in the present epoch the universe is expanding with
acceleration, earlier, at redshifts z > 0.5, its expansion was
decelerating according to the evidence obtained by the
Hubble space telescope. The Hubble team studied distant
type Ia supernovas, from whose distribution the dynamics of
cosmological expansion can be learned. Of all the supernovas
studied by the Hubble telescope, 170 had been detected earlier
by other telescopes, and 16 others had been observed for the
first time (6 of these being among the most distant supernovas
known). A possible explanation of the observed nature of
cosmological expansion is that the energy of universe would
be divided up as follows: 29% in the form of matter, and 71%
as `dark energy' (cosmological constant or quintessence).
Source: astro-ph/0402512
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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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