4,778 research outputs found
Properties of contact matrices induced by pairwise interactions in proteins
The total conformational energy is assumed to consist of pairwise interaction
energies between atoms or residues, each of which is expressed as a product of
a conformation-dependent function (an element of a contact matrix, C-matrix)
and a sequence-dependent energy parameter (an element of a contact energy
matrix, E-matrix). Such pairwise interactions in proteins force native
C-matrices to be in a relationship as if the interactions are a Go-like
potential [N. Go, Annu. Rev. Biophys. Bioeng. 12. 183 (1983)] for the native
C-matrix, because the lowest bound of the total energy function is equal to the
total energy of the native conformation interacting in a Go-like pairwise
potential. This relationship between C- and E-matrices corresponds to (a) a
parallel relationship between the eigenvectors of the C- and E-matrices and a
linear relationship between their eigenvalues, and (b) a parallel relationship
between a contact number vector and the principal eigenvectors of the C- and
E-matrices; the E-matrix is expanded in a series of eigenspaces with an
additional constant term, which corresponds to a threshold of contact energy
that approximately separates native contacts from non-native ones. These
relationships are confirmed in 182 representatives from each family of the SCOP
database by examining inner products between the principal eigenvector of the
C-matrix, that of the E-matrix evaluated with a statistical contact potential,
and a contact number vector. In addition, the spectral representation of C- and
E-matrices reveals that pairwise residue-residue interactions, which depends
only on the types of interacting amino acids but not on other residues in a
protein, are insufficient and other interactions including residue
connectivities and steric hindrance are needed to make native structures the
unique lowest energy conformations.Comment: Errata in DOI:10.1103/PhysRevE.77.051910 has been corrected in the
present versio
Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating into
an ambient plasma. We find small differences in the results for no ambient and
modest ambient magnetic fields. Simulations show that the Weibel instability
created in the collisionless shock front accelerates jet and ambient particles
both perpendicular and parallel to the jet propagation direction. The small
scale magnetic field structure generated by the Weibel instability is
appropriate to the generation of ``jitter'' radiation from deflected electrons
(positrons) as opposed to synchrotron radiation. The jitter radiation resulting
from small scale magnetic field structures may be important for understanding
the complex time structure and spectral evolution observed in gamma-ray bursts
or other astrophysical sources containing relativistic jets and relativistic
collisionless shocks.Comment: 6 pages, 1 figure, revised and accepted for Advances in Space
Research (35th COSPAR Scientific Assembly, Paris, 18-25 July 2004
Particle Acceleration in Relativistic Jets due to Weibel Instability
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
two-streaming instability, and the Weibel instability) created in the shocks
are responsible for particle (electron, positron, and ion) acceleration. Using
a 3-D relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating
through an ambient plasma with and without initial magnetic fields. We find
only small differences in the results between no ambient and weak ambient
magnetic fields. Simulations show that the Weibel instability created in the
collisionless shock front accelerates particles perpendicular and parallel to
the jet propagation direction. While some Fermi acceleration may occur at the
jet front, the majority of electron acceleration takes place behind the jet
front and cannot be characterized as Fermi acceleration. The simulation results
show that this instability is responsible for generating and amplifying highly
nonuniform, small-scale magnetic fields, which contribute to the electron's
transverse deflection behind the jet head. The ``jitter'' radiation (Medvedev
2000) from deflected electrons has different properties than synchrotron
radiation which is calculated in a uniform magnetic field. This jitter
radiation may be important to understanding the complex time evolution and/or
spectral structure in gamma-ray bursts, relativistic jets, and supernova
remnants.Comment: ApJ, in press, Sept. 20, 2003 (figures with better resolution:
http://gammaray.nsstc.nasa.gov/~nishikawa/apjweib.pdf
Particle Acceleration and Magnetic Field Generation in Electron-Positron Relativistic Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic electron-positron jet
front propagating into an ambient electron-positron plasma with and without
initial magnetic fields. We find small differences in the results for no
ambient and modest ambient magnetic fields. New simulations show that the
Weibel instability created in the collisionless shock front accelerates jet and
ambient particles both perpendicular and parallel to the jet propagation
direction. Furthermore, the non-linear fluctuation amplitudes of densities,
currents, electric, and magnetic fields in the electron-positron shock are
larger than those found in the electron-ion shock studied in a previous paper
at the comparable simulation time. This comes from the fact that both electrons
and positrons contribute to generation of the Weibel instability. Additionally,
we have performed simulations with different electron skin depths. We find that
growth times scale inversely with the plasma frequency, and the sizes of
structures created by the Weibel instability scale proportional to the electron
skin depth. This is the expected result and indicates that the simulations have
sufficient grid resolution. The simulation results show that the Weibel
instability is responsible for generating and amplifying nonuniform,
small-scale magnetic fields which contribute to the electron's (positron's)
transverse deflection behind the jet head.Comment: 18 pages, 8 figures, revised and accepted for ApJ, A full resolution
of the paper can be found at
http://gammaray.nsstc.nasa.gov/~nishikawa/apjep1.pd
Particle Acceleration and Radiation associated with Magnetic Field Generation from Relativistic Collisionless Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
two-streaming instability, and the Weibel instability) created in the shocks
are responsible for particle (electron, positron, and ion) acceleration. Using
a 3-D relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating
through an ambient plasma with and without initial magnetic fields. We find
only small differences in the results between no ambient and weak ambient
magnetic fields. Simulations show that the Weibel instability created in the
collisionless shock front accelerates particles perpendicular and parallel to
the jet propagation direction. The simulation results show that this
instability is responsible for generating and amplifying highly nonuniform,
small-scale magnetic fields, which contribute to the electron's transverse
deflection behind the jet head. The ``jitter'' radiation from deflected
electrons has different properties than synchrotron radiation which is
calculated in a uniform magnetic field. This jitter radiation may be important
to understanding the complex time evolution and/or spectral structure in
gamma-ray bursts, relativistic jets, and supernova remnants.Comment: 4 pages, 1 figure, submitted to Proceedings of 2003 Gamma Ray Burst
Conferenc
Renormalized parameters and perturbation theory for an n-channel Anderson model with Hund's rule coupling: Asymmetric case
We explore the predictions of the renormalized perturbation theory for an
n-channel Anderson model, both with and without Hund's rule coupling, in the
regime away from particle-hole symmetry. For the model with n=2 we deduce the
renormalized parameters from numerical renormalization group calculations, and
plot them as a function of the occupation at the impurity site, nd. From these
we deduce the spin, orbital and charge susceptibilities, Wilson ratios and
quasiparticle density of states at T=0, in the different parameter regimes,
which gives a comprehensive overview of the low energy behavior of the model.
We compare the difference in Kondo behaviors at the points where nd=1 and nd=2.
One unexpected feature of the results is the suppression of the charge
susceptibility in the strong correlation regime over the occupation number
range 1 <nd <3.Comment: 9 pages, 17 figure
Acceleration Mechanics in Relativistic Shocks by the Weibel Instability
Plasma instabilities (e.g., Buneman, Weibel and other two-stream
instabilities) created in collisionless shocks may be responsible for particle
(electron, positron, and ion) acceleration. Using a 3-D relativistic
electromagnetic particle (REMP) code, we have investigated long-term particle
acceleration associated with relativistic electron-ion or electron-positron jet
fronts propagating into an unmagnetized ambient electron-ion or
electron-positron plasma. These simulations have been performed with a longer
simulation system than our previous simulations in order to investigate the
nonlinear stage of the Weibel instability and its particle acceleration
mechanism. The current channels generated by the Weibel instability are
surrounded by toroidal magnetic fields and radial electric fields. This radial
electric field is quasi stationary and accelerates particles which are then
deflected by the magnetic field.Comment: 17 pages, 5 figures, accepted for publication in ApJ, A full
resolution ot the paper can be found at
http://gammaray.nsstc.nasa.gov/~nishikawa/accmec.pd
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