344 research outputs found
Quasienergy spectrum and tunneling current in ac-driven triple quantum dot shuttles
The dynamics of electrons in ac driven double quantum dots have been
extensively analyzed by means of Floquet theory. In these systems, coherent
destruction of tunneling has been shown to occur for certain ac field
parameters. In the present work we analyze, by means of Floquet theory, the
electron dynamics of a triple quantum dot in series attached to electric
contacts, where the central dot position oscillates. In particular, we analyze
the quasienergy spectrum of this ac driven nanoelectromechanical system, as a
function of the intensity and frequency of the ac field and of external dc
voltages. For strong driving fields, we derive, by means of perturbation
theory, analytical expressions for the quasienergies of the driven oscillator
system. From this analysis we discuss the conditions for coherent destruction
of tunneling (CDT) to occur as a function of detuning and field parameters. For
zero detuning, and from the invariance of the Floquet Hamiltonian under a
generalized parity transformation, we find analytical expressions describing
the symmetry properties of the Fourier components of the Floquet states under
such transformation. By using these expressions, we show that in the vicinity
of the CDT condition, the quasienergy spectrum exhibits exact crossings which
can be characterized by the parity properties of the corresponding
eigenvectors
Dynamical nuclear spin polarization induced by electronic current through double quantum dots
We analyze electron spin relaxation in electronic transport through
coherently coupled double quantum dots in the spin blockade regime. In
particular, we focus on hyperfine interaction as the spin relaxation mechanism.
We pay special attention to the effect of the dynamical nuclear spin
polarization induced by the electronic current on the nuclear environment. We
discuss the behaviour of the electronic current and the induced nuclear spin
polarization versus an external magnetic field for different hyperfine coupling
intensities and interdot tunnelling strengths. We take into account, for each
magnetic field, all hyperfine mediated spin relaxation processes coming from
the different opposite spin levels approaches. We find that the current as a
function of the external magnetic field shows a peak or a dip, and that the
transition from a current dip to a current peak behaviour is obtained by
decreasing the hyperfine coupling or by increasing the interdot tunnelling
strength. We give a physical picture in terms of the interplay between the
electrons tunnelling out of the double quantum dot and the spin flip processes
due to the nuclear environment.Comment: 25 pages and 8 figures. To be published in New Journal of Physic
Bipolar spin blockade and coherent state superpositions in a triple quantum dot
Spin qubits based on interacting spins in double quantum dots have been
successfully demonstrated. Readout of the qubit state involves a conversion of
spin to charge information, universally achieved by taking advantage of a spin
blockade phenomenon resulting from Pauli's exclusion principle. The archetypal
spin blockade transport signature in double quantum dots takes the form of a
rectified current. Currently more complex spin qubit circuits including triple
quantum dots are being developed. Here we show both experimentally and
theoretically (a) that in a linear triple quantum dot circuit, the spin
blockade becomes bipolar with current strongly suppressed in both bias
directions and (b) that a new quantum coherent mechanism becomes relevant.
Within this mechanism charge is transferred non-intuitively via coherent states
from one end of the linear triple dot circuit to the other without involving
the centre site. Our results have implications in future complex
nano-spintronic circuits.Comment: 21 pages, 7 figure
Transport properties of a molecule embedded in an Aharonov-Bohm interferometer
We theoretically investigate the transport properties of a molecule embedded
in one arm of a mesoscopic Aharonov-Bohm interferometer. Due to the presence of
phonons the molecule level position () and the electron-electron
interaction () undergo a \emph{polaronic shift} which affects dramatically
the electronic transport through the molecular junction. When the
electron-phonon interaction is weak the linear conductance presents Fano-line
shapes as long as the direct channel between the electrodes is opened. The
observed Fano resonances in the linear conductance are originated from the
interference between the spin Kondo state and the direct path. For strong
enough electron-phonon interaction, the electron-electron interaction is
renormalized towards negative values, {\it i.e.} becomes effectively
attractive. This scenario favors fluctuations between the empty and doubly
occupied charge states and therefore promotes a charge Kondo effect. However,
the direct path between the contacts breaks the electron-hole symmetry which
can efficiently suppress this charge Kondo effect. Nevertheless, we show that a
proper tuning of the gate voltage is able to revive the Kondo resonance. Our
results are obtained by using the Numerical Renormalization approximation to
compute the electronic spectral function and the linear conductance.Comment: 17 pages, 12 figure
Voltage dip generator for testing wind turbines connected to electrical networks
This paper describes a new voltage dip generator that allows the shape of the time profile of the voltage generated to be configured. The use of this device as a tool to test the fault ride-through capability of wind turbines connected to the electricity grid can provide some remarkable benefits: First, this system offers the possibility of adapting the main features of the time–voltage profile generated (dip depth, dip duration, the ramp slope during the recovery process after clearing fault, etc.) to the specific requirements set forth by the grid operation codes, in accordance with different network electrical systems standards. Second, another remarkable ability of this system is to provide sinusoidal voltage and current wave forms during the overall testing process without the presence of harmonic components. This is made possible by the absence of electronic converters. Finally, the paper includes results and a discussion on the experimental data obtained with the use of a reduced size laboratory prototype that was constructed to validate the operating features of this new device
Spin correlations in spin blockade
We investigate spin currents and spin-current correlations for double quantum
dots in the spin blockade regime. By analysing the time evolution of the
density matrix, we obtain the spin resolved currents and derive from a
generating function an expression for the fluctuations and correlations. Both
the charge current and the spin current turn out to be generally
super-Poissonian. Moreover, we study the influence of ac fields acting upon the
transported electrons. In particular, we focus on fields that cause spin
rotation or photon-assisted tunnelling.Comment: 14 pages, 9 figure
Gain in quantum cascade lasers and superlattices: A quantum transport theory
Gain in current-driven semiconductor heterostructure devices is calculated
within the theory of nonequilibrium Green functions. In order to treat the
nonequilibrium distribution self-consistently the full two-time structure of
the theory is employed without relying on any sort of Kadanoff-Baym Ansatz. The
results are independent of the choice of the electromagnetic field if the
variation of the self-energy is taken into account. Excellent quantitative
agreement is obtained with the experimental gain spectrum of a quantum cascade
laser. Calculations for semiconductor superlattices show that the simple 2-time
miniband transport model gives reliable results for large miniband widths at
room temperatureComment: 8 Pages, 4 Figures directly included, to appear in Physical Review
Canted phase in double quantum dots
We perform a Hartree-Fock calculation in order to describe the ground state
of a vertical double quantum dot in the absence of magnetic fields parallel to
the growth direction. Intra- and interdot exchange interactions determine the
singlet or triplet character of the system as the tunneling is tuned. At finite
Zeeman splittings due to in-plane magnetic fields, we observe the continuous
quantum phase transition from ferromagnetic to symmetric phase through a canted
antiferromagnetic state. The latter is obtained even at zero Zeeman energy for
an odd electron number.Comment: 5 pages, 3 figure
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