681 research outputs found
Determining Ages of APOGEE Giants with Known Distances
We present a sample of local red giant stars observed using the New Mexico
State University 1 m telescope with the APOGEE spectrograph, for which we
estimate stellar ages and the age distribution from the high-resolution
spectroscopic stellar parameters and accurate distance measurements from
Hipparcos. The high-resolution (R ~ 23,000), near infrared (H-band, 1.5-1.7
micron) APOGEE spectra provide measurements of the stellar atmospheric
parameters (temperature, surface gravity, [M/H], and [alpha/M]). Due to the
smaller uncertainties in surface gravity possible with high-resolution spectra
and accurate Hipparcos distance measurements, we are able to calculate the
stellar masses to within 40%. For red giants, the relatively rapid evolution of
stars up the red giant branch allows the age to be constrained based on the
mass. We examine methods of estimating age using both the mass-age relation
directly and a Bayesian isochrone matching of measured parameters, assuming a
constant star formation history (SFH). To improve the prior on the SFH, we use
a hierarchical modeling approach to constrain the parameters of a model SFH
from the age probability distribution functions of the data. The results of an
alpha dependent Gaussian SFH model shows a clear relation between age and
[alpha/M] at all ages. Using this SFH model as the prior for an empirical
Bayesian analysis, we construct a full age probability distribution function
and determine ages for individual stars. The age-metallicity relation is flat,
with a slight decrease in [M/H] at the oldest ages and a ~ 0.5 dex spread in
metallicity. For stars with ages < 1 Gyr we find a smaller spread, consistent
with radial migration having a smaller effect on these young stars than on the
older stars.Comment: 14 page, 18 figures, accepted to ApJ with minor revisions, full
electronic table of data available upon publicatio
Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density.Comment: 7 pages with 3 figures + supplemental material (4 pages and 5
figures
Ant Colony Search Algorithm for Optimal Generators Startup during Power System Restoration
Generators startup sequence plays a significant role in achieving a suitable and effective restoration strategy. This paper outlines an ant colony search algorithm in order to determine the generator starting times during the bulk power system restoration. The algorithm attempts to maximize the system generation capability over a restoration period, where the dynamic characteristics of different types of units and system constraints are considered. Applying this method for the 39-bus New England test system, and comparing the results with backtracking-search and P/t methods, it is found that proposed algorithm improved generation capability
Application of the ant colony search algorithm to reactive power pricing in an open electricity market
a b s t r a c t Reactive power management is essential to transfer real energy and support power system security. Developing an accurate and feasible method for reactive power pricing is important in the electricity market. In conventional optimal power flow models the production cost of reactive power was ignored. In this paper, the production cost of reactive power and investment cost of capacitor banks were included into the objective function of the OPF problem. Then, using ant colony search algorithm, the optimal problem was solved. Marginal price theory was used for calculation of the cost of active and reactive power at each bus in competitive electric markets. Application of the proposed method on IEEE 14-bus system confirms its validity and effectiveness. Results from several case studies show clearly the effects of various factors on reactive power price
A New Approach to Nonsinusoidal Steady-State Power System Analysis
A new analysis method using wavelet domain for steady-state operating condition of power system is developed and introduced. Based on wavelet-Galerkin theory, the system components such as resistor, inductor, capacitor, transmission lines, and switching devices are modeled in discrete wavelet domain for the purpose of steady-state analysis. To solve system equations, they are transferred to wavelet domain by forming algebraic matrix-vector relations using the wavelet transform coefficients and the equivalent circuit is thus built for system simulation. After describing the new algorithm, two-case studies are presented and compared with the simulations in the time domain to verify the accuracy and computational performance
Multi-terminal spin-dependent transport in ballistic carbon nanotubes
We study theoretically nonlocal spin transport in a ballistic carbon nanotube
contacted to two ferromagnetic leads and two normal-metal leads. When the
magnetizations of the two ferromagnets are changed from a parallel to an
antiparallel configuration, the circuit shows a hysteretic behavior which is
specific to the few-channel regime. In the coherent limit, the amplitude of the
magnetic signals is strongly enhanced due to resonance effects occurring inside
the nanotube. Our calculations pave the way for experiments on low-dimensional
nonlocal spin transport, which should give results remarkably different from
the experiments realized so far in the multichannel diffusive incoherent
regime.Comment: 9 pages, 8 figure
Competition between electron pairing and phase coherence in superconducting interfaces
In LaAlO3/SrTiO3 heterostructures, a gate tunable superconducting electron gas is confined in a quantum well at the interface between two insulating oxides. Remarkably, the gas coexists with both magnetism and strong Rashba spin–orbit coupling. However, both the origin of superconductivity and the nature of the transition to the normal state over the whole doping range remain elusive. Here we use resonant microwave transport to extract the superfluid stiffness and the superconducting gap energy of the LaAlO3/SrTiO3 interface as a function of carrier density. We show that the superconducting phase diagram of this system is controlled by the competition between electron pairing and phase coherence. The analysis of the superfluid density reveals that only a very small fraction of the electrons condenses into the superconducting state. We propose that this corresponds to the weak filling of high- energy dxz/dyz bands in the quantum well, more apt to host superconductivity
Structural recovery of ion implanted ZnO nanowires
5 pagesInternational audienceIon implantation is an interesting method to dope semiconducting materials such as zinc oxide provided that the implantation-induced defects can be subsequently removed. Nitrogen implantation followed by anneals under O2 were carried out on zinc oxide nanowires in the same conditions as in a previous study on bulk ZnO [J. Appl.Phys. 109, 023513 (2011)], allowing a direct comparison of the defect recovery mechanisms. Transmission electron microscopy and cathodoluminescence were carried out to assess the effects of nitrogen implantation and of subsequent anneals on the structural and optical properties of ZnO nanowires. Defect recovery is shown to be more effective in nanowires compared with bulk material due to the proximity of free surfaces. Nevertheless, the optical emission of implanted and annealed nanowires deteriorated compared to as-grown nanowires, as also observed for unimplanted and annealed nanowires. This is tentatively attributed to the dissociation of excitons in the space charge region induced by O2 adsorption on the nanowire surface
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