Two-impurity Anderson Hamiltonian Research Articles

Spin-spin correlations of magnetic adatoms on graphene

We study the interaction between two magnetic adatom impurities in graphene using the Anderson model. The two-impurity Anderson Hamiltonian is solved numerically by using the quantum Monte Carlo technique. We find that the interimpurity spin susceptibility is strongly enhanced at low temperatures, significantly diverging from the well-known Ruderman-Kittel-Kasuya-Yoshida result which decays as ${R}^{\ensuremath{-}3}$.

Dimensionality effects in the local density of states of ferromagnetic hosts probed via STM: Spin-polarized quantum beats and spin filtering

We theoretically investigate the local density of states (LDOS) probed by a STM tip of ferromagnetic metals hosting a single adatom and a subsurface impurity. We model the system via the two-impurity Anderson Hamiltonian. By using the equation of motion with the relevant Green functions, we derive analytical expressions for the LDOS of two host types: a surface and a quantum wire. The LDOS reveals Friedel-like oscillations and Fano interference as a function of the STM tip position. These oscillations strongly depend on the host dimension. Interestingly, we find that the spin-dependent Fermi wave numbers of the hosts give rise to spin-polarized quantum beats in the LDOS. While the LDOS for the metallic surface shows a damped beating pattern, it exhibits an opposite behavior in the quantum wire. Due to this absence of damping, the wire operates as a spatially resolved spin filter with a high efficiency.

Spin exchange interaction with tunable range between graphene quantum dots

We study the spin exchange between two electrons localized in separate quantum dots in graphene. The electronic states in the conduction band are coupled indirectly by tunneling to a common continuum of delocalized states in the valence band. As a model, we use a two-impurity Anderson Hamiltonian which we subsequently transform into an effective spin Hamiltonian by way of a two-stage Schrieffer-Wolff transformation. We then compare our result to that from a Coqblin-Schrieffer approach as well as to fourth order perturbation theory.

Kondo effect in parallel double quantum dots embedded in a mesoscopic ring

We theoretically studied the properties of the ground state of the parallel-coup led double quantum dot embedded in a mesoscopic ring in the Kondo regime by mean s of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means o f the slave-boson mean-field theory. Our results show that when the system goes into the strong coupling regime, two parallel dots can be coupled coherently, wh ich leads to an enhanced Kondo effect and a giant persistent current emerging in this system. This double quantum dot device can be a candidate for future devic e applications.

Magnetic polarization currents in parallel-coupled double-quantum-dot devices

We theoretically study the properties of the magnetic polarization currents (MPC) in parallel-coupled double-quantum-dot devices by means of the two-impurity Anderson Hamiltonian and Green functions. It is shown that at zero temperature, the Kondo effect of the system becomes weaker with the increase of the coupling strength. At low temperature, by varying some conditions such as temperature and bias voltage, the direction and magnitude of MPC can be changed, which implies that the magnetic states of the device can be mode up-, non- or down-polarized.

Controlling Fano and Dicke effects via a magnetic flux in a two-site Anderson model

The electronic transport through a parallel double quantum-dot molecule attached asymmetrically to leads is studied under a magnetic field. We model the system by means of a non interacting two-impurity Anderson Hamiltonian. We find that the conductance shows Fano and Dicke effects that can be controlled by the magnetic flux.

Shot noise in strongly correlated double quantum dots

We have investigated theoretically the shot noise of strongly correlated double quantum dot systems in the regime where the Kondo effect and the interdot antiferromagnetic exchange do compete. Our calculations are based on a generalization to nonequilibrium transport of the two-impurity Anderson Hamiltonian which is solved by the slave-boson mean-field theory. This mean-field approach together with nonequilibrium Green's-function techniques allow us to obtain closed formulas for the shot noise in the strong-coupling regime. Two configurations are considered: series and parallel. When the two quantum dots are in series, we obtain at low voltages, a nonmonotonic behavior of the Fano factor, as a function of the interdot tunneling coupling. When the competition between the Kondo effect and the interdot antiferromagnetic exchange starts to play a role this picture is strongly modified. In a parallel configuration, the functional form of the Fano factor as a function of the applied voltage ${V}_{\text{dc}}$ changes abruptly, depending on whether the ratio between the antiferromagnetic exchange constant $J$ and the Kondo temperature ${T}_{K}$ is below or above ${(J∕{T}_{K})}_{c}\ensuremath{\simeq}2.5$. Below ${(J∕{T}_{K})}_{c}$, the Fano factor behaves as in the single quantum dot case (monotonic dependence which saturates at large voltages). Above ${(J∕{T}_{K})}_{c}$, the Fano factor exhibits a nonmonotonic dependence on voltage, with a kink at $e{V}_{\text{dc}}=J$.

Properties of the ground state of parallel double quantum dots embedded in a mesoscopic ring

We theoretically study the properties of the ground state of the parallel-coupled double quantum dot embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. Our results that when the system goes into the strong coupling regime, two parallel dots can are coupled coherently, which leads to an enhanced Kondo effect and a giant persistent current emerging in this system. This double quantum dot device might be a possible candidate for future device applications.

Giant Persistent Current in a Mesoscopic Ring withParallel-Coupled Double Quantum Dots

We theoretically study the properties of the ground state of theparallel-coupled double quantum dots embedded in a mesoscopic ring inthe Kondo regime by means of the two-impurity Anderson Hamiltonian. TheHamiltonian is solved by means of the slave-boson mean-field theory.Our results show that in this system, the persistent current dependssensitively on both the parity of this system and the size of the ring.Two dots can be coupled coherently, which is reflected in the giantcurrent peak in the strong coupling regime. This system might be acandidate for future device applications.

Coherent Coupling of Double Quantum Dots Embedded in aMesoscopic Ring

We theoretically study the properties of the ground state of aseries-coupled double quantum dot embedded in a mesoscopic ring in theKondo regime by means of the two-impurity Anderson Hamiltonian. TheHamiltonian is solved by means of the slave-boson mean-field theory. Itis shown that two dots can be coupled coherently, which is reflected inthe appearance of parity effects and the complex current-phase relationin this system. This system might be a possible candidate for futuredevice applications.

Kondo effect in coupled quantum dots: A noncrossing approximation study

The out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime are studied theoretically by means of a two-impurity Anderson Hamiltonian with interimpurity hopping. The Hamiltonian, formulated in slave-boson language, is solved by means of a generalization of the noncrossing approximation (NCA) to the present problem. We provide benchmark calculations of the predictions of the NCA for the linear and nonlinear transport properties of coupled quantum dots in the Kondo regime. We give a series of predictions that can be observed experimentally in linear and nonlinear transport measurements through coupled quantum dots. Importantly, it is demonstrated that measurements of the differential conductance $\mathcal{G}=dI/dV,$ for the appropriate values of voltages and interdot tunneling couplings, can give a direct observation of the coherent superposition between the many-body Kondo states of each dot. This coherence can be also detected in the linear transport through the system: the curve linear conductance vs temperature is nonmonotonic, with a maximum at a temperature ${T}^{*}$ characterizing quantum coherence between both the Kondo states.

Kondo effect and bistability in a double quantum dot

We study theoretically the out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime. We model the system by means of a two-impurity Anderson Hamiltonian. The transport properties are characterized by Kondo effect properties, however, superimposed them, the system possesses novel non-linear bistability behavior.

An exactly solvable model for the two-impurity Anderson Hamiltonian

Abstract We consider a two-impurity Anderson Hamiltonian that includes hybridization both between and into the same site and Coulomb and exchange interactions between the sites. An exact diagonalization of this Hamiltonian is performed. The existence of the Kondo effect depends strongly on the ratio between the on-site and inter-site hybridizations. No divergence of the staggered susceptibility was found at the critical J value, for non-zero temperatures.