Noninteracting Dot Research Articles

Waiting Time Distributions of Transport through a Two-Channel Quantum System

In this work, the waiting time distribution (WTD) statistics of electron transport through a two-channel quantum system in a strong Coulomb blockade regime and non-interacting dots are investigated by employing a particle-number resolved master equation with the Born–Markov approximation. The results show that the phase difference between the two channels, the asymmetry of the dot-state couplings to the left and right electrodes, and Coulomb repulsion have obvious effects on the WTD statistics of the system. In a certain parameter range, the system manifests the coherent oscillatory behavior of WTDs in the strong Coulomb blockade regime, and the phase difference between the two channels is clearly reflected in the oscillation phase of the WTDs. The two-channel quantum dot (QD) system for non-interacting dots manifests nonrenewal characteristics, and the electron waiting time of the system is negatively correlated. The different phase differences between the two channels can clearly enhance the negative correlation. These results deepen our understanding of the WTD statistical properties of electron transport through a mesoscopic QD system and help pave a new path toward constructing nanostructured QD electronic devices.

Enhanced performance of a quantum-dot-based nanomotor due to Coulomb interactions

We study the relation between quantum pumping of charge and the work exchanged with the driving potentials in a strongly interacting ac-driven quantum dot. We work in the large-interaction limit and in the adiabatic pumping regime, and we develop a treatment that combines the time-dependent slave-boson approximation with linear response in the rate of change of the ac-potentials. We find that the time evolution of the system can be described in terms of equilibrium solutions at every time. We analyze the effect of the electronic interactions on the performance of the dot when operating as a quantum motor. The main two effects of the interactions are a shift of the resonance and an enhancement of the efficiency with respect to a non-interacting dot. This is due to the appearance of additional ac-parameters accounting for the interactions that increase the pumping of particles while decreasing the conductance.

Time-dependent resonant tunneling transport: Keldysh and Kadanoff-Baym nonequilibrium Green's functions in an analytically soluble problem

Here we address two nonequilibrium Green's functions approaches for a resonant tunneling structure under a sudden switch of a bias. Our aim is to stress that the time-dependent Keldysh formulation of Jauho, Wingreen and Meir, and the partition-free scheme of Stefanucci and Almbladh are formally equivalent in the ubiquitous case of wide-band limit and noninteracting electrons, if leads and dot are in equilibrium before the time-dependent perturbation. We develop explicit closed formulas of the lesser Green's function and time-dependent current, reminding that the different integration limits preclude a face-to-face comparison of two approaches. This study sheds light on both practices, which are of great interest to the mesoscopic transport community.

Zero-field splitting of the Kondo resonance and quantum criticality in triple quantum dots

We consider a triple-quantum-dot (TQD) system composed by an interacting quantum dot connected to two effectively non-interacting dots, which in turn are both connected in parallel to metallic leads. As we show, this system can be mapped onto a single-impurity Anderson model with a non-trivial density of states. The TQD's transport properties are investigated under a continuous tuning of the non-interacting dots' energy-levels, employing the Numerical Renormalization Group technique. Interference between single and many-particle resonances splits the Kondo peak, fulfilling a generalized Friedel sum rule. In addition, a particular configuration in which one of the non-interacting dots is held out of resonance with the leads allows to access a pseudogap regime where a Kosterlitz-Thouless type quantum-phase-transition (QPT) occur, separating the Kondo and non-Kondo behavior. Within this same configuration, the TQD exhibits traces of the Fano-Kondo effect, which is in turn, strongly affected by the QPT. Signatures of all these phenomena are neatly displayed by the calculated linear conductance.

Microwave absorption properties of permalloy nanodots in the vortex and quasi-uniform magnetization states

When the in-plane bias magnetic field acting on a flat circular magnetic dot is smaller than the saturation field, there are two stable competing magnetization configurations of the dot: the vortex and the quasi-uniform (C-state). We measured microwave absorption properties in an array of non-interacting permalloy dots in the frequency range 1–8 GHz when the in-plane bias magnetic field was varied in the region of the dot magnetization state bi-stability. We found that the microwave absorption properties in the vortex and quasi-uniform stable states are substantially different, so that switching between these states in a fixed bias field can be used for the development of reconfigurable microwave magnetic materials.

Nonequilibrium transport of helical Luttinger liquids through a quantum dot

We study a steady state non-equilibrium transport between two interacting helical edge states of a two dimensional topological insulator, described by helical Luttinger liquids, through a quantum dot. For non-interacting dot the current is obtained analytically by including the self-energy correction to the dot Green's function. For interacting dot we use equation of motion method to study the influence of weak on-site Coulomb interaction on the transport. We find the metal-to-insulator quantum phase transition for attractive or repulsive interactions in the leads when the magnitude of the interaction strength characterized by a charge sector Luttinger parameter $K$ goes beyond a critical value. The critical Luttinger parameter $K_{cr}$ depends on the hoping strength between dot and the leads as well as the energy level of the dot with respect to the Fermi levels of the leads, ranging from weak interaction regime for dot level off resonance to strong interaction regime for dot in resonance with the equilibrium Fermi level. Nearby the transition various singular behaviors of current noise, dot density of state, and the decoherence rate (inverse of lifetime) of the dot are briefly discussed.

Spin-polarized conductance in double quantum dots: Interplay of Kondo, Zeeman, and interference effects

We study the effect of a magnetic field in the Kondo regime of a double-quantum-dot system consisting of a strongly correlated dot (the ``side dot'') coupled to a second, noninteracting dot that also connects two external leads. We show, using the numerical renormalization group, that application of an in-plane magnetic field sets up a subtle interplay between electronic interference, Kondo physics, and Zeeman splitting with nontrivial consequences for spectral and transport properties. The value of the side-dot spectral function at the Fermi level exhibits a nonuniversal field dependence that can be understood using a form of the Friedel sum rule that appropriately accounts for the presence of an energy- and spin-dependent hybridization function. The applied field also accentuates the exchange-mediated interdot coupling, which dominates the ground state at intermediate fields leading to the formation of antiparallel magnetic moments on the dots. By tuning gate voltages and the magnetic field, one can achieve complete spin polarization of the linear conductance between the leads, raising the prospect of applications of the device as a highly tunable spin filter. The system's low-energy properties are qualitatively unchanged by the presence of weak on-site Coulomb repulsion within the second dot.

Spin selective pseudogap Kondo effect in a double quantum dot interferometer with Rashba interaction

A system composed of two quantum dots, i.e. a strongly interacting Kondo dot and a noninteracting one, placed in the arms of the Aharonov–Bohm ring, is investigated theoretically. The ring is coupled to normal leads. This configuration is mapped on the system of a correlated impurity embedded in a host with energy and flux dependent density of states. Additionally, the presence of the Rashba field allows a spin selective opening of the pseudogap in the density of states of the host, when the level of the noninteracting dot is tuned to the Fermi energy. This selectively diminishes electron correlations in the Kondo dot and creates resultant spin polarization at the Fermi level. It is shown that this polarization arises in the absence of any exchange field. Interestingly, this Rashba-correlation-induced spin polarization reaches its maximum for the position of the Kondo dot level corresponding to the Kondo temperature of the Anderson impurity in the host with constant density of states.

Noise spectra of an interacting quantum dot

We study the noise spectra of a many-level quantum dot coupled to two electron reservoirs, when interactions are taken into account only on the dot within the Hartree-Fock approximation. The dependence of the noise spectra on the interaction strength, the coupling to the leads, and the chemical potential is derived. For zero bias and zero temperature, we find that as a function of the (external) frequency, the noise exhibits steps and dips at frequencies reflecting the internal structure of the energy levels on the dot. Modifications due to a finite bias and finite temperatures are investigated for a non-interacting two-level dot. Possible relations to experiments are pointed out.

Tuning the Kondo and Fano effects in double quantum dots

We demonstrate delicate control over the Kondo effect and its interplay with quantum interference in an Aharonov-Bohm interferometer containing one Kondo dot and one noninteracting dot. It is shown that the Kondo resonance undergoes a dramatic evolution as the interdot tunnel coupling progressively increases. A novel triple Kondo splitting occurs from the interference between constant and Lorentzian conduction bands that cooperate in forming the Kondo singlet. The device also manifests a highly controllable Fano-Kondo effect in coherent electronic transport, and can be tuned to a regime where the coupled dots behave as decoupled dots.

Finite-temperature conductance signatures of quantum criticality in double quantum dots

We study the linear conductance through a double-quantum-dot system consisting of an interacting dot in its Kondo regime and an effectively noninteracting dot connected in parallel to metallic leads. Signatures in the zero-bias conductance at temperatures $Tg0$ mark a pair of quantum $(T=0)$ phase transitions between a Kondo-screened many-body ground state and non-Kondo ground states. Notably, the conductance features become more prominent with increasing $T$, which enhances the experimental prospects for accessing the quantum-critical region through tuning of gate voltages in a single device.

Noise of quantum dots in the ac Kondo regime: Slave-boson mean-field method and Floquet theorem

The noise of quantum dots in the ac Kondo regime is investigated by means of slave-boson mean-field method and Floquet theorem. A nonadiabatical formula for the noise power is obtained in the Floquet-Green formalism to take into account interaction effects. We show that, as a manifestation of many-particle correlations, the singularities of photon-assisted noise for noninteracting dots cannot survive in the ac Kondo regime. These peculiar features of the photon-assisted noise thus provide us an alternative probe to distinguish the noninteracting resonant tunneling and the many-particle Kondo resonance.

Transport properties of a Kondo dot with a larger side-coupled noninteracting quantumdot

We investigate theoretically linear and nonlinear quantum transport through a smallerquantum dot in a Kondo regime connected to two leads in the presence of a largerside-coupled noninteracting quantum dot, without tunneling coupling to the leads. To dothis we employ the slave boson mean field theory with the help of the KeldyshGreen’s function at zero temperature. The numerical results show that the Kondoconductance peak may develop multiple resonance peaks and multiple zero points in theconductance spectrum owing to constructive and destructive quantum interferenceeffects when the energy levels of the large side-coupled noninteracting dot arelocated in the vicinity of the Fermi level in the leads. As the coupling strengthbetween two quantum dots increases, the tunneling current through the quantumdevice as a function of gate voltage applied across the two leads is suppressed.The spin-dependent transport properties of two parallel coupled quantum dotsconnected to two ferromagnetic leads are also investigated. The numerical results showthat, for the parallel configuration, the spin current or linear spin differentialconductance are enhanced when the polarization strength in the two leads is increased.

Computational models for interpretation of wave function imaging in cross-sectional STM of quantum dots

Computational models are used to investigate the role of electron–electron interactions in cross-sectional STM of cleaved quantum dots. If correlation effects are weak, the tunnelling current reflects the nodal structure of the non-interacting dot states. If correlation is strong, peaks in the current for tunnelling out of the dot tend to coincide with peaks in the electron density, while the current for tunnelling into the dot tends to be largest between peaks in the electron density.

Vortex state and effect of anisotropy in sub-100-nm magnetic nanodots

Magnetic properties of Fe nanodots are simulated using a scaling technique and Monte Carlo method, in good agreement with experimental results. For the 20-nm-thick dots with diameters larger than 60nm, the magnetization reversal via vortex state is observed. The role of magnetic interaction between dots in arrays in the reversal process is studied as a function of nanometric center-to-center distance. When this distance is more than twice the dot diameter, the interaction can be neglected and the magnetic properties of the entire array are determined by the magnetic configuration of the individual dots. The effect of crystalline anisotropy on the vortex state is investigated. For arrays of noninteracting dots, the anisotropy strongly affects the vortex nucleation field and coercivity, and only slightly affects the vortex annihilation field.

Zero-Field Kondo Splitting and Quantum-Critical Transition in Double Quantum Dots

Double quantum dots offer unique possibilities for the study of many-body correlations. A system containing one Kondo dot and one effectively noninteracting dot maps onto a single-impurity Anderson model with a structured (nonconstant) density of states. Numerical renormalization-group calculations show that, while band filtering through the resonant dot splits the Kondo resonance, the singlet ground state is robust. The system can also be continuously tuned to create a pseudogapped density of states and access a quantum-critical point separating Kondo and non-Kondo phases.

Fano regime of one-dot Aharonov-Bohm interferometers

We use the Landauer-B\"uttiker formalism to study the mesoscopic Fano effect in Aharonov-Bohm rings with an embedded two-dimensional noninteracting dot. The magnetic field dependence of the dot levels leads to a global shift of the Fano lines which becomes important for small ring/dot area ratios. As the magnetic field is varied the Fano dips move periodically from one side of the peak to the other, as reported by Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)]. We show that this effect appears due to a specific magnetic control of the difference between the phase of the single nonresonant path via the free arm of the ring and the global phase of all trajectories involving resonant tunnelings through the dot.

High-order current correlation functions in the Kondo systems

We examine the statistics of current fluctuations in a junction with a quantum dot described by Kondo Hamiltonian. With the help of modified Keldish technique we calculate the third current cumulant. As a function of ratio $v=eV/T_{K}$ the 3rd cumulant was obtained for three different regimes: Fermi liquid regime (v<1), crossover interval ($v\geq1$) and RG limit (v>>1). Unlike the case of noninteracting dot, 3rd cumulant shows strong non-linear voltage dependence. Only in the asymptotical limit of large voltages the linear dependence on $V$ is recovered.

Interplay between the Mesoscopic Stoner and Kondo Effects in Quantum Dots

We consider electrons confined to a quantum dot interacting antiferromagnetically with a spin-1 / 2 Kondo impurity. The electrons also interact among themselves ferromagnetically with a dimensionless coupling J , where J =1 denotes the bulk Stoner transition. We show that as J approaches 1 there is a regime with enhanced Kondo correlations, followed by one where the Kondo effect is destroyed and impurity is spin polarized opposite to the dot electrons. The most striking signature of the first, Stoner-enhanced Kondo regime is that a Zeeman field increases the Kondo scale, in contrast to the case for noninteracting dot electrons. Implications for experiments are discussed.

Long-range radiative interaction between semiconductor quantum dots

We develop a Maxwell-Schr\"odinger formalism in order to describe the radiative interaction mechanism between semiconductor quantum dots. We solve the Maxwell equations for the electromagnetic field coupled to the polarization field of a quantum dot ensemble through a linear nonlocal susceptibility and compute the polariton resonances of the system. The radiative coupling, mediated by both radiative and surface photon modes, causes the emergence of collective modes whose lifetimes are longer or shorter compared to the ones of noninteracting dots. The magnitude of the coupling and the collective-mode energies depend on the detuning and on the mutual quantum dot distance. The spatial range of this coupling mechanism is of the order of the wavelength. This coupling should therefore be accounted for when considering quantum dots as building blocks of integrated systems for quantum information processing.