154 research outputs found
Energy and power fluctuations in ac-driven coherent conductors
Using a scattering matrix approach we study transport in coherent conductors
driven by a time-periodic bias voltage. We investigate the role of
electron-like and hole-like excitations created by the driving in the energy
current noise and we reconcile previous studies on charge current noise in this
kind of systems. The energy noise reveals additional features due to
electron-hole correlations. These features should be observable in power
fluctuations. In particular, we show results for the case of a harmonic and
bi-harmonic driving and of Lorentzian pulses applied to a two-terminal
conductor, addressing the recent experiments of Refs. 1 and 2.Comment: 12 pages, 5 figure
Correlations between charge and energy current in ac-driven coherent conductors
We study transport in coherent conductors driven by a time-periodic bias
voltage. We present results of the charge and energy noise and complement them
by a study of the mixed noise, namely the zero-frequency correlator between
charge and energy current. The mixed noise presents interference contributions
and transport contributions, showing features different from those of charge
and energy noise. The mixed noise can be accessed by measuring the correlator
between the fluctuations of the power provided to the system and the charge
current.Comment: 8 pages, 1 figur
Self impedance matched Hall-effect gyrators and circulators
We present a model study of an alternative implementation of a two-port
Hall-effect microwave gyrator. Our set-up involves three electrodes, one of
which acts as a common ground for the others. Based on the capacitive-coupling
model of Viola and DiVincenzo, we analyze the performance of the device and we
predict that ideal gyration can be achieved at specific frequencies.
Interestingly, the impedance of the three-terminal gyrator can be made
arbitrarily small for certain coupling strengths, so that no auxiliary
impedance matching is required. Although the bandwidth of the device shrinks as
the impedance decreases, it can be improved by reducing the magnetic field; it
can be realistically increased up to at
by working at filling factor . We examine also the effects of the
parasitic capacitive coupling between electrodes and we find that, although in
general they strongly influence the response of device, their effect is
negligible at low impedance. Finally, we analyze an interferometric
implementation of a circulator, which incorporates the gyrator in a
Mach-Zender-like construction. Perfect circulation in both directions can be
achieved, depending on frequency and on the details of the interferometer
Spin transport and tunable Gilbert damping in a single-molecule magnet junction
We study time-dependent electronic and spin transport through an electronic
level connected to two leads and coupled with a single-molecule magnet via
exchange interaction. The molecular spin is treated as a classical variable and
precesses around an external magnetic field. We derive expressions for charge
and spin currents by means of the Keldysh non-equilibrium Green's functions
technique in linear order with respect to the time-dependent magnetic field
created by this precession. The coupling between the electronic spins and the
magnetization dynamics of the molecule creates inelastic tunneling processes
which contribute to the spin currents. The inelastic spin currents, in turn,
generate a spin-transfer torque acting on the molecular spin. This back-action
includes a contribution to the Gilbert damping and a modification of the
precession frequency. The Gilbert damping coefficient can be controlled by the
bias and gate voltages or via the external magnetic field and has a
non-monotonic dependence on the tunneling rates.Comment: 11 pages, 9 figures, final version including corrections to published
articl
Nonequilibrium phonon backaction on the current noise in atomic-sized junctions
We study backaction effects of phonon heating due to tunneling electrons on
the current noise in atomic-sized junctions. Deriving a generalized kinetic
approximation within the extended Keldysh Green's functions technique, we
demonstrate the existence of a characteristic backaction contribution to the
noise in case of low external phonon damping. We provide a physically intuitive
interpretation of this contribution at large voltage in terms of slow
fluctuations of the phonon occupation, and show that it generally gives a
significant correction to the noise above the phonon emission threshold.Comment: v2 - update: 4 pages, 1 figure, minor changes in the main text nearly
identical to the published version; the manuscript is supplemented by an
updated Mathematica notebook and a new supplementary note in PDF, which are
parts of the associated .zip bundl
Phonon-assisted current noise in molecular junctions
We investigate the effects of phonon scattering on the electronic current
noise through nanojunctions using the non-equilibrium Green's functions
formalism extended to include the counting field. In the case of weak
electron-phonon coupling and a single broad electronic level we derive an
analytic expression for the current noise at arbitrary temperature and identify
physically distinct contributions based on their voltage dependence. We apply
our theory to the experimentally relevant case of a deuterium molecule placed
in a break-junction and predict a significant inelastic contribution to the
current noise.Comment: 4 pages, 3 figures; updated versio
Transfer of a quantum state from a photonic qubit to a gate-defined quantum dot
Interconnecting well-functioning, scalable stationary qubits and photonic
qubits could substantially advance quantum communication applications and serve
to link future quantum processors. Here, we present two protocols for
transferring the state of a photonic qubit to a single-spin and to a two-spin
qubit hosted in gate-defined quantum dots (GDQD). Both protocols are based on
using a localized exciton as intermediary between the photonic and the spin
qubit. We use effective Hamiltonian models to describe the hybrid systems
formed by the the exciton and the GDQDs and apply simple but realistic noise
models to analyze the viability of the proposed protocols. Using realistic
parameters, we find that the protocols can be completed with a success
probability ranging between 85-97%
Thermoelectric performance of a driven double quantum dot
In this paper we investigate the thermoelectric performance of a double-dot
device driven by time-dependently modulated gate voltages. We show that if the
modulation frequency {\Omega} is sufficiently small, not only quantized charge
pumping can be realized, but also the heat current flowing in the leads is
quantized and exhibits plateaux in units of kB T ln2 {\Omega}/2{\pi}. The
factor ln2 stems from the degeneracy of the double-dot states involved into
transport. This opens the possibility of using the pumping cycle to transfer
heat against a temperature gradient or to extract work from a hot reservoir
with Carnot efficiency. However, the performance of a realistic device is
limited by dissipative effects due to leakage currents and finite-frequency
operation, which we take into account rigorously by means of a generalized
master equation approach in the regime where the double dot is weakly coupled
to the leads. We show that despite these effects, the efficiency of a
double-dot charge pump performing work against a dc-source can reach of up to
70% of the ideal value.Comment: 16 pages, 9 figure
Anomalous suppression of the shot noise in a nanoelectromechanical system
In this paper we report a relaxation-induced suppression of the noise for a
single level quantum dot coupled to an oscillator with incoherent dynamics in
the sequential tunneling regime. It is shown that relaxation induces
qualitative changes in the transport properties of the dot, depending on the
strength of the electron-phonon coupling and on the applied voltage. In
particular, critical thresholds in voltage and relaxation are found such that a
suppression below 1/2 of the Fano factor is possible. Additionally, the current
is either enhanced or suppressed by increasing relaxation, depending on bias
being greater or smaller than the above threshold. These results exist for any
strength of the electron-phonon coupling and are confirmed by a four states toy
model.Comment: 7 pages, 7 eps figures, submitted to PRB; minor changes in the
introductio
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