Quantum Electron Transport Research Articles

Design of graphene waveguides: Effect of edge orientation and waveguide configuration

Electron transport in a graphene quantum well can be analogous to photon transmission in an optical fiber. In this work, we present a detailed theoretical analysis to study the transport characteristics of graphene waveguides under the influence of different edge orientations. Non-equilibrium Green's function approach in combination with tight-binding Hamiltonian has been utilized to investigate the conductance properties of straight armchair and zigzag oriented graphene waveguides. Conductance plateaus at integer steps of $4e^2/h$ have been observed in both orientations while the zigzag oriented waveguides present a wider first quantized plateau compared to that in the armchair oriented ones. Using various geometric and physical parameters, including side-barrier and waveguide width, and the metallic properties of terminals, we investigate the conductance profile of waveguides. In addition to the observation of valley-symmetry in both edge orientations, this article explores the critical influence of drain contacts on waveguide conductance. Furthermore, we extended our transport study to three different highly bent waveguide configurations, such as U-shape, L-shape and split-shape waveguides, in order to explore their applications in graphene-based ballistic integrated circuit devices. In the end, we also calculated the conductance of larger graphene waveguides using the scalable tight-binding model, in order to compare the results obtained from the original model.

Solar UV irradiation effects on photosynthetic performance, biochemical markers, and gene expression in highbush blueberry (Vaccinium corymbosum L.) cultivars

Solar UV irradiation allows life on Earth, but becomes harmful at high levels due to UV-B and UV-A irradiation, generating negative effects for all organisms, including plants. We determined the effects of solar UV irradiation on photosynthetic performance, biochemical markers, and gene expression in five-year-old plants of two highbush blueberry (Vaccinium corymbosum L.) cultivars, Legacy and Bluegold. Plants growing under field conditions were subjected to three solar UV irradiation filter treatments: UV-transmitting filter (+UV-A/B), UV-B blocking filter (-UV-B), and UV-A/B blocking filter (-UV-A/B) during two seasons. Net photosynthesis did not change among solar radiation filter treatments in either cultivar. Bluegold showed a significant reduction in stomatal conductance (gs) during both seasons under +UV-A/B compared to -UV-A/B (control treatment); while effective quantum yield (ΦPSII) and electron transport rate (ETR) did not change in Legacy, Bluegold showed a reduction in ΦPSII and ETR during the 2014 season. The highest significant increase (P ≤ 0.05) in total phenol concentration was observed in Bluegold (40%) under +UV-A/B in the 2013 season compared to Legacy. A similar tendency in flavonoids and anthocyanin was observed but in both seasons. In conclusion, an interaction genotype-environment was observed, where Legacy and Bluegold cultivars were differentially affected by solar UV irradiation and seasons in field conditions. Although Bluegold maintains CO2 assimilation, activating antioxidant defense, gene expression (MYBPA1, CHS, and F3’H) and metabolites (flavonoid and anthocyanins) to counteract +UV-A/B treatment, this cultivar was no able to reduce oxidative stress and to recover photochemical efficiency of PSII compared to Legacy. Our findings contribute to the understanding of blueberry responses to solar UV irradiation at physiological, molecular, and metabolite levels under field conditions.

Spin coherent quantum transport of electrons between defects in diamond

Abstract The nitrogen-vacancy (NV) color center in diamond has rapidly emerged as an important solid-state system for quantum information processing. Whereas individual spin registers have been used to implement small-scale diamond quantum computing, the realization of a large-scale device requires the development of an on-chip quantum bus for transporting information between distant qubits. Here, we propose a method for coherent quantum transport of an electron and its spin state between distant NV centers. Transport is achieved by the implementation of spatial stimulated adiabatic Raman passage through the optical control of the NV center charge states and the confined conduction states of a diamond nanostructure. Our models show that, for two NV centers in a diamond nanowire, high-fidelity transport can be achieved over distances of order hundreds of nanometers in timescales of order hundreds of nanoseconds. Spatial adiabatic passage is therefore a promising option for realizing an on-chip spin quantum bus.

Quantum dynamics of single-photon detection using functionalized quantum transport electronic channels

Single photon detectors have historically consisted of macroscopic-sized materials but recent experimental and theoretical progress suggests new approaches based on nanoscale and molecular electronics. Here we present a theoretical study of photodetection in a system composed of a quantum electronic transport channel functionalized by a photon absorber. Notably, the photon field, absorption process, transduction mechanism, and measurement process are all treated as part of one fully-coupled quantum system, with explicit interactions. Using non-equilibrium, time-dependent quantum transport simulations, we reveal the unique temporal signatures of the single photon detection process, and show that the system can be described using optical Bloch equations, with a new non-linearity as a consequence of time-dependent detuning caused by the backaction from the transport channel via the dynamical Stark effect. We compute the photodetector signal-to-noise ratio and demonstrate that single photon detection at high count rate is possible for realistic parameters by exploiting a novel non-equilibrium control of backaction.

Interfacial charge transfers and interactions drive rectifying and negative differential resistance behaviors in InAs/graphene van der Waals heterostructure

Vertical heterostructures of two-dimensional (2D) materials are excellent candidates for designing next-generation electronic devices with superior performance. By using both ab initio electronic calculations and quantum transport simulations, we investigated the electronic and transport properties of InAs/graphene heterostructure. The results reveal that electrons and holes accumulate at different layers after the adsorption of InAs layer, forming the built-in electronic field at the interface. The electrostatic potential energy of InAs layer is higher than that of graphene, and it favors more electrons transferring from InAs to graphene layer. Comparing the comment methods by introducing impurity and carriers' injection, rectifying and negative differential resistance behaviors can also be realized by the combined effects of electron-hole distribution, interfacial hybridization, and contact barrier in InAs/graphene heterostructure device. It shows that the rectifying ration gradually increases with bias voltage, and the negative differential resistance effect happens at either positive or at negative bias voltage regions. Electrostatic potential distribution and contact barrier play important roles in determining transport properties. Increasing interfacial hybridization is helpful for transmission enhancement in the weak interlayer interaction van der Waals heterostructure.

Aluminum stress differentially affects physiological performance and metabolic compounds in cultivars of highbush blueberry

Aluminum (Al) toxicity is one of the major factors that limit the growth and production of crops in acid soils. Highbush blueberry (Vaccinium corymbosum L.) cultivars differing in resistance to Al toxicity regarding root growth and photosynthetic performance were used. In this study, we compared the physiological and metabolic strategies to cope with Al toxicity among the highbush blueberry cultivars [two new ones (Camellia and Cargo) and three established ones (Brigitta (Al-resistant), Star and Duke)]. Aluminum concentration in roots and leaves increased in all cultivars after 24 and 48 h of exposure to Al, but less so in roots of cultivar Camellia and leaves of cultivar Cargo. These two cultivars displayed minor effects of Al exposure in terms of photosynthetic activity in comparison with the established cultivars. Furthermore, Cargo did not vary fluorescence parameters, whereas Camellia exhibited a decrease in effective quantum yield (ΦPSII) and electron transport rate (ETR) and a change in non-photochemical quenching (NPQ) and maximum quantum yield (Fv/Fm) under Al after 48 h. The Al treatment increased total phenols in leaves of Brigitta, Cargo, and Camellia, whereas antioxidant activity increased in Star and Cargo after 48 h. Aluminum exposure decreased malate concentration in roots of all cultivars, but no change was noted in fumarate concentration. The antioxidant activity correlated with photosynthetic performance and the total phenol concentration in the leaves of new cultivars exposed to Al, suggesting enhanced resistance in the short-term experiment. The principal component analysis separated the new from the established cultivars. In conclusion, the new cultivars appear to be more Al-resistant than the established ones, with Star being most Al-sensitive. Regarding the Al-resistance mechanisms of the new cultivars, it is suggested that Camellia could have a root Al-exclusion mechanism under Al toxicity. This mechanism could be explained by low Al concentration in roots, suggesting that this cultivar could exude organic acid, allowing to chelate Al in the rhizosphere. Nonetheless, further researches are needed to confirm this assumption.

Transport gap engineering in zigzag graphene nanoribbons through topological design of deposited oxygen atoms: a new way to control the quantum transport in graphene-like materials

In this paper, we propose a new look at the problem of creating 2D graphene-like materials with adjustable zero conductance zones (transport gaps) in the profile of the electron transmission function T(E) through oxidation. With this purpose, a numerical study of quantum electron transport in oxidized zigzag graphene nanoribbons (ZGNRs) with a chaotic and ordered arrangement of oxygen (O) atoms is carried out. Regularities of the influence of chaotically arranged O-atoms on the function T(E) depending on their concentration and topology are analyzed. The extent of the influence of distance between O-atoms and their concentration on the profile of the function T(E) is determined. For the first time, it is shown that the chaotic deposition of O-atoms ‘switched off’ the localized edge states from the electron transport process of narrow 6-ZGNR, 7-ZGNR and 8-ZGNR nanoribbons even at a concentration of O-atoms of ∼0.2%, and the ordered linear arrangement of O-atoms retains them even at a concentration of O-atoms of 0.411%, regardless of the nanoribbon width. A method for forming and controlling the transport gaps in the profile of the function T(E) of 2D graphene-like materials without significantly deteriorating its electro-conductive properties is proposed based on obtained results.

Quantum Electron Transport in Ohmic Edge Contacts between Two-Dimensional Materials

The metal-semiconductor contact is a major factor limiting the shrinking of transistor dimension to further increase device performance. In-plane edge contacts have the potential to achieve lower contact resistance due to stronger orbital hybridization compared to conventional top contacts. However, a quantitative understanding of the electron transport properties in the edge contact is still lacking. In this work, we present full-band atomistic quantum transport simulations of the graphene/MoS$_2$ edge contact. By using a Wannier function basis to accurately describe the electronic bands, together with a full self-consistent solution of the electrostatics, we are able to efficiently model device structures on a micron scale, but with atomic level accuracy. We find that the potential barrier created by trapped charges decays fast with distance away from the interface, and is thus thin enough to enable efficient injection of electrons. This results in Ohmic behavior in its I-V characteristics, which agrees with experiments. Our results demonstrate the role played by trapped charges in the formation of a Schottky barrier, and how one can reduce the Schottky barrier height (SBH) by adjusting the relevant parameters of the edge contact system. Our framework can be extended conveniently to incorporate more general nanostructure geometries. For example, a full 3D solution of the electrostatics will also lead to better modeling of the electrical potential. Furthermore, better ab-initio calculations can be conveniently added to our methods to further improve their accuracy.

Interfacial Properties of Monolayer Antimonene Devices

As an emerging two-dimensional (2D) semiconductor, monolayer antimonene---think graphene, but made of Sb instead of C, and not so flat---is of great potential for (opto)electronic devices. High-quality electrode contact is critical to developing these devices. In this article, calculations of electronic structure and quantum transport are used to study the interfacial properties of antimonene-metal contacts. This comprehensive investigation provides a theoretical basis for selecting favorable electrodes in 2D antimonene devices. The optimal $n$-type electrode is 2D Hf${}_{2}$N(OH)${}_{2}$, while the optimal $p$-type electrode is graphene-Pt, both of which provide Ohmic contact.

One-Dimensional Transport of Ultracold Bosons

Advances in cooling and trapping of atoms have enabled unprecedented experimental control of many-body quantum systems. This led to the observation of numerous quantum phenomena, important for fundamental science, indispensable for high-precision simulations of condensed-matter systems and promising for technological applications. However, transport measurements in neutral quantum gases are still in their infancy in contrast to the central role they play in electronics. In these lectures, after reviewing nascent experiments on quantum fermionic transport, I will focus on our theoretical prediction sand the possibility of experimental observations of qualitatively new phenomena in transport of ultracold bosons which do not have a direct counterpart in quantum electronic transport in condensed matter systems. The description of this transport is based on the Luttinger liquid (LL) theory. So in the first part of the lectures I will introduce main concepts of the LL based on the functional bosonisation approach.

An order N numerical method to efficiently calculate the transport properties of large systems: An algorithm optimized for sparse linear solvers

We present a self-contained description of the wave-function matching (WFM) method to calculate electronic quantum transport properties of nanostructures using the Landauer-Büttiker approach. The method is based on a partition of the system between a central region (“conductor”) containing NS sites and an asymptotic region (“leads”) characterized by NP open channels. The two subsystems are linearly coupled and solved simultaneously using an efficient sparse linear solver. Invoking the sparsity of the Hamiltonian matrix representation of the central region, we show that the number of operations required by the WFM method in conductance calculations scales linearly with the number of sites, more precisely with ∼NS×NP for large NS, faster than previously claimed.

930 kA/cm2 peak tunneling current density in GaN/AlN resonant tunneling diodes grown on MOCVD GaN-on-sapphire template

We report on the design and fabrication of ultrahigh current density GaN/AlN double barrier resonant tunneling diodes grown via rf-plasma assisted molecular-beam epitaxy. The device structure was grown on a metal-organic chemical vapor deposition GaN-on-sapphire template. The devices displayed repeatable room temperature negative differential resistance with peak tunneling current densities (Jp) between 637 and 930 kA/cm2. Analysis of temperature dependent measurements revealed the presence of severe self-heating effects, which allow strong phonon scattering that deteriorates the electron quantum transport. Finally, a qualitative comparison to the same structure grown on a low dislocation density freestanding GaN substrate has shown that sapphire-based templates are a feasible alternative.

Magnetic structure and magnetic transport properties of armchair arsenene nanoribbons

Inducing magnetism for arsenene is important for developing high-performance spintronic devices. Here, we investigate the electronic and magnetic properties of armchair arsenene nanoribbons doped with transition metal Mn atom by the ab initio electronic structure calculations and quantum transport simulations. It is found that such structures are stable by calculated formation energy and molecular dynamics simulation. Magnetic states can be obtained by doping Mn atom in such ribbons at different doping-positions with the distance from the Mn atom to the ribbon edges. AAsNR-Mn experiences a transition from a half-semiconductor to a half-metal by applying an external electric field. Moreover, it possesses an outstanding magnetic device nature, such as perfect spin filtering effect (100%), and a spin-valve feature with giant magnetoresistance (GMR) approaching 106%. Our calculation indicates AAsNRs-Mn has a promising application for multifunctional spintronic devices.

Initial simulated acid rain impacts reactive oxygen species metabolism and photosynthetic abilities in Cinnamonum camphora undergoing high temperature

Cinnamomum camphora has great economic value for extensive applications. It is widely planted in the south of China, and is often exposed to acid rain following high temperature (HT). To study the effects of acid rain on C. camphora undergoing HT, the reactive oxygen species metabolism and photosynthetic abilities in the treatment with initial simulated acid rain (SiAR) and following HT (40 °C) were investigated. After SiAR pretreatment, the levels of O2− and H2O2 and lipid peroxidation in C. camphora increased under HT stress compared with normal temperature (28 °C). High temperature after SiAR pretreatment promoted the inactivation of enzymatic antioxidants, including superoxide dismutase, peroxidase, catalase, ascorbate (AsA) peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione (GSH) reductase, and further decreased the content of non-enzymatic antioxidants, including AsA, GSH and carotenoids. Photosystem II (PSII) efficiency in C. camphora declined under individual SiAR and HT stress, and HT after SiAR stress aggravated the decrease by aggravating the reduction of photosynthetic pigment content, inhibition on quantum production and electron transport in PSII. It can be deduced that initial acid rain cannot enhance C. camphora tolerance to the following HT, and the impacts of the two stresses will depend on the pH of acid rain.

Anisotropic Transport on Monolayer and Multilayer Phosphorene in the Presence of an Electric Field**Supported by the National Natural Science Foundation of China under Grant No 11374002, the Scientific Research Fund of Hunan Provincial Education Department under Grant No 17A001, and the Hunan Provincial Key Laboratory of Flexible Electronic Materials Genome Engineering of Changsha University of Science and Technology.

We demonstrate theoretically the anisotropic quantum transport of electrons through an electric field on monolayer and multilayer phosphorene. Using the long-wavelength Hamiltonian with continuum approximation, we find that the transmission probability for transport through an electric field is an oscillating function of incident angle, electric field intensity, as well as the incident energy of electrons. By tuning the electric field intensity and incident angle, the channels can be transited from opaque to transparent. The conductance through the quantum waveguides depends sensitively on the transport direction because of the anisotropic effective mass, and the anisotropy of the conductance can be tuned by the electric field intensity and the number of layers. These behaviors provide us an efficient way to control the transport of phosphorene-based microstructures.

Quantum Transport Properties of Two-Dimensional Quantum Lattices under Synthetic Magnetic Fields

Motivated by recent experimental progress, we study the quantum transport properties of two-dimensional electron gases under high perpendicular magnetic fields. We use a simple tight-binding model to model the system and open-source software to simulate quantum electronic transport properties such as band structure variations and conductance-flux relationships in such systems. Dependence of quantum transport properties on two-dimensional square, triangular and kagome lattice shapes were studied adding a Gaussian noise to account for the impurities. Numerical simulations are presented to predict the emergence of physical effects related to quantum Hall effect, such as the existence of Landau levels and edge states. The kagome lattice exhibits a different band structure giving rise to a flat band, due to its trihexagonal geometry. The peak conductance value increases with decreasing lattice constant due to higher transmission probability. The transport properties vary significantly with lattice geometries, both with the lattice type and the lattice constant.

Electron-Quantum Transport in Pseudomorphic and Metamorphic In0.2Ga0.8As-Based Quantum Wells

Metamorphic high-electron-mobility transistor (HEMT) structures based on deep In0.2Ga0.8As/In0.2Al0.8As quantum wells (0.7 eV for Γ electrons) with different metamorphic buffer designs are implemented and investigated for the first time. The electronic properties of metamorphic and pseudomorphic HEMT structures with the same doping are compared. It is found that, over a temperature range of 4–300 K, both the electron mobility and concentration in the HEMT structure with a linear metamorphic buffer are higher than those in the pseudomorphic HEMT structure due to an increase in the depth of the quantum well. Low-temperature magnetotransport measurements demonstrate that the quantum momentum-relaxation time decreases considerably in metamorphic HEMT structures because of enhanced small-angle scattering resulting from structural defects and inhomogeneities, while the dominant scattering mechanism in structures of both types is still due to remote ionized impurities.

Quantum Transport Properties of Two-Dimensional Quantum Lattices under Synthetic Magnetic

Motivated by recent experimental progress, we study the quantum transport properties of two-dimensional electron gases under high perpendicular magnetic fields. We use a simple tight-binding model to model the system and open-source software to simulate quantum electronic transport properties such as band structure variations and conductance-flux relationships in such systems. Dependence of quantum transport properties on two-dimensional square, triangular and kagome lattice shapes were studied adding a Gaussian noise to account for the impurities. Numerical simulations are presented to predict the emergence of physical effects related to quantum Hall effect, such as the existence of Landau levels and edge states. The kagome lattice exhibits a different band structure giving rise to a flat band, due to its trihexagonal geometry. The peak conductance value increases with decreasing lattice constant due to higher transmission probability. The transport properties vary significantly with lattice geometries, both with the lattice type and the lattice constant.

Ground state cooling of nanomechanical resonators by electron transport

We discuss two theoretical proposals for controlling the nonequilibrium steady state of nanomechanical resonators using quantum electronic transport. Specifically?, we analyse two approaches to achieve the ground-state cooling of the mechanical vibration coupled to a quantum dot embedded between (i) spin-polarised contacts or (ii) a normal metal and a superconducting contact. Assuming a suitable coupling between the vibrational modes and the charge or spin of the electrons in the quantum dot, we show that ground-state cooling of the mechanical oscillator is within the state of the art for suspended carbon nanotube quantum dots operating as electromechanical devices.

Air‐Stable Symmetric Ambipolar Field‐Effect Transistors Based on Reduced Graphene Oxide‐OTS Self‐Assembled Monolayer Heterostructure

AbstractGraphene has attracted great interest in the past decade due to its unique band structure and physical properties such as quantum electronic transport, extremely high mobility, high elasticity and optical properties. Graphene is generally obtained by mechanical exfoliation, epitaxial growth, chemical vapor deposition and chemical synthesis approaches, wherein the reduction of graphene oxide (GO) is a practical approach to synthesize large‐scale graphene. Via reduction, highly conductive reduced graphene oxide (RGO) can be obtained, which has electrical and optical properties analogous to those of graphene. However, Field‐effect transistors (FETs) based RGO typically exhibit p‐type in ambient rather than the ambipolar behavior of intrinsic graphene, due to the doping effect induced by substrate and the air. Meanwhile, large hysteresis accompanies with the p‐type transistor. Here, a self‐assembled monolayer (SAM) of octadecyltrichlorosilane (OTS)‐RGO heterostructure is developed, in which RGO is sandwiched between OTS SAMs. The heterostructure passivates the RGO device and eliminates the doping of graphene from the surroundings. The obtained RGO FETs exhibit air‐stable symmetric ambipolar behavior with Dirac point at 0 V gate voltage and negligible hysteresis. This approach will also be applicable to pristine graphene and other two‐dimensional materials to obtain the intrinsic properties under ambient conditions, thereby promoting the research and applications of two‐dimensional materials.