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Element 117 has been synthesized
1 May 2010
A team of researchers from Russia and the U.S. under the guidance of academician
Yu.Ts. Oganesyan have synthesized for the first time six nuclei of the chemical
element with atomic number 117. Its isotopes 293117 and 294117 were
created on a heavy ion accelerator — the U-400 cyclotron at the JINR (Dubna,
Russia) in collisions of the beam of nuclei 48Ca with a target of the
radioactive isotope 249Bk. 22.2 mg of 249Bk nuclei were produced at
the Oak Ridge National Laboratory (USA) and prepared for use at the RIAR
(Dimitrovgrad, Russia); they were incorporated into the target by deposition of
oxide BkO2 onto titanium foil. The experimental data was processed at JINR,
at the E. Lawrence Livermore National Laboratory (Berkeley, USA) and at two
American universities. The method used for the identification of nuclei was the
determination of spatio-temporal correlations in the gas-filled fragment recoil
separator; it revealed the characteristic chain of α-decays of nuclei
which included the sequence of 11 intermediate neutron-rich isotopes. The
half-lives of the nuclei 293117 and 294117 are about 14 and 78 ms,
respectively. The properties of the observed chains of decay indicate that the
experiments have now moved closer to the border of the “island of stability”
— that region of long-lived superheavy nuclei whose existence was predicted
theoretically. Among the nuclei produced by now, the atomic number is the
highest in the nucleus of the element 118 obtained at JINR in collaboration with the
Lawrence Livermore National Laboratory in 2006 (see
Phys. Usp. 49 1221 (2006)).
Source: Phys. Rev. Lett. 104 142502 (2010)
Superconductivity on the nanoscale
1 May 2010
Researchers at Ohio State University (USA) together with colleagues from Japan
and Germany found that superconductivity can emerge in samples consisting of
only four pairs of molecules of organic salt (BETS)2GaCl4 where BETS is a
complex-structure organic compound bis(ethylenedithio)tetraselenafulvalene
acting as donor of charges in the salt molecule. The temperature of
superconducting transition in a macroscopic specimen of this substance is
Tc ≈ 8 Ź; specimens possessed two-dimensional layered structure
resembling the structure of high-Tc cuprate superconductors. The electron
spectrum of a single layer of (BETS)2GaCl4 on silver substrate at
temperatures from 5.8 to 15 K has been studied by scanning tunneling
spectroscopy. The superconducting gap was established; its width was a function
of temperature and specimen size (length of paired molecular chains of
(BETS)2GaCl4). With the shortening of molecular chains down from 50 nm,
the gap diminished and superconducting properties decreased correspondingly.
However, the gap persisted even in samples measuring about 3.5 nm and consisting
of only four pairs of molecules (BETS)2GaCl4. The mechanism of
superconductivity in (BETS)2GaCl4 has not yet been identified. It is
possible that these molecular-scale superconductors may find applications in
nanoelectronics.
Source: Nature Nanotechnology 5 261 (2010)
Ionization of atoms near the nanotube
1 May 2010
Ionization of rubidium atoms by electric field created by carbon nanotube has
been studied at the Harvard University (USA). Rubidium atoms cooled in a
magneto-optical trap to a temperature of 200 µK were sent to the nanotube in
the plane perpendicular to its axis. If the angular momentum of the atom with
respect to the nanotube did not exceed a certain critical value which was a
function of electric potential of the nanotube, the atom began to rotate around
the nanotube, approaching it along a spiral trajectory and at the same time
greatly accelerating. Then one of the outer electrons of the atom tunneled to
the nanotube while the resulting ion felt the Coulomb repulsion and accelerated away
from the nanotube at a high velocity. Measurements of the momentum distribution
in receding ions as a function of the potential of carbon nanotubes showed good
agreement with the predictions of the theoretical model. The effect of
ionization of atoms near the nanotube can be used to create high-sensitivity
detectors of ultracold atoms.
Source: Phys. Rev. Lett. 104 133002 (2010)
Adiabatic Landau – Zener transition in Rydberg atoms
1 May 2010
N. Saquet and his colleagues at the Aime Cotton Laboratory (Orsay, France)
studied the dipole-dipole interaction of sodium atoms in Rydberg states,
accompanied with electron transitions ns + ns → np + (n – 1)p in the vicinity of
n = 48 driven by the Landau – Zener mechanism. A beam of sodium atoms was created
by laser-driven ablation of atoms from the surface of a solid specimen.
Resonance excitation and irradiation by pulses of infrared laser in the beam
produced a concentration of 108 cm-3 of rubidium atoms in Rydberg
ns-state. Dipole-dipole interaction of Rydberg atoms combined with a slowly
varying external electric field resulted in adiabatic transitions to np levels
in the Landau – Zener configuration of nearly intersecting potential energy
surfaces of two electronic states. The number of atoms in np states in the beam
was measured at the output. This experiment succeeded in inducing Landau – Zener
transitions in Rydberg atoms in a controlled manner; a similar technique could be
used in the future to produce Rydberg atoms in quantum-entangled states.
Source: Phys. Rev. Lett. 104 133003 (2010)
White dwarfs in the Galactic halo
1 May 2010
M. Kilic and his colleagues from the U.S. and Germany have discovered using
ground-based optical telescopes three white dwarfs at a distances of 70-80 pc
from the Sun; in all likelihood, they belong to the population of very old stars
from the galactic halo. Once white dwarfs are formed, they start to cool down
and their observed temperature is a measure of their age. The study of white
dwarfs is thus an important source of information about the history of star
formation in various subsystems in the Galaxy. The three white dwarfs found, two
of which form a binary, are among the coldest of the known white dwarfs. Their
effective temperature is 3700-4100 K, which corresponds to the age of stars
approximately 10-11 billion years. The type of motion of white dwarfs shows that they
are likely to belong to the Galactic halo, while their belonging to the disk is
rejected at the 2 σ confidence level. A comparison with the stars of the
galactic disk led to a conclusion that there was a time gap of one to two
billion years between the epoch of star formation in the halo and the beginning
of star formation in the galactic disk. A similar result was obtained earlier using
Hubble telescope observations of cold white dwarfs in two globular star clusters.
Source: arXiv:1004.0958v1 [astro-ph.GA]
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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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