Nonstationary effects in the system of coupled quantum dots influenced by Coulomb correlations

We investigate the time evolution of filling numbers of localized electrons in the system of two coupled single-level quantum dots (QDs) connected with the continuous-spectrum states in the presence of Coulomb interaction. We consider correlation functions of all orders for electrons in the QDs by decoupling higher-order correlations between localized and band electrons in the reservoir. We analyze different initial charge configurations and consider Coulomb correlations between localized electrons both within the dots and between the different dots. We reveal the presence of a dynamical charge trapping effect in the first QD in the situation where both dots are occupied at the initial instant. We also find an analytic solution for the time-dependent filling numbers of the localized electrons for a particular configuration of the dots.     

电子关联效应对平行双量子点系统磁输运性质的影响

从理论上研究了平行双量子点系统中的电子关联效应对该系统磁输运性质的影响. 基于广义主方程方法, 计算了通过此系统的电流、微分电导和隧穿磁阻. 计算结果表明: 电子自旋关联效应可以促发一个很大的隧穿磁阻, 而电子库仑关联效应不仅可以压制电子自旋关联效应, 还可以导致负隧穿磁阻和负微分电导的出现. 对相关的基本物理问题进行了讨论.     

Structural relaxation and aging scaling in the Coulomb and Bose glass models

We employ Monte Carlo simulations to study the relaxation properties of the two-dimensional Coulomb glass in disordered semiconductors and the three-dimensional Bose glass in type-II superconductors in the presence of extended linear defects. We investigate the effects of adding non-zero random on-site energies from different distributions on the properties of the correlation-induced Coulomb gap in the density of states (DOS) and on the non-equilibrium aging kinetics highlighted by the density autocorrelation functions. We also probe the sensitivity of the system’s equilibrium and non-equilibrium relaxation properties to instantaneous changes in the density of charge carriers in the Coulomb glass or flux lines in the Bose glass.     

The effect of coulomb correlations on the nonequilibrium charge redistribution tuned by the tunneling current

We demonstrate that the tunneling current flowing through a system with Coulomb correlations leads to a charge redistribution between the different localized states. A simple model consisting of two electron levels is analyzed by means of the Heisenberg equations of motion taking correlations of electron filling numbers in localized states into account exactly in all orders. We consider various relations between the Coulomb interaction and localized electron energies. Sudden jumps of the electron density at each level in a certain range of the applied bias are found. We find that for some parameter range, inverse occupation in the two-level system appears due to Coulomb correlations. It is also shown that Coulomb correlations lead to the appearance of negative tunneling conductivity at a certain relation between the values of tunneling rates from the two electron levels.     

Magnetic and charge orders in zigzag nanographene ribbons

We theoretically study the electronic states in graphene ribbons which are strongly affected by the edge states, the peculiar non-bonding molecular orbitals localized along the zigzag edges of the ribbons. New kinds of edge localized electronic states with spin and charge polarizations are found in the mean field solutions of the extended Hubbard model with onsite and nearest-neighbor Coulomb repulsions. These novel states appear due to the interplay between the edge states and the Fermi instabilities. We also examine the competition between the charge polarized state and the spin polarized state to draw a phase diagram depending on Coulomb parameters. The results obtained by the mean field calculations with the extended Hubbard model modified to include Coulomb integrals provide useful insights to understand and functionalize the nanoscale materials.     

Diagnostics of many-particle electronic states: Non-stationary currents and residual charge dynamics

We propose the method for identifying many particle electronic states in the system of coupled quantum dots (impurities) with Coulomb correlations. We demonstrate that different electronic states can be distinguished by the complex analysis of localized charge dynamics and non-stationary characteristics. We show that localized charge time evolution strongly depends on the properties of initial state and analyze different time scales in charge kinetics for initially prepared singlet and triplet states. We reveal the conditions for existence of charge trapping effects governed by the selection rules for electron transitions between the states with different occupation numbers.     

Correlation induced switching of the local spatial charge distribution in a two-level system

It was found that tunneling current through a nanometer scale structure with strongly coupled localized states causes spatial redistribution of localized charges induced by Coulomb correlations. We present here theoretical investigation of this effect by means of Heisenberg equations for localized states electron filling numbers. This method makes it possible to take into account pair correlations of local electron density exactly. It is shown that inverse occupation of the two-level system caused by Coulomb correlations appears in particular range of applied bias. Described effects can give a possibility of charge manipulation in the proposed system. We also expect that described results can be observed in tunneling structures with impurities or with small quantum dots.     

Optical Bloch equations with dual photon excitation explain the polarization dependence of four wave mixing quantum beats

We explain the polarization dependence of four wave mixing (FWM) quantum beats for semiconductors as essentially due to the spin phase correlations of photo-excited electrons, rather than to Coulomb interaction between the electrons. A theoretical analysis is given within the framework of optical Bloch equations for the light–semiconductor interactions and the Luttinger–Kohn model for the band structure. Residual Coulomb interactions between charge carriers are ignored. The results suggest that the polarization dependence of FWM quantum beats is a purely coherent effect of dual photon excitations, rather than, e.g., exciton–exciton Coulomb interaction. We show that the coherence transfer between the excited states is responsible for the FWM in a configuration with orthogonally polarized pump and probe.     

Effect of Coulomb repulsion on spin and orbital magnetism of cobalt–benzene complex: A relativistic density functional study

We have demonstrated electronic configurations and magnetic properties of single Co adatom on benzene (Bz) molecule in the framework of relativistic density functional theory. A sequence of fixed spin moment (FSM) calculations were carried out with and without Coulomb repulsion (U). We have investigated that varying the strength of Coulomb repulsion results to different equilibrium positions for the Co adatom on benzene molecule. It was shown that inclusion of the on-site Coulomb repulsion in the Co 3d orbitals affects significantly the geometry of Co–Bz complex. We also found two stable low-spin and high-spin multiplicities for the complex. The nature of the high-spin configuration was explained according to the Hubbard electron–electron correlation in 3d shell of the Co adatom. Our FSM results indicate that the high-spin state is a global minimum in the presence of Hubbard parameter U. The relativistic spin–orbit coupling and using orbital polarization correction induce considerable orbital magnetism in both low and high spin states of the Co–Bz complex. We have also calculated magnetic anisotropy energies for two spin states and we found out that an out-of-plane magnetic orientation of Co adatom is more favorable in the low spin state whereas the Coulomb repulsion (U = 2 eV and U = 4 eV) predicts an in-plane magnetic orientation for Co adatom. Our findings can be implicitly taken into account for the extended system of added single Co atom on graphene.     

Spin-dependent thermoelectric transport through double quantum dots

We study the thermoelectric transport through a double-quantum-dot system with spin-dependent interdot coupling and ferromagnetic electrodes by means of the non-equilibrium Green’s function in the linear response regime.It is found that the thermoelectric coefficients are strongly dependent on the splitting of the interdot coupling,the relative magnetic configurations,and the spin polarization of leads.In particular,the thermoelectric efficiency can reach a considerable value in the parallel configuration when the effective interdot coupling and the tunnel coupling between the quantum dots and the leads for the spin-down electrons are small.Moreover,the thermoelectric efficiency increases with the intradot Coulomb interaction increasing and can reach very high values at appropriate temperatures.In the presence of the magnetic field,the spin accumulation in the leads strongly suppresses the thermoelectric efficiency,and a pure spin thermopower can be obtained.     

Non-equilibrium spin accumulation in ferromagnetic single-electron transistors

We study transport in ferromagnetic single-electron transistors. The non-equilibrium spin accumulation on the island caused by a finite current through the system is described by a generalized theory of the Coulomb blockade. It enhances the tunnel magnetoresistance and has a drastic effect on the time-dependent transport properties. A transient decay of the spin accumulation may reverse the electric current on time scales of the order of the spin-flip relaxation time. This can be used as an experimental signature of the non-equilibrium spin accumulation. Received 6 May 1998     

Nonequilibrium transport through a vertical quantum dot in the absence of spin-flip energy relaxation

We investigate nonequilibrium transport in the absence of spin-flip energy relaxation in a few-electron quantum dot artificial atom. Novel nonequilibrium tunneling processes involving high-spin states, which cannot be excited from the ground state because of spin blockade, and other processes involving more than two charge states are observed. These processes cannot be explained by orthodox Coulomb blockade theory. The absence of effective spin relaxation induces considerable fluctuation of the spin, charge, and total energy of the quantum dot. Although these features are revealed clearly by pulse excitation measurements, they are also observed in conventional dc current characteristics of quantum dots.     

Adiabatic charge and spin pumping through interacting quantum dots

In this paper we investigate adiabatic charge and spin pumping through interacting quantum dots using non-equilibrium Green's function techniques and the equation-of-motion method. We treat the electronic correlations inside the dot using a Hartree-Fock approximation and succeed in obtaining closed analytic expressions for the Keldysh Green's functions. These allow us to compute charge and spin currents through the quantum dot. Depending on the parameters of the quantum dot and its coupling to the reservoirs, we show that it can be found in two different regimes: the magnetic regime and the non-magnetic regime. In the magnetic regime we find a non-vanishing spin current in addition to the charge current present in both cases.     

Critical temperature of a van Hove superconductor with spin–charge separation

We studied a spin–charge separation superconductor with a van Hove density of states. The critical temperature and the influence of the Coulomb interaction have been calculated in the mean field model. We also calculated the Ginzburg–Landau coefficients as a function of temperature.     

Direct coulomb and exchange interaction in artificial atoms

We determine contributions from the direct Coulomb and exchange interactions to the total interaction in artificial semiconductor atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. The electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle-state degeneracy, and find that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.     

Power output and efficiency of quantum dot attached to ferromagnetic electrodes with non-collinear magnetic moments

Non-linear charge and heat transport through a single-level quantum dot in the Coulomb blockade regime is investigated within the framework of non-equilibrium Green function formalism and power output and efficiency of the device are studied. It is found that maximum power as well as efficiency depends on the relative orientation of magnetic moments in electrodes and can vary with polarization factor p. In general, power output is suppressed in magnetic systems and decreases with polarization. The highest efficiency can be attained in antiparallel configuration, and moreover, it does not depend on p. Spin power as well as spin efficiency of the system is introduced and discussed. It is also shown that in the Coulomb blockade regime the (spin) efficiency of the device operating under maximum power conditions varies with temperature bias in a non-monotonic way and shows a flat maximum for low ΔT.     

Microscopic analysis of the coherent optical generation and the decay of charge and spin currents in semiconductor heterostructures

The coherent optical injection and temporal decay of spin and charge currents in semiconductor heterostructures is described microscopically, including excitonic effects, many-body Coulomb correlations, and the carrier LO-phonon coupling on the second-order Born-Markov level, as well as nonperturbative light-field-induced intraband and interband excitations. A nonmonotonic dependence of the currents on the intensities of the laser beams is predicted. Enhanced damping of the spin current relative to the charge current is obtained as a consequence of Coulomb scattering.     

Incommmensurability and unconventional superconductor to insulator transition in the hubbard model with bond-charge interaction

We determine the quantum phase diagram of the one-dimensional Hubbard model with bond-charge interaction X in addition to the usual Coulomb repulsion U>0 at half-filling. For large enough X相似文献   

Effect of Coulomb correlations on the electronic structure of PuCoGa(5)

We investigate the effect of strong Coulomb correlations on the electronic structure of the Pu-based superconductor PuCoGa5 by employing the relativistic local spin density approximation+ Hubbard U (LSDA+U) method. The inclusion of intra-atomic Coulomb U and exchange J parameters leads to a significant reconstruction of the f states electronic structure over that given by the LSDA approach. At variance with the LSDA, the LSDA+U suggests "jj"-like coupling for the Pu 5f manifold.