Time-Dependent Transport Through Molecular Junctions

We investigate transport properties of molecular junctions under two types of bias--a short time pulse or an AC bias--by combining a solution for the Green functions in the time domain with electronic structure information coming from ab initio density functional calculations. We find that the short time response depends on lead structure, bias voltage, and barrier heights both at the molecule-lead contacts and within molecules. Under a low frequency AC bias, the electron flow either tracks or leads the bias signal (capacitive or resistive response) depending on whether the junction is perfectly conducting or not. For high frequency, the current lags the bias signal due to the kinetic inductance. The transition frequency is an intrinsic property of the junctions.Comment: 5 pages, 9 figure

The Role of the Exchange-Correlation Potential in ab initio Electron Transport Calculations

The effect of the exchange-correlation potential in ab initio electron transport calculations is investigated by constructing optimized effective potentials (OEP) using different energy functionals or the electron density from second-order perturbation theory. We calculate electron transmission through two atomic chain systems, one with charge transfer and one without. Dramatic effects are caused by two factors: changes in the energy gap and the self-interaction error. The error in conductance caused by the former is about one order of magnitude while that caused by the latter ranges from several times to two orders of magnitude, depending on the coupling strength and charge transfer. The implications for accurate quantum transport calculations are discussed.Comment: 4 pages, published version, substantially revised discussion and revisions for clarit

Intermolecular Effect in Molecular Electronics

We investigate the effects of lateral interactions on the conductance of two molecules connected in parallel to semi-infinite leads. The method we use combines a Green function approach to quantum transport with density functional theory for the electronic properties. The system, modeled after a self-assembled monolayer, consists of benzylmercaptane molecules sandwiched between gold electrodes. We find that the conductance increases when intermolecular interaction comes into play. The source of this increase is the indirect interaction through the gold substrate rather than direct molecule-molecule interaction. A striking resonance is produced only 0.3 eV above the Fermi energy.Comment: 4 pages, 5 figure

Nanotube-Metal Junctions: 2- and 3- Terminal Electrical Transport

We address the quality of electrical contact between carbon nanotubes and metallic electrodes by performing first-principles calculations for the electron transmission through ideal 2- and 3-terminal junctions, thus revealing the physical limit of tube-metal conduction. The structural model constructed involves surrounding the tube by the metal atoms of the electrode as in most experiments; we consider metallic (5,5) and n-doped semiconducting (10,0) tubes surrounded by Au or Pd. In the case of metallic tubes, the contact conductance is shown to approach the ideal 4e^2/h in the limit of large contact area. For three-terminals, the division of flux among the different transmission channels depends strongly on the metal material. A Pd electrode has nearly perfect tube-electrode transmission and therefore turns off the straight transport along the tube. Our results are in good agreement with some recent experimental reports and clarify a fundamental discrepancy between theory and experiment.Comment: 5 pages, 5 figures, published version: some modified figures and clarifications in the tex

Europeanization and the Promotion of Political Development in Poland

From introduction: "This study has considered the impacts of EU accession that have resulted in the promotion of political stability and economic transformation in the Central and Eastern European countries (hereafter CEECs). The main target for the EU’s eastern policy after the end of the Cold War is to create a zone of western democracies and liberal economy entities. Given an opportunity to swap the EU’s resources for ideal transformation in the CEECs, Brussels is willing to accept."(...

Divergent Mating Behaviors and the Evolution of Reproductive Isolation

Sexual selection can cause rapid co-divergence of mating traits and mate preferences, generate reproductive barriers among individuals bearing divergent mating traits, and potentially lead to speciation. In my dissertation, I focused on two emerging topics that challenge this traditional speciation-by-mate-choice paradigm. First, sexual selection encompasses both mate preferences and intrasexual competition, yet speciation research disproportionally focused on the role of the former. Second, sexual behaviors are usually assumed to be genetically inherited, but they may often be shaped by learning instead, which can generate very different evolutionary trajectories for traits and preferences. Using studies of the highly polymorphic strawberry poison frogs (Oophaga pumilio), I demonstrated how incorporating (i) male-male competition and (ii) behavioral learning can enhance our understanding of the potential for speciation to be driven by sexual selection. I first characterized behavioral patterns across a natural contact zone between color morphs and showed that coloration (the divergent mating trait) mediates both female choice and male-male competition. Females often prefer males of their own (local) color over a novel color, and males, when defending territories, are more aggressive against their own color morph. I then tested how these color-mediated female preferences and male aggression biases interact to determine mating patterns. I conducted a controlled breeding experiment in which male-male competition and female mate choice act either in same or in opposing directions. In this study, females reproduced more often with the territorial male over the non-territorial male, regardless of the males’ coloration. This challenges the common assumption that knowledge of female preferences for male mating traits is sufficient to predict mating patterns. Finally, I discovered that learning from mothers during the tadpole stage shapes both female mate preferences and male aggression biases in O. pumilio. Based on this finding, I built a population genetic model and used it to demonstrate a simple and elegant mechanism by which sexual selection alone has the potential to initiate speciation. My research highlights the importance of considering interactions between mate choice, intrasexual competition, and behavioral learning, for studies of mating trait evolution and sexual selection’s role in speciation