28,182 research outputs found
Thon rings from amorphous ice and implications of beam-induced Brownian motion in single particle electron cryo-microscopy
We have recorded dose-fractionated electron cryo-microscope images of thin
films of pure flash-frozen amorphous ice and pre-irradiated amorphous carbon on
a Falcon~II direct electron detector using 300 keV electrons. We observe Thon
rings \cite{Thon1966} in both the power spectrum of the summed frames and the
sum of power spectra from the individual frames. The Thon rings from amorphous
carbon images are always more visible in the power spectrum of the summed
frames whereas those of amorphous ice are more visible in the sum of power
spectra from the individual frames. This difference indicates that while
pre-irradiated carbon behaves like a solid during the exposure, amorphous ice
behaves like a fluid with the individual water molecules undergoing
beam-induced motion. Using the measured variation in the power spectra
amplitude with number of electrons per image we deduce that water molecules are
randomly displaced by mean squared distance of 1.1 \AA for every
incident 300 keV e/\AA. The induced motion leads to an optimal
exposure with 300 keV electrons of 4.0 e/\AA per image with which to
see Thon rings centred around the strong 3.7{\AA} scattering peak from
amorphous ice. The beam-induced movement of the water molecules generates
pseudo-Brownian motion of embedded macromolecules. The resulting blurring of
single particle images contributes an additional term, on top of that from
radiation damage, to the minimum achievable B-factor for macromolecular
structure determination.Comment: 11 pages, 6 figures, Supplementary information 6 pages with 5 figure
Extent of regretted sexual intercourse among young teenagers in Scotland: a cross sectional survey
No abstract available
Experimental Study of the Role of Atomic Interactions on Quantum Transport
We report an experimental study of quantum transport for atoms confined in a
periodic potential and compare between thermal and BEC initial conditions. We
observe ballistic transport for all values of well depth and initial
conditions, and the measured expansion velocity for thermal atoms is in
excellent agreement with a single-particle model. For weak wells, the expansion
of the BEC is also in excellent agreement with single-particle theory, using an
effective temperature. We observe a crossover to a new regime for the BEC case
as the well depth is increased, indicating the importance of interactions on
quantum transport.Comment: 4 pages, 3 figure
Yang-Mills Theory on a Cylinder Coupled to Point Particles
We study a model of quantum Yang-Mills theory with a finite number of gauge
invariant degrees of freedom. The gauge field has only a finite number of
degrees of freedom since we assume that space-time is a two dimensional
cylinder. We couple the gauge field to matter, modeled by either one or two
nonrelativistic point particles. These problems can be solved {\it without any
gauge fixing}, by generalizing the canonical quantization methods of
Ref.\[rajeev] to the case including matter. For this, we make use of the
geometry of the space of connections, which has the structure of a Principal
Fiber Bundle with an infinite dimensional fiber. We are able to reduce both
problems to finite dimensional, exactly solvable, quantum mechanics problems.
In the case of one particle, we find that the ground state energy will diverge
in the limit of infinite radius of space, consistent with confinement. In the
case of two particles, this does not happen if they can form a color singlet
bound state (`meson').Comment: 37 pages, UR-1327 ER-40685-77
Multiparticle Schrodinger operators with point interactions in the plane
We study a system of N bosons in the plane interacting with delta function
potentials. After a coupling constant renormalization we show that the
Hamiltonian defines a self-adjoint operator and obtain a lower bound for the
energy. The same results hold if one includes a regular inter-particle
potential.Comment: 17 pages, Late
Experimental demonstration of painting arbitrary and dynamic potentials for Bose-Einstein condensates
There is a pressing need for robust and straightforward methods to create
potentials for trapping Bose-Einstein condensates which are simultaneously
dynamic, fully arbitrary, and sufficiently stable to not heat the ultracold
gas. We show here how to accomplish these goals, using a rapidly-moving laser
beam that "paints" a time-averaged optical dipole potential in which we create
BECs in a variety of geometries, including toroids, ring lattices, and square
lattices. Matter wave interference patterns confirm that the trapped gas is a
condensate. As a simple illustration of dynamics, we show that the technique
can transform a toroidal condensate into a ring lattice and back into a toroid.
The technique is general and should work with any sufficiently polarizable
low-energy particles.Comment: Minor text changes and three references added. This is the final
version published in New Journal of Physic
Bose-Einstein Condensate Driven by a Kicked Rotor in a Finite Box
We study the effect of different heating rates of a dilute Bose gas confined
in a quasi-1D finite, leaky box. An optical kicked-rotor is used to transfer
energy to the atoms while two repulsive optical beams are used to confine the
atoms. The average energy of the atoms is localized after a large number of
kicks and the system reaches a nonequilibrium steady state. A numerical
simulation of the experimental data suggests that the localization is due to
energetic atoms leaking over the barrier. Our data also indicates a correlation
between collisions and the destruction of the Bose-Einstein condensate
fraction.Comment: 7 pages, 8 figure
Framing Expectations in Early HIV Cure Research
Language used to describe clinical research represents a powerful opportunity to educate volunteers. In the case of HIV cure research there is an emerging need to manage expectations by using the term ‘experiment’. Cure experiments are proof-of-concept studies designed to evaluate novel paradigms to reduce persistent HIV-1 reservoirs, without any expectation of medical benefit
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