3,237 research outputs found
Far-infrared spectra of lateral quantum dot molecules
We study effects of electron-electron interactions and confinement potential
on the magneto-optical absorption spectrum in the far-infrared range of lateral
quantum dot molecules. We calculate far-infrared (FIR) spectra for three
different quantum dot molecule confinement potentials. We use accurate exact
diagonalization technique for two interacting electrons and calculate
dipole-transitions between two-body levels with perturbation theory. We
conclude that the two-electron FIR spectra directly reflect the symmetry of the
confinement potential and interactions cause only small shifts in the spectra.
These predictions could be tested in experiments with nonparabolic quantum dots
by changing the number of confined electrons. We also calculate FIR spectra for
up to six noninteracting electrons and observe some additional features in the
spectrum.Comment: For better quality Figs download manuscript from
http://www.fyslab.hut.fi/~mma/FIR/Helle_qdmfir.ps.g
Influence of shape of quantum dots on their far-infrared absorption
We investigate the effects of the shape of quantum dots on their far-infrared
absorption in an external magnetic field by a model calculation. We focus our
attention on dots with a parabolic confinement potential deviating from the
common circular symmetry, and dots having circular doughnut shape. For a
confinement where the generalized Kohn theorem does not hold we are able to
interprete the results in terms of a mixture of a center-of-mass mode and
collective modes reflecting an excitation of relative motion of the electrons.
The calculations are performed within the time-dependent Hartree approximation
and the results are compared to available experimental results.Comment: RevTeX, 16 pages with 10 postscript figures included. Submitted to
Phys. Rev.
Afterglow Light Curves and Broken Power Laws: A Statistical Study
In gamma-ray burst research it is quite common to fit the afterglow light
curves with a broken power law to interpret the data. We apply this method to a
computer simulated population of afterglows and find systematic differences
between the known model parameters of the population and the ones derived from
the power law fits. In general, the slope of the electron energy distribution
is overestimated from the pre-break light curve slope while being
underestimated from the post-break slope. We also find that the jet opening
angle derived from the fits is overestimated in narrow jets and underestimated
in wider ones. Results from fitting afterglow light curves with broken power
laws must therefore be interpreted with caution since the uncertainties in the
derived parameters might be larger than estimated from the fit. This may have
implications for Hubble diagrams constructed using gamma-ray burst data.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
Spin effects in a confined 2DEG: Enhancement of the g-factor, spin-inversion states and their far-infrared absorption
We investigate several spin-related phenomena in a confined two-dimensional
electron gas (2DEG) using the Hartree-Fock approximation for the mutual Coulomb
interaction of the electrons. The exchange term of the interaction causes a
large splitting of the spin levels whenever the chemical potential lies within
a Landau band (LB). This splitting can be reinterpreted as an enhancement of an
effective g-factor, g*. The increase of g* when a LB is half filled can be
accompanied by a spontaneous formation of a static spin-inversion state (SIS)
whose details depend on the system sision state (SIS) whose details depend on
the system size. The coupling of the states of higher LB's into the lowest band
by the Coulomb interaction of the 2DEG is essential for the SIS to occur. The
far-infrared absorption of the system, relatively insensitive to the spin
splitting, develops clear signs of the SIS.Comment: 7 figure
Hartree-Fock dynamics in highly excited quantum dots
Time-dependent Hartree-Fock theory is used to describe density oscillations
of symmetry-unrestricted two-dimensional nanostructures. In the small amplitude
limit the results reproduce those obtained within a perturbative approach such
as the linearized time-dependent Hartree-Fock one. The nonlinear regime is
explored by studying large amplitude oscillations in a non-parabolic potential,
which are shown to introduce a strong coupling with internal degrees of
freedom. This excitation of internal modes, mainly of monopole and quadrupole
character, results in sizeable modifications of the dipole absorption.Comment: 4 pages, 4 embedded figure
Magnetization of noncircular quantum dots
We calculate the magnetization of quantum dots deviating from circular
symmetry for noninteracting electrons or electrons interacting according to the
Hartree approximation. For few electrons the magnetization is found to depend
on their number, and the shape of the dot. The magnetization is an ideal probe
into the many-electron state of a quantum dot.Comment: 11 RevTeX pages with 6 included Postscript figure
Generalized Master equation approach to mesoscopic time-dependent transport
We use a generalized Master equation (GME) formalism to describe the
non-equilibrium time-dependent transport through a short quantum wire connected
to semi-infinite biased leads. The contact strength between the leads and the
wire are modulated by out-of-phase time-dependent functions which simulate a
turnstile device. One lead is fixed at one end of the sample whereas the other
lead has a variable placement. The system is described by a lattice model. We
find that the currents in both leads depend on the placement of the second
lead. In the rather small bias regime we obtain transient currents flowing
against the bias for short time intervals. The GME is solved numerically in
small time steps without resorting to the traditional Markov and rotating wave
approximations. The Coulomb interaction between the electrons in the sample is
included via the exact diagonalization method
Energy Injection Episodes in Gamma Ray Bursts: The Light Curves and Polarization Properties of GRB 021004
Several GRB afterglow light curves deviate strongly from the power law decay
observed in most bursts. We show that these variations can be accounted for by
including refreshed shocks in the standard fireball model previously used to
interpret the overall afterglow behavior. As an example we consider GRB 021004
that exhibited strong light curve variations and has a reasonably well
time-resolved polarimetry. We show that the light curves in the R-band, X-rays
and in the radio can be accounted for by four energy injection episodes in
addition to the initial event. The polarization variations are shown to be a
consequence of the injections.Comment: 4 pages, 2 figures. To appear in ApJ
Manifestation of the Hofstadter butterfly in far-infrared absorption
The far-infrared absorption of a two-dimensional electron gas with a
square-lattice modulation in a perpendicular constant magnetic field is
calculated self-consistently within the Hartree approximation. For strong
modulation and short period we obtain intra- and intersubband magnetoplasmon
modes reflecting the subbands of the Hofstadter butterfly in two or more Landau
bands. The character of the absorption and the correlation of the peaks to the
number of flux quanta through each unit cell of the periodic potential depends
strongly on the location of the chemical potential with respect to the
subbands, or what is the same, on the density of electrons in the system.Comment: RevTeX file + 4 postscript figures, to be published Phys. Rev. B
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