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Superconductivity in multiwalled nanotubes
1 March 2006
I Takesue and his colleagues from Japan studied the superconducting
properties of multi-walled carbon nanotubes, i. e., of ones
composed of concentric layers of carbon atoms. The superconducting
transition temperature of these tubes, Tc, was measured to be about
12 K, 30 times lower than for their single-walled counterparts.
These latter have their superconductivity greatly suppressed due to
the formation of a Tomonaga-Luttinger liquid phase, in which an
additional repulsion between electrons breaks up the Cooper pairs.
The Japanese experiment proceeded by synthesizing multi-walled
nanotubes on a substrate of porous aluminum oxide, cutting all of
them off at the same level using ultrasound and etching techniques,
and then evaporating silver electrodes on the resulting cuts. This
ensured that the electrodes be in electrical contact with all the
inner layers of carbon atoms - unlike previous experiments, where
the electrodes made contact only with outer layers and the
nanotubes did not superconduct. In the new experiment, with all of
the carbon layers connected by electrodes and electrically active,
superconductivity turns out to overwhelm the Tomonaga-Luttinger
effect. The magnitude of the superconducting gap is exactly as
predicted by the Bardeen-Cooper-Schrieffer theory, and the critical
current - the current above which the material's
superconductivity breaks down - has a temperature dependence
described by the Ginzburg-Landau formula.
Source: Phys. Rev. Lett. 96 057001 (2006)
Nuclear molecules
1 March 2006
M Freer of the University of Birmingham in the UK and his
colleagues have established that the nucleus of 10Be consists of two
alpha-particles and two neutrons in the space between them - a
diatomic molecular structure in a sense, with the neutrons creating
attractive forces between the alpha-particles. The cluster structure is
found in many nuclei, but that in 10Be is the most manifest one. Such
`nuclear molecules' measure a few fermis across (1fermi=10-15m),
and have a lifetime of only 10-21s. The 10Be nuclei were produced by
bombarding a gaseous 4He target with a beam of 6He nuclei,
preliminarily produced in proton-lithium collisions. The nuclear
molecules were found to decay into the same original 4He and 6He.
Measuring the kinematics of these decay products provided the
rotational characteristics of the nuclear molecules, from which
their structure was inferred.
Source: Physics News Update, Number 762
Electron velocity in a conductor
1 March 2006
M. Drescher, N. Kaplan, and E. Dormann have performed an experiment
at Jerusalem University to measure the average velocity of
electrons in a conductor. Because electrons undergo scattering,
their average velocity (i. e., the current velocity) and
instantaneous velocity are not the same thing. The team used a
technique known as magnetic resonance imaging to study the electron
spin motion in a magnetic field that varies along the conductor.
The researchers started with a zero-electric-current calibration
procedure, by measuring the radio echo from the spins they had made
precess in phase using radio pulses, and then repeated their
measurements with a current flowing through the conductor. From the
observed shape of the echo the researchers determined the average
electron velocity as a function of current and found it to be in
good agreement with the predictions of classical theory. The major
challenge in these measurements lies in the random nature of spin
precession. For the specific conductors - salt crystals - used
in the Jerusalem experiment, the randomization time was of order of
a few microseconds. It is not yet clear whether metals, with their
nanosecond randomization times, can be used in such experiments.
Source: Phys. Rev. Lett. 96 037601 (2006)
Ball lightning in the lab?
1 March 2006
The formation in the laboratory of spherically shaped, glowing,
lightning-ball-like gaseous structures has been investigated by E
Jerby and V Dikhtyar of Tel Aviv University. Unlike gas discharge
experiments, the Tel Aviv researchers were concerned with the
formation of hot plasma balls near a solid - specifically
silicate - surface, a small portion of which was heated and
evaporated by a radio-frequency pulse. The experiment involved
directing a 600-watt magnetron pulse into a rectangular waveguide,
inside which the silicate sample and a metal needle pointing to it
were placed, an arrangement that produced high-intensity radiation
between the needle's tip and the sample's surface. To see what is
going on, special holes had been made in the waveguide's walls.
What the researchers saw was that a ball of partially ionized gas
formed under the tip in the region where fast melting and
evaporation processes took place, which lifted itself from the
surface and then either was hanging above it for some time or moved
along the waveguide over a distance of 0.5 m or so at 0.3ms-1.
The balls absorbed radiation very efficiently from the waveguide.
The concentration of electromagnetic radiation in a ball is
believed to be due to the plasmon resonance phenomenon - as in
the previously proposed models of the natural fireball. There are,
however, substantial, mostly scale related, differences between the
laboratory balls and their natural counterparts, which move much
faster, are larger in size, and live longer.
Source: Phys. Rev. Lett. 96 045002 (2006)
A new class of neutron stars
1 March 2006
Astronomers using Australia's Parkes radio telescope have detected
11 radio transient sources of a new type in the Galactic plane,
which go invisible for a long period of time after emitting for 2-30 ms.
With the `dark spell' lasting 2 to 30 ms depending on the
source, the observation of these sources is limited to less than 1
s per day. Ten of these eleven sources were found to emit in a
periodic pattern - with a period of 0.4-7.0 s - strongly
suggesting that the new sources are spinning neutron stars. On the
other hand, because no signs of orbital motion are seen in the
signals from them, these neutron stars should be lone rather than
members of binary star systems. Given that radio transients are
very difficult to detect, the Galaxy may actually contain several
times as many such neutron stars as conventional, continuously
emitting pulsars. The Parkes findings may change the existing
supernova rate estimates and call for developing models of how
neutron star radiate.
Source: Nature 439 817 (2006)
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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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