10,368 research outputs found
Theory of semi-ballistic wave propagation
Wave propagation through waveguides, quantum wires or films with a modest
amount of disorder is in the semi-ballistic regime when in the transversal
direction(s) almost no scattering occurs, while in the long direction(s) there
is so much scattering that the transport is diffusive. For such systems
randomness is modelled by an inhomogeneous density of point-like scatterers.
These are first considered in the second order Born approximation and then
beyond that approximation. In the latter case it is found that attractive point
scatterers in a cavity always have geometric resonances, even for Schr\"odinger
wave scattering. In the long sample limit the transport equation is solved
analytically. Various geometries are considered: waveguides, films, and
tunneling geometries such as Fabry-P\'erot interferometers and double barrier
quantum wells. The predictions are compared with new and existing numerical
data and with experiment. The agreement is quite satisfactory.Comment: 24 pages Revtex; 10 figure
Zero-conductance resonances and spin-filtering effects in ring conductors subject to Rashba coupling
We investigate the effect of Rashba spin-orbit coupling and of a tunnel
barrier on the zero conduc- tance resonances appearing in a one-dimensional
conducting Aharonov-Bohm (AB) ring symmet- rically coupled to two leads. The
transmission function of the corresponding one-electron problem is derived
within the scattering matrix approach and analyzed in the complex energy plane
with focus on the role of the tunnel barrier strength on the zero-pole
structure characteristic of trans- mission (anti)resonances. The lifting of the
real conductance zeros is related to the breaking of the spin-reversal symmetry
and time-reversal symmetry of Aharonov-Casher (AC)and AB rings, as well as to
rotational symmetry breaking in presence of a tunnel barrier. We show that the
polarization direction of transmitted electrons can be controlled via the
tunnel barrier strength and discuss a novel spin-filtering design in
one-dimensional rings with tunable spin-orbit interaction.Comment: 13 pages, 8 figure
Quantum Size Effect in Conductivity of Multilayer Metal Films
Conductivity of quantized multilayer metal films is analyzed with an emphasis
on scattering by rough interlayer interfaces. Three different types of quantum
size effect (QSE) in conductivity are predicted. Two of these QSE are similar
to those in films with scattering by rough walls. The third type of QSE is
unique and is observed only for certain positions of the interface. The
corresponding peaks in conductivity are very narrow and high with a finite
cutoff which is due only to some other scattering mechanism or the smearing of
the interface. There are two classes of these geometric resonances. Some of the
resonance positions of the interface are universal and do not depend on the
strength of the interface potential while the others are sensitive to this
potential. This geometric QSE gradually disappears with an increase in the
width of the interlayer potential barrier.Comment: 12 pages, 10 figures, RevTeX4, to be published in Phys. Rev B (April
2003
A Unified Theory of Quasibound States
We have developed a formalism that includes both quasibound states with real
energies and quantum resonances within the same theoretical framework, and that
admits a clean and unambiguous distinction between these states and the states
of the embedding continuum. States described broadly as 'quasibound' are
defined as having a connectedness (in the mathematical sense) to true bound
states through the growth of some parameter. The approach taken here builds on
our earlier work by clarifying several crucial points and extending the
formalism to encompass a variety of continuous spectra, including those with
degenerate energy levels. The result is a comprehensive framework for the study
of quasibound states. The theory is illustrated by examining several cases
pertinent to applications widely discussed in the literature
Tunable superlattice p-i-n photodetectors: characteristics, theory, and application
Extended measurements and theory on the recently developed monolithic wavelength demultiplexer consisting of voltage-tunable superlattice p-i-n photodetectors in a waveguide confirmation are discussed. It is shown that the device is able to demultiplex and detect two optical signals with a wavelength separation of 20 nm directly into different electrical channels at a data rate of 1 Gb/s and with a crosstalk attenuation varying between 20 and 28 dB, depending on the polarization. The minimum acceptable crosstalk attenuation at a data rate of 100 Mb/s is determined to be 10 dB. The feasibility of using the device as a polarization angle sensor for linearly polarized light is also demonstrated. A theory for the emission of photogenerated carriers out of the quantum wells is included, since this is potentially a speed limiting mechanism in these detectors. It is shown that a theory of thermally assisted tunneling by polar optical phonon interaction is able to predict emission times consistent with the observed temporal response
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