Extracts from the Internet

The coherent quantum phase slip

O.V. Astafiev (Institute of Physical and Chemical Research, RIKEN, Japan) et al. reported the first experimental demonstration of the effect of coherent quantum phase slip (CQPS) in tunneling of quanta of magnetic field (vortices) from one insulator to another across a thin layer of superconductor which acts, due to the Meissner effect, as a potential barrier. The CQPS effect is a close analogue of the Josephson effect in which electrons tunnel between two superconducting contacts through a thin insulating layer. The difficulty for observing the CQPS is the dissipation of quasiparticles. In this experiment, this problem was overcome by using a special disordered superconductor — a loop of indium oxide measuring 5 µm in size and having a narrow segment 40 nm wide through which vortices tunneled. The loop has been integrated into a waveguide and resonances were observed in the spectrum of the transmitted microwave radiation at certain intensities of external magnetic field perpendicular to the plane of the loop. The spectrum showed an energy gap and theoretically predicted resonances. The resonances corresponded to coherent tunneling of an integral number of magnetic flux quanta under conditions of superposition of states with different numbers of photons. Source: Nature 484 355 (2012)

New quasiparticles

Majorana fermion. L.P. Kouwenhoven (University of Technology, Delft, The Netherlands) and his colleagues observed the formation of Majorana quasiparticles in magnetic field in the region of tunnel junctions of superconductors with nanowire made of InSb; this material possesses properties of a topological insulator with strong spin-orbital interaction. The formation of a Majorana fermion was signaled by the presence of two characteristic peaks in the current-voltage characteristic of the system; peak positions depend in a specific manner on magnetic field. A Majorana fermion is a complex quantum state obeying the Fermi – Dirac statistics but having no charge. In elementary particle physics Majorana particles (named after Ettore Majorana who theoretically predicted them in 1937) refer to hypothetical, not yet observed, particles which are their own antiparticles. Source: arXiv:1204.2792v1 [cond-mat.mes-hall]

Orbiton . Experimenters have already observed separation of electrons into quasiparticles, spinons and holons, which responsible for the spin and charge of the electron, respectively. The separation of the electron into a spinon and an orbiton was observed for the first time in a new experiment carried out in the Paul Scherrer Institute (Switzerland). The orbiton it is a quasiparticle constituting a quantum of the orbital wave of the electron cloud. The act of separation of the electron in the quasi-one-dimensional compound Sr₂CuO₃ in the Mott insulator state was stimulated by electron transitions to excited levels caused by x-ray pulses. The dispersion curves of quasiparticles were measured on the spectrum of inelastic scattering of x rays. The experimental results are in good agreement with calculations based on the Kugel – Khomskii model. This study may be useful, for instance, to improve understanding of the properties of high-temperature superconducting cuprates. Source: www.sciencedaily.com

Graphene monoxide

Е.С. Mattson (University of Wisconsin, USA) et al. discovered a new carbon compound which may find applications in microelectronics. Ordinary graphene oxide, which is a disordered insulator, was heated in vacuum to a temperature of 750°C. It was assumed that annealing will only lead to a loss of oxygen atoms but in the experiment this compound partly converted to a new ordered phase which lacked many of the initially present oxygen-containing functional groups. Graphene monoxide has semiconductor properties and, according to the data reported, its energy spectrum shows a gap of about 0.9 eV. Inspection in electron microscope revealed on the surface of the specimen a number of alternating areas of unoxidated graphite and carbon monoxide. It was also found that crystals of graphene monoxide are built of quasihexagonal unit cells. Source: www.sciencedaily.com

Lightning-produced neutrons

A.V. Gurevich (P.N. Lebedev Physical Institute, Russian Academy of Sciences) and his colleagues from Russia and Kazakhstan reported the registration of significant low-energy neutron fluxes during lightning discharges at the Tien-Shan high altitude research station. Lightning-produced neutrons were already reported in other experiment as early as 1985 but the accuracy was then rather poor. The Tien-Shan experiment involved three low-energy detectors based on using the reaction ³He(n,p)t, plus a neutron monitor sensitive to high-energy neutrons. In contrast to neutrons produced by cosmic rays, the energies of lightning-produced neutrons are low. The time resolution of neutron detectors was about one minute. Within this accuracy, surges of neutron emission coincided with lightning strikes which were recorded by electrostatic and radio frequency detectors. The neutron signal generated by lightning in the detector in open air was typically 2-3 times stronger than the mean natural background. It is not clear so far which mechanism is responsible for the generation of neutrons since the flux of γ-quanta in a lightning is three orders of magnitude lower than is necessary for the corresponding photonuclear reactions; nor do muons of cosmic-rays travelling along the discharge channel provide an explanation . Source: Phys. Rev. Lett. 108 125001 (2012)

Search for dark matter using kinematics of stars in solar neighborhood

Telescopes of the European Southern Observatory in Chile were used to study the motion of 400 red giant stars at distances of 1.5-4 kpc from the galactic plane in the vicinity of the Solar System. It was found that the observed kinematics can be explained without assuming the presence of some additional gravitating mass on top of the mass of the stars, gas and dust. These data exclude the presence of a spherical halo of dark matter with the local density ≈ 0.3 GeV cm^-3 at the 4 σ, and show that the dark matter density near the solar system is probably at least five times lower than usually assumed. To explain the discovered deficit of dark matter, one has to posit an exotic combination of assumptions, such as for example a halo stretched more than twice along the axis. Hypotheses of a ring of dark matter or an additional compact disk are equally improbable. The existence of dark matter was confirmed in a number of other observations, and it is difficult to find an explanation why it should be lacking in the vicinity of the Sun. Assuming the reported results are correct, the flux of dark matter particles in experiments directly searching for them shall be considerably less intense than is currently expected. Source: arXiv:1204.3924v1 [astro-ph.GA]