4,374 research outputs found
A spin pump turnstile: parametric pumping of a spin-polarized current through a nearly-closed quantum dot
We investigate parametric pumping of a spin-polarized current through a
nearly-closed quantum dot in a perpendicular magnetic field. Pumping is
achieved by tuning the tunnel couplings to the left and right lead - thereby
operating the quantum dot as a turnstile - and changing either the magnetic
field or a gate-voltage. We analyze the quantum dynamics of a pumping cycle and
the limiting time scales for operating the quantum dot turnstile as a pure spin
pump. The proposed device can be used as a fully controllable double-sided and
bipolar spin filter and to inject spins "on demand".Comment: 5 pages, 2 figures, one reference correcte
Rectification of displacement currents in an adiabatic electron pump
Rectification of ac displacement currents generated by periodic variation of
two independent gate voltages of a quantum dot can lead to a dc voltage linear
in the frequency. The presence of this rectified displacement current could
account for the magnetic field symmetry observed in a recent measurement on an
adiabatic quantum electron pump by Switkes et al. [Science 283, 1905 (1999)].Comment: 2 pages, RevTeX; 1 figur
The effects of Chern-Simons gravity on bodies orbiting the Earth
One of the possible low-energy consequences of string theory is the addition
of a Chern-Simons term to the standard Einstein-Hilbert action of general
relativity. It can be argued that the quintessence field should couple to this
Chern-Simons term, and if so, it drives in the linearized theory a
parity-violating interaction between the gravito-electric and gravitomagnetic
fields. In this paper, the linearized spacetime for Chern-Simons gravity around
a massive spinning body is found to include new modifications to the
gravitomagnetic field that have not appeared in previous work. The orbits of
test bodies and the precession of gyroscopes in this spacetime are calculated,
leading to new constraints on the Chern-Simons parameter space due to current
satellite experiments.Comment: 9 pages, 2 figures; minor corrections made; to appear in PR
Current induced transverse spin-wave instability in thin ferromagnets: beyond linear stability analysis
A sufficiently large unpolarized current can cause a spin-wave instability in
thin nanomagnets with asymmetric contacts. The dynamics beyond the instability
is understood in the perturbative regime of small spin-wave amplitudes, as well
as by numerically solving a discretized model. In the absence of an applied
magnetic field, our numerical simulations reveal a hierarchy of instabilities,
leading to chaotic magnetization dynamics for the largest current densities we
consider.Comment: 14 pages, 10 figures; revtex
Andreev interference in adiabatic pumping
Within the scattering approach, we develop a model for adiabatic quantum
pumping in hybrid normal/superconductor systems where several superconducting
leads are present. This is exploited to study Andreev-interference effects on
adiabatically pumped charge in a 3-arm beam splitter attached to one normal and
two superconducting leads with different phases of the order parameters. We
derive expressions for the pumped charge through the normal lead for different
parameters for the scattering region, and elucidate the effects due to Andreev
interference. In contrast to what happens for voltage-driven transport, Andreev
interference does not yield in general a pumped current which is a symmetric
function of the superconducting-phase difference.Comment: 4 pages, 1 figur
Photonic excess noise and wave localization
This is a theory for the effect of localization on the super-Poissonian noise
of radiation propagating through an absorbing disordered waveguide.
Localization suppresses both the mean photon current I and the noise power P,
but the Fano factor P/I is found to remain unaffected. For strong absorption
the Fano factor has the universal value 1+3f/2 (with f the Bose-Einstein
function), regardless of whether the waveguide is long or short compared to the
localization length.Comment: 3 pages including 3 figure
Effects of interaction on an adiabatic quantum electron pump
We study the effects of inter-electron interactions on the charge pumped
through an adiabatic quantum electron pump. The pumping is through a system of
barriers, whose heights are deformed adiabatically. (Weak) interaction effects
are introduced through a renormalisation group flow of the scattering matrices
and the pumped charge is shown to {\it always} approach a quantised value at
low temperatures or long length scales. The maximum value of the pumped charge
is set by the number of barriers and is given by . The
correlation between the transmission and the charge pumped is studied by seeing
how much of the transmission is enclosed by the pumping contour. The (integer)
value of the pumped charge at low temperatures is determined by the number of
transmission maxima enclosed by the pumping contour. The dissipation at finite
temperatures leading to the non-quantised values of the pumped charge scales as
a power law with the temperature (), or with
the system size (), where is a
measure of the interactions and vanishes at . For a double
barrier system, our result agrees with the quantisation of pumped charge seen
in Luttinger liquids.Comment: 9 pages, 9 figures, better quality figures available on request from
author
Distributions of the Conductance and its Parametric Derivatives in Quantum Dots
Full distributions of conductance through quantum dots with single-mode leads
are reported for both broken and unbroken time-reversal symmetry. Distributions
are nongaussian and agree well with random matrix theory calculations that
account for a finite dephasing time, , once broadening due to finite
temperature is also included. Full distributions of the derivatives of
conductance with respect to gate voltage are also investigated.Comment: 4 pages (REVTeX), 4 eps figure
Voltage-probe and imaginary potential models for dephasing in a chaotic quantum dot
We compare two widely used models for dephasing in a chaotic quantum dot: The
introduction of a fictitious voltage probe into the scattering matrix and the
addition of an imaginary potential to the Hamiltonian. We identify the limit in
which the two models are equivalent and compute the distribution of the
conductance in that limit. Our analysis explains why previous treatments of
dephasing gave different results. The distribution remains non-Gaussian for
strong dephasing if the coupling of the quantum dot to the electron reservoirs
is via ballistic single-mode point contacts, but becomes Gaussian if the
coupling is via tunneling contacts.Comment: 9 pages, RevTeX, 6 figures. Mistake in Eq. (35) correcte
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