670 research outputs found
Wigner Function Description of the A.C.-Transport Through a Two-Dimensional Quantum Point Contact
We have calculated the admittance of a two-dimensional quantum point contact
(QPC) using a novel variant of the Wigner distribution function (WDF)
formalism. In the semiclassical approximation, a Boltzman-like equation is
derived for the partial WDF describing both propagating and nonpropagating
electron modes in an effective potential generated by the adiabatic QPC. We
show that this quantum kinetic approach leads to the well-known stepwise
behavior of the real part of the admittance (the conductance), and of the
imaginary part of the admittance (the emittance), in agreement with the latest
results, which is determined by the number of propagating electron modes. It is
shown, that the emittance is sensitive to the geometry of the QPC, and can be
controlled by the gate voltage. We established that the emittance has
contributions corresponding to both quantum inductance and quantum capacitance.
Stepwise oscillations in the quantum inductance are determined by the harmonic
mean of the velocities for the propagating modes, whereas the quantum
capacitance is a significant mesoscopic manifestation of the non-propagating
(reflecting) modes.Comment: 23 pages (latex), 3 figure
Constructing Spin Interference Devices from Nanometric Rings
The study of nanospintronic devices utilizing coherent transport through
molecular scale multiply-connected geometries in the presence of moderate
magnetic fields is presented. It is shown how two types of simple devices, spin
filters and spin splitters (or Stern-Gerlach devices) may be constructed from
molecular nanometric rings utilizing the Aharonov-Bohm effect. The current is
calculated within a single electron approximation and within a many-body master
equation approach where charging effects are accounted for in the Coulomb
Blockade regime. We provide rules and tools to develop and analyze efficient
spintronic devices based on nanometric interferometers.Comment: 16 pages, 8 figures, submitted to Phys. Rev.
Hypersensitivity to Perturbations in the Quantum Baker's Map
We analyze a randomly perturbed quantum version of the baker's
transformation, a prototype of an area-conserving chaotic map. By numerically
simulating the perturbed evolution, we estimate the information needed to
follow a perturbed Hilbert-space vector in time. We find that the Landauer
erasure cost associated with this information grows very rapidly and becomes
much larger than the maximum statistical entropy given by the logarithm of the
dimension of Hilbert space. The quantum baker's map thus displays a
hypersensitivity to perturbations that is analogous to behavior found earlier
in the classical case. This hypersensitivity characterizes ``quantum chaos'' in
a way that is directly relevant to statistical physics.Comment: 8 pages, LATEX, 3 Postscript figures appended as uuencoded fil
Landauer Conductance without Two Chemical Potentials
We present a theory of the four--terminal conductance for the multi-channel
tunneling barrier, which is based on the self-consistent solution of
Shrodinger, Poisson and continuity equations. We derive new results for the
case of a barrier embedded in a long wire with and without disorder. We also
recover known expressions for the conductance of the barrier placed into a
ballistic constriction. Our approach avoids a problematic use of two chemical
potentials in the same system.Comment: 12 page
Backflow and dissipation during the quantum decay of a metastable Fermi liquid
The particle current in a metastable Fermi liquid against a first-order phase
transition is calculated at zero temperature. During fluctuations of a droplet
of the stable phase, in accordance with the conservation law, not only does an
unperturbed current arise from the continuity at the boundary, but a backflow
is induced by the density response. Quasiparticles carrying these currents are
scattered by the boundary, yielding a dissipative backflow around the droplet.
An energy of the hydrodynamic mass flow of the liquid and a friction force
exerted on the droplet by the quasiparticles have been obtained in terms of a
potential of their interaction with the droplet.Comment: 5 pages (REVTeX), to be published in Phys. Rev.
Analog quantum error correction
Quantum error-correction routines are developed for continuous quantum
variables such as position and momentum. The result of such analog quantum
error correction is the construction of composite continuous quantum variables
that are largely immune to the effects of noise and decoherence.Comment: Ten pages, Te
N-particle scattering matrix for electrons interacting on a quantum dot
We present a non-perturbative expression for the scattering matrix of
particles interacting inside a quantum dot. Characterizing the dot by its
resonances, we find a compact form for the scattering matrix in a real-time
representation. We study the transmission probabilities and interaction-induced
orbital entanglement of two electrons incident on the dot in a spin-singlet
state.Comment: 4 page
The Escape Problem for Irreversible Systems
The problem of noise-induced escape from a metastable state arises in
physics, chemistry, biology, systems engineering, and other areas. The problem
is well understood when the underlying dynamics of the system obey detailed
balance. When this assumption fails many of the results of classical
transition-rate theory no longer apply, and no general method exists for
computing the weak-noise asymptotics of fundamental quantities such as the mean
escape time. In this paper we present a general technique for analysing the
weak-noise limit of a wide range of stochastically perturbed continuous-time
nonlinear dynamical systems. We simplify the original problem, which involves
solving a partial differential equation, into one in which only ordinary
differential equations need be solved. This allows us to resolve some old
issues for the case when detailed balance holds. When it does not hold, we show
how the formula for the mean escape time asymptotics depends on the dynamics of
the system along the most probable escape path. We also present new results on
short-time behavior and discuss the possibility of focusing along the escape
path.Comment: 24 pages, APS revtex macros (version 2.1) now available from PBB via
`get oldrevtex.sty
The capacity of the noisy quantum channel
An upper limit is given to the amount of quantum information that can be
transmitted reliably down a noisy, decoherent quantum channel. A class of
quantum error-correcting codes is presented that allow the information
transmitted to attain this limit. The result is the quantum analog of Shannon's
bound and code for the noisy classical channel.Comment: 19 pages, Submitted to Science. Replaced give correct references to
work of Schumacher, to add a figure and an appendix, and to correct minor
mistake
Generalized Drude model: Unification of ballistic and diffusive electron transport
For electron transport in parallel-plane semiconducting structures, a model
is developed that unifies ballistic and diffusive transport and thus
generalizes the Drude model. The unified model is valid for arbitrary magnitude
of the mean free path and arbitrary shape of the conduction band edge profile.
Universal formulas are obtained for the current-voltage characteristic in the
nondegenerate case and for the zero-bias conductance in the degenerate case,
which describe in a transparent manner the interplay of ballistic and diffusive
transport. The semiclassical approach is adopted, but quantum corrections
allowing for tunneling are included. Examples are considered, in particular the
case of chains of grains in polycrystalline or microcrystalline semiconductors
with grain size comparable to, or smaller than, the mean free path. Substantial
deviations of the results of the unified model from those of the ballistic
thermionic-emission model and of the drift-diffusion model are found. The
formulation of the model is one-dimensional, but it is argued that its results
should not differ substantially from those of a fully three-dimensional
treatment.Comment: 14 pages, 5 figures, REVTEX file, to appear in J. Phys.: Condens.
Matte
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