Transport through quantum dots in mesoscopic circuits

We study the transport through a quantum dot, in the Kondo Coulomb blockade valley, embedded in a mesoscopic device with finite wires. The quantization of states in the circuit that hosts the quantum dot gives rise to finite size effects. These effects make the conductance sensitive to the ratio of the Kondo screening length to the wires length and provide a way of measuring the Kondo cloud. We present results obtained with the numerical renormalization group for a wide range of physically accessible parameters.Comment: 4 pages, 5 figure

Many Body Effects on the Transport Properties of Single-Molecule Devices

The conductance through a molecular device including electron-electron and electron-phonon interactions is calculated using the Numerical Renormalization Group method. At low temperatures and weak electron-phonon coupling the properties of the conductance can be explained in terms of the standard Kondo model with renormalized parameters. At large electron-phonon coupling a charge analog of the Kondo effect takes place that can be mapped into an anisotropic Kondo model. In this regime the molecule is strongly polarized by a gate voltage which leads to rectification in the current-voltage characteristics of the molecular junction.Comment: 4 pages, 4 figures, minor changes, added reference

Electronic Transport through Magnetic Molecules with Soft Vibrating Modes

The low-temperature transport properties of a molecule are studied in the field-effect transitor geometry. The molecule has an internal mechanical mode that modulates its electronic levels and renormalizes both the interactions and the coupling to the electrodes. For a soft mechanical mode the spin fluctuations in the molecule are dominated by the bare couplings while the valence changes are determined by the dressed energies. In this case, the transport properties present an anomalous behavior and the Kondo temperature has a weak gate voltage dependence. These observations are in agreement with recent experimental data.Comment: 4 pages, 3 figures, accepted in PRB R

Thermopower of an SU(4) Kondo resonance under an SU(2) symmetry-breaking field

We calculate the thermopower of a quantum dot described by two doublets hybridized with two degenerate bands of two conducting leads, conserving orbital (band) and spin quantum numbers, as a function of the temperature $T$ and a splitting $\delta$ of the quantum dot levels which breaks the SU(4) symmetry. The splitting can be regarded as a Zeeman (spin) or valley (orbital) splitting. We use the non-crossing approximation (NCA), the slave bosons in the mean-field approximation (SBMFA) and also the numerical renormalization group (NRG) for large $\delta$. The model describes transport through clean C nanotubes %with weak disorder and in Si fin-type field effect transistors, under an applied magnetic field. The thermopower as a function of temperature $S(T)$ displays two dips that correspond to the energy scales given by the Kondo temperature $T_K$ and $\delta$ and one peak when $k_BT$ reaches the charge-transfer energy. These features are much more pronounced than the corresponding ones in the conductance, indicating that the thermopower is a more sensitive probe of the electronic structure at intermediate or high energies. At low temperatures ($T \ll T_K$) $T_K S(T)/T$ is a constant that increases strongly near the degeneracy point $\delta=0$. We find that the SBMFA fails to provide an accurate description of the thermopower for large $\delta$. Instead, a combination of Fermi liquid relations with the quantum-dot occupations calculated within the NCA gives reliable results for $T \ll T_K$.Comment: 8 pages, 7 figure

Transport through side-coupled double quantum dots: from weak to strong interdot coupling

We report low-temperature transport measurements through a double quantum dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak coupling regime can be understood by considering a single level on each dot, in the intermediate and strong coupling regimes, the multi-level nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong and weak coupling regimes, the double quantum dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate coupling regime the device presents a single dot-like molecular behavior as the molecular wavefunctions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed.Comment: 9 pages, 5 figure

Enhanced Kondo Effect in an Electron System Dynamically Coupled with Local Optical Phonon

We discuss Kondo behavior of a conduction electron system coupled with local optical phonon by analyzing the Anderson-Holstein model with the use of a numerical renormalization group (NRG) method. There appear three typical regions due to the balance between Coulomb interaction $U_{\rm ee}$ and phonon-mediated attraction $U_{\rm ph}$. For $U_{\rm ee}>U_{\rm ph}$, we observe the standard Kondo effect concerning spin degree of freedom. Since the Coulomb interaction is effectively reduced as $U_{\rm ee}-U_{\rm ph}$, the Kondo temperature $T_{\rm K}$ is increased when $U_{\rm ph}$ is increased. On the other hand, for $U_{\rm ee}<U_{\rm ph}$, there occurs the Kondo effect concerning charge degree of freedom, since vacant and double occupied states play roles of pseudo-spins. Note that in this case, $T_{\rm K}$ is decreased with the increase of $U_{\rm ph}$. Namely, $T_{\rm K}$ should be maximized for $U_{\rm ee} \approx U_{\rm ph}$. Then, we analyze in detail the Kondo behavior at $U_{\rm ee}=U_{\rm ph}$, which is found to be explained by the polaron Anderson model with reduced hybridization of polaron and residual repulsive interaction among polarons. By comparing the NRG results of the polaron Anderson model with those of the original Anderson-Holstein model, we clarify the Kondo behavior in the competing region of $U_{\rm ee} \approx U_{\rm ph}$.Comment: 8 pages, 8 figure

Carbapenem-resistant Pseudomonas aeruginosa with acquired bla(vim) metallo-beta-lactamase determinants, Italy.

6nonenoneROSSOLINI G.M.; RICCIO M.L.; CORNAGLIA G.; PAGANI L.; LAGATOLLA C.; SELAN L. AND FONTANA R.Rossolini, G. M.; Riccio, M. L.; Cornaglia, G.; Pagani, L.; Lagatolla, Cristina; Selan, L. AND FONTANA R

Nuclear magnetic resonance probes for the Kondo scenario for the 0.7 feature in semiconductor quantum point contact devices

We propose a probe based on nuclear relaxation and Knight shift measurements for the Kondo scenario for the "0.7 feature" in semiconductor quantum point contact (QPC) devices. We show that the presence of a bound electron in the QPC would lead to a much higher rate of nuclear relaxation compared to nuclear relaxation through exchange of spin with conduction electrons. Furthermore, we show that the temperature dependence of this nuclear relaxation is very non-monotonic as opposed to the linear-T relaxation from coupling with conduction electrons. We present a qualitative analysis for the additional relaxation due to nuclear spin diffusion (NSD) and study the extent to which NSD affects the range of validity of our method. The conclusion is that nuclear relaxation, in combination with Knight shift measurements, can be used to verify whether the 0.7 feature is indeed due to the presence of a bound electron in the QPC.Comment: Published version. Appears in a Special Section on the 0.7 Feature and Interactions in One-Dimensional Systems. 16 page

Universal transport signatures in two-electron molecular quantum dots: gate-tunable Hund's rule, underscreened Kondo effect and quantum phase transitions

We review here some universal aspects of the physics of two-electron molecular transistors in the absence of strong spin-orbit effects. Several recent quantum dots experiments have shown that an electrostatic backgate could be used to control the energy dispersion of magnetic levels. We discuss how the generically asymmetric coupling of the metallic contacts to two different molecular orbitals can indeed lead to a gate-tunable Hund's rule in the presence of singlet and triplet states in the quantum dot. For gate voltages such that the singlet constitutes the (non-magnetic) ground state, one generally observes a suppression of low voltage transport, which can yet be restored in the form of enhanced cotunneling features at finite bias. More interestingly, when the gate voltage is controlled to obtain the triplet configuration, spin S=1 Kondo anomalies appear at zero-bias, with non-Fermi liquid features related to the underscreening of a spin larger than 1/2. Finally, the small bare singlet-triplet splitting in our device allows to fine-tune with the gate between these two magnetic configurations, leading to an unscreening quantum phase transition. This transition occurs between the non-magnetic singlet phase, where a two-stage Kondo effect occurs, and the triplet phase, where the partially compensated (underscreened) moment is akin to a magnetically "ordered" state. These observations are put theoretically into a consistent global picture by using new Numerical Renormalization Group simulations, taylored to capture sharp finie-voltage cotunneling features within the Coulomb diamonds, together with complementary out-of-equilibrium diagrammatic calculations on the two-orbital Anderson model. This work should shed further light on the complicated puzzle still raised by multi-orbital extensions of the classic Kondo problem.Comment: Review article. 16 pages, 17 figures. Minor corrections and extra references added in V