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Physica status solidi. C, Conferences and critical reviews/Physica status solidi. C, Current topics in solid state physics

Pauli blockade transport in the cotunneling regime through a double quantum dot

Hongwu Liu, T. Fujisawa, T. Hayashi, Y. Hirayama

Published Nov 22, 2006• 0 citations
Publisher

Abstract

Abstract Pauli spin blockade in the cotunneling transport through an effective two‐electron double quantum dot (™10 electrons in each dot) has been presented. In the cotunneling regime where the two‐electron (1,1) and (0,2) states (ordered pairs denote the effective electron numbers in each dot) are far below the Fermi energy of the leads, we can concentrate on the two‐electron tunnelling transitions and detect singlet‐triplet splitting arising from the exchange interaction between the two electron spins with improved resolution. We discuss the cotunneling transport by considering the coherent and incoherent coupling between the dots in connection with the singlet‐triplet splitting. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssc.200671532

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Abstract Pauli spin blockade in the cotunneling transport through an effective two‐electron double quantum dot (™10 electrons in each dot) has been presented. In the cotunneling regime where the two‐electron (1,1) and (0,2) states (ordered pairs denote the effective electron numbers in each dot) are far below the Fermi energy of the leads, we can concentrate on the two‐electron tunnelling transitions and detect singlet‐triplet splitting arising from the exchange interaction between the two electron spins with improved resolution. We discuss the cotunneling transport by considering the coherent and incoherent coupling between the dots in connection with the singlet‐triplet splitting. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Addressing the feasibility of quantum communication with electrons we consider entangled spin states of electrons in a double-dot which is weakly coupled to leads. We show that the entanglement of two electrons in the double-dot can be detected in mesoscopic transport and noise measurements. In the Coulomb blockade and cotunneling regime the singlet and triplet states lead to phase-coherent current and noise contributions of opposite signs and to Aharonov-Bohm and Berry phase oscillations. These oscillations are a genuine two-particle effect and provide a direct measure of nonlocality in entangled states. We show that the ratio of zero-frequency noise to current is equal to the electron charge.
We investigate the influence of thermal energy on the current flow and electron spin states in double quantum dots in series. The quadruplet Pauli spin blockade, which is caused by the quadruplet and doublet states, occurs at low temperatures affecting the transport properties. As the temperature increases, the quadruplet Pauli spin blockade occurs as a result of the thermal energy, even in regions where it does not occur at low temperatures. This is because the triplet state is formed in one dot as a result of the gradual change of the Fermi distribution function of the electrodes with increasing temperature. Moreover, the thermally assisted Pauli spin blockade results in coexistence of the Coulomb and Pauli spin blockades. Conversely, for the standard triplet Pauli spin blockade, which occurs as a result of the triplet and singlet states, the current through the double dots monotonously smears out as the temperature increases. Therefore, the thermally assisted Pauli spin blockade is not clearly observed. However, the coexistence of the Coulomb and triplet Pauli spin blockades as a result of the thermal energy is clearly obtained in the calculation of the probability of the spin state in the double dots.
In this thesis, I study quantum transport of electron (e.g., current and noise) in quantum dots exploring microscopic processes responsible for spin-relaxation in double quantum dots in Pauli spin blockade regime. This is a regime where current is blocked due to the spin configuration of electrons in the dot. The Pauli \nspin blockade provides a means for preparation, manipulation and readout in spin qubits. Hence, understanding the underlying mechanism which lifts this blockade is extremely important. \n \nFirst, I have developed a theory of spin-flip cotunneling (higher order tunneling) processes in double quantum dots in the Pauli spin blockade regime. Utilizing this \ntheory, I have calculated the full analytical dependence of the stationary current on applied magnetic fields, gate voltages, and an inter-dot tunnel coupling in Pauli \nspin blockade. This work is important for understanding the nature of leakage, especially in systems where other spin-flip mechanisms (due, e.g., hyperfine coupling \nto nuclear spins or spin-orbit coupling) are weak, including silicon and carbon nanotube or graphene quantum dots. This theory explains recent experiments on \ncarbon nanotubes and silicon double quantum dot. \n \nIn addition, I propose a new scheme based on the current noise to probe spin relaxation mechanisms in double quantum dot in the Pauli spin blockade regime, where spin-selection rule applies. As a result, I provide a simple closed-form expression which can be used to fit experimental data to extract multiple spin-relaxation \nrates, even at very low energy splitting. This method allows for the characterization of different aspects of decay process in these systems.
We use a vertical double quantum dot (QD) to study spin blockade (SB) for the two-to five-electron states. SB observed for the two- and four-electron states is both assigned to Pauli exclusion with formation of a spin triplet state, and lifted by singlet-triplet admixing due to fluctuating nuclear field. SB observed for the five-electron state is caused by combined Pauli effect and Hund's rule. We observe a hysteretic behavior of the SB leakage current for up and down sweep of magnetic field, and argue that SB and its lifting by hyperfine interaction are subtle with the spin configuration and modified depending on the inter-dot detuning and number of electrons.