Electrical current and coupled electron-nuclear spin dynamics in double quantum dots

We examine electronic transport in a spin-blockaded double quantum dot. We show that by tuning the strength of the spin-orbit interaction the current flowing through the double dot exhibits a dip at zero magnetic field or a peak at a magnetic field for which the two-electron energy levels anticross. This behaviour is due to the dependence of the singlet-triplet mixing on the field and spin-orbit amplitude. We derive approximate expressions for the current as a function of the amplitudes of the states involved in the transport. We also consider an alternative model that takes into account a finite number of nuclear spins and study the resulting coupled dynamics between electron and nuclear spins. We show that if the spin ensemble is in a thermal state there are regular oscillations in the transient current followed by quasi-chaotic revivals akin to those seen in a thermal Jaynes-Cummings model.Comment: 13 pages, 7 fig

Analysis of Possible Quantum Metastable States in Ballistic Graphene-based Josephson Junctions

Graphene is a relatively new material (2004) made of atomic layers of carbon arranged in a honeycomb lattice. Josephson junction devices are made from graphene by depositing two parallel superconducting leads on a graphene flake. These devices have hysteretic current-voltage characteristics with a supercurrent branch and Shapiro steps appear when irradiated with microwaves. These properties motivate us to investigate the presence of quantum metastable states similar to those found in conventional current-biased Josephson junctions. We present work investigating the nature of these metastable states for ballistic graphene Josephson junctions. We model the effective Washboard potential for these devices and estimate parameters, such as energy level spacing and critical currents, to deduce the design needed to observe metastable states. We propose devices consisting of a parallel on-chip capacitor and suspended graphene. The capacitor is needed to lower the energy level spacing down to the experimentally accessible range of 1-20 GHz. The suspended graphene helps reduce the noise that may otherwise come from two-level states in the insulating oxide layer. Moreover, back-gate voltage control of its critical current introduces another knob for quantum control. We will also report on current experimental progress in the area of fabrication of this proposed device.Comment: 4 pages, 5 figures, Accepted for publication in IEEE Transactions on Applied Superconductivity from ASC 2010. Additional figures, additional calculation

HD 179821 (V1427 Aql, IRAS 19114+0002) -- A Massive Post-Red Supergiant Star?

We have derived elemental abundances of a remarkable star, HD 179821, with unusual composition (e.g. [Na/Fe]=1.0$\pm$0.2 dex) and extra-ordinary spectral characteristics. Its metallicity at [Fe/H]=0.4 dex places it among the most metal-rich stars yet analyzed. The abundance analysis of this luminous star is based on high resolution and high quality (S/N$\approx$120--420) optical echelle spectra from McDonald Observatory and Special Astronomy Observatory. The data includes five years of observations over twenty-one epochs. Standard 1D {\sc LTE} analysis provides a fresh determination of the atmospheric parameters over all epochs: \Teff = 7350$\pm$200 \kelvin, \logg = +0.6$\pm$0.3, and a microturbulent velocity $\xi =$ 6.6$\pm$1.6 km s$^{\rm -1}$ and [Fe/H] = 0.4$\pm$0.2, and a carbon abundance [C/Fe]= $-$0.19$\pm$0.30. We find oxygen abundance [O/Fe]= $-$0.25$\pm$0.28 and an enhancement of 0.9 dex in N. A supersonic macroturbulent velocity of 22.0 $\pm$ 2.0 km s$^{\rm -1}$ is determined from both strong and weak Fe\,{\sc i} and Fe\,{\sc ii} lines. Elemental abundances are obtained for 22 elements. HD 179821 is not enriched in s-process products. Eu is overabundant relative to the anticipated [X/Fe] $\approx$ 0.0. Some peculiarities of its optical spectrum (e.g. variability in the spectral line shapes) is noticed. This includes the line profile variations for H$\alpha$ line. Based on its estimated luminosity, effective temperature and surface gravity, HD 179821 is a massive star evolving to become a red supergiant and finally a Type II supernova.Comment: 19 pages, 8 figures, accepted for publication in MNRA

The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A

The isotope ratio, 85Rb/87Rb, places constraints on models of the nucleosynthesis of heavy elements, but there is no precise determination of the ratio for material beyond the Solar System. We report the first measurement of the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma) that differs significantly from the meteoritic value of 2.59. The Rb/K elemental abundance ratio for the cloud also is lower than that seen in meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values indicates that the interstellar 85Rb abundance in this direction is lower than the Solar System abundance. We attribute the lower abundance to a reduced contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are consistent with much less r-process synthesis for the solar neighborhood compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter

Demonstration of a sterilizable solid rocket motor system

A solid propellant rocket motor containing 60.9 Kg (134-lb) of propellant was successfully static fired after being subjected to eight heat sterilization cycles (three 54-hour cycles plus five 40-hour cycles) at 125 C (257 F). The test motor, a modified SVM-3 chamber, incorporated a flexible grain retention system of EPR rubber to relieve thermal shrinkage stresses. The propellant used in the motor was ANB-3438, and 84 wt% solids system (18 wt% aluminum) containing 66 wt% stabilized ammonium perchlorate oxidizer and a saturated hydroxylterminated polybutadiene binder. Bonding of the propellant to the EPR insulation (GenGard V-4030) was provided by the use of SD-886, an epoxy urethane restriction

U.S. Corn Producer’s Response to Increased Energy Prices: Evidence from Producer Surveys in 2001 and 2005

Crop Production/Industries, Risk and Uncertainty,

Ultraviolet spectroscopy of old novae and symbiotic stars

The IUE spectra are presented for two old novae and for two of the symbiotic variables. Prominent emission line spectra are revealed as a continuum whose appearance is effected by the system inclination. These data provide evidence for hot companions in the symbiotic stars, making plausible the binary model for these peculiar stars. Recent IUE spectra of dwarf novae provide additional support for the existence of optically thick accretion disks in active binary systems. The ultraviolet data of the eclipsing dwarf novae EX Hya and BV Cen appear flatter than for the noneclipsing systems, an effect which could be ascribed to the system inclination

Rubidium and lead abundances in giant stars of the globular clusters M 13 and NGC 6752

We present measurements of the neutron-capture elements Rb and Pb in five giant stars of the globular cluster NGC 6752 and Pb measurements in four giants of the globular cluster M 13. The abundances were derived by comparing synthetic spectra with high resolution, high signal-to-noise ratio spectra obtained using HDS on the Subaru telescope and MIKE on the Magellan telescope. The program stars span the range of the O-Al abundance variation. In NGC 6752, the mean abundances are [Rb/Fe] = -0.17 +/- 0.06 (sigma = 0.14), [Rb/Zr] = -0.12 +/- 0.06 (sigma = 0.13), and [Pb/Fe] = -0.17 +/- 0.04 (sigma = 0.08). In M 13 the mean abundance is [Pb/Fe] = -0.28 +/- 0.03 (sigma = 0.06). Within the measurement uncertainties, we find no evidence for a star-to-star variation for either Rb or Pb within these clusters. None of the abundance ratios [Rb/Fe], [Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 may have slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to the small sample of field stars at the same metallicity. For M 13 and NGC 6752 the Pb abundances are in accord with predictions from a Galactic chemical evolution model. If metal-poor intermediate-mass asymptotic giant branch stars did produce the globular cluster abundance anomalies, then such stars do not synthesize significant quantities of Rb or Pb. Alternatively, if such stars do synthesize large amounts of Rb or Pb, then they are not responsible for the abundance anomalies seen in globular clusters.Comment: Accepted for publication in Ap

Quantum-limited mass flow of liquid 3^{3}He

We consider theoretically the possibility of observing unusual quantum fluid behavior in liquid $^{3}$He and solutions of $^{3}$He in $^{4}$He systems confined to nano-channels. In the case of pure ballistic flow at very low temperature conductance will be quantized in units of $2m^{2}/h$. We show that these steps should be sensitive to increases in temperature. We also use of a random scattering matrix simulation to study flow with diffusive wall scattering. Universal conductance fluctuations analogous to those seen in electron systems should then be observable. Finally we consider the possibility of the cross-over to a one-dimensional system at sufficiently low temperature where the system could form a Luttinger liquid