630 research outputs found
Harmonics generation in electron-ion collisions in a short laser pulse
Anomalously high generation efficiency of coherent higher field-harmonics in
collisions between {\em oppositely charged particles} in the field of
femtosecond lasers is predicted. This is based on rigorous numerical solutions
of a quantum kinetic equation for dense laser plasmas which overcomes
limitations of previous investigations.Comment: 4 pages, 4 eps-figures include
Dispersion in a relativistic degenerate electron gas
Relativistic effects on dispersion in a degenerate electron gas are discussed
by comparing known response functions derived relativistically (by Jancovici)
and nonrelativistically (by Lindhard). The main distinguishing feature is
one-photon pair creation, which leads to logarithmic singularities in the
response functions. Dispersion curves for longitudinal waves have a similar
tongue-like appearance in the relativistic and nonrelativistic case, with the
main relativistic effects being on the Fermi speed and the cutoff frequency.
For transverse waves the nonrelativistic treatment has a nonphysical feature
near the cutoff frequency for large Fermi momenta, and this is attributed to an
incorrect treatment of the electron spin. We find (with two important provisos)
that one-photon pair creation is allowed in superdense plasmas, implying
relatively strong coupling between transverse waves and pair creation.Comment: 17 pages, 9 figures. Submitted to Physical Review
Pore-scale mechanisms of gas flow in tight sand reservoirs
Tight gas sands are unconventional hydrocarbon energy resource storing large volume of natural gas. Microscopy and 3D imaging of reservoir samples at different scales and resolutions provide insights into the coaredo not significantly smaller in size than conventional sandstones, the extremely dense grain packing makes the pore space tortuous, and the porosity is small. In some cases the inter-granular void space is presented by micron-scale slits, whose geometry requires imaging at submicron resolutions. Maximal Inscribed Spheres computations simulate different scenarios of capillary-equilibrium two-phase fluid displacement. For tight sands, the simulations predict an unusually low wetting fluid saturation threshold, at which the non-wetting phase becomes disconnected. Flow simulations in combination with Maximal Inscribed Spheres computations evaluate relative permeability curves. The computations show that at the threshold saturation, when the nonwetting fluid becomes disconnected, the flow of both fluids is practically blocked. The nonwetting phase is immobile due to the disconnectedness, while the permeability to the wetting phase remains essentially equal to zero due to the pore space geometry. This observation explains the Permeability Jail, which was defined earlier by others. The gas is trapped by capillarity, and the brine is immobile due to the dynamic effects. At the same time, in drainage, simulations predict that the mobility of at least one of the fluids is greater than zero at all saturations. A pore-scale model of gas condensate dropout predicts the rate to be proportional to the scalar product of the fluid velocity and pressure gradient. The narrowest constriction in the flow path is subject to the highest rate of condensation. The pore-scale model naturally upscales to the Panfilov's Darcy-scale model, which implies that the condensate dropout rate is proportional to the pressure gradient squared. Pressure gradient is the greatest near the matrix-fracture interface. The distinctive two-phase flow properties of tight sand imply that a small amount of gas condensate can seriously affect the recovery rate by blocking gas flow. Dry gas injection, pressure maintenance, or heating can help to preserve the mobility of gas phase. A small amount of water can increase the mobility of gas condensate
Quantum kinetic theory of the filamentation instability
The quantum electromagnetic dielectric tensor for a multi species plasma is
re-derived from the gauge invariant Wigner-Maxwell system and presented under a
form very similar to the classical one. The resulting expression is then
applied to a quantum kinetic theory of the electromagnetic filamentation
instability. Comparison is made with the quantum fluid theory including a Bohm
pressure term, and with the cold classical plasma result. A number of
analytical expressions are derived for the cutoff wave vector, the largest
growth rate and the most unstable wave vector
True Dielectric and Ideal Conductor in Theory of the Dielectric Function for Coulomb System
On the basis of the exact relations the general formula for the static
dielectric permittivity e(q,0) for Coulomb system is found in the region of
small wave vectors q. The obtained formuladescribes the dielectric function
e(q,0) of the Coulomb system in both states in the "metallic" state and in the
"dielectric" one. The parameter which determines possible states of the Coulomb
system - from the "true" dielectric till the "ideal" conductor is found. The
exact relation for the pair correlation function for two-component system of
electrons and nuclei g_ei(r) is found for the arbitrary thermodynamic
parameters.Comment: 5 pages, no figure
Planar Heterostructure Graphene -- Narrow-Gap Semiconductor -- Graphene
We investigate a planar heterostructure composed of two graphene films
separated by a narrow-gap semiconductor ribbon. We show that there is no the
Klein paradox when the Dirac points of the Brillouin zone of graphene are in a
band gap of a narrow-gap semiconductor. There is the energy range depending on
an angle of incidence, in which the above-barrier damped solution exists.
Therefore, this heterostructure is a "filter" transmitting particles in a
certain range of angles of incidence upon a potential barrier. We discuss the
possibility of an application of this heterostructure as a "switch".Comment: 9 pages, 2 figure
Theory of transverse spin dynamics in a polarized Fermi liquid and an itinerant ferromagnet
The linear equations for transverse spin dynamics in a weakly polarized
degenerate Fermi liquid with arbitrary relationship between temperature and
polarization are derived from Landau-Silin phenomenological kinetic equation
with general form of two-particle collision integral. Unlike the previous
treatment where Fermi velocity and density of states have been taken as
constants independent of polarization here we made derivation free from this
assumption. The obtained equations are applicable for description of spin
dynamics in paramagnetic Fermi liquid with finite polarization as well in an
itinerant ferromagnet. In both cases transverse spin wave frequency is found to
be proportional to the square of the wave vector with complex constant of
proportionality (diffusion coefficient) such that the damping has a finite
value at T=0. The polarization dependence of the diffusion coefficient is found
to be different for a polarized Fermi liquid and for an itinerant ferromagnet.
These conclusions are confirmed by derivation of transverse spin wave
dispersion law in frame of field theoretical methods from the integral equation
for the vortex function. It is shown that similar derivation taking into
consideration the divergency of static transverse susceptibility also leads to
the same attenuating spin wave spectrum.Comment: 7 pages, no figure
Boundary States in Graphene Heterojunctions
A new type of states in graphene-based planar heterojunctions has been
studied in the envelope wave function approximation. The condition for the
formation of these states is the intersection between the dispersion curves of
graphene and its gap modification. This type of states can also occur in smooth
graphene-based heterojunctions.Comment: 5 pages, 3 figure
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