Enhancement of spin polarization in asymmetrically coupled CdSe and CdZnMnSe quantum dots in ZnSe matrix

An asymmetrically coupled double quantum dot (QD) system consisting of adjacent CdSe and CdZnMnSe QD layers in a ZnSe matrix was investigated using polarization-selective magneto-photoluminescence (PL). Two well-resolved PL peaks are observed corresponding, respectively, to the CdSe and the CdZnMnSe QDs. The peaks exhibit significant change in the intensity and energy position when a magnetic field is applied. The enhancement of the degree of σ⁻ circular polarization emitted by the non-magnetic CdSe QDs is observed in the double layer system, as compared to that observed in CdSe QDs without the influence of neighboring CdZnMnSe QDs. This behavior was discussed in terms of antiferromagnetic interaction between carrier spins localized in pairs of CdSe and CdZnMnSe QDs that are electronically coupled.     

Spin-dependent Kondo effect induced by Rashba spin-orbit interaction in parallel coupled double quantum dots

Electronic transport through parallel coupled double quantum dots (DQD) with Rashba spin-orbit (RSO) interaction is investigated in Kondo regime by means of the slave-boson mean field approximation at zero temperature. By the co-action of the phase factor deduced by RSO interaction and the magnetic flux penetrating the parallel DQD, an interesting spin-dependent Kondo effect emerges. The molecular state representation theory is used to obtain a detailed understanding of the spin-dependent Kondo effect. It is shown that Quantum interference between the bonding Kondo state and antibonding state, which is modulated by the RSO interaction, plays a crucial role to the density of states and the linear conductance. The magnitude of each spin component conductance can be modulated by the RSO interaction strength. The conductance of each spin component exhibits 4π-periodic function with respect to φR. Moreover, the swap operation in the parallel DQD system can be implemented by tuning the RSO interaction.     

Magneto-optical study of nonmagnetic quantum dots coupled to a magnetic semiconductor quantum well

We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs.     

Electron–phonon-induced spin relaxation in InAs quantum dots

We have calculated spin-relaxation rates in parabolic quantum dots due to the phonon modulation of the spin–orbit interaction in the presence of an external magnetic field. Both deformation potential and piezoelectric electron–phonon coupling mechanisms are included within the Pavlov–Firsov spin–phonon Hamiltonian. Our results have demonstrated that, in narrow gap materials, the electron–phonon deformation potential and piezoelectric coupling give comparable contributions to spin-relaxation processes. For large dots, the deformation potential interaction becomes dominant. This behavior is not observed in wide or intermediate gap semiconductors, where the piezoelectric coupling, in general, governs the spin-relaxation processes. We have also demonstrated that spin-relaxation rates are particularly sensitive to the Landé g-factor.     

Quantum spin and charge pumping through double quantum dots with ferromagnetic leads

The pumping of electrons through double quantum dots (DQDs) attached to ferromagnetic leads have been theoretically investigated by using the nonequilibrium Green?s function method. It is found that an oscillating electric field applied to the quantum dot may give rise to the pumped charge and spin currents. In the case that both leads are ferromagnet, a pure spin current can be generated in the antiparallel magnetization configuration, where no net charge current exists. The possibility of manipulating the pumped spin current is explored by tuning the dot level and the ac field. By making use of various tunings, the magnitude and direction of the pumped spin current can be well controlled. For the case that only one lead is ferromagnetic, both of the charge and spin currents can be pumped and flow in opposite directions on the average. The control of the magnitude and direction of the pumped charge and spin currents is also discussed by means of the magnetic flux threading through the DQD ring.     

Photocurrent and spin manipulation in quantum dots

In this paper we use a density matrix formalism to model the spin photocurrent obtained from a single self-assembled quantum dot photodiode under the influence of an applied strong polarized electromagnetic pulse and a gate voltage. We show that the degree of polarization of the output photocurrent generated by a circularly polarized pulse in a strongly anisotropic quantum dot can be switched as we increase the pulse intensity. A similar effect is observed in a quantum dot with weak anisotropic electron–hole exchange interaction by using an elliptically polarized pulse. In the latter, a shorter pulse is needed, which creates an effective exchange channel through the biexciton. This phenomenon can be used as a dynamical switch to invert the spin-polarization of the extracted current.     

Electrically controllable spin transport in bilayer graphene with Rashba spin-orbit interaction

We study the spin transport in bilayer graphene nanoribbons (BGNs) in the presence of Rashba spin-orbit interaction (SOI) and external gate voltages. It is found that the spin polarization can be significantly enhanced by the interlayer asymmetry or longitudinal mirror asymmetry produced by external gate voltages. Rashba SOI alone in BGNs can only generate current with spin polarization along the in-plane y direction, but the polarization components can be found along the x, y and z directions when a gate voltage is applied. High spin polarization with flexible orientation is obtained in the proposed device. Our findings shed new light on the generation of highly spin-polarized current in BGNs without external magnetic fields, which could have useful applications in spintronics device design.     

Disappearance of quantum chaos in coupled chaotic quantum dots

Statistical properties of the single electron levels confined in the semiconductor (InAs/GaAs, Si/SiO₂) double quantum dots (DQDs) are considered. We demonstrate that in the electronically coupled chaotic quantum dots the chaos with its level repulsion disappears and the nearest neighbor level statistics becomes Poissonian. This result is discussed in the light of the recently predicted “huge conductance peak” by R.S. Whitney et al. [Phys. Rev. Lett. 102 (2009) 186802] in the mirror symmetric DQDs.     

Two-dimensional probe absorption in coupled quantum dots

We investigate the two-dimensional (2D) probe absorption in coupled quantum dots. It is found that, due to the position-dependent quantum interference effect, the 2D optical absorption spectrum can be easily controlled via adjusting the system parameters. Thus, our scheme may provide some technological applications in solid-state quantum communication.     

Optical nuclear spin polarization in quantum dots

Hyperfine interaction between electron spin and randomly oriented nuclear spins is a key issue of electron coherence for quantum information/computation. We propose an efficient way to establish high polarization of nuclear spins and reduce the intrinsic nuclear spin fluctuations. Here, we polarize the nuclear spins in semiconductor quantum dot(QD) by the coherent population trapping(CPT) and the electric dipole spin resonance(EDSR) induced by optical fields and ac electric fields. By tuning the optical fields, we can obtain a powerful cooling background based on CPT for nuclear spin polarization. The EDSR can enhance the spin flip–flop rate which may increase the cooling efficiency. With the help of CPT and EDSR, an enhancement of 1300 times of the electron coherence time can be obtained after a 10-ns preparation time.     

Flux-dependent tunnel magnetoresistance in parallel-coupled double quantum dots

The tunnel magnetoresistance (TMR) in an Aharonov–Bohm interferometer with two quantum dots inserted in its arms, which is attached to ferromagnetic leads with parallel and antiparallel magnetic configurations, is theoretically studied by means of the nonequilibrium Green’s function technique. We pay particular attention to the influence of an applied magnetic flux on the characteristics of the TMR. In the linear response regime (the external bias voltage V→0) and when the electrons are free from intradot Coulomb interaction, the magnetic flux only changes the peak or dip positions of the TMR. But in the presence of intradot Coulomb repulsion, its peak or dip positions, signs and magnitude are tuned by the magnetic flux. For the nonlinear response regime (V≠0), the TMR is symmetric with respect to zero bias voltage and the magnetic flux can influence its magnitude, signs and the peak positions regardless of the existence of intradot Coulomb interaction. The behavior of the TMR is interpreted in terms of the quantum interference (Fano) effect.     

Quantum entanglement formation by repeated spin blockade measurements in a spin field-effect transistor structure embedded with quantum dots

We propose a method of operating a quantum state machine made of stacked quantum dots buried in adjacent to the channel of a spin field-effect transistor (FET) [S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665; K. Yoh, et al., Proceedings of the 23rd International Conference on Physics of Semiconductors (ICPS) 2004; H. Ohno, K. Yoh et al., Jpn. J. Appl. Phys. 42 (2003) L87; K. Yoh, J. Konda, S. Shiina, N. Nishiguchi, Jpn. J. Appl. Phys. 36 (1997) 4134]. In this method, a spin blockade measurement extracts the quantum state of a nearest quantum dot through Coulomb blockade [K. Yoh, J. Konda, S. Shiina, N. Nishiguchi, Jpn. J. Appl. Phys. 36 (1997) 4134; K. Yoh, H. Kazama, Physica E 7 (2000) 440] of the adjacent channel conductance. Repeated quantum Zeno-like (QZ) measurements [H. Nakazato, et al., Phys. Rev. Lett. 90 (2003) 060401] of the spin blockade is shown to purify the quantum dot states within several repetitions. The growth constraints of the stacked InAs quantum dots are shown to provide an exchange interaction energy in the range of 0.01–1 meV [S. Itoh, et al., Jpn. J. Appl. Phys. 38 (1999) L917; A. Tackeuchi, et al., Jpn. J. Appl. Phys. 42 (2003) 4278]. We have verified that one can reach the fidelity of 90% by repeating the measurement twice, and that of 99.9% by repeating only eleven QZ measurements. Entangled states with two and three vertically stacked dots are achieved with the sampling frequency of the order of 100 MHz.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.     

Thermopower in parallel double quantum dots with Rashba spin-orbit interaction

Based on the Green’s function technique and the equation of motion approach,this paper theoretically studies the thermoelectric effect in parallel coupled double quantum dots (DQDs),in which Rashba spin-orbit interaction is taken into account.Rashba spin-orbit interaction contributions,even in a magnetic field,are exhibited obviously in the double quantum dots system for the thermoelectric effect.The periodic oscillation of thermopower can be controlled by tunning the Rashba spin-orbit interaction induced phase.The interesting spin-dependent thermoelectric effects will arise which has important influence on thermoelectric properties of the studied system.     

Dimensional effects and magnetic field influence on excitons in coupled quantum dots and coupled quantum wells

Direct and indirect excitons in coupled quantum wells and in coupled quantum dots are studied. We consider excitons with two-dimensional, quasi-two-dimensional and three-dimensional carriers. Problems were investigated for a wide range of characteristic parameters—confining to potential steepness, distances between quantum wells or dots, effective width of wells and magnetic fields. The mutual influence of the controlling parameters of the problem on exciton properties is analyzed. Energy and wave function spectra were calculated and dispersion law and effective masses were obtained.     

Entanglement and the Kondo effect in serially coupled double quantum dots

We investigate entanglement between electrons in serially coupled double quantum dots attached to noninteracting leads. In addition to local repulsion we consider the influence of capacitive inter-dot interaction. We show how the competition between extended Kondo and local singlet phases determines the ground state and thereby the entanglement. The results are additionally discussed in connection with the linear conductance through the system.     

Mid-infrared luminescence from coupled quantum dots and wells

Coupled nanostructures have been developed in the InAs/InSb/GaSb materials system in order to extend the emission wavelength further into the infrared, beyond 2 μm. The samples studied consist of a single narrow InAs quantum well grown below a layer of InSb quantum dots in a GaSb matrix, in which the coupling has been altered by changing the thickness of a GaSb spacer layer. The overall transition energy of the combined dot–well system is generally reduced with respect to the dots and well only but the dependence on spacer thickness is more complex than that expected from a simple envelope function model.     

Fano effect and Andreev bound states in T-shape double quantum dots

In this Letter, we investigate the transport through a T-shaped double quantum dot coupled to two normal metal leads left and right and a superconducting lead. Analytical expressions of Andreev transmission and local density of states of the system at zero temperature have been obtained. We study the role of the superconducting lead in the quantum interferometric features of the double quantum dot. We report for first time the Fano effect produced by Andreev bound states in a side quantum dot. Our results show that as a consequence of quantum interference and proximity effect, the transmission from normal to normal lead exhibits Fano resonances due to Andreev bound states. We find that this interference effect allows us to study the Andreev bound states in the changes in the conductance between two normal leads.