7,355 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
Quantum dot dephasing by edge states
We calculate the dephasing rate of an electron state in a pinched quantum
dot, due to Coulomb interactions between the electron in the dot and electrons
in a nearby voltage biased ballistic nanostructure. The dephasing is caused by
nonequilibrium time fluctuations of the electron density in the nanostructure,
which create random electric fields in the dot. As a result, the electron level
in the dot fluctuates in time, and the coherent part of the resonant
transmission through the dot is suppressed
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Design, characterization, and fabrication of solar-retroreflective cool-wall materials
Raising urban albedo increases the fraction of incident sunlight returned to outer space, cooling cities and their buildings. We evaluated the angular distribution of solar radiation incident on exterior walls in 17 U S. climates to develop performance parameters for solar-retroreflective walls, then applied first-principle physics and ray-tracing simulations to explore designs. Our analysis indicates that retroreflective walls must function at large incidence angles to reflect a substantial portion of summer sunlight, and that this will be difficult to attain with materials that rely on total internal reflection. Gonio-spectrophotometer measurements of the solar spectral bi-directional reflectivity of a bicycle reflector showed little to no retroreflection at large incidence angles. Visual comparisons of retroreflection to specular first-surface reflection for four different retroreflective safety films using violet and green lasers suggest their retroreflection to be no greater than 0.09 at incidence angles up to 45°, and no greater than 0.30 at incidence angles of up to 70°. Attempts to produce a two-surface retroreflector with orthogonal mirror grooves by cutting and polishing an aluminum block indicate that residual surface roughness impedes retroreflection. Ongoing efforts focus on forming orthogonal surfaces with aluminized Mylar film, a material with very high specular reflectance across the solar spectrum. We investigated (1) folding or stamping a free film; (2) adhering the film to a pre-shaped substrate; or (3) attaching the film to a flat ductile substrate, then shaping. The latter two methods were more successful but yielded imperfect right angles
Stress Induced Anisotropy in Pressurized Thick Walled Cylinders
The most important mechanical features of propellants arise
from the presence of a highly packed array of granular particles (filler), and a distribution of adhesive strengths between the rubbery binder and these particles. The first factor leads to dilatation and the formation of voids in any stress field other than pure hydrostatic compression. The second factor virtually guarantees that the pullaway of the binder from the filler is nonuniform, leading in extreme cases to the so-called "zebra-stripe" effect, or localized dewetting. This factor also is associated with stress relaxation due to the slow flow of the binder from regions of high strain concentration into regions of low concentration or into voids. Finally, because the binder is incompressible, and the filler is for all practical purposes infinitely rigid, most of the macroscopically applied load is concentrated as large strains near the binder-filler interfaces leading to non-linear behavior. At ambient temperature or thereabouts,
viscoelasticity as associated with polymer chain uncoiling plays no role in the mechanical behavior of the propellant. Summarizing, the important mechanical features to be expected are
1. Dilatation with void formation when the stress is tensile.
2. Localized dilatation because of nonuniformity of adhesion
strengths.
3. Stress relaxation due to binder flow and perhaps due to
particle movement at a very slow rate determined by frictional and adhesive effects .
4. Nonlinear stress-strain relations due to high local strains at binder-filler interfaces
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
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
Entanglement, measurement, and conditional evolution of the Kondo singlet interacting with a mesoscopic detector
We investigate various aspects of the Kondo singlet in a quantum dot (QD)
electrostatically coupled to a mesoscopic detector. The two subsystems are
represented by an entangled state between the Kondo singlet and the
charge-dependent detector state. We show that the phase-coherence of the Kondo
singlet is destroyed in a way that is sensitive to the charge-state information
restored both in the magnitude and in the phase of the scattering coefficients
of the detector. We also introduce the notion of the `conditional evolution' of
the Kondo singlet under projective measurement on the detector. Our study
reveals that the state of the composite system is disentangled upon this
measurement. The Kondo singlet evolves into a particular state with a fixed
number of electrons in the quantum dot. Its relaxation time is shown to be
sensitive only to the QD-charge dependence of the transmission probability in
the detector, which implies that the phase information is erased in this
conditional evolution process. We discuss implications of our observations in
view of the possible experimental realization.Comment: Focus issue on "Interference in Mesoscopic Systems" of New J. Phy
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