P. Bokes

Ground-state reconstruction of the Si(0 0 1) surface: symmetric versus buckled dimers

An extensive computational study is presented with the quest to investigate the nature of the ground-state geometry of the Si(0 0 1) surface, a subject of recent experimental controversy. We analyze for the first time in detail the possible sources of errors which would arise in any correlated calculation for a system size of interest here. For this purpose, we present a detailed analysis of the cluster model of the surface at the DFT and MCSCF level of theory. Estimates of errors arising from the use of pseudopotential, finite cluster size, and biased (method dependent) choice of ground-state geometry are given. The resulting error is estimated to be comparable to the energy scale of interest. On the other hand, the energy variation due to negative thermal expansion at low temperature is found to be qualitatively consistent with dimer symmetrization.

Stroboscopic wave-packet description of nonequilibrium many-electron problems.

We introduce the construction of an orthogonal wave-packet basis set, using the concept of stroboscopic time propagation, tailored to the efficient description of nonequilibrium extended electronic systems. Thanks to three desirable properties of this basis, significant insight is provided into nonequilibrium processes (both time-dependent and steady-state), and reliable physical estimates of various many-electron quantities such as density, current, and spin polarization can be obtained. Use of the wave-packet basis provides new results for time-dependent switching-on of the bias in quantum transport, and for current-induced spin accumulation at the edge of a 2D doped semiconductor caused by edge-induced spin-orbit interaction.

Comment on "Dynamical corrections to the DFT-LDA electron conductance in nanoscale systems"

In a recent paper Sai et al. [1] identified a correctionRdyn to the DC conductance of nanoscale junctions arising from dynamical exchange-correlation ( XC) effects within timedependent density functional theory. This quantity contri butes to the total resistance through R= Rs+ Rdyn whereRs is the resistance evaluated in the absence of dynamical XC effects. In this Comment we show that the numerical estimation of Rdyn in example systems of the type they considered should be considerably reduced, once a more appropriate form for the shear electron viscosity η is used. Saiet al.’s expression for Rdyn, based on electron-liquid theory [2], is a one-dimensional integral between the two electrodes

Ab initio formulation of the four-point conductance of interacting electronic systems

We derive an expression for the four-point conductance of a general quantum junction in terms of the density response function. Our formulation allows us to show that the four-point conductance of an interacting electronic system possessing either a geometrical constriction and/or an opaque barrier becomes identical to the macroscopically measurable two-point conductance. Within time-dependent density-functional theory the formulation leads to a direct identification of the functional form of the exchange-correlation kernel that is important for the conductance. We demonstrate the practical implementation of our formula for a metalvacuum-metal interface.

Tunneling through Al/AlOx/Al junction: Analytical models and first-principles simulations

We study from first principles the transport properties of Al/AlOx/Al tunnel junctions. On this basis, we analyze the reliability of two analytical models for the conductance, namely the trapezoid potential barrier model and a tight-binding model. Our findings show that (i) the interface width used in the models is determined by the electronic density profile, and it is shorter than the width one expects from the atomic arrangements; (ii) the effective mass}, found to be about on third of the free electron mass, can be determined from the oxide band-structure calculations, and (iii) the barrier height is given by one fourth of the bandgap in the oxide, which explains the apparently small values found for these junctions experimentally.

First-principles conductance of nanoscale junctions from the polarizability of finite systems

A method for the calculation of the conductance of nanoscale electrical junctions is extended to ab initio electronic structure methods that make use of the periodic supercell technique and applied to realistic models of metallic wires and break junctions of sodium and gold. The method is systematically controllable and convergeable and can be straightforwardly extended to include more complex processes and interactions. Important issues, about the order in which the thermodynamic and static (small field) limits are taken, are clarified, and characterized further through comparisons to model systems.

Current-constraining variational approaches to quantum transport

Presently, the main methods for describing a nonequilibrium charge-transporting steady state are based on time-evolving it from the initial zero-current situation. An alternative class of theories would give the statistical nonequilibrium density operator from principles of statistical mechanics, in a spirit close to Gibbs ensembles for equilibrium systems, leading to a variational principle for the nonequilibrium steady state. We discuss the existing attempts to achieve this using the maximum entropy principle based on constraining the average current. We show that the current-constrained theories result in a zero-induced drop in electrostatic potential, so that such ensembles cannot correspond to the time-evolved density matrix, unless left- and right-going scattering states are mutually incoherent.

Asymptotic self-consistency in quantum transport calculations

Ab initio simulations of quantum transport commonly focus on a central region which is considered to be connected to infinite leads through which the current flows. The electronic structure of these distant leads is normally obtained from an equilibrium calculation, ignoring the self-consistent response of the leads to the current. We examine the consequences of this, and show that the electrostatic potential Delta phi is effectively being approximated by the difference between electrochemical potentials Delta mu, and that this approximation is incompatible with asymptotic charge neutrality. In a test calculation for a simple metal-vacuum-metal junction, we find significant errors in the nonequilibrium properties calculated with this approximation, in the limit of small vacuum gaps. We provide a scheme by which these errors may be corrected.

Maximum-entropy theory of steady-state quantum transport

We develop a theoretical framework for describing steady-state quantum transport phenomena, based on the general maximum-entropy principle of nonequilibrium statistical mechanics. The general form of the many-body density matrix is derived, which contains the invariant part of the current operator that guarantees the nonequilibrium and steady-state character of the ensemble. Several examples of the theory are given, demonstrating the relationship of the present treatment to the widely used scattering-state occupation schemes at the level of the self-consistent single-particle approximation. The latter schemes are shown not to maximize the entropy, except in certain limits.

Rigidity of the conductance of an anchored dithioazobenzene optomechanical switch

Reversible opto-mechanical molecular switch based on a single azobenzene molecule suspended via thiolate links between realistic models of gold tips is investigated. Using a combination of the transfer matrix technique and density functional theory we focus on conductance of the nano-device in the two (meta)stable cis and trans junction conformations. We find the conductance of both conformations to be broadly similar. In qualitative agreement with related experiments, we find that the same nano-device with one/two methylene linker group(s) inserted on one/both ends of the azobenzene molecule is driven into tunneling regime and reduces the conductances by up to two orders of magnitude, again almost uniformly for both conformations. These results clarify the huge differences in switching ratios found previously and indicate that this nano-device is not particularly suited for use as a molecular switch based on conductance change.