Magnetic field driven instability of charged center in graphene

It is shown that a magnetic field dramatically affects the problem of supercritical charge in graphene making any charge in gapless theory supercritical. The cases of radially symmetric potential well and Coulomb center in an homogeneous magnetic field are considered. The local density of states and polarization charge density are calculated in the first order of perturbation theory. It is argued that the magnetically induced instability of the supercritical Coulomb center can be considered as a quantum mechanical counterpart of the magnetic catalysis phenomenon in graphene.Comment: 10 pages, 4 figures; to be published in PR

Light Element Isotopic Compositions of Cometary Matter Returned by the STARDUST Mission

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild2 particle fragments, however extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Non-terrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is {sup 16}O-enriched like refractory inclusions in meteorites, suggesting formation in the hot inner solar nebula and large-scale radial transport prior to comet accretion in the outer solar system

Irradiation Records in Regolith Materials, II: Solar-Wind and Solar-Energetic-Particle Components in Helium, Neon, and Argon Extracted from Single Lunar Mineral Grains and from the Kapoeta Howardite by Stepwise Pulse-Heating

High-resolution stepped heating has been used to extract light noble gases implanted in a suite of 13 individual lunar ilmenite and iron grains and in the Kapoeta howardite by solar wind (SW) and solar energetic particle (SEP) irradiation. Isotopic analyses of gases evolved at low temperatures from the lunar grains confirm the neon and argon compositions obtained by Pepin et al. (Pepin R. O., Becker R. H., and Schlutter D. J., “Irradiation records in regolith materials, I: Isotopic compositions of solar-wind neon and argon in single lunar regolith grains”, Geochim. Cosmochim. Acta63, 2145–2162, 1999) in an initial study of 11 regolith grains, primarily ilmenites. Combination of the data sets from both investigations yields 20Ne/22Ne = 13.85 ± 0.04, 21Ne/22Ne = 0.0334 ± 0.0003, and 36Ar/38Ar = 5.80 ± 0.06 for the lunar samples; the corresponding 36Ar/38Ar ratio in Kapoeta is 5.74 ± 0.06. The neon ratios agree well with those measured by Benkert et al. (Benkert J.-P., Baur H., Signer P., and Wieler R., “He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes”, J. Geophys. Res. (Planets)98, 13147–13162, 1993) in gases extracted from bulk lunar ilmenite samples by stepped acid etching and attributed by them to the SW. The 36Ar/38Ar ratios, however, are significantly above both Benkert et al.’s (1993) proposed SW value of 5.48 ± 0.05 and a later estimate of 5.58 ± 0.03 from an acid-etch analysis of Kapoeta (Becker R. H., Schlutter D. J., Rider P. E., and Pepin R. O., “An acid-etch study of the Kapoeta achondrite: Implications for the argon-36/argon-38 ratio in the solar wind”, Meteorit. Planet. Sci.33, 109–113, 1998). We believe, for reasons discussed here and in our earlier report, that 5.80 ± 0.06 ratio most nearly represents the wind composition. The 3He/4He ratio in low-temperature gas releases, not measured in the first particle suite, is found in several grains to be indistinguishable from Benkert et al.’s (1993) SW estimate. Elemental ratios of He, Ne, and Ar initially released from grain-surface SW implantation zones are solar-like, as found earlier by Pepin et al. (1999). Gases evolved from these reservoirs at higher temperatures show evidence for perturbations from solar elemental compositions by prior He loss, thermal mobilization of excess Ne from fractionated SW components, or both. Attention in this second investigation was focused on estimating the isotopic compositions of both the SW and the more deeply sited SEP components in regolith grains. Several high-temperature “isotopic plateaus”—approximately constant isotopic ratios in gas fractions released over a number of consecutive heating steps—were observed in the close vicinities of the SEP ratios for He, Ne, and Ar reported by Benkert et al. (1993). Arguments presented in the text suggest that these plateaus are relatively free of interferences from multicomponent mixing artifacts that can mimic pure component signatures. Average SEP compositions derived from the stepped-heating plateau measurements are in remarkable agreement with the Zürich acid-etch values for all three gases

A partial wave analysis of the (Kˉ0π+π−)(\bar{K}^{0}\pi^{+}\pi^{-}) system produced in K−p→Kˉ0π+π−nK^{-}p \rightarrow \bar{K}^{0}\pi^{+}\pi^{-}n at 10 GeV/c

A partial wave analysis of the non-diffractively produced (K/sup 0/ pi /sup +/ pi /sup -/) system has been performed. The system was produced in the reaction K/sup -/p to K/sup 0/ pi /sup +/ pi /sup -/n at 10 GeV /c, measured in the CERN Omega spectrometer. Besides the well-known K* (1420) resonance, the authors find good evidence for the production of Q/sub 2/(1400) and some indication for Q/sub 1/(1290) production in J /sup P/=1/sup +/. In addition a bump in the 1800 MeV region is clearly observed, the properties of which are discussed

Helium and neon abundances and compositions in cometary matter

International audienc

Helium and Neon in a Stardust Track Wall

Materials trapped and preserved in comets date from the earliest history of the solar system. Here we present and discuss recent measurements of light noble gases carried in a particle captured from comet Wild2 by the Stardust mission