AC-field-induced quantum phase transitions in density of states

We investigate the joint effects of the intralead electron interaction and an externalalternating gate voltage on the time-averaged local density of states (DOSs) of a quantumdot coupled to two Luttinger-liquid leads in the Kondo regime. A rich dependence of theDOSs on the driving amplitude and intralead interaction is demonstrated. We show that thefeature is quite different for different interaction strengths in the presence of the acfield. It is shown that the photon-assisted transport processes cause an additionalsplitting of the Kondo peak or dip, which exhibits photon-assisted single-channel (1CK) ortwo-channel Kondo (2CK) physics behavior. The phase transition between photon-assisted 1CKand 2CK physics occurs when the interaction strength is moderately strong. The inelasticchannels associated with photon-assisted electron tunneling can dominate electrontransport for weak interaction when the ac amplitude is greater than the frequency by oneorder of magnitude. In the limit of strong interaction the DOSs scale as a power-lawbehavior which is strongly affected by the ac field.     

Non-equilibrium STLS approach to transport properties of single impurity Anderson model

In this work, using the non-equilibrium Keldysh formalism, we study the effects of the electron–electron interaction and the electron-spin correlation on the non-equilibrium Kondo effect and the transport properties of the symmetric single impurity Anderson model (SIAM) at zero temperature by generalizing the self-consistent method of Singwi, Tosi, Land, and Sjolander (STLS) for a single-band tight-binding model with Hubbard type interaction to out of equilibrium steady-states. We at first determine in a self-consistent manner the non-equilibrium spin correlation function, the effective Hubbard interaction, and the double-occupancy at the impurity site. Then, using the non-equilibrium STLS spin polarization function in the non-equilibrium formalism of the iterative perturbation theory (IPT) of Yosida and Yamada, and Horvatic and Zlatic, we compute the spectral density, the current–voltage characteristics and the differential conductance as functions of the applied bias and the strength of on-site Hubbard interaction. We compare our spectral densities at zero bias with the results of numerical renormalization group (NRG) and depict the effects of the electron–electron interaction and electron-spin correlation at the impurity site on the aforementioned properties by comparing our numerical result with the order U²$U^2$ IPT. Finally, we show that the obtained numerical results on the differential conductance have a quadratic universal scaling behavior and the resulting Kondo temperature shows an exponential behavior.     

The shot noise of a strongly correlated quantum dot coupled to the Luttinger liquid leads

We study the shot noise of a strongly correlated quantum dot weakly coupled to Luttinger liquid leads in the Kondo regime by means of the extended equation of motion method. A general zero-frequency shot noise formula with good convergence is derived. The shot noise exhibits a non-monotonic dependence on voltage for weak intralead interaction. There is a peak around the Kondo temperature at low voltage when the interaction is very weak, and its height decreases rapidly with the intralead interaction increasing. When the interaction is moderately strong the peak disappears and the shot noise scales as a power law in bias voltage, indicating that the intralead electron interaction suppresses the shot noise. It is possible that the measurements of the shot noise spectrum can extract the information of the intralead interaction.     

Conductance and shot noise of inelastic tunneling through a quantum dot in the Kondo regime

We investigate the inelastic transport properties of a quantum dot connected to two leads, based on the combination of a recently developed nonperturbative technique and slave-boson methods involving the approximate mapping of the many-body electron–phonon coupling problem onto a multichannel scattering problem in the Kondo regime. The nonequilibrium Green's function method is adopted in calculations for the inelastic transport processes of electrons in the limit of large Coulomb interaction U→∞$U\to \infty$ under nonequilibrium conditions. The electron–phonon interactions, which are the main source of the inelasticity, are taken into account. For a single quantum dot, we find that the differential conductance and the shot noise exhibit new structures of peaks and dips which are absent in the case without electron–phonon interactions.     

The exchange interaction effect in the scanning tunneling spectroscopy of a two-orbital Anderson impurity on metallic surface

We investigate the scanning tunneling spectroscopy (STS) of a two-orbital Anderson impurity adsorbed on a metallic surface by using the numerical renormalization group (NRG) method. The density of state of magnetic impurity and the local conduction electron are calculated. We obtain the Fano resonance line shape in the STM conductance at zero temperature. For the impurity atom with antiferromagnetic inter-orbital exchange interaction and a spin singlet ground state, we show that a dip in the STM spectra around zero bias voltage regime and side peaks of spin excitation can be observed. The spin excitation energy is proportional to the exchange interaction strength. As the exchange interaction is ferromagnetic, the underscreened Kondo effect dominates the low energy properties of this system, and it gives rise to drastically different STM spectra as compared with the spin singlet case.     

Effects of intradot electron–electron interaction on the photon-assisted Andreev tunneling through a finite-sized carbon-nanotube system

The effects of intradot electron–electron interaction on the photon-assisted Andreev tunneling of a superconductor/carbon-nanotube/superconductor system are studied by using nonequilibrium Green's function technique. The inverse supercurrent reflecting the π-junction transition emerges in the spin-split energy-levels regime polarized by the Coulomb interaction. For the positive tunneling case, the supercurrent reaches its maximum when the spin-degenerate energy-levels are nearest to the Fermi surface. Conversely, for the negative tunneling case, the supercurrent reaches its maximum when two split energy-levels are symmetric with respect of the Fermi surface. The sign and the amplitude of the Andreev tunneling depend distinctly on the energy-level spacing tuned by photon-assisted tunneling. In order to fully understand the transport characteristics, the current-carrying density of states are investigated, which clearly shows the enhancement, suppression or even reversion of the supercurrent.     

Density of states of a strongly correlated quantum dot coupled to Luttinger liquid leads

We theoretically investigate the density of states (DOS) of a quantum dot weakly coupled to Luttinger liquid (LL) leads in the Kondo regime by use of the equation-of-motion technique of the nonequilibrium Green functions. At zero temperature, the Kondo peak in DOS is suppressed by the intralead interaction. When the LL interaction parameter K   is about 0.77, a moderately strong interaction, the Kondo peak disappears and then a dip develops, a signature of the intermediate two-channel Kondo (2CK) physics. This shows that the condition for the 2CK to occur ever addressed is not necessary. Applying a finite voltage bias splits the dip in the DOS. Each split dip is located at the chemical potential of a LL lead. This again appears the stabilized 2CK physics for moderately strong interaction K<1$K<1$.     

Phonon-assisted Kondo effect in single-molecule quantum dots coupled to ferromagnetic leads

Based on the infinite-U Anderson model spin-polarized transport through the tunnel magnetoresistance (TMR) system of single-molecule quantum dot is investigated under the interplay of strong electron correlation and electron-phonon (e-ph) coupling. The spectral density and the nonlinear differential conductance are studied using the extended non-equilibrium Green's function method through calculating the dot-level splitting self-consistently. The results exhibit that a serial of peaks emerge on the two sides of the main Kondo peak for the antiparallel magnetic configuration of electrodes, while for the parallel case both the main and phonon-assisted satellite Kondo peaks all split up into two asymmetric peaks even at zero-bias. Correspondingly, the nonlinear differential conductance displays a set of satellite-peaks around the Kondo-peak in the presence of the e-ph interaction. Furthermore, extra maxima and minima appear in the TMR curve. The TMR alternates between the positive and the negative values along with the variation of bias voltage.     

Dynamic transport characteristics of side-coupled double-quantum-impurity systems

A systematic study is performed on time-dependent dynamic transport characteristics of a side-coupled double-quantum-impurity system based on the hierarchical equations of motion. It is found that the transport current behaves like a single quantum dot when the coupling strength is low during tunneling or Coulomb coupling. For the case of only tunneling transition, the dynamic current oscillates due to the temporal coherence of the electron tunneling device. The oscillation frequency of the transport current is related to the step voltage applied by the lead, while temperature $T$, electron--electron interaction $U$ and the bandwidth $W$ have little influence. The amplitude of the current oscillation exists in positive correlation with $W$ and negative correlation with $U$. With the increase in coupling $t_{12}$ between impurities, the ground state of the system changes from a Kondo singlet of one impurity to a spin singlet of two impurities. Moreover, lowering the temperature could promote the Kondo effect to intensify the oscillation of the dynamic current. When only the Coulomb transition is coupled, it is found that the two split-off Hubbard peaks move upward and have different interference effects on the Kondo peak at the Fermi surface with the increase in $U_{12}$, from the dynamics point of view.     

Electron tunneling through a coupled system of electron and phonon

We study the electron tunneling through a single level quantum dot in the presence of electron–phonon interaction. By using the Green’s function and canonical transformation methods, we calculated exactly the current. It is found that the current vs dot level exhibits satellite peaks even without occurring of phonon-assisted tunneling processes, and properties of the current are affected heavily by the strength of electron–phonon interaction and phonon temperature.     

Spin-dependent shot noise of inelastic transport through molecular quantum dots

Here we present a theoretical analysis of the effect of inelastic electron scattering on spin-dependent transport characteristics (conductance, current–voltage dependence, magnetoresistance, shot noise spectrum, Fano factor) for magnetic nanojunction. Such device is composed of molecular quantum dot (with discrete energy levels) connected to ferromagnetic electrodes (treated within the wide-band approximation), where molecular vibrations are modeled as dispersionless phonons. Non-perturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT–MT), which transforms the many-body electron–phonon interaction problem into a single-electron multi-channel scattering problem. The consequence of the localized electron–phonon coupling is polaron formation. It is shown that polaron shift and additional peaks in the transmission function completely change the shape of considered transport characteristics.     

Enhancement of the shot noise of a quantum dot–Luttinger lead system

We investigate the joint effects of the intralead electron interaction and Coulombic dot–lead interaction on the shot noise of a quantum dot coupled to Luttinger liquid leads. A formula of the shot noise is derived by applying the nonequilibrium Green function technique. The shot noise is enhanced by the dot–lead interaction. For a weak or moderately strong interaction the differential shot noise demonstrates resonant-like behavior as a function of bias and gate voltages. In the limit of strong interaction resonant behavior disappears and the differential shot noise and Fano factor scale as a power law in bias voltage. Under some parameters, the differential shot noise may become negative around resonant peaks, and the physical reason is analyzed.     

Spin filters with Fano dots

We compute the zero bias conductance of electrons through a single ballistic channel weakly coupled to a side quantum dot with Coulomb interaction. In contrast to the standard setup which is designed to measure the transport through the dot, the channel conductance reveals Coulomb blockade dips rather then peaks due to the Fano-like backscattering. At zero temperature the Kondo effect leads to the formation of broad valleys of small conductance corresponding to an odd number of electrons on the dot. By applying a magnetic field in the dot region we find two dips corresponding to a total suppression in the conductance of spins up and down separated by an energy of the order of the Coulomb interaction. This provides a possibility of a perfect spin filter.Received: 6 November 2003, Published online: 2 April 2004PACS: 72.15.Qm Scattering mechanisms and Kondo effect - 73.23.Ad Ballistic transport - 72.25.-b Spin polarized transport     

Spin-Polarized Transport Through a Quantum Dot Coupled to Ferromagnetic Leads:Kondo Correlation Effect

We investigate the linear and nonlinear transport through a single level quantum dot connected to two ferromagnetic leads in Kondo regime, using the slave-boson mean-field approach for finite on-site Coulomb repulsion. We find that for antiparallel alignment of the spin orientations in the leads, a single zero-bias Kondo peak always appears in the voltage-dependent differential conductance with peak height going down to zero as the polarization grows to P=1. For parallel configuration, with increasing polarization from zero, the Kondo peak descends and greatly widens with the appearance of shoulders, and finally splits into two peaks on both sides of the bias voltage around P～0.7 until disappearing at even larger polarization strength. At any spin orientation angle θ, the linear conductance generally drops with growing polarization strength. For a given finite polarization, the minimum linear conductance always appears at θ=π.     

Full counting statistics for the Kondo dot

The generating function for the cumulants of charge current distribution is calculated for two generalized Majorana resonant level models: the Kondo dot at the Toulouse point and the resonant level embedded in a Luttinger liquid with the interaction parameter g=1/2. We find that the low-temperature nonequilibrium transport in the Kondo case occurs via tunneling of physical electrons as well as by coherent transmission of electron pairs. We calculate the third cumulant ("skewness") explicitly and analyze it for different couplings, temperatures, and magnetic fields. For the g=1/2 setup the statistics simplifies and is given by a modified version of the Levitov-Lesovik formula.     

量子线/铁基超导隧道结中隧道谱的研究

考虑铁基超导中能带间的相互作用和界面对每一个能带的散射作用, 利用推广的Blonder-Tinkham-Klapwijk模型, 并通过求解Bogoliubov-de Gennes 方程研究了具有不同类型双能隙系统的量子线/铁基超导隧道结中准粒子的输运系数和隧道谱. 研究表明: 1)在弹道极限时, 随着带间相互作用的增大, s_± 波隧道谱中零偏压附近的平台演变成电导峰; s₊₊ 波的平台演变成凹陷; p波的零偏压电导峰被压低. 2)界面对两个能带的散射作用不为零时, 随着带间相互作用的增大, s_± 波和s₊₊ 波两能隙处的峰值将降低, 而两峰间的凹陷值将变大; p波的零偏压电导峰被压低, 非零偏压电导增大. 3)界面对每个能带的散射, 可使其产生的电导峰变得更加尖锐, 但可压低和抹平另一个带产生的电导峰值. 这些结果对于澄清铁基超导体的能隙结构和区别不同类型铁基超导体有所帮助.     

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.     

相互作用量子点系统中的场致自旋电流

从理论上研究了相互作用量子点在外部旋转磁场下的非平衡自旋输运性质，研究结果表明，量子点中的相干自旋振荡可以导致自旋电流的产生，当计入库仑关联相互作用后，近藤共振效应受外部进动磁场的影响很强，特别是当磁场的进动频率与塞曼能移满足共振条件时，每个自旋近藤峰就会劈裂为两个自旋共振峰的叠加，在低温强耦合区，这种近藤型共隧穿过程对自旋电流带来重要贡献。     

Tunneling Conductance in Quantum-Wire/Ferromagnetic-Insulator/d-Wave Superconductor Junction

We have studied the quasiparticle transport in quantum-wire /ferromagnetic-insulator/d wave superconductor Junction (q/FI/d) in the framework of the Blonder-Tinkham-Klapwijk model. We calculate the tunneling conductance in q/FI/d as a function of the bias voltage at zero temperature and finite temperature based on Bogoliubov-de Gennes equations. Different from the case in normal-metal/insulator/d wave superconductor Junctions, the zero-bias conductance peaks vanish for the single-mode case. The tunneling conductance spectra depend on the magnitude of the exchange interaction at the ferromagnetic-insulator.     

Improving performance of resonant tunneling devices in asymmetric structures

Based on the global coherent tunneling model, we present a self-consistent calculation and show that structural asymmetry of double barrier resonant tunneling structures (DBRTSs) significantly modifies the current–voltage characteristics compared to the symmetric structures. Within the framework of the dielectric continuum model, we further investigate the phonon-assisted tunneling (PAT) current in symmetric and asymmetric DBRTSs. Both the interface modes and the confined bulk-like longitudinal-optical phonons are considered. The results indicate that the four higher-frequency interface phonon modes (especially the one which has the largest electron–phonon interaction at either interface of the emitter barrier) dominate the PAT processes. We show that a suitably designed asymmetric structure can produce much larger peak current and absolute value of the negative differential conductivity than its commonly used symmetric counterpart.