9 research outputs found
Floquet scattering in parametric electron pumps
A Floquet scattering approach to parametric electron pumps is presented and
compared with Brouwer's adiabatic scattering approach [Phys. Rev. B 58, R10135
(1998)] for a simple scattering model with two harmonically oscillating
delta-function barriers. For small strength of oscillating potentials these two
approaches give exactly equivalent results while for large strength, these
clearly deviate from each other. The validity of the adiabatic theory is also
discussed by using the Wigner delay time obtained from the Floquet scattering
matrix.Comment: 10 pages, 7 figure
High-harmonic generation from a confined atom
The order of high harmonics emitted by an atom in an intense laser field is
limited by the so-called cutoff frequency. Solving the time-dependent
Schr\"odinger equation, we show that this frequency can be increased
considerably by a parabolic confining potential, if the confinement parameters
are suitably chosen.
Furthermore, due to confinement, the radiation intensity remains high
throughout the extended emission range. All features observed can be explained
with classical arguments.Comment: 4 pages(tex files), 4 figures(eps files); added references and
comment
Quantum spin pumping with adiabatically modulated magnetic barrier's
A quantum pump device involving magnetic barriers produced by the deposition
of ferro magnetic stripes on hetero-structure's is investigated. The device for
dc- transport does not provide spin-polarized currents, but in the adiabatic
regime, when one modulates two independent parameters of this device, spin-up
and spin-down electrons are driven in opposite directions, with the net result
being that a finite net spin current is transported with negligible charge
current. We also analyze our proposed device for inelastic-scattering and
spin-orbit scattering. Strong spin-orbit scattering and more so inelastic
scattering have a somewhat detrimental effect on spin/charge ratio especially
in the strong pumping regime. Further we show our pump to be almost noiseless,
implying an optimal quantum spin pump.Comment: 14 pages, 9 figures. Manuscript revised with additional new material
on spin-orbit scattering and inelastic scattering. Further new additions on
noiseless pumping and analytical results with distinction between weak and
strong pumping regimes. Accepted for publication in Physical Review
Shortest paths on systems with power-law distributed long-range connections
We discuss shortest-path lengths on periodic rings of size L
supplemented with an average of pL randomly located long-range links whose
lengths are distributed according to P_l \sim l^{-\xpn}. Using rescaling
arguments and numerical simulation on systems of up to sites, we show
that a characteristic length exists such that for
. For small p we find
that the shortest-path length satisfies the scaling relation
\ell(r,\xpn,p)/\xi = f(\xpn,r/\xi). Three regions with different asymptotic
behaviors are found, respectively: a) \xpn>2 where , b)
1<\xpn<2 where 0<\theta_s(\xpn)<1/2 and, c) \xpn<1 where
behaves logarithmically, i.e. . The characteristic length is
of the form with \nu=1/(2-\xpn) in region b), but depends
on L as well in region c). A directed model of shortest-paths is solved and
compared with numerical results.Comment: 10 pages, 10 figures, revtex4. Submitted to PR
Enhancement of bichromatic high-harmonic generation with a high-frequency field
Using a high-frequency field superposed to a linearly polarized bichromatic
laser field composed by a wave with frequency and a wave with
frequency , we show it is possible to enhance the intensity of a
group of high harmonics in orders of magnitude. These harmonics have
frequencies about 30% higher than the monochromatic-cutoff frequency, and,
within the three-step-model framework, correspond to a set of electron
trajectories for which tunneling ionization is strongly suppressed. Particular
features in the observed enhancement suggest that the high-frequency field
provides an additional mechanism for the electron to reach the continuum. This
interpretation is supported by a time-frequency analysis of the harmonic yield.
The additional high frequency field permits the control of this group of
harmonics leaving all other sets of harmonics practically unchanged, which is
an advantage over schemes involving only bichromatic fields.Comment: 6 pages RevTex, 5 figures (ps files), Changes in text, figures,
references and equations include
Pauli blocking factors in quantum pumps
We investigate the Pauli blocking factor in quantum pumps using Floquet
formalism. Even though the time dependent potentials in quantum pumping can not
only cause inelastic scatterings but also break the micro-reversibility, i.e.
, the Pauli blocking factor is unnecessary when the
scattering process through the scatterer is coherent. The well defined
scattering states extending from one reservoir to the others form a complete
non-orthogonal set. Regardless of the non-orthogonality one can obtain the
pumped currents using the field operator formalism. The current expression
finally obtained do not contain Pauli blocking factor.Comment: 4 pages, 2 figure