Pauli Spin Blockade Research Articles

Effects of different geometries on the conductance, shot noise, and tunnel magnetoresistance of double quantum dots

The spin-polarized transport through a coherent strongly coupled double quantum dot (DQD) system is analyzed theoretically in the sequential and cotunneling regimes. Using the real-time diagrammatic technique, we analyze the current, differential conductance, shot noise, and tunnel magnetoresistance (TMR) as a function of both the bias and gate voltages for double quantum dots coupled in series, in parallel, as well as for $T$-shaped systems. For DQDs coupled in series, we find a strong dependence of the TMR on the number of electrons occupying the double dot, and super-Poissonian shot noise in the Coulomb blockade regime. In addition, for asymmetric DQDs, we analyze transport in the Pauli-spin blockade regime and explain the existence of the leakage current in terms of cotunneling and spin-flip cotunneling-assisted sequential tunneling. For DQDs coupled in parallel, we show that the transport characteristics in the weak-coupling regime are qualitatively similar to those of DQDs coupled in series. On the other hand, in the case of $T$-shaped quantum dots we predict a large super-Poissonian shot noise and TMR enhanced above the Julliere value due to increased occupation of the decoupled quantum dot. We also discuss the possibility of determining the geometry of the double dot from transport characteristics. Furthermore, where possible, we compare our results with existing experimental data on nonmagnetic systems and find qualitative agreement.

Pauli spin blockade in carbon nanotube double quantum dots

We report Pauli spin blockade in a carbon nanotube double quantum dot defined by tunnel barriers at the contacts and a structural defect in the nanotube. We observe a pronounced current suppression for negative source-drain bias voltages, which is investigated for both symmetric and asymmetric coupling of the quantum dots to the leads. The measured differential conductance agrees well with a theoretical model of a double quantum dot system in the spin-blockade regime, which allows us to estimate the occupation probabilities of the relevant singlet and triplet states. This work shows that effective spin-to-charge conversion in nanotube quantum dots is feasible and opens the possibility of single-spin readout in a material that is not limited by hyperfine interaction with nuclear spins.

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.

Pauli-spin-blockade transport through a silicon double quantum dot

We present measurements of resonant tunneling through discrete energy levels of a silicon double quantum dot formed in a thin silicon-on-insulator layer. In the absence of piezoelectric phonon coupling, spontaneous phonon emission with deformation-potential coupling accounts for inelastic tunneling through the ground states of the two dots. Such transport measurements enable us to observe a Pauli spin blockade due to effective two-electron spin-triplet correlations, evident in a distinct bias-polarity dependence of resonant tunneling through the ground states. The blockade is lifted by the excited-state resonance by virtue of efficient phonon emission between the ground states. Our experiment demonstrates considerable potential for investigating silicon-based spin dynamics and spin-based quantum information processing.

Singlet–triplet transition induced by Zeeman energy in weakly coupled vertical double quantum dots

We use Pauli-spin blockade (P-SB) in a vertical double quantum dot to study the interplay between the nuclear spin effect and exchange interaction. We observe a specific P-SB lifting, reflecting coexistence of singlet–triplet admixing due to fluctuating nuclear spin field and singlet–triplet transition induced by the Zeeman effect when the exchange energy and the fluctuating nuclear field energy are comparable. From measurement of the Zeeman-induced P-SB lifting, we derive the exchange energy as a function of inter-dot level detuning. We confirm enhancement of exchange energy by resonance, and use Hubbard model calculation to account for it.

Analytical expression of leakage current through double quantum dots in Pauli spin blockade

We theoretically examine a small current through double quantum dots with Pauli spin blockade, in which electrons cannot be transported without spin–flip or higher-order tunnel processes (cotunneling). Analytical expressions for the current are given in both cases of sequential tunneling and resonant tunneling between the quantum dots and leads. In the former case, the cotunneling weakens the spin blockade and increases a coherent component of the current. When the cotunneling can be disregarded, effective magnetic fields created by nuclear spins (Overhauser fields) can be detected by the observation of the current. In the case of resonant tunneling with Zeeman splitting in an external magnetic field ( ∼ 1 T ) , small Overhauser fields ( ∼ 5 mT ) could be detected by the fine tuning of the resonant condition.

Spin blockade of quantum cellular automata effects in a few electron triple quantum dot

It has been previously demonstrated, employing charge detection techniques, that quantum cellular automata (QCA) processes exist in the vicinity of quadruple degeneracy points in both ring and serial arrangements of lateral triple quantum dots. The effect is primarily an electrostatic one. In this paper, we report on transport measurements through a triple dot potential and study experimentally the interplay between these QCA phenomena and the Pauli (spin) blockade effect. We demonstrate experimentally that the interaction between these processes leads to a higher order and indirect form of spin blockade in which the QCA effect itself is blockaded.

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.

Transport through two-level quantum dots weakly coupled to ferromagnetic leads

Spin-dependent transport through a two-level quantum dot in the sequential tunnellingregime is analysed theoretically by means of a real-time diagrammatic technique. It isshown that the current, tunnel magnetoresistance, and shot noise (Fano factor) stronglydepend on the transport regime, providing detailed information on the electronic structureof quantum dots and their coupling to external leads. When the dot is asymmetricallycoupled to the leads, a negative differential conductance may occur in certain biasregions, which is associated with a super-Poissonian shot noise. In the case of aquantum dot coupled to one half-metallic and one nonmagnetic lead, one findscharacteristic Pauli spin blockade effects. Transport may also be suppressed whenthe dot levels are coupled to the leads with different coupling strengths. Theinfluence of an external magnetic field on transport properties is also discussed.

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.

Non-equilibrium triplet blockade in parallel coupled quantum dots

It is theoretically demonstrated that parallel weakly tunnel coupled quantum dots exhibit non-equilibrium blockade regimes caused by a full occupation in the spin triplet state, in analogy to the Pauli spin blockade in serially weakly coupled quantum dots. Charge tends to accumulate in the two-electron triplet for bias voltages that support transitions between the singlet and three-electron states.

Pauli spin blockade in weakly coupled double quantum dots

In a two-level system, constituted by two serially coupled single level quantum dots, coupled to external leads we find that the current is suppressed in one direction of biasing caused by a fully occupied two-electron triplet state in the interacting region. The efficiency of the current suppression is governed by the ratio between the interdot tunnelling rate and the level off-set. In the opposite bias direction, the occupation of the two-electron triplet is lifted which allows a larger current to flow through the system, where the conductance is provided by transitions between one-electron states and two-electron singlet states. Is is also shown that a finite ferromagnetic interdot exchange interaction provides an extended range of the current suppression, while an anti-ferromagnetic exchange leads to a decreased range of the blockade regime.

Pauli spin blockade in cotunneling transport through a double quantum dot

We study spin correlation in a double quantum dot containing a few electrons in each dot (around 10). Clear current rectification with negative differential conductance is observed in the cotunneling regime, which is well explained by a Pauli spin blockade. The current feature is discussed in connection with exchange singlet-triplet splitting based on two simple models, one of which takes coherent interdot coupling and the other incoherent interdot coupling into account.