2,361 research outputs found
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.
Coulomb interaction and transient charging of excited states in open nanosystems
We obtain and analyze the effect of electron-electron Coulomb interaction on
the time dependent current flowing through a mesoscopic system connected to
biased semi-infinite leads. We assume the contact is gradually switched on in
time and we calculate the time dependent reduced density operator of the sample
using the generalized master equation. The many-electron states (MES) of the
isolated sample are derived with the exact diagonalization method. The chemical
potentials of the two leads create a bias window which determines which MES are
relevant to the charging and discharging of the sample and to the currents,
during the transient or steady states. We discuss the contribution of the MES
with fixed number of electrons N and we find that in the transient regime there
are excited states more active than the ground state even for N=1. This is a
dynamical signature of the Coulomb blockade phenomenon. We discuss numerical
results for three sample models: short 1D chain, 2D lattice, and 2D parabolic
quantum wire.Comment: 12 pages, 12 figure
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
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
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
Regional scaling of annual mean precipitation and water availability with global temperature change
Changes in regional water availability belong to the most crucial potential impacts of anthropogenic climate change, but are highly uncertain. It is thus of key importance for stakeholders to assess the possible implications of different global temperature thresholds on these quantities. Using a subset of climate model simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5), we derive here the sensitivity of regional changes in precipitation and in precipitation minus evapotranspiration to global temperature changes. The simulations span the full range of available emission scenarios, and the sensitivities are derived using a modified pattern scaling approach. The applied approach assumes linear relationships on global temperature changes while thoroughly addressing associated uncertainties via resampling methods. This allows us to assess the full distribution of the simulations in a probabilistic sense. Northern high-latitude regions display robust responses towards wetting, while subtropical regions display a tendency towards drying but with a large range of responses. Even though both internal variability and the scenario choice play an important role in the overall spread of the simulations, the uncertainty stemming from the climate model choice usually accounts for about half of the total uncertainty in most regions. We additionally assess the implications of limiting global mean temperature warming to values below (i) 2 K or (ii) 1.5 K (as stated within the 2015 Paris Agreement). We show that opting for the 1.5 K target might just slightly influence the mean response, but could substantially reduce the risk of experiencing extreme changes in regional water availability
α2-macroglobulin and α1-inhibitor-3 mRNA expression in the rat liver after slow interleukin-1 stimulation
In this study we have investigated total fiver RNA and the expression of mRNA in the rat fiver in vivo after a slow stimulation of interleukin-1. A total dose of 4 μg interleukin-1β was administered via a subcutaneously implanted osmotic minipump over a period of 7 days. Plasma concentrations of α2-macroglobulin manifested a rapid increase, reaching a peak on day 2, while α1-inhibitor-3 manifested a marked initial decrease to 50% of the baseline level, followed by a tendency to increase again. For measurement of total RNA and specific mRNAs from the fiver, rats were sacrificed at different times during the experimental period. Total RNA peaked at 6 h, the level being approximately 60% higher than baseline value. Specific mRNA from the liver for α2-macroglobulin and α1-inhibitor-3 were quantified using laser densitometry on slot blots. The amounts measured during the experimental period agreed with the pattern of corresponding plasma protein levels. From barely detectable amounts at baseline, α2-macroglobulin mRNA peaked on day 1, and then declined. Levels of α1-inhibitor-3 mRNA manifested an initial increase at 3 h, but then declined and remained low until day 5 when there was a tendency towards an increase. It was concluded that the levels of plasma concentrations of α2-macroglobulin and α1-inhibitor-3 are mainly regulated at the protein synthesis level, and that long-term interleukin-1β release could not override the initial acute phase protein counteracting mechanism triggered
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
Searching for the quark-gluon plasma
The claims for production of high energy densities and possible new states of matter in collisions of nuclei by George F. Bertsch (Science, {\bf 265} (1994) 480-481) are examined and compared with simple explanations of the data which have appeared in the literature
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