Nonequilibrium Green function modelling of transport in mesoscopic systems

A generalized Landauer formula, derived with the methods due to Keldysh, and Baym and Kadanoff, is gaining widespread use in the modeling of transport in a large number of different mesoscopic systems. We review some of the recent developments, including transport in semiconductor superlattices, calculation of noise, and nanoelectromechanical systems.Comment: Contribution to "Progress in Nonequilibrium Green Functions", Dresden, Germany, 19-22 August, Editor: Michael Bonit

Time-dependent transport in interacting mesoscopic systems

We review recent applications of the nonequilibrium Green function technique to time-dependent transport in mesoscopic systems.Comment: Submitted to "Progress in Nonequilibrium Green's Functions", Ed. M. Bonitz, World Scientific; 24 pages, 7 figures, uses sprocl.sty (included) Revised version (misprints corrected, added two references

Signatures of adatom effects in the quasiparticle spectrum of Li-doped graphene

We study the spectral function and quasiparticle scattering in Li-decorated graphene (Li@graphene) with an atomistic $T$-matrix formalism and uncover adatom-induced spectral effects which shed light on experimentally observed angle-resolved photoemission spectroscopy (ARPES) features. From transport studies, alkali adatoms are known to introduce charged-impurity scattering limiting the carrier mobility. Here, we demonstrate that Li adatoms furthermore give rise to a low-energy impurity band centered at the $\Gamma$ point which originates from the hybridization between the atomic 2s state of the Li adatoms and graphene "surface" states. We show that the impurity band is strongly dependent on the concentration $c_\mathrm{Li}$ of Li adatoms, and aligns with the Li-induced Fermi level on the Dirac cone at $c_\mathrm{Li}\sim 8\,\%$ ($E_F\approx 1.1\,\mathrm{eV}$). Finally, we show that adatom-induced quasiparticle scattering increases dramatically at energies above $\sim 1\,\mathrm{eV}$ close to the van Hove singularity in the graphene density of states (DOS), giving rise to a large linewidth broadening on the Dirac cone with a concomitant downshift and a characteristic kink in the conduction band. Our findings are highly relevant for future studies of ARPES, transport, and superconductivity in adatom-doped graphene.Comment: 6 pages, 4 figures, and supplemental material. Published versio

Mesoscopic photon heat transistor

We show that the heat transport between two bodies, mediated by electromagnetic fluctuations, can be controlled with an intermediate quantum circuit - leading to the device concept Mesoscopic Photon Heat Transistor (MPHT). Our theoretical analysis is based on a novel Meir-Wingreen-Landauer type of conductance formula, which gives the photonic heat current through an arbitrary circuit element coupled to two dissipative reservoirs at finite temperatures. As an illustration we present an exact solution for the case when the intermediate circuit can be described as an electromagnetic resonator. We discuss in detail how the MPHT can be implemented experimentally in terms of a flux-controlled SQUID circuit.Comment: 4 pages, 3 figure

Dynamical Franz-Keldysh effect

We introduce and analyze the properties of dynamical Franz-Keldysh effect, i.e. the change of density-of-states, or absorption spectra, of semiconductors under the influence of {\it time-dependent} electric fields. In the case of a harmonic time-dependence, we predict the occurence of significant fine structure, both below and above the zero-field band-gap, which should be experimentally observable.Comment: 4 pages, REVTEX 3.0, uses epsf, 5 figures attached as Z-compressed .tar fil

Correlated Coulomb drag in capacitively coupled quantum-dot structures

We study theoretically Coulomb drag in capacitively coupled quantum dots (CQDs) -- a biasdriven dot coupled to an unbiased dot where transport is due to Coulomb mediated energy transfer drag. To this end, we introduce a master-equation approach which accounts for higher-order tunneling (cotunneling) processes as well as energy-dependent lead couplings, and identify a mesoscopic Coulomb drag mechanism driven by nonlocal multi-electron cotunneling processes. Our theory establishes the conditions for a nonzero drag as well as the direction of the drag current in terms of microscopic system parameters. Interestingly, the direction of the drag current is not determined by the drive current, but by an interplay between the energy-dependent lead couplings. Studying the drag mechanism in a graphene-based CQD heterostructure, we show that the predictions of our theory are consistent with recent experiments on Coulomb drag in CQD systems.Comment: 6 pages, 4 figures + supplementary. Published versio

Quantum transport: The link between standard approaches in superlattices

Theories describing electrical transport in semiconductor superlattices can essentially be divided in three disjoint categories: i) transport in a miniband; ii) hopping between Wannier-Stark ladders; and iii) sequential tunneling. We present a quantum transport model, based on nonequilibrium Green functions, which, in the appropriate limits, reproduces the three conventional theories, and describes the transport in the previously unaccessible region of the parameter space.Comment: 4 Page

Phase measurement of photon-assisted tunneling through a quantum dot

Recent double-slit interference experiments have demonstrated the possibility of probing the phase of the complex transmission coefficient of a quantum dot via the Aharonov-Bohm effect. We propose an extension of these experiments: an ac voltage imposed on the side gate with the concomitant photonic sidebands leads to additional structure both in the amplitude and in the phase of the Aharonov-Bohm signal. Observation of these effects would be a definitive proof of coherent absorption and reemission of photons from the ac source.Comment: 6 pages using latex2e and EuroPhys.sty. Uses epsf to include 5 figures (submitted to Europhys. Lett.

Current responsivity of semiconductor superlattice THz-photon detectors

The current responsivity of a semiconductor superlattice THz-photon detector is calculated using an equivalent circuit model which takes into account the finite matching efficiency between a detector antenna and the superlattice in the presence of parasitic losses. Calculations performed for currently available superlattice diodes show that both the magnitudes and the roll-off frequencies of the responsivity are strongly influenced by an excitation of hybrid plasma-Bloch oscillations which are found to be eigenmodes of the system in the THz- frequency band. The expected room temperature values of the responsivity (2-3 A/W in the 1-3 THz-frequency band) range up to several percents of the quantum efficiency $e/\hbar\omega$ of an ideal superconductor tunnel junction detector. Properly designed semiconductor superlattice detectors may thus demonstrate better room temperature THz-photon responsivity than conventional Schottky junction devices.Comment: Revtex file, uses epsf, 11 pages. 11 eps-figures; EPS-files generated by scanner, original higher resolution line drawings available from [email protected] by regular mail or fa

Simple models suffice for the single dot quantum shuttle

A quantum shuttle is an archetypical nanoelectromechanical device, where the mechanical degree of freedom is quantized. Using a full-scale numerical solution of the generalized master equation describing the shuttle, we have recently shown [Novotn\'{y} {\it et al.}, Phys. Rev. Lett. {\bf 92}, 248302 (2004)] that for certain limits of the shuttle parameters one can distinguish three distinct charge transport mechanisms: (i) an incoherent tunneling regime, (ii) a shuttling regime, where the charge transport is synchronous with the mechanical motion, and (iii) a coexistence regime, where the device switches between the tunneling and shuttling regimes. While a study of the cross-over between these three regimes requires the full numerics, we show here that by identifying the appropriate time-scales it is possible to derive vastly simpler equations for each of the three regimes. The simplified equations allow a clear physical interpretation, are easily solved, and are in good agreement with the full numerics in their respective domains of validity.Comment: 23 pages, 14 figures, invited paper for the Focus issue of the New Journal of Physics on Nano-electromechanical system