2,805 research outputs found
Spatiotemporal discrete multicolor solitons
We have found various families of two-dimensional spatiotemporal solitons in
quadratically nonlinear waveguide arrays. The families of unstaggered odd, even
and twisted stationary solutions are thoroughly characterized and their
stability against perturbations is investigated. We show that the twisted and
even solutions display instability, while most of the odd solitons show
remarkable stability upon evolution.Comment: 18 pages,7 figures. To appear in Physical Review
Magneto-Roton Modes of the Ultra Quantum Crystal: Numerical Study
The Field Induced Spin Density Wave phases observed in quasi-one-dimensional
conductors of the Bechgaard salts family under magnetic field exhibit both Spin
Density Wave order and a Quantized Hall Effect, which may exhibit sign
reversals. The original nature of the condensed phases is evidenced by the
collective mode spectrum. Besides the Goldstone modes, a quasi periodic
structure of Magneto-Roton modes, predicted to exist for a monotonic sequence
of Hall Quantum numbers, is confirmed, and a second mode is shown to exist
within the single particle gap. We present numerical estimates of the
Magneto-Roton mode energies in a generic case of the monotonic sequence. The
mass anisotropy of the collective mode is calculated. We show how differently
the MR spectrum evolves with magnetic field at low and high fields. The
collective mode spectrum should have specific features, in the sign reversed
"Ribault Phase", as compared to modes of the majority sign phases. We
investigate numerically the collective mode in the Ribault Phase.Comment: this paper incorporates material contained in a previous cond-mat
preprint cond-mat/9709210, but cannot be described as a replaced version,
because it contains a significant amount of new material dealing with the
instability line and with the topic of Ribault Phases. It contains 13 figures
(.ps files
Development of a dynamic pressure calibration technique
The report deals with work continuing on the development of a method of producing sinusoidally varying pressures of at least 34 kPa zero-to-peak with amplitude variations within plus or minus 5% up to 2 kHz for the dynamic calibration of pressure transducers. Sinusoidally varying pressures of 34 kPa zero-to-peak were produced between 40 Hz and 750 Hz by vibrating a 10-cm column of a dimethyl siloxane liquid at 36gn zero-to-peak. Damping of the liquid column was accomplished by packing the fixture tube with a number of smaller diameter tubes
Tunable Graphene Antennas for Selective Enhancement of THz-Emission
In this paper, we will introduce THz graphene antennas that strongly enhance
the emission rate of quantum systems at specific frequencies. The tunability of
these antennas can be used to selectively enhance individual spectral features.
We will show as an example that any weak transition in the spectrum of coronene
can become the dominant contribution. This selective and tunable enhancement
establishes a new class of graphene-based THz devices, which will find
applications in sensors, novel light sources, spectroscopy, and quantum
communication devices
Development of dynamic calibration methods for POGO pressure transducers
Two dynamic pressure sources are described for the calibration of pogo pressure transducers used to measure oscillatory pressures generated in the propulsion system of the space shuttle. Rotation of a mercury-filled tube in a vertical plane at frequencies below 5 Hz generates sinusoidal pressures up to 48 kPa, peak-to-peak; vibrating the same mercury-filled tube sinusoidally in the vertical plane extends the frequency response from 5 Hz to 100 Hz at pressures up to 140 kPa, peak-to-peak. The sinusoidal pressure fluctuations can be generated by both methods in the presence of high pressures (bias) up to 55 MPa. Calibration procedures are given in detail for the use of both sources. The dynamic performance of selected transducers was evaluated using these procedures; the results of these calibrations are presented. Calibrations made with the two sources near 5 Hz agree to within 3% of each other
A dynamic pressure source for the calibration of pressure transducers
A dynamic pressure source is described for producing sinusoidally varying pressures of up to 34 kPa zero to peak, over the frequency range of approximately 50 Hz to 2 kHz. The source is intended for the dynamic calibration of pressure transducers. The transducer to be calibrated is mounted near the base of the thick walled aluminum tube forming the vessel so that the pressure sensitive element is in contact with the liquid in the tube. A section of the tube is filled with small steel balls to damp the motion of the 10-St dimethyl siloxane working fluid in order to extend the useful frquency range to higher frequencies than would be provided by an undamped system. The dynamic response of six transducers provided by the sponsor was evaluated using the pressure sources; the results of these calibrations are given
Phonon-mediated tuning of instabilities in the Hubbard model at half-filling
We obtain the phase diagram of the half-filled two-dimensional Hubbard model
on a square lattice in the presence of Einstein phonons. We find that the
interplay between the instantaneous electron-electron repulsion and
electron-phonon interaction leads to new phases. In particular, a
d-wave superconducting phase emerges when both anisotropic phonons
and repulsive Hubbard interaction are present. For large electron-phonon
couplings, charge-density-wave and s-wave superconducting regions also appear
in the phase diagram, and the widths of these regions are strongly dependent on
the phonon frequency, indicating that retardation effects play an important
role. Since at half-filling the Fermi surface is nested, spin-density-wave is
recovered when the repulsive interaction dominates. We employ a functional
multiscale renormalization-group method that includes both electron-electron
and electron-phonon interactions, and take retardation effects fully into
account.Comment: 8 pages, 5 figure
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