3D Conduction Research Articles

Crossing and anticrossing of 1D subbands in a quantum point contact with in-plane side gates

The conductance of a single GaAs quantum point contact (QPC) with in-plane side gates separated from a channel by lithographic trenches is experimentally studied. It is shown that, in such a trench-type QPC, the conducting channel splits into two channels, each demonstrating a conductance quantization. A phase diagram of the 1D subband population in separate channels as a function of the voltages at side gates is experimentally obtained. The obtained data show that the electron transport regime undergoes the transition from the case of two independent parallel 1D conductors to the case of strongly correlated double-row transport. The transition occurs with the increase in the number of populated 1D subbands in the channels and manifests itself in the phase diagram by the change of behavior of the lines separating the regions corresponding to different numbers. The crossings of these lines at small numbers turn into anticrossings at large numbers. The anticrossings are explained by the splitting of degenerate levels in a double-well confinement potential formed inside the QPC constriction.

New signatures of the spin gap in quantum point contacts

One dimensional semiconductor systems with strong spin-orbit interaction are both of fundamental interest and have potential applications to topological quantum computing. Applying a magnetic field can open a spin gap, a pre-requisite for Majorana zero modes. The spin gap is predicted to manifest as a field dependent dip on the first 1D conductance plateau. However, disorder and interaction effects make identifying spin gap signatures challenging. Here we study experimentally and numerically the 1D channel in a series of low disorder p-type GaAs quantum point contacts, where spin-orbit and hole-hole interactions are strong. We demonstrate an alternative signature for probing spin gaps, which is insensitive to disorder, based on the linear and non-linear response to the orientation of the applied magnetic field, and extract a spin-orbit gap ΔE ≈ 500 μeV. This approach could enable one-dimensional hole systems to be developed as a scalable and reproducible platform for topological quantum applications.

EMT simulation and effect of TTI anisotropic media in EMT signal

An axisymmetric finite difference method is employed for the simulations of electromagnetic telemetry in the homogeneous and layered underground formation. In this method, we defined the anisotropy property using extensive 2D conductivity tensor and solved it in the transverse magnetic mode. Significant simplification arises in the decoupling of the anisotropic parameter. The developed method is cost-efficient, more straightforward in modeling anisotropic media, and easy to be implemented. In addition, we solved the integral operation in the estimation of measured surface voltage using Gaussian quadrature technique. We performed a series of numerical modeling of EM telemetry signals in both isotropic and anisotropic models. Experiment with 2D tilt transverse isotropic media characterized by the tilt axis and anisotropy parameters shows an increase in the EMT signal with an increase in the angle of tilt of the principal axis for a moderate coefficient of anisotropy. We show that the effect of the tilt of the subsurface medium can be observed with sufficient accuracy and that it is an order of magnitude of 5 over the tilt of 90 degrees. Lastly, consistent results with existing field data were obtained by employing the Gaussian quadrature rule for the computation of surface measured signal.

Energy usage modeling for heating and cooling of off-grid shelters

Temporary shelters are widely used in various missions, whether military or non-military, to meet the needs of low-cost and quick deployment of structures. The shelters are unlikely to be connected to conventional electrical grids, hence are known as ‘off-grid shelters’ and require independent energy sources such as gas-fueled generators. Since the supply of the fuel is often limited, planning efficient energy usage is crucial for their operations, and having energy models of structures will greatly support the planning process. This research presents a method for modeling energy usage in off-grid soft-wall shelters, and propose a systematic model validation process. The model is generated for a soft-wall shelter using the OpenStudio software, and the thermal properties of the materials are measured using 1D conduction analysis. Air leakage is measured using modified blower door tests, and a ground coupling model is used to account for ground temperatures. Finally, the model is validated with field measured data in terms of HVAC load, which showed an average difference of 0.64 kWh over a mean value of 2.43 kWh over a 7 day period. The models were then used to conduct parametric studies using advanced materials to seek energy efficiency improvements. The integration of aerogel insulation showed 0.98% reduction, while radiant barrier showed 3.6% reduction in cooling energy usage. The findings from this study provide a method of reliable energy modeling for less-studied area of soft-wall shelters which can be subsequently used to efficiently plan the energy requirement of off-grid shelters prior to their deployments.

Non-Born effects in the scattering of electrons in a conducting strip with a low concentration of impurities

We extend the theory of non-Born effects in resistivity $\rho$ of clean conducting tubes (developed in our previous work arXiv:1810.00426) to ``strips'' -- quasi-one-dimensional structures in 2D conductors. Here also an original Van Hove singularity in dependence of $\rho$ on the position of chemical potential $\varepsilon$ is asymmetrically split in two peaks for attracting impurities. However, since amplitudes of scattering at impurities depend on their positions, these peaks are inhomogeneously broadened. Strongest broadening occurs in the left peak, arising, for attracting impurities, due to scattering at quasistationary levels. In contrast with the case of tube these levels form not a unique sharp line, but a relatively broad impurity band with a weak quasi-Van Hove feature on its lower edge. Different parts of $\rho(\varepsilon)$ are dominated by different groups of impurities: close to the minimum the most effective scatterers, paradoxically are the ``weakest'' impurities -- those, located close to nodes of the electronic wave-function, so that the bare scattering matrix elements are suppressed. The quasi-Van Hove feature at left maximum is dominated by strongest impurites, located close to antinodes.

Nanometer-Thick Bismuth Nanocrystal Films for Sensoric Applications

The present article is concerned with investigations of the structural, surface morphological, and magnetotransport properties of DC magnetron-sputtered nanometer-thick Bi nanocrystal films on Si(111) substrates. Crystal structure and surface morphology were studied with X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, and atomic force microscopy. For the samples deposited at the melting point of Bi, 271 °C, equilibrium crystals formed and according to Wulff theorem acquire a specific shape determined by the surface tension. These crystals were investigated for different film thicknesses and deposition temperatures varying from 25 to 300 °C. Furthermore, magnetotransport characterization was carried out in steady and pulsed magnetic fields of up to 9 and 70 T, respectively. At low temperatures, clear weak antilocalization behavior is observed, attributed to 2D conduction channels. A nonlinear Hall resistance is also confirmed, ascribed to the coexistence of two types of carriers (p and n). This study contributes to the elucidation of the transport properties of the Bi thin films and opens new perspectives for their exploitation in modern applications such as sensorics.

Protected helical transport in magnetically doped quantum wires: Beyond the one-dimensional paradigm

One-dimensional (1D) quantum wires, which are functionalized by magnetic ad-atoms, can host ballistic helical transport. Helicity protects transport from an undesirable influence of material imperfections, and it makes the magnetically doped wire a very promising element for nanoelectronics and spintronics. However, fabricating purely 1D conductors is experimentally very challenging and not always feasible. In this paper, we show that the protected helical transport can exist even in quasi-1D wires. We model the quasi-1D magnetically doped wire as two coupled dense 1D Kondo chains. Each chain consists of itinerant electrons interacting with localized quantum magnetic moments -- Kondo impurities. We have analyzed the regimes of weak-, intermediate-, and strong inter-chain coupling, and we found conditions necessary for the origin of the aforementioned protected transport. Our results may pave the way for experimental realizations of helical states in magnetically doped wires.

Effect of the pore radius on the effective conductivity of UO2 in 2D and 3D: A computational approach

We performed 2D and 3D steady-state heat conduction simulations with a Crank–Nicolson implicit scheme implementation. The parameter ϕ is introduced, with a value of 1 within the matrix and 0 inside a pore. At the interface between a matrix and a pore, this parameter continuously changes from 0 to 1 or vice versa. We investigated the effect of not only the porosity but also the pore radius on the effective thermal conductivity of UO2. To perform large-scale 2D and 3D (10.242μm2, 10.243μm3) simulations, we utilized CUDA-accelerated parallel computing, which consumed only approximately 10% of the time required by serial computation to obtain the same results in 3D. We found that the pore radius, which has not previously been considered a notable microstructural parameter of porous UO2 nuclear fuel, significantly affects the effective thermal conductivity of UO2.

Self‐Formed, Conducting LaAlO3/SrTiO3 Micro‐Membranes

AbstractThe discovery of 2D conductivity at the LaAlO3/SrTiO3 interface has been linking, for over a decade, two of the major current research fields in materials science: correlated transition‐metal‐oxide systems and low‐dimensional systems. Notably, despite the 2D nature of the interfacial electron gas, the samples are 3D objects with thickness in the mm range. This prevented researchers so far from adopting strategies that are only viable for fully 2D materials, or from effectively exploiting degrees of freedom related to strain, strain gradient and curvature. Here a method based on pure strain engineering for obtaining freestanding LaAlO3/SrTiO3 membranes with micrometer lateral dimensions is demonstrated. Detailed transmission electron microscopy investigations show that the membranes are fully epitaxial and that their curvature results in a huge strain gradient, each layer showing a mixed compressive/tensile strain state. Electronic devices are fabricated by realizing ad hoc circuits for individual micro‐membranes transferred on silicon chips. The samples exhibit metallic conductivity and electrostatic field effect like 2D‐electron systems in bulk heterostructures. The results open a new path for adding oxide functionalities into semiconductor electronics, potentially allowing for ultra‐low voltage gating of a superconducting transistors, micromechanical control of the 2D electron gas mediated by ferroelectricity and flexoelectricity, and on‐chip straintronics.

Mobility-Fluctuation-Controlled Linear Positive Magnetoresistance in 2D Semiconductor Bi2O2Se Nanoplates.

Linear magnetoresistance is generally observed in polycrystalline zero-gap semimetals and polycrystalline Dirac semimetals with ultrahigh carrier mobility. We report the observation of positive and linear magnetoresistance in a single-crystalline semiconductor Bi2O2Se grown by chemical vapor deposition. Both Se-poor and Se-rich Bi2O2Se single-crystalline nanoplates display a linear magnetoresistance at high fields. The Se-poor Bi2O2Se exhibits a typical 2D conduction feature with a small effective mass of 0.032m0. The average transport Hall mobility, which is lower than 5500 cm2 V-1 s-1, is significantly reduced, compared with the ultrahigh quantum mobility as high as 16260 cm2 V-1 s-1. More interestingly, the pronounced Shubnikov-de Hass oscillations can be clearly observed from the very large and nearly linear magnetoresistance (>500% at 14 T and 2 K) in Se-poor Bi2O2Se. A close analysis of the results reveals that the large and linear magnetoresistance observed can be ascribed to the spatial mobility fluctuation, which is strongly supported by Fermi energy inhomogeneity in the nanoplate samples detected using an electrostatic force microscopy images and multiple frequencies in a Shubnikov-de Hass oscillation. On the contrary, the Se-rich Bi2O2Se exhibits a transport mobility (<300 cm2 V-1 s-1) much smaller than that observed in Se-poor samples and shows a much smaller linear magnetoresistance ratio (less than 150% at 14 T and 2 K). More strikingly, no Shubnikov-de Hass oscillations can be observed. Therefore, the linear magnetoresistance in Se-rich Bi2O2Se is governed by the average mobility rather than the mobility fluctuation.

Evidence of higher-order topology in multilayer WTe2 from Josephson coupling through anisotropic hinge states

Td-WTe2 (non-centrosymmetric and orthorhombic), a type-II Weyl semimetal, is expected to have higher-order topological phases with topologically protected, helical one-dimensional hinge states when its Weyl points are annihilated. However, the detection of these hinge states is difficult due to the semimetallic behaviour of the bulk. In this study, we have spatially resolved the hinge states by analysing the magnetic field interference of the supercurrent in Nb-WTe2-Nb proximity Josephson junctions. The Josephson current along the a axis of the WTe2 crystal, but not along the b axis, showed a sharp enhancement at the edges of the junction, and the amount of enhanced Josephson current was comparable to the upper limits of a single one-dimensional helical channel. Our experimental observations suggest a higher-order topological phase in WTe2 and its corresponding anisotropic topological hinge states, in agreement with theoretical calculations. Our work paves the way for the study of hinge states in topological transition-metal dichalcogenides and analogous phases.

Integrated geophysical investigation techniques for mapping cassava effluent leachate contamination plume, at a dumpsite in Ilero, southwestern Nigeria

The subsurface illuminating strength of integrating Electrical Resistivity (ER) and Frequency Domain Electromagnetic (FDEM) geophysical investigation techniques to evaluate the level of subsurface leachate contamination plume generated by indiscriminate dumping of cassava product processing effluents and wastes around a dumpsite near cassava processing mill at Ilero southwestern Nigeria, has been demonstrated in this study. The lateral spread as well as depth extent of the cassava waste effluent leachate plume was determined from the subsurface resistivity and conductivity models generated from combined ER and FDEM geophysical surveys carried out through eight (8) Electrical Resistivity Tomography (ERT), eight (8) FDEM and (2) Vertical Electrical Sounding (VES) measurement profiles and stations purposively established across the dumpsite.Quality checked, inverted and depth matched ER and FDEM data generated 1D, 2D, and 3D resistivity and conductivity distribution in the form of subsurface logs, sections, constant depth maps and earth model employed to characterize the study area. Electrical resistivity result presents resistivity distribution that range in value between 21 Ωm to 213 Ωm from the topsoil to depths beyond 7.5 m. The FDEM sections and depth maps show conductivity distribution range between 4 mS/m – 286 mS/m across the dump site and provided information up to a depth of 60 m. The VES results generally delineate three (3) georesistivity layers, which include topsoil, weathered and fractured basement layers, occurring at depth between 0.4 – 10.0 m with characteristic resistivity values from 33.5 Ωm – 37.9 Ωm. The 3D resistivity and conductivity models generated by integrating the various results indicate anomalously low resistivity and relatively high conductivity (21 – 30 Ωm; 11 mS/m – 40 mS/m) zone which defines contamination plume in the central part of the cassava effluents and waste dumpsite.This study has through integrated electrical resistivity and frequency domain electromagnetic geophysical methods of investigation highlighted the subsurface contaminating potential of indiscriminating dumping of agricultural processing effluent and wastes, especially of cassava effluent, which has toxic cyanogenicglucosides constituent that is deleterious to the environment.

A density gradient approach to topology optimization under design-dependent boundary loading

The paper proposes a density gradient based approach to topology optimization under design-dependent boundary loading. Since there is no explicit boundary representation in density-based topology optimization, the design-dependent boundary loads are implicitly imposed through a domain integration of spatial gradient of the density field. The density gradient based loading is formally derived and justified based on the smoothed boundary method for numerical solution of PDE. The Heaviside projection, density filtering and local refinement are combined to efficiently control the thickness of the loading interface and the number of elements within it. For problems with both loading and non-loading boundary, an auxiliary density field is introduced to keep track of the loading boundary. Both 2D and 3D heat conduction problems, linear elasticity problems and coupled thermoelastic problems under design-dependent boundary loading are presented to illustrate the effectiveness and efficiency of the method.

Coenzyme Coupling Boosts Charge Transport through Single Bioactive Enzyme Junctions.

SummaryOxidation of formate to CO2 is catalyzed via the donation of electrons from formate dehydrogenase (FDH) to nicotinamide adenine dinucleotide (NAD+), and thus the charge transport characteristics of FDH become essential but remain unexplored. Here, we investigated the charge transport through single-enzyme junctions of FDH using the scanning tunneling microscope break junction technique (STM-BJ). We found that the coupling of NAD+ with FDH boosts the charge transport by ∼2,100%, and the single-enzyme conductance highly correlates with the enzyme activity. The combined flicker noise analysis demonstrated the switching of the coenzyme-mediated charge transport pathway and supported by the significantly reduced HOMO-LUMO gap from calculations. Site-specific mutagenesis analysis demonstrated that FDH-NAD+ stably combined own higher bioactivity and boosts charge transport, and the coupling has been optimized via the natural selection. Our work provides evidence of hydrogen bond coupling in bioactivity but also bridges the charge transport through single-enzyme junctions and enzyme activities.

On the study of industrial waste sites on the Karelian Isthmus/Russia using the RMT and CSRMT methods

Radiomagnetotelluric (RMT) and Controlled Source Radiomagnetotelluric (CSRMT) measurements were carried out on landfills of industrial wastes located on the Karelian Isthmus/Russia. The RMT and CSRMT data were interpreted using the 2D inversion techniques, and the 2D conductivity structures of the investigated industrial waste sites were derived. The RMT method is based on measurements of electromagnetic fields of remote radio transmitters in a frequency range of 10–1000 kHz or in a frequency range of 1–1000 kHz by using a controlled source (CSRMT method). A landfill in the settlement Morozova near St. Petersburg was formed as a result of a long-term warehousing of toxic wastes by the sulfuric acid manufacture. Soils and ground waters were polluted with salts of heavy metals. In the adjacent to the landfill territory, investigations were realized by the RMT method and a polluted zone was determined. This zone is traced up to the coast of the Ladoga Lake and the Neva River. It constitutes a potential danger because the Neva River is used for the water supply of St. Petersburg. Another landfill of debris in a former sandy career is located in the Vyborg area. The high conductivity and a relative large thickness of the wastes did not allow us to map the bottom of the landfill by the RMT method using the frequency range of 10–1000 kHz. Therefore, CSRMT measurements with an extended frequency range up to 1 kHz were carried out on this landfill, and the 2D inversion of the CSRMT data clearly showed the lateral and vertical extension of it.

Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors.

We report the fabrication and electrical characterization of germanium arsenide (GeAs) field-effect transistors with ultrathin channels. The electrical transport is investigated in the 20–280 K temperature range, revealing that the p-type electrical conductivity and the field-effect mobility are growing functions of temperature. An unexpected peak is observed in the temperature dependence of the carrier density per area at ∼75 K. Such a feature is explained considering that the increased carrier concentration at higher temperatures and the vertical band bending combined with the gate field lead to the formation of a two-dimensional (2D) conducting channel, limited to few interfacial GeAs layers, which dominates the channel conductance. The conductivity follows the variable-range hopping model at low temperatures and becomes the band-type at higher temperatures when the 2D channel is formed. The formation of the 2D channel is validated through a numerical simulation that shows excellent agreement with the experimental data.

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.

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.

Augmenting 1D conductivity depth sections to include information pertaining to 2D/3D conductors

SummaryConductivity depth data presentations derived using 1D approximations have become a very popular way to present airborne electromagnetic (AEM) data. While data profiles and a single error limit per station can accompany these presentations there are more information contained in data. This work proposes some ideas for augmenting 1D derived data presentations to highlight two and three-dimensional conductors as well as more detailed error level indication. The 1D conductivity depth algorithm used is an extended version of the S-layer differential transform. The depths calculated with the transform is used also present decay constant values corrected for conductive background responses. Fraser filtered X-component data are used to indicate structural complexity and data fit error levels are used to grey out areas on the depth section presentations where the results are less reliable. To demonstrate these ideas a line of Xcite data over the Abra deposit in Western Australia is analysed. The augmentations add valuable information to the conductivity depth section in the sense of drawing attention the where the 1D assumptions are not valid and should encourage interpreters to pursue 2D or 3D modelling techniques for successfully mapping the ore deposit.

A new quasi-one-dimensional compound Ba3TiTe5 and superconductivity induced by pressure

We report systematic studies of a new quasi-one-dimensional (quasi-1D) compound, Ba3TiTe5, and the high-pressure induced superconductivity therein. Ba3TiTe5 was synthesized at high pressure and high temperature. It crystallizes into a hexagonal structure (P63/mcm), which consists of infinite face-sharing octahedral TiTe6 chains and Te chains along the c axis, exhibiting a strong 1D characteristic structure. The first-principles calculations demonstrate that Ba3TiTe5 is a well-defined 1D conductor; thus, it can be considered a starting point to explore the exotic physics induced by pressure by enhancing the interchain hopping to move the 1D conductor to a high-dimensional metal. For Ba3TiTe5, high-pressure techniques were employed to study the emerging physics dependent on interchain hopping, such as the Umklapp scattering effect, spin/charge density wave (SDW/CDW), superconductivity and non-Fermi liquid behavior. Finally, a complete phase diagram was plotted. The superconductivity emerges at 8.8 GPa, near which the Umklapp gap is mostly suppressed. Tc is enhanced and reaches a maximum of ~6 K at ~36.7 GPa, where the SDW/CDW is completely suppressed, and a non-Fermi liquid behavior appears. Our results suggest that the appearance of superconductivity is associated with the fluctuation due to the suppression of the Umklapp gap and that the enhancement of the Tc is related to the fluctuation of the SDW/CDW.