ac-Driven double quantum dots as spin pumps and spin filters

Quantum interference effects in multi-channel correlated tunneling structures

In multi-channel tunneling systems quantum interference effects modify tunneling conductance spectra due to Fano effect. We investigated the impact of Hubbard type Coulomb interaction on tunneling conductance spectra for the system formed by several interacting impurity atoms or quantum dots localised between the contact leads. It was shown that the Fano shape of tunneling conductance spectra strongly changes in the presence of on-site Coulomb interaction between localised electrons in the intermediate system. The main effect which determines the shape of the tunneling peaks could be not Fano interference but mostly nonequilibrium dependence of the occupation numbers on bias voltage.

Fluctuation relations for adiabatic pumping

We derive an extended fluctuation relation for an open system coupled with two reservoirs under adiabatic one-cycle modulation. We confirm that the geometrical phase caused by the Berry-Sinitsyn-Nemenman curvature in the parameter space generates non-Gaussian fluctuations. This non-Gaussianity is enhanced for the instantaneous fluctuation relation when the bias between the two reservoirs disappears.

Nonadiabatic Control of Geometric Pumping

We study nonadiabatic effects of geometric pumping. With arbitrary choices of periodic control parameters, we go beyond the adiabatic approximation to obtain the exact pumping current. We find that a geometrical interpretation for the nontrivial part of the current is possible even in the nonadiabatic regime. The exact result allows us to find a smooth connection between the adiabatic Berry phase theory at low frequencies and the Floquet theory at high frequencies. We also study how to control the geometric current. Using the method of shortcuts to adiabaticity with the aid of an assisting field, we illustrate that it enhances the current.

Correlated impurity complex in the asymmetric tunneling contact: an ideal system to observe negative tunneling conductivity

We studied theoretically electron transport through the impurity complex localized between the tunneling contact leads by means of the generalized Keldysh diagram technique. The formation of multiple well pronounced regions with negative tunneling conductivity in the I-V characteristics was revealed. The appearance of negative tunneling conductivity is caused by the presence of both strong Coulomb correlations and the asymmetry of tunneling rates, which lead to the blockade of the electron transport through the system for a certain values of applied bias. The developed theory and obtained results may be useful for the application of impurity (dopant) atoms as a basic elements in modern nanoelectronic circuits.

Nonadiabaticity in Quantum Pumping Phenomena under Relaxation

The ability to control quanta shown by quantum pumping has been intensively studied, aiming to further develop nano fabrication. In accordance with the fast progress of the experimental techniques, the focus on quantum pumping extends to include the quicker transport. For this purpose, it is necessary to remove the “adiabatic” or “slow” condition, which has been the central concept of quantum pumping since its first proposal for a closed system. In this article, we review the studies which go beyond the conventional adiabatic approximation for open quantum systems to transfer energy quanta and electron spins with using the full counting statistics. We also discuss the recent developments of the nonadiabatic treatments of quantum pumping.

Probing and driving of spin and charge states in double quantum dot under the quench

We have analyzed theoretically quenched dynamics of correlated double quantum dot (DQD) due to the switching “on” and “off” coupling to reservoirs. The possibility for controllable manipulation of charge and spin states in the double quantum dot was revealed and discussed. The proposed experimental scheme allows to prepare in DQD maximally entangled pure triplet state and to drive it to another entangled singlet state by tuning both applied bias and gate voltage. It was also demonstrated that the symmetry properties of the total system (double quantum dot coupled to electron reservoirs) allow to resolve the initially prepared two-electron states by detecting non-stationary spin-polarized currents flowing in both reservoirs and controlling the residual charge.

Collective spin correlations and entangled state dynamics in coupled quantum dots

Here we demonstrate that the dynamics of few-electron states in a correlated quantum-dot system coupled to an electronic reservoir is governed by the symmetry properties of the total system leading to the collective behavior of all the electrons. Time evolution of two-electron states in a correlated double quantum dot after coupling to the reservoir has been analyzed by means of kinetic equations for pseudoparticle occupation numbers with constraint on possible physical states. It was revealed that the absolute value of the spin correlation function and the degree of entanglement for two-electron states could considerably increase after coupling to the reservoir. The obtained results demonstrate the possibility of a controllable tuning of both the spin correlation function and the concurrence value in a coupled quantum-dot system by changing of the gate voltage applied to the barrier separating the dots.

Geometric fluctuation theorem for a spin-boson system

We derive an extended fluctuation theorem for geometric pumping of a spin-boson system under periodic control of environmental temperatures by using a Markovian quantum master equation. We obtain the current distribution, the average current, and the fluctuation in terms of the Monte Carlo simulation. To explain the results of our simulation we derive an extended fluctuation theorem. This fluctuation theorem leads to the fluctuation dissipation relations but the absence of the conventional reciprocal relation.

Optically induced transport through semiconductor-based molecular electronics

A tight binding model is used to investigate photoinduced tunneling current through a molecular bridge coupled to two semiconductor electrodes. A quantum master equation is developed within a non-Markovian theory based on second-order perturbation theory with respect to the molecule-semiconductor electrode coupling. The spectral functions are generated using a one dimensional alternating bond model, and the coupling between the molecule and the electrodes is expressed through a corresponding correlation function. Since the molecular bridge orbitals are inside the bandgap between the conduction and valence bands, charge carrier tunneling is inhibited in the dark. Subject to the dipole interaction with the laser field, virtual molecular states are generated via the absorption and emission of photons, and new tunneling channels open. Interesting phenomena arising from memory are noted. Such a phenomenon could serve as a switch.

A proposal of a spin cell using light on magnetic tunneling junctions

We propose and theoretically investigate a spin cell using light as the power source. Such a device can be realized when a quantum dot is connected to two ferromagnetic electrodes. In the case of identical electrodes, a pure spin current (PSC) can be generated when the light is shone on the quantum dot. Moreover, the PSC can be tuned continuously from zero to the maximum when the magnetic moment orientations of the two electrodes are changed from parallel to anti-parallel. The output spin bias is linear with the light power in the low power region, while it approaches the theoretical limit when the power is extremely high because of the electrodes' renormalization by the spin transfer torque. This effect implies that light energy can be transferred to electron spin directly, which may be applicable in future opto-spintronics.

First-principles study of spin-dependent transport through graphene/BNC/graphene structure

: First-principles study on the electronic structure and transport property of the boron nitride sheet (BNC) structure, in which a triangular graphene flake surrounded by a hexagonal boron nitride sheet, is implemented. As the graphene flake becomes small and is more isolated by the boron nitride region, the magnetic ordering of the flake increases. When the BNC structure is connected to the graphene electrodes, the spin-polarized charge-density distribution appears only at the triangular graphene flake region, and the electronic structure of the graphene electrode is not spin polarized. First-principles transport calculation reveals that the transport property of the BNC structure is spin dependent.

Tunable Kondo effect in a double quantum dot coupled to ferromagnetic contacts

We investigate the effects induced by ferromagnetic contacts attached to a serial double quantum dot. Spin polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The superexchange interaction J(AFM), tuned by the interdot tunneling rate t, can be used to compensate the effective fields and restore the Kondo resonance when the contact polarizations are aligned. As a consequence, the direction of the spin conductance can be controlled and even reversed using electrostatic gates alone. Our results demonstrate a new approach for controlling spin-dependent transport in carbon nanotube double dot devices.

Triple quantum dots as charge rectifiers

We theoretically analyze electronic spin transport through a triple quantum dot in series, attached to electrical contacts, where the drain contact is coupled to the central dot. We show that current rectification is observed in the device due to current blockade. The current blocking mechanism is originated by a destructive interference of the electronic wavefunction at the drain dot. There, the electrons are coherently trapped in a singlet two-electron dark state, which is a coherent superposition of the electronic wavefunction in the source dot and in the dot isolated from the contacts. Its formation gives rise to zero current and current rectification as the voltage is swept. We analyze this behavior analytically and numerically for both zero and finite magnetic dc fields. On top of that, we include phenomenologically a finite spin relaxation rate and calculate the current numerically. Our results show that triple dots in series can be designed to behave as quantum charge rectifiers.

Spin and charge pumping in a quantum wire: the role of spin-flip scattering and Zeeman splitting

We investigate theoretically charge and spin pumps based on a linear configuration of quantum dots (quantum wire) which are disturbed by an external time-dependent perturbation. This perturbation forms an impulse which moves as a train pulse through the wire. It is found that the charge pumped through the system depends non-monotonically on the wire length, N. In the presence of the Zeeman splitting pure spin current flowing through the wire can be generated in the absence of charge current. Moreover, we observe electron pumping in a direction which does not coincide with the propagation direction of the pulse and the spin pumping direction (spin-charge separation). Additionally, on-site spin-flip processes significantly influence electron transport through the system and can also reverse the charge current direction.

Interaction-induced adiabatic nonlinear transport

We calculate the time-dependent nonlinear transport current through an interacting quantum dot in the single-electron tunneling (SET) regime. We show that an additional dc current is generated by the electron-electron interaction by adiabatic out-of-phase modulation of the gate and bias voltage. This current can arise only when two SET resonance conditions are simultaneously satisfied. We propose an adiabatic transport spectroscopy where lock-in measurement of a "time-averaged stability diagram" probes interactions, tunnel asymmetries, and changes in the ground state spin degeneracy.

Nonadiabatic pumping through interacting quantum dots

We study nonadiabatic two-parameter charge and spin pumping through a single-level quantum dot with Coulomb interaction. For the limit of weak tunnel coupling and in the regime of pumping frequencies up to the tunneling rates, Omega less, similar Gamma/variant Planck's over 2pi, we perform an exact resummation of contributions of all orders in the pumping frequency. As striking nonadiabatic signatures, we find frequency-dependent phase shifts in the charge and spin currents, which opens the possibility to control charge and spin currents by tuning the pumping frequency. This includes the realization of an effective single-parameter pumping as well as pure spin without charge currents.

Phase coherence, inelastic scattering, and interaction corrections in pumping through quantum dots

Adiabatic quantum pumping in noninteracting, phase coherent quantum dots is elegantly described by Brouwer's formula. Interactions within the dot, while suppressing phase coherence, make Brouwer's formalism inapplicable. In this Letter, we discuss the nature of the physical processes forcing a description of pumping beyond Brouwer's formula, and develop, using a controlled adiabatic expansion, a useful formalism to study the effect of interactions within a generic perturbative scheme. The pumped current consists of a first contribution, analogous to Brouwer's formula and accounting for the remanent coherence, and of interaction corrections describing inelastic scattering. We apply the formalism to study the effect of interaction with a bosonic bath on a resonant level pump and discuss the robustness of the quantization of the pumped charge in turnstile cycles.

Photoinduced removal of the franck-condon blockade in single-electron inelastic charge transmission

A new mechanism of charge transmission through a metal-molecule-metal junction is suggested that is based on optical driving of electronic transitions in the neutral and singly charged molecular state. The effects of strong electron vibrational coupling, intramolecular vibrational energy redistribution, and molecular de-excitation caused by electron-hole pair formation in the leads are taken into account. It is shown that current suppression due to the Franck-Condon blockade can be overcome by opening new transmission channels via photoexcitation.

Optical control in coupled two-electron quantum dots

The dynamics of two electrons in a 2-dimensional quantum dot molecule in the presence of a time-dependent electromagnetic field is calculated from first principles. We show that carefully selected microwave pulses can exclusively populate a single state of the first excitation band and that the transition time can be further decreased by optimal pulse control. Finally we demonstrate that an oscillating charge localized state may be created by multiple transitions using a sequence of pulses.

Adiabatic pumping through interacting quantum dots

We present a general formalism to study adiabatic pumping through interacting quantum dots. We derive a formula that relates the pumped charge to the local, instantaneous Green's function of the dot. This formula is then applied to the infinite-U Anderson model for both weak and strong tunnel-coupling strengths.

Nonadiabatic electron pumping: maximal current with minimal noise

The noise properties of pump currents through an open double-quantum-dot setup with nonadiabatic ac driving are investigated. Driving frequencies close to the internal resonances of the double-dot system mark the optimal working points at which the pump current assumes a maximum while its noise power possesses a remarkably low minimum. A rotating-wave approximation provides analytical expressions for the current and its noise power and allows to optimize the noise characteristics. The analytical results are compared to numerical results from a Floquet transport theory.