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Register a new userSuperconducting proximity effect and zero-bias anomaly in transport through quantum dots weakly attached to ferromagnetic leads
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The Andreev transport through a quantum dot coupled to two external ferromagnetic leads and one superconducting lead is studied theoretically by means of the real-time diagrammatic technique in the sequential and cotunneling regimes. We show that the tunnel magnetoresistance (TMR) of the Andreev current displays a nontrivial dependence on the bias voltage and the level detuning, and can be described by analytical formulas in the zero temperature limit. The cotunneling processes lead to a strong modification of the TMR, which is most visible in the Coulomb blockade regime. We find a zero-bias anomaly of the Andreev differential conductance in the parallel configuration, which is associated with a nonequilibrium spin accumulation in the dot triggered by Andreev processes.
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The zero-bias anomaly at low temperatures, originated by the Kondo effect when an electric current flows through a system formed by a spin-$1/2$ quantum dot and two metallic contacts is theoretically investigated. In particular, we compare the width of this anomaly $2T_{rm NE}$ with that of the Kondo resonance in the spectral density of states $2T_{K}^{rho}$, obtained from a Fano fit of the corresponding curves and also with the Kondo temperature, $T_K^G$, defined from the temperature evolution of the equilibrium conductance $G(T)$. In contrast to $T_K^G$ and $2T_{K}^{rho}$, we found that the scale $2T_{rm NE}$ strongly depends on the asymmetry between the couplings of the quantum dot to the leads while the total hybridization is kept constant. While the three scales are of the same order of magnitude, $2T_{rm NE}$ and $T_{K}^{rho}$ agree only in the case of large asymmetry between the different tunneling couplings of the contacts and the quantum dot. On the other hand, for similar couplings, $T_{rm NE}$ becomes larger than $T_{K}^{rho}$, reaching the maximum deviation, of the order of $30%$, for identical couplings. The fact that an additional parameter to $T_{rm NE}$ is needed to characterize the Kondo effect, weakenig the universality properties, points that some caution should be taken in the usual identification in experiments of the low temperature width of the zero-bias anomaly with the Kondo scale. Furthermore, our results indicate that the ratios $T_{rm NE}/T_K^G$ and $T_{K}^{rho}/T_K^G$ depend on the range used for the fitting.
Electronic transport properties of two quantum dots side-coupled to a quantum wire are studied by means of the two impurity Anderson Hamiltonian. The conductance is found to be a superposition of a Fano and a Breit-Wigner resonances as a function of the Fermi energy, when the gate voltages of the quantum dots are slightly different. Under this condition, we analyze the time evolution of a Gaussian-shaped superposition of plane waves incoming from the source lead, and found that the wave packet can be splitted into three packets at the drain lead. This spatial pattern manifests in a direct way the peculiarities of the conductance in energy space. We conclude that the device acts as a quantum electron splitter.
We investigate quantum dots in clean single-wall carbon nanotubes with ferromagnetic PdNi-leads in the Kondo regime. In most odd Coulomb valleys the Kondo resonance exhibits a pronounced splitting, which depends on the tunnel coupling to the leads and an external magnetic field $B$, and only weakly on gate voltage. Using numerical renormalization group calculations, we demonstrate that all salient features of the data can be understood using a simple model for the magnetic properties of the leads. The magnetoconductance at zero bias and low temperature depends in a universal way on $g mu_B (B-B_c) / k_B T_K$, where $T_K$ is the Kondo temperature and $B_c$ the external field compensating the splitting.
We investigate the spin-resolved transport properties, such as the linear conductance and the tunnel magnetoresistance, of a double quantum dot device attached to ferromagnetic leads and look for signatures of SU(4) symmetry in the Kondo regime. We show that the transport behavior greatly depends on the magnetic configuration of the device, and the spin-SU(2) as well as the orbital and spin-SU(4) Kondo effects become generally suppressed when the magnetic configuration of the leads varies from the antiparallel to the parallel one. Furthermore, a finite spin polarization of the leads lifts the spin degeneracy and drives the system from the SU(4) to an orbital-SU(2) Kondo state. We analyze in detail the crossover and show that the Kondo temperature between the two fixed points has a non-monotonic dependence on the degree of spin polarization of the leads. In terms of methods used, we characterize transport by using a combination of analytical and numerical renormalization group approaches.
We study the spin-resolved transport through single-level quantum dots strongly coupled to ferromagnetic leads in the Kondo regime, with a focus on contact and material asymmetry-related effects. By using the numerical renormalization group method, we analyze the dependence of relevant spectral functions, linear conductance and tunnel magnetoresistance on the system asymmetry parameters. In the parallel magnetic configuration of the device the Kondo effect is generally suppressed due to the presence of exchange field, irrespective of systems asymmetry. In the antiparallel configuration, on the other hand, the Kondo effect can develop if the system is symmetric. We show that even relatively weak asymmetry may lead to the suppression of the Kondo resonance in the antiparallel configuration and thus give rise to nontrivial behavior of the tunnel magnetoresistance. In addition, by using the second-order perturbation theory we derive general formulas for the exchange field in both magnetic configurations of the system.
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