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Quantum model of “Zitterbewegung”
1 February 2010
Ñ. Roos and his group at the Institute for Quantum Optics and Quantum
Information (Innsbruck, Austria) carried out an experiment with 40Ca+ ions
in which they recorded the “quivering motion” effect (Zitterbewegung)
predicted for electrons by E. Schrodinger in 1930 using the Dirac equation.
Rapid spatial oscillations of a particle arise owing to a superposition of
states with positive and negative energies. The experiment with ions is a
quantum model of the “Zitterbewegung” of electrons; in other words, the joint
evolution of the vibrational degrees of freedom and the internal spin state of the
ion is described by the same equation as the spatial trajectory of free
electrons. Laser pulses transferred ions into a prescribed initial state and a
short time later their fluorescent radiation was observed. The properties of
this radiation yielded the characteristics of the “Zitterbewegung”. The
“Zitterbewegung” can be experimentally observed with a quantum model in which
it is linked to the spin state of the ion. At the moment it is quite unfeasible
to detect this effect with real electrons since its amplitude is only ≈ 10-10 cm and its frequency is ≈ 1021 Hz. The idea of quantum models
was suggested by R. Feynman in 1982 for those cases in which the complex
behavior is beyond any hope of direct or numerical computer experiments.
Source: Nature 463 68 (2010)
The E8 group in crystals
1 February 2010
R. Coldea (Oxford University) and his colleagues in Germany and Great Britain
discovered in neutron scattering experiments that the distribution of spins in
quasi-one-dimensional ferromagnetic crystal of cobalt niobate CoNb2O6 at
low temperature and in high magnetic field obeys the symmetry group E8. In 1989
A.B. Zamolodchikov (The L.D. Landau Institute of Theoretical Physics) showed
that in certain cases this Lie group describes the spectrum of excitations for
the Ising model. In recent years the E8 group was also discussed in
elementary particles theory but has never been observed in real physical
systems. The phase transition in quasi-one-dimensional Ising model occurs
when the magnetic field directed along the atomic chains increases above a certain
critical value which separates the magnetically ordered and the paramagnetic phases. The
experiment studied the spin excitations in the two above phases and also investigated the
properties of crystals in the immediate vicinity of the phase transition point. The measured
ratio of the two lower resonance frequencies (two meson states) of atomic chains close to the
critical magnetic field of ≈ 5 T was found to lie close to the golden ratio 1.618, in
agreement with the prediction of A.B. Zamolodchikov's theory.
Source: Science 327 177 (2010)
Goos – Hanchen effect for neutrons
1 February 2010
Researchers at the Delft University of Technology (The Netherlands) and the
Rutherford Appleton Laboratory (Great Britain) were able for the first time to
detect the Goos – Hanchen effect in the reflection of spin-polarized neutrons by
a potential barrier. This effect which was already predicted by Isaac Newton for
light, produces a gap between the points of incidence and reflection of the
beam. In a manner of speech, we can say that the beam penetrates into the sample
and is reflected at a certain depth under the reflecting surface. The effect was
first measured for light by F. Goos and H. Hanchen in 1947. In the case of
neutrons, reflection imparts a certain phase shift to the wave function of a
particle, which may be represented by a spatial translation along the reflecting
surface. The experiment above made it possible to measure the Goos – Hanchen
effect owing to the fact that the height of the relecting potential barrier in
magnetic field depends on neutron spin direction (and on the orienttion of the neutron
magnetic moment) so that neutrons of different degree of spin polarization are
reflected at a different efficiency. The neutron beam was reflected by a layer
of permalloy Fe0.2Ni0.8, 3µm thick. The choice of this material with
high magnetic permeability was predicated on the need to create magnetic field
under the reflecting surface and thus produce phase-shifted wave function. Reflected neutrons were
recorded by a neutron reflectometer OffSpec. A small change in the polarization of
the reflected beam was an indication of nonzero Goos – Hanchen effect. The
Goos – Hanchen effect may be used for practical applications, such as generation of
coherent neutron beams, and for creating neutron “waveguides”.
Source: Phys. Rev. Lett. 104 010401 (2010)
Electron “liquid crystal” in iron-based superconductor
1 February 2010
J.C. Davis (Brookhaven National Laboratory), Ð.Ñ. Canfield (Ames Laboratory) and
co-workers conducted a scanning tunneling spectroscopy study of the compound
Ca(Fe1-xCox)2As2 (x is the fraction of dopant atoms) and
discovered a static spatially ordered state of electron wave functions whose
structure resembles that of liquid crystals. This compound is a parent of one
class of superconductors — layered iron-based high-temperature superconductors
(see review papers in
Phys. Usp. 51 1201, 1229, 1261 (2008)). Electron wave functions
of elongated shape were observed on the specimen surface, stretching along one
of the crystal axes. Their longitudianl size was approximately eight time the
distance between two neighboring iron atoms, and they were in spatial correlation with
the dopant atoms. It was conjectured that the “liquid crystal” state of electrons is
critically important for the high-temperature superconductivity to emerge. Similar
ordered electron states have already been reported in cuprate superconductors. This
may be an indication that the superconductivity mechanism in iron-based superconductors
is closer to the currently unknown superconductivity mechanism in cuprates than to the
Bardeen – Cooper – Schrieffer mechanism in low-temperature superconductors.
Source: Science 327 181 (2010)
Gamma-radiation background at intermediate galactic latitudes
1 February 2010
The results have been published of observing cosmic gamma radiation at
intermediate galactic latitudes 10° ≤ |b| ≤ 20° in the energy
range 100 MeV to 10 GeV, obtained by the Large Area Telescope (LAT) on board the
cosmic Fermi gamma observatory during the first five months after launch.
New background measurements do not confirm some earlier observations conducted
by the EGRET gamma telescope on the orbital Compton Observatory in
1991 – 2000. According to the standard model of the origin of the diffuse galactic
gamma radiation, it is generated when charged particles of cosmic rays interact
with interstellar gas and radiation. This model is calibrated using cosmic rays
data and provides sufficiently specific predictions for the gamma radiation
background. The EGRET has earlier detected an excess of gamma radiation flux in
comparison with the standard model outlined above. This information gave rise to
conjectures on additional contributions to gamma background from particles of
dark matter and other hypothetical sources. The LAT is more sensitive than the
EGRET by approximately an order of magnitude but the LAT data reveal no excess
in the gamma background and thus the measured spectrum supports with high
accuracy the predictions of the standard model of background generation.
Source: Phys. Rev. Lett. 103 251101 (2009)
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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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