2,585 research outputs found
Ehrenfest-time dependence of weak localization in open quantum dots
Semiclassical theory predicts that the weak localization correction to the
conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time
exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys.
Rev. B {\bf 54}, 14424 (1996)]. We report numerical simulations of weak
localization in the open quantum kicked rotator that confirm this prediction.
Our results disagree with the `effective random matrix theory' of transport
through ballistic chaotic cavities.Comment: 4 pages, 2 figure
Classical limit of transport in quantum kicked maps
We investigate the behavior of weak localization, conductance fluctuations,
and shot noise of a chaotic scatterer in the semiclassical limit. Time resolved
numerical results, obtained by truncating the time-evolution of a kicked
quantum map after a certain number of iterations, are compared to semiclassical
theory. Considering how the appearance of quantum effects is delayed as a
function of the Ehrenfest time gives a new method to compare theory and
numerical simulations. We find that both weak localization and shot noise agree
with semiclassical theory, which predicts exponential suppression with
increasing Ehrenfest time. However, conductance fluctuations exhibit different
behavior, with only a slight dependence on the Ehrenfest time.Comment: 17 pages, 13 figures. Final versio
Impurity-assisted Andreev reflection at a spin-active half-metal-superconductor interface
The Andreev reflection amplitude at a clean interface between a half-metallic
ferromagnet (H) and a superconductor (S) for which the half metal's
magnetization has a gradient perpendicular to the interface is proportional to
the excitation energy and vanishes at [B\'{e}ri
{\em et al.}, Phys.\ Rev.\ B {\bf 79}, 024517 (2009)]. Here we show that the
presence of impurities at or in the immediate vicinity of the HS interface
leads to a finite Andreev reflection amplitude at . This
impurity-assisted Andreev reflection dominates the low-bias conductance of a HS
junction and the Josephson current of an SHS junction in the long-junction
limit.Comment: 12 pages, 2 figure
Coherent Electron Optics with Ballistically Coupled Quantum Point Contacts
The realization of integrated quantum circuits requires precise on-chip control of charge carriers. Aiming at the coherent coupling of distant nanostructures at zero magnetic field, here we study the ballistic electron transport through two quantum point contacts (QPCs) in series in a three terminal configuration. We enhance the coupling between the QPCs by electrostatic focusing using a field effect lens. To study the emission and collection properties of QPCs in detail we combine the electrostatic focusing with magnetic deflection. Comparing our measurements with quantum mechanical and classical calculations we discuss generic features of the quantum circuit and demonstrate how the coher
How many young star clusters exist in the Galactic center?
We study the evolution and observability of young compact star clusters
within about 200pc of the Galactic center. Calculations are performed using
direct N-body integration on the GRAPE-4, including the effects of both stellar
and binary evolution and the external influence of the Galaxy. The results of
these detailed calculations are used to calibrate a simplified model applicable
over a wider range of cluster initial conditions. We find that clusters within
200 pc from the Galactic center dissolve within about 70 Myr. However, their
projected densities drop below the background density in the direction of the
Galactic center within 20 Myr, effectively making these clusters undetectable
after that time. Clusters farther from the Galactic center but at the same
projected distance are more strongly affected by this selection effect, and may
go undetected for their entire lifetimes. Based on these findings, we conclude
that the region within 200 pc of the Galactic center could easily harbor some
50 clusters with properties similar to those of the Arches or the Quintuplet
systems.Comment: ApJ Letters in pres
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Impact of Nuclear Energy Futures on Advanced Fuel Cycle Options
The Nuclear Waste Policy Act requires the Secretary of Energy to inform Congress before 2010 on the need for a second geologic repository for spent nuclear fuel. By that time, the spent fuel discharged from current commercial reactors will exceed the statutory limit of the first repository. There are several approaches to eliminate the need for another repository in this century. This paper presents a high-level analysis of these spent fuel management options in the context of a full range of possible nuclear energy futures. The analysis indicates the best option to implement varies depending on the nuclear energy future selected
Nonequilibrium theory of Coulomb blockade in open quantum dots
We develop a non-equilibrium theory to describe weak Coulomb blockade effects
in open quantum dots. Working within the bosonized description of electrons in
the point contacts, we expose deficiencies in earlier applications of this
method, and address them using a 1/N expansion in the inverse number of
channels. At leading order this yields the self-consistent potential for the
charging interaction. Coulomb blockade effects arise as quantum corrections to
transport at the next order. Our approach unifies the phase functional and
bosonization approaches to the problem, as well as providing a simple picture
for the conductance corrections in terms of renormalization of the dot's
elastic scattering matrix, which is obtained also by elementary perturbation
theory. For the case of ideal contacts, a symmetry argument immediately allows
us to conclude that interactions give no signature in the averaged conductance.
Non-equilibrium applications to the pumped current in a quantum pump are worked
out in detail.Comment: Published versio
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