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| Issue 11, 2024 |
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Extracts from the Internet | ||
Rydberg atom in a molecule1 June 2009V. Bendkowsky and her coworkers at the Universities of Stuttgart and Oklahoma prepared and then investigated Rb2 molecules in which one of the rubidium atoms is in a Rydberg state with the outer electron in the s-state and n = 34-40 while the second atom remains in the ground state. The bonding between these atoms is caused by the scattering of the Rydberg electron on the electrons of the second atom. As Enrico Fermi established in 1934 the interaction potential in this sort of scattering of low-energy electrons may be attractive (negative scattering length). Since the frequency of the electron in the Rydberg atom is much higher than the characteristic frequency of interaction, the wave function is practically undistorted by scattering. The molecules obtained have a lifetime before decay of about 18 µs and are about 100 nm in size, which is 1900 times the Bohr radius. The molecules were created by laser-excitation of rubidium atoms in a Ioffe – Pritchard magnetic trap at a temperature of 3.5 µK. Oscillation spectra of molecules were measured in the ground and first excited states. The results obtained are in good agreement with the results of theoretical calculations. The researchers expressed hope that similar molecules may soon be created with the Rydberg electron in p-state, as well as a three-atom molecule and a molecule belonging to a class of what is known as trylobite molecules in which the Rydberg electron has a high angular momentum. Source: Nature 458 1005 (2009) Superconductivity of europium1 June 2009Atoms of bivalent rare-earth element europium at normal atmospheric pressure possess high magnetic moments; this is an obstacle to europium's transition to superconducting state. However, researchers of the Washington University in St. Louis J.S. Schilling and Ì. Debessai established that europium becomes weak Van Vleck paramagnetic under pressure of 80 GPa and acquires superconducting properties. Its superconducting transition temperature is Tc = 1,8 K, and Tc grows linearly with increasing pressure, reaching Tc = 2,75 K at 1142 GPa. Eu specimens were studied in a diamond anvil cell. The superconducting transition was identified by measuring electric conductivity and electric susceptibility in ac field. Electron levels of europium get distorted under high pressure so it is transformed from a bivalent to a trivalent element. A similar effect has already been observed in Americium that also becomes Van Vleck paramagnetic and superconducting under pressure (Tc = 0,79 K). Europium is the 53rd element which manifested superconducting properties in elemental state, and the 23rd element in which superconductivity arises only on application of high pressure. Sources: Phys. Rev. Lett. 102 197002 (2009) ; arXiv:0903.1808v1 [cond-mat.supr-con] “Incandescent lamp” made of a nanotube1 June 2009B.C. Regan and colleagues at the California NanoSystems Institute at UCLA (Los-Angeles) studied the incandescence of current-heated carbon nanotube. This experiment allowed them to test Planck's law of radiation almost to the limit of applicability of thermodynamics. Two gold contacts were lithographically attached to the ends of a nanotube placed over a hole in the silicon substrate in vacuum. The multilayer carbon nanotube was about 100 atoms thick. When electric current was passed through it, it heated up and began to glow. The emission wavelength was on the order of the nanotube length and much larger than the tube thickness. The radiation spectrum of nanotube emission was studied at different temperatures (different currents) by using a microscope and a set of optical filters. The classical thermodynamics used together with Max Planck's quantum hypothesis to derive the spectrum of blackbody radiation is applicable only to macroscopic systems consisting of a very large number of particles. The spectrum of thermal emission of a nanotube was nevertheless found after corrections for geometric factors to be in good agreement with the Planck formula. A nanotube has microscopic dimensions but is still large enough for a statistical description. At the same time, quantum properties of a microscopic system already manifest themselves sufficiently well. This experiment therefore dealt with the properties at the borderline between the thermodynamic and quantum regimes. Source: Phys. Rev. Lett. 102 187402 (2009) Topological Hall effect in MnSi1 June 2009A number of neutron scattering experiments provided indirect evidence that the distribution of electron spins in the compound MnSi contains a nontrivial topological structure composed of quasiparticles known as skyrmions. Skyrmions are described by chiral soliton models suggested by Tony Skyrme in 1961. It is assumed that topological excitations we call skyrmions arise in MnSi due to the spin-orbital interaction and form a lattice composed of three helices. Two independent groups of experimenters carried out new experiments that confirm this picture. Ì. Lee and his colleagues in the USA and Japan studied the Hall effect in a specimen of MnSi under pressure of 6-12 kbar. They found an additional contribution to conductivity detected at the magnetic field in the interval 0.1-0.45 T, which is not typical of the conventional Hall effect. In the model with skyrmions this contribution is known as the topological Hall effect. In another experiment A. Neubauer and his colleagues in Germany also studied the Hall effect in MnSi, but in a different segment of the phase diagram of this compound. Similar features of the Hall effect were discovered and could be explained in terms of skyrmion distribution. Source: Phys. Rev. Lett. 102 186601 (2009) Refined Hubble constant1 June 2009240 Cepheid variables and several supernova explosions of class Ia in remote galaxies were observed using the Hubble Space Telescope (HST) which made it possible to improve the accuracy of the Hubble constant by a factor of more than two, reaching the level of ≈5%. Some of the observed Cepheids (variable stars with a known period vs. luminosity curve) were located in the same galaxies as six of supernovas of class Ia; this made it possible to carry out direct joint calibration of various distance units. The team also recorded Cepheids in the galaxy NGC 4258 which contains cosmic masers. Using the geometry of the maser emission, the distance to the galaxy was measured with sufficient accuracy. Furthermore, the Hubble telescope measured the parallax of ten Cepheids in our Galaxy. This led to further refinement of the accuracy of calibration of distance measurement. The importance of calibration is illustrated by the fact that the relative accuracy of the Hubble diagram for supernovas of class Ia is less than 1% but the presence of systematic errors does not yet permit reaching the same accuracy in measuring the absolute value of the Hubble constant. Likewise the observation of baryonic oscillations in the spectrum of microwave background radiation does not yet reach better accuracy without the assumption that the Universe is flat plus the hypothesis that the parameter of the equation of state of dark energy is w = p/(ρ c2) = –1 (the cosmological constant). According to the new HST data, the refined value of the Hubble constant is H0 = 74,2 ± 3,6 km s-1Mpc-1. Together with the WMAP data over the last five years this gives w = –1,12 ± 0,12. Source: arXiv:0905.0695v1 [astro-ph.CO] |
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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