Ab Initio Nonequilibrium Research Articles

Accurate ab initio determination of ballistic electron emission spectroscopy: Application to Au/Ge

Ab initio nonequilibrium Keldysh formalism based on an $N$-order renormalization technique is used to compute $I(V)$ ballistic electron emission microscopy characteristics at the Au/Ge(001) interface. Such a formalism quantitatively reproduces precise experimental measurements under ultrahigh vacuum and low-temperature conditions. At $T=0$ K, the ballistic current follows the law ${(V\ensuremath{-}{V}_{\mathrm{SB}})}^{2.1}$, ${V}_{\mathrm{SB}}$ being the Schottky barrier. At $Tg0$ K, temperature effects become significant near the onset and must be taken into account to identify an accurate value for ${V}_{\mathrm{SB}}$ from a best-fit procedure. We find two values for ${V}_{\mathrm{SB}}$, 0.67 and 0.75 eV, which we associate with two different atomic registries at the interface.

Microscopic theory and quantum simulation of atomic heat transport

Quantum simulation methods based on density-functional theory are currently deemed unfit to cope with atomic heat transport within the Green-Kubo formalism, because quantum-mechanical energy densities and currents are inherently ill-defined at the atomic scale. We show that, while this difficulty would also affect classical simulations, thermal conductivity is indeed insensitive to such ill-definedness by virtue of a sort of gauge invariance resulting from energy extensivity and conservation. Based on these findings, we derive an expression for the adiabatic energy flux from density-functional theory, which allows heat transport to be simulated using ab-initio equilibrium molecular dynamics. Our methodology is demonstrated by comparing its predictions with those of classical equilibrium and ab-initio non-equilibrium (M\"uller-Plathe) simulations for a liquid-Argon model, and finally applied to heavy water at ambient conditions.

Ab Initio Nonequilibrium Thermodynamic and Transport Properties of Ultrafast Laser Irradiated 316L Stainless Steel

We present calculations of transient behavior of thermodynamic and transport coefficients on the time scale of electron–phonon relaxation upon ultrashort laser excitation of ferrous alloys. Their role defining energy deposition and primary microscopic material response to the laser irradiation is outlined. Nonequilibrium thermodynamic properties of 316L stainless steel are determined from first-principles calculations. Taking into account the complexity of multimetallic materials, the density functional theory is first applied to describe the electronic density of states of an alloy stainless steel matrix as a function of electronic heating. An increase of the localization degree of the charge density was found to be responsible for the modification of the electronic structure upon electronic heating, with consequences on chemical potential, electronic capacity, and pressure. It is shown that the electronic temperature dependence of stainless steel thermodynamic properties are consistent with the behavior...

The Role of the Electrode Size of to the Elecronic Transport Properties

We study the size effects of the electrode to the elecronic transport properties via a molecule of benzene sandwiched between two graphene electrodes using an ab initio nonequilibrium Green's function method and density function theory. More specially, two types of electrodes are selected: one ring wide electrodes and two rings wide electrodes. We find that the current cross the system that is with two rings wide electrodes is bigger than the current that is with one ring wide electrodes. Detailed analyses of the projected density of states and the transmission spectra of the system reveals the mechanism: with wider electrodes, transmission coeffients walk nearer to the fermi energy and may make contribution to the current at applied bias voltages.

Electrostatic and dynamical effects of an aqueous solution on the zero-bias conductance of a single molecule: A first-principles study

The electrostatic and dynamical effects of an aqueous solution on the conductance of benzene-1,4-dithiolate are examined using the ab initio nonequilibrium Green's function method and Car-Parrinello molecular dynamics. The peak of the conductance histogram shifts downward by $0.01--0.02\text{ }{\text{G}}_{0}$, which is attributed to the electrostatic effects of the aqueous solution. Dynamical changes in molecular motion induced by the aqueous solution are observed in the molecular dynamics, resulting in emergence/disappearance of new peaks in the conductance histograms.

Efficientab initiomethod for inelastic transport in nanoscale devices: Analysis of inelastic electron tunneling spectroscopy

We describe the ab initio nonequilibrium Green's function method for electron-transport calculations in nanoscale devices based on the ``efficient molecular-orbital approach.'' This is implemented in the density-functional theory code SIESTA with the additional option of including effects originating from electron-phonon coupling. We also derive simple expressions for the conductance and the inelastic electron tunneling spectrum (IETS) based on the rigorous lowest-order expansion formalism. In order to illustrate our method, we have performed calculations of inelastic transport in a linear gold atomic wire and a benzene-dithiol molecule both sandwiched between gold electrodes. In the latter case the leads have been constrained to maintain an overall ${D}_{2h}$ symmetry, as typical of both high- and low-conductance systems. The shapes of the IETS, the effect of the temperature, and of the symmetry of the IETS signals are analyzed in details.

Simple STM Tip Functionalization for Rapid DNA Sequencing: An Ab Initio Green's Function Study

An ab initio nonequilibrium Green's function study of the electron-transport properties of the adenine, thymine, cytosine, and guanine DNA bases located between gold electrodes has been performed. One-electron transmission spectra were calculated for both gold and sulfur-modified gold electrodes, which are model conditions of scanning tunneling microscopy (STM) experiments with the different tips. It is shown that the nature of chemical bonding between molecules and metal electrodes plays the most significant role in the overall conductance of the systems. The distance between electrodes and the size of molecules are less important, at least when both sides of the molecule form chemical contact with the electrodes. On the basis of the obtained results, a simple two-pass DNA sequencing scheme is suggested.

First principles study on the effects of the H2O molecules on the transport properties of a carbon wire

We investigate the effects of H2O molecule environment on the transport properties of a seven-atom carbon wire coupled to two Al(100) electrodes based on a recently developed ab initio nonequilibrium Green function formalism. Our results show that the transport properties are sensitive to the variation of the number and the position of the H2O molecule on the carbon wire. Especially, the equilibrium conductance of the carbon wire with single H2O molecule exhibits an oscillatory behavior with the shift of positions of the H2O molecule. For the case of two H2O molecules, the contribution of the third eigenchannel becomes larger in some configurations. The calculated current-voltage curves show different behaviors with the variation of the positions of the H2O molecules. In certain cases, large negative differential resistance is found, but not in other cases.

Current rectification by asymmetric molecules: An ab initio study

Current rectification effect in an asymmetric molecule HCOO-C6H4-(CH2)n sandwiched between two aluminum electrodes has been studied using an ab initio nonequilibrium Green's function method. The conductance of the system decreases exponentially with the increasing number n of CH2. The phenomenon of current rectification is observed such that a very small current appears at negative bias and a sharp negative differential resistance at a critical positive bias when n>or=2. The rectification effect arises from the asymmetric structure of the molecule and the molecule-electrode couplings. A significant rectification ratio of approximately 38 can be achieved when n=5.

A wave function based ab initio nonequilibrium Green’s function approach to charge transport

We present an ab initio nonequilibrium approach to calculate the current across a molecular junction. The method rests upon a wave function based description of the central region of the junction combined with a tight binding approximation for the electrodes in the frame of the Keldysh Green’s function formalism. In addition we present an extension so as to include effects of the two-particle propagator. Our procedure is demonstrated for a dithiolbenzene molecule between silver electrodes. The full current-voltage characteristic is calculated. Specific conclusions for the contribution of correlation and two-particle effects are derived. The latter are found to contribute about 5% to the current. The order of magnitude of the current coincides with experiments.

Effect of gating on the transport properties of aSi4cluster

The effect of gate voltage on the transport properties of ${\mathrm{Si}}_{4}$ cluster coupled with two atomic scale Al(100) electrodes has been investigated using a recently developed ab initio nonequilibrium Green's function technique. Motivated by a recent experiment [Champagne et al. Nano Lett. 5, 305 (2005)], the gating effect is probed in three cases with different contact distances. The equilibrium conductance is found to oscillate with the gate potential for all contact distances. We elucidate that this oscillatory behavior is very closely related to the variation of the system density of state at Fermi level. Moreover, we also demonstrate that the charge transfer is not the key factor to determine such a conductance oscillatory behavior. Through the analysis of the eigenchannel decomposition of the conductance, we suggest that gating does not change the eigenchannel number, and that the appearance of the conductance maximum results from the saturation of one or two eigenchannels. Even with the modification of gating, ${\mathrm{Si}}_{4}$ cluster still exhibits a negative differential resistance behavior. In particular, it also displays a negative transconductance behavior for a small fixed drain bias.

Quantum transport through short semiconducting nanotubes: A complex band structure analysis

With a recently developed ab initio nonequilibrium Green's function formalism, we have examined the problem of quantum transport through prototypical, short, semiconducting nanotube devices. Metallic behavior is predicted for very short nanotubes, which crosses over to semiconducting behavior as the tube length is increased. This behavior finds its origins in the evanescent modes that are present in these finite-sized systems, which cannot be ignored. A complex band structure analysis makes the contributions of these modes particularly transparent.

Charge transport through small silicon clusters

With a recently developed ab initio nonequilibrium Green's-function formalism, we have investigated the transport behavior of small ${\mathrm{Si}}_{n},$ $n=1--10,$ 13, and 20 nanoclusters between atomistic Al and Au leads. All of the clusters display metallic $I\ensuremath{-}V$ characteristics, with typical conductances ranging between two and three (units of ${G}_{o}{=2e}^{2}/h).$ The transport properties of these cluster junctions may be understood in terms of both the band structure of the electrodes, and the molecular electronic states of the clusters as modified by the lead environment. In addition, the quantum transport properties of Si nanoclusters doped with a Na atom are also analyzed.