Kondo transport through a quantum dot coupled with side quantum-dot structures

This paper investigates Kondo transport properties in a quadruple quantum dot (QD) based on the slave-boson mean field theory and the non-equilibrium Green’s function.In the quadruple QD structure one Kondo-type QD sandwiched between two leads is side coupled to two separate QD structures:a single-QD atom and a double-QD molecule.It shows that the conductance valleys and peaks always appear in pairs and by tuning the energy levels in three side QDs,the one-,two-,or three-valley conductance pattern can be obtained.Furthermore,it finds that whether the valley and the peak can appear is closely dependent on the specific values of the interdot couplings and the energy level difference between the two QDs in the molecule.More interestingly,an extra novel conductance peak can be produced by the coexistence of the two different kinds of side QD structures.     

Cooling effect of thermal bias on a current-carrying nanodevice

We investigate the heat generation in a quantum dot (QD) coupled to two normal leads with different temperatures. It is found that heat in the QD can be conducted efficiently away through electron–phonon interaction in the QD when the QD is coupled stronger to colder lead than to the hotter one. As temperature of the colder lead is close to zero, the current through the QD peaks at the very QD level position, where the heat generation is zero, which helps to keep the stability of a working nanodevice. Then an ideal condition for nanodevice operation can be found.     

Transmission phase of a quantum dot with Kondo correlation near the unitary limit

The complex transmission amplitude of a quantum dot (QD) with Kondo correlation was measured near the unitary limit. The transmission phase was observed to evolve almost linearly over a range of about 1.5 pi when the Fermi energy was scanned through a spin degenerate energy level of the QD. Surprisingly, the phase in the Coulomb Blockade regime, with one more electron entering the dot, was strongly affected by a preexistence of Kondo correlation. These results suggest that a full explanation of the Kondo effect may go beyond the framework of the Anderson model.     

Electronic transport through a periodic array of quantum-dot rings

Using the tight-binding approximation and the transfer matrix method, this paper studies the electronic transport properties through a periodic array of quantum-dot (QD) rings threaded by a magnetic flux. It demonstrates that the even--odd parity of the QD number in a single ring and the number of the QD rings in the array play a crucial role in the electron transmission. For a single QD ring, the resonance and antiresonance transmission depend not only on the applied magnetic flux but also on the difference between the number of QDs on the two arms of the ring. For an array of QD rings, the transmission properties are related not only to the even--odd parity of the number $N_{0}$ of QDs in the single ring but also to the even--odd parity of the ring number $N$ in the array. When the incident electron energy is aligned with the site energy, for the array of $N$ rings with $N_{0}={\rm odd}$ the antiresonance transmission cannot occur but the resonance transmission may occur and the transmission spectrum has $N$ resonance peaks ($N-1$ resonance peaks) in a period for $N={\rm odd}$ (for $N={\rm even}$). For the array of $N$ rings with $N_{0}={\rm even}$ the transmission properties depend on the flux threading the ring and the QD number on one arm of the ring. These results may be helpful in designing QD devices.     

Spin-dependent conductance in FM/quantum-dot/FM tunneling system

We present a theoretical study of the spin-dependent conductance spectra in a FM/semiconductor quantum-dot (QD)/FM system. Both the Rashba spin-orbit (SO) coupling in the QD and spin-flip scattering caused by magnetic barrier impurities are taken into account. It is found that in the single-level QD system with parallel magnetic moments in the two FM leads, due to the interference between different tunneling paths through the spin-degenerate level, a dip or a narrow resonant peak can appear in the conductance spectra, which depends on the property of the spin-flip scattering. When the magnetizations of the two FM leads are noncollinear, the resonant peak can be transformed into a dip. The Rashba SO coupling manifests itself by a Rashba phase factor, which changes the phase information of every tunneling path and can greatly modulate the conductance. When the QD has multiple levels, the Rashba interlevel spin-flip effect appears, which changes the topological property of the structure. Its interplay with the Rashba phase can directly tune the coupling strengths between dot and leads, and can result in switching from resonance into antiresonance in the conductance spectra.     

Persistent current and transmission probability in the Aharonov-Bohm ring with an embedded quantum dot

Persistent current and transmission probability in the Aharonov-Bohm (AB) ring with an embedded quantum dot (QD) are studied using the technique of the scattering matrix. For the first time, we find that the persistent current can arise in the absence of magnetic flux in the ring with an embedded QD. The persistent current and the transmission probability are sensitive to the lead-ring coupling and the short-range potential barrier. It is shown that increasing the lead-ring coupling or the short-range potential barrier causes the suppression of the persistent current and the increasing resonance width of the transmission probability. The effect of the potential barrier on the number of the transmission peaks is also investigated. The dependence of the persistent current and the transmission probability on the magnetic flux exhibits a periodic property with period of the flux quantum.     

Enhancement of the Kerr effect for a quantum dot in a cavity

Nonlinear susceptibility of a quantum dot (QD) embedded in a two-sided cavity, is studied theoretically from a weak-coupling to a strong-coupling regime. In the relevance of a quantum logic gate, the corresponding nonlinear phase shifts (Kerr effect) are estimated for coherent wavepackets including one photon on average. In the weak-coupling regime, the phase shift enhances strongly as a function of a coupling constant between the cavity photon and QD, and eventually saturates in the strong-coupling regime. We also show transmission spectra to evaluate the efficiency of the phase shift. Although the efficiency decreases monotonically in the weak-coupling regime, it rises in the strong-coupling regime.     

Detection of spin current through a quantum dot with Majorana bound states

The spin transport properties are theoretically investigated when a quantum dot(QD) is side-coupled to Majorana bound states(MBSs) driven by a symmetric dipolar spin battery. It is found that MBSs have a great effect on spin transport properties. The peak-to-valley ratio of the spin current decreases as the coupling strength between the MBS and the QD increases. Moreover, a non-zero charge current with two resonance peaks appears in the system. In the extreme case where the dot–MBS coupling strength is strong enough, the spin current and the charge current are both constants in the non-resonance peak range. When considering the effect of the Zeeman energy, it is interesting that the resonance peak at the higher energy appears one shoulder. And the shoulder turns into a peak when the Zeeman energy is big enough. In addition, the coupling strength between the two MBSs weakens their effects on the currents of the system. These results are helpful for understanding the MBSs signature in the transport spectra.     

Tunneling current of the Coulomb-coupled quantum dots embedded in n-n junction

Taking account of the electron--electron (hole) and electron--hole interactions, the tunneling processes of the main quantum dot (QD) Coulomb-coupled with a second quantum dot embedded in n--n junction have been investigated. The eighteen resonance mechanisms involved in the tunneling processes of the system have been identified. It is found that the tunneling current depends sensitively on the electron occupation number in the second quantum dot. When the electron occupation number in the second dot is tiny, both the tunneling current peaks and the occupation number plateaus in the main QD are determined by the intra-resonance mechanism. The increase of the electron occupation number in the second dot makes the inter-resonance mechanism participate in the transport processes. The competition between the inter and intra resonance mechanisms persists until the electron occupation number in the second dot reaches around unity, leading to the consequence that the inter-resonance mechanisms completely dominate the tunneling processes.     

双光子成像观测衰老对小鼠卵母细胞内DNA甲基转移酶表达的影响

量子点(QD)具有抗漂白能力强、量子产率高、激发谱宽和发射谱窄等优点,在生物荧光标记领域有较广泛的应用。利用QD 585和Hoechst 33342 (H342)双标记小鼠卵母细胞中DNA甲基转移酶(Dnmt)和染色体,通过双光子成像同时双通道探测它们的荧光图像,获得了Dnmt蛋白表达的三维空间分布。发现老龄小鼠卵母细胞不适合体外培养成熟：该成熟方式不仅引起老龄小鼠卵母细胞Dnmt蛋白含量的变化,而且还改变了它们在胞质中的空间分布。这些改变可能与异常的甲基化修饰有关。     

Nanoscale solid-state single photon router

A novel scheme for a solid-state single-photon router based on a single quantum dot (QD) coupled to a nanomechanical resonator (NR) is proposed and analyzed theoretically. It relies on the coherent coupling between the quantum dot and the NR. We demonstrate that when a single-photon signal is tuned on resonance with the exciton in the QD, one can use a strong pump field to choose to what output port of this signal field is delivered, which is based on the analogue of electromagnetically induced transparency (EIT) effect which we refer it as phonon induced transparency (PIT) in this coupled system. The path between the reflection output port and the transmission output port can be achieved by simply turning off and on the pump field. The numerical results also indicate that this router can operate efficiently in the optical regime and at ultralow pump power as well as short switching time (～ns). This nanoscale router presented here will offer potential applications in scalable solid-state quantum networks and quantum information.     

Suppression of indefinite peaks in InAs/GaAs quantum dot spectrum by low temperature capping in the indium-flush method

We have investigated effects of growth temperature of thin GaAs capping layer in the initial stage of indium-flush process using atomic force microscopy and microscopic photoluminescence (μ-PL) methods. The shape of capped InAs quantum dot (QD) and its μ-PL properties are sensitive to the growth temperature of thin GaAs capping layer. In the case of the high temperature cap, the QD shape in initial capping stage is elongated along the [1 1 −0] direction, and μ-PL spectrum shows several peaks accompanied with indefinite peaks. On the other hand, the low temperature case, the QD shape is kept in isotropic and μ-PL spectrum shows distinctive emissions from excitonic states of the QD with suppressed indefinite peaks. These results indicate that the low temperature capping is effective to keep an isotropic shape of QD and suppress indefinite peaks.     

Effect of Intra-Dot Coulomb Interaction on Andreev Reflection in Normal-Metal/Quantum-Dot/Superconductor System

We investigate the effect of intra-dot Coulomb interaction on the Andreev reflection in a normal-metal/quantum-dot/superconductor (N-QD-S) system with multiple levels in the quantum dot, in the regime where the intra-dot interacting constant is comparable to the energy gap of superconducting lead. By using nonequilibrium Green function method, the averaged occupation of electrons in the quantum dot and the Andreev reflection (AR) current are studied. Comparing to the case of non-interacting quantum dot, the system shows significant changes for the averaged occupation of electrons in QD (〈n〉) and the AR current (I). (i) In the linear response regime, 〈n〉-V_g exhibits a two-step-like behavior; and the I-V_g shows two groups of peaks, separated by U and with equal heights, where V_g is the gate voltage and U denotes the intra-dot Coulomb interaction constant. (ii) For finite bias voltage, dips, superposed on the step-like 〈n〉-V_g curve, and the current peaks appear simultaneously, both originate from the AR processes. For V≥U/2, extra AR current peaks occur between the two groups of the peaks. Besides, the properties of the heights of the AR current peaks are more complicated.     

Strain interactions and defect formation in stacked InGaAs quantum dot and dot-in-well structures

The growth of high-quality stacked quantum dot (QD) structures represents one of the key challenges for future device applications. Electronic coupling between QDs requires closely separated electronic levels and thin barrier layers, requiring near identical composition and shape, despite strong strain interactions. This paper presents a detailed characterization study of stacked InGaAs QD and InAs/InGaAs dot-in-well (DWELL) structures using cross-sectional transmission electron microscopy. For In_.5Ga_.5As/GaAs QD structures we have observed optimized stacking using a barrier thickness 12 nm.We also report studies of stacking in DWELL laser structures. Despite reports of very low threshold currents in such lasers, designed for 1.3 μm emission, performance is limited by gain saturation and thermal excitation effects. We have explored solutions to these problems by stacking multiple DWELL layers of three, five and 10 repeats. Initial attempts at stacked multilayer structures, particularly samples with a large number of repeats, produced variable results, with a number of the final devices characterized by poor emission and electrical characteristics. Analysis by transmission electron microscopy has identified the presence of large defective regions arising from the complex interaction of dots on several planes and propagating threading dislocations into the cladding layers. The origin of this defect is identified as the coalescence of QDs at very high density and the resulting dislocation propagating to higher dot planes. An effective modified method to reduce the defect density by growing the barrier layer at higher temperature will be discussed. Finally, we report the growth of a stacked 10-layer structure using relatively thin barriers, grown using this technique.     

Combined Aharonov-Bohm and Zeeman spin-polarization effects in a double quantum dot ring

A mesoscale Aharonov-Bohm (AB) ring with a quantum dot (QD) embedded in each arm is computationally modeled for unique transmission properties arising from a combination of AB effects and Zeeman splitting of the QD energy levels. A tight-binding Hamiltonian is solved, providing analytical expressions for the transmission as a function of system parameters. Transmission resonances with spin-polarized output are presented for cases involving either a perpendicular field, or a parallel field, or both. The combination of the AB-effect with Zeeman splitting allows sensitive control of the output resonances of the device, manifesting in spin-polarized states which separate and cross as a function of applied field. In the case with perpendicular flux, the AB-oscillations exhibit atypical non-periodicity, and Fano-type resonances appear as a function of magnetic flux due to the flux-dependent shift in the QD energy levels via the Zeeman effect.     

Electron states in diluted magnetic semiconductors

One of the remarkable properties of the II–VI diluted magnetic semiconductor (DMS) quantum dot (QD) is the giant Zeeman splitting of the carrier states under application of a magnetic field. This splitting reveals strong exchange interaction between the magnetic ion moment and electronic spins in the QD. A theoretical study of the electron spectrum and of its relaxation to the ground state via the emission of a longitudinal optical (LO) phonon, in a CdSe/ZnMnSe self-assembled quantum dot, is proposed in this work. Numerical calculations showed that the strength of this interaction increases as a function of the magnetic field to become more than 30 meV and allows some level crossings. We have also shown that the electron is more localized in this DMS QD and its relaxation to the ground state via the emission of one LO phonon is allowed.     

Microwave-mediated heat transport in a quantum dot attached to leads

The thermoelectric effect in a quantum dot (QD) attached to two leads in the presence of microwave fields is studied by using the Keldysh nonequilibrium Green function technique. When the microwave is applied only on the QD and in the linear response regime, the main peaks in the thermoelectric figure of merit and the thermopower are found to decrease, with the emergence of a set of photon-induced peaks. Under this condition the microwave field cannot generate heat current or electrical bias voltage. Surprisingly, when the microwave field is applied only to one (bright) lead and not to the other (dark) lead or the QD, heat flows mostly from the dark to the bright lead, almost irrespective of the direction of the thermal gradient. We attribute this effect to microwave-induced opening of additional transport channels below the Fermi energy. The microwave field can change both the magnitude and the sign of the electrical bias voltage induced by the temperature gradient.     

Generating and reversing spin accumulation by temperature gradient in a quantum dot attached to ferromagnetic leads

We propose to generate and reverse the spin accumulation in a quantum dot (QD) by using the temperature difference between the two ferromagnetic leads connected to the dot. The electrons are driven purely by the temperature gradient in the absence of an electric bias and a magnetic field. In the Coulomb blockade regime, we find two ways to reverse the spin accumulation. One is by adjusting the QD energy level with a fixed temperature gradient, and the other is by reversing the temperature gradient direction for a fixed value of the dot level. The spin accumulation in the QD can be enhanced by the magnitudes of both the leads’ spin polarization and the asymmetry of the dot-lead coupling strengths. The present device is quite simple, and the obtained results may have practical usage in spintronics or quantum information processing.     

Direct interband light absorption in a cylindrical quantum dot in quantizing magnetic field

In this paper the direct interband transitions in cylindrical quantum dot (QD) made of GaAs are studied in the presence of a magnetic field. Two models of QD confinement potential are discussed. For both models the expressions for absorption coefficients and dependencies of effective threshold frequencies of absorption on the value of applied magnetic field and on geometrical sizes of QD are obtained. The selection rules corresponding to different transitions between quantum levels are found.     

光学微盘腔与三能级量子点系统中的模耦合研究

用一种全量子理论方法研究了波导、光学微盘腔与三能级量子点耦合系统的动力学过程,求出其耦合后的透射模和反射模的解析解. 由于微腔表面粗糙引起反向散射,在微腔内形成两简并回音壁耦合共振模,其耦合率为β;量子点的两激发态分别以耦合率g₁,g₂与回音壁耦合共振模产生耦合. 在实数空间里,得出透射光谱和反射光谱的数值解,这些三能级模型结果比二能级模型结果更接近真实光学微盘腔系统,能更好地显示耦合系统的动力学特性. 关键词： 模耦合 光学微盘腔 三能级量子点 全量子理论