Landauer Formalism Research Articles

An Explicit Quantum-Mechanical Compact Model for the <italic>I-V</italic> Characteristics of Cylindrical Nanowire MOSFETs

In this paper, we developed an analytical quantum-mechanical model for the I-V characteristics of cylindrical nanowire junctionless MOSFETs. Our compact model is based on the Landauer formalism to describe the ballistic current transport, as well as in a novel approach to solve the coupled Schrodinger and Poisson equations, in which the radial potential distribution is described as a perturbed harmonic oscillator potential. This approach reduces the complexity of the model derivation and provides an analytical expression for the eigenenergies within the quantum wire. As a consequence, our simplified model takes into account the relevant quantum effects arising from the cylindrical carried confinement but provides explicit and easy to handle expressions for the device characteristics. The proposed model is validated in comparison to available experimental results, with agreement equivalent to much more complex approaches.

Phonon transport effects in one-dimensional width-modulated graphene nanoribbons

We investigate the thermal conductance of one-dimensional periodic width-modulated graphene nanoribbons using lattice dynamics for the phonon spectrum and the Landauer formalism for phonon transport. We conduct a full investigation considering all relevant geometrical features, i.e., the various lengths and widths of the narrow and wide regions that form the channel. In all cases that we examine, we find that width-modulation suppresses the thermal conductance at values even up to ∼70% below those of the corresponding uniform narrow nanoribbon. We show that this can be explained by the fact that the phonon spectrum of the width-modulated channels acquires less dispersive bands with lower group velocities and several narrow bandgaps, which reduce the phonon transmission function significantly. The largest degradation in thermal conductance is determined by the geometry of the narrow regions. The geometry of the wider regions also influences thermal conductance, although modestly. Our results add to the ongoing efforts in understanding the details of phonon transport at the nanoscale, and our conclusions are generic and could also apply to other one-dimensional channel materials.

Electronic and thermal transport study of sinusoidally corrugated nanowires aiming to improve thermoelectric efficiency

Improvement of thermoelectric efficiency has been very challenging in the solid-state industry due to the interplay among transport coefficients which measure the efficiency. In this work, we modulate the geometry of nanowires to interrupt thermal transport with causing only a minimal impact on electronic transport properties, thereby maximizing the thermoelectric power generation. As it is essential to scrutinize comprehensively both electronic and thermal transport behaviors for nano-scale thermoelectric devices, we investigate the Seebeck coefficient, the electrical conductance, and the thermal conductivity of sinusoidally corrugated silicon nanowires and eventually look into an enhancement of the thermoelectric figure-of-merit from the modulated nanowires over typical straight nanowires. A loss in the electronic transport coefficient is calculated with the recursive Green function along with the Landauer formalism, and the thermal transport is simulated with the molecular dynamics. In contrast to a small influence on the thermopower and the electrical conductance of the geometry-modulated nanowires, a large reduction of the thermal conductivity yields an enhancement of the efficiency by 10% to 35% from the typical nanowires. We find that this approach can be easily extended to various structures and materials as we consider the geometrical modulation as a sole source of perturbation to the system.

Spin-polarized transport utilizing d0 ferromagnetism: an ab initio study of CaC/MgS/CaC (0 0 1) heterojunctions

The electronic structure and magnetic properties of rocksalt-type CaC compound are studied by means of first-principles calculations. The B1-type calcium monocarbide is a metastable, nearly half-metallic ferromagnetic compound, with a magnetic moment of 1.81 μB/f.u. and a negative Fermi level spin-polarization of 50%. The estimated Curie temperature exceeds the room temperature. The electronic, magnetic and spin-polarized transport properties of CaC/MgS/CaC (0 0 1) heterostructures are investigated by using a first-principles Green’s function technique for layered structures in conjunction with the Kubo–Landauer formalism. The magnetism of CaC (0 0 1) electrodes is robust while the Fermi level spin-polarization of interfacial CaC layers at CaC/MgS (0 0 1) interfaces increases over the bulk value. The localized states formed at CaC/MgS (0 0 1) interfaces strongly enhance the minority-spin ferromagnetic state conductance. The current spin-polarization reaches values as high as 99%. Very large magnetoresistance ratios, above 104%, are in evidence. The highly magnetoresistive effect observed for CaC/MgS/CaC (0 0 1) heterojunctions together with the robust ferromagnetism of CaC (0 0 1) electrodes make this system attractive in the context of spin-electronics.

Effect of Molecular Rotation on Charge Transport Phenomena

The study of electron transport properties of molecular systems could be explained on the basis of the Landauer formalism. Unfortunately, due to the complexity of the experimental setup, most of these measurements have no control over the details of the electrode geometry, rotation of molecules, variation in angle of contacts, effect of fano resonances associated with side groups attached to rigid backbones, which results in a spectrum of IV-characteristics. Theoretical models can therefore help to understand and helps to develop new applications such as molecular sensors, etc. Thus we used simulation methods that generate the required structural ensemble, which is then analyzed with Green’s function methods to characterize the electronic transport properties. In present work we had discussed applications of this approach to understand the conductance in molecular system in the direction of controlling electron transport through molecules and studied the effect of rotation of sandwiched molecule.

Spin-Polarized Transport Through a Zigzag-Edge Graphene Flake Embedded between Two Armchair Nanoribbon Electrodes

We study the coherent spin-polarized transport through a zigzag-edge graphene flake (ZGF), using Hubbard model in the nearest neighbor approximation within the framework of the Green function’s technique and Landauer formalism. The system considered consists of electrode/(ZGF)/electrode, in which the electrodes are chosen to be armchair nanoribbons. The study was performed for two types of electrodes i.e., armchair-edge graphene nanoribbons (AGNRs) and armchair-edge boron-nitride nanoribbons (ABNNRs). Our calculations of the electronic and transport properties of these systems show that both systems posses spin filtering properties, while the spin filtering is higher in the full graphene system than the second system with boron nitride nanoribbon electrodes. The spin filtering in these systems is due to the interaction of the spin of the carriers with the local zigzag-edge magnetism in ZGF. However, the reduction of the spin filtering property in the case of the system with boron-nitride nanoribbon electrodes could be due to the decrease in the effective spin interaction of boron atoms in the contact sites and magnetization of the zigzag-edge of the ZGF.

Physical properties of Zener tunnelling nano‐devices in graphene

By considering the direction of charge carriers and the conservation of probablity current the transmission properties of graphene Zener tunnelling nano‐devices were obtained. The scattering properties were then used with an adaptation of the Landauer formalism to calculate an analytical expression for current and conductance. The numerical results of the IV characteristics were then briefly discussed for the graphene step and Zener barrier. A comparison between the theoretical model and experimental results shows the similarities of graphene nanoribbons and infinite sheet graphene.

Landauer, Kubo, and microcanonical approaches to quantum transport and noise: A comparison and implications for cold-atom dynamics

We compare the Landauer, Kubo, and microcanonical [J. Phys.: Condens. Matter 16, 8025 (2005)] approaches to quantum transport for the average current, the entanglement entropy, and the semiclassical full-counting statistics (FCS). Our focus is on the applicability of these approaches to isolated quantum systems such as ultracold atoms in engineered optical potentials. For two lattices connected by a junction, we find that the current and particle number fluctuations from the microcanonical approach compare well with the values predicted by the Landauer formalism and FCS assuming a binomial distribution. However, we demonstrate that well-defined reservoirs (i.e., particles in Fermi-Dirac distributions) are not present for a substantial duration of the quasi-steady state. Thus, on the one hand, the Landauer assumption of reservoirs and/or inelastic effects is not necessary for establishing a quasi-steady state. Maintaining such a state indefinitely requires an infinite system, and in this limit well-defined Fermi-Dirac distributions can occur. On the other hand, as we show, the existence of a finite speed of particle propagation preserves the quasi-steady state irrespective of the existence of well-defined reservoirs. This indicates that global observables in finite systems may be substantially different from those predicted by an uncritical application of the Landauer formalism, with its underlying thermodynamic limit. Therefore, the microcanonical formalism which is designed for closed, finite-size quantum systems seems more suitable for studying particle dynamics in ultracold atoms. Our results highlight both the connection and differences with more traditional approaches to calculating transport properties in condensed matter systems, and will help guide the way to their simulations in cold-atom systems.

Electron propagator theory approach to ab initio calculations of electron transfer rate and molecular conductance

The ab initio theoretical approach to calculations of molecular electron transfer rate and molecular junction conductance is presented. The implemented approach is founded on Landauer formalism coupled to electron propagator theory (Green-Keldysh functions formalism) of solving the Dyson equation for molecular junction. The electron transfer rate calculation algorithm implementing quantum chemistry computational software output is proposed. The conductance through a single molecular orbital is found to be equal to a quantum of conductance, 2e2/h. The existence of the maximal value of electron transfer rate through a single molecular orbital is proposed, (π2/4h)ΓLΓRF, whose order-of-magnitude estimate is 1011–1013s−1.

Thermal transport properties of metal/MoS2 interfaces from first principles

Thermal transport properties at the metal/MoS2 interfaces are analyzed by using an atomistic phonon transport model based on the Landauer formalism and first-principles calculations. The considered structures include chemisorbed Sc(0001)/MoS2 and Ru(0001)/MoS2, physisorbed Au(111)/MoS2, as well as Pd(111)/MoS2 with intermediate characteristics. Calculated results illustrate a distinctive dependence of thermal transfer on the details of interfacial microstructures. More specifically, the chemisorbed case with a stronger bonding exhibits a generally smaller interfacial thermal resistance than the physisorbed. Comparison between metal/MoS2 and metal/graphene systems suggests that metal/MoS2 is significantly more resistive. Further examination of lattice dynamics identifies the presence of multiple distinct atomic planes and bonding patterns at the interface as the key origins of the observed large thermal resistance.

Electron transport through metallic single wall carbon nanotubes with adsorbed NO2 and NH3 molecules: A first-principles study

In this Letter, variations in the transport characteristics of metallic single wall carbon nanotubes (SWCNTs) due to the absorption of NO2 and NH3 molecules on the surface are investigated using the Landauer formalism combined with the non-equilibrium Green’s function techniques and ab initio electronic structure obtained using density functional theory (DFT). The electronic structure, charge distribution, transmission spectrum and I–V characteristics of the SWCNT are significantly changed on adsorption which is reflected in the conductance and transport characteristics of the SWCNT. Thus, we conclude that these systems can be used in CNT based NO2 and NH3 gas sensors.

Non-perturbative study of impurity effects on the Kubo conductivity in macroscopic periodic and quasiperiodic lattices

In this paper, we analyze the effects of site and bond impurities on the electrical conductance of periodic and quasiperiodic systems with macroscopic length by means of a real-space renormalization plus a convolution method developed for the Kubo–Greenwood formula. All analyzed systems are connected to semi-infinite periodic leads. Analytical and numerical conductivity spectra are obtained for one and two site impurities in a periodic chain, where the separation between impurities determines the number of maximums in the spectra. We also found transparent states at the zero chemical potential in Fibonacci chains of every three generations with bond impurities, whose existence was confirmed by an analytical analysis within the Landauer formalism. For many impurities, the spectral average of the conductivity versus the system length reveals a power-law behavior, when the distance between impurities follows the Fibonacci sequence. Finally, we present an analysis of the conductance spectra of segmented periodic and Fibonacci chains and nanowires.

Ballistic transport properties in pristine/doped/pristine graphene junctions

We investigate the ballistic electron transport in a monolayer graphene with configurational averaged impurities, located between two clean graphene leads. It is shown that the electron transmission are strongly dependent on the concentration of impurities and the incident energy. In turn, the conductance computed using the Landauer formalism shows a similar behavior to those found in experimental works as a function of the applied voltage for different concentrations of impurities in the limit of low temperatures. In the limit of zero bias voltage, the conductance shows a minimum value which reduces to zero for high concentration of impurities which disentangle graphene sublattices. These results can be very helpful for exploring the tunneling mechanism of electrons through doped thermodynamically stable graphene.

Quantum conductance of zigzag graphene oxide nanoribbons

The electronic properties of zigzag graphene oxide nanoribbons (ZGOR) are presented. The results show interesting behaviors which are considerably different from the properties of the perfect graphene nanoribbons (GNRs). The theoretical methods include a Huckel-tight binding approach, a Green's function methodology, and the Landauer formalism. The presence of oxygen on the edge results in band bending, a noticeable change in density of states and thus the conductance. Consequently, the occupation in the valence bands increase for the next neighboring carbon atom in the unit cell. Conductance drops in both the conduction and valence band regions are due to the reduction of allowed k modes resulting from band bending. The asymmetry of the energy band structure of the ZGOR is due to the energy differences of the atoms. The inclusion of a foreign atom's orbital energies changes the dispersion relation of the eigenvalues in energy space. These novel characteristics are important and valuable in the study of quantum transport of GNRs.

Aromatic molecules as spintronic devices

In this paper, we study the spin-dependent electron transport through aromatic molecular chains attached to two semi-infinite leads. We model this system taking into account different geometrical configurations which are all characterized by a tight binding Hamiltonian. Based on the Green's function approach with a Landauer formalism, we find spin-dependent transport in short aromatic molecules by applying external magnetic fields. Additionally, we find that the magnetoresistance of aromatic molecules can reach different values, which are dependent on the variations in the applied magnetic field, length of the molecules, and the interactions between the contacts and the aromatic molecule.

Numerical study on electronic properties of a molecular wire based on BC3 zigzag nanotube

Electronic properties of a molecular wire based on a single-walled zigzag BC3 nanotube (n, 0) are investigated using tight-binding model combined with Green’s function theory and Landauer formalism. Our results indicate that these nanotubes have semiconducting properties. By increasing n, band gap of single-walled BC3 nanotubes are inversely proportional to nanotube diameter. On the other hand, a semiconductor–metal transition is demonstrated at a specific diameter in (n, 0) BC3 (if n is not in multiple of 3). Furthermore, the obtained results indicate that transmission probability mimics the behavior of density of states at all energy ranges. The current–voltage curves have nonlinear behavior and staircase structure. There is also a voltage drop in the first step on I–V characteristics with an increase in the diameter of the nanotube.

Probing the effect of electric field in 9,10-dimethoxy-2,6-bis(2-p-tolylethynyl)anthracene molecular nanowire using quantum chemical and charge density analysis

The effect of electric field (EF) in a newly designed molecular nanowire 9,10-dimethoxy-2,6-bis(2-p-tolylethynyl)anthracene has been analysed theoretically from the structural and electronic charge transport properties using quantum chemical and charge density calculations. The applied EF (0–0.36 VÅ− 1) alters the molecular conformation, charge density distribution, electrostatic properties and the electronic energy levels of the molecule. Furthermore, the applied EF decreases the highest occupied molecular orbital–lowest unoccupied molecular orbital gap significantly from 1.775 to 0.258 eV and it also induces polarisation in the molecule, which leads to increase the dipole moment of the molecule. The electrostatic potential for various levels of applied EF reveals the charge-accumulated regions of the molecule. The I–V characteristics of the molecule have been studied against various applied fields using Landauer formalism.

Quantized Thermal Conductance of Nanowires at Room Temperature Due to Zenneck Surface-Phonon Polaritons

Based on the Landauer formalism, we demonstrate that the thermal conductance due to the propagation of Zenneck surface-phonon polaritons along a polar nanowire is independent of the material characteristics and is given by π2kB2T/3h. The giant propagation length of these energy carriers establishes that this quantization holds not only for a temperature much smaller than 1K, as is the case for electrons and phonons, but also for temperatures comparable to room temperature, which can significantly facilitate its observation and application in the thermal management of nanoscale electronics and photonics.

Edge proximity-induced magnetoresistance and spin polarization in ferromagnetic gated bilayer graphene nanoribbon

Coherent spin-dependent transport through a junction containing normal/ferromagnetic/normal bilayer graphene nanoribbon with zigzag edges is investigated by using Landauer formalism. In a more realistic set-up, the exchange field is induced by two ferromagnetic insulator strips deposited on the ribbon edges while a perpendicular electric field is applied by the top gated electrodes. Our results show that, for antiparallel configuration, a band gap is opened giving rise to a semiconducting behavior, while for parallel configuration, the band structure has no band gap. As a result, a giant magnetoresistance is achievable by changing the alignment of induced magnetization. Application of a perpendicular electric field on the parallel configuration results in a spin field-effect transistor where a fully spin polarization occurs around the Dirac point. To compare our results with the one for monolayer graphene, we demonstrate that the reflection symmetry and so the parity conservation fail in bilayer graphene nanoribbons with the zigzag edges.

Conductance of Tailored Molecular Segments: A Rudimentary Assessment by Landauer Formulation

One of the strengths of molecular electronics is the synthetic ability of tuning the electric properties by the derivatization and reshaping of the functional moieties. However, after the quantitative measurements of single-molecule resistance became available, it was soon apparent that the assumption of negligible influence of the headgroup-electrode contact on the molecular resistance was oversimplified. Due to the measurement scheme of the metal--molecule-metal configuration, the contact resistance is always involved in the reported values. Consequently the electrical behavior of the tailored molecular moiety can only be conceptually inferred by the tunneling decay constant (βn in Rmeasured = R(n=0)e(βnN), where N is the number of repeated units), available only for compounds with a homologous series. This limitation hampers the exploration of novel structures for molecular devices. Based on the Landauer formula, we propose that the single-molecule resistance of the molecular backbones can be extracted. This simplified evaluation scheme is cross-examined by electrode materials of Au, Pd, and Pt and by anchoring groups of thiol (-SH), nitrile (-CN), and isothiocyanate (-NCS). The resistance values of molecular backbones for polymethylenes (n = 4, 6, 8, and 10) and phenyl (-C6H4-) moieties are found independent of the anchoring groups and electrode materials. The finding justifies the proposed approach that the resistance of functional moieties can be quantitatively evaluated from the measured values even for compounds without repeated units.