量子点环的电子输运研究

利用非平衡格林函数方法，研究了量子点环的相干输运性质. 结果表明：与一维量子点阵列 相比，量子点环中的电子出现更多新的准束缚能级. 量子点间耦合的增强会使微分电导振荡 出现退相干现象. 关键词： 量子点 电导     

Andreev reflection current through a molecule quantum dot in the presence of the electron-phonon interaction and the spin-flip scattering

With the aid of the nonequilibrium Green's function and the Lang-Firsov canonical transformation, we investigate the joint effects of a phononic environment and the spin-flip scattering on the Andreev reflection (AR) in a ferromagnet/single-molecular quantum dot/superconductor (FM/MQD/SC) system. In the presence of the strong electron-phonon interaction (EPI), it is found that the EPI strongly suppresses the AR current (called the Franck-Condon blockade). When the coherent spin-flip (similar to a transverse magnetic field) is taken into account within the MQD, the AR current is significantly enhanced, furthermore, the spin-polarized AR current or even the pure spin-polarized AR current can be generated. By tuning the system parameters, the amplitude and direction of the AR current can be changed, this provides an efficient mechanism for controlling the AR process.     

Polar optical phonon states and their electron-phonon coupling properties in a wurtzite nitride quantum dot

Within the framework of the macroscopic dielectric continuum model, the interface-optical-propagating (IO-PR) mixing phonon modes of a quasi-zero-dimensional (Q0D) wurtzite cylindrical quantum dot (QD) structure are derived and studied. The approximative analytical-phonon-states of IO-PR mixing modes are given. It is found that there are two types of IO-PR mixing phonon modes, i.e. ρ-IO/z-PR mixing modes and the z-IO/ρ-PR mixing modesexisting in Q0D wurtzite QDs. And each IO-PR mixing modes also have symmetrical and antisymmetrical forms. Via a standard procedure of field quantization, the Fröhlich Hamiltonians of electron-(IO-PR) mixing phonons interaction are obtained. And the orthogonal relations of polarization eigenvectors for these IO-PR mixing modes are also deduced. Numerical calculations of dispersive relation and electron-phonon coupling properties on a wurtzite GaN cylindrical QD are carried out. The behaviors that the IO-PR mixing phonon modes in wurtzite QDs reduce to the IO modes and PR modes in wurtzite QW and QWR systems are analyzed deeply from both of the viewpoints of physics and mathematics. The result shows that the present theories of polar mixing phonon modes in wurtzite cylindrical QDs are consistent with the phonon modes theories in wurtzite QWs and QWR systems. The coupling properties of electron-(IO-PR) mixing modes interactions are studied and analyzed in detail. An abnormal increase of electron-phonon coupling strength are observed as the azimuthal quantum numbers and order of phonon modes increase, which is ascribed to the modulation effect of different dielectric functions of wurtzite crystals in radius- and axial-directions. The analytical electron-phonon interaction Hamiltonians obtained here are useful for further investigating phonon influence on optoelectronics properties of wurtzite Q0D QD structures.     

Hydrogenic impurity bound polaron in a parabolic quantum dot with arbitrary electron-phonon coupling strength

A variational approach is employed to obtain the ground and the first excited state binding energies of an electron bound to a hydrogenic impurity in a polar semiconductor quantum dot (QD) with symmetric parabolic confinement in both two and three-dimensions. We perform calculations for the entire range of the electron-phonon coupling constant and the Coulomb binding parameter and for arbitrary confinement length. It is found that the binding energy of ground and first excited state is larger in a two-dimension (2D) dot than in a three-dimension (3D) dot and this trend is more pronounced with the increase of the electron-phonon coupling constant for the same value of the Coulomb binding parameter and confinement length. Furthermore, the ground and the first excited state binding energy increases with increasing the Coulomb binding parameter in both 2D and 3D QDs for the same electron-phonon coupling constant.     

Electron transport through a quantum cavity

Electron transport through a quantum cavity coupled with two one-dimensional waveguides is studied using a generalized scattering matrix method. In a symmetric N-channel cavity model, we are able to obtain an exact solution that predicts the electron energies at which the transmission of electron waves become zero. We found that the zero of transmission is closely related to the longitudinal resonance through inter-channel scattering, in particular, to the resonance of the highest propagating mode inside the cavity. This model provides a simple way to calculate the electron transmission through a cavity which could be useful in quantum waveguide engineering.     

Frequency-dependent transport through a quantum dot in the Kondo regime

We study the ac conductance and equilibrium current fluctuations of a Coulomb-blockaded quantum dot in the Kondo regime. To this end we have developed an extension of the numerical renormalization group suitable for the nonperturbative calculation of finite-frequency transport properties. We demonstrate that ac transport gives access to the many-body resonance in the equilibrium spectral density. It provides a new route for measuring this key signature of Kondo physics, which so far has defied direct experimental observation.     

Entanglement and transport through correlated quantum dot

We study quantum entanglement in a single-level quantum dot in the linear-response regime. The results show, that the maximal quantum value of the conductance 2e²/h not always match the maximal entanglement. The pairwise entanglement between the quantum dot and the nearest atom of the lead is also analyzed by utilizing the Wootters formula for charge and spin degrees of freedom separately. The coexistence of zero concurrence and the maximal conductance is observed for low values of the dot-lead hybridization. Moreover, the pairwise concurrence vanish simultaneously for charge and spin degrees of freedom, when the Kondo resonance is present in the system. The values of a Kondo temperature, corresponding to the zero-concurrence boundary, are also provided.     

Thermoelectric transport properties through a T-shaped single quantum dot

We study the thermopower, thermal conductance, electric conductance and the thermoelectric figure of merit for a gate-defined T-shaped single quantum dot (QD). The QD is solved in the limit of strong Coulombian repulsion U→∞, inside the dot, and the quantum wire is modeled on a tight-binding linear chain. We employ the X-boson approach for the Anderson impurity model to describe the localized level within the quantum dot. Our results are in qualitative agreement with recent experimental reports and other theoretical researches for the case of a quantum dot embedded into a conduction channel, employing analogies between the two systems. The results for the thermopower sign as a function of the gate voltage (associated with the quantum dot energy) are in agreement with a recent experimental result obtained for a suspended quantum dot. The thermoelectric figure of merit times temperature results indicates that, at low temperatures and in the crossover between the intermediate valence and Kondo regimes, the system might have practical applicability in the development of thermoelectric devices.     

Photon-assisted transport through a quantum dot coupled to superconductors

Photon-assisted electron transport for resonant tunneling has been investigated by using a current formula developed based on the nonequilibrium Green’s function technique. We have studied the external frequency dependence as well as the energy level position dependence for the resonant ac tunneling through the quantum dot coupled to two superconducting reservoirs.     

Spin-flip transport through an interacting quantum dot

The spin-flip associated transport based on the Anderson model is studied. It is found that the electrons are scattered due to spin-flip effect via the normal, mixed and Kondo channels. The spin-flip scattering via Kondo channel enhances the Kondo resonance peak and causes a slight blue shift. The conductance is suppressed by the spin-flip scattering. This is attributed to the reason that electrons with energy near Fermi level are scattered by Kondo channel.     

Inelastic transport through double quantum dot systems

Inelastic transport through double quantum dot systems with coupling between electronic and vibrational degrees of freedom is examined by means of a master equation approach. The current and the conductance are analyzed for both weak and strong interdot couplings. The results show that an asymmetry in the current-voltage characteristic and appearance of negative differential conductance due to electron-phonon interaction. The influence of temperature on the current is studied and found that increasing temperature gives rise to eliminating the current blockade and, thus, removing the Coulomb diamonds in the conductance spectra.     

Thermoelectric transport through a quantum dot with a magnetic impurity

We study the thermoelectric effect in a small quantum dot with a magnetic impurity in the Coulomb blockade regime.The electrical conductance,thermal conductance,thermopower,and the thermoelectrical figure of merit(FOM)are calculated by using Green’s function method.It is found that the peaks in the electrical conductance are split by the exchange coupling between the electron entering into the dot and the magnetic impurity inside the dot,accompanied by the decrease in the height of peaks.As a result,the resonances in the thermoelectric quantities,such as the thermal conductance,thermopower,and the FOM,are all split,opening some effective new working regions.Despite of the significant reduction in the height of the electrical conductance peaks induced by the exchange coupling,the values of the FOM and the thermopower can be as large as those in the case of zero exchange coupling.We also find that the thermoelectric efficiency,characterized by the magnitude of the FOM,can be enhanced by adjusting the left–right asymmetry of the electrode–dot coupling or by optimizing the system’s temperature.     

Time-dependent transport through a quantum dot: influence of the Coulomb interaction on the quantum dot charge states

We consider the electron transport through one-level quantum dot, out of the Kondo regime, under the influence of the external microwave fields. The influence of the intra-dot Coulomb electron-electron interaction is studied using the equation of motion method for appropriate correlation functions. The formula for the current and the closed set of the integro-differential equations for the expectation values of the quantum dot charge states are given. The most characteristic feature of these time-averaged expectation values is an appearance of the additional structure (sidebands) on the curves of the derivatives of the expectation values with respect to the gate voltage. The sidebands structure formed on both sides of the ‘ionization’ and ‘affinity’ quantum dot levels are also found on the current and differential conductance curves.     

Electron transport in a double quantum dot governed by a nuclear magnetic field

We investigate theoretically electron transfer in a double dot in a situation where spin blockade is lifted by nuclear magnetic field: this has been recently achieved in experiment [F. Koppens, Science 309, 1346 (2005)]. We show that for a given realization of nuclear magnetic field spin blockade can be restored by tuning external magnetic field; this may be useful for quantum manipulation of the device.     

Phonon-dependent transport through a serially coupled double quantum dot system

Using Keldysh nonequilibrium Green function formalism and mapping a many-body electron–phonon interaction onto a one body problem, the electron transport through a serially coupled double quantum dot system is analyzed. The influence of the electron–phonon interaction, temperature, detuning, and interdot tunneling on the transmission coefficient and current is studied. Our results show that the electron–phonon interaction results in the appearance of the side peaks in the transmission coefficient, whose height is strongly dependent on the phonon temperature. We have also found that the inequality of the electron–phonon interaction strength in two dots gives rise to an asymmetry in the current–voltage characteristic. In addition, the temperature difference between the phonon and electron subsystems results in the reduction of the saturated current and the destruction of the step-like behavior of the current. It is also observed that the detuning can improve the magnitude of the current by compensating the mismatch of the quantum dots energy levels induced by the electron–phonon interaction.     

Spin-flip mesoscopic transport through a carbon nanotube quantum dot system

The pure spin transport in an entire metallic single-wall carbon-nanotube (SWCN) interacting quantum dot (QD) system is investigated by using non-equilibrium Green's function (NEGF) technique. The novel spin current performance introduced by one constant and one rotating magnetic fields shows the unique four-fold degenerate electron shell structure which exists the SWCN QD sensitively. Spin transport properties can be designed by tuning the orbital and Zeeman configuration in the central resonant region, which are greatly influenced by the Coulomb interaction and the magnetic fields.     

Theory of resonant tunneling through quantum dots in the presence of electron-phonon interaction

Structures where the electrons of a two-dimensional electron gas are confined to disconnected regions can be fabricated by the use of appropriate gate geometries. The transport between these electrostatically defined quantum dots takes place by tunneling. Using the tunneling Hamiltonian approach we present a theoretical model of the system including electron-phonon interaction. The relevant coupling constants are determined from realistic wave functions for the expected confinement potentials. The phonon part of the Hamiltonian is diagonalized using a canonical transformation. Starting from the determination of the transmission matrix for the interacting system we calculate the current-voltage characteristics for different temperatures and phonon coupling strengths.     

Nonequilibrium transport through a spinful quantum dot with superconducting leads

We study the nonlinear cotunneling current through a spinful quantum dot contacted by two superconducting leads. Applying a general nonequilibrium Green function formalism to an effective Kondo model, we study the rich variation in the IV characteristics with varying asymmetry in the tunnel coupling to source and drain electrodes. The current is found to be carried, respectively, by multiple Andreev reflections in the symmetric limit, and by spin-induced Yu-Shiba-Rusinov bound states in the strongly asymmetric limit. The interplay between these two mechanisms leads to qualitatively different IV characteristics in the crossover regime of intermediate symmetry, consistent with recent experimental observations of negative differential conductance and repositioned conductance peaks in subgap cotunneling spectroscopy.