Contact resistance in the scanning tunneling microscope at very small distances

Non-Markovian transients in transport across chiral quantum wires using space-time non-equilibrium Green functions

We study a system of two non-interacting quantum wires with fermions of opposite chirality with a point contact junction at the origin across which tunneling can take place when an arbitrary time-dependent bias between the wires is applied. We obtain the exact dynamical non-equilibrium Green function by solving Dyson's equation analytically. Both the space-time dependent two and four-point functions are written down in a closed form in terms of simple functions of position and time. This allows us to obtain, among other things, theI-Vcharacteristics for an arbitrary time-dependent bias. Our method is a superior alternative to competing approaches to non-equilibrium as we are able to account for transient phenomena as well as the steady state. We study the approach to steady state by computing the time evolution of the equal-time one-particle Green function. Our method can be easily applied to the problem of a double barrier contact whose internal properties can be adjusted to induce resonant tunneling leading to a conductance maximum. We then consider the case of a finite bandwidth in the point contact and calculate the non-equilibrium transport properties which exhibit non-Markovian behaviour. When a subsequently constant bias is suddenly switched on, the current shows a transient build up before approaching its steady state value in contrast to the infinite bandwidth case. This transient property is consistent with numerical simulations of lattice systems using time-dependent density matrix renormalization group suggesting thereby that this transient build up is merely due to the presence of a short distance cutoff in the problem description and not on the other details.

Electron transport in ultra-thin films and ballistic electron emission microscopy

We have developed a calculation scheme for the elastic electron current in ultra-thin epitaxial heterostructures. Our model uses a Keldysh's non-equilibrium Green's function formalism and a layer-by-layer construction of the epitaxial film. Such an approach is appropriate to describe the current in a ballistic electron emission microscope (BEEM) where the metal base layer is ultra-thin and generalizes a previous one based on a decimation technique appropriated for thick slabs. This formalism allows a full quantum mechanical description of the transmission across the epitaxial heterostructure interface, including multiple scattering via the Dyson equation, which is deemed a crucial ingredient to describe interfaces of ultra-thin layers properly in the future. We introduce a theoretical formulation needed for ultra-thin layers and we compare with results obtained for thick Au(1 1 1) metal layers. An interesting effect takes place for a width of about ten layers: a BEEM current can propagate via the center of the reciprocal space ([Formula: see text]) along the Au(1 1 1) direction. We associate this current to a coherent interference finite-width effect that cannot be found using a decimation technique. Finally, we have tested the validity of the handy semiclassical formalism to describe the BEEM current.

Theory for STM images of resonances in the near-field regime: application to adsorbates and local defects on graphene

We analyze the STM current through electronic resonances on a substrate as a function of tip-surface distance. We show that when the tip approaches the surface a maximum of the density of states of the electronic resonance at some energy can lead to a dip of the STM signal dI/dV. This phenomenon is a Fano effect resulting from the coupling of the electronic states of the STM tip with the resonance. In graphene such resonances can be produced by local defects or adsorbates and we analyze the cases of top and hollow configurations of adsorbates.

Spin-dependent electronic conduction along zigzag graphene nanoribbons bearing adsorbed Ni and Fe nanostructures

Using SMEAGOL, an ab initio computational method that combines the non-equilibrium Green's function formalism with density-functional theory, we calculated spin-specific electronic conduction in systems consisting of single Fen and Nin nanostructures (n = 1-4) adsorbed on a hydrogen-passivated zigzag graphene nanoribbon. For each cluster we considered both ferromagnetically and antiferromagnetically coupled ribbon edges (Ferro-F and Ferro-A systems, respectively). Adstructures located laterally on Ferro-A ribbons caused significant transmittance loss at energies 0.6-0.25 eV below the Fermi level for one spin and 0.2-0.4 eV above the Fermi level for the other, allowing the potential use of these systems in transistors to create a moderately spin-polarized current of one or the other sign depending on the gate voltage. Ni3 and Ni4 clusters located at the centre of Ferro-F ribbons exhibited a strong spin-filtering effect in a narrow energy window around the Fermi level.

Negative differential resistance in nanoscale transport in the Coulomb blockade regime

Motivated by recent experiments, we have studied the transport behavior of coupled quantum dot systems in the Coulomb blockade regime using the master (rate) equation approach. We explore how electron-electron interactions in a donor-acceptor system, resembling weakly coupled quantum dots with varying charging energy, can modify the system's response to an external bias, taking it from normal Coulomb blockade behavior to negative differential resistance (NDR) in the current-voltage characteristics.

Conductance oscillations in zigzag platinum chains

Using first principles simulations we perform a detailed study of the structural, electronic, and transport properties of monatomic platinum chains, sandwiched between platinum electrodes. First, we demonstrate that the most stable atomic configuration corresponds to a zigzag arrangement that gradually straightens as the chains are stretched. Second, we find that the averaged conductance shows slight parity oscillations with the number of atoms in the chain. Additionally, the conductance of chains of fixed oscillates as the end atoms are pulled apart, due to the gradual closing and opening of conductance channels as the chain straightens.

Towards molecular spintronics

The ability to manipulate electron spin in organic molecular materials offers a new and extremely tantalizing route towards spin electronics, both from fundamental and technological points of view. This is mainly due to the unquestionable advantage of weak spin-orbit and hyperfine interactions in organic molecules, which leads to the possibility of preserving spin-coherence over times and distances much longer than in conventional metals or semiconductors. Here we demonstrate theoretically that organic spin valves, obtained by sandwiching an organic molecule between magnetic contacts, can show a large bias-dependent magnetoresistance and that this can be engineered by an appropriate choice of molecules and anchoring groups. Our results, obtained through a combination of state-of-the-art non-equilibrium transport methods and density functional theory, show that although the magnitude of the effect varies with the details of the molecule, large magnetoresistance can be found both in the tunnelling and the metallic limit.