Conductance Asymmetry Research Articles

Conductance asymmetry in proximitized magnetic topological insulator junctions with Majorana modes

Conductance asymmetry in proximitized magnetic topological insulator junctions with Majorana modes

Diagnostics of the Ionospheric Conductivity Based on Spacecraft Observations of the Magnetospheric ULF Waves

AbstractIn 27 October 2012 an Alfvén wave of the Pc4 range was observed with Van Allen Probe A. The parallel Poynting flux of the wave was directed toward the Northern ionosphere. Assuming that this may be caused by the asymmetry of the ionospheric conductivity between the Northern and Southern hemispheres, its effect on the standing structure of the Alfvén wave was investigated. For this purpose, the analytical model with straight magnetic field lines was used. As a result, the method for estimation of the Northern and Southern ionospheric conductivity was developed. It allows to reconstruct the parallel structure of Alfvén waves under various conditions of the ionospheric conductivity. With the method developed, the ionospheric conductivity for the 27 October 2012 event was evaluated and compared with ionosphere model IRI‐2016 results.

Enhancement of thermal rectification by asymmetry engineering of thermal conductivity and geometric structure for multi-segment thermal rectifier

Thermal rectification is an exotic thermal transport phenomenon, an analog to electrical rectification, in which heat flux along one direction is larger than that in the other direction and is of significant interest in electronic device applications. However, achieving high thermal rectification efficiency or rectification ratio is still a scientific challenge. In this work, we performed a systematic simulation of thermal rectification by considering both efforts of thermal conductivity asymmetry and geometrical asymmetry in a multi-segment thermal rectifier. It is found that the high asymmetry of thermal conductivity and the asymmetry of the geometric structure of multi-segment thermal rectifiers can significantly enhance the thermal rectification, and the combination of both thermal conductivity asymmetry and geometrical asymmetry can further improve thermal rectification efficiency. This work suggests a possible way for improving thermal rectification devices by asymmetry engineering.

Functionalization of Quasi-Two-Dimensional Materials: Chemical and Strain-Induced Modifications

Among the family of currently known promising quasi-two-dimensional (2D) materials, the authors of this survey concentrate on the problem of functionalization of the graphene- and phosphorene-based structures. In most cases, the modification of their properties occurs through the covalent or noncovalent surface functionalization and mechanical affects. The atomic structures and some physicochemical features of 2D materials possessing novel properties as compared to their bulk counterparts are analysed. Their main advantages are the thickness of one or more atoms, the absence of surface-broken bonds, high mobility of charge carriers, the flexibility, the ability to be combined artificially into coplanar (lateral) or lamellar heterostructures, as well as the possibility to manipulate widely the band-gap changing from the semi-conducting state even into the semi-metallic one (or vice versa) when needed. In order to reveal new factors affecting the electronic properties of 2D materials by means of the computational experiment using the author’s (self-constructed) software code, a series of studies are carried out. They are the calculations of the spatial distribution of valence electrons’ density, the electron densities of states, the band-gap widths, Coulomb potentials along selected directions, the charge values in regions of different-size material, the dielectric matrices, the macroscopic relative permittivities, and absorption spectra. A series of recent studies, which the authors carried out modelling the electronic and transport properties of single- or multilayer graphene films subjected to deformation or/and magnetic fields and containing different-type (point- or/and linear-acting) defects is reviewed. Analysing the obtained results and revealed effects, it is claimed that the uniaxial tensile deformations or shear deformations along with their combinations as well as the structural imperfections (mainly, the mutually configured defects) can be useful for achieving the new level of functionalization of graphene. So, for modification of its electrotransport properties through tuning the band-gap value as much as it is enough to achieve the graphene transformation from the zero-band-gap semi-metallic state into the semi-conducting state and even reach the gap values, which are substantially higher than that for some materials (including silicon) currently used widely in the nanoelectronic devices. The strain- and defect-induced electron–hole asymmetry and anisotropy of conductivity and its nonmonotony as a function of deformation suggest a confidence in manipulating the electrotransport properties of graphene-like and beyond quasi-2D materials through a variety of both strains and defects. The use of reviewed and analysed results serves as a significant step in improving the properties of the considered materials in order to implement the multifunctional applications of them in the immediate prospect.

Thermal Diodes Based on Fractal Structures with Tunable Thermal Threshold

AbstractThe asymmetry of heat conduction in thermal diodes is attributed to the thermal regulation in thermal logic circuits, thermal control devices, etc. Currently, thermal rectification devices, however, are constrained by either specific on/off switching thresholds, or low sensitivity to manufacturing errors, as well as the restrictions for specific roughness and material characteristics. Here, a thermal diode based on fractal structures (inverted tapered branch channel, 5.7 mm branch length, and 4 branches) with liquid metal/insulator interface is proposed to avoid these existing constraints. The thermal rectification coefficient reaches 0.4704 (16.43 times larger than that of the basic structures) when the filling fraction only increases 1.33 times. The maximum temperature difference for the optimized thermal diodes is 18.31 °C when the heating temperature is 80 °C. By different arrangements of two thermal diodes mounted on chambers’ surfaces, the interior temperatures range from 16.06 to 52.34 °C in the same ambient conditions (23.49 °C), indicating the excellent thermal regulation effect of the thermal diodes. These controlled temperature chambers can be applied in greenhouses or cold chain storage devices, more combinations with higher degree of thermal‐threshold tunability offer possibilities in solving complicated thermal management problems and promote the energy utilization efficiency.

Unraveling the electronic properties of graphene with substitutional oxygen

We show abrupt changes in the electronic properties of graphene with different types of binding to oxygen. Whereas oxygen bonded to the basal plane in the form of functional groups p-type dopes graphene, we prove that substitutional (i.e. in-plane) oxygen n-type dopes it. Moreover, we determine that impurity scattering potentials introduced by these substitutional atoms are notably larger than those of conventional donors, e.g. nitrogen. Both facts ultimately result in a conduction asymmetry in the system with holes being scattered more strongly than electrons. These findings provide essential insights into the impact of oxygen in carbon nanomaterials such as graphene oxide, oxidized carbon nanotubes or novel two-dimensional π-conjugated organic frameworks, promising compounds for a wide range of applications including flexible electronics, catalysis, energy storage or biomedicine.

Radiative thermal rectification in many-body systems

Radiative thermal diodes based on two-element structures rectify heat flows thanks to a temperature dependence of material optical properties. The heat transport asymmetry through these systems, however, remains weak without a significant change in material properties with the temperature. Here we explore the heat transport in three-element radiative systems and demonstrate that a strong asymmetry in the thermal conductance can appear because of many-body interactions, without any dependence of optical properties on the temperature. The analysis of transport in three-body systems made with polar dielectrics and metallic layers reveals that rectification coefficients exceeding 50 % can be achieved in the near-field regime with temperature differences of about 200 K. This work paves the way for compact devices to rectify near field radiative heat fluxes over a broad temperature range and could have important applications in the domain of nanoscale thermal management.

Statistical Analysis of Throat Aurora Using Long Term DMSP/SSUSI Observation

AbstractThroat aurora is believed to be associated with magnetopause indentations and has direct implications on magnetopause reconnection. In this study, for the first time, we use Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observations over ∼14 years to characterize the throat aurora occurrence, latitudinal extent, seasonal, and its solar cycle dependence. We identified 386 throat aurora cases during the different passes of the satellite over the northern hemisphere. The latitudinal extent of these throat aurorae are estimated and are divided into small, medium, and large categories. The small and medium latitudinal extent throat aurorae account for about 91% of the total cases. The throat aurorae are found to occur most frequently in the post‐noon hours. The throat aurorae were also observed more frequently during winter, likely due to summer‐winter asymmetry in the ionospheric conductance. The occurrence of the throat aurora is also found to be anti‐correlated with the solar cycle, likely because low solar activity gives good chances for sporadic reconnection that favors the occurrence of throat aurora. The dependence on interplanetary magnetic field reveals positive Bx favoring the occurrence of throat aurora. It also shows positive Bx (negative By) supports pre‐noon (post‐noon) occurrence. The difference in the trend and average values of background interplanetary magnetic field conditions with the throat aurora confirms the favorable conditions for the occurrence of throat aurora.

Asymmetric thermal transport in strained Weyl semimetals due to axial anomaly

We show that strained Weyl semimetals exhibit a directional asymmetry of the heat transfer along the strain axis. The forward/backward asymmetry of the thermal conductivity appears as a result of the axial anomaly, which leads to the emergence of unidirectionally propagating ``chiral'' sound modes. The chiral phonons modify, in a directionally asymmetric manner, the spectrum of the acoustic lattice vibrations, and both types of phonons contribute to the thermal asymmetry of the material. We estimate the magnitude of the effect and discuss its possible experimental detection.

Asymmetric carrier transport and weak localization in few layer graphene grown directly on a dielectric substrate.

Temperature-dependent electrical and magneto-transport measurements have been performed on devices composed of few layer (4L) graphene grown directly on SiO2/Si substrates using the CVD method. An intrinsic energy band-gap of 4.6 meV in 4L graphene is observed, which primarily dictates the current transport at T <50 K. Unusual temperature dependent electron-hole conduction asymmetry is observed at T >50 K, which can be explained in the framework of the defect scattering of relativistic charge carriers. Magneto-transport measurements reveal a weak localization effect sustainable till T >200 K. The coexistence of phonon mediated carrier mobility and defect induced weak localization effects in measuring devices suggests low disorder and impurity scattering.

Phonon-boundary scattering and thermal transport in AlxGa1−xN: Effect of layer thickness

Thermal conductivity of AlxGa1−xN layers with 0≤x≤0.96 and variable thicknesses is systematically studied by combined thermoreflectance measurements and a modified Callaway model. We find a reduction in the thermal conductivity of AlxGa1−xN by more than one order of magnitude compared to that of GaN, which indicates a strong effect of phonon-alloy scattering. It is shown that the short-mean free path phonons are strongly scattered, which leads to a major contribution of the long-mean free path phonons to the thermal conductivity. In thin layers, the long-mean free path phonons become efficiently scattered by the boundaries, resulting in a further decrease in the thermal conductivity. Also, an asymmetry of thermal conductivity as a function of Al content is experimentally observed and attributed to the mass difference between Ga and Al host atoms.

Sensitivity to strains and defects for manipulating the conductivity of graphene

Implementing the quantum-mechanical Kubo-Greenwood formalism for the numerical calculation of dc conductivity, we demonstrate that the electron transport properties of a graphene layer can be tailored through the combined effect of defects (point and line scatterers) and strains (uniaxial tension and shear), which are commonly present in a graphene sample due to the features of its growth procedure and when the sample is used in devices. Motivated by two experimental works (He X. et al. Appl. Phys. Lett., 104 (2014) 243108; 105 (2014) 083108), where authors did not observe the transport gap even at large (22.5% of tensile and 16.7% of shear) deformations, we explain possible reasons, emphasizing on graphene's strain and defect sensing. The strain- and defect-induced electron-hole asymmetry and anisotropy of conductivity, and its nonmonotony as a function of deformation suggest perspectives for the strain-defect engineering of electrotransport properties of graphene and related 2D materials.

Doping dependence of thermopower in cuprate superconductors

The doping dependence of the thermopower of cuprate superconductors in the normal-state is studied within the [Formula: see text]–[Formula: see text] model. It is shown that with a proper modification of the bare electron dispersion in the [Formula: see text]–[Formula: see text] model, the experimental results of the doping dependence of the normal-state thermopower are qualitatively reproduced. In particular, the theory shows that a pseudogap-generated split of the van Hove peak in the density of states appears in the underdoped and optimally doped regimes, however, this split is absent from the overdoped regime. Concomitantly, the strong asymmetry of the spectral conductivity near the electron Fermi surface emerges, where the peak in the spectral conductivity appears always below the electron Fermi surface in the underdoped and optimally doped regimes, while it appears above the electron Fermi surface in the overdoped regime. This strong asymmetry of the spectral conductivity leads to the unusual behaviors of the normal-state thermopower from the underdoped regime to the overdoped regime.

Determining the Orientation of Porins in Planar Lipid Bilayers.

Single-channel planar lipid bilayer (PLB) recording of bacterial porins has revealed molecular details of transport across the outer membrane of Gram-negative bacteria, including antibiotic permeation and protein translocation. To explore directional transport processes across cellular membranes, the orientation of porins or other pore-forming proteins must be established in a lipid bilayer prior to experimentation. Here, we describe a direct method for determining the orientation of porins in a PLB-with a focus on E. coli OmpF-by using targeted covalent modification of cysteine mutants. Each of the two possible orientations can be correlated with the porin conductance asymmetry, such that thereafter an I-V curve taken at the start of an experiment will suffice to establish orientation.

Electron-hole asymmetry in electrical conductivity of low-fluorinated graphene: numerical study

By using the real-space Green-Kubo formalism we study numerically the electron transport properties of low-fluorinated graphene. At low temperatures the diffuse transport regime is expected to be prevalent, and we found a pronounced electron-hole asymmetry in electrical conductivity as a result of quasi-resonant scattering on the localized states. For the finite temperatures in the variable-range hopping transport regime the interpretation of numerical results leads to the appearance of local minima and maxima of the resistance near the energies of the localized states. A comparison with the experimental measurements of the resistance in graphene samples with various fluorination degrees is discussed.

Design of a unidirectional water valve in Tillandsia

The bromeliad Tillandsia landbeckii thrives in the Atacama desert of Chile using the fog captured by specialized leaf trichomes to satisfy its water needs. However, it is still unclear how the trichome of T. landbeckii and other Tillandsia species is able to absorb fine water droplets during intermittent fog events while also preventing evaporation when the plant is exposed to the desert’s hyperarid conditions. Here, we explain how a 5800-fold asymmetry in water conductance arises from a clever juxtaposition of a thick hygroscopic wall and a semipermeable membrane. While absorption is achieved by osmosis of liquid water, evaporation under dry external conditions shifts the liquid-gas interface forcing water to diffuse through the thick trichome wall in the vapor phase. We confirm this mechanism by fabricating artificial composite membranes mimicking the trichome structure. The reliance on intrinsic material properties instead of moving parts makes the trichome a promising basis for the development of microfluidics valves.

Electronic and optical properties of SnX2(X = S, Se)—InSe van der Waal’s heterostructures from first-principle calculations

In this work from first-principles simulations we investigate bilayer van der Waal’s heterostructures (vdWh) of emerging 2-dimensional optical materials SnS2 and SnSe2 with monolayer InSe. With density functional theory calculations, we study the structural, electronic, optical and carrier transport properties of the SnX2 (X = S, Se)-InSe vdWh. Calculations show SnX2-InSe in its most stable stacking form (named AB-1) to be a material with a small (0.6–0.7 eV) indirect band-gap. The bilayer vdWh shows broad spectrum optical response, with number of peaks in the infra-red to visible region. In terms of carrier transport properties, asymmetry in conductance was observed with respect to the transport direction and electron and hole transmission. The findings are promising from the viewpoint of nanoelectronics and photonics.

Effect of boundary conditions on reduction during early stage flash sintering of YSZ

The onset of flash sintering is generally considered to be predominantly a function of the conductivity of a given ceramic, the furnace temperature, and the electric field. However, the evolution of the point defect profiles in ionic conductors such as yttria-stabilized zirconia (YSZ) can complicate the picture of homogeneous Joule heating and thermal runaway during DC flash sintering. Here, 8 mol% YSZ pellets were partially flash sintered under a DC bias using various current densities and hold times. The electrode geometry was varied to modulate the oxygen ion flux available to the cathode to compare the effects on the resulting oxygen vacancy inhomogeneity. The contribution from fundamental and experimental factors to the cathodic reduction reaction is also discussed, such as interface reaction kinetics and sample geometry. Local reduction of the ceramic was inevitably observed under all current densities and the resulting microstructural inhomogeneity was explained as the result of a transient conductivity asymmetry. This asymmetry was the result of the enhanced electronic conductivity in the cathode region to a value significantly greater than the ionic conductivity of near-stoichiometric YSZ. The link between the local conductivity, voltage and Joule heating is mathematically demonstrated to result in an asymmetric heating profile.

Theoretical study of anisotropic tunnelling conductance in iron-based orbitally ordered superconductors

We address here the role of Jahn-Tellar (JT) distortion on the superconducting (SC) gap in iron-based superconductors taking into account of the first and second nearest-neighbour (NN) electron hoppings in the square lattice within one band model approach. The Green's functions are calculated by using Zubarev's Green's function technique. The temperature dependent superconducting gap and the lattice strain are calculated from the correlation functions of the corresponding Green's functions and are computed self-consistently. The temperature dependent gap equations show that the superconducting gap is enhanced with the decrease of second nearest-neighbour hopping accompanied by the suppression of lattice strain near the superconducting transition temperature. The second nearest hopping introduces asymmetry in the tunnelling conductance.

Controlling the Current Conduction Asymmetry of HfO2 Metal–Insulator–Metal Diodes by Interposing Al2O3 Layer

Metal-insulator-metal (MIM) diodes with bulk-limited current conduction asymmetry employing interposed Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer in 6-nm-thick HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film have been investigated. The current density is decreased as the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer is located far from the electron injection electrode (i.e., cathode). Also, the work function difference of the electrode does not affect the asymmetry of the MIM diodes. Asymmetry of current conduction is attributed to the control of bulk-limited current conduction by the interposed Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer. Physicochemical analysis also confirms the dependence of the location of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer on the bulk-limited current asymmetry.