6,151 research outputs found
Umklapp scattering of pairs in BCS superconductivity theory
The BCS theory of superconductivity is extended to recognize pairing of
electrons by both normal and umklapp scattering. Application of the variational
approach shows that coexistence of normal and umklapp scattering frustrates
superconductivity.Comment: 9 pages, 5 figures. to be published in Journal of Physics: Condensed
Matte
Fractional and unquantized dc voltage generation in THz-driven semiconductor superlattices
We consider the spontaneous creation of a dc voltage across a strongly
coupled semiconductor superlattice subjected to THz radiation. We show that the
dc voltage may be approximately proportional either to an integer or to a half-
integer multiple of the frequency of the applied ac field, depending on the
ratio of the characteristic scattering rates of conducting electrons. For the
case of an ac field frequency less than the characteristic scattering rates, we
demonstrate the generation of an unquantized dc voltage.Comment: 6 pages, 3 figures, RevTEX, EPSF. Revised version v3: corrected typo
New Use for a Corn By-product
Iowa farmers can expect satisfactory results when Iowa-produced hydrol is fed to dairy cows
Symmetry-breaking and chaos in electron transport in semiconductor superlattices
We study the motion of electrons in a single miniband of a semiconductor
superlattice driven by THz electric field polarized along the growth direction.
We work in the semiclassical balance-equation model, including different
elastic and inelastic scattering rates, and incorporating the self-consistent
electric field generated by electron motion. We explore regions of complex
dynamics, which can include chaotic behaviour and symmetry-breaking. We
estimate the magnitudes of dc current and dc voltage that spontaneously appear
in regions of broken-symmetry for parameters characteristic of modern
semiconductor superlattices. This work complements PRL 80(1998)2669 [
cond-mat/9709026 ].Comment: 4 pages, 3 figures, RevTEX, EPS
Z-Selective Cross-Metathesis and Homodimerization of 3E-1,3-Dienes: Reaction Optimization, Computational Analysis, and Synthetic Applications
Olefin metathesis reactions with 3E-1,3-dienes using Z-selective cyclometalated ruthenium benzylidene catalysts are described. In particular, a procedure for employing 3E-1,3-dienes in Z-selective homodimerization and cross-metathesis with terminal alkenes is detailed. The reaction takes advantage of the pronounced chemoselectivity of a recently reported ruthenium-based catalyst containing a cyclometalated NHC ligand for terminal alkenes in the presence of internal E-alkenes. A wide array of commonly encountered functional groups can be tolerated, and only a small excess (1.5 equiv) of the diene coupling partner is required to achieve high yields of the desired internal E,Z-diene cross-metathesis product. Computational studies have been performed to elucidate the reaction mechanism. The computations are consistent with a diene-first pathway. The reaction can be used to quickly assemble structurally complex targets. The power of this cross-metathesis reaction is demonstrated by the concise syntheses of two insect pheromones
The NIRSPEC Brown Dwarf Spectroscopic Survey. I. Low-Resolution Near-Infrared Spectra
We present the first results of a near-infrared (0.96-2.31 micron)
spectroscopic survey of M, L, and T dwarfs obtained with NIRSPEC on the Keck II
telescope. Our new survey has a resolving power of R = 2000 and is comprised of
two major data sets: 53 J-band (1.14-1.36 micron) spectra covering all spectral
types from M6 to T8 with at least two members in each spectral subclass
(wherever possible), and 25 flux-calibrated spectra from 1.14 to 2.31 microns
for most spectral classes between M6 and T8. Sixteen of these 25 objects have
additional spectral coverage from 0.96-1.14 microns to provide overlap with
optical spectra. Spectral flux ratio indexes for prominent molecular bands are
derived and equivalent widths (EWs) for several atomic lines are measured. We
find that a combination of four H2O and two CH4 band strengths can be used for
spectral classification. Weak (EW~1-2 angstrom) atomic lines of Al I and Ca I
disappear at the boundary between M and L types.Comment: 60 pages, 25 figures. To appear in the Astrophysical Journal Vol 596.
Received 2003 March 31; accepted 2003 June 20. Web site at
http://www.astro.ucla.edu/~mclean/BDSSarchiv
A Unified Near Infrared Spectral Classification Scheme for T Dwarfs
A revised near infrared classification scheme for T dwarfs is presented,
based on and superseding prior schemes developed by Burgasser et al. and
Geballe et al., and defined following the precepts of the MK Process. Drawing
from two large spectroscopic libraries of T dwarfs identified largely in the
Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary
spectral standards and five alternate standards spanning spectral types T0 to
T8 are identified that match criteria of spectral character, brightness,
absence of a resolved companion and accessibility from both northern and
southern hemispheres. The classification of T dwarfs is formally made by the
direct comparison of near infrared spectral data of equivalent resolution to
the spectra of these standards. Alternately, we have redefined five key
spectral indices measuring the strengths of the major HO and CH bands
in the 1-2.5 micron region that may be used as a proxy to direct spectral
comparison. Two methods of determining T spectral type using these indices are
outlined and yield equivalent results. These classifications are also
equivalent to those from prior schemes, implying that no revision of existing
spectral type trends is required. The one-dimensional scheme presented here
provides a first step toward the observational characterization of the lowest
luminosity brown dwarfs currently known. Future extensions to incorporate
spectral variations arising from differences in photospheric dust content,
gravity and metallicity are briefly discussed. A compendium of all currently
known T dwarfs with updated classifications is presented.Comment: 52 pages, 11 figures; accepted for publication to Ap
Photonic superdiffusive motion in resonance line radiation trapping - partial frequency redistribution effects
The relation between the jump length probability distribution function and
the spectral line profile in resonance atomic radiation trapping is considered
for Partial Frequency Redistribution (PFR) between absorbed and reemitted
radiation. The single line Opacity Distribution Function [M.N. Berberan-Santos
et.al. J.Chem.Phys. 125, 174308 (2006)] is generalized for PFR and used to
discuss several possible redistribution mechanisms (pure Doppler broadening,
combined natural and Doppler broadening and combined Doppler, natural and
collisional broadening). It is shown that there are two coexisting scales with
a different behavior: the small scale is controlled by the intricate PFR
details while the large scale is essentially given by the atom rest frame
redistribution asymptotic. The pure Doppler and combined natural, Doppler and
collisional broadening are characterized by both small and large scale
superdiffusive Levy flight behaviors while the combined natural and Doppler
case has an anomalous small scale behavior but a diffusive large scale
asymptotic. The common practice of assuming complete redistribution in core
radiation and frequency coherence in the wings of the spectral distribution is
incompatible with the breakdown of superdiffusion in combined natural and
Doppler broadening conditions
Magnetic Fluctuations in a Charge Ordered State of the One-Dimensional Extended Hubbard Model with a Half-Filled Band
Magnetic properties in a charge ordered state are examined for the extended
Hubbard model at half-filling. Magnetic excitations, magnetic susceptibilities
and a nuclear spin relaxation rate are calculated with taking account of
fluctuations around the mean-field solution. The relevance of the present
results to the observation in the 1:1 organic conductors, (TTM-TTP)I, is
discussed.Comment: 4 pages, 3 figures, to be published in J. Phys. Soc. Jpn. Vol.71
(2002) No.
Blue Straggler Stars: Early Observations that Failed to Solve the Problem
In this chapter, I describe early ideas on blue stragglers, and various
observations (some published, some not) that promised but failed to resolve the
question of their origin. I review the data and ideas that were circulating
from Allan Sandage's original discovery in 1953 of "anomalous blue stars" in
the globular cluster M3, up until about 1992, when what seems to have been the
only previous meeting devoted to Blue Straggler Stars (BSSs) was held at the
Space Telescope Science Institute.Comment: Chapter 2, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
- …