5,763 research outputs found
Quantized adiabatic quantum pumping due to interference
Recent theoretical calculations, demonstrating that quantized charge transfer
due to adiabatically modulated potentials in mesoscopic devices can result
purely from the interference of the electron wave functions (without invoking
electron-electron interactions) are reviewed: (1) A new formula is derived for
the pumped charge Q (per period); It reproduces the Brouwer formula without a
bias, and also yields the effect of the modulating potential on the Landauer
formula in the presence of a bias. (2) For a turnstile geometry, with
time-dependent gate voltages V_L(t) and V_R(t), the magnitude and sign of Q are
determined by the relative position and orientation of the closed contour
traversed by the system in the {V_L-V_R} plane, relative to the transmission
resonances in that plane. Integer values of Q (in units of e) are achieved when
a transmission peak falls inside the contour, and are given by the winding
number of the contour. (3) When the modulating potential is due to surface
acoustic waves, Q exhibits a staircase structure, with integer values,
reminiscent of experimental observations.Comment: Invited talk, Localization, Tokyo, August 200
Transmission of two interacting electrons
The transmission of two electrons through a region where they interact is
found to be enhanced by a renormalization of the repulsive interaction. For a
specific example of the single-particle Hamiltonian, which includes a strongly
attractive potential, the renormalized interaction becomes attractive, and the
transmission has a pronounced maximum as function of the depth of the
single-electron attractive potential. The results apply directly to a simple
model of scattering of two interacting electrons by a quantum dot.Comment: 12 pages, 2 figure
Velocity shift of surface acoustic waves due to interaction with composite fermions in a modulated structure
We study the effect of a periodic density modulation on surface acoustic wave
(SAW) propagation along a 2D electron gas near Landau level filling .
Within the composite fermion theory, the problem is described in terms of
fermions subject to a spatially modulated magnetic field and scattered by a
random magnetic field. We find that a few percent modulation induces a large
peak in the SAW velocity shift, as has been observed recently by Willett et al.
As further support of this theory we find the dc resistivity to be in good
agreement with recent data of Smet et al.Comment: revised version resubmitted to PRL. Part concerning dc transport
corrected and extended. A new figure showing dc resistivity in comparison
with experiment of Smet et al include
Acoustoelectric current and pumping in a ballistic quantum point contact
The acoustoelectric current induced by a surface acoustic wave (SAW) in a
ballistic quantum point contact is considered using a quantum approach. We find
that the current is of the "pumping" type and is not related to drag, i.e. to
the momentum transfer from the wave to the electron gas. At gate voltages
corresponding to the plateaus of the quantized conductance the current is
small. It is peaked at the conductance step voltages. The peak current
oscillates and decays with increasing SAW wavenumber for short wavelengths.
These results contradict previous calculations, based on the classical
Boltzmann equation.Comment: 4 pages, 1 figur
Acoustoelectric pumping through a ballistic point contact in the presence of magnetic fields
The acoustoelectric current, J, induced in a ballistic point contact (PC) by
a surface acoustic wave is calculated in the presence of a perpendicular
magnetic field, B. It is found that the dependence of the current on the Fermi
energy in the terminals is strongly correlated with that of the PC conductance:
J is small at the conductance plateaus, and is large at the steps. Like the
conductance, the acoustoelectric current has the same functional behavior as in
the absence of the field, but with renormalized energy scales, which depend on
the strength of the magnetic field, | B|.Comment: 7 page
Approximated maximum likelihood estimation in multifractal random walks
We present an approximated maximum likelihood method for the multifractal
random walk processes of [E. Bacry et al., Phys. Rev. E 64, 026103 (2001)]. The
likelihood is computed using a Laplace approximation and a truncation in the
dependency structure for the latent volatility. The procedure is implemented as
a package in the R computer language. Its performance is tested on synthetic
data and compared to an inference approach based on the generalized method of
moments. The method is applied to estimate parameters for various financial
stock indices.Comment: 8 pages, 3 figures, 2 table
Collimation and Radiative Deceleration of Jets in TeV AGNs
We consider some implications of the rapid X-ray and TeV variability observed
in M87 and the TeV blazars. We outline a model for jet focusing and demonstrate
that modest radiative cooling can lead to recollimation of a relativistic jet
in a nozzle having a very small cross-sectional radius. Such a configuration
can produce rapid variability at large distances from the central engine and
may explain recent observations of the HST-1 knot in M87. Possible applications
of this model to TeV blazars are discussed. We also discuss a scenario for the
very rapid TeV flares observed with HESS and MAGIC in some blazars, that
accommodates the relatively small Doppler factors inferred from radio
observations.Comment: 5 pages. Proceedings of "High Energy Phenomena in Relativistic
Outflows", held in Dublin, Ireland, September 24-28, 200
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