Spin Blockade Regime Research Articles

Erratum: Current-voltage characteristics of semiconductor/ferromagnet junctions in the spin-blockade regime [Phys. Rev. B77, 073301 (2008)

Erratum: Current-voltage characteristics of semiconductor/ferromagnet junctions in the spin-blockade regime [Phys. Rev. B<b>77</b>, 073301 (2008)

Magnetic and Electrical Control of Electron-Nuclear Spin Coupling in GaAs Double Quantum Dots

We magnetically and electrically control spin coupling between electrons and nuclei in a vertical GaAs double quantum dot in the Pauli spin blockade regime to characterize the two-electron exchange energy and the dynamic nuclear polarization (DNP). The Pauli blockade is lifted by a hyperfine-mediated transition from an electron triplet state to the singlet state. This transition progressively occurs when the triplet-singlet separation or exchange energy J is compensated by tuning either Zeeman energy with external magnetic field (i.e., magnetic control) or inter-dot energy detuning with bias voltage (i.e., electrical control). The blockade is then lifted with a current step for sweeping up an external magnetic field. The J value, which is evaluated from the step position for various inter-dot detuning values, is consistent with calculation. DNP takes place through the hyperfine coupling near the current step, leading to a shifted current step to lower external magnetic field for the down-sweep. For the electrical control, a similar current step appears for sweeping bias voltage. In this case we are able to control the two-electron state energies in a relevant manner based on real time before DNP is significantly influenced, because the voltage sweep is much faster than the magnetic field sweep. We use this technique to distinguish the current step with and without DNP contributions, and therefore quantify the nuclear Overhauser fields with external magnetic field as a parameter. We present a simple phenomenological model to reproduce the experiment and also discuss a possible approach to realize the full DNP.

NEMO-3D based atomistic simulation of a double quantum dot structure for spin-blockaded transport

This work combines an atomistic electronic structure calculation with many-body rate equations to simulate the current-voltage (I–V) characteristics of a weakly-coupled Double Quantum Dot (DQD) system in the spin-blockade regime. Here we performed a NEMO-3D based, atomistic simulation of the geometry of the DQD to obtain its single electron eigen-states, hopping parameters, and Coulomb integrals followed by the evaluation of I–V characteristics with the many-electron spectrum of the DQD system, derived from this single-electron parameter set. The many-electron spectra and wave-functions are evaluated by exact-diagonalization of the many-electron system. The Hamiltonian is constructed from single electron eigen-states, hopping parameters and Coulomb integrals derived from atomistic NEMO 3-D simulations. Calculated I–V characteristics exhibit multiple regions of prominent Negative Differential Resistances (NDRs) that resemble the experimental trends. Unlike resonant tunnelling devices, however, level crossings in DQDs are negligible, and the NDRs result from a delicate interplay of delocalization, orbital offset and Coulomb interaction.

Current-voltage characteristics of semiconductor/ferromagnet junctions in the spin-blockade regime

It was recently predicted [Phys. Rev. B 75, 193301 (2007)] that spin blockade may develop at nonmagnetic semiconductor/perfect ferromagnet junctions when the electron flow is directed from the semiconductor into the ferromagnet. Here we consider current-voltage characteristics of such junctions. By taking into account the contact resistance, we demonstrate a current stabilization effect: by increasing the applied voltage, the current density through the junction saturates at a specific value. The transient behavior of the current density is also investigated. We show that an abrupt change in the applied voltage is accompanied by a spike in the current density. It is anticipated that this is a common dynamical behavior of the current density in structures with conductivity depending on the level of spin polarization.

Hysteretic behavior in weakly coupled double-dot transport in the spin blockade regime

Double quantum dot systems in the spin blockade regime exhibit leakage currents that have been attributed to the hyperfine interaction. We model weakly coupled double-dot transport using a rate equation approach which accounts for hyperfine flip-flop transitions. The rate equations allow us to self-consistently obtain the time evolution for electronic charge occupations and for the nuclei polarizations in each dot. We analyze the current in the spin blockade region as a function of magnetic field and observe hysteretic behavior for fields corresponding to the crossing between triplet and singlet states.

Spin-state mixing in InAs double quantum dots

We quantify the contributions of hyperfine and spin-orbit-mediated singlet-triplet mixing in weakly coupled InAs quantum dots by electron transport spectroscopy in the Pauli spin-blockade regime. In contrast to double dots in GaAs, the spin-orbit coupling is found to be more than two orders of magnitudes larger than the hyperfine mixing energy. It is already effective at magnetic fields of a few mT, where deviations from pure hyperfine mixing are observed.

Magnetic field-induced effects in the high source–drain bias current of weakly coupled vertical quantum dot molecules

We report on the basic properties of recently observed magnetic field resonance, induced time-dependent oscillation, and hysteresis effects in the current flowing through two weakly coupled vertical quantum dots at high source–drain bias (up to a few tens of mV). These effects bare some similarity to those reported in the N=2 spin-blockade regime, usually for weak in-plane magnetic field, of quantum dot molecules and attributed to hyperfine coupling, except here the measurements are conducted outside of the spin-blockade regime and the out-of-plane magnetic field is up to ∼6 T.

Large Nuclear Overhauser Fields Detected in Vertically Coupled Double Quantum Dots

We report the electrical induction and detection of dynamic nuclear polarization in the spin-blockade regime of double GaAs vertical quantum dots. The nuclear Overhauser field measurement relies on bias voltage control of the interdot spin exchange coupling and measurement of dc current at variable external magnetic fields. The largest Overhauser field observed was about 4 T, corresponding to a nuclear polarization approximately 40% for the electronic g factor typical of these devices, |g*| approximately 0.25. A phenomenological model is proposed to explain these observations.

Electronic transport through a double quantum dot in the spin-blockade regime: Theoretical models

We analyzed the electronic transport through a double quantum dot in the spin blockade regime. Experiments of current rectification by Pauli exclusion principle in double quantum dots were discussed. The electron and nuclei spin dynamics and their interplay due to the hyperfine interaction were self-consistently analyzed within the framework of rate equations. Our results show that the current leakage experimentally observed in the spin-blockade region is due to spin-flip processes induced by hyperfine interaction through Overhauser effect. We show as well how a magnetic field applied parallel to the current allows excited states to participate in the electronic current and removes spin blockade. Our model includes also a self-consistent description of inelastic transitions where the energy is exchanged through interactions with acoustic phonons in the environment. It accounts for spontaneous emission of phonons which results in additional features in the current characteristics. We develop a microscopical model to treat the hyperfine interaction in each dot. Using this model, we study the dynamical nuclear polarization as a function of the applied voltage.

Suppression of Spin Relaxation in an InAs Nanowire Double Quantum Dot

We investigate the triplet-singlet relaxation in a double quantum dot defined by top gates in an InAs nanowire. In the Pauli spin blockade regime, the leakage current can be mainly attributed to spin relaxation. While at weak and strong interdot coupling relaxation is dominated by two individual mechanisms, the relaxation is strongly reduced at intermediate coupling and finite magnetic field. In addition we observe a characteristic bistability of the spin-nonconserving current as a function of magnetic field. We propose a model where these features are explained by the polarization of nuclear spins enabled by the interplay between hyperfine and spin-orbit mediated relaxation.

Phonon‐assisted transport through a double quantum dot: magnetic field dependence in a spin blockade regime

AbstractWe analyze the electronic transport through a double quantum dot in the regime where spin blockade occurs. Experiments of current rectification by Pauli exclusion principle in double quantum dots are discussed. Our model includes a self‐consistent description of inelastic transitions where the energy is exchanged through interactions with acoustic phonons in the environment. It accounts for spontaneous emission of phonons which results in additional features in the current characteristics as observed experimentally due to the opening of new current channels. We show as well how a magnetic field applied parallel to the current allows excited states to participate in the electronic current and removes spin blockade. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Formation of pure two-electron triplet states in weakly coupled quantum dots attached toferromagnetic leads

Weakly coupled quantum dots in the Pauli spin blockade regime are considered withrespect to spin-dependent transport. By attaching one half-metallic and one non-magneticlead, the Pauli spin blockade is formed by a pure triplet state with spin momentSz = 1 or−1. Furthermore, additional spin blockade regimes emerge because of full occupation in stateswith opposite spin to that of the half-metallic lead.

Nuclear-spin-induced oscillatory current in spin-blockaded quantum dots.

We show experimentally that electron transport through GaAs-based double quantum dots can be affected by ambient nuclear spin states in a certain regime where transport is blocked in the absence of electron spin flip. Current through the dots oscillates in time with a period up to 200 s depending on magnetic field. Oscillation is quenched by application of a continuous wave ac magnetic field which can induce nuclear magnetic resonance in 71Ga or 69Ga. A possible mechanism for dynamically polarizing the nuclear spins is proposed.

High-frequency manipulation of few-electron double quantum dots—toward spin qubits

We use a photon-assisted tunneling (PAT) technique to study the high-frequency response of one- and two-electron states in a semiconductor vertically coupled double-dot system. In particular, PAT associated with two-electron spin states in the spin-blockade regime is observed up to the absorption of 10 photons, indicating the preservation of long relaxation times and hence the robustness of our electron spin device under strong microwave irradiation. An alternative double-dot structure with greater flexibility in tuning the inter-dot coupling is presented and its transport characteristics are discussed. This structure is proposed for high-frequency control of two-electron spin states, as required for quantum computation schemes using electron spins in quantum dots.

Electronic–nuclear dynamics in double quantum dots in the spin-blockade regime

We have theoretically investigated the nonlinear dissipative dynamics of coupled electronic–nuclear systems in double quantum dots in the spin-blockade regime. The result shows multiple stability and oscillation (limit cycle) in the magnetic field regime where the singlet and triplet two-electron levels cross each other. These instabilities explain the hysteresis and oscillation of leakage current observed recently by Ono et al.

Dynamics of Electrons and Nuclei in Double Quantum Dots in the Spin Blockade Regime

Electron-nuclear interaction is a classic subject of condensed matter physics, studied extensively over decades by using nuclear magnetic resonance. This seemingly mature field has been attracting renewed interest recently due mainly to the discovery of novel nuclearinduced electronic phenomena in nanostructures. In the theoretical treatment of such effects, it has often been done to start from a phenominological equation of motion for macroscopic variables, introduced in a rather ad-hoc mannner. This approach is of limited usefulness, and a systematic method to derive macroscopic equations from microscopic Hamiltonians is desired. We have developed such a method applicable to confined electronic systems interacting with nuclei. The present paper outlines this method and discusses its application to double quantum dots in the spin-blockade regime.