36,917 research outputs found
An ab initio HCN and HNC rotational vibrationallinelist for astronomy
We present a new ab initio HCN/HNC linelist. The linelist forms themost accurate and extensive HCN/HNC data set presently in existence.The data contain rotation vibration line frequencies and line strengths,for all transitions that are not rigorously dipole forbidden, between stateswith energy less than 18 000 cm-1 and with J ? 60
A review of fracture mechanics life technology
Current lifetime prediction technology for structural components subjected to cyclic loads was reviewed. The central objectives of the project were to report the current state of and recommend future development of fracture mechanics-based analytical tools for modeling and forecasting subcritical fatigue crack growth in structures. Of special interest to NASA was the ability to apply these tools to practical engineering problems and the developmental steps necessary to bring vital technologies to this stage. A survey of published literature and numerous discussions with experts in the field of fracture mechanics life technology were conducted. One of the key points made is that fracture mechanics analyses of crack growth often involve consideration of fatigue and fracture under extreme conditions. Therefore, inaccuracies in predicting component lifetime will be dominated by inaccuracies in environment and fatigue crack growth relations, stress intensity factor solutions, and methods used to model given loads and stresses. Suggestions made for reducing these inaccuracies include: development of improved models of subcritical crack growth, research efforts aimed at better characterizing residual and assembly stresses that can be introduced during fabrication, and more widespread and uniform use of the best existing methods
Evaluation of active control technology for short haul aircraft
An evaluation of the economics of short-haul aircraft designed with active controls technology and low wing-loading to achieve short field performance with good ride quality is presented. Results indicate that for such a system incorporating gust load alleviation and augmented stability the direct operating cost is better than for aircraft without active controls
Interacting Dark Resonances: Interference Effects Induced by Coherently Altered Quantum Superpositions
We predict the possibility of sharp, high-contrast resonances in the optical
response of a broad class of systems, wherein interference effects are
generated by coherent perturbation or interaction of dark states. The
properties of these resonances can be manipulated to design a desired atomic
response.Comment: 4 pages, 3 figures, RevTeX, submitted to PRL; changed three numbers
in Fig. 3 (caption
Photonic band gap via quantum coherence in vortex lattices of Bose gases
We investigate the optical response of an atomic Bose-Einstein condensate
with a vortex lattice. We find that it is possible for the vortex lattice to
act as a photonic crystal and create photonic band gaps, by enhancing the
refractive index of the condensate via a quantum coherent scheme. If high
enough index contrast between the vortex core and the atomic sample is
achieved, a photonic band gap arises depending on the healing length and the
lattice spacing. A wide range of experimentally accessible parameters are
examined and band gaps in the visible region of the electromagnetic spectrum
are found. We also show how directional band gaps can be used to directly
measure the rotation frequency of the condensate.Comment: 4 pages, 4 figures, Final version to appear in PR
Multi-band quantum ratchets
We investigate directed motion in non-adiabatically rocked ratchet systems
sustaining few bands below the barrier. Upon restricting the dynamics to the
lowest M bands, the total system-plus-bath Hamiltonian is mapped onto a
discrete tight-binding model containing all the information both on the intra-
and inter-well tunneling motion. A closed form for the current in the
incoherent tunneling regime is obtained. In effective single-band ratchets, no
current rectification occurs. We apply our theory to describe rectification
effects in vortex quantum ratchets devices. Current reversals upon variation of
the ac-field amplitude or frequency are predicted.Comment: Accepted for publication in Physical Review Letter
Diluted Networks of Nonlinear Resistors and Fractal Dimensions of Percolation Clusters
We study random networks of nonlinear resistors, which obey a generalized
Ohm's law, . Our renormalized field theory, which thrives on an
interpretation of the involved Feynman Diagrams as being resistor networks
themselves, is presented in detail. By considering distinct values of the
nonlinearity r, we calculate several fractal dimensions characterizing
percolation clusters. For the dimension associated with the red bonds we show
that at least to order {\sl O} (\epsilon^4),
with being the correlation length exponent, and , where d
denotes the spatial dimension. This result agrees with a rigorous one by
Coniglio. Our result for the chemical distance, d_{\scriptsize min} = 2 -
\epsilon /6 - [ 937/588 + 45/49 (\ln 2 -9/10 \ln 3)] (\epsilon /6)^2 + {\sl O}
(\epsilon^3) verifies a previous calculation by one of us. For the backbone
dimension we find D_B = 2 + \epsilon /21 - 172 \epsilon^2 /9261 + 2 (- 74639 +
22680 \zeta (3))\epsilon^3 /4084101 + {\sl O} (\epsilon^4), where , in agreement to second order in with a two-loop
calculation by Harris and Lubensky.Comment: 29 pages, 7 figure
Towards a Microscopic Model of Magnetoelectric Interactions in Ni3V2O8
We develop a microscopic magnetoelectric coupling in NiVO (NVO)
which gives rise to the trilinear phenomenological coupling used previously to
explain the phase transition in which magnetic and ferroelectric order
parameters appear simultaneously. Using combined neutron scattering
measurements and first-principles calculations of the phonons in NVO, we
determine eleven phonons which can induce the observed spontaneous
polarization. Among these eleven phonons, we find that a few of them can
actually induce a significant dipole moment. Using the calculated atomic
charges, we find that the required distortion to induce the observed dipole
moment is very small (~0.001 \AA) and therefore it would be very difficult to
observe the distortion by neutron-powder diffraction. Finally, we identify the
derivatives of the exchange tensor with respect to atomic displacements which
are needed for a microscopic model of a spin-phonon coupling in NVO and which
we hope will be obtained from a fundamental quantum calculation such as LDA+U.
We also analyze two toy models to illustrate that the Dzyaloskinskii-Moriya
interaction is very important for coexisting of magnetic and ferroelectric
order but it is not the only mechanism when the local site symmetry of the
system is low enough.Comment: 20 pages, 10 figure
Quantum spin liquids: a large-S route
This paper explores the large-S route to quantum disorder in the Heisenberg
antiferromagnet on the pyrochlore lattice and its homologues in lower
dimensions. It is shown that zero-point fluctuations of spins shape up a
valence-bond solid at low temperatures for one two-dimensional lattice and a
liquid with very short-range valence-bond correlations for another. A
one-dimensional model demonstrates potential significance of quantum
interference effects (as in Haldane's gap): the quantum melting of a
valence-bond order yields different valence-bond liquids for integer and
half-integer values of S.Comment: Proceedings of Highly Frustrated Magnetism 2003 (Grenoble), 6 LaTeX
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