5,478 research outputs found
Back-reaction of perturbation wave packets on gray solitons
Within the Bogoliubov-de Gennes linearization theory of quantum or classical
perturbations around a background solution to the one-dimensional nonlinear
Schr\"odinger equation, we study the back-reaction of wave packet perturbations
on a gray soliton background. From our recently published exact solutions, we
determine that a wave packet effectively jumps ahead as it passes through a
soliton, emerging with a wavelength-dependent forward translation in comparison
to its motion in absence of the soliton. From this and from the full theory's
exact momentum conservation, we deduce that post-Bogoliubov back-reaction must
include a commensurate forward advance by the soliton itself. We quantify this
effect with a simple theory, and confirm that it agrees with full numerical
solution of the classical nonlinear Schr\"odinger equation. We briefly discuss
the implications of this effect for quantum behavior of solitons in
quasi-condensed dilute gases at finite temperature.Comment: 12 pages, 2 figure
Electron transport through multilevel quantum dot
Quantum transport properties through some multilevel quantum dots sandwiched
between two metallic contacts are investigated by the use of Green's function
technique. Here we do parametric calculations, based on the tight-binding
model, to study the transport properties through such bridge systems. The
electron transport properties are significantly influenced by (a) number of
quantized energy levels in the dots, (b) dot-to-electrode coupling strength,
(c) location of the equilibrium Fermi energy and (d) surface disorder. In
the limit of weak-coupling, the conductance () shows sharp resonant peaks
associated with the quantized energy levels in the dots, while, they get
substantial broadening in the strong-coupling limit. The behavior of the
electron transfer through these systems becomes much more clearly visible from
our study of current-voltage (-) characteristics. In this context we also
describe the noise power of current fluctuations () and determine the Fano
factor () which provides an important information about the electron
correlation among the charge carriers. Finally, we explore a novel transport
phenomenon by studying the surface disorder effect in which the current
amplitude increases with the increase of the surface disorder strength in the
strong disorder regime, while, the amplitude decreases in the limit of weak
disorder. Such an anomalous behavior is completely opposite to that of bulk
disordered system where the current amplitude always decreases with the
disorder strength. It is also observed that the current amplitude strongly
depends on the system size which reveals the finite quantum size effect.Comment: 12 pages, 7 figure
A mesoscopic ring as a XNOR gate: An exact result
We describe XNOR gate response in a mesoscopic ring threaded by a magnetic
flux . The ring is attached symmetrically to two semi-infinite
one-dimensional metallic electrodes and two gate voltages, viz, and
, are applied in one arm of the ring which are treated as the inputs of
the XNOR gate. The calculations are based on the tight-binding model and the
Green's function method, which numerically compute the conductance-energy and
current-voltage characteristics as functions of the ring-to-electrode coupling
strength, magnetic flux and gate voltages. Our theoretical study shows that,
for a particular value of () (, the elementary
flux-quantum), a high output current (1) (in the logical sense) appears if both
the two inputs to the gate are the same, while if one but not both inputs are
high (1), a low output current (0) results. It clearly exhibits the XNOR gate
behavior and this aspect may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure
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