2,076 research outputs found
A Study of Fluctuations In Coyote Numbers and Possible Causes
This paper reports the findings of a study of coyote numbers, with particular reference to fluctuations, their periodicity, if any, and their possible causes
Sherwin B. Nuland: 10-04-2000
The non-fiction writer, surgeon, and former professor at Yale School of Medicine Sherwin B. Nuland begins the interview by reading a passage from his book The Mystery Within. The interview continues with Nuland discussing his writing process and how English gave him autonomy, as no one in his household knew the language. He goes on to discuss the reaction his book received and ends the interview by discussing how uncertainty has influenced his career as a surgeon and as a writer.https://digitalcommons.brockport.edu/writers_videos/1000/thumbnail.jp
Electromagnetic Fields in Jets
The magnetic fields and energy flows in an astronomical jet described by our
earlier model are calculated in detail. Though the field distribution varies
with the external pressure function p(z), it depends only weakly on the other
boundary conditions. Individual fieldlines were plotted; the lines become
nearly vertical at the bottom and are twisted at the top. An animation of a
fieldline's motion was made, which shows the line being wound up by the
accretion disc's differential rotation and rising as a result of this. The
distribution of Poynting flux within the jet indicates that much of the energy
flows up the jet from the inside of the accretion disc but a substantial
fraction flows back down to the outside.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Optical frequency combs from high-order sideband generation
We report on the generation of frequency combs from the recently-discovered
phenomenon of high-order sideband generation (HSG). A near-band gap
continuous-wave (cw) laser with frequency was transmitted
through an epitaxial layer containing GaAs/AlGaAs quantum wells that were
driven by quasi-cw in-plane electric fields between 4 and 50
kV/cm oscillating at frequencies between 240 and 640 GHz.
Frequency combs with teeth at
( even) were produced, with maximum reported , corresponding to a
maximum comb span THz. Comb spectra with the identical product
were found to have similar spans and shapes
in most cases, as expected from the picture of HSG as a scattering-limited
electron-hole recollision phenomenon. The HSG combs were used to measure the
frequency and linewidth of our THz source as a demonstration of potential
applications
Relaminarisation of Re_Ï„=100 channel flow with globally stabilising linear feedback control
The problems of nonlinearity and high dimension have so far prevented a complete solution of the control of turbulent flow. Addressing the problem of nonlinearity, we propose a flow control strategy which ensures that the energy of any perturbation to the target profile decays monotonically. The controller’s estimate of the flow state is similarly guaranteed to converge to the true value. We present a one-time off-line synthesis procedure, which generalises to accommodate more restrictive actuation and sensing arrangements, with conditions for existence for the controller given in this case. The control is tested in turbulent channel flow (Re_τ = 100) using full-domain sensing and actuation on the wall-normal velocity. Concentrated at the point of maximum inflection in the mean profile, the control directly counters the supply of turbulence energy arising from the interaction of the wall-normal perturbations with the flow shear. It is found that the control is only required for the larger-scale motions, specifically those above the scale of the mean streak spacing. Minimal control effort is required once laminar flow is achieved. The response of the near-wall flow is examined in detail, with particular emphasis on the pressure and wall-normal velocity fields, in the context of Landahl’s theory of sheared turbulence
Relaminarisation of Re_{\tau} = 100 channel flow with globally stabilising linear feedback control
The problems of nonlinearity and high dimension have so far prevented a
complete solution of the control of turbulent flow. Addressing the problem of
nonlinearity, we propose a flow control strategy which ensures that the energy
of any perturbation to the target profile decays monotonically. The
controller's estimate of the flow state is similarly guaranteed to converge to
the true value. We present a one-time off-line synthesis procedure, which
generalises to accommodate more restrictive actuation and sensing arrangements,
with conditions for existence for the controller given in this case. The
control is tested in turbulent channel flow () using full-domain
sensing and actuation on the wall-normal velocity. Concentrated at the point of
maximum inflection in the mean profile, the control directly counters the
supply of turbulence energy arising from the interaction of the wall-normal
perturbations with the flow shear. It is found that the control is only
required for the larger-scale motions, specifically those above the scale of
the mean streak spacing. Minimal control effort is required once laminar flow
is achieved. The response of the near-wall flow is examined in detail, with
particular emphasis on the pressure and wall-normal velocity fields, in the
context of Landahl's theory of sheared turbulence
Dynamical birefringence: Electron-hole recollisions as probes of Berry curvature
The direct measurement of Berry phases is still a great challenge in
condensed matter systems. The bottleneck has been the ability to adiabatically
drive an electron coherently across a large portion of the Brillouin zone in a
solid where the scattering is strong and complicated. We break through this
bottleneck and show that high-order sideband generation (HSG) in semiconductors
is intimately affected by Berry phases. Electron-hole recollisions and HSG
occur when a near-band gap laser beam excites a semiconductor that is driven by
sufficiently strong terahertz (THz)-frequency electric fields. We carried out
experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum
wells. The observed HSG spectra contain sidebands up to the 90th order, to our
knowledge the highest-order optical nonlinearity observed in solids. The
highest-order sidebands are associated with electron-hole pairs driven
coherently across roughly 10% of the Brillouin zone around the \Gamma point.
The principal experimental claim is a dynamical birefringence: the sidebands,
when the order is high enough (> 20), are usually stronger when the exciting
near-infrared (NIR) and the THz electric fields are polarized perpendicular
than parallel; the sideband intensities depend on the angles between the THz
field and the crystal axes in samples with sufficiently weak quenched disorder;
and the sidebands exhibit significant ellipticity that increases with
increasing sideband order, despite nearly linear excitation and driving fields.
We explain dynamical birefringence by generalizing the three-step model for
high order harmonic generation. The hole accumulates Berry phases due to
variation of its internal state as the quasi-momentum changes under the THz
field. Dynamical birefringence arises from quantum interference between
time-reversed pairs of electron-hole recollision pathways
Nonlinear electromagnetic response of graphene: Frequency multiplication and the self-consistent-field effects
Graphene is a recently discovered carbon based material with unique physical
properties. This is a monolayer of graphite, and the two-dimensional electrons
and holes in it are described by the effective Dirac equation with a vanishing
effective mass. As a consequence, electromagnetic response of graphene is
predicted to be strongly non-linear. We develop a quasi-classical kinetic
theory of the non-linear electromagnetic response of graphene, taking into
account the self-consistent-field effects. Response of the system to both
harmonic and pulse excitation is considered. The frequency multiplication
effect, resulting from the non-linearity of the electromagnetic response, is
studied under realistic experimental conditions. The frequency up-conversion
efficiency is analysed as a function of the applied electric field and
parameters of the samples. Possible applications of graphene in terahertz
electronics are discussed.Comment: 14 pages, 7 figures, invited paper written for a special issue of
JPCM "Terahertz emitters
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