Non-Linear Transport through a Molecular Nanojunction

We present a simple model of electrical transport through a metal-molecule-metal nanojunction that includes charging effects as well as aspects of the electronic structure of the molecule. The interplay of a large charging energy and an asymmetry of the metal-molecule coupling can lead to various effects in non-linear electrical transport. In particular, strong negative differential conductance is observed under certain conditions.Comment: 7 pages, 5 figures, accepted by Europhys. Let

Super-poissonian noise, negative differential conductance, and relaxation effects in transport through molecules, quantum dots and nanotubes

We consider charge transport through a nanoscopic object, e.g. single molecules, short nanotubes, or quantum dots, that is weakly coupled to metallic electrodes. We account for several levels of the molecule/quantum dot with level-dependent coupling strengths, and allow for relaxation of the excited states. The current-voltage characteristics as well as the current noise are calculated within first-order perturbation expansion in the coupling strengths. For the case of asymmetric coupling to the leads we predict negative-differential-conductance accompanied with super-poissonian noise. Both effects are destroyed by fast relaxation processes. The non-monotonic behavior of the shot noise as a function of bias and relaxation rate reflects the details of the electronic structure and level-dependent coupling strengths.Comment: 8 pages, 7 figures, submitted to Phys. Rev. B, added reference

Co-tunneling current and shot noise in quantum dots

We derive general expressions for the current and shot noise, taking into account non-Markovian memory effects. In generalization of previous approaches our theory is valid for arbitrary Coulomb interaction and coupling strength and is applicable to quantum dots and more complex systems like molecules. A diagrammatic expansion up to second-order in the coupling strength, taking into account co-tunneling processes, allows for a study of transport in a regime relevant to many experiments. As an example, we consider a single-level quantum dot, focusing on the Coulomb-blockade regime. We find super-Poissonian shot noise due to spin-flip co-tunneling processes at an energy scale different from the one expected from first-order calculations, with a sensitive dependence on the coupling strength.Comment: 4 pages, three figures, submitted to PR

Co-tunneling assisted sequential tunneling in multi-level quantum dots

We investigate the conductance and zero-frequency shot noise of interacting, multi-level quantum dots coupled to leads. We observe that co-tunneling assisted sequential tunneling (CAST) processes play a dominant role in the transition region from Coulomb blockade to sequential tunneling. We analyze for intermediate coupling strength the dependence of the conductance due to CAST processes on temperature, coupling constant, and gate voltage. Remarkably, the width of the CAST transport feature scales only with temperature, but not with the coupling constant. While the onset of inelastic co-tunneling is associated with a super-Poissonian noise, the noise is even stronger above the threshold for CAST processesComment: 10 pages including 3 color figure

Основи теорії спотворень

У цьому навчальному посібнику викладено основні положення теорії спотворень, що виникають під час зображення поверхні еліпсоїда і поверхні кулі на площині, а також поверхні еліпсоїда на поверхні кулі. Висвітлено загальні відомості про геоцентричні координати і різні загальноземні еліпсоїди та системи координат у світі. Наведено завдання і приклади виконання лабораторних робіт для студентів. Наприкінці кожного розділу подано контрольні питання для самостійної перевірки засвоєного матеріалу, а в кінці навчального посібника – тести і критерії оцінювання знань студентів. Посібник призначено для студентів, аспірантів, спеціалістів і викладачів, які займаються дослідженнями математичної основи карт і картографічних проекцій