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Parity violation in electron-positron interactions
1 June 2004
The violation of parity (reflection invariance) was discovered
in the decays of cobalt nuclei in 1956 and has since been
observed in many other processes. The E158 Collaboration at
the Stanford Linear Accelerator Center (SLAC) has for the
first time detected parity nonconservation in elastic electron-positron
collisions at low energies (in Muller scattering). The
violation of P invariance is caused by weak neutral currents,
but Z0-boson exchange, because of the large mass of the
Z0-boson, contributes very little to the scattering cross section
compared to electromagnetic processes: the reason the effect
has not been observed in previous experiments. The new
experiment studied the scattering of a beam of (longitudinally
polarized) electrons by a liquid hydrogen target. The quantity
measured was the cross section difference between electrons
with spins aligned along their motion and those with spins
aligned opposite. The experimental technique allowed a very
accurate determination of various types of corrections and
errors involved. The measured cross section asymmetry
is in excellent agreement with Weinberg angle measurements
and with what the Standard Model of elementary particles
predicts.
Source: Phys. Rev. Lett. 92 181602 (2004)
Magnetic properties of a superconductorat room temperature
1 June 2004
Thus far, no material has been discovered which possesses
superconducting properties at room temperature. There are
recent indications, however, that high-temperature super-
conductors (HTSCs) in the nonsuperconducting state retain
certain properties previously thought only to be possible for
the superconducting phase at low temperatures. One such
possible property is the existence of Cooper pairs in anHTSC
at room temperature. C Panagopoulos and his colleagues at
the University of Cambridge (Great Britain) have discovered
yet another new property. It is known that the magnetization
of a superconductor is different in the following two cases: the
superconducting transition takes place in the external
magnetic field applied, and the field is only furnished after
the transition. This difference, it was believed, exists only in
the superconducting state and has to do with the properties of
magnetic vortices. Panagopoulos and his colleagues have for
the first time discovered that the dependence of the magneti-
zation on whether the magnetic field or cooling is applied first
is to a small extent retained in HTSCs (lanthanum and
strontium cuprates) even at room temperature, when they
are not superconducting. The phenomenon of interest is
possibly due to the presence of magnetic vortices in an
HTSC at temperatures above the critical temperature.
Magnetic vortices were in fact observed in the nonsupercon-
ducting phase of an HTSC, although much below room
temperature. Explaining the new phenomenon could help in
understanding the mechanism of high-temperature super-
conductivity.
Source: Phys. Rev. B 69 144508 (2004)
C50 molecules
1 June 2004
Spherical carbon molecules known as fullerenes are suffi-
ciently stable only when each pentagonal face of the molecule
is surrounded by five hexagonal faces. This `isolated pentagon
rule' cannot be satisfied for molecules consisting of fewer than
60 carbon atoms and is the reason why small fullerenes do not
form solid crystals and have previously only been observed in
the gas phase. However, when combined with other sub-
stances, fullerenes may acquire additional stability. This has
been demonstrated by L S Zheng and colleagues at Xiamen
University, and co-workers at the Chinese Academy of
Sciences in Beijing and Wuhan who have for the first time
obtained C50 molecules in the form of a solid compound with
chlorine, C50Cl10.
The C50Cl10 molecules were synthesized from
carbon tetrachloride in a helium atmosphere using a graphite
arc-discharge technique. From the 90 g of the resulting
sediment, about 2 mg of solid C50Cl10 was obtained follow-
ing a purifying procedure. Interesting chemical and physical
properties are expected to be found in C50 molecules.
Sources: Science 304 699 (2004);
http://physicsweb.org/article/news/8/4/14
Magnetization reversal rate
1 June 2004
The magnetization of a ferromagnet varies due to its magnetic
domains changing the directions of their dipole moments. A
team of researchers from the Stanford Synchrotron Radia-
tion Laboratory, the L D Landau Institute for Theoretical
Physics, and Seagate Technology has measured the rate of
this process. The team used 10T, 2ps magnetic field pulses
produced by ultrashort bunches of high-energy electrons
from an SLAC accelerator. Various ferromagnetic materials
were exposed to the magnetic pulses. It turned out that these
short pulses do not have enough time to cause a noticeable
reversion in the magnetization of a grain of the material.
Thus, the magnetization reversal rate is at least 1,000 times
less than previously thought. This result also imposes a severe
limitation on the speed at which information can be recorded
onto a magnetic medium.
Source: http://physicsweb.org/article/news/8/4/10
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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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