12,104 research outputs found
Ultra-Short Optical Pulse Generation with Single-Layer Graphene
Pulses as short as 260 fs have been generated in a diode-pumped low-gain
Er:Yb:glass laser by exploiting the nonlinear optical response of single-layer
graphene. The application of this novel material to solid-state bulk lasers
opens up a way to compact and robust lasers with ultrahigh repetition rates.Comment: 6 pages, 3 figures, to appear in Journal of Nonlinear Optical Physics
& Material
Laser Velocimeter Measurements in the Leakage Annulus of a Whirling Shrouded Centrifugal Pump
Previous experiments conducted in the Rotor Force Test Facility at the California Institute of Technology have thoroughly examined the effect of leakage flows on the rotordynamic forces on a centrifugal pump impeller undergoing a prescribed circular whirl. These leakage flows have been shown to contribute substantially to the total fluid induced forces acting on a pump. However, to date nothing is known of the flow field in the leakage annulus of shrouded centrifugal pumps. No attempt has been made to qualitatively or quantitatively examine the velocity field in the leakage annulus. Hence the test objective of this experiment is to acquire fluid velocity data for a geometry representative of the leakage annulus of a shrouded centrifugal pump while the rotor is whirling using laser velocimetry. Tests are performed over a range of whirl ratios and a flowrate typical of Space Shuttle Turbopump designs. In addition to a qualitive study of the flow field, the velocity data can be used to anchor flow models
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An unfolded protein-induced conformational switch activates mammalian IRE1.
The unfolded protein response (UPR) adjusts the cell's protein folding capacity in the endoplasmic reticulum (ER) according to need. IRE1 is the most conserved UPR sensor in eukaryotic cells. It has remained controversial, however, whether mammalian and yeast IRE1 use a common mechanism for ER stress sensing. Here, we show that similar to yeast, human IRE1α's ER-lumenal domain (hIRE1α LD) binds peptides with a characteristic amino acid bias. Peptides and unfolded proteins bind to hIRE1α LD's MHC-like groove and induce allosteric changes that lead to its oligomerization. Mutation of a hydrophobic patch at the oligomerization interface decoupled peptide binding to hIRE1α LD from its oligomerization, yet retained peptide-induced allosteric coupling within the domain. Importantly, impairing oligomerization of hIRE1α LD abolished IRE1's activity in living cells. Our results provide evidence for a unifying mechanism of IRE1 activation that relies on unfolded protein binding-induced oligomerization
Periodic Chaotic Billiards: Quantum-Classical Correspondence in Energy Space
We investigate the properties of eigenstates and local density of states
(LDOS) for a periodic 2D rippled billiard, focusing on their quantum-classical
correspondence in energy representation. To construct the classical
counterparts of LDOS and the structure of eigenstates (SES), the effects of the
boundary are first incorporated (via a canonical transformation) into an
effective potential, rendering the one-particle motion in the 2D rippled
billiard equivalent to that of two-interacting particles in 1D geometry. We
show that classical counterparts of SES and LDOS in the case of strong chaotic
motion reveal quite a good correspondence with the quantum quantities. We also
show that the main features of the SES and LDOS can be explained in terms of
the underlying classical dynamics, in particular of certain periodic orbits. On
the other hand, statistical properties of eigenstates and LDOS turn out to be
different from those prescribed by random matrix theory. We discuss the quantum
effects responsible for the non-ergodic character of the eigenstates and
individual LDOS that seem to be generic for this type of billiards with a large
number of transverse channels.Comment: 13 pages, 18 figure
Nonlocal interactions in doped cuprates: correlated motion of Zhang-Rice polarons
In-plane, inter-carrier correlations in hole doped cuprates are investigated
by ab initio multiconfiguration calculations. The dressed carriers display
features that are reminiscent of both Zhang-Rice (ZR) CuO4 singlet states and
Jahn-Teller polarons. The interaction between these quasiparticles is
repulsive. At doping levels that are high enough, the interplay between
long-range unscreened Coulomb interactions and long-range phase coherence among
the O-ion half-breathing vibrations on the ZR plaquettes may lead to a strong
reduction of the effective adiabatic energy barrier associated to each
polaronic state. Tunneling effects cannot be neglected for a relatively flat,
multi-well energy landscape. We suggest that the coherent, superconducting
quantum state is the result of such coherent quantum lattice fluctuations
involving the in-plane O ions. Our findings appear to support models where the
superconductivity is related to a lowering of the in-plane kinetic energy
Top quark physics in hadron collisions
The top quark is the heaviest elementary particle observed to date. Its large
mass makes the top quark an ideal laboratory to test predictions of
perturbation theory concerning heavy quark production at hadron colliders. The
top quark is also a powerful probe for new phenomena beyond the Standard Model
of particle physics. In addition, the top quark mass is a crucial parameter for
scrutinizing the Standard Model in electroweak precision tests and for
predicting the mass of the yet unobserved Higgs boson. Ten years after the
discovery of the top quark at the Fermilab Tevatron top quark physics has
entered an era where detailed measurements of top quark properties are
undertaken. In this review article an introduction to the phenomenology of top
quark production in hadron collisions is given, the lessons learned in Tevatron
Run I are summarized, and first Run II results are discussed. A brief outlook
to the possibilities of top quark research a the Large Hadron Collider,
currently under construction at CERN, is included.Comment: 84 pages, 32 figures, accepted for publication by Reports on Progress
in Physic
Effects of Defoliation Frequency on Foraging Selective Behaviour in Pasture with Paspalum Dilatatum
The objective was to assess defoliation effects exerted by sheep grazing on a pasture composed by falaris and dallisgrass, at two phenological stages during spring and summer. A sown pasture of Phalaris aquatica and Paspalum dilatatum were grazed at two phenological stages: stage I (SI), with no more than 5% of reproductive tillers, and stage II (SII), with at least 70% of reproductive tillers. Tillers of both grass species were marked with plastic colour rings along two transects in each plot. Phenological stage of tillers, and the number, age and length of each leaf per tiller were registered. Tiller defoliation rates (TDR), Leaf defoliation rates (LDR) of young and old leaves and Defoliated tiller percent (DTP) were estimated for both species. The results obtained demonstrated that selective forage behaviour of grazing sheep showed the existence of mechanisms towards maximum forage consumption
A device to characterize optical fibres
ATLAS is a general purpose experiment approved for the LHC collider at CERN.
An important component of the detector is the central hadronic calorimeter; for
its construction more than 600,000 Wave Length Shifting (WLS) fibres
(corresponding to a total length of 1,120 Km) have been used.
We have built and put into operation a dedicated instrument for the
measurement of light yield and attenuation length over groups of 20 fibres at a
time.
The overall accuracy achieved in the measurement of light yield
(attenuation length) is 1.5% (3%).
We also report the results obtained using this method in the quality control
of a large sample of fibres.Comment: 17 pages 20 figeres submitted to NIM journa
On recent puzzles in the production of heavy quarkonia
Recently, several surprising experimental observations in the production of
heavy quarkonium have been reported. In annihilation at
GeV, Belle Collaboration finds that mesons are
predominantly produced in association with an extra pair, with
, and the BaBar collaboration reports that the
produced 's have mostly longitudinal polarization. In
collisions at the Tevatron, the CDF Collaboration reported an excess of
and mesons at high over the perturbative
QCD predictions; non--perturbative approach of NRQCD can accomodate the
magnitude of the production cross section but not the observed experimentally
polarization of quarkonia. In this note we propose possible solutions to these
puzzles, and devise further experimental tests.Comment: 3 pages, 2 figure
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