We describe a semiempirical atomic basis extended Hückel theoretical ͑EHT͒ technique that can be used to calculate bulk band structure, surface density of states, electronic transmission, and interfacial chemistry of various materials within the same computational platform. We apply this method to study multiple technologically important systems, starting with carbon nanotubes and their interfaces and silicon-based heterostructures in our follow-up paper ͓D. Kienle et al., J. Appl. Phys. 100, 043715 ͑2006͒, following paper͔. We find that when it comes to quantum transport through interesting, complex heterostructures including gas molecules adsorbed on nanotubes, the Hückel band structure offers a fair and practical compromise between orthogonal tight-binding theories with limited transferability between environments under large distortion and density functional theories that are computationally quite expensive for the same purpose.
Deformations of a polymer fixed at one end and subjected to a uniform flow are investigated with special emphasis on the hydrodynamic interaction (HI) between polymer segments. The so-called non-draining effect, which results from the collective hydrodynamic back-flow caused by all segments, is calculated for the first time with fluctuating hydrodynamic interactions. HI reduces the viscous drag and the flow partially penetrates the polymer coil. Therefore neither the free-draining nor the non-draining models discussed previously describe the polymer-flow interaction appropriately. Accordingly the f-shell blob model is introduced, describing the partial draining and the transition to a free-draining polymer with increasing flow velocity, similarly as in simulations.
We present an approach for time-dependent quantum transport based on a self-consistent nonequilibrium Green function formalism. The technique is applied to a ballistic carbon nanotube transistor in the presence of a time harmonic signal at the gate. In the ON state the dynamic conductance exhibits plasmonic resonant peaks at terahertz frequencies. These vanish in the OFF state, and the dynamic conductance displays smooth oscillations, a signature of single particle quantum effects. We show that the nanotube kinetic inductance plays an essential role in the high-frequency behavior.
We report on theoretical investigations of scanning tunneling spectroscopy ͑STM͒ image heights on Si͑100͒. Calculations are performed using density functional theory ͑DFT͒ within the Keldysh nonequilibrium Green's function ͑NEGF͒ formalism. The nonequilibrium potential drop between Si͑100͒ and a STM tip is determined self-consistently. This potential drop is found to play an important role in the calculated image height characteristics of adsorbed hydrocarbons by lowering the vacuum barrier and shifting molecular levels. Numerical data collected for image heights of styrene against a hydrogen passivated Si͑100͒ background are found to agree quantitatively with the corresponding experimental results. We also present a comparison between results obtained by the NEGF-DFT formalism and the Tersoff-Hamann approximation, showing that nonequilibrium analysis can be important in the study of STM image heights of molecules.
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