12,085 research outputs found
Periodic orbit theory for the H\'enon-Heiles system in the continuum region
We investigate the resonance spectrum of the H\'enon-Heiles potential up to
twice the barrier energy. The quantum spectrum is obtained by the method of
complex coordinate rotation. We use periodic orbit theory to approximate the
oscillating part of the resonance spectrum semiclassically and Strutinsky
smoothing to obtain its smooth part. Although the system in this energy range
is almost chaotic, it still contains stable periodic orbits. Using Gutzwiller's
trace formula, complemented by a uniform approximation for a codimension-two
bifurcation scenario, we are able to reproduce the coarse-grained
quantum-mechanical density of states very accurately, including only a few
stable and unstable orbits.Comment: LaTeX (v3): 10 pages, 9 figures (new figure 6 added), 1 table; final
version for Phys. Rev. E (in print
Semiflexible polymers under external fields confined to two dimensions
The non-equilibrium structural and dynamical properties of semiflexible
polymers confined to two dimensions are investigated by molecular dynamics
simulations. Three different scenarios are considered: The force-extension
relation of tethered polymers, the relaxation of an initially stretched
semiflexible polymer, and semiflexible polymers under shear flow. We find
quantitative agreement with theoretical predictions for the force-extension
relation and the time dependence of the entropically contracting polymer. The
semiflexible polymers under shear flow exhibit significant conformational
changes at large shear rates, where less stiff polymers are extended by the
flow, whereas rather stiff polymers are contracted. In addition, the polymers
are aligned by the flow, thereby the two-dimensional semiflexible polymers
behave similarly to flexible polymers in three dimensions. The tumbling times
display a power-law dependence at high shear rate rates with an exponent
comparable to the one of flexible polymers in three-dimensional systems.Comment: Accepted for publication in J. Chem. Phy
Optical orientation of electron spins in GaAs quantum wells
We present a detailed experimental and theoretical analysis of the optical
orientation of electron spins in GaAs/AlAs quantum wells. Using time and
polarization resolved photoluminescence excitation spectroscopy, the initial
degree of electron spin polarization is measured as a function of excitation
energy for a sequence of quantum wells with well widths between 63 Ang and 198
Ang. The experimental results are compared with an accurate theory of excitonic
absorption taking fully into account electron-hole Coulomb correlations and
heavy-hole light-hole coupling. We find in wide quantum wells that the measured
initial degree of polarization of the luminescence follows closely the spin
polarization of the optically excited electrons calculated as a function of
energy. This implies that the orientation of the electron spins is essentially
preserved when the electrons relax from the optically excited high-energy
states to quasi-thermal equilibrium of their momenta. Due to initial spin
relaxation, the measured polarization in narrow quantum wells is reduced by a
constant factor that does not depend on the excitation energy.Comment: 12 pages, 9 figure
Level density of the H\'enon-Heiles system above the critical barrier Energy
We discuss the coarse-grained level density of the H\'enon-Heiles system
above the barrier energy, where the system is nearly chaotic. We use periodic
orbit theory to approximate its oscillating part semiclassically via
Gutzwiller's semiclassical trace formula (extended by uniform approximations
for the contributions of bifurcating orbits). Including only a few stable and
unstable orbits, we reproduce the quantum-mechanical density of states very
accurately. We also present a perturbative calculation of the stabilities of
two infinite series of orbits (R and L), emanating from the shortest
librating straight-line orbit (A) in a bifurcation cascade just below the
barrier, which at the barrier have two common asymptotic Lyapunov exponents
and .Comment: LaTeX, style FBS (Few-Body Systems), 6pp. 2 Figures; invited talk at
"Critical stability of few-body quantum systems", MPI-PKS Dresden, Oct.
17-21, 2005; corrected version: passages around eq. (6) and eqs. (12),(13)
improve
X-Ray Emitting Ejecta of Supernova Remnant N132D
The brightest supernova remnant in the Magellanic Clouds, N132D, belongs to
the rare class of oxygen-rich remnants, about a dozen objects that show optical
emission from pure heavy-element ejecta. They originate in explosions of
massive stars that produce large amounts of O, although only a tiny fraction of
that O is found to emit at optical wavelengths. We report the detection of
substantial amounts of O at X-ray wavelengths in a recent 100 ks Chandra ACIS
observation of N132D. A comparison between subarcsecond-resolution Chandra and
Hubble images reveals a good match between clumpy X-ray and optically emitting
ejecta on large (but not small) scales. Ejecta spectra are dominated by strong
lines of He- and H-like O; they exhibit substantial spatial variations
partially caused by patchy absorption within the LMC. Because optical ejecta
are concentrated in a 5 pc radius elliptical expanding shell, the detected
ejecta X-ray emission also originates in this shell.Comment: 5 pages, 6 figures, ApJ Letters, in pres
Band structure and magnetotransport of a two-dimensional electron gas in the presence of spin-orbit interaction
The band structure and magnetotransport of a two-dimensional electron gas
(2DEG), in the presence of the Rashba (RSOI) and Dresselhaus (DSOI) terms of
the spin-orbit interaction and of a perpendicular magnetic field, is
investigated. Exact and approximate analytical expressions for the band
structure are obtained and used to calculate the density of states (DOS) and
the longitudinal magnetoresitivity assuming a Gaussian type of level
broadening. The interplay between the Zeeman coupling and the two terms of the
SOI is discussed. If the strengths and , of the RSOI and DSOI,
respectively, are equal and the factor vanishes, the two spin states are
degenerate and a shifted Landau-level structure appears. With the increase of
the difference , a novel beating pattern of the DOS and of the
Shubnikov-de Haas (SdH) oscillations appears distinctly different from that
occurring when one of these strengths vanishes
Role of finite layer thickness in spin-polarization of GaAs 2D electrons in strong parallel magnetic fields
We report measurements and calculations of the spin-polarization, induced by
a parallel magnetic field, of interacting, dilute, two-dimensional electron
systems confined to GaAs/AlGaAs heterostructures. The results reveal the
crucial role the non-zero electron layer thickness plays: it causes a
deformation of the energy surface in the presence of a parallel field, leading
to enhanced values for the effective mass and g-factor and a non-linear
spin-polarization with field.Comment: 4 pages, 4 figures, Fig. 4 has been replaced from the previous
version, minor changes in the tex
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