2,416 research outputs found
Herbivore induction of the glucosinolate-myrosinase defense system: major trends, biochemical bases and ecological significance
Like many other plant defense compounds, glucosinolates are present constitutively in plant tissues, but are also induced to higher levels by herbivore attack. Of the major glucosinolate types, indolic glucosinolates are most frequently induced regardl
Model for triggering of non-volcanic tremor by earthquakes
There is evidence of tremor triggering by seismic waves emanating from
distant large earthquakes. The frequency content of both triggered and ambient
tremor are largely identical, suggesting that this property does not depend
directly on the nature of the source. We show here that the model of plate
dynamics developed earlier by us is an appropriate tool for describing tremor
triggering. In the framework of this model, tremor is an internal response of a
fault to a failure triggered by external disturbances. The model predicts
generation of radiation in a frequency range defined by the fault parameters.
Thus, although the amplitude and duration of a tremor burst may reflect the
"personality" of the source, the frequency content does not. The model also
explains why a tremor has no clear impulsive phase, in contrast to earthquakes.
The relationship between tremor and low frequency earthquakes is discussed.Comment: 9 pages, 1 figur
Model of deep non-volcanic tremor part I: ambient and triggered tremor
There is evidence of triggering of tremor by seismic waves emanating from
distant large earthquakes. The frequency contents of triggered and ambient
tremor are largely identical, suggesting that tremor does not depend directly
on the nature of the source. We show here that the model of plate dynamics
developed earlier by us is an appropriate tool for describing the onset of
tremor. In the framework of this model, tremor is an internal response of a
fault to a failure triggered by external disturbances. The model predicts
generation of radiation in a frequency range defined by the fault parameters.
Other specific features predicted are: the upper limit of the size of the
emitting area is a few dozen km; tremor accompanies earthquakes and aseismic
slip; the frequency content of tremor depends on the type of failure. The model
also explains why a tremor has no clear impulsive phase, in contrast to
earthquakes. A comparatively small effective normal stress (hence a high fluid
pressure) is required to make the model consistent with observed tremor
parameters. Our model indicates that tremor is not necessarily a superposition
of low frequency earthquakes, as commonly assumed, although the latter may
trigger them. The approach developed complements the conventional viewpoint
which assumes that tremor reflects a frictional process with low rupture speed.
Essentially our model adds the hypothesis that resonant-type oscillations exist
inside a fault. This addition may change our understanding of the nature of
tremor in general, and the methods of its identification and location in
particular.Comment: 32 pages, 16 figures. arXiv admin note: text overlap with
arXiv:1202.091
How does dissipation affect the transition from static to dynamic macroscopic friction?
Description of the transitional process from a static to a dynamic frictional
regime is a fundamental problem of modern physics. Previously we developed a
model based on the well-known Frenkel-Kontorova model to describe dry
macroscopic friction. Here this model has been modified to include the effect
of dissipation in derived relations between the kinematic and dynamic
parameters of a transition process. The main (somewhat counterintuitive) result
is a demonstration that the rupture (i.e. detachment front) velocity of the
slip pulse which arises during the transition does not depend on friction. The
only parameter (besides the elastic and plastic properties of the medium)
controlling the rupture velocity is the shear to normal stress ratio. In
contrast to the rupture velocity, the slip velocity does depend on friction.
The model we have developed describes these processes over a wide range of
rupture and slip velocities (up to 7 orders of magnitude) allowing, in
particular, the consideration of seismic events ranging from regular
earthquakes, with rupture velocities on the order of a few km/s, to slow slip
events, with rupture velocities of a few km/day.Comment: 21 pages, 12 figure
Understanding the Impact of Open-Framework Conglomerates on Water-Oil Displacements: Victor Interval of the Ivishak Reservoir, Prudhoe Bay Field, Alaska
The Victor Unit of the Ivishak Formation in the Prudhoe Bay Oilfield is
characterized by high net-to-gross fluvial sandstones and conglomerates. The
highest permeability is found within sets of cross-strata of open-framework
conglomerate (OFC). They are preserved within unit bar deposits and assemblages
of unit bar deposits within compound (braid) bar deposits. They are thief zones
limiting enhanced oil recovery. We incorporate recent research that has
quantified important attributes of their sedimentary architecture within
preserved deposits. We use high-resolution models to demonstrate the
fundamental aspects of their control on oil production rate, water breakthrough
time, and spatial and temporal distribution of residual oil saturation. We
found that when the pressure gradient is oriented perpendicular to the
paleoflow direction, the total oil production and the water breakthrough time
are larger, and remaining oil saturation is smaller, than when it is oriented
parallel to paleoflow. The pressure difference between production and injection
wells does not affect sweep efficiency, although the spatial distribution of
oil remaining in the reservoir critically depends on this value. Oil sweep
efficiency decreases slightly with increase in the proportion of OFC
cross-strata. Whether or not clusters of connected OFC span the domain does not
visibly affect sweep efficiency.Comment: 27 pages including 14 figure
Noise Thermal Impedance of a Diffusive Wire
The current noise density S of a conductor in equilibrium, the Johnson noise,
is determined by its temperature T: S=4kTG with G the conductance. The sample's
noise temperature Tn=S/(4kG) generalizes T for a system out of equilibrium. We
introduce the "noise thermal impedance" of a sample as the amplitude of the
oscillation of Tn when heated by an oscillating power. For a macroscopic
sample, it is the usual thermal impedance. We show for a diffusive wire how
this (complex) frequency-dependent quantity gives access to the electron-phonon
interaction time in a long wire and to the diffusion time in a shorter one, and
how its real part may also give access to the electron-electron inelastic time.
These times are not simply accessible from the frequency dependence of S
itself.Comment: 4 pages, 2 figure
Full characterization and analysis of a terahertz heterodyne receiver based on a NbN hot electron bolometer
We present a complete experimental characterization of a quasioptical twin-slot antenna coupled small area (1.0×0.15 µm^2) NbN hot electron bolometer (HEB) mixer compatible with currently available solid state tunable local oscillator (LO) sources. The required LO power absorbed in the HEB is analyzed in detail and equals only 25 nW. Due to the small HEB volume and wide antenna bandwidth, an unwanted direct detection effect is observed which decreases the apparent sensitivity. Correcting for this effect results in a receiver noise temperature of 700 K at 1.46 THz. The intermediate frequency (IF) gain bandwidth is 2.3 GHz and the IF noise bandwidth is 4 GHz. The single channel receiver stability is limited to 0.2–0.3 s in a 50 MHz bandwidth
Macroscopic coherence effects in a mesoscopic system: Weak localization of thin silver films in an undergraduate lab
We present an undergraduate lab that investigates weak localization in thin
silver films. The films prepared in our lab have thickness, , between 60-200
\AA, a mesoscopic length scale. At low temperatures, the inelastic dephasing
length for electrons, , exceeds the thickness of the film (), and the films are then quasi-2D in nature. In this situation, theory
predicts specific corrections to the Drude conductivity due to coherent
interference between conducting electrons' wavefunctions, a macroscopically
observable effect known as weak localization. This correction can be destroyed
with the application of a magnetic field, and the resulting magnetoresistance
curve provides information about electron transport in the film. This lab is
suitable for Junior or Senior level students in an advanced undergraduate lab
course.Comment: 16 pages, 9 figures. Replaces earlier version of paper rejected by
Am. J. Phys. because of too much content on vacuum systems. New version deals
with the undergraduate experiment on weak localization onl
Transition from static to dynamic macroscopic friction in the framework of the Frenkel-Kontorova model
A new generation of experiments on dry macroscopic friction has revealed that
the transition from static to dynamic friction is essentially a spatially and
temporally non-uniform process, initiated by a rupture-like detachment front.
We show the suitability of the Frenkel-Kontorova model for describing this
transition. The model predicts the existence of two types of detachment fronts,
explaining both the variability and abrupt change of velocity observed in
experiments. The quantitative relation obtained between the velocity of the
detachment front and the ratio of shear to normal stress is consistent with
experiments. The model provides a functional dependence between slip velocity
and shear stress, and predicts that slip velocity is independent of normal
stress. Paradoxically, the transition from static to dynamic friction does not
depend explicitly on ether the static or the dynamic friction coefficient,
although the beginning and end of transition process are controlled by these
coefficients.Comment: 16 pages, 8 figure
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