Shot noise in a quantum dot coupled to non-magnetic leads: effects of Coulomb interaction

We study electron transport through a quantum dot, connected to non-magnetic leads, in a magnetic field. A super-Poissonian electron noise due to the effects of both interacting localized states and dynamic channel blockade is found when the Coulomb blockade is partially lifted. This is sharp contrast to the sub-Poissonian shot noise found in the previous studies for a large bias voltage, where the Coulomb blockade is completely lifted. Moreover, we show that the super-Poissonian shot noise can be suppressed by applying an electron spin resonance (ESR) driving field. For a sufficiently strong ESR driving field strength, the super-Poissonian shot noise will change to be sub-Poissonian.     

Discrete quantum Fourier transform in coupled semiconductor double quantum dot molecules

In this Letter, we present a physical scheme for implementing the discrete quantum Fourier transform in a coupled semiconductor double quantum dot system. The main controlled-R gate operation can be decomposed into many simple and feasible unitary transformations. The current scheme would be a useful step towards the realization of complex quantum algorithms in the quantum dot system.     

Commensurate photon-irradiated Fano-Kondo tunneling through a quantum dot embedded Aharonov-Bohm interferometer

The commensurate photon-irradiated mesoscopic transport in a strongly correlated quantum dot (QD) embedded Aharonov-Bohm (AB) interferometer has been investigated. We focus our investigation on the dynamic Kondo and Fano cooperated effect affected by the double commensurate MWFs with q=ω₂/ω₁ being an arbitrary integer, where ω₁ and ω₂ are the two frequencies of the fields. The general tunneling current formula is derived by employing the nonequilibrium Green's function technique, and the different photon absorption and emission processes induced nonlinear properties have been studied to compare with the single-field system where q=0. Our numerical calculations are performed for the special cases with two commensurate fields possessing q=1,2. The Kondo peak can be suppressed to be a Kondo valley for the case where the commensurate number q=1, and the Fano asymmetric structure exhibits in the differential conductance quite evidently. Different commensurate number q contributes different photon absorption and emission effects. However, the conductance for the case of q=2 possesses more peaks and heavier asymmetric structure than the situations of q=0,1. The enhancement of satellite peaks behaves quite differently for the two cases with q=1, and q=2. The asymmetric peak-valley structure is adjusted by the gate voltage, commensurate MWFs, AB flux, source-drain bias, and non-resonant tunneling strength to form novel Fano and Kondo resonant tunneling.     

Phase Interference in a Multi-level Quantum-Dot System

Considering phase interference, we investigate coherent transport in a quantum dot by using a thermopower. In the single process of the electronic transport through the quantum dot, it is shown that the phase interference between the levels of a quantum dot is like the Aharonov-Bohm effect. The result indicates that the thermopower is very sensitive to phase interference. It is also found that the phase-difference change of the different levels of the quantum dot can determine the shape of the thermopower.     

Bound states of Dirac electrons in a graphene-based magnetic quantum dot

We investigate the magnetically confined states of the massless Dirac fermions in a graphene quantum dot formed by the inhomogeneous distributions of the magnetic fields inside and outside the dot. The calculated energy spectrum exhibits quite different features with and without the magnetic field inside the dot. It is found that the degeneracy of the relativistic Landau level with negative angular momenta can be lifted, and this degeneracy breaking can be modulated by the magnetic field inside the dot. Moreover, such a system can form the strongly localized states within the dot and along its boundary, especially with the magnetic field inside the dot.     

Shot noise in a quantum dot coupled to carbon nanotube terminals applied with a microwave field

We have investigated the spectral density of shot noise for the system of a quantum dot (QD) coupled to two single-wall carbon nanotube terminals irradiated with a microwave field on the QD. The terminal features are involved in the shot noise through modifying the self-energy of QD. The contributions of carbon nanotube terminals to the shot noise exhibit obvious behaviors. The novel side peaks are associated with the photon absorption and emission procedure accompanying the suppression of shot noise. The shot noise in balanced absorption belongs to sub-Poissonian, and it is symmetric with respect to the gate voltage. The differential shot noise displays intimate relation with the nature of carbon nanotubes and the applied microwave field. It exhibits asymmetric behavior for the unbalanced absorption case versus gate voltage. The Fano factor of the system exhibits the deviation of shot noise from the Schottky formula, and the structures of terminals obviously contribute to it. The super-Poissonian and sub-Poissonian shot noise can be achieved in the unbalanced absorption in different regime of source-drain bias.     

Fano effect in a T-shaped double quantum dot structure in the presence of Rashba spin-orbit coupling

The influence of Rashba spin-orbit coupling on the Fano lineshape of the conductance spectrum in a T-shaped double quantum dot structure is theoretically studied. By second-quantizing the electron Hamiltonian in this structure, it is found that the Rashba interaction brings about a spin-flip interdot hopping term. With the enhancement of the Rashba interaction, this term separates the two resonant peaks in the conductance spectrum from each other. More importantly, it causes the broadening of the narrow Fano peak, and the narrowing of the broader peak. Finally, the asymmetric Fano lineshape changes into a symmetric profile in the global conductance spectrum.     

ac field-induced Fano resonances in a parallel-coupled double quantum dot system

The effects of an ac electric field on the Fano resonance in a parallel-coupled double quantum dot system are investigated theoretically. The field can induce the photon-assisted Fano resonances for both symmetrical and asymmetrical parallel configurations. The magnitude and position of the photon-assisted Fano peak can be tuned by the ac field strength and frequency, respectively. Furthermore, the Fano resonance can appear with increasing the field frequency for both the symmetrical and asymmetrical configurations. This provides an efficient mechanism to control the Fano resonance. The photon-electron pumping effects for the symmetrical and asymmetrical cases are also studied in the weak- and strong-coupling regime.     

Maximal Heat Generation in Nanoscale Systems

We investigate the heat generation in a nanoscale system coupled to normal leads and find that it is maximal when the average occupation of the electrons in the nanoscale system is 0.5, no matter what mechanism induces the heat generation.     

Kicked quantum circuits with charge discreteness: Resonance condition

In this work we study a quantum electrical circuit with charge discreteness perturbed by periodic external kicks. Time evolution equations, for energy and electrical current, are solved analytically. Time evolution fluctuations are also studied and they become bounded. Resonances are well characterized including arbitrary (generic) quantum circuits with charge discreteness.     

Tunability of the Kondo effect in an asymmetrically tunnel coupled quantum dot

The transport properties of a single quantum dot were measured at low temperature in a regime of strong asymmetric tunnel coupling to leads. By tuning this asymmetry, the two parameters of the Kondo effect in a quantum dot, the Kondo temperature and the zero-bias zero-temperature conductance, were independently controlled. A careful analysis of the Coulomb energies and of the tunnel couplings was performed. It allowed an estimate of the Kondo temperature independently of its value obtained via the temperature dependence of the conductance. Both are in good agreement. We finally compared our experimental data with an exact solution of the Kondo problem which provides the dependence of the differential conductance on temperature and source-drain voltage. Theoretical expectations fit quite well our experimental data in the equilibrium and out-of-equilibrium regimes.     

Logic gates scheme based on Coulomb blockade in metallic nanoclusters with organic ligands

We propose a logic gates scheme based on the electron transfer through metallic nanoclusters linked to organic ligands and discuss theoretically the characteristics needed for practical implementation. As a proof-of-the-concept, we demonstrate the OR, AND and NOT gates and study the performance in terms of temperature, applied voltage, and noise.     

Electron States in Parallel Double Quantum Dots with a Tunable Inter-Dot Coupling

We study the electron states on lateral double quantum dots coupled in parallel. The charge stability diagrams are given in terms of the gate voltages of both dots. We discover that the two electron states translate from separated states to coupled states continuously by increasing the inter-dot coupling strength. Our results demonstrate that the parallel-quantum-dot tunability bodes well for future quantum computing applications.     

Energy of exciton in quantum dot for a potential containing Coulomb and quadratic terms

We theoretically study the energy levels of an exciton in a quantum dot. We take in to account both quadratic and Coulomb terms. Next, we use the method of series to solve the Schrödinger equation exactly. Using this formalism, we have calculated the exciton energy in both ground and excited states. The results are comparable to those of variational exact diagonolization, full configuration interaction, Hartree-Fock and 1/N methods. Our approach could be fitted for any desired material.     

Discrete-charge quantum circuits and electrical resistance

From the theory of quantum LC circuits with discrete charge, and semiclassical considerations, we obtain approximate energy eigenvalues, depending on the parameter . Next, we include electrical resistance for the quantum RLC circuit, obtaining a relation that strongly reminds us of the Landauer formula.     

Charge and spin currents tunnelling through a toroidal carbon nanotube side-coupled with a quantum dot

We have investigated the mesoscopic transport through the system with a quantum dot (QD) side-coupled to a toroidal carbon nanotube (TCN) in the presence of spin-flip effect. The coupled QD contributes to the mesoscopic transport significantly through adjusting the gate voltage and Zeeman field applied to the QD. The compound TCN-QD microstructure is related to the separate subsystems, the applied external magnetic fields, as well as the combination of subsystems. The spin current component I_z^s is independent on time, while the spin current components I_x^s and I_y^s evolve with time sinusoidally. The rotating magnetic field induces novel levels due to the spin splitting and photon absorption procedures. The suppression and enhancement of resonant peaks, and semiconductor-metal phase transition are observed by studying the differential conductance through tuning the source-drain bias and photon energy. The magnetic flux induces Aharonov-Bohm oscillation, and it controls the tunnelling behavior due to adjusting the flux. The Fano type of multi-resonant behaviors are displayed in the conductance structures by adjusting the gate voltage V_g and the Zeeman field applied to the QD.     

Spin Accumulation in a Double Quantum Dot Aharonov-Bohm Interferometer

We investigate the spin accumulation in a double quantum dot Aharonov-Bohm （AB） interferometer in which both the Rashba spin-orbit （RSO） interaction and intradot Coulomb interaction are taken into account. Due to the existence of the RSO interaction, the electron, flowing through different arms of the AB ring, will acquire a spin-dependent phase factor in the tunnel-coupling strengths. This phase factor will induce various interesting interference phenomena. It is found that the electrons of the different spin directions can accumulate in the two dots by properly adjusting the bias and the intradot level with a fixed RSO interaction strength. Moreover, both the magnitude and direction of the spin accumulation in each dot can be conveniently controlled and tuned by the gate voltage acting on the dot or the bias on the lead.     

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.     

Charge dynamics effects in conductance through a large semi-open quantum dot

Fano lineshapes in resonant transmission in a quantum dot imply interference between localized and extended states. The influence of the charge accumulated at the localized levels, which screens the external gate voltage acting on the conduction channel is investigated. The modified Fano q parameter and the resonant conduction is derived starting from a microscopic Hamiltonian. The latest experiments on ‘charge sensing’ and ‘Coulomb modified Fano sensing’ compare well with the results of the present model.     

Coulomb blockade without potential barriers

We study transport through a strongly correlated quantum dot and show that Coulomb blockade can appear even in the presence of perfect contacts. This conclusion arises from numerical calculations of the conductance for a microscopic model of spinless fermions in an interacting chain connected to each lead via a completely open channel. The dependence of the conductance on the gate voltage shows well defined Coulomb blockade peaks which are sharpened as the interaction strength is increased. Our numerics is based on the embedding method and the DMRG algorithm. We explain the emergence of Coulomb blockade with perfect contacts by a reduction of the effective coupling matrix elements between many-body states corresponding to successive particle numbers in the interacting region. A perturbative approach, valid in the strong interaction limit, yields an analytic expression for the interaction-induced suppression of the conductance in the Coulomb blockade regime.