Conduction through a quantum dot near a singlet-triplet transition

Kondo effect in the vicinity of a singlet-triplet transition in a vertical quantum dot is considered. This system is shown to map onto a special version of the two-impurity Kondo model. At any value of the control parameter, the system has a Fermi-liquid ground state. Explicit expressions for the linear conductance as a function of the control parameter and temperature T are obtained. At T = 0, the conductance reaches the unitary limit approximately 4e(2)/h at the triplet side of the transition, and decreases with the increasing distance to the transition at the singlet side. At finite temperature, the conductance exhibits a peak near the transition point.     

Singlet-triplet transition in a quantum dot: effect on mesoscopic transport

The T=0 transport properties of a wire interacting with a lateral two-level quantum dot are studied by using an exact numerical calculation. The wire conductance, the spin–spin correlation and the Kondo temperature are obtained as a function of the dot level energy spacing. When the dot has two electrons and spin S_D1, the wire current is totally quenched by the S=1 Kondo effect. The Kondo temperature is maximum at the singlet–triplet transition and its dependence upon the dot energy spacing follows a non-universal scaling law.     

Kondo effect from a tunable bound state within a quantum wire

We investigate the conductance of quantum wires with a variable open quantum dot geometry, displaying an exceptionally strong Kondo effect and most of the 0.7 structure characteristics. Our results indicate that the 0.7 structure is not a manifestation of the singlet Kondo effect. However, specific similarities between our devices and many of the clean quantum wires reported in the literature suggest a weakly bound state is often present in real quantum wires.     

Transport through a quantum dot subject to spin and charge bias

Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron-phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.     

Kondo effect for electron transport through an artificial quantum dot

We have calculated the transport properties of electron through an artificial quantum dot by using the numerical renormalization group technique in this paper. We obtain the conductance for the system of a quantum dot which is embedded in a one-dimensional chain in zero and finite temperature cases. The external magnetic field gives rise to a negative magnetoconductance in the zero temperature case. It increases as the external magnetic field increases. We obtain the relation between the coupling coefficient and conductance. If the interaction is big enough to prevent conduction electrons from tunnelling through the dot, the dispersion effect is dominant in this case. In the Kondo temperature regime, we obtain the conductivity of a quantum dot system with Kondo correlation.     

Nonequilibrium Keldysh formalism for interacting leads—Application to quantum dot transport driven by spin bias

The conductance through a mesoscopic system of interacting electrons coupled to two adjacent leads is conventionally derived via the Keldysh nonequilibrium Green’s function technique, in the limit of noninteracting leads [Y. Meir, N.S. Wingreen, Phys. Rev. Lett. 68 (1992) 2512]. We extend the standard formalism to cater for a quantum dot system with Coulombic interactions between the quantum dot and the leads. The general current expression is obtained by considering the equation of motion of the time-ordered Green’s function of the system. The nonequilibrium effects of the interacting leads are then incorporated by determining the contour-ordered Green’s function over the Keldysh loop and applying Langreth’s theorem. The dot–lead interactions significantly increase the height of the Kondo peaks in density of states of the quantum dot. This translates into two Kondo peaks in the spin differential conductance when the magnitude of the spin bias equals that of the Zeeman splitting. There also exists a plateau in the charge differential conductance due to the combined effect of spin bias and the Zeeman splitting. The low-bias conductance plateau with sharp edges is also a characteristic of the Kondo effect. The conductance plateau disappears for the case of asymmetric dot–lead interaction.     

Spin fractionalization of an even number of electrons in a quantum Dot

We find that Kondo resonant conductance can occur in a quantum dot in the Coulomb blockade regime with an even number of electrons N. The contacts are attached to the dot in a pillar configuration, and a magnetic field B( perpendicular) along the axis is applied. B( perpendicular) lifts the spin degeneracy of the dot energies. Usually, this prevents the system from developing the Kondo effect. Tuning B( perpendicular) to the value B(*) where levels with different total spin cross restores both the degeneracy and the Kondo effect. We analyze a dot charged with N = 2 electrons. Coupling to the contacts is antiferromagnetic due to a spin selection rule and, in the Kondo state, the charge is unchanged while the total spin on the dot is S = 1/2.     

Multipeak Kondo effect in one- and two-electron quantum dots

We have fabricated a few-electron quantum dot that can be tuned down to zero electrons while maintaining strong coupling to the leads. Using a nearby quantum point contact as a charge sensor, we can determine the absolute number of electrons in the quantum dot. We find several sharp peaks in the differential conductance, occurring at both zero and finite source-drain bias, for the one- and two-electron quantum dot. We attribute the peaks at finite bias to a Kondo effect through excited states of the quantum dot and investigate the magnetic field dependence of these Kondo resonances.     

Universal scaling in nonequilibrium transport through a single channel Kondo dot

Scaling laws and universality play an important role in our understanding of critical phenomena and the Kondo effect. We present measurements of nonequilibrium transport through a single-channel Kondo quantum dot at low temperature and bias. We find that the low-energy Kondo conductance is consistent with universality between temperature and bias and is characterized by a quadratic scaling exponent, as expected for the spin-1/2 Kondo effect. We show that the nonequilibrium Kondo transport measurements are well described by a universal scaling function with two scaling parameters.     

Finite-size effects in conductance measurements on quantum dots

The observation of the Kondo effect in quantum dots has provided new opportunities to finally observe the controversial Kondo screening cloud. Here we study the conductance of a quantum dot embedded in a finite length quantum wire, predicting a change in behavior when the length of the wire is comparable to the size of the screening cloud.     

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.     

Quantum phase transition and emergent symmetry in a quadruple quantum dot system

We propose a system of four quantum dots designed to study the competition between three types of interactions: Heisenberg, Kondo, and Ising. We find a rich phase diagram containing two sharp features: a quantum phase transition (QPT) between charge-ordered and charge-liquid phases and a dramatic resonance in the charge liquid visible in the conductance. The QPT is of the Kosterlitz-Thouless type with a discontinuous jump in the conductance at the transition. We connect the resonance phenomenon with the degeneracy of three levels in the isolated quadruple dot and argue that this leads to a Kondo-like emergent symmetry from left-right Z2 to U(1).     

Observation of the Kondo effect in a spin-3/2 hole quantum dot

We report the observation of Kondo physics in a spin-3/2 hole quantum dot. The dot is formed close to pinch-off in a hole quantum wire defined in an undoped AlGaAs/GaAs heterostructure. We clearly observe two distinctive hallmarks of quantum dot Kondo physics. First, the Zeeman spin splitting of the zero-bias peak in the differential conductance is independent of the gate voltage. Second, this splitting is twice as large as the splitting for the lowest one-dimensional subband. We show that the Zeeman splitting of the zero-bias peak is highly anisotropic and attribute this to the strong spin-orbit interaction for holes in GaAs.     

Tunable Kondo effect in a double quantum dot coupled to ferromagnetic contacts

We investigate the effects induced by ferromagnetic contacts attached to a serial double quantum dot. Spin polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The superexchange interaction J(AFM), tuned by the interdot tunneling rate t, can be used to compensate the effective fields and restore the Kondo resonance when the contact polarizations are aligned. As a consequence, the direction of the spin conductance can be controlled and even reversed using electrostatic gates alone. Our results demonstrate a new approach for controlling spin-dependent transport in carbon nanotube double dot devices.     

Quantum transport across multilevel quantum dot

The conductance across a quantum dot can be influenced by levels localized in the dot and having little hybridization with the conduction channel. Fano lineshapes arising in resonant transmission measurements, imply interference between the localized and extended states. By applying a magnetic orthogonal field, the total spin of a quantum dot can be tuned. Electron correlations drive the dot through level crossings to higher spin states. Such crossings can give rise to Kondo conductance when the dot is at Coulomb blockade close to a magnetic field induced level degeneracy. In a previous work [P. Stefański, A. Tagliacozzo, B.R. Bulka, Phys. Rev. Lett. 93 (2004) 186805] we have shown that a Fano-like pattern also appears when the continuum of the conduction states originates from a broad Kondo resonance. A bunch of localized core levels, weakly coupled to the Kondo resonance, imprints the broad Kondo peak with Fano lineshapes. A signature of the presence of correlations in the quantum dot is discussed.     

Magnetically induced chessboard pattern in the conductance of a Kondo quantum dot

We quantitatively describe the main features of the magnetically induced conductance modulation of a Kondo quantum dot-or chessboard pattern-in terms of a constant-interaction double quantum dot model. We show that the analogy with a double dot holds down to remarkably low magnetic fields. The analysis is extended by full 3D spin density functional calculations. Introducing an effective Kondo coupling parameter, the chessboard pattern is self-consistently computed as a function of magnetic field and electron number, which enables us to explain our experimental data quantitatively.     

Quantum dots with even number of electrons: kondo effect in a finite magnetic field

We show that the Kondo effect can be induced by an external magnetic field in quantum dots with an even number of electrons. If the Zeeman energy B is close to the single-particle level spacing Delta in the dot, the scattering of the conduction electrons from the dot is dominated by an anisotropic exchange interaction. A Kondo resonance then occurs despite the fact that B exceeds by far the Kondo temperature T(K). As a result, at low temperatures T相似文献   

Electron Transport Through Double-Dot Aharonov-Bohm Interferometer in the Kondo Regime

By applying the slave boson technique, we have studied the electron transport through double-dot Aharonov-Bohm interferometer in the Kondo regime. For the system with symmetric quantum dots, the linear conductance is shown to be enhanced by Kondo effect, but it is suppressed in the deep dot level regime in the presence of nonzero magnetic flux. The Aharonov-Bohm oscillations of the conductance are also investigated.     

Transition between quantum states in a parallel-coupled double quantum dot

Strong electron and spin correlations in a double quantum dot (DQD) can give rise to different quantum states. We observe a continuous transition from a Kondo state exhibiting a single-peak Kondo resonance to another exhibiting a double peak by increasing the interdot coupling (t) in a parallel-coupled DQD. The transition into the double-peak state provides evidence for spin entanglement between the excess electrons on each dot. Toward the transition, the peak splitting merges and becomes substantially smaller than t because of strong Coulomb effects. Our device tunability bodes well for future quantum computation applications.     

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.