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| Issue 12, 2024 |
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Extracts from the Internet | ||
Mobility of electrons in graphene1 March 2008Researchers at the Institute of Problems of Technology of Microelectronics and Ultrapure Materials of the Russian Academy of Sciences (IPTM RAS, Chernogolovka, Russia) S.V. Morozov and K.S. Novoselov in collaboration with colleagues in Great Britain, The Netherlands and the USA discovered that mobility of electrons in graphene (one-atom-thick layer of carbon) greatly exceeds the known values of electron mobility in all other materials and at room temperature is close to 2 × 105 cm2 V-1 s-1. This means that electrons in graphene can travel over distances of thousands of interatomic unit length without undergoing scattering. As temperature increases, especially above 200 K, mobility decreases owing to scattering by phonons. The unique electron properties of graphene may prove useful for applications in microelectronics, e.g. for designing generators and detectors of radio waves in the THz wavelength range. However, for practical applications of graphene it will be necessary to work out a technique for preparing specimens of graphene with a lower concentration of defects than is possible at the moment. The unique properties of graphene were outlined in detail in reports of Yu.Ye. Lozovik and S.V. Morozov at the session of the Division of Physical Sciences of the RAS on February 27, 2008. These reports and other materials of the session are to be published in one of the nearest future issues of the UFN. Source: Phys. Rev. Lett. 100 016602 (2008) Spontaneous coherence of Bose – Einstein condensate of magnons1 March 2008V.E. Demidov and his colleagues in Germany, Ukraine and the USA studied the dynamics of the Bose-Einstein condensate of magnons (quasiparticles corresponding to spin wave quanta) by the Mandelshtam – Brillouin scattering technique. Single-crystalline 5µm thick ribbons of yttrium compounds (YIG) were irradiated by 30 ns laser pulses. If the magnon concentration in a specimen exceeded a certain critical level, irradiation transferred some of magnons coherently into the Bose – Einstein condensate state. The time of magnon thermalization and the condensate decay time were much longer than 30 ns so that most of the time the evolution of the condensate was unaffected by radiation. Data on Bose – Einstein condensation of magnons at low temperature and under continuous exposure to external factors were reported earlier. In this experiment a free magnon condensate was observed for the first time at room temperature and direct confirmation of the quantum nature of magnon condensation was obtained. Source: Phys. Rev. Lett. 100 047205 (2008) A mixture of different degenerate Fermi gases1 March 2008W. Ketterle and colleagues at the Massachusetts Institute of Technology were earlier able to create ultracold mixtures of atoms of one element in two different spin states. Ì. Taglieber and his colleagues in Germany are the first to create a mixture of degenerate Fermi gases of different atoms, namely 6Li è 40K. The problem in preparing such mixtures lies in the difficulties involved in simultaneously cooling different Fermi atoms: they have different masses and low cross section of scattering by each other. The new experiment used the technique of sympathetic cooling in which evaporatively cooled boson atoms of 87Rb were placed into the trap together with 6Li and 40K. Owing to a high interaction cross section between the atoms of 87Rb and 40K, the latter were efficiently cooled. The atoms of 87Rb acted as catalysts for the interaction between 6Li è 40K and this made it possible to cool the gas of 6Li atoms to a degenerate state. Experiments with a mixture of degenerate atoms open up important horizons for further research. For instance, there are plans to create inhomogeneous superconducting states in gas mixtures; some theoretical computations indicate that such systems exist in certain high-temperature superconductors. It may also be possible to synthesize ultracold molecules consisting of different fermion atoms. Source: Phys. Rev. Lett. 100 010401 (2008) Antiatoms in a trap1 March 2008The ATRAP collaboration at CERN obtained anti-hydrogen in a trap. Previous experiments already produced antiatoms but it was not possible until now to hold antiprotons and positrons in a trap long enough to form trapped antiatoms. The new experiment operated a combined trap composed of Penning and Ioffe atomic traps. Both traps used electric and magnetic fields for confining. A Penning trap confined accelerator-produced antiprotons and positrons from a radioactive source. An Ioffe trap confined the resulting antiatoms which were then studied by spectroscopic instruments. Source: Physics News Update Number 856 Intermediate-mass black hole1 March 2008The data obtained by the Hubble Space Telescope and Gemini Observatory point to
an intermediate-mass black hole in a globular stellar cluster ω Centauri.
This cluster is one of the most massive globular clusters in the Milky Way
Galaxy and displays a record-high dispersion of stellar velocities. Furthermore,
stars in this cluster differ in chemical composition and age. These properties
may constitute an indication that the cluster ω Centauri is in fact the
nucleus of a small galaxy captured into our Galaxy. An alternative hypotheses
was proposed, namely that ω Centauri was formed as a result of merging of
two globular clusters. According to the new observations, the dispersion of
stellar velocities and the density of the invisible dark matter increase toward
the center of the cluster. An analysis of dynamic models both with isotropic and
with anisotropic distributions of stellar velocities showed that in all
likelihood this growth is caused by the presence of a black hole with a mass of
approximately 4 × 104 M |
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. | |
at the center of the cluster.
Another scenario (the presence of a dense cluster of neutron stars and
stellar-mass black holes) in the central region of ω Centauri is rather
improbable as this cluster does not manifest any signs of strong dynamic
relaxation. In view of this, the scenario with no central black hole was
rejected with the significance level of 99%. The mass of the black hole falls
somewhat above the linear “mass-velocity dispersion” curve (but within the
distribution width) known for massive galaxies and extrapolated to the area of
small masses.
Source: |
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