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Register a new userUnhiding a concealed resonance by multiple Kondo transitions in a quantum dot
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Kondo correlations are responsible for the emergence of a zero-bias peak in the low temperature differential conductance of Coulomb blockaded quantum dots. In the presence of a global SU(2)$otimes$SU(2) symmetry, which can be realized in carbon nanotubes, they also inhibit inelastic transitions which preserve the Kramers pseudospins associated to the symmetry. We report on magnetotransport experiments on a Kondo correlated carbon nanotube where resonant features at the bias corresponding to the pseudospin-preserving transitions are observed. We attribute this effect to a simultaneous enhancement of pseudospin-non-preserving transitions occurring at that bias. This process is boosted by asymmetric tunneling couplings of the two Kramers doublets to the leads and by asymmetries in the potential drops at the leads. Hence, the present work discloses a fundamental microscopic mechanisms ruling transport in Kondo systems far from equilibrium.
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Phonon-assisted electronic tunnelings through a vibrating quantum dot embedded between normal and superconducting leads are studied in the Kondo regime. In such a hybrid device, with the bias applied to the normal lead, we find a series of Kondo sidebands separated by half a phonon energy in the differential conductance, which are distinct from the phonon-assisted sidebands previously observed in the conventional Andreev tunnelings and in systems with only normal leads. These Kondo sidebands originate from the Kondo-Andreev cooperative cotunneling mediated by phonons, which exhibit a novel Kondo transport behavior due to the interplay of the Kondo effect, the Andreev tunnelings, and the mechanical vibrations. Our result could be observed in a recent experiment setup [J. Gramich emph{et al.}, PRL textbf{115}, 216801 (2015)], provided that their carbon nanotube device reaches the Kondo regime at low temperatures.
Over-screened Kondo effect is feasible in carbon nanotube quantum dot junction hosting a spin $tfrac{1}{2}$ atom with single $s$-wave valence electron (e.g Au). The idea is to use the two valleys as two symmetry protected flavor quantum numbers $xi={bf K}, {bf K}$. Perturbative RG analysis exposes the finite weak-coupling two-channel fixed point, where the Kondo temperature is estimated to be around $0.5div5$~K. Remarkably, occurrence of two different scaling regimes implies a non-monotonic dependence of the conductance as function of temperature.
Spin exchange between a single-electron charged quantum dot and itinerant electrons leads to an emergence of Kondo correlations. When the quantum dot is driven resonantly by weak laser light, the resulting emission spectrum allows for a direct probe of these correlations. In the opposite limit of vanishing exchange interaction and strong laser drive, the quantum dot exhibits coherent oscillations between the single-spin and optically excited states. Here, we show that the interplay between strong exchange and non-perturbative laser coupling leads to the formation of a new nonequilibrium quantum-correlated state, characterized by the emergence of a laser-induced secondary spin screening cloud, and examine the implications for the emission spectrum.
We calculate the conductance through a single quantum dot coupled to metallic leads, modeled by the spin 1/2 Anderson model. We adopt the finite-U extension of the noncrossing approximation method. Our results are in good agreement with exact numerical renormalization group results both in the high temperature and in the Kondo (low temperature) regime. Thanks to this approach, we were able to fit fairly well recently reported measurements by S. De Franceschi et al. in a quantum dot device. We show that, contrarily to what previously suggested, the conductance of this particular device can be understood within the spin-1/2 Anderson model, in which the effects of the multilevel structure of the dot are neglected.
We study the Kondo effect in a CNT(left lead)-CNT(QD)-CNT(right lead) structure. Here CNT is a single-wall metallic carbon nanotube, for which 1) the valence and conduction bands of electrons with zero orbital angular momentum ($m=0$) coalesc at the two valley points ${bf{K}}$ and ${bf{K}}$ of the first Brillouin zone and 2) the energy spectrum of electrons with $m e 0$ has a gap whose size is proportional to $|m|$. Following adsorption of hydrogen atoms and application of an appropriately designed gate potential, electron energy levels in the CNT(QD) are tunable to have: 1) two-fold spin degeneracy; 2) two-fold isospin (valley) degeneracy; 3) three-fold orbital degeneracy $m=0,pm1$. As a result, an SU(12) Kondo effect is realized with remarkably high Kondo temperature. Unlike the SU(2) case, the low temperature conductance and magnetic susceptibility have a peak at finite temperature. Moreover, the magnetic susceptibilities for parallel and perpendicular magnetic fields (WRT the tube axis) display anisotropy with a universal ratio $chi_{rm{imp}}^parallel / chi_{rm{imp}}^perp=eta$ that depends only on the electrons orbital and spin $g$ factors.
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