Single Wall Nanotubes: Atomic Like Behaviour and Microscopic Approach

Recent experiments about the low temperature behaviour of a Single Wall Carbon Nanotube (SWCNT) showed typical Coulomb Blockade (CB) peaks in the zero bias conductance and allowed us to investigate the energy levels of interacting electrons. Other experiments confirmed the theoretical prediction about the crucial role which the long range nature of the Coulomb interaction plays in the correlated electronic transport through a SWCNT with two intramolecular tunneling barriers. In order to investigate the effects on low dimensional electron systems due to the range of electron electron repulsion, we introduce a model for the interaction which interpolates well between short and long range regimes. Our results could be compared with experimental data obtained in SWCNTs and with those obtained for an ideal vertical Quantum Dot (QD). For a better understanding of some experimental results we also discuss how defects and doping can break some symmetries of the bandstructure of a SWCNT.Comment: 8 pages, 4 figure

Measuring Temperature Gradients over Nanometer Length Scales

When a quantum dot is subjected to a thermal gradient, the temperature of electrons entering the dot can be determined from the dot's thermocurrent if the conductance spectrum and background temperature are known. We demonstrate this technique by measuring the temperature difference across a 15 nm quantum dot embedded in a nanowire. This technique can be used when the dot's energy states are separated by many kT and will enable future quantitative investigations of electron-phonon interaction, nonlinear thermoelectric effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure

Signatures of Chaos in the Statistical Distribution of Conductance Peaks in Quantum Dots

Analytical expressions for the width and conductance peak distributions of irregularly shaped quantum dots in the Coulomb blockade regime are presented in the limits of conserved and broken time-reversal symmetry. The results are obtained using random matrix theory and are valid in general for any number of non-equivalent and correlated channels, assuming that the underlying classical dynamic of the electrons in the dot is chaotic or that the dot is weakly disordered. The results are expressed in terms of the channel correlation matrix which for chaotic systems is given in closed form for both point-like contacts and extended leads. We study the dependence of the distributions on the number of channels and their correlations. The theoretical distributions are in good agreement with those computed in a dynamical model of a chaotic billiard.Comment: 19 pages, RevTex, 11 Postscript figure

The Anderson Model out of equilibrium: Time dependent perturbations

The influence of high-frequency fields on quantum transport through a quantum dot is studied in the low-temperature regime. We generalize the non crossing approximation for the infinite-U Anderson model to the time-dependent case. The dc spectral density shows asymmetric Kondo side peaks due to photon-assisted resonant tunneling. As a consequence we predict an electron-photon pump at zero bias which is purely based on the Kondo effect. In contrast to the resonant level model and the time-independent case we observe asymmetric peak amplitudes in the Coulomb oscillations and the differential conductance versus bias voltage shows resonant side peaks with a width much smaller than the tunneling rate. All the effects might be used to clarify the question whether quantum dots indeed show the Kondo effect.Comment: 13 pages, REVTEX 3.0, 5 figure

Electronic Structure of Dinuclear Gold(I) Complexes

Cyclic voltammetry (CV) experiments on LL(AuSR∗)2 complexes [LL = diphenylphosphinomethane (dppm), diphenylphosphinopentane (dpppn); R* = p-SC6H4CH3] show anodic sweeps that broaden by about 25 mV on going from the longer (dpppn) to the shorter (dppm) bidentate phosphine ligand. Changing concentrations had no effect on the shape of the waveform. The result suggests a weak intramolecular metal-metal interaction in dppm(AuSR∗)2 that correlates well with rate acceleration occurring in the reaction of dppm(AuSR∗)2 with organic disulfides. Quantum yields for cis-dppee(AuX)2 [dppee = 1,2-bis(diphenylphosphino)ethylene; X = Cl, Br, I] complexes, (disappearance) Φ , are significantly higher in complexes with a softer X ligand, a trend that correlates well with aurophilicity. This result also suggests that electronic perturbation caused by Au(I)-Au(I) interactions is important in explaining the reactivity of some dinuclear gold(I) complexes. The crystal structure for cis-dppee(Aul)2 shows short intramolecular Au(I)-Au(I) interactions of 2.9526 (6) A°, while the structure of trans-dppee(AuI)2 , shows intermolecular Au(I)-Au(I) interactions of 3.2292 (9) A°. The substitution of .As for P results in a ligand, cis-diphenylarsinoethylene (cis-dpaee), that is photochemically active, in contrast to the cis-dppee ligand. The complexes, cis-dpaee(AuX)2, are also photochemically active but with lower quantum yields than the cis-dppee(AuX)2 complexes

From the Kondo Regime to the Mixed-Valence Regime in a Single-Electron Transistor

We demonstrate that the conductance through a single-electron transistor at low temperature is in quantitative agreement with predictions of the equilibrium Anderson model. When an unpaired electron is localized within the transistor, the Kondo effect is observed. Tuning the unpaired electron's energy toward the Fermi level in nearby leads produces a cross-over between the Kondo and mixed-valence regimes of the Anderson model.Comment: 3 pages plus one 2 page postscript file of 5 figures. Submitted to PR

Modeling Cell Gradient Sensing and Migration in Competing Chemoattractant Fields

Directed cell migration mediates physiological and pathological processes. In particular, immune cell trafficking in tissues is crucial for inducing immune responses and is coordinated by multiple environmental cues such as chemoattractant gradients. Although the chemotaxis mechanism has been extensively studied, how cells integrate multiple chemotactic signals for effective trafficking and positioning in tissues is not clearly defined. Results from previous neutrophil chemotaxis experiments and modeling studies suggested that ligand-induced homologous receptor desensitization may provide an important mechanism for cell migration in competing chemoattractant gradients. However, the previous mathematical model is oversimplified to cell gradient sensing in one-dimensional (1-D) environment. To better understand the receptor desensitization mechanism for chemotactic navigation, we further developed the model to test the role of homologous receptor desensitization in regulating both cell gradient sensing and migration in different configurations of chemoattractant fields in two-dimension (2-D). Our results show that cells expressing normal desensitizable receptors preferentially orient and migrate toward the distant gradient in the presence of a second local competing gradient, which are consistent with the experimentally observed preferential migration of cells toward the distant attractant source and confirm the requirement of receptor desensitization for such migratory behaviors. Furthermore, our results are in qualitative agreement with the experimentally observed cell migration patterns in different configurations of competing chemoattractant fields

Observation of Quantum Fluctuations of Charge on a Quantum Dot

We have incorporated an aluminum single electron transistor directly into the defining gate structure of a semiconductor quantum dot, permitting precise measurement of the charge in the dot. Voltage biasing a gate draws charge from a reservoir into the dot through a single point contact. The charge in the dot increases continuously for large point contact conductance and in a step-like manner in units of single electrons with the contact nearly closed. We measure the corresponding capacitance lineshapes for the full range of point contact conductances. The lineshapes are described well by perturbation theory and not by theories in which the dot charging energy is altered by the barrier conductance.Comment: Revtex, 5 pages, 3 figures, few minor corrections to the reference