Novel Kondo anomaly in quantum dots (original) (raw)

Kondo effect in an integer-spin quantum dot

Arxiv preprint cond-mat/ …, 2000

The Kondo effect is a key many-body phenomenon in condensed matter physics. It concerns the interaction between a localised spin and free electrons. Discovered in metals containing small amounts of magnetic impurities, it is now a fundamental mechanism in a wide class of correlated electron systems 1,2. Control over single, localised spins has become relevant also in fabricated structures due to the rapid developments in nano-electronics 3,4. Experiments have already demonstrated artificial realisations of isolated magnetic impurities at metallic surfaces 5,6 , nanometer-scale magnets 7 , controlled transitions between two-electron singlet and triplet states 8 , and a tunable Kondo effect in semiconductor quantum dots 9-12. Here, we report an unexpected Kondo effect realised in a few-electron quantum dot containing singlet and triplet spin states whose energy difference can be tuned with a magnetic field. This effect occurs for an even number of electrons at the degeneracy between singlet and triplet states. The characteristic energy scale is found to be much larger than for the ordinary spin-1/2 case. Quantum dots are small electronic devices 13 , which confine a well-defined number of electrons, N. The total spin is zero or an integer for N = even and half-integer for N = odd. The latter case constitutes the canonical example for the Kondo effect 14,15 when all electrons can be ignored, except for the one with the highest energy; i.e. the case of a single, isolated spin, S = 1/2 (see Fig. 1a). Although the energy level ε o is well below the Fermi energies of the two leads, Heisenberg uncertainty allows the electron on the dot to tunnel to one of the leads when it is replaced quickly by another electron. The time scale for such a co-tunneling process 17 is ~c/U, where h = 2πc is Planck's constant and U is the on-site Coulomb energy. Figure 1a illustrates that particle exchange by co-tunneling can effectively flip the spin on the dot. At low temperature, the coherent superposition of all possible co-tunneling processes involving spin flip can result in a time-averaged spin equal to zero. The whole system, i.e. quantum dot plus electrodes, forms a spin singlet. The

Kondo effect in quantum dots coupled to ferromagnetic leads

2002

We study the Kondo effect in a quantum dot which is coupled to ferromagnetic leads and analyse its properties as a function of the spin polarization of the leads. Based on a scaling approach we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation-of-motion technique we study nonlinear transport through the dot. For parallel alignment the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetric coupling, the peak is split only in the presence of a magnetic field, but shows a characteristic asymmetry in amplitude and position.

Spintronic transport and Kondo effect in quantum dots

2005

We investigate the spin-dependent transport properties of quantum-dot based structures where Kondo correlations dominate the electronic dynamics. The coupling to ferromagnetic leads with parallel magnetizations is known to give rise to nontrivial effects in the local density of states of a single quantum dot. We show that this influence strongly depends on whether charge fluctuations are present or absent in the dot. This result is confirmed with numerical renormalization group calculations and perturbation theory in the on-site interaction. In the Fermi-liquid fixed point, we determine the correlations of the electric current at zero temperature (shot noise) and demonstrate that the Fano factor is suppressed below the Poissonian limit for the symmetric point of the Anderson Hamiltonian even for nonzero lead magnetizations. We discuss possible avenues of future research in this field: coupling to the low energy excitations of the ferromagnets (magnons), extension to double quantum dot systems with interdot antiferromagnetic interaction and effect of spin-polarized currents on higher symmetry Kondo states such as SU(4).

Orbital and spin Kondo effects in a double quantum dot

Europhysics Letters (EPL), 2001

Motivated by recent experiments, in which the Kondo effect has been observed for the first time in a double quantum-dot structure, we study electron transport through a system consisting of two ultrasmall, capacitively-coupled dots with large level spacing and charging energy. Due to strong interdot Coulomb correlations, the Kondo effect has two possible sources, the spin and orbital degeneracies, and it is maximized when both occur simultaneously. The large number of tunable parameters allows a range of manipulations of the Kondo physics-in particular, the Kondo effect in each dot is sensitive to changes in the state of the other dot. For a thorough account of the system dynamics, the linear and nonlinear conductance is calculated in perturbative and non-perturbative approaches. In addition, the temperature dependence of the resonant peak heights is evaluated in the framework of a renormalization group analysis.

Kondo effect and singlet-triplet splitting in coupled quantum dots in a magnetic field

Europhysics Letters (EPL), 2003

We study two tunnel-coupled quantum dots each with a spin 1/2 and attached to leads in the Coulomb blockade regime. We study the interplay between Kondo correlations and the singlettriplet exchange splitting K between the two spins. We calculate the cotunneling current with elastic and inelastic contributions and its renormalization due to Kondo correlations, away and at the degeneracy point K = 0. We show that these Kondo correlations induce pronounced peaks in the conductance as function of magnetic field B, inter-dot coupling t0, and temperature. Moreover, the long-range part of the Coulomb interaction becomes visibile due to Kondo correlations resulting in an additional peak in the conductance vs t0 with a strong B-field dependence. These conductance peaks thus provide direct experimental access to K, and thus to a crucial control parameter for spin-based qubits and entanglement.

Quantum Dots with Even Number of Electrons: Kondo Effect in a Finite Magnetic Field

Physical Review Letters, 2000

We study a small spin-degenerate quantum dot with even number of electrons, weakly connected by point contacts to the metallic electrodes, and subject to an external magnetic field. If the Zeeman energy B is equal to the single-particle level spacing ∆ in the dot, the system exhibits Kondo effect, despite the fact that B exceeds by far the Kondo temperature TK. A possible realization of this in tunneling experiments is discussed.

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

Solid State Communications, 2003

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 of the problem. PACS numbers: PACS number(s): 73.21.La; 73.63.Kv; 85.35.Be

Observation of the Kondo Effect in a Spin-3/2 Hole Quantum Dot

Physical Review Letters, 2011

We have studied the zero-bias anomaly (ZBA) in a ballistic hole quantum wire in the low conductance limit between 10 −3 and 0.5 × 2e 2 /h. Application of a magnetic field along the wire causes a splitting of the ZBA, whereas there is no splitting when the field is oriented in-plane and parallel to the wire. The splitting rate is independent of gate voltage, and is twice as large as the splitting rate for the lowest one-dimensional subband in the quantum wire. These observations are consistent with the Kondo effect, which could occur due to the formation of an open quantum dot system.

Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinear magnetizations

Physica B: Condensed Matter, 2006

Novel Kondo anomaly in quantum dots (original) (raw)

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