We present an atomistic theory of electronic transport through single organic molecules that reproduces the important features of the current-voltage ( I-V) characteristics observed in recent experiments. We trace these features to their origin in the electronic structure of the molecules and their local atomic environment. We demonstrate how conduction channels arise from the molecular orbitals and elucidate the contributions of individual orbitals to the current. We find that in thiol-bridged aromatic molecules many molecular orbitals contribute to a single conduction channel and discuss the implications of this result for the design of molecular devices.

Many experiments have shown that the conductance histograms of metallic atomic-sized contacts exhibit a peak structure, which is characteristic of the corresponding material. The origin of these peaks still remains as an open problem. In order to shed some light on this issue, we present a theoretical analysis of the conductance histograms of Au atomic contacts. We have combined classical molecular dynamics simulations of the breaking of nanocontacts with conductance calculations based on a tight-binding model. This combination gives us access to crucial information such as contact geometries, forces, minimum cross-section, total conductance and transmission coefficients of the individual conduction channels. The ensemble of our results suggests that the low temperature Au conductance histograms are a consequence of a subtle interplay between mechanical and electrical properties of these nanocontacts. At variance with other suggestions in the literature, our results indicate that the peaks in the Au conductance histograms are not a simple consequence of conductance quantization or the existence of exceptionally stable radii. We show that the main peak in the histogram close to one quantum of conductance is due to the formation of single-atom contacts and chains of gold atoms. Moreover, we present a detailed comparison with experimental results on Au atomic contacts where the individual channel transmissions have been determined.

We present a theoretical analysis of the electronic transport through atomic and molecular junctions. The main goal of this work is to show how the electronic structure of single atoms and molecules controls the macroscopic electrical properties of the circuits in which they are used as building blocks. In particular, we review our work on three basic problems that have received special experimental attention in recent years: (i) the conductance of a single-atom contact, (ii) the conductance of a hydrogen molecule and (iii) the current through single organic molecules.

We analyze the current in a superconducting point contact of arbitrary transmission in the presence of a microwave radiation. The interplay between the ac Josephson current and the microwave signal gives rise to Shapiro steps at voltages V = (m/n)hωr/2e, where n, m are integer numbers and ωr is the radiation frequency. The subharmonic steps (n = 1) are a consequence of multiple Andreev reflections (MAR) and provide a signature of the peculiar ac Josephson effect at high transmission. Moreover, the dc current exhibits a rich subgap structure due to photon-assisted MARs.Introduction.-Our understanding of the electronic transport through superconducting nanostructures has experienced a notable development in last few years [1]. Partly, this has been due to the appearance on scene of the metallic atomic-size contacts, which can be produced by means of scanning tunneling microscope and breakjunction techniques [2][3][4][5]. These nanowires have turned out to be ideal systems to test the modern transport theories in mesoscopic superconductors. Thus, for instance Scheer and coworkers [3] found a quantitative agreement between the measurements of the current-voltage characteristics of different atomic contacts and the predictions of the theory for a single-channel superconducting contact [6,7]. These experiments not only helped to clarify the structure of the subgap current in superconducting contacts, but also showed that the set of the transmission coefficients in an atomic-size contact is amenable to measurement. This possibility has recently allowed a set of experiments that confirm the theoretical predictions for transport properties like supercurrent [4] and noise [5]. From these combined theoretical and experimental efforts a coherent picture of transport in superconducting point contacts has emerged with multiple Andreev reflections (MAR) [8] as a central concept. However, in spite of these recent successes, one of the most remarkable predictions of MAR theory remains to be confirmed, namely the ac Josephson effect. The theory says that in a constant voltage biased superconducting point contact, the time-dependent current is given by I(t) = n I n e inω0t . This means that the occurrence of MARs gives rise to the appearance of alternating currents that oscillate not only with the Josephson frequency ω 0 = 2eV /h, V being the voltage, as in the case of tunnel junctions, but also with all its harmonics. So far there is no experimental evidence of the existence of such components.In this paper, we present a theoretical analysis of the current in a superconducting point contact under a microwave radiation. We show that the interplay between the ac Josephson current components and a microwave signal leads to the appearance of Shapiro steps at voltages V = (m/n)hω r /2e, where n, m are integer numbers and ω r is the frequency of the radiation. This means that in

We demonstrate that weak external magnetic fields generate dissipationless spin currents in the ground state of systems with spiral magnetic order. Our conclusions are based on phenomenological considerations and on microscopic mean-field theory calculations for an illustrative toy model. We speculate on possible applications of this effect in spintronic devices.