Quantum Transport Through a Stretched Spin--1 Molecule

We analyze the electronic transport through a model spin-1 molecule as a function of temperature, magnetic field and bias voltage. We consider the effect of magnetic anisotropy, which can be generated experimentally by stretching the molecule. In the experimentally relevant regime the conductance of the unstretched molecule reaches the unitary limit of the underscreened spin- 1 Kondo effect at low temperatures. The magnetic anisotropy generates an antiferromagnetic coupling between the remaining spin 1/2 and a singular density of quasiparticles, producing a second Kondo effect and a reduced conductance. The results explain recent measurements in spin-1 molecules [Science 328 1370 (2010)].Comment: 5 pages, 3 figures, minor changes, accepted for publication in EP

Universal scaling in transport out of equilibrium through a single quantum dot using the noncrossing approximation

The universal scaling behavior is studied for nonequilibrium transport through a quantum dot. To describe the dot we use the standard Anderson impurity model and use the non-equilibrium non-crossing approximation in the limit of infinite Coulomb repulsion. After solving de hamiltonian, we calculate the conductance through the system as a function of temperature $T$ and bias voltage $V$ in the Kondo and in the mixed valent regime. We obtain a good scaling function in both regimes. In particular, in the mixed valent regime, we find excellent agreement with recent experiments and previous theoretical works.Comment: 6 pages, 5 figures, Accepted Physical Review

Impact of capacitance and tunneling asymmetries on Coulomb blockade edges and Kondo peaks in non-equilibrium transport through molecular quantum dots

We investigate theorerically the non-equilibrium transport through a molecular quantum dot as a function of gate and bias voltage, taking into account the typical situation in molecular electronics. In this respect, our study includes asymmetries both in the capacitances and tunneling rates to the source and drain electrodes, as well as an infinitely large charging energy on the molecule. Our calculations are based on the out-of-equilibrium Non-Crossing-Approximation (NCA), which is a reliable technique in the regime under consideration. We find that Coulomb blockade edges and Kondo peaks display strong renormalization in their width and intensity as a function of these asymmetries, and that basic expectations from Coulomb blockade theory must be taken with care in general, expecially when Kondo physics is at play. In order to help comparison of theory to experiments, we also propose a simple phenomenological model which reproduces semi-quantitatively the Coulomb blockade edges that were numerically computed from the NCA in all regimes of parameters.Comment: 9 pages, 8 figure

Interplay between quantum interference and Kondo effects in nonequilibrium transport through nanoscopic systems

We calculate the finite temperature and non-equilibrium electric current through systems described generically at low energy by a singlet and \emph{two} spin doublets for $N$ and $N \pm 1$ electrons respectively, coupled asymmetrically to two conducting leads, which allows for destructive interference in the conductance. The model is suitable for studying transport in a great variety of systems such us aromatic molecules, different geometries of quantum dots and rings with applied magnetic flux. As a consequence of the interplay between interference and Kondo effect, we find changes by several orders of magnitude in the values of the conductance and its temperature dependence as the doublet level splitting is changed by some external parameter. The differential conductance at finite bias is negative for some parameters.Comment: 4 pages, 5 figure

Selfconsistent hybridization expansions for static properties of the Anderson impurity model

By means of a projector-operator formalism we derive an approximation based on a self consistent hybridization expansion to study the ground state properties of the Anderson Impurity model. We applied the approximation to the general case of finite Coulomb repulsion $U$, extending previous work with the same formalism in the infinite-$U$ case. The treatment provides a very accurate calculation of the ground state energy and their related zero temperature properties in the case in which $U$ is large enough, but still finite, as compared with the rest of energy scales involved in the model. The results for the valence of the impurity are compared with exact results that we obtain from equations derived using the Bethe ansatz and with a perturbative approach. The magnetization and magnetic susceptibility is also compared with Bethe ansatz results. In order to do this comparison, we also show how to regularize the Bethe ansatz integral equations necessary to calculate the impurity valence, for arbitrary values of the parameters.Comment: 8 pages, 5 figure

Nonequilibrium transport through magnetic vibrating molecules

We calculate the nonequilibrium conductance through a molecule or a quantum dot in which the occupation of the relevant electronic level is coupled with intensity $\lambda$ to a phonon mode, and also to two conducting leads. The system is described by the Anderson-Holstein Hamiltonian. We solve the problem using the Keldysh formalism and the non-crossing approximation (NCA) for both, the electron-electron and the electron-phonon interactions. We obtain a moderate decrease of the Kondo temperature $T_K$ with $\lambda$ for fixed renormalized energy of the localized level $\tilde{E_d}$. The meaning and value of $\tilde{E_d}$ are discussed. The spectral density of localized electrons shows in addition to the Kondo peak of width $2 T_K$, satellites of this peak shifted by multiples of the phonon frequency $\omega_0$. The nonequilibrium conductance as a function of bias voltage $V_b$ at small temperatures, also displays peaks at multiples of $\omega_0$ in addition to the central dominant Kondo peak near $V_b=0$.Comment: 11 pages, 13 figures, accepted in Phys. Rev.