Spin Filter Effect in a Parallel Double Quantum Dot

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-dependent conductance is calculated at low temperatures. We analyze the conditions such that the device would operate as a bipolar spin filter by only varying the incident electron Fermi energy from non-magnetic leads.     

Spin Filter Effect in a Parallel Double Quantum Dot

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-dependent conductance is calculated at low temperatures. We analyze the conditions such that the device would operate as a bipolar spin filter by only varying the incident electron Fermi energy from non-magnetic leads.     

Tunable Spin Current Through a Vibrating Molecular Quantum Dot with Spin Bias and Spin Flip Scattering

By applying the Lang-Firsov canonical transformation and the Keldysh nonequilibrium Green’s function approach, the effect of the spin-flip scattering on the spin current through a vibrating molecular quantum dot with spin bias is theoretically investigated. We can obtain the spin current from the output terminal, and find that the sign of the spin current can be changed by adjusting the spin-flip strength, and a pure spin current can be generated via the charge bias and the spin bias. In the presence of the electron-phonon interaction, the positions of the current peaks are shifted and the spin current is remarkably suppressed, which leads to the Franck-Condon blockade. Furthermore, it is found that the competition between the EPI and the spin-flip scattering jointly determines the character of the spin current. These results offer us a way to manipulate the spin current in the spin current setup. The proposed device should be realizable with use of the present technology at low temperature.     

Field-Controllable Spin Filter Based on Parallel Quantum Dot Systems

In this paper, we investigate the spin-polarized transport through parallel N?dot (N = 1, 2, 3) systems in the strongly correlated regime. We focus our attention on the responses of the N _{t o t} = i n t e g e r states to an increasing magnetic field B, where N _{t o t} is the total charge number on the dots. We show that when the charge level ?? is chosen at the particle-hole (p-h) symmetric case, spin filtering is difficult to occur. While if ?? is beyond the p-h symmetric point, perfect spin-polarized currents could be achieved, and the spin directions can be easily manipulated by tuning external electric and/or magnetic fields, making it easy to be realized in future experiments of the ideas. To approach these problems, the celebrated numerical renormalization group (NRG) technique is implemented, the dynamical properties and the quantum fluctuations are shown.     

Switching of Current Spin Polarization by Electron-Phonon Interaction in a Quantum Dot Device

We study spin-polarized electron transport through a quantum dot coupled to one normal metal lead and one ferromagnetic lead. Both the intradot Coulomb correlation and the electron-phonon interaction are taken into account in the framework of nonequilibrium Green’s function theory. We find that due to the interplay of the Coulomb blockade effect and the phonon-induced extra electron transport channels, the spin polarization of the electron current driven by external bias voltage is enhanced in a range of negative biases in which the current is flowing from the ferromagnetic lead to the normal metal one. While for the corresponding positive biases, the current polarization is suppressed to negative values where the current is flowing from the normal metal lead to the ferromagnetic one. The device thus operates as a current polarization switcher without the need of a magnetic field or spin-orbit interaction, and may find use in low-power spintronic devices with the help of phonon engineering techniques.     

Spin Effects on Heat Current Through a Quantum Dot Attached to Ferromagnetic Leads

A heat current originating from electron–phonon coupling in a quantum dot (QD) molecule connected to ferromagnetic leads is studied by the non-equilibrium Green’s function technique. The system is driven out of equilibrium by a temperature gradient (thermal bias) applied across the two terminals of the structure. We find that when the magnetic moments of the two leads are arranged in parallel configuration, the heat current is not sensitive to the leads’ ferromagnetism, whereas in the case of antiparallel configuration, the magnitude of the heat current increases with increasing spin polarization of the leads, with the reduction of the electric current’s intensity. We also find that the ferromagnetism on the leads can amplify the heat rectification effect occurring for some particular dot levels, i.e., the strength of the heat flowing between the QD and the phonon bath can be very small for one direction of the temperature gradient, while it becomes quite large when the corresponding direction of the temperature gradient is reversed.     

Quantum Dot in the Pseudo-gap Sate with Spin–Orbit Interaction

We study the linear conductance in quantum dot with spin–orbit interaction coupled to Fermi liquid leads with a power-low density of states. The conductance at zero temperature is calculated as a function of the power exponent from the density of state ρ(ω)∼|ω−E _F | ^r at the Fermi energy E _F and the different energy rates. The phase shift of the conduction electrons is also r-dependent. The model can be used in the study of the quantum phase transition.     

Detailed Measurements of Nuclear Spin Polarizations in a Single InAlAs Quantum Dot Through Overhauser Shift of Photoluminescence

We investigated optical pumping of nuclear spin polarizations in a single self-assembled In_0.75Al_0.25As/Al_0.3Ga_0.7As quantum dot. The nuclear spin polarization exhibits the abrupt jump and hysteresis in the excitation power dependence at a particular excitation polarization. Measurement of circular polarization rate of the photoluminescence reveals that the abrupt change of the nuclear spin polarization is created mainly by the spin flip-flop process between nuclei and an electron of a positive charged exciton in this single quantum dot. Model calculation explains well the experimentally observed bistable behavior in InAlAs quantum dot. By using this abrupt change, the sign and magnitude of electron and hole g-factors in z-direction are verified.      

Effects of the Spin Heat Accumulation on the Heat Generation in a Quantum Dot Coupled to Leads

Heat generation by a spin-polarized current in a single-level quantum dot (QD) subjected to spin heat accumulation (SHA), which denotes the spin-dependent electron temperature, is studied by using the nonequilibrium Green’s function technique. The heat generation originates from the energy exchange between the conduction electrons and the phonon reservoir coupled to the QD. Due to the SHA, the spin-up and spin-down heat generations are opposite in sign, and each has a maximum when the QD level is aligned to the chemical potentials of the leads, where the electric current is zero. Under a magnetic field, the maxima of the spin-up and spin-down heat generations are shifted to different dot level regimes. Now total negative heat generation emerges, indicating that the electron absorbs heat from the phonon reservoir to the dot. By tuning the dot levels and the system temperature, the magnitude of the negative heat generation can be enhanced accompanied by weakened electric current, an ideal condition for the realization of nanorefrigerator.     

Nanoscale Thermometry with a Quantum Dot

A two-terminal quantum dot bridging a temperature difference can operate as a thermometer by probing the Fermi-Dirac distributions of the electron gas on both sides of the quantum dot. This thermometry requires that the dot’s energy levels are spaced in energy by many kT and that the intrinsic energetic width of the energy resonances are either much larger or much smaller than kT. We compare these two regimes of operation and discuss the intermediate regime which separates them. Quantum-dot thermometry can assist in mesoscopic thermal experiments where short-distance temperature gradients elude most thermometry techniques.     

Detecting Spin Heat Accumulation by Sign Reversion of Thermopower in a Quantum Dot Side-Coupled to Majorana Bound States

We propose a scheme to detect the spin heat accumulation (SHA), an effective spin-dependent electron temperature, via sign reversion of thermopower induced by the Majorana bound states (MBSs) coupled to a quantum dot (QD). The SHA is generated in either a nonmagnetic material or a ferromagnet serving as an electrode connected to the QD and leads the spin-up and spin-down thermopowers to change signs at different temperatures with the help of QD-MBSs coupling. The existence of the SHA then can be detected by the variation of the spin-polarized or even charge thermopower with respect to the magnitude of the SHA. Our numerical results show that the transition temperature of the thermopower is sensitive to QD-MBSs coupling strength, hybridization between the MBSs, and the ferromagnetism on the leads. Around the transition temperature, either 100% spin-polarized or pure spin thermopower can be generated by the combined effects of SHA and MBSs. We also find that the intradot Coulomb interaction does not change the qualitative results of the present scheme.

     

基于扫描隧道显微镜的单原子自旋共振技术

近年来,应用扫描隧道显微镜技术已经可以测量单个原子的电子自旋共振谱线,为实现原子尺度量子磁性的探测与操控迈出了重要一步。电子自旋共振扫描隧道显微镜具有原子分辨能力和几十个纳电子伏的超高能量分辨率,可以实现微弱信号的原子尺度探测,例如可以测量固体表面相距几纳米的两个原子之间的微弱磁偶极相互作用、单个原子的电子与核自旋之间的超精细相互作用,以及人工自旋阵列的量子涨落等。借助脉冲式电子自旋共振技术,可以进一步实现固体表面单个磁性原子以及耦合原子的量子相干操控,测量其拉比振荡、拉姆齐干涉条纹和自旋回波信号等。单原子脉冲式电子自旋共振的实现为应用单原子量子探针进行量子探测奠定了重要基础。另外,对具有原子级精度的人工自旋结构的量子相干操控,为多体系统的量子模拟提供了重要的固态实验平台。     

Influence of Magnetic Field and LO Phonon Effects on the Spin Polarization State Energy of Strong-Coupling Bipolaron in a Quantum Dot

On the basis of Lee–Low–Pines unitary transformation, the influence of magnetic field and LO phonon effects on the energy of spin polarization states of strong-coupling bipolarons in a quantum dot (QD) is studied by using the variational method of Pekar type. The variations of the ground state energy $E_0$ and the first excited state the energy $E_1$ of bipolarons in a two-dimensional QD with the confinement strength of QDs $\omega _0$ , dielectric constant ratio $\eta $ , electron–phonon coupling strength $\alpha $ and cyclotron resonance frequency of the magnetic field $\omega _{c}$ are derived when the influence of the spin and external magnetic field is taken into account. The results show that both energies of the ground and first excited states ( $E_0$ and $E_1)$ consist of four parts: the single-particle energy of electrons $E_\mathrm{e}$ , Coulomb interaction energy between two electrons $E_\mathrm{c}$ , interaction energy between the electron spin and magnetic field $E_\mathrm{S}$ and interaction energy between the electron and phonon $E_{\mathrm{e-ph}}$ ; the energy level of the first excited state $E_1$ splits into two lines as $E_1^{(1+1)}$ and $E_1^{(1-1)}$ due to the interaction between the single-particle “orbital” motion and magnetic field, and each energy level of the ground and first excited states splits into three “fine structures” caused by the interaction between the electron spin and magnetic field; the value of $E_{\mathrm{e-ph}}$ is always less than zero and its absolute value increases with increasing $\omega _0$ , $\alpha $ and $\omega _c$ ; the effect of the interaction between the electron and phonon is favorable to forming the binding bipolaron, but the existence of the confinement potential and Coulomb repulsive energy between electrons goes against that; the bipolaron with energy $E_1^{(1-1)}$ is easier and more stable in the binding state than that with $E_1^{(1+1)}$ .     

Hyperfine Interactions and Electron Spin-Dynamics in a Quantum Dot

We study the dynamics of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei. The decoherence is caused by the spatial variation of the electron wave function within the dot, leading to a nonuniform hyperfine coupling A. We evaluate the spin correlation function and find that the decay is not exponential but rather power (inverse logarithm) law-like. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by N/A, where N is the number of nuclei inside the dot, and the amplitude of precession decays to a finite value. We show that there is a striking difference between the decoherence time for a single dot and the dephasing time for an ensemble of dots.     

Voltage-Controlled Electron-Hole Interaction in a Single Quantum Dot

The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.     

Voltage-Controlled Electron-Hole Interaction in a Single Quantum Dot

The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.     

Stability of Quantum Dots,Quantum Dot Films,and Quantum Dot Light‐Emitting Diodes for Display Applications

Quantum dots (QDs) are being highlighted in display applications for their excellent optical properties, including tunable bandgaps, narrow emission bandwidth, and high efficiency. However, issues with their stability must be overcome to achieve the next level of development. QDs are utilized in display applications for their photoluminescence (PL) and electroluminescence. The PL characteristics of QDs are applied to display or lighting applications in the form of color‐conversion QD films, and the electroluminescence of QDs is utilized in quantum dot light‐emitting diodes (QLEDs). Studies on the stability of QDs and QD devices in display applications are reviewed herein. QDs can be degraded by oxygen, water, thermal heating, and UV exposure. Various approaches have been developed to protect QDs from degradation by controlling the composition of their shells and ligands. Phosphorescent QDs have been protected by bulky ligands, physical incorporation in polymer matrices, and covalent bonding with polymer matrices. The stability of electroluminescent QLEDs can be enhanced by using inorganic charge transport layers and by improving charge balance. As understanding of the degradation mechanisms of QDs increases and more stable QDs and display devices are developed, QDs are expected to play critical roles in advanced display applications.     

Anisotropic Quantum Transport through a Single Spin Channel in the Magnetic Semiconductor EuTiO3

Magnetic semiconductors are a vital component in the understanding of quantum transport phenomena. To explore such delicate, yet fundamentally important, effects, it is crucial to maintain a high carrier mobility in the presence of magnetic moments. In practice, however, magnetization often diminishes the carrier mobility. Here, it is shown that EuTiO₃ is a rare example of a magnetic semiconductor that can be desirably grown using the molecular beam epitaxy to possess a high carrier mobility exceeding 3000 cm² V⁻¹ s⁻¹ at 2 K, while intrinsically hosting a large magnetization value, 7 μ_B per formula unit. This is demonstrated by measuring the Shubnikov–de Haas (SdH) oscillations in the ferromagnetic state of EuTiO₃ films with various carrier densities. Using first-principles calculations, it is shown that the observed SdH oscillations originate genuinely from Ti 3d-t_2g states which are fully spin-polarized due to their energetical proximity to the in-gap Eu 4f bands. Such an exchange coupling is further shown to have a profound effect on the effective mass and fermiology of the Ti 3d-t_2g electrons, manifested by a directional anisotropy in the SdH oscillations. These findings suggest that EuTiO₃ film is an ideal magnetic semiconductor, offering a fertile field to explore quantum phenomena suitable for spintronic applications.     

Tunneling of Holes in Spin Polaron Theory

The single-particle tunneling current is calculated for normal metal–superconductor and superconductor–superconductor junctions using the spin polaron theory.