We present a transport model for molecular conduction involving an extended Hückel theoretical treatment of the molecular chemistry combined with a nonequilibrium Green's function treatment of quantum transport. The self-consistent potential is approximated by CNDO (complete neglect of differential overlap) method and the electrostatic effects of metallic leads (bias and image charges) are included through a three-dimensional finite element method. This allows us to capture spatial details of the electrostatic potential profile, including effects of charging, screening, and complicated electrode configurations employing only a single adjustable parameter to locate the Fermi energy. As this model is based on semiempirical methods it is computationally inexpensive and flexible compared to ab initio models, yet at the same time it is able to capture salient qualitative features as well as several relevant quantitative details of transport. We apply our model to investigate recent experimental data on alkane dithiol molecules obtained in a nanopore setup. We also present a comparison study of single molecule transistors and identify electronic properties that control their performance.

We explore electronic transport in a nanotube quantum dot strongly coupled with vibrations and weakly with leads and the thermal environment. We show that the recent observation of anomalous conductance signatures in single-walled carbon nanotube quantum dots ͓B. J. LeRoy et al., Nature ͑London͒ 395, 371 ͑2004͒ and B. J. LeRoy et al., Phys. Rev. B 72, 075413 ͑2005͔͒ can be understood quantitatively in terms of current driven "hot phonons" that are strongly correlated with electrons. Using rate equations in the many-body configuration space for the joint electron-phonon distribution, we argue that the variations are indicative of strong electronphonon coupling requiring an analysis beyond the traditional uncorrelated phonon-assisted transport ͑Tien-Gordon͒ approach.

A recent experiment reports a non-local spin-signal that shows oscillatory behavior as a function of gate voltage when the contacts are magnetized along the direction of current flow, but not when they are magnetized perpendicular to the current, in agreement with the predictions from a simple theory. In this paper we first present a straightforward extension of this theory to include the angular spectrum of electrons and the extended injecting and detecting contacts. The results are in good agreement with those from a non-equilibrium Green function (NEGF)-based model with contact parameters adjusted to fit the experimental contact conductances. They also describe certain aspects of the experiment fairly well, but other aspects deserve further investigation.

In this second paper, we develop transferable semi-empirical parameters for the technologically important material, silicon, using Extended Hückel Theory (EHT) to calculate its electronic structure. The EHT-parameters are optimized to experimental target values of the band dispersion of bulk-silicon. We obtain a very good quantitative match to the bandstructure characteristics such as bandedges and effective masses, which are competitive with the values obtained within an sp 3 d 5 s * orthogonal-tight binding model for silicon 9 . The transferability of the parameters is investigated applying them to different physical and chemical environments by calculating the bandstructure of two reconstructed surfaces with different orientations: Si(100) (2x1) and Si(111) (2x1). The reproduced π-and π * -surface bands agree in part quantitatively with DFT-GW calculations and PES/IPES experiments demonstrating their robustness to environmental changes. We further apply the silicon-parameters to describe the 1D band dispersion of a unrelaxed rectangular silicon nanowire (SiNW) and demonstrate the EHT-approach of surface passivation using hydrogen. Our EHT-parameters thus provide a quantitative model of bulk-silicon and silicon-based materials such as contacts and surfaces, which are essential ingredients towards a quantitative quantum transport simulation through silicon-based heterostructures.

The spin dynamics of dilute paramagnetic impurities embedded in a semiconductor GaAs channel of a conventional lateral spin valve has been investigated. It is observed that the electron spin of paramagnetic Mn atoms can be polarized electrically when driven by a spin valve in the antiparallel configuration. The transient current through the MnAs/GaAs/MnAs spin valve bears the signature of the underlying spin dynamics driven by the exchange interaction between the conduction band electrons in GaAs and the localized Mn electron spins. The time constant for this interaction is observed to be dependent on temperature and is estimated to be 80 ns at 15 K.