arXiv subjects

Kurt Stokbro

Publications and source records attributed to Kurt Stokbro.

32 records · Page 2Linked to original sources

arXiv2013

Electron transport across a metal-organic interface

We simulate the electron transport across the Au(111)-pentacene interface using non-equilibrium Green's functions and density-functional theory (NEGF-DFT), and calculate the bias-dependent electron transmission. We find that the electrical contact resistance is dominated by the formation of a Schottky barrier at the interface, and show that the conventional semiconductor transport models across Schottky barriers need to be modified in order to describe the simulation data. We present an extension of the conventional Schottky barrier transport model, which can describe our simulation results and rationalize recent experimental data.

cond-mat.mtrl-sci↗

arXiv2012

Atomic-scale model for the contact resistance of the nickel-graphene interface

We perform first-principles calculations of electron transport across a nickel-graphene interface. Four different geometries are considered, where the contact area, graphene and nickel surface orientations and the passivation of the terminating graphene edge are varied. We find covalent bond formation between the graphene layer and the nickel surface, in agreement with other theoretical studies. We calculate the energy-dependent electron transmission for the four systems and find that the systems have very similar edge contact resistance, independent of the contact area between nickel and graphene, and in excellent agreement with recent experimental data. A simple model where graphene is bonded with a metal surface shows that the results are generic for covalently bonded graphene, and the minimum attainable edge contact resistance is twice the ideal edge quantum contact resistance of graphene.

cond-mat.mes-hall↗

arXiv2010

First-principles modelling of molecular single-electron transistors

We present a first-principles method for calculating the charging energy of a molecular single-electron transistor operating in the Coulomb blockade regime. The properties of the molecule are modeled using density-functional theory, the environment is described by a continuum model, and the interaction between the molecule and the environment are included through the Poisson equation. The model is used to calculate the charge stability diagrams of a benzene and C$_{60}$ molecular single-electron transistor.

cond-mat.mes-hall↗

arXiv2010

Semi-Empirical Model for Nano-Scale Device Simulations

We present a new semi-empirical model for calculating electron transport in atomic-scale devices. The model is an extension of the Extended Hückel method with a self-consistent Hartree potential. This potential models the effect of an external bias and corresponding charge re-arrangements in the device. It is also possible to include the effect of external gate potentials and continuum dielectric regions in the device. The model is used to study the electron transport through an organic molecule between gold surfaces, and it is demonstrated that the results are in closer agreement with experiments than ab initio approaches provide. In another example, we study the transition from tunneling to thermionic emission in a transistor structure based on graphene nanoribbons.

cond-mat.mes-hall↗

arXiv2009

Efficient wave function matching approach for quantum transport calculations

The Wave Function Matching (WFM) technique has recently been developed for the calculation of electronic transport in quantum two-probe systems. In terms of efficiency it is comparable with the widely used Green's function approach. The WFM formalism presented so far requires the evaluation of all the propagating and evanescent bulk modes of the left and right electrodes in order to obtain the correct coupling between device and electrode regions. In this paper we will describe a modified WFM approach that allows for the exclusion of the vast majority of the evanescent modes in all parts of the calculation. This approach makes it feasible to apply iterative techniques to efficiently determine the few required bulk modes, which allows for a significant reduction of the computational expense of the WFM method. We illustrate the efficiency of the method on a carbon nanotube field-effect-transistor (FET) device displaying band-to-band tunneling and modeled within the semi-empirical Extended Hückel theory (EHT) framework.

cond-mat.mtrl-sci↗

arXiv2008

Ab initio study of spin-dependent transport in carbon nanotubes with iron and vanadium adatoms

We present an ab initio study of spin dependent transport in armchair carbon nanotubes with transition metal adsorbates, iron or vanadium. We neglect the effect of tube curvature and model the nanotube by graphene with periodic boundary conditions. A density functional theory based nonequilibrium Green's function method is used to compute the electronic structure and zero-bias conductance. The presence of the adsorbate causes a strong scattering of electrons of one spin type only. The scattering is shown to be due to coupling of the two armchair band states to the metal 3d orbitals with matching symmetry causing Fano resonances appearing as dips in the transmission function. The spin type (majority/minority) being scattered depends on the adsorbate and is explained in terms of d-state filling. The results are qualitatively reproduced using a simple tight-binding model, which is then used to investigate the dependence of the transmission on the nanotube width. We find a decrease in the width of the transmission dip as the tube-size increases.

cond-mat.mtrl-sci↗

arXiv2003

Origin Of Current-Induced Forces In An Atomic Gold Wire: A First Principles Study

We address the microscopic origin of the current-induced forces by analyzing results of first principles density functional calculations of atomic gold wires connected to two gold electrodes with different electrochemical potentials. We find that current induced forces are closely related to the chemical bonding, and arise from the rearrangement of bond charge due to the current flow. We explain the current induced bond weakening/strengthening by introducing bond charges decomposed into electrode components.

cond-mat.mtrl-sci↗

arXiv2002

Conductance switching in a molecular device: the role of sidegroups and intermolecular interactio

We report first-principles studies of electronic transport in monolayers of Tour wires functionalized with different side groups. An analysis of the scattering states and transmission eigenchannels suggests that the functionalization does not strongly affect the resonances responsible for current flow through the monolayer. However, functionalization has a significant effect on the interactions within the monolayer, so that monolayers with NO$_2$ side groups exhibit local minima associated with twisted conformations of the molecules. We use our results to interpret observations of negative differential resistance and molecular memory in monolayers of NO$_2$ functionalized molecules in terms of a twisting of the central ring induced by an applied bias potential.

cond-mat.mtrl-sci↗

arXiv2002

Density functional method for nonequilibrium electron transport

We describe an ab initio method for calculating the electronic structure, electronic transport, and forces acting on the atoms, for atomic scale systems connected to semi-infinite electrodes and with an applied voltage bias. Our method is based on the density functional theory (DFT) as implemented in the well tested Siesta approach (which uses non-local norm-conserving pseudopotentials to describe the effect of the core electrons, and linear combination of finite-range numerical atomic orbitals to describe the valence states). We fully deal with the atomistic structure of the whole system, treating both the contact and the electrodes on the same footing. The effect of the finite bias (including selfconsistency and the solution of the electrostatic problem) is taken into account using nonequilibrium Green's functions. We relate the nonequilibrium Green's function expressions to the more transparent scheme involving the scattering states. As an illustration, the method is applied to three systems where we are able to compare our results to earlier ab initio DFT calculations or experiments, and we point out differences between this method and existing schemes. The systems considered are: (1) single atom carbon wires connected to aluminum electrodes with extended or finite cross section, (2) single atom gold wires, and finally (3) large carbon nanotube systems with point defects.

cond-mat.mtrl-sci↗

arXiv1997

Electric field effects in STM imaging

We present a high voltage extension of the Tersoff-Hamann theory of STM images, which includes the effect of the electric field between the tip and the sample. The theoretical model is based on first principles electronic structure calculations and has no adjustable parameters. We use the method to calculate theoretical STM images of the monohydrate Si(100)-H(2$\times$1) surface with missing hydrogen defects at $- 2$~V and find an enhanced corrugation due to the electric field, in good agreement with experimental images.

cond-mat.mtrl-sci↗

arXiv1996

The surface chemistry of metal-oxygen interactions: a first-principles study of O:Rh(110)

We report on a computational study of the clean and oxygen-covered Rh(110) surface, based on density-functional theory within the local-density approximation. We have used plane-wave basis sets and Vanderbilt ultra-soft pseudopotentials. For the clean surface, we present results for the equilibrium structure, surface energy, and surface stress of the unreconstructed and $(1\times 2)$ reconstructed structures. For the oxygen-covered surface we have performed a geometry optimization at $1\over 2$, 1, and 2 monolayer oxygen coverages, and we present results for the equilibrium configurations, workfunctions and oxygen chemisorption energies. At half monolayer coverage, we find that oxygen induces a $(1\times 2)$ reconstruction of the surface, while at one monolayer coverage the chemisorption energy is highest for the unreconstructed surface. Our results are rationalized by a simple tight-binding description of the interaction between the O$-2p$ orbitals and the metal valence states. The resulting bonds are stronger when established with low coordinated metal atoms, and give rise to an effective adsorbate-adsorbate interaction when two oxygen atoms are bound to the same metal orbital.

cond-mat↗

arXiv1996

A mixed ultrasoft/normconserved pseudopotential scheme

A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the normconservation is released for only one or a few angular channels, is presented. Within this scheme some difficulties of the truly ultrasoft pseudopotentials are overcome without sacrificing the pseudopotential softness. i) Ghost states are easily avoided without including semicore shells. ii) The ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The number of nonlocal operators is reduced. The scheme will be most useful for transition metals, and the feasibility and accuracy of the scheme is demonstrated for the 4d transition metal rhodium.

cond-mat↗

arXiv1995

Formation Energy, Stress, and Relaxations of Low-Index Rhodium Surfaces

Ab initio local-density-functional-theory calculations of formation energies, surface stress, and multilayer relaxations are reported for the (111), (100), and (110) surfaces of Rh. The study is performed using ultrasoft pseudopotentials and plane waves in a parallel implementation. Calculated values are in good agreement with previous studies where they exist. In particular, we confirm recent results on surface stress anisotropy of transition metals.

mtrl-th↗

arXiv1994

Effective Medium Tight-Binding Theory

In this thesis a new model for calculating the total energy of atomic and solid systems is presented. The model is used to study the properties of the Si(100) surface and dislocation dynamics in the diamond structure. For the Si(100) surface the main result is the finding of an incomplete melted state at $1550^0 K$. The studied dislocation is the $90^0$ partial and it is found that the barriers for kink formation and migration are in good accordance with experimental observations. The general idea for calculating the total energy for a given system is to relate a reference system to each atom, an effective medium for which the total energy is known and then only calculate the energy difference between the two systems. Using transferable potentials and densities a pair potential is constructed which describe the electro-static and exchange-correlation part of the energy difference. The remaining part, the one-electron difference, is calculated with an LMTO tight-binding model with the potential parameters determined from the effective potential of the reference system.

cond-mat↗