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.     

Signature of Kondo effect in silicon quantum dots

We report observation of the Kondo effect in the Coulomb blockade oscillations of an impurity quantum dot (IQD). This IQD is formed in the channel of a 100 nm gate length Silicon MOSFET. The quantitative analysis of the anomalous temperature and voltage dependence for the drain-source current over a series of Coulomb blockade oscillations is performed. It strongly supports the Kondo explanation for the conductance behavior at very low temperature in this standard microelectronics device. Received 13 November 2001 and Received in final form 18 February 2002     

Measurement-induced decoherence in electronic interferometry at nanoscale

We introduce a theoretical formalism describing a wide class of ‘Which Path’ experiments in mesoscopic/nanoscopic transport. The physical system involves a mesoscopic interferometer (e.g. an Aharonov-Bohm ring with embedded dots or a side-coupled quantum dot) which is electrostatically coupled to a nearby quantum point constriction. Due to the charge sensing effect the latter acts as a charge detector. Therefore the interference pattern can be monitored indirectly by looking at the current characteristics of the detector as shown in the experimental work of Buks et al. [E. Buks, R. Schuster, M. Heiblum, D. Mahalu, V. Umansky, Nature (London) 391 (1998) 871]. We use the non-equilibrium Green-Keldysh formalism and a second order perturbative treatment of the Coulomb interaction in order to compute the relevant transport properties. It is shown that in the presence of the Coulomb interaction the current through the detector exhibits oscillations as a function of the magnetic field applied on a single-dot AB interferometer. We also discuss the dependence of the visibility of the Aharonov-Bohm oscillations on the gate potential applied to the dot.     

Mesoscopic Fano effect modulated by Rashba spin–orbit coupling and external magnetic field

By employing non-equilibrium Green's function method, the mesoscopic Fano effect modulated by Rashba spin–orbit (SO) coupling and external magnetic field has been elucidated for electron transport through a hybrid system composed of a quantum dot (QD) and an Aharonov–Bohm (AB) ring. The results show that the orientation of the Fano line shape is modulated by the Rashba spin–orbit interaction kRL$k_{R}L$ variation, which reveals that the Fano parameter q will be extended to a complex number, although the system maintains time-reversal symmetry (TRS) under the Rashba SO interaction. Furthermore, it is shown that the modulation of the external magnetic field, which is applied not only inside the frame, but also on the QD, leads to the Fano resonance split due to Zeeman effect, which indicates that the hybrid is an ideal candidate for the spin readout device.     

Photon-assisted shot noise due to the charging effect in a quantum dot device

We have investigated the spectral density of shot noise for an ultra-small quantum dot(QD) system in the Coulomb blockade regime when irradiated with microwave fields (MWFs) by employing a nonequilibrium Green’s function technique. The shot noise is sensitive to Coulomb interaction, and the photon-assisted Coulomb blockade behaviour strongly modifies the mesoscopic transport. We have calculated the first and second derivatives of shot noise in the strong and weak coupling regimes to compare the theoretical results with existing experimental results. In the strong coupling regime, the first and second derivatives of shot noise display Fano type peak-valley structures around the charging channel 2E _c due to Coulomb interaction. When the magnitudes of the MWFs are sufficiently large, the system displays channel blockade due to photon irradiation. The photon-assisted and Coulomb blockade steps in the noise — as well as the resonant behaviour in the differential noise — are smeared by increasing temperature. The Coulomb interaction suppresses the shot noise, but the ac fields can either suppress the shot noise(balanced case) or enhance the shot noise(unbalanced case). The suppression of shot noise caused by ac fields in the balanced case is greater than that caused by Coulomb interaction in our system. Super-Poissonian shot noise may be induced due to the compound effects of strong Coulomb interaction and photon absorption-emission processes.     

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.     

Background charges and quantum effects in quantum dots transport spectroscopy

We extend a simple model of a charge trap coupled to a single-electron box to energy ranges and parameters such that it gives new insights and predictions readily observable in many experimental systems. We show that a single background charge is enough to give lines of differential conductance in the stability diagram of the quantum dot, even within undistorted Coulomb diamonds. It also suppresses the current near degeneracy of the impurity charge, and yields negative differential lines far from this degeneracy. We compare this picture to two other accepted explanations for lines in diamonds, based respectively on the excitation spectrum of a quantum dot and on fluctuations of the density-of-states in the contacts. In order to discriminate between these models, we emphasize the specific features related to environmental charge traps. Finally we show that our model accounts very well for all the anomalous features observed in silicon nanowire quantum dots.     

Bound states in the continuum in two-dimensional serial structures

We investigate the occurrence of bound states in the continuum (BICs) in serial structures of quantum dots coupled to an external waveguide, when some characteristic length of the system is changed. By resorting to a multichannel scattering-matrix approach, we show that BICs do actually occur in two-dimensional serial structures, and that they are a robust effect. When a BIC is produced in a two-dot system, BICs also occur for several coupled dots. We also show that the complex dependence of the conductance upon the geometry of the multi-dot system allows for a simple picture in terms of the resonance pole motion in the multi-sheeted Riemann energy surface. Finally, we show that in correspondence to zero-width states for the open system one has a multiplet of degenerate eigenenergies for the associated closed serial system, thereby generalizing results previously obtained for single dots and two-dot structures.     

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.     

Multi-photon behaviors of shot noise in a quantum dot system under the perturbation of two microwave fields

We have investigated the shot noise affected by the perturbation of two microwave fields (MWFs) with frequencies ω₁ and ω₂, which can be classified as the commensurate and incommensurate external ac fields. The time-dependent current correlation function and the spectral density of shot noise have been obtained. They are very different compared with the single-field applied system in the nonlinear regime of the ac potentials. The different photon absorption and emission processes induce different kinds of noise spectral density. We have performed the numerical calculations for both commensurate balanced and unbalanced photon absorptions and emissions. The multi-photon procedure can be seen clearly from the resonance of shot noise. Different commensurate number q = ω₂/ω₁ contributes to different photon absorption and emission behaviors. It is found that the asymmetric configuration of shot noise is intimately associated with the commensurate number q. The differential conductance appears symmetric and asymmetric behaviors, and the channel blockade exhibits. The shot noise is large enough to surpass its saturated value for the unbalanced photon absorption case. The sensitive behaviors of Fano factor associated with different commensurate numbers and amplitudes of ac fields signify that the shot noise can be controlled by external MWFs significantly.     

Kondo effect in a double quantum-dot molecule under the effect of an electric and magnetic field

Electron tunneling through a double quantum-dot molecule, in the Kondo regime, under the effect of a magnetic field and an applied voltage, is studied. This system possesses a complex response to the applied fields characterized by a tristable solution for the conductance. The different nature of the solutions are studied in and out thermodynamical equilibrium. It is shown that the interdot coupling and the fields can be used to control the region of multistability. The mean-field slave-boson formalism is used to obtain the solution for the problem.     

Supercurrent and its Fano effect in a Josephson Aharonov-Bohm ring

The Josephson current through an Aharonov-Bohm (AB) interferometer, in which a quantum dot (QD) is situated on one arm and a magnetic flux Φ threads through the ring, has been investigated. In the presence of the magnetic flux, the relation between the Josephson current and the superconductor phase is complex, and the system can be adjusted to π junction by either modulating the magnetic flux or the QD’s energy level ε_d. Due to the electron-hole symmetry, the Josephson current I has the property I(ε_d,Φ)=I(-ε_d,Φ+π). The Josephson current exhibits a jump when a pair of Andreev bound states aligns with the Fermi energy. The condition for the current jump is given. Particularly, we find that the position of the current jump and the position of the maximum value of the critical current I_c are identical. Due to the interference between the two paths, the critical current I_c versus the QD’s level ε_d shows a typical Fano shape, which is similar to the Fano effect in the corresponding normal device. However they also show some differences. For example, the critical current never reaches zero for any parameters, while the current in the normal device can reach zero at the destruction point.     

Analytical result of the effect of spin scattering and impurity-assistance on magnetoresistance in doped magnetic tunneling junctions

An extended tunneling Hamiltonian method is proposed to study the temperature-dependent tunneling magnetoresistance (TMR) in doped magnetic tunnel junctions. It is found that for nonmagnetic dopants (Si), impurity-assisted tunneling is mainly elastic, giving rise to a weak spin polarization, thereby reduces the overall TMR, while for magnetic ions (Ni), the collective excitation of local spins in δ-doped magnetic layer contributes to the severe drop of TMR and the behavior of the variation of TMR with temperature different from that for Si-doping. The theoretical results can reproduce the main characteristic features of experiments. Received 13 January 2002 / Received in final form 30 November 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: yctao12@163.com     

Coexistence of anomalous field effect and mesoscopic conductance fluctuations in granular aluminium

We perform electrical field effect measurements at 4 K on insulating granular aluminium thin films. When the samples size is reduced below ≃100 μm, reproducible and stable conductance fluctuations are seen as a function of the gate voltage. Our results suggest that these fluctuations reflect the incomplete self-averaging of largely distributed microscopic resistances. We also study the anomalous field effect (conductance dip) already known to exist in large samples and its slow relaxation in the presence of the conductance fluctuations. Within our measurements accuracy, the two phenomena appear to be independent of each other, like two additive contributions to the conductance. We discuss the possible physical meaning of this independence and in particular whether or not this observation is in favor of an electron glass interpretation of slow conductance anomaly relaxations.     

Correlations of conductance peaks and transmission phases in deformed quantum dots

We investigate the Coulomb blockade resonances and the phase of the transmission amplitude of a deformed ballistic quantum dot weakly coupled to leads. We show that preferred single-particle levels exist which stay close to the Fermi energy for a wide range of values of the gate voltage. These states give rise to sequences of Coulomb blockade resonances with correlated peak heights and transmission phases. The correlation of the peak heights becomes stronger with increasing temperature. The phase of the transmission amplitude shows lapses by between the resonances. Implications for recent experiments on ballistic quantum dots are discussed. Received 17 July 1998     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

Phonon effects in tunnelling through a double quantum dot molecule

Phonon effects in tunnelling through a double quantum dot molecule are investigated by use of a recently developed technique, which is based on an exact mapping of a many-body electron-phonon interaction problem onto a multichannel one-body problem. The molecule is sandwiched between two ideal electrodes and the electron at each dot of the molecule interacts independently with Einstein phonons. Single-electron transmission rates through the molecule are computed and the nonlinear spectrum obtained shows a structure with many more satellite peaks due to the excitations of phonons. The strength of resonant peaks is found to be strongly dependent on the number of excited phonons. The effects of electron-phonon interaction on the current and shot noise, depending on the voltage bias applied at the two electrodes as well as the potential energy of the molecule, are discussed.     

Kondo effect in side coupled double quantum-dot molecule

Electron tunneling through a double quantum-dot molecule side attached to a quantum wire, in the Kondo regime, is studied. The mean-field finite-U slave-boson formalism is used to obtain the solution of the problem. We found conductance cancellations when the molecular energies of the side attached double quantum-dot cross the Fermi energy. We investigate the many body molecular Kondo states and its interplay with the inter-dot antiferromagnetic correlation as a function of the parameters of the system.     

Analytic study of electron transmission through serial mesoscopic metallic rings

We investigate the electron transmission through a structure of serial mesoscopic metallic rings coupled to two external leads. A set of analytical expressions based on the quantum waveguide transport and the transfer matrix method are derived and used to discuss the effects of geometric configurations on transmission probabilities. It is found that in the contact ring case the existence of an applied magnetic flux is necessary to create transmission gaps, while in the non-contact ring case transmission gaps always appear irrespective of whether there is an applied magnetic flux or not. The transmissions for periodic rings with a defect ring and periodic rings built by two sorts of rings are also briefly studied. It is also found that the transmission periodicity with wave vector must be ensured by the commensurability of two characteristic lengths, i.e., of the half perimeter of a ring and the connecting wire between two adjacent rings. The special points of wave vector and magnetic flux which give rise to the transmission resonance and antiresonance are analyzed in detail.     

Mechanism of fluorescence energy transfer in aqueous solution cadmium sulfide quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) prepared by a convenient chemical method have been characterized using absorption, fluorescence, and photoluminescence excitation techniques. The photoluminescence excitation studies show that there is an electron transfer from the surface adsorbate (thiourea) to CdS QDs in aqueous solution. The excitation band with peak maximum at 5.8 eV is assigned to the electronic transitions in the chemisorbed thiourea, whereas the excitation band between 3.45 and 3.7 eV corresponds to the band-to-band transition within the nanocrystalline CdS host. The absorption spectroscopy of the CdS QD solutions shows a strong absorption peak which is generated from thiourea. The band-edge fluorescence of the CdS QDs has also been investigated. It is shown that the fluorescence property of the CdS QDs can be enhanced by adding cadmium chloride (CdCl₂) solution.