Migration reversal of soft particles in vertical flows

Non-neutrally buoyant soft particles in vertical microflows are investigated. We find, soft particles lighter than the liquid migrate to off-center streamlines in a downward Poiseuille flow (buoyancy-force antiparallel to flow). In contrast, heavy soft particles migrate to the center of the downward (and vanishing) Poiseuille flow. A reversal of the flow direction causes in both cases a reversal of the migration direction, i. e. heavier (lighter) particles migrate away from (to) the center of a parabolic flow profile. Non-neutrally buoyant particles migrate also in a linear shear flow across the parallel streamlines: heavy (light) particles migrate along (antiparallel to) the local shear gradient. This surprising, flow-dependent migration is characterized by simulations and analytical calculations for small particle deformations, confirming our plausible explanation of the effect. This density dependent migration reversal may be useful for separating particles.Comment: 8 pages, 7 figure

Cross-stream transport of asymmetric particles driven by oscillating shear

We study the dynamics of asymmetric, deformable particles in oscillatory, linear shear flow. By simulating the motion of a dumbbell, a ring polymer, and a capsule we show that cross-stream migration occurs for asymmetric elastic particles even in linear shear flow if the shear rate varies in time. The migration is generic as it does not depend on the particle dimension. Importantly, the migration velocity and migration direction are robust to variations of the initial particle orientation, making our proposed scheme suitable for sorting particles with asymmetric material properties.Comment: 5 pages, 4 figure

Self-consistent ac quantum transport using nonequilibrium Green functions

We develop an approach for self-consistent ac quantum transport in the presence of time-dependent potentials at non-transport terminals. We apply the approach to calculate the high-frequency characteristics of a nanotube transistor with the ac signal applied at the gate terminal. We show that the self-consistent feedback between the ac charge and potential is essential to properly capture the transport properties of the system. In the on-state, this feedback leads to the excitation of plasmons, which appear as pronounced divergent peaks in the dynamic conductance at terahertz frequencies. In the off-state, these collective features vanish, and the conductance exhibits smooth oscillations, a signature of single-particle excitations. The proposed approach is general and will allow the study of the high-frequency characteristics of many other low-dimensional nanoscale materials such as nanowires and graphene-based systems, which are attractive for terahertz devices, including those that exploit plasmonic excitations.Comment: 11 pages, 5 figures, accepted in Physical Review

Incoherent Transport through Molecules on Silicon in the vicinity of a Dangling Bond

We theoretically study the effect of a localized unpaired dangling bond (DB) on occupied molecular orbital conduction through a styrene molecule bonded to a n++ H:Si(001)-(2x1) surface. For molecules relatively far from the DB, we find good agreement with the reported experiment using a model that accounts for the electrostatic contribution of the DB, provided we include some dephasing due to low lying phonon modes. However, for molecules within 10 angstrom to the DB, we have to include electronic contribution as well along with higher dephasing to explain the transport features.Comment: 9 pages, 5 figure

Dependence of DC characteristics of CNT MOSFETs on bandstructure models

Since their discovery in the early 1990s, the interest in carbon nanotube (CNT) electronics has exploded. One main factor that controls the device performance of CNT field-effect transistors (CNT MOSFETs) is the electronic structure of the nanotube. In this paper we use three different bandstructure models. 1) extended Huckel theory (EHT); 2) orthogonal p(z) tight-binding (OTB); and 3) parabolic effective mass model (EFM) to investigate the bandstructure effects on the device characteristics of a CNT MOSFET using semiclassical and quantum treatments of transport. We find that, after proper calibration, the OTB model is essentially identical to the EHT over the energy range of interest. We also find that an even simpler parabolic EFM facilitates CNT MOSFET simulations within practically applied bias ranges

Influence of defects on nanotube transistor performance

We study the effect of vacancies and charged impurities on the performance of carbon nanotube transistors by self-consistently solving the three-dimensional Poisson and Schrödinger equations. We find that a single vacancy or charged impurity can decrease the drive current by more than 25% from the ballistic current. The threshold voltage shift in the case of charged impurities can be as large as 40 mV