The role of many-particle excitations in Coulomb blockaded transport

We discuss the role of electron-electron and electron-phonon correlations in current flow in the Coulomb blockade regime, focusing specifically on non-trivial signatures arising from the breakdown of mean-field theory. By solving transport equations directly in Fock space, we show that electron-electron interactions manifest as gateable excitations experimentally observed in the current-voltage characteristic. While these excitations might merge into an incoherent sum that allows occasional simplifications, a clear separation of excitations into slow 'traps' and fast 'channels' can lead to further novelties such as negative differential resistance, hysteresis and random telegraph signals. Analogous novelties for electron-phonon correlation include the breakdown of commonly anticipated Stokes-anti-Stokes intensities, and an anomalous decrease in phonon population upon heating due to reabsorption of emitted phonons.

Identifying contact effects in electronic conduction through C60 on silicon

We present a theory of current conduction through buckyball (C(60)) molecules on silicon by coupling a density functional treatment of the molecular levels embedded in a semiempirical treatment of the silicon surface with a nonequilibrium Green's function treatment of quantum transport. Several experimental variations in conductance-voltage characteristics are quantitatively accounted for by varying the detailed molecule-silicon bonding geometries. We identify how variations in contact surface microstructure influence the number, positions, and shapes of the conductance peaks, while varying separations of the scanning probe from the molecules influence their peak amplitudes.

A self-consistent transport model for molecular conduction based on extended Hückel theory with full three-dimensional electrostatics

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.

Breaking of general rotational symmetries by multidimensional classical ratchets

We demonstrate that a particle driven by a set of spatially uncorrelated, independent colored noise forces in a bounded, multidimensional potential exhibits rotations that are independent of the initial conditions. We calculate the particle currents in terms of the noise statistics and the potential asymmetries by deriving an n-dimensional Fokker-Planck equation in the small correlation time limit. We analyze a variety of flow patterns for various potential structures, generating various combinations of laminar and rotational flows.

Bloch oscillations in the presence of plasmons and phonons

The coupling between Bloch oscillating electrons and longitudinal optical phonons in a superlattice leads to resonant phonon excitation but no gap in the Bloch-phonon spectrum. In addition, we predict a sharp transition from plasma to Bloch oscillations at nu(B) = 2nu(P). From a microscopic description with phenomenological dampings, we numerically map out the behavior of coupled Bloch-plasmon-phonon modes for a wide range of parameters, and mimic experimental conditions. Our results are in good agreement with recent experiments by Dekorsy et al. [Phys. Rev. Lett. 85, 1080 (2000)].

Rotation in an asymmetric multidimensional periodic potential due to colored noise

We analyze the motion of an overdamped classical particle in a multidimensional periodic potential, driven by an external noise. We demonstrate that in the steady state the presence of temporal correlations in the noise and spatial asymmetry within a period of the potential could lead to particle rotation. The rotation is a direct consequence of a change in the sign of the noise-induced drift motion in each dimension. By choosing different potentials, we can generate a variety of flow patterns from laminar drifts to rotations.