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Two-loop QED corrections
1 January 2006
The first measurement of two-loop corrections to the Lamb shift has
been made at the Lawrence Livermore National Laboratory. The Lamb
shift, i. e., the shift of electronic energy levels in an atom, is
due to the interaction of electrons with virtual electron-positron
pairs and photons that appear from the vacuum. To accurately
calculate this shift requires that interactions between various
types of virtual particles be taken into account which, in terms of
the Feynman diagrammatic technique, means adding loop contributions
to the probability of the process. In the hydrogen atom, the
relative two-loop correction to the Lamb shift is as small as 10-6,
which is beyond current measurement abilities. In the highly
ionized atoms of heavy elements, however, the strong electric field
of the nucleus increases these corrections considerably. The
Livermore team studied the Lamb shift in uranium ions that had only
three electrons left. In these ions the corresponding corrections
are already as large as about one-third percent of the total
effect. The measurements were made by accumulating ions in a trap
and detecting the photons emitted in transitions. The two-loop
corrections were measured to an accuracy of about 10% according to
the team. Source: Physics News Update, Number 756
Josephson junction capacitance
1 January 2006
Two independent teams of researchers have measured the electrical
conductance of a Josephson junction for the first time. A Josephson
junction is two layers of superconductor separated by a thin
insulating film through which Cooper pairs can tunnel without
losing their quantum properties. It has been theoretically
predicted that when in electrical circuits, Josephson junctions
have quantum capacitance in addition to the ordinary geometrical
capacitance. In both experiments the quantum capacitance was
measured in much the same way, by detecting the phase shift of a
radio frequency signal in a resonant circuit containing Josephson
junction(s) cooled to mK temperatures. The single-junction
experiment by P.Hakonen and colleagues at Helsinki University and
the Landau Institute of Theoretical Physics (Yu.G.Makhlin); and the
double-junction experiment by P Delsing and colleagues at Chalmers
University of Technology in Sweden produced results that are in
good agreement with the theoretical calculations. The Josephson
junction is a promising device for storing single units of
information (known as qubits). The fact that quantum capacitance
was measured without destroying the quantum state of the junction
makes the technique potentially useful for future quantum computer
technologies. Source: Phys. Rev. Lett. 95 206806 (2005); Phys. Rev. Lett. 95 206807 (2005)
Pseudogap in a ferromagnet
1 January 2006
According to the Bardeen-Cooper-Schreiffer theory of
superconductivity, the energy gap in a superconducting material is
associated with the binding energy of the electron Cooper pairs.
However, a similar spectral feature, known as a pseudogap, is also
observed in high-temperature superconductors above the
superconducting transition temperature. N.Mannella and colleagues
first found the pseudogap in the energy spectrum of the La1.2Sr1.8Mn2O7
manganite, a material which is not a high-temperature
superconductor. The interesting point about La1.2Sr1.8Mn2O7 is that close to the
Curie temperature it makes an abrupt transition from the
ferromagnetic metallic state to the paramagnetic non-conducting
state. The spectrum of La1.2Sr1.8Mn2O7 in its ferromagnetic phase was measured at
Berkeley National Lab using the technique known as angle-resolved
photoemission spectroscopy (ARPES). Along with the pseudogap, team
also observed an anisotropy in the material's spectrum in momentum
space, which is another characteristic feature of a high-temperature superconductor. The findings imply that the pseudogap
is not specific to high-temperature superconductors but is in fact
a more general phenomenon yet to be explained theoretically.
Source: Nature 438 474 (2005)
Positronium molecules
1 January 2006
Positronium is a hydrogen-like system consisting of an electron and
a positron. A.Mills and colleagues from the University of
California may have discovered bound molecular states of
positronium atoms. In their experiment, positrons traveling in a
beam through porous silicon dioxide caught silicon atom' electrons
to form positronium atoms which remained trapped within the pores
for some time before positron-electron annihilation took place. The
photons emitted in the annihilation process were registered by a
detector. Under the conditions of high-dense positronium gas as
obtained in the experiment, frequent positronium-positronium
collisions led to frequent transitions from the angular-momentum-one state to short-lived angular-momentum-zero state, thus
increasing the annihilation rate. It was found that at high density
of positronium in the original beam the annihilation rate was four
times higher than predicted by theoretical calculations. One
possible explanation for this is the presence of defects in the
material. It is also hypothesized that when at high concentrations,
positronium atoms start to interact with one another to form a
short-lived bound state, the molecule of positronium. Because
experiment cannot as yet establish for certain which theory is
correct, further investigations are needed. In the future, plans
are to experiment on the creation of the Bose-Einstein condensate
of positronium molecules and possibly to develop the electron-positron annihilation laser.
Source: Phys. Rev. Lett. 95 195006 (2005)
Laser nuclear fusion
1 January 2006
A new test experiment for a fusion technology based on the use of
a high-power laser has been performed at the Lawrence Livermore
National Laboratory. In this experiment, light from 192 lasers was
passed through a hole into a sphere whose inner surface was covered
with gold. The resulting heating caused the gold to emit powerful
x-ray radiation which was focused to the center of the sphere. So
far the researchers have only been concerned with the device's X-ray production and focusing properties. Future plans are to obtain
a 100-fold increase in laser power and to place a deuterium-tritium
capsule at the center of the sphere. The heating and evaporation of
the capsule's shell will produce a large pressure rise in its core,
causing fusion reactions to occur. It is believed that the
Livermore idea can potentially be the basis for future fusion
electric power plants.
Source: Physics News Update, Number 755
Galaxy cluster lensing
1 January 2006
A new Hubble Space telescope study has been made of the galaxy
cluster MS 1054-0321 observed at a redshift z=0.83 (corresponding to the
time when the Universe was about half its current age). Using data
on the weak gravitational lensing of the line of sight galaxies
provided unprecedented accuracy in determining the distribution of
mass in the cluster. In particular, in addition to the three main
clumps of dark matter (or hidden mass), a few smaller clumps were
seen as satellites around the cluster. Comparison with Chandra's X-ray images of the same cluster showed that only two of the three
larger clumps are luminous in X-rays and that the X-ray maxima are
off the centers of the dark matter clumps, presumably due to the
gas pressure driven motions of the hot matter. The new finding are
important for understanding the structure and formation of galaxy
clusters. Source: astro-ph/0508044
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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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