3,554 research outputs found
Heterogeneous perturbations in the Doppler-free S1 ← S0 two-photon spectrum of benzene: Evidence for intrastate coupling
Rotational perturbations are identified in Doppler-free two-photon spectra of the 1410 and 1410110 vibronic bands in C6H6. Evidence is found that Coriolis coupling between some of the rotational levels of two distinct vibrational states within S1 is the mechanism responsible. This coupling mechanism is thought to be responsible for irreversible intramolecular relaxation at higher excess energies and higher vibrational state densities
Investigation of direct solar-to-microwave energy conversion techniques
Identification of alternative methods of producing microwave energy from solar radiation for purposes of directing power to the Earth from space is investigated. Specifically, methods of conversion of optical radiation into microwave radiation by the most direct means are investigated. Approaches based on demonstrated device functioning and basic phenomenologies are developed. There is no system concept developed, that is competitive with current baseline concepts. The most direct methods of conversion appear to require an initial step of production of coherent laser radiation. Other methods generally require production of electron streams for use in solid-state or cavity-oscillator systems. Further development is suggested to be worthwhile for suggested devices and on concepts utilizing a free-electron stream for the intraspace station power transport mechanism
Carbon Ignition in Type Ia Supernovae: An Analytic Model
The observable properties of a Type Ia supernova are sensitive to how the
nuclear runaway ignites in a Chandrasekhar mass white dwarf - at a single point
at its center, off-center, or at multiple points and times. We present a simple
analytic model for the runaway based upon a combination of stellar
mixing-length theory and recent advances in understanding Rayleigh-Benard
convection. The convective flow just prior to runaway is likely to have a
strong dipolar component, though higher multipoles may contribute appreciably
at the very high Rayleigh number (10) appropriate to the white dwarf
core. A likely outcome is multi-point ignition with an exponentially increasing
number of ignition points during the few tenths of a second that it takes the
runaway to develop. The first sparks ignite approximately 150 - 200 km off
center, followed by ignition at smaller radii. Rotation may be important to
break the dipole asymmetry of the ignition and give a healthy explosion.Comment: 14 pages, 0 figures, submitted to ApJ, corrected typo in first
author's nam
The fragmentation of expanding shells III: Oligarchic accretion and the mass spectrum of fragments
We use SPH simulations to investigate the gravitational fragmentation of
expanding shells through the linear and non--linear regimes. The results are
analysed using spherical harmonic decomposition to capture the initiation of
structure during the linear regime; the potential-based method of Smith et al.
(2009) to follow the development of clumps in the mildly non-linear regime; and
sink particles to capture the properties of the final bound objects during the
highly non-linear regime. In the early, mildly non--linear phase of
fragmentation, we find that the clump mass function still agrees quite well
with the mass function predicted by the analytic model. However, the sink mass
function is quite different, in the sense of being skewed towards high-mass
objects. This is because, once the growth of a condensation becomes non-linear,
it tends to be growing non-competitively from its own essentially separate
reservoir; we call this Oligarchic Accretion.Comment: 14 pages, accepted for publication in MNRA
Dynamical polarization, screening, and plasmons in gapped graphene
The one-loop polarization function of graphene has been calculated at zero
temperature for arbitrary wavevector, frequency, chemical potential (doping),
and band gap. The result is expressed in terms of elementary functions and is
used to find the dispersion of the plasmon mode and the static screening within
the random phase approximation. At long wavelengths the usual square root
behaviour of plasmon spectra for two-dimensional (2D) systems is obtained. The
presence of a small (compared to a chemical potential) gap leads to the
appearance of a new undamped plasmon mode. At greater values of the gap this
mode merges with the long-wavelength one, and vanishes when the Fermi level
enters the gap. The screening of charged impurities at large distances differs
from that in gapless graphene by slower decay of Friedel oscillations (
instead of ), similarly to conventional 2D systems.Comment: 8 pages, 8 figures, v2: to match published versio
Pulsed Doppler-free two-photon spectroscopy of polyatomic molecules
Doppler-free two-photon electronic spectra of a large polyatomic molecule are recorded for the first time with pulsed laser radiation of near Fourier-transform limited bandwidth (Δvnot, vert, similar100 MHz). The resolution obtained is sufficient to resolve individual rotational lines. Due to the high density of these rotational transitions a strong Doppler-broadened background is observed, which is, however, subtantially reduced by suitable choice of photon polarizations. Different vibronic bands of benzene (C6H6) are investigated and very accurate rotational constants are found
Neural-Network Vector Controller for Permanent-Magnet Synchronous Motor Drives: Simulated and Hardware-Validated Results
This paper focuses on current control in a permanentmagnet synchronous motor (PMSM). The paper has two main objectives: The first objective is to develop a neural-network (NN) vector controller to overcome the decoupling inaccuracy problem associated with conventional PI-based vector-control methods. The NN is developed using the full dynamic equation of a PMSM, and trained to implement optimal control based on approximate dynamic programming. The second objective is to evaluate the robust and adaptive performance of the NN controller against that of the conventional standard vector controller under motor parameter variation and dynamic control conditions by (a) simulating the behavior of a PMSM typically used in realistic electric vehicle applications and (b) building an experimental system for hardware validation as well as combined hardware and simulation evaluation. The results demonstrate that the NN controller outperforms conventional vector controllers in both simulation and hardware implementation
Double Quantum Dots in Carbon Nanotubes
We study the two-electron eigenspectrum of a carbon-nanotube double quantum
dot with spin-orbit coupling. Exact calculation are combined with a simple
model to provide an intuitive and accurate description of single-particle and
interaction effects. For symmetric dots and weak magnetic fields, the
two-electron ground state is antisymmetric in the spin-valley degree of freedom
and is not a pure spin-singlet state. When double occupation of one dot is
favored by increasing the detuning between the dots, the Coulomb interaction
causes strong correlation effects realized by higher orbital-level mixing.
Changes in the double-dot configuration affect the relative strength of the
electron-electron interactions and can lead to different ground state
transitions. In particular, they can favor a ferromagnetic ground state both in
spin and valley degrees of freedom. The strong suppression of the energy gap
can cause the disappearance of the Pauli blockade in transport experiments and
thereby can also limit the stability of spin-qubits in quantum information
proposals. Our analysis is generalized to an array of coupled dots which is
expected to exhibit rich many-body behavior.Comment: 14 pages, 11 pages and 1 table. Typos in text and Figs.4 and 6
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