2,437 research outputs found
On the Spectrum of the Resonant Quantum Kicked Rotor
It is proven that none of the bands in the quasi-energy spectrum of the
Quantum Kicked Rotor is flat at any primitive resonance of any order.
Perturbative estimates of bandwidths at small kick strength are established for
the case of primitive resonances of prime order. Different bands scale with
different powers of the kick strength, due to degeneracies in the spectrum of
the free rotor.Comment: Description of related published work has been expanded in the
Introductio
Particle Acceleration, Magnetic Field Generation, and Associated Emission in Collisionless Relativistic Jets
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray
bursts (GRBs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations using injected relativistic electron-ion
(electro-positron) jets show that acceleration occurs within the downstream
jet. Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
other two-streaming instability, and the Weibel instability) created in the
shocks are responsible for particle (electron, positron, and ion) acceleration.
The simulation results show that the Weibel instability is responsible for
generating and amplifying highly nonuniform, small-scale magnetic fields. These
magnetic fields contribute to the electron's transverse deflection behind the
jet head. The ``jitter'' radiation from deflected electrons has different
properties than synchrotron radiation which assumes 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, 3 figures, contributed talk at the workshop: High Energy
Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007.
Fig. 3 is replaced by the correct versio
A numerical and symbolical approximation of the Nonlinear Anderson Model
A modified perturbation theory in the strength of the nonlinear term is used
to solve the Nonlinear Schroedinger Equation with a random potential. It is
demonstrated that in some cases it is more efficient than other methods.
Moreover we obtain error estimates. This approach can be useful for the
solution of other nonlinear differential equations of physical relevance.Comment: 21 pages and 7 figure
Antiresonance and Localization in Quantum Dynamics
The phenomenon of quantum antiresonance (QAR), i.e., exactly periodic
recurrences in quantum dynamics, is studied in a large class of nonintegrable
systems, the modulated kicked rotors (MKRs). It is shown that asymptotic
exponential localization generally occurs for (a scaled ) in the
infinitesimal vicinity of QAR points . The localization length
is determined from the analytical properties of the kicking potential. This
``QAR-localization" is associated in some cases with an integrable limit of the
corresponding classical systems. The MKR dynamical problem is mapped into
pseudorandom tight-binding models, exhibiting dynamical localization (DL). By
considering exactly-solvable cases, numerical evidence is given that
QAR-localization is an excellent approximation to DL sufficiently close to QAR.
The transition from QAR-localization to DL in a semiclassical regime, as
is varied, is studied. It is shown that this transition takes place via a
gradual reduction of the influence of the analyticity of the potential on the
analyticity of the eigenstates, as the level of chaos is increased.Comment: To appear in Physical Review E. 51 pre-print pages + 9 postscript
figure
Dimer Decimation and Intricately Nested Localized-Ballistic Phases of Kicked Harper
Dimer decimation scheme is introduced in order to study the kicked quantum
systems exhibiting localization transition. The tight-binding representation of
the model is mapped to a vectorized dimer where an asymptotic dissociation of
the dimer is shown to correspond to the vanishing of the transmission
coefficient thru the system. The method unveils an intricate nesting of
extended and localized phases in two-dimensional parameter space. In addition
to computing transport characteristics with extremely high precision, the
renormalization tools also provide a new method to compute quasienergy
spectrum.Comment: There are five postscript figures. Only half of the figure (3) is
shown to reduce file size. However, missing part is the mirror image of the
part show
The possibility of a metal insulator transition in antidot arrays induced by an external driving
It is shown that a family of models associated with the kicked Harper model
is relevant for cyclotron resonance experiments in an antidot array. For this
purpose a simplified model for electronic motion in a related model system in
presence of a magnetic field and an AC electric field is developed. In the
limit of strong magnetic field it reduces to a model similar to the kicked
Harper model. This model is studied numerically and is found to be extremely
sensitive to the strength of the electric field. In particular, as the strength
of the electric field is varied a metal -- insulator transition may be found.
The experimental conditions required for this transition are discussed.Comment: 6 files: kharp.tex, fig1.ps fig2.ps fi3.ps fig4.ps fig5.p
Manipulation of the Spin Memory of Electrons in n-GaAs
We report on the optical manipulation of the electron spin relaxation time in
a GaAs based heterostructure. Experimental and theoretical study shows that the
average electron spin relaxes through hyperfine interaction with the lattice
nuclei, and that the rate can be controlled by the electron-electron
interactions. This time has been changed from 300 ns down to 5 ns by variation
of the laser frequency. This modification originates in the optically induced
depletion of n-GaAs layer
Magnetic properties of the S=1/2 quasi square lattice antiferromagnet CuF2(H2O)2(pyz) (pyz=pyrazine) investigated by neutron scattering
We have performed elastic and inelastic neutron experiments on single crystal
samples of the coordination polymer compound CuF2(H2O)2(pyz) (pyz=pyrazine) to
study the magnetic structure and excitations. The elastic neutron diffraction
measurements indicate a collinear antiferromagnetic structure with moments
oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60
+/- 0.03 muB/Cu. This value is significantly smaller than the single ion
magnetic moment, reflecting the presence of strong quantum fluctuations. The
spin wave dispersion from magnetic zone center to the zone boundary points (0.5
1.5 0) and (0.5 0 1.5) can be described by a two dimensional Heisenberg model
with a nearest neighbor magnetic exchange constant J2d = 0.934 +/-0.0025 meV.
The inter-layer interaction Jperp in this compound is less than 1.5% of J2d.
The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced
when compared to the (0.5 1 0.5) zone boundary point by ~10.3 +/- 1.4 %. This
zone boundary dispersion is consistent with quantum Monte Carlo and series
expansion calculations which include corrections for quantum fluctuations to
linear spin wave theory.Comment: 7 pages, 6 figure
- …