Landauer Approach Research Articles

GUANINE- DNA NUCLEOTIDE FOR SINGLE MOLECULE ELECTRONIC DEVICES

Theoretical framework of single-molecule electronics is introduced in this paper. Two-terminal device is the fundamental device for single molecule electronics. We have designed the two-probe system in which guanine is sandwiched between two semi-infinite gold electrodes .Tunneling current is computed using a density functional theory combined with Non- Equilibrium Green’s function theory of quantum transport based on landauer approach. We have investigated the guanine electrical properties in two-terminal as well as three-terminal device that is we are using the gate as third terminal. The resulting two and three-terminal set-up have shown a negative differential region in I-V Characteristics of guanine. The peak to valley current increases on increasing the gate voltage from negative to positive source from -4Vg to +4Vg but with a very small amount of IE-15A. The guanine molecules have shown very good conducting properties which can be used in designing two and three terminal nanoelectronic devices.

Charge transport in desolvated DNA

The conductivity of DNA in molecular junctions is often probed experimentally under dry conditions, but it is unclear how much of the solvent remains attached to the DNA and how this impacts its structure, electronic states, and conductivity. Classical MD simulations show that DNA is unstable if the solvent is removed completely, while a micro-hydrated system with few water molecules shows similar charge transport properties as fully solvated DNA does. This surprising effect is analyzed in detail by mapping the density functional theory-based electronic structure to a tight-binding Hamiltonian, allowing for an estimate of conductivity of various DNA sequences with snapshot-averaged Landauer's approach. The characteristics of DNA charge transport turn out to be determined by the nearest hydration shell(s), and the removal of bulk solvent has little effect on the transport.

Treatment of time‐dependent effects in molecular junctions

The current through molecular junctions can be influenced by various time‐dependent perturbations. Among these are external (laser) fields and fluctuating environments. Both types of external perturbations can lead to large and fast fluctuations of the site energies and electronic couplings involved in the charge transfer. In these cases, theoretical approaches have to be utilized which can accurately and efficiently determine the resulting strongly time‐dependent charge current. To this end we review different formalisms based on the decomposition of the spectral density and the Fermi function, i.e., a standard perturbative approach to second and fourth‐order, a hierarchical theory as well as a nonequilibrium Green's function formalism. The accuracy of the different approaches for the single resonant level model is numerically studied. In an additional test, the electronic level is driven in order to analyze the accuracy for time‐dependent scenarios. Moreover, the results for a multi‐site setup with strongly fluctuating levels are analyzed and compared to a snapshot‐averaged Landauer approach.

Modeling of Temperature and Field-Dependent Electron Mobility in a Single-Layer Graphene Sheet

In this paper, we address a physics-based analytical model of electric-field-dependent electron mobility (μ) in a single-layer graphene sheet using the formulation of Landauer and Mc Kelvey's carrier flux approach under finite temperature and quasi-ballistic regime. The energy-dependent, near-elastic scattering rate of in-plane and out-of-plane (flexural) phonons with the electrons are considered to estimate μ over a wide range of temperature. We also demonstrate the variation of μ with carrier concentration as well as the longitudinal electric field. We find that at high electric field , the mobility falls sharply, exhibiting the scattering between the electrons and flexural phonons. We also note here that under quasi-ballistic transport, the mobility tends to a constant value at low temperature, rather than in between T-2 and T-1 in strongly diffusive regime. Our analytical results agree well with the available experimental data, while the methodologies are put forward to estimate the other carrier-transmission-dependent transport properties.

Conductance features and quasi-bound states in a longitudinal asymmetric quantum point contact

We calculate, in the Büttiker–Landauer approach, the ballistic conductance in the quasi-one-dimensional structure with longitudinal asymmetric potential. We have shown that there is a plateau of conductance in the area typical for the “0.7 anomaly”. Using the computational method proposed, we have calculated the energies and lifetimes of the electron resonance states in the system considered. It is shown that in this case the conductance plateau is generated by resonances of Breit–Wigner type.

Landau levels, edge states, and magnetoconductance in GaAs/AlGaAs core-shell nanowires

Magnetic states of the electron gas confined in modulation-doped core-shell nanowires are calculated for a transverse field of arbitrary strength and orientation. Magnetoconductance is predicted within the Landauer approach. The modeling takes fully into account the radial material modulation, the prismatic symmetry, and the doping profile of realistic GaAs/AlGaAs devices within an envelope-function approach, and electron-electron interaction is included in a mean-field self-consistent approach. Calculations show that in the low free-carrier density regime, magnetic states can be described in terms of Landau levels and edge states, similar to planar two-dimensional electron gases in a Hall bar. However, at higher carrier density, the dominating electron-electron interaction leads to a strongly inhomogeneous localization at the prismatic heterointerface. This gives rise to a complex band dispersion, with local minima at finite values of the longitudinal wave vector, and a region of negative magnetoresistance. The predicted marked anisotropy of the magnetoconductance with field direction is a direct probe of the inhomogeneous electron gas localization of the conductive channel induced by the prismatic geometry.

A computational study of the thermoelectric performance of ultrathin Bi2Te3 films

The ballistic thermoelectric performance of ultrathin films of Bi2Te3, ranging in thickness from 1 to 6 quintuple layers, is analyzed using density functional theory combined with the Landauer approach. Our results show that the thinnest film, corresponding to a single quintuple layer, has an intrinsic advantage originating from the particular shape of its valence band, leading to a large power factor and figure-of-merit exceeding bulk Bi2Te3. The interaction between the top and bottom topological surface states is key. The thinnest film yields a six-fold increase in power factor compared to bulk.

Numerical analysis of electrical conduction in carbon nanostructures

Electric conduction in structures containing fullerenes was simulated. The electric current was calculated as a function of the bias voltage using the Landauer approach, thereby connecting charge transport with the transmission of electrons. To obtain the transmission matrix components, the supplementary problem of the electron scattering by the model potential was solved. Three types of structures were considered: a single fullerene molecule, a dimer of fullerenes, and a cluster of eight fullerenes. The current-voltage characteristics of these structures may have negative-conductance regions which make it possible to use them as active components of nanoelectronic devices.

Towards a dynamical approach to the calculation of the figure of merit of thermoelectric nanoscale devices

Research on thermoelectrical energy conversion, the reuse of waste heat produced by some mechanical or chemical processes to generate electricity, has recently gained some momentum. The calculation of the electronic parameters entering the figure of merit of this energy conversion, and therefore the discovery of efficient materials, is usually performed starting from the Landauer's approach to quantum transport coupled with the Onsager's linear response theory. As it is well known, that approach suffers of certain serious drawbacks. Here, we discuss alternative dynamical methods that can go beyond the validity of the Landauer's/Onsager's approach for electronic transport. They can be used to validate the predictions of the Landauer's/Onsager's approach and to investigate systems for which that approach has shown to be unsatisfactory.

A theoretical model for metal–graphene contact resistance using a DFT–NEGF method

The contact resistance (R(c)) between graphene and metal electrodes is of crucial importance for achieving potentially high performances for graphene devices. However, previous analytical models based on Landauer's approach have failed to include the Fermi velocity difference between the graphene under the metal and the pure graphene channel. Hereby we report a theoretical model to estimate the R(c) using density-functional theory and non-equilibrium Green's function methods. Our model not only presents a clear physical picture of the metal-graphene contacts, but also generates R(c) values which are in good agreement with the experimental results: 210 Ω μm for double-sided Pd contacts compared with 403 Ω μm for single-sided Pd contact.

Abnormal electronic transport in disordered four-terminal graphene nanodevice

In this paper, a numerical study of quantum transport in a disordered four-terminal graphene nanodevice is investigated based on the Landauer approach. The effects of impurity on transmission coefficient of the electron injected into the system are studied using tight-binding model. In this manner, we emphasize that when the disorder density is sufficiently large, the transmission coefficients and the current reduce due to multiscattering phenomenon. We have found that the perfectly conducting channel develops in four-terminal device in its zigzag edge if the range of impurity gets exponentially wider. The theoretical results obtained can be a base for the development in designing graphene nanodevice.

Time-Dependent View of Sequential Transport through Molecules with Rapidly Fluctuating Bridges

Molecules in junctions often fluctuate considerably, especially when subject to the influence of solvent molecules. These fluctuations in site energies and couplings can be sampled, for example, by using molecular dynamics simulations, and can lead to incoherent effects in charge transport. To this end, a popular snapshot-averaged Landauer approach is compared to a time-dependent Green's function scheme. Since sequential transport dominates in systems with rapidly varying bridges, schemes not taking the time order of conformations into account, such as the Landauer approach, are inappropriate.

DC Compact Model for SOI Tunnel Field-Effect Transistors

A physics-based dc compact model for SOI tunnel field-effect transistors (TFETs) has been developed in this paper utilizing Landauer approach. The important transistor electrical parameters, i.e., threshold voltage Vth, charge in the channel Q, gate capacitance CG, drain current IDS, subthreshold swing S, transconductance gm, and output conductance gDS, have been modeled. The model predicts the low subthreshold swing values (less than 60 mV/dec) observed in TFETs and shows a good match with the technology computer aided design (TCAD) results. Model validation was carried out using TCAD simulation for different TFET structures with abrupt junctions, i.e., 40-nm Si nTFET and pTFET, a 0.4-μm Si nTFET, and a 40-nm Ge nTFET. The compact model predictions are in good agreement with the TCAD simulation results.

Phonon engineering in nanostructures: Controlling interfacial thermal resistance in multilayer-graphene/dielectric heterojunctions

Using calculations from first principles and the Landauer approach for phonon transport, we study the Kapitza resistance in selected multilayer graphene/dielectric heterojunctions (hexagonal BN and wurtzite SiC) and demonstrate (i) the resistance variability (∼50−700×10−10 m2K/W) induced by vertical coupling, dimensionality, and atomistic structure of the system and (ii) the ability of understanding the intensity of the thermal transmittance in terms of the phonon distribution at the interface. Our results pave the way to the fundamental understanding of active phonon engineering by microscopic geometry design.

Probing Charge Transport in Oxidatively Damaged DNA Sequences under the Influence of Structural Fluctuations

We present a detailed study of the charge transport characteristics of double-stranded DNA oligomers including the oxidative damage 7,8-dihydro-8-oxoguanine (8-oxoG). The problem is treated by a hybrid methodology combining classical molecular dynamics simulations and semiempirical electronic structure calculations to formulate a coarse-grained charge transport model. The influence of solvent- and DNA-mediated structural fluctuations is encoded in the obtained time series of the electronic charge transfer parameters. Within the Landauer approach to charge transport, we perform a detailed analysis of the conductance and current time series obtained by sampling the electronic structure along the molecular dynamics trajectory, and find that the inclusion of 8-oxoG damages into the DNA sequence can induce a change in the electrical response of the system. However, solvent-induced fluctuations tend to mask the effect, so that a detection of such sequence modifications via electrical transport measurements in a liquid environment seems to be difficult to achieve.

Thermal conductivity of bulk and thin-film silicon: A Landauer approach

The question of what fraction of the total heat flow is transported by phonons with different mean-free-paths is addressed using a Landauer approach with a full dispersion description of phonons to evaluate the thermal conductivities of bulk and thin film silicon. For bulk Si, the results reproduce those of a recent molecular dynamic treatment showing that about 50% of the heat conduction is carried by phonons with a mean-free-path greater than about 1 μm. For the in-plane thermal conductivity of thin Si films, we find that about 50% of the heat is carried by phonons with mean-free-paths shorter than in the bulk. When the film thickness is smaller than ∼0.2 μm, 50% of the heat is carried by phonons with mean-free-paths longer than the film thickness. The cross-plane thermal conductivity of thin-films, where quasi-ballistic phonon transport becomes important, is also examined. For ballistic transport, the results reduce to the well-known Casimir limit [H. B. G. Casimir, Physica 5, 495–500 (1938)]. These results shed light on phonon transport in bulk and thin-film silicon and demonstrate that the Landauer approach provides a relatively simple but accurate technique to treat phonon transport from the ballistic to diffusive regimes.

Non-equilibrium Landauer transport model for Hawking radiation from a black hole

We propose that the Hawking radiation energy and entropy flow rates from a black hole can be viewed as a one-dimensional (1D), non-equilibrium Landauer transport process. Support for this viewpoint comes from previous calculations invoking conformal symmetry in the near-horizon region, which give radiation rates that are identical to those of a single 1D quantum channel connected to a thermal reservoir at the Hawking temperature. The Landauer approach shows in a direct way the particle statistics independence of the energy and entropy fluxes of a black hole radiating into vacuum, as well as one near thermal equilibrium with its environment. As an application of the Landauer approach, we show that Hawking radiation gives a net entropy production that is 50% larger than that obtained assuming standard 3D emission into vacuum.

Crossover between universality classes in a magnetically disordered metallic wire

In this paper, we present the numerical results of conduction in a disordered quasi-one-dimensional wire in the possible presence of magnetic impurities. Our analysis leads us to the study of universal properties in different conduction regimes, such as the localized and metallic ones. In particular, we analyze the crossover between universality classes occurring when the strength of magnetic disorder is increased. For this purpose, we use a numerical Landauer approach, and derive the scattering matrix of the wire from the electron's Green's function.

Effect of the band structure topology on the minimal conductivity for bilayer graphene with symmetry breaking

Using the Kubo formula we develop a general and simple expression for the minimal conductivity in systems described by a two by two Hamiltonian. As an application we derive an analytical expression for the minimal conductivity tensor of bilayer graphene as a function of a complex parameter $w$ related to recently proposed symmetry breaking mechanisms resulting from electron-electron interaction or strain applied to the sample. The number of Dirac points changes with varying parameter w, this directly affect the minimal conductivity. Our analytic expression is confirmed using an independent calculation based on Landauer approach and we find remarkably good agreement between the two methods. We demonstrate that the minimal conductivity is very sensitive to the change of the parameter $w$ and the orientation of the electrodes with respect to the sample. Our results show that the minimal conductivity is closely related to the topology of the low energy band structure.

An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach

The Landauer approach provides a conceptually simple way to calculate the intrinsic thermoelectric (TE) parameters of materials from the ballistic to the diffusive transport regime. The approach relies on the calculation of the number of propagating modes and the mean free path for each mode. The modes are calculated from the energy dispersion (E(k)) of the materials, which require heavy computation and are often restricted to energy relations on sparse momentum (k) grids. Here an efficient method to calculate the distribution of modes (DOM) from a given E(k) relationship is presented. The two main features of this algorithm are: (i) its ability to work on sparse dispersion data, and (ii) creation of an energy grid for the DOM that is almost independent of the dispersion data therefore allowing for efficient and fast calculation of TE parameters. The effect of K-grid sparsity on the compute time for DOM and on the sensitivity of the calculated TE results are provided. The algorithm calculates the TE parameters within 5% accuracy when the K-grid sparsity is increased up to 60% for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation is strongly influenced by the transverse K density (K perpendicular to transport direction) but is almost independent of the transport K density (along the transport direction). The DOM and TE results from the algorithm are bench-marked with, (i) analytical calculations for parabolic bands, and (ii) realistic electronic and phonon results for Bi2Te3.