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Replication of states in a quantum dot
1 June 2010
A.M. Burke and his colleagues at the Arizona State University found that
magnetoresistance in an open quantum dot is distributed in the form of rhomboid
periodic structures. A quantum dot of dimensions 1.1 × 1.1 µm was
prepared by etching and electron-beam lithography in a thin layer of InAs. The
scanning gate microscope used in these experiments fixed the variations of
magnetoresistance in the quantum dot (produced in response to perturbations
introduced by the field of the microscope tip) as a function of the magnetic
field. The repeated periodic structures identified in a quantum dot can be
described by the the so-called scar wave function of electrons. The data agree
well with the concept of “Quantum Darwinism” proposed by W. Zurek (Los Alamos
National Laboratory) in 2003. In this model, which describes the transition from
the quantum state to the classical one, the decoherence of the very stable quantum state (pointer state) is accompanied with its replication in
the classical environment. Repetitive structures in the quantum dot are
interpreted by the authors as classical copies of the same pointer state that
went through decoherence.
Source: Phys. Rev. Lett. 104 176801 (2010)
Dipolar intermolecular interactions in ultracold gases
1 June 2010
Researchers at the University of Colorado and National Institute of Standards
and Technologies (NIST) studied for the first time the properties of the
ultracold gas consisting of polar fermion molecules 40K87Rb
characterized by long-range potential of dipole-dipole pair interaction.
Molecules in one of the states of hyperfine splitting of nuclear levels were
obtained by sending laser light onto a mixture of potassium and rubidium atoms
cooled in nonuniform magnetic field. The induced electric dipole moment of
40K87Rb molecules of magnitude up to 0.22 D is dictated by the
intensity of the external electric field and this relationship creates the
possiblities for controlling the properties of the gas. The molecules of
40K87Rb placed in electric field interact anisotropically. If a
collision occurs with relative velocity directed along the field, the
interactions between molecules become attractive as these are collisions between
opposite charges of dipoles oriented along the field. In collisions in the
transverse direction, molecules are more often repelled, and the rate of
inelastic processes (chemical reactions) is lower. The anisotropy of pair
interactions of molecules manifested itself, in particular, in anisotropic
thermalization of the gas, i.e. in different relaxation rates of the components
of velocity along and transversely to the electric field.
Source: Nature 464 1324 (2010)
Melting mechanism in plasma-dust crystals
1 June 2010
L. Couedel and his colleagues at the Max Planck Institute for Extraterrestrial
Physics (Germany) found that the melting of two-dimensional plasma-dust crystals
takes place in view of the crossing and the resonance relations of the two
branches of the dusty-plasma oscillations. The mechanism of melting was
theoretically predicted by A.V. Ivlev and G. Morfil in 2000 for the
one-dimensional case. Plasma-dust crystals are defined as an ordered state of
dust particles in dusty plasmas (for details see V.E. Fortov et al.
Phys. Usp. 47 447 (2004)). In this experiment dusty plasmas were created in the chamber above a
planar electrode generating high-frequency discharge at a frequency of
13.56 MHz. The discharge-ionized argon was kept at at a pressure of 0.4 Pa; it had an
admixture of dust, namely, particles of melamine formaldehyde 9 µm in size.
The structure of the obtained plasma-dust crystals was studied using high-speed
video recording in reflected laser light. The development of instability was
accompanied by an increase in the kinetic energy of the particles of dust during the melting of plasma-dust crystals, which was caused by the decrease in the discharge power or by
lowering of pressure of the gas. Three branches of dusty plasma oscillations were
observed in these experiments. Crossing and the resonance coupling of the
transverse mode (relative to the plane of the electrode) and one of the two
longitudinal modes of oscillation were observed near the melting point, which
corresponds exactly to the theoretical model.
Source: Phys. Rev. Lett. 104 195001 (2010)
Intergalactic magnetic field
1 June 2010
S. Ando (California Institute of Technology) and A. Kusenko (University of
California and University of Tokio) determined for the first time the strength
and characteristic scale of intergalactic magnetic fields. The magnetic field
was found from the effect of diffuse emission (halo) of gamma radiation
around the active galactic nuclei which themselves are pointlike gamma sources. The
halo is due to deflections in the magnetic field of charged particles of the
electromagnetic cascades produced by high-energy gamma-ray photons interacting
with background radiation. The observed angular scale and brightness of the halo
correspond to the magnitude of the magnetic field B ≈ 10-15(λB/1 kpc)-1/2 gauss where λB < 10-100 kpc is the correlation
length of magnetic fields; in fact, larger scales are not excluded. It is rather
difficult to measure reliably the profile of the gamma halo around an individual
active galactic nucleus and the decisive factor proved to be that the
statistical study used a set of 170 bright active nuclei observed by the Space
Telescope Fermi-LAT. It was established that the gamma-ray halo is not produced
by observation errors or by the emission of extended objects around galactic
nuclei since the scale of such objects would be about 5-15 Mpc. The
knowledge of intergalactic magnetic fields is important for cosmic ray physics
and gamma-ray astronomy. It is possible that these fields are remnants of a very early
universe (of an inflationary stage or a cosmological phase transition), and that
primary magnetic fields in galaxies and stars have been strengthened by the dynamo mechanism.
Source: arXiv:1005.1924v1 [astro-ph.HE]
Black hole shifted along the jet in M87
1 June 2010
Observations using the Hubble Space Telescope revealed that the supermassive
black hole in M87 is shifted from the galactic center by 6.8 ± 0.8 pc.
Isophotes of the central bulge of the galaxy were plotted and their geometric
center was determined. The black hole which is a pointlike source of radiation is
displaced from the above center along the line of the relativistic jet towards
the less bright counterjet. Several possible causes of the displacement of the
black hole are discussed. It is possible that the black hole gets the recoil in
response to jet emission (this mechanism of acceleration of black holes was
suggested by I.S. Shklovsky in 1982) while the depth of the gravitational
potential well in the nucleus of the galaxy M87 is insufficiently large for retaining
the black hole at its center. The black hole can form a binary system with
another black hole or feel the gravitational attraction of a massive star
cluster, which would also have led to its displacement away from the center of
the galaxy. A black hole could have formed by a merger of two black holes and
get the recoil momentum via the anisotropic emission of gravitational waves.
The latter scenario and I.S. Shklovsky's mechanism yield a correct order of
magnitude for the displacement and are regarded as the most likely, but the exact cause
of the displacement of the black hole remains unclear.
Source: arXiv:1005.2173v1 [astro-ph.CO]
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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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