4,923 research outputs found
Seasonal trends in response to inoculation of coast live oak with Phytophthora ramorum
We developed a branch cutting inoculation method to provide a controlled system for studying variation in response to inoculation of coast live oak (Quercus agrifolia) with Phytophthora ramorum. This method has advantages over inoculations of trees in the field, in containing the inoculum and in allowing high levels of replication and the possibility of time series of responses. We previously reported significant tree-to-tree variation, with little population variation in lesion size using this method (Dodd and others 2005). Here we report on a time series in which branch cuttings were collected from the same trees at eight dates through a full year cycle. Branch cuttings were sampled from 33 trees from two sites at China Camp in Marin County, California, including 18 trees from one site that had suffered heavy mortality from this disease (Miwok Meadows) and 15 trees from a second site that has had little infection (Chicken Coop Island)
Backaction-Driven Transport of Bloch Oscillating Atoms in Ring Cavities
We predict that an atomic Bose-Einstein condensate strongly coupled to an
intracavity optical lattice can undergo resonant tunneling and directed
transport when a constant and uniform bias force is applied. The bias force
induces Bloch oscillations, causing amplitude and phase modulation of the
lattice which resonantly modifies the site-to-site tunneling. For the right
choice of parameters a net atomic current is generated. The transport velocity
can be oriented oppositely to the bias force, with its amplitude and direction
controlled by the detuning between the pump laser and the cavity. The transport
can also be enhanced through imbalanced pumping of the two counter-propagating
running wave cavity modes. Our results add to the cold atoms quantum simulation
toolbox, with implications for quantum sensing and metrology.Comment: Published version: 5 pages, 4 figures; Supplementary Material
include
Using ACIS on the Chandra X-ray Observatory as a particle radiation monitor
The Advanced CCD Imaging Spectrometer (ACIS) is one of two focal-plane
instruments on the Chandra X-ray Observatory. During initial radiation-belt
passes, the exposed ACIS suffered significant radiation damage from trapped
soft protons scattering off the x-ray telescope's mirrors. The primary effect
of this damage was to increase the charge-transfer inefficiency (CTI) of the
ACIS 8 front-illuminated CCDs. Subsequently, the Chandra team implemented
procedures to remove the ACIS from the telescope's focus during high-radiation
events: planned protection during radiation-belt transits; autonomous
protection triggered by an on-board radiation monitor; and manual intervention
based upon assessment of space-weather conditions. However, as Chandra's
multilayer insulation ages, elevated temperatures have reduced the
effectiveness of the on-board radiation monitor for autonomous protection. Here
we investigate using the ACIS CCDs themselves as a radiation monitor. We
explore the 10-year database to evaluate the CCDs' response to particle
radiation and to compare this response with other radiation data and
environment models.Comment: 10 pages, 5 figures. To appear in Proc. SPIE vol. 773
Alternative derivation of the Feigel effect and call for its experimental verification
A recent theory by Feigel [Phys. Rev. Lett. {\bf 92}, 020404 (2004)] predicts
the finite transfer of momentum from the quantum vacuum to a fluid placed in
strong perpendicular electric and magnetic fields. The momentum transfer arises
because of the optically anisotropic magnetoelectric response induced in the
fluid by the fields. After summarising Feigel's original assumptions and
derivation (corrected of trivial mistakes), we rederive the same result by a
simpler route, validating Feigel's semi-classical approach. We then derive the
stress exerted by the vacuum on the fluid which, if the Feigel hypothesis is
correct, should induce a Poiseuille flow in a tube with maximum speed m/s (2000 times larger than Feigel's original prediction). An experiment
is suggested to test this prediction for an organometallic fluid in a tube
passing through the bore of a high strength magnet. The predicted flow can be
measured directly by tracking microscopy or indirectly by measuring the flow
rate (ml/min) corresponding to the Poiseuille flow. A second
experiment is also proposed whereby a `vacuum radiometer' is used to test a
recent prediction that the net force on a magnetoelectric slab in the vacuum
should be zero.Comment: 20 pages, 1 figures. revised and improved versio
Classical versus quantum dynamics of the atomic Josephson junction
We compare the classical (mean-field) dynamics with the quantum dynamics of
atomic Bose-Einstein condensates in double-well potentials. The quantum
dynamics are computed using a simple scheme based upon the Raman-Nath
equations. Two different methods for exciting a non-equilbrium state are
considered: an asymmetry between the wells which is suddenly removed, and a
periodic time oscillating asymmetry. The first method generates wave packets
that lead to collapses and revivals of the expectation values of the
macroscopic variables, and we calculate the time scale for these revivals. The
second method permits the excitation of a single energy eigenstate of the
many-particle system, including Schroedinger cat states. We also discuss a band
theory interpretation of the energy level structure of an asymmetric
double-well, thereby identifying analogies to Bloch oscillations and Bragg
resonances. Both the Bloch and Bragg dynamics are purely quantum and are not
contained in the mean-field treatment.Comment: 31 pages, 14 figure
Impurity in a Bose-Einstein condensate in a double well
We compare and contrast the mean-field and many-body properties of a
Bose-Einstein condensate trapped in a double well potential with a single
impurity atom. The mean-field solutions display a rich structure of
bifurcations as parameters such as the boson-impurity interaction strength and
the tilt between the two wells are varied. In particular, we study a pitchfork
bifurcation in the lowest mean-field stationary solution which occurs when the
boson-impurity interaction exceeds a critical magnitude. This bifurcation,
which is present for both repulsive and attractive boson-impurity interactions,
corresponds to the spontaneous formation of an imbalance in the number of
particles between the two wells. If the boson-impurity interaction is large,
the bifurcation is associated with the onset of a Schroedinger cat state in the
many-body ground state. We calculate the coherence and number fluctuations
between the two wells, and also the entanglement entropy between the bosons and
the impurity. We find that the coherence can be greatly enhanced at the
bifurcation.Comment: 19 pages, 17 figures. The second version contains minor corrections
and some better figures (thicker lines
Anisotropic and long-range vortex interactions in two-dimensional dipolar Bose gases
We perform a theoretical study into how dipole-dipole interactions modify the
properties of superfluid vortices within the context of a two-dimensional
atomic Bose gas of co-oriented dipoles. The reduced density at a vortex acts
like a giant anti-dipole, changing the density profile and generating an
effective dipolar potential centred at the vortex core whose most slowly
decaying terms go as and . These effects modify
the vortex-vortex interaction which, in particular, becomes anisotropic for
dipoles polarized in the plane. Striking modifications to vortex-vortex
dynamics are demonstrated, i.e. anisotropic co-rotation dynamics and the
suppression of vortex annihilation.Comment: PRL accepted, 6 pages, 5 figure
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