Quantum Electron Transport Research Articles

First-principles study of the electronic structure and transport properties of armchair graphene nanoribbons with adsorbed super-halogen LiF2 and super-alkali Li3 clusters

The electronic structure and quantum transport properties of pristine armchair graphene nanoribbons (AGNRs) and AGNRs adsorbing super-halogen LiF2 and super-alkaline Li3 clusters (Li3/AGNRs/LiF2) were investigated using density functional theory and non-equilibrium Green's function calculations. It was found that LiF2 and Li3 clusters are stably adsorbed on the AGNRs, and the adsorption of Li3 and LiF2 endows AGNRs with the characteristics of n-type and p-type semiconductors, respectively. The Li3/AGNRs/LiF2 structure reduces the band gap and the turn-on voltage, and improves the transmission coefficient of the ANGRs device. This structure also exhibit the rectification characteristics of a pn junction with the forward bias current greater than the reverse bias current. This shows that adsorption of super-alkali and super-halogen clusters in different regions of AGNRs is a feasible approach for obtaining AGNRs with pn junction characteristics.

Coherent electron quantum transport in hbox {In}_{0.53}hbox {Ga}_{0.47}As/GaAs_{0.51}hbox {Sb}_{0.49} double barrier resonant tunnelling structures

In this work, we have modelled and computed the transport properties of the double-barrier InGaAs/GaAsSb structure from the resonant tunnelling point of view. Based on the classical Tsu-Esaki formula for the tunnelling current, we have calculated the current density-voltage characteristic of a Al-free type-II hbox {In}_{0.53}hbox {Ga}_{0.47}hbox {As/GaAs}_{0.51}hbox {Sb}_{0.49} structure at different cryogenic and elevated temperatures. The tunnelling coefficient has been calculated in the framework of effective mass approximation with nonparabolicity included, using the transfer matrix approach. A good qualitative agreement of the position of resonant current peaks with the existing experimental data was achieved. Our calculation shows a very high sensitivity of the tunnelling current peak on monolayer-scale layer structure fluctuation which strongly affects peak to valley ratio in the resonant tunnelling structure.

The effect of contact resistance on electrical conductivity in filled elastomer materials

This paper is focused on studying the charge transport between the particles in filled elastomer composites, and suggests a new modeling approach that would allow predicting the electrical conductivity of this material. Our approach is based on a combination of non-equilibrium Green's function (DFTB-NEGF) analysis for atomic-level electron quantum transport, and effective medium theory for calculating the said macroscopic transport properties of the material. The model can be used for predicting macroscopic material conductivity based on the information of material microstructure received by electron microscopy; explain changes of conductivity in deformed material and with temperature variation, and finally it can assist in finding the optimal material parameters such as particle shape, size distribution, and volume fraction to maximize the performance. The results of our modeling re-attest the known phenomenon that the electrical conductivity in percolated network is always dominated by direct particle-to-particle charge transport. We studied the electrical conductivity in a medium of high volume fraction of metallic particles dispersed in polymer matrix where thin polymer matrix layer covalently attached to the metallic particle contacts, and the thin polymer layer undergoes high local shear strain when hard particles slide along each other during deformation. These conditions allow the redistribution of electron density from metal to the polymer, and the conductance through normally insulating matrix can become comparable with the pinpoint conductance through the metal. These conditions can only occur in the place of direct particle contacts, therefore all criteria for percolated conductive network hold valid in this case. The integral conductance of thin “sandwiched” polymer layer strongly, but predictably depends on local contact geometry, mostly on particle surface curvature at place of contact. This allows predicting composite conductivity from the properties of each phase, loading, and particle shape and size distributions. Further, the conductance of polymer chain can significantly increase with the fluctuations of molecular shape caused by temperature and/or deformation. We infer that this effect is responsible for the major part of strain and temperature dependence of the conductivity that has been reported in the literature, although particle orientation and stacking at larger deformations may also contribute to the strain dependence.

Optimized Operation of Quantum-Dot Intermediate-Band Solar Cells Deduced from Electronic Transport Modeling

So far physics of quantum electronic transport has not tackled the problems raised by quantum dot intermediate-band solar cells. Our study shows that this physics imposes design rules for the inter-subband transition. We developed an analytical model that correctly treats, from a quantum point-of-view, the trade-off between the absorption, the recombination and the electronic transport occurring in this transition. Our results clearly indicate that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor. For that, we propose to assume the dot in a tunnel-shell whose main characteristics can be obtained by a simple analytical formula. Moreover, we show that in a realistic case, the energy transition only needs to be larger than 0.28 eV to obtain a quasi Fermi-level splitting. This quite small value designates the quantum dot solar cell as a serious candidate to be an efficient intermediate-band solar cell. This work gives a framework to design efficient inter-subband transitions and then opens new opportunities for quantum dot intermediate-band solar cells.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Quantized conductance in a one-dimensional ballistic oxide nanodevice

The electric-field effect control of two-dimensional electron gases (2-DEGs) has allowed nanoscale electron quantum transport to be explored in semiconductors. Structures based on transition metal oxides have electronic states that favour the emergence of novel quantum orders that are absent in conventional semiconductors and the 2-DEG formed at a LaAlO3/SrTiO3 interface—a structure in which superconductivity and spin–orbit coupling can coexist—is a promising platform to develop devices for spintronics and topological electronics. However, field-effect control of the properties of this interface at the nanoscale remains challenging. Here we show that a quantum point contact can be formed in a LaAlO3/SrTiO3 interface through electrostatic confinement of the 2-DEG using a split gate. Our device exhibits a quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels. Under a magnetic field, the direct observation of the Zeeman splitting between spin-polarized bands allows the determination of the Lande g-factor, whose value differs strongly from that of the free electrons. Through source–drain voltage measurements, we also performed a spectroscopic investigation of the 3d energy levels inside the quantum point contact. The LaAlO3/SrTiO3 quantum point contact could potentially be used as a spectrometer to probe Majorana states in an oxide 2-DEG. A quantum point contact formed in the two-dimensional electron gas of a LaAlO3/SrTiO3 interface exhibits quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels.

Implications of elevated carbon dioxide on the susceptibility of the globally invasive weed, Parthenium hysterophorus, to glyphosate herbicide.

The noxious annual herb, Parthenium hysterophorus L. (Asteraceae), is an invasive weed of global significance, threatening food security, biodiversity and human health. In South Africa, chemical control is frequently used to manage P. hysterophorus, however, concern surrounds increasing atmospheric CO2 levels, which may reduce the efficacy of glyphosate against the weed. Therefore, this study aimed to determine the susceptibility of P. hysterophorus to glyphosate (1L/ha: recommended) after being grown for five generations in Convirons under ambient (400 ppm) and elevated (600 and 800 ppm) CO2 . Glyphosate efficacy decreased with increasing CO2 , with mortalities of 100, 83 and 75% recorded at 400, 600 and 800 ppm, respectively. Parthenium hysterophorus experienced enhanced growth and reproduction under elevated CO2, however, glyphosate application was highly damaging, reducing the growth and flowering of plants across all CO2 treatments. Physiologically, glyphosate-treated plants, in all CO2 treatments, suffered severe declines of >90% in chlorophyll content, maximum quantum efficiency (F v /Fm ), photon absorption (ABS/RC), electron transport (ET 0 /RC) and performance index (PI ABS ), albeit at slower rates for plants grown under elevated CO2 . Low levels of recovery from glyphosate were documented only for plants grown under elevated CO2 and was attributed to their increased biomass. These results suggest that increasing CO2 levels may hinder chemical control efforts used against P. hysterophorus in the future, advocating for further investigation using multigenerational CO2 studies and the maintenance of effective spraying programs at present. © 2020 Society of Chemical Industry.

Coulomb blockade and hopping transport behaviors of donor-induced quantum dots in junctionless transistors**Project supported by the National Key R&D Program of China (Grant No. 2016YFA0200503) and the National Natural Science Foundation of China (Grant Nos. 11947115, 61376096, 61327813, and 61404126).

The ionized dopants, working as quantum dots in silicon nanowires, exhibit potential advantages for the development of atomic-scale transistors. We investigate single electron tunneling through a phosphorus dopant induced quantum dots array in heavily n-doped junctionless nanowire transistors. Several subpeaks splittings in current oscillations are clearly observed due to the coupling of the quantum dots at the temperature of 6 K. The transport behaviors change from resonance tunneling to hoping conduction with increased temperature. The charging energy of the phosphorus donors is approximately 12.8 meV. This work helps clear the basic mechanism of electron transport through donor-induced quantum dots and electron transport properties in the heavily doped nanowire through dopant engineering.

Seasonal acclimation of thermal performance in two species of reef-building corals

Thermal performance curves describe the relationship between temperature and the rate of biological processes. These relationships can vary among species and environments, allowing organisms to acclimatize to their local thermal regime. This study quantified the seasonal variation in the thermal performance of several coral and symbiont-dominated physiological traits for the thermally tolerant coral speciesPorites cylindricaand the thermally sensitive coral speciesAcropora valenciennesi. Photosynthesis rates, respiration rates, maximum photosystem II (PSII) quantum yield and electron transport rates were measured in winter and summer on coral fragments exposed to an acute temperature increase and decrease up to 5°C above and below the average seawater temperature in each season. Results showed that colonies ofA. valenciennesiacclimated primarily by shifting their optimal temperature to a higher temperature in summer, whereas colonies ofP. cylindricahad broader thermal breadth during summer. For symbionts within both species, performance was higher at all temperatures in summer, while the thermal optima and performance breadth remained unchanged. Despite these changes in thermal performance, the thermal optima of most traits did not match the ambient environmental temperature, but fell between the summer and winter temperatures. Overall, these results showed that both coral species were physiologically plastic in response to temperature change, but that there are constraints on the rate or capacity for acclimation that prevent a perfect match between the average temperature of the environment and the thermal optimum of the species.

KITE: high-performance accurate modelling of electronic structure and response functions of large molecules, disordered crystals and heterostructures.

We present KITE, a general purpose open-source tight-binding software for accurate real-space simulations of electronic structure and quantum transport properties of large-scale molecular and condensed systems with tens of billions of atomic orbitals (N ∼ 1010). KITE’s core is written in C++, with a versatile Python-based interface, and is fully optimized for shared memory multi-node CPU architectures, thus scalable, efficient and fast. At the core of KITE is a seamless spectral expansion of lattice Green’s functions, which enables large-scale calculations of generic target functions with uniform convergence and fine control over energy resolution. Several functionalities are demonstrated, ranging from simulations of local density of states and photo-emission spectroscopy of disordered materials to large-scale computations of optical conductivity tensors and real-space wave-packet propagation in the presence of magneto-static fields and spin–orbit coupling. On-the-fly calculations of real-space Green’s functions are carried out with an efficient domain decomposition technique, allowing KITE to achieve nearly ideal linear scaling in its multi-threading performance. Crystalline defects and disorder, including vacancies, adsorbates and charged impurity centres, can be easily set up with KITE’s intuitive interface, paving the way to user-friendly large-scale quantum simulations of equilibrium and non-equilibrium properties of molecules, disordered crystals and heterostructures subject to a variety of perturbations and external conditions.

Generalized quadrature for finite temperature Green’s function methods

In electronic structure and quantum transport calculations, many physical quantities are integrations over the electron energy, weighted by the Fermi–Dirac distribution function. Green’s function based approaches commonly circumvent the numerically difficult real energy integration by extending the integrand analytically into the complex energy plane, and using a Gaussian quadrature integration over a complex energy contour for zero temperature. For finite temperatures, a much slower convergent sum over the Matsubara frequencies is necessary. We present a generalized quadrature method that uses orthogonal polynomials on the manifold of Matsubara frequencies to enable rapid convergence. Both Gaussian quadrature integration and Matsubara frequency summation methods are shown to be limiting cases of the generalized method. Tests on an all-electron ab initio code and total energy calculation of interacting Anderson impurity model show convergence with a small number of energy mesh points.

Ground Based Hyperspectral Imaging to Characterize Canopy-Level Photosynthetic Activities

Improving plant photosynthesis provides the best possibility for increasing crop yield potential, which is considered a crucial effort for global food security. Chlorophyll fluorescence is an important indicator for the study of plant photosynthesis. Previous studies have intensively examined the use of spectrometer, airborne, and spaceborne spectral data to retrieve solar induced fluorescence (SIF) for estimating gross primary productivity and carbon fixation. None of the methods, however, had a spatial resolution and a scanning throughput suitable for applications at the canopy and sub-canopy levels, thereby limiting photosynthesis analysis for breeding programs and genetics/genomics studies. The goal of this study was to develop a hyperspectral imaging approach to characterize plant photosynthesis at the canopy level. An experimental field was planted with two cotton cultivars that received two different treatments (control and herbicide treated), with each cultivar-treatment combination having eight replicate 10 m plots. A ground mobile sensing system (GPhenoVision) was configured with a hyperspectral module consisting of a spectrometer and a hyperspectral camera that covered the spectral range from 400 to 1000 nm with a spectral sampling resolution of 2 nm. The system acquired downwelling irradiance spectra from the spectrometer and reflected radiance spectral images from the hyperspectral camera. On the day after 24 h of the DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) application, the system was used to conduct six data collection trials in the experiment field from 08:00 to 18:00 with an interval of two hours. A data processing pipeline was developed to measure SIF using the irradiance and radiance spectral data. Diurnal SIF measurements were used to estimate the effective quantum yield and electron transport rate, deriving rapid light curves (RLCs) to characterize photosynthetic efficiency at the group and plot levels. Experimental results showed that the effective quantum yields estimated by the developed method highly correlated with those measured by a pulse amplitude modulation (PAM) fluorometer. In addition, RLC characteristics calculated using the developed method showed similar statistical trends with those derived using the PAM data. Both the RLC and PAM data agreed with destructive growth analyses. This suggests that the developed method can be used as an effective tool for future breeding programs and genetics/genomics studies to characterize plant photosynthesis at the canopy level.

Giant Conductance Enhancement of Intramolecular Circuits through Interchannel Gating

Summary For neutral intramolecular circuits with two constitutionally identical branches, a maximum 4-fold increase in total conductance can be obtained according to constructive quantum interference (CQI). For charged intramolecular circuits, however, the strong electrostatic interactions entangle the quantum states of these two parallel pathways, thus introducing complicated transport behavior that warrants experimental investigation of the intramolecular circuit rules. Here, we report that a tetracationic cyclophane with parallel channels exhibits a 50-fold conductance enhancement compared with that of a single-channel control, an observation that supplements intramolecular circuit law in systems with strong Coulombic interactions. Flicker noise measurements and theoretical calculations show that strong electrostatic interactions between charged parallel channels—serving as the chemical gate to promote the effective conductance of each channel—and CQI boosts the total conductance of the two-channel circuit. The molecular design presented herein constitutes a proof-of-principle approach to charged intramolecular circuits that are desirable for quantum circuits and devices.

DFT coupled with NEGF study of the electronic properties and ballistic transport performances of 2D SbSiTe3.

Identifying novel 2D semiconductors with promising electronic properties and transport performances for the development of electronic and optoelectronic applications is of utmost importance. Here, we show a detailed study of the electronic properties and ballistic quantum transport performance of a new 2D semiconductor, SbSiTe3, based on density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism. Promisingly, monolayer SbSiTe3 owns an indirect band gap of 1.61 eV with a light electron effective mass (0.13m0) and an anisotropic hole effective mass (0.49m0 and 1.34m0). The ballistic performance simulations indicate that the 10 nm monolayer SbSiTe3 n- and p-MOSFETs display a steep subthreshold swing of about 80 mV dec-1 and a high on/off ratio (106), which indicate a good gate-controlling capability. As the channel length of SbSiTe3 decreases to 5 nm, its p-MOSFET also effectively suppresses the intra-band tunneling. Therefore, 2D SbSiTe3 is a potential semiconductor for future nanoelectronics.

Timescale separation solution of the Kadanoff-Baym equations for quantum transport in time-dependent fields

The interaction with time-dependent external fields, especially the interplay between time-dependent driving and quantum correlations, changes the familiar picture of electron transport through nanoscale systems. Although the exact solution of the problem of AC quantum transport of noninteracting electrons has been known for more than two decades, the treatment of correlated particles presents a significant theoretical challenge. In this paper, using the perturbative separation of fast electron tunnelling and slow driving time-scales, we developed a practical approach for time-dependent quantum transport with nonequilibrium Green's functions. The fast electronic dynamics is associated with relative time whilst the slow driving is related to the central time in the Green's functions. The ratio of characteristic electron tunneling time over the period of harmonic driving is used as a small parameter in the theory to obtain a convergent time-derivative expansions of the Green's functions. This enables the algebraic solution of the Kadanoff-Baym equations in Wigner space. Consequently, we produced analytical expressions for dynamical corrections to advanced, retarded, and lesser Green's functions, as well as an improved expression for AC electric current. The method developed is applicable to the general case of multi-channel electron transport through a correlated central region. The theory is applied to different transport scenarios: time-dependent transport through a driven single-resonant level is compared to exact results; and electron transport through a molecular junction described by the Holstein model with a time-oscillating voltage bias is also investigated.

Electric-Field-Induced Connectivity Switching in Single-Molecule Junctions.

SummaryThe manipulation of molecule-electrode interaction is essential for the fabrication of molecular devices and determines the connectivity from electrodes to molecular components. Although the connectivity of molecular devices could be controlled by molecular design to place anchor groups in different positions of molecule backbones, the reversible switching of such connectivities remains challenging. Here, we develop an electric-field-induced strategy to switch the connectivity of single-molecule junctions reversibly, leading to the manipulation of different connectivities in the same molecular backbone. Our results offer a new concept of single-molecule manipulation and provide a feasible strategy to regulate molecule-electrode interaction.

Quantum localization and electronic transport in covalently functionalized carbon nanotubes

Carbon nanotubes are of central importance for applications in nano-electronics thanks to their exceptional transport properties. They can be used as sensors, for example in biological applications, provided that they are functionalized to detect specific molecules. Due to their one-dimensional geometry the carbon nanotubes are very sensitive to the phenomenon of Anderson localization and it is therefore essential to know how the functionalization modifies their conduction properties and if they remain good conductors. Here we present a study of the quantum localization induced by functionalization in metallic single walled carbon nanotubes (SWCNT) with circumferences up to 15 nm. We consider resonant and non-resonant adsorbates that represent two types of covalently functionalized groups with strong and moderate scattering properties. The present study provides a detailed analysis of the localization behaviour and shows that the localization length can decrease down to 20–50 nm at concentrations of about 1 percent of adsorbates. On this basis we discuss the possible electronic transport mechanisms which can be either metallic like or insulating like with variable range hopping.

Stochastic Schrödinger Equations and Conditional States: A General Non-Markovian Quantum Electron Transport Simulator for THz Electronics

A prominent tool to study the dynamics of open quantum systems is the reduced density matrix. Yet, approaching open quantum systems by means of state vectors has well known computational advantages. In this respect, the physical meaning of the so-called conditional states in Markovian and non-Markovian scenarios has been a topic of recent debate in the construction of stochastic Schrödinger equations. We shed light on this discussion by acknowledging the Bohmian conditional wavefunction (linked to the corresponding Bohmian trajectory) as the proper mathematical object to represent, in terms of state vectors, an arbitrary subset of degrees of freedom. As an example of the practical utility of these states, we present a time-dependent quantum Monte Carlo algorithm to describe electron transport in open quantum systems under general (Markovian or non-Markovian) conditions. By making the most of trajectory-based and wavefunction methods, the resulting simulation technique extends to the quantum regime, the computational capabilities that the Monte Carlo solution of the Boltzmann transport equation offers for semi-classical electron devices.

Ohmic contact formation mechanisms of TiN film on 4H–SiC

The atomic structure, interfacial charge distribution, bonding nature, and interfacial electronic states of a 4H–SiC/TiN interface are systematically investigated to understand the Ohmic contact formation mechanisms of TiN to 4H–SiC. The experiment results clearly demonstrate that the well-arranged TiN (111)-oriented lattice planes are parallel to the (0001) SiC-oriented substrate, which is in line with the XRD results. In addition, the interface is coherent without any secondary phase layers, amorphous layers, or transition regions, which confirms the direct contact of TiN to SiC at the atomic scale, exhibiting a linear current–voltage relationship. Quantitatively, first-principle calculations reveal that the Schottky barrier height (SBH) is as low as 0.03 eV and that the band gap nearly vanishes at the interface, indicating an excellent Ohmic contact of TiN to 4H–SiC. Furthermore, the SBH is significantly reduced through the interfacial charge polarization effect and strong coupling of interfacial electronic states, enhancing the quantum electron transport. The present results provide insight into the complicated electronic effects of the Ohmic contact interface and indicate that TiN is a promising SiC Ohmic contact material for advanced next-generation power device applications.

Ideal Weyl semimetal induced by magnetic exchange

Weyl semimetals exhibit exceptional quantum electronic transport due to the presence of topologically-protected band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material specific. Following the initial discoveries there is now a need for better material realizations, ideally comprising a single pair of Weyl nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. Here we propose the layered intermetallic EuCd$_2$As$_2$ to be such a system. We show that Weyl nodes in EuCd$_2$As$_2$ are magnetically-induced via exchange coupling, emerging when the Eu spins are aligned by a small external magnetic field. The identification of EuCd$_2$As$_2$ as a model magnetic Weyl semimetal, evidenced here by ab initio calculations, photoemission spectroscopy, quantum oscillations and anomalous Hall transport measurements, opens the door to fundamental tests of Weyl physics.