3,686 research outputs found
Predicted Abundances of Carbon Compounds in Volcanic Gases on Io
We use chemical equilibrium calculations to model the speciation of carbon in
volcanic gases on Io. The calculations cover wide temperature (500-2000 K),
pressure (10^-8 to 10^+2 bars), and composition ranges (bulk O/S atomic ratios
\~0 to 3), which overlap the nominal conditions at Pele (1760 K, 0.01 bar, O/S
~ 1.5). Bulk C/S atomic ratios ranging from 10^-6 to 10^-1 in volcanic gases
are used with a nominal value of 10^-3 based upon upper limits from Voyager for
carbon in the Loki plume on Io. Carbon monoxide and CO2 are the two major
carbon gases under all conditions studied. Carbonyl sulfide and CS2 are orders
of magnitude less abundant. Consideration of different loss processes
(photolysis, condensation, kinetic reactions in the plume) indicates that
photolysis is probably the major loss process for all gases. Both CO and CO2
should be observable in volcanic plumes and in Io's atmosphere at abundances of
several hundred parts per million by volume for a bulk C/S ratio of 10^-3.Comment: 21 pages, 4 figures, 4 tables; accepted by Astrophysical Journa
Dynamic phenomena in superconducting oxides by ESR
Dynamic electron spin resonance (ESR) measurements compare the paramagnetic and antiferromagnetic (AF) properties of superconducting oxides in the range 4 K to room temperature, at 8 MHz and 9.36 GHz. Two are derivatives of YBa2Cu30 7: 1: Nd(Nd0.05Ba0.95 )2Cu30 7, Te0 =72 K and II: Y0.2Cao.8Sr2[Cu2(Tlo.5Pb0.5 )]07, Te0 =108 K and two are cases where AF ordering dominates the weak superconductivity: III: Nb01.1\u3e 1. 25 ~Teo~ 10 K and IV: La2Ni04.00, 70 K :::: Teo:::: 40 K. At temperatures 298:::: T:::: 64 K, the ESR absorption by I indicates orthorhombic symmetry. The peaks at Ke =2.06, gb =2.13, and Ka =2.24 are identified with the presence of 5% Nd3+( 41912 ) in the Ba layer because the characteristic Cu2+ impurity hyperfine structure is absent and the ESR signal disappears several degrees below Te. Near Te the ESR absorption is reduced by two orders of magnitude. Proximity effects give rise to interference fringes with period r1 ( T) independent of the field B and the rate of sweep dBzldt. ESR is observed below Te because flux penetrates the superconductor. The temperature dependence of r1 leads to an activation energy for the flux motion E0 (1)/R ~ 16 K and Ea (111)/R ~3 K =Te /4. In the superconducting state a coherent flux expulsion response to a change in B. from 500 mT to zero is observed in times T, = 8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the samples are cooled through Te in a magnetic field, varies from Tnoise=3.5 s for III to 21 s for IV. The microwave absorption spectra identify three temperature regimes: (i) For 3.5 K \u3c T \u3c T m T* \u3c Teo superconducting behavior was confirmed by the energy loss near zero magnetic field and the kinetics of high-field noise due to flux expulsion. Near g =2.00 ESR absorption is observed for all materials. A broad absorption near 50 to 100 mT at 9.36 GHz has been attributed to AF resonance. (ii) T m T* ~ T ~ Te identifies the range where flux motion gives rise to interference fringes in the ESR absorption. (iii) ESR and AF resonance are observed immediately after warming above Tc
Zener double exchange from local valence fluctuations in magnetite
Magnetite (FeO) is a mixed valent system where electronic
conductivity occurs on the B-site (octahedral) iron sublattice of the spinel
structure. Below K, a metal-insulator transition occurs which is
argued to arise from the charge ordering of 2+ and 3+ iron valences on the
B-sites (Verwey transition). Inelastic neutron scattering measurements show
that optical spin waves propagating on the B-site sublattice (80 meV) are
shifted upwards in energy above due to the occurrence of B-B
ferromagnetic double exchange in the mixed valent metallic phase. The double
exchange interaction affects only spin waves of symmetry, not all
modes, indicating that valence fluctuations are slow and the double exchange is
constrained by electron correlations above .Comment: 4 pages, 5 figure
High sensitivity sensor for moderate pressures
The metal-insulator transition of (V_(0.99)Ti_(0.01))_2O_3 is marked by dramatic changes in the electrical resistivity and the magnetic susceptibility, with a linear pressure variation of -6.06 K/kbar for P≤15 kbar. We propose its use as the sensing element of a manometer in applications where the superconducting transition of soft metals has been traditional
Development of portable NMR polarimeter system for polarized HD target
A portable NMR polarimeter system has been developed to measure the
polarization of a polarized Hydrogen-Deuteride (HD) target for hadron
photoproduction experiments at SPring-8. The polarized HD target is produced at
the Research Center for Nuclear Physics (RCNP), Osaka university and is
transported to SPring-8. The HD polarization should be monitored at both
places. We have constructed the portable NMR polarimeter system by replacing
the devices in the conventional system with the software system with PCI
eXtensions for Instrumentation (PXI). The weight of the NMR system is downsized
from 80 kg to 7 kg, and the cost is reduced to 25%. We check the performance of
the portable NMR polarimeter system. The signal-to-noise (S/N) ratio of the NMR
signal for the portable system is about 50% of that for the conventional NMR
system. This performance of the portable NMR system is proved to be compatible
with the conventional NMR system for the polarization measurement.Comment: 6 page, 8 figures, 2011/Mar/9 Replace Author
Characterization of nanometer-sized, mechanically exfoliated graphene on the H-passivated Si(100) surface using scanning tunnelling microscopy
We have developed a method for depositing graphene monolayers and bilayers
with minimum lateral dimensions of 2-10 nm by the mechanical exfoliation of
graphite onto the Si(100)-2x1:H surface. Room temperature, ultra-high vacuum
(UHV) tunnelling spectroscopy measurements of nanometer-sized single-layer
graphene reveal a size dependent energy gap ranging from 0.1-1 eV. Furthermore,
the number of graphene layers can be directly determined from scanning
tunnelling microscopy (STM) topographic contours. This atomistic study provides
an experimental basis for probing the electronic structure of nanometer-sized
graphene which can assist the development of graphene-based nanoelectronics.Comment: Accepted for publication in Nanotechnolog
Mass enhancement and magnetic order at the Mott-Hubbard transition
We study the evolution with pressure P and band filling y of the heat capacity, Hall coefficient, and resistivity at the approach to the T→0 Mott-Hubbard metal-insulator transition (MIT) in highly correlated V_(2-y)O_3. Under P, the electronic effective mass m* diverges at the MIT with a negligible change in carrier concentration n away from half-filling. Conversely, in the doped system m* actually decreases as the MIT is approached, while n increases linearly with y. The low-T magnetic order in the metal helps us deconvolute contributions from charge correlations and spin fluctuations
Effect of correlations and disorder on electron states in the Mott-Hubbard insulator V_2O_3
We compare vanadium-deficient (nonstoichiometric) and titanium-doped vanadium sesquioxide through measurements of the electrical resistivity at a series of hydrostatic pressures, the magnetic susceptibility, and the low-temperature specific heat: all as a function of T. The pressure dependence of the critical temperature for this discontinuous metal-antiferromagnetic-insulator transition as well as the temperature dependence of the magnetic susceptibility track in the two cases. However, the pressure dependence of the Hubbard gap, the slower than exponential form of the low-temperature resistivity, and the concentration of two-level systems are markedly different for V_(1.9967)O_3 and (V_(0.99)Ti_(0.01))_2O_3. We discuss our results in terms of the intra-atomic Coulomb repulsion, which is of comparable magnitude to the bare bandwidth of the vanadium 3d states. The band splitting in the antiferromagnetic insulating state is argued to cross over to a Slater-type splitting between the subbands narrowed by correlations with a sufficient degree of oxygen nonstoichiometry or Ti doping
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