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Role of phonons in the mechanism of high-temperaturesuperconductivity
1 September 2006
S.Davis of Cornell University and his colleagues from the US and
Japan have presented new experimental evidence of the key role of
phonons (quantized vibrations of the crystal lattice) in the
mechanism of high-temperature superconductivity, namely in the
process where electrons form Cooper pairs as they interact with
phonons. While this effect has been observed previously in
low-temperature superconductors using tunneling spectroscopy, no
definitive results have been obtained in similar studies on
high-temperature superconductors. One possible explanation for this is
that electron pairing in a high-temperature superconductor occurs
only in small-size regions that are distributed chaotically in
space - necessitating measurements on many atomic-scale regions
along the surface of the semiconductor. S.Davis and his colleagues
used a scanning tunneling microscope to perform new measurements of
tunneling current on a nanometer scale in the superconducting
compound Bi2Sr2CaCu2O8+d. The interaction of electrons with phonons was
identified by peaks on the second derivative plot of current vs
voltage. Additional studies showed that this
interaction has nothing to do with magnetic-structure details of
the sample; and that on substituting the isotope 18O for 16O the
phonon mode is displaced in energy by 6%, consistent with what
theory predicts. Also, the photon mode energy and the magnitude of
the superconducting gap anticorrelate with each other at each point
of the sample surface - again pointing to the interrelation
between superconductivity and phonons.
Source: Nature 442 657 (2006)
Bloch oscillations in the field of a light wave
1 September 2006
A new technique to study the force of gravity on micrometer scale
has been developed at the University of Florence which determines
the force by measuring the Bloch oscillation frequency of atoms in
an optical trap consisting of a vertical standing light wave (with
periodicity in the vertical direction). The quantum-mechanical
phenomenon of Bloch oscillations arises when in addition to a
periodic potential, a particle also experiences a static force (as
exemplified by electron oscillations in a crystal in an external
magnetic field). In the experiment by G.Ferrari and colleagues, the
role of the potential was played by the dipole interaction of atoms
with the periodic field of the light wave, the role of the external
force - by the force of gravity. Because the ultracold bosonic
atoms of 88Sr that were used in the experiment had the spin and
nuclear magnetic moment zero in their ground state, they interacted
weakly with one another - allowing coherent Bloch oscillations to
be observed for a record time of about 10 seconds. The plans for
using this technique include superaccurate measurements of the
force of gravity on micrometer scale and the verification of
Newton's law of gravity at small scales.
Source: Phys. Rev. Lett. 97 060402 (2006)
Direct evidence for dark matter
1 September 2006
Direct observations of the galaxy cluster 1E0657-558 have been
made using the Chandra X-Ray space telescope and the Hubble
telescope, the cluster consisting in fact of two separate clusters
just starting flying apart after their collision. The hot gas
present in the clusters was observed via its X-ray radiation,
whereas the mass distribution was inferred from the way the optical
images of galaxies far beyond the cluster were distorted. Because
the gravitational lensing of light makes galaxies oblong in shape,
it proves possible statistically to derive the distribution of mass
in the lense. In this way it was found that the gas cores of the
clusters are located other than at the centers of mass of the
galaxies flying apart - giving the impression that the gas lags,
as it were, behind invisible gas concentrations and galaxies flying
apart, and thus proving in fact the existence of dark matter. The
finding is given the following interpretation. In an isolated
galaxy cluster the core is located at approximately the center of
the more extended dark matter halo. As the clusters collided and
flied through each other, their gas clouds experienced mutual
friction and so slowed down, whereas the collisionless dark matter
and the galaxies continued to move by inertia - leading to a
spatial separation of the clusters' baryonic component and dark
matter. This separation, although not complete, was established
conclusively to exist by comparing the X-ray image and the dark
matter distribution as obtained from the optical observation of
gravitational lensing. While until now various modifications of the
law of gravity have been suggested to explain galaxy rotation
curves and the high temperature of gas in galaxy clusters, the
reported findings make them unnecessary and indeed most of them
fail to account for the cluster collisions observed. It is not yet
clear exactly what dark matter is made of. According to the most
likely hypothesis, it is made of elementary particles that have not
yet been detected experimentally. A quite definitive solution would
be the detection of such particles by a terrestrial detector -
which is where most laboratory efforts are currently being
concentrated and plans for future experimental work focused on.
Source: astro-ph/0608407
Lithium in the Universe
1 September 2006
The exact primordial nucleosynthesis calculation of the initial
(prestellar) chemical composition of the Universe has now become
possible due to the latest cosmic microwave anisotropy data from
the WMAP satellite. However, the theory is found to overestimate by
a factor of 2 to 3 the abundance of lithium as obtained from the
astronomical data on the chemical composition of stars. A.Korn and
his colleagues may have removed this discrepancy by
spectroscopically studying 18 stars in the globular star cluster
NGC6397 using the 8.2-m VLT telescope in Chile. The oldest stellar
population in the Galaxy, the globular-cluster stars differ in
their mass and temperature but are all of the same age and initial
chemical composition, allowing a comparison of the observational
data with what the theory of stellar evolution predicts. The amount
of lithium near the visible surface of a star first remains nearly
constant with increasing temperature (which is seen as a plateau on
the plot) and then sharply drops. This dependence is described by
a theory which assumes that, subject to a diffusion process with
the presence of turbulent mixing, lithium atoms move down to the
center of a (sufficiently hot) star and are destroyed there in
nuclear reactions under high-temperature conditions. As a result,
the near-surface region of a star is lithium depleted by about 78%
during the star's lifetime, whereas the initial amount of lithium
is within one standard deviation from the prediction of primordial
nucleosynthesis theory.
Source: Nature 442 657 (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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