Showing papers on "Conductance published in 2020"

Light-induced anomalous Hall effect in graphene.

Abstract: Many non-equilibrium phenomena have been discovered or predicted in optically driven quantum solids1. Examples include light-induced superconductivity2,3 and Floquet-engineered topological phases4–8. These are short-lived effects that should lead to measurable changes in electrical transport, which can be characterized using an ultrafast device architecture based on photoconductive switches9. Here, we report the observation of a light-induced anomalous Hall effect in monolayer graphene driven by a femtosecond pulse of circularly polarized light. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that reflect a Floquet-engineered topological band structure4,5, similar to the band structure originally proposed by Haldane10. This includes an approximately 60 meV wide conductance plateau centred at the Dirac point, where a gap of equal magnitude is predicted to open. We find that when the Fermi level lies within this plateau the estimated anomalous Hall conductance saturates around 1.8 ± 0.4 e2/h. A transient topological response in graphene is driven by a short pulse of light. When the Fermi energy is in the predicted band gap the Hall conductance is around two conductance quanta. An ultrafast detection technique enables the measurement.

Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor

Abstract: Majorana zero modes (MZMs) are spatially localized, zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold promise for topological quantum computing. Owing to the particle-antiparticle equivalence, MZMs exhibit quantized conductance at low temperature. By using variable-tunnel–coupled scanning tunneling spectroscopy, we studied tunneling conductance of vortex bound states on FeTe0.55Se0.45 superconductors. We report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bound states with values close to or even reaching the 2e2/h quantum conductance (where e is the electron charge and h is Planck’s constant). By contrast, no plateaus were observed on either finite energy vortex bound states or in the continuum of electronic states outside the superconducting gap. This behavior of the zero-mode conductance supports the existence of MZMs in FeTe0.55Se0.45.

Generic quantized zero-bias conductance peaks in superconductor-semiconductor hybrid structures

Abstract: We show theoretically that quantized zero-bias conductance peaks should be ubiquitous in superconductor-semiconductor hybrids by employing a zero-dimensional random matrix model with continuous tuning parameters. We demonstrate that a normal metal-superconductor (NS) junction conductance spectra can be generically obtained in this model replicating all features seen in recent experimental results. The theoretical quantized conductance peaks, which explicitly do not arise from spatially isolated Majorana zero modes, are easily found by preparing a contour plot of conductance over several independent tuning parameters, mimicking the effect of Zeeman splitting and voltages on gates near the junction. This suggests that, even stable apparently quantized conductance peaks need not correspond to isolated Majorana modes; rather, the a priori expectation should be that such quantized peaks generically occupy a significant fraction of the high-dimensional tuning parameter space that characterizes the NS tunneling experiments.

Conductance-Matrix Symmetries of a Three-Terminal Hybrid Device.

Abstract: We present conductance-matrix measurements of a three-terminal superconductor-semiconductor hybrid device consisting of two normal leads and one superconducting lead. Using a symmetry decomposition of the conductance, we find that antisymmetric components of pairs of local and nonlocal conductances qualitatively match at energies below the superconducting gap, and we compare this finding with symmetry relations based on a noninteracting scattering matrix approach. Further, the local charge character of Andreev bound states is extracted from the symmetry-decomposed conductance data and is found to be similar at both ends of the device and tunable with gate voltage. Finally, we measure the conductance matrix as a function of magnetic field and identify correlated splittings in low-energy features, demonstrating how conductance-matrix measurements can complement traditional single-probe measurements in the search for Majorana zero modes.

Nonlocal Conductance Spectroscopy of Andreev Bound States: Symmetry Relations and BCS Charges.

Abstract: Two-terminal conductance spectroscopy of superconducting devices is a common tool for probing Andreev and Majorana bound states. Here, we study theoretically a three-terminal setup, with two normal leads coupled to a grounded superconducting terminal. Using a single-electron scattering matrix, we derive the subgap conductance matrix for the normal leads and discuss its symmetries. In particular, we show that the local and the nonlocal elements of the conductance matrix have pairwise identical antisymmetric components. Moreover, we find that the nonlocal elements are directly related to the local BCS charges of the bound states close to the normal probes and we show how the BCS charge of overlapping Majorana bound states can be extracted from experiments.

Pascal conductance series in ballistic one-dimensional LaAlO3/SrTiO3 channels.

Abstract: One-dimensional electronic systems can support exotic collective phases because of the enhanced role of electron correlations. We describe the experimental observation of a series of quantized conductance steps within strongly interacting electron waveguides formed at the lanthanum aluminate–strontium titanate (LaAlO3/SrTiO3) interface. The waveguide conductance follows a characteristic sequence within Pascal’s triangle: (1, 3, 6, 10, 15, …) ⋅ e2/h, where e is the electron charge and h is the Planck constant. This behavior is consistent with the existence of a family of degenerate quantum liquids formed from bound states of n = 2, 3, 4, … electrons. Our experimental setup could provide a setting for solid-state analogs of a wide range of composite fermionic phases.

Mechanically Tunable Quantum Interference in Ferrocene-Based Single-Molecule Junctions.

Abstract: Ferrocenes are ubiquitous organometallic building blocks that comprise a Fe atom sandwiched between two cyclopentadienyl (Cp) rings that rotate freely at room temperature. Of widespread interest in fundamental studies and real-world applications, they have also attracted some interest as functional elements of molecular-scale devices. Here we investigate the impact of the configurational degrees of freedom of a ferrocene derivative on its single-molecule junction conductance. Measurements indicate that the conductance of the ferrocene derivative, which is suppressed by 2 orders of magnitude as compared to a fully conjugated analogue, can be modulated by altering the junction configuration. Ab initio transport calculations show that the low conductance is a consequence of destructive quantum interference effects of the Fano type that arise from the hybridization of localized metal-based d-orbitals and the delocalized ligand-based π-system. By rotation of the Cp rings, the hybridization, and thus the quantum interference, can be mechanically controlled, resulting in a conductance modulation that is seen experimentally.

Cumulene Wires Display Increasing Conductance with Increasing Length.

Abstract: One-dimensional sp-hybridized carbon wires, including cumulenes and polyynes, can be regarded as finite versions of carbynes. They are likely to be good candidates for molecular-scale conducting wires as they are predicted to have a high-conductance. In this study, we first characterize the single-molecule conductance of a series of cumulenes and polyynes with a backbone ranging in length from 4 to 8 carbon atoms, including [7]cumulene, the longest cumulenic carbon wire studied to date for molecular electronics. We observe different length dependence of conductance when comparing these two forms of carbon wires. Polyynes exhibit conductance decays with increasing molecular length, while cumulenes show a conductance increase with increasing molecular length. Their distinct conducting behaviors are attributed to their different bond length alternation, which is supported by theoretical calculations. This study confirms the long-standing theoretical predictions on sp-hybridized carbon wires and demonstrates that cumulenes can form highly conducting molecular wires.

Giant Conductance Enhancement of Intramolecular Circuits through Interchannel Gating

Abstract: 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.

Ion Transport in Electrically Imperfect Nanopores.

Abstract: Ionic transport through a charged nanopore at low ion concentration is governed by the surface conductance. Several experiments have reported various power-law relations between the surface conductance and ion concentration, i.e., Gsurf ∝ c0α. However, the physical origin of the varying exponent, α, is not yet clearly understood. By performing extensive coarse-grained Molecular Dynamics simulations for various pore diameters, lengths, and surface charge densities, we observe varying power-law exponents even with a constant surface charge and show that α depends on how electrically "perfect" the nanopore is. Specifically, when the net charge of the solution in the pore is insufficient to ensure electroneutrality, the pore is electrically "imperfect" and such nanopores can exhibit varying α depending on the degree of "imperfectness". We present an ionic conductance theory for electrically "imperfect" nanopores that not only explains the various power-law relationships but also describes most of the experimental data available in the literature.

Electronic Conductance Resonance in Non-Redox-Active Proteins.

Abstract: Bioelectronics research has mainly focused on redox-active proteins because of their role in biological charge transport. In these proteins, electronic conductance is a maximum when electrons are injected at the known redox potential of the protein. It has been shown recently that many non-redox-active proteins are good electronic conductors, though the mechanism of conduction is not yet understood. Here, we report single-molecule measurements of the conductance of three non-redox-active proteins, maintained under potential control in solution, as a function of electron injection energy. All three proteins show a conductance resonance at a potential ∼0.7 V removed from the nearest oxidation potential of their constituent amino acids. If this shift reflects a reduction of reorganization energy in the interior of the protein, it would account for the long-range conductance observed when carriers are injected into the interior of a protein.

Single-Molecule Conductance through an Isoelectronic B–N Substituted Phenanthrene Junction

Abstract: Single-molecule conductance of a B–N substituted phenanthrene derivative and its isoelectronic C═C counterpart was investigated by the scanning tunneling microscopy break junction (STM-BJ) techniqu...

Dielectric and electrical properties of annealed ZnS thin films. The appearance of the OLPT conduction mechanism in chalcogenides

Abstract: The annealing temperature (Ta) dependence of the structural, morphological, electrical and dielectric properties of ZnS thin films was investigated. In this work, we consider the as-deposited and annealed ZnS thin films at different temperatures. The as-deposited films were amorphous in nature. However, the films annealed at Ta ≥ 673 K, exhibited a hexagonal structure with (002) preferential orientation. The post annealing caused an improvement in crystallinity. The best one was observed at Ta = 723 K. Grain size increased from 7 nm to 25 nm as annealing temperature was increased from 673 K to 723 K. The surface of annealed samples is homogenous and uniform and the rms roughness is dependent on the annealing temperature: it increases with temperature within the range 5–50 nm. The film electrical conductance is found to be dependent on frequency measurement and annealing temperature: the dc conductance exhibits semi-conductor behavior for all ZnS films over the explored range of temperature and the conductance was found to enhance with increasing annealing temperature up to 623 K. In addition, it was observed that the highest conductance and lowest activation energy of ZnS films were obtained at an annealing temperature of 623 K. The mechanism of alternating current ac conductance can be reasonably explained in terms of the overlapping-large polaron tunnelling (OLPT) model for samples annealed at 623 K and 673 K. To our knowledge, this conduction mechanism was rarely found in chalcogenide materials. A significant change of Nyquist plot with annealing temperature was noted permitting the correlation between the microstructure and its electrical properties. The impedance analysis investigated that the relaxation process is well pronounced for the both annealed films at 623 K and 673 K. The dielectric behavior was associated to the polarization effect, an improvement on the dielectric constant e′ and dielectric loss e′′ with annealing was noticed.

Achieving Efficient Multichannel Conductance in Through-Space Conjugated Single-Molecule Parallel Circuits

Abstract: Constructing single-molecule parallel circuits with multiple conduction channels is an effective strategy to improve the conductance of a single molecular junction, but rarely reported. We present a novel through-space conjugated single-molecule parallel circuit (f-4Ph-4SMe) comprised of a pair of closely parallelly aligned p-quaterphenyl chains tethered by a vinyl bridge and end-capped with four SMe anchoring groups. Scanning-tunneling-microscopy-based break junction (STM-BJ) and transmission calculations demonstrate that f-4Ph-4SMe holds multiple conductance states owing to different contact configurations. When four SMe groups are in contact with two electrodes at the same time, the through-bond and through-space conduction channels work synergistically, resulting in a conductance much larger than those of analogous molecules with two SMe groups or the sum of two p-quaterphenyl chains. The system is an ideal model for understanding electron transport through parallel π-stacked molecular systems and may serve as a key component for integrated molecular circuits with controllable conductance.

pH-Activated Single Molecule Conductance and Binding Mechanism of Imidazole on Gold.

Abstract: We identify imidazole as a pH-activated linker for forming stable single molecule–gold junctions with several distinct configurations and reproducible electrical characteristics. Using a scanning t...

Conductance and Configuration of Molecular Gold-Water-Gold Junctions under Electric Fields

Abstract: Water molecules can mediate charge transfer in biological and chemical reactions by forming electronic coupling pathways. Understanding the mechanism requires a molecular-level electrical characterization of water. Here, we describe the measurement of single water molecular conductance at room temperature, characterize the structure of water molecules using infrared spectroscopy, and perform theoretical studies to assist in the interpretation of the experimental data. The study reveals two distinct states of water, corresponding to a parallel and perpendicular orientation of the molecules. Water molecules switch from parallel to perpendicular orientations on applying an electric field, producing switching from high to low conductance states, thus enabling the determination of single water molecular dipole moments. The work further shows that water-water interactions affect the atomic scale configuration and conductance of water molecules. These findings demonstrate the importance of the discrete nature of water molecules in electron transfer and set limits on water-mediated electron transfer rates.

Improvement of Conductance Modulation Linearity in a Cu2+-Doped KNbO3 Memristor through the Increase of the Number of Oxygen Vacancies.

Abstract: The Pt/KNbO3/TiN/Si (KN) memristor exhibits various biological synaptic properties. However, it also displays nonlinear conductance modulation with the application of identical pulses, indicating t...

Non-equilibrium Steady State Conductivity in Cyclo[18]carbon and Its Boron Nitride Analogue

Abstract: A ring-shaped carbon allotrope was recently synthesized for the first time, reinvigorating theoretical interest in this class of molecules. The dual $\pi$ structure of these molecules allows for the possibility of novel electronic properties. In this work we use reduced density matrix theory to study the electronic structure and conductivity of cyclo[18]carbon and its boron nitride analogue, B\textsubscript{9}N\textsubscript{9}. The variational 2RDM method replicates the experimental polyynic geometry of cyclo[18]carbon. We use a current-constrained 1-electron reduced density matrix (1-RDM) theory with Hartree-Fock molecular orbitals and energies to compute the molecular conductance in two cases: (1) conductance in the plane of the molecule and (2) conductance around the molecular ring as potentially driven by a magnetic field through the molecule's center. In-plane conductance is greater than conductance around the ring, but cyclo[18]carbon is slightly more conductive than B\textsubscript{9}N\textsubscript{9} for both in-the-plane and in-the-ring conduction. The computed conductance per molecular orbital provides insight into how the orbitals---their energies and densities---drive the conduction.

Hybrid Molecular-Junction Mapping Technique for Simultaneous Measurements of Single-Molecule Electronic Conductance and Its Corresponding Binding Geometry in a Tunneling Junction.

Abstract: Improving the techniques for single-molecule conductance measurements is important for the progress of molecular electronics. In this report, a novel technique, which is named molecular-junction mapping (MJM) technique, is demonstrated to be able to simultaneously measure the electronic conductance of single molecules and their corresponding conformations in an electrode gap. Measured conductances of a few model molecules yield a much narrower distribution as compared with the results obtained using conventional break-junction technique, indicating that better defined metal-molecule contacts can be achieved using this new technique. In addition, multiple binding states of an alkanedithiol molecule in an electrode gap, which give rise to multiple conductance states, are efficiently revealed by this hybrid technique, with the results being consistent with those in former reports. This newly demonstrated technique opens up a new avenue for the study of single-molecule electronic properties and will instantly add significant assets to the tool library available for researchers in molecular electronics.

Quantitative Study of the Energy Changes in Voltage-Controlled Spin Crossover Molecular Thin Films.

Abstract: Voltage-controlled nonvolatile isothermal spin state switching of a [Fe{H2B(pz)2}2(bipy)] (pz = tris(pyrazol-1-1y)-borohydride, bipy = 2,2'-bipyridine) film, more than 40 to 50 molecular layers thick, is possible when it is adsorbed onto a molecular ferroelectric substrate. Accompanying this high-spin and low-spin state switching, at room temperature, we observe a remarkable change in conductance, thereby allowing not only nonvolatile voltage control of the spin state ("write") but also current sensing of the molecular spin state ("read"). Monte Carlo Ising model simulations of the high-spin state occupancy, extracted from X-ray absorption spectroscopy, indicate that the energy difference between the low-spin and high-spin state is modified by 110 meV. Transport measurements demonstrate that four terminal voltage-controlled devices can be realized using this system.

Carbazole-Based Tetrapodal Anchor Groups for Gold Surfaces: Synthesis and Conductance Properties.

Abstract: As the field of molecular‐scale electronics matures and the prospect of devices incorporating molecular wires becomes more feasible, it is necessary to progress from the simple anchor groups used in fundamental conductance studies to more elaborate anchors designed with device stability in mind. This study presents a series of oligo(phenylene‐ethynylene) wires with one tetrapodal anchor and a phenyl or pyridyl head group. The new anchors are designed to bind strongly to gold surfaces without disrupting the conductance pathway of the wires. Conductive probe atomic force microscopy (cAFM) was used to determine the conductance of self‐assembled monolayers (SAMs) of the wires in Au−SAM−Pt and Au−SAM−graphene junctions, from which the conductance per molecule was derived. For tolane‐type wires, mean conductances per molecule of up to 10 −4.37 G 0 (Pt) and 10 −3.78 G 0 (graphene) were measured, despite limited electronic coupling to the Au electrode, demonstrating the potential of this approach. Computational studies of the surface binding geometry and transport properties rationalise and support the experimental results.

Effect of alkyl chain on micellization properties of dodecylbenzenesulfonate based surface active ionic liquids using conductance, surface tension, and spectroscopic techniques

Abstract: Two surface active ionic liquids (ILs) having 1-alkyl-3-methylimidazolium cationic moiety and dodecylbenzenesulfonate based anionic moiety i.e. [C5mim][DBS] and [C7mim][DBS], have been synthesized....

Ubiquitous Electron Transport in Non-Electron Transfer Proteins.

Abstract: Many proteins that have no known role in electron transfer processes are excellent electronic conductors. This surprising characteristic is not generally evident in bulk aggregates or crystals, or in isolated, solvated peptides, because the outer hydrophilic shell of the protein presents a barrier to charge injection. Ligands that penetrate this barrier make excellent electrical contacts, yielding conductivities on the order of a S/m. The Fermi Energy of metal electrodes is aligned with the energy of internal electronic states of the protein, as evidenced by resonant transmission peaks at about 0.3V on the Normal Hydrogen Electrode scale. This energy is about 0.7 V less than the oxidation potential of aromatic amino acids, indicating a large reduction in electrostatic reorganization energy losses in the interior of the proteins. Consistent with a possible biological role for this conductance, there is a strong dependence on protein conformation. Thus, direct measurement of conductance is a powerful new way to read out protein conformation in real time, opening the way to new types of single molecule sensors and sequencing devices.

Fabrication and Characterization of TiO x Memristor for Synaptic Device Application

Abstract: In this work, a two-terminal TiO x -based memristor device has been fabricated and the methods for controlling its conductance are demonstrated. The fabricated memristor device exhibits bipolar analog resistive-switching characteristics and the conductance margin over 10 fold between the highest and the lowest resistance states (RS). It is revealed that the conductance can be adjusted with high resolution by either continuous voltage sweep mode or pulse mode. In the former mode, the conductance is controlled as set/reset sweep stop voltages are changed by −0.2 V/ 0.2 V, respectively. In the latter method, the conductance is controlled by modulating the pulse width and amplitude. When the fabricated device is utilized as a synaptic device, consequently, the potentiation and depression operations start at voltages below −1.8 V and over 1.0 V, respectively. It has been found that the conductance changes and nonlinearity characteristics of weight update are tuned with various pulse widths and amplitudes. These results support that the fabricated memristor in a highly simple material configuration of Al/TiO x /Al can be a strong candidate for a synaptic device for the hardware-driven neuromorphic system as well as a novel nonvolatile memory device by the accurate conductance adjustability.

Intermolecular Effects on Tunneling through Acenes in Large-Area and Single-Molecule Junctions

Abstract: This paper describes the conductance of single-molecules and self-assembled monolayers comprising an oligophenyleneethynylene core, functionalized with acenes of increasing length that extend conjugation perpendicular to the path of tunneling electrons. In the Mechanically Controlled Break Junction (MCBJ) experiment, multiple conductance plateaus were identified. The high conductance plateau, which we attribute to the single molecule conformation, shows an increase of conductance as a function of acene length, in good agreement with theoretical predictions. The lower plateau is attributed to multiple molecules bridging the junctions with intermolecular interactions playing a role. In junctions comprising a self-assembled monolayer with eutectic Ga-In top-contacts (EGaIn), the pentacene derivative exhibits unusually low conductance, which we ascribe to the inability of these molecules to pack in a monolayer without introducing significant intermolecular contacts. This hypothesis is supported by the MCBJ data and theoretical calculations showing suppressed conductance through the PC films. These results highlight the role of intermolecular effects and junction geometries in the observed fluctuations of conductance values between single-molecule and ensemble junctions, and the importance of studying molecules in both platforms.

Magnetic properties and impedance spectroscopic analysis in Pr0.7Ca0.3Mn0.95Fe0.05O3 perovskite ceramic

Abstract: The orthorhombic Pr0.7Ca0.3Mn0.95Fe0.05O3 manganite is subject to magnetic and impedance spectroscopy measurements. This sample shows a paramagnetic to ferromagnetic phase transition at about 90 K. According to the Banerjee criterion, the nature of the magnetic transition is found to be of second order.The conductance spectra for Pr0.7Ca0.3Mn0.95Fe0.05O3 ceramic obey the power law variation for characterizing the hopping dynamics of charge carriers. The activation energies extracted from the dc conductance and hopping frequency show a positive correlation. Using the scaling approach, the conductance spectra are merged into a single master curve, confirming the validity of the time–temperature superposition principle. Equally, a strong deviation from the Summerfield scaling is observed. The random barrier model (RBM) is applied to correct this anomaly and a single master curve is constructed with a positive value of the scaling parameter α. Such parameter indicates a coulomb exchange between the interacting particles. Impedance results confirm the contribution of the resistive grain boundary on the electrical properties and the appearance of multiple electrical relaxation phenomena in Pr0.7Ca0.3Mn0.95Fe0.05O3 sample. From the evolution of the derivative ANC with temperature, we confirm the presence of various conduction mechanisms. The Nyquist plots show that the increase in temperature is followed by a decrease in the grain resistance (Rg) and the grain boundary resistance (Rgb) values. Such monotonic decrease confirms a predominant role of grain boundary contribution in governing the transport properties of Pr0.7Ca0.3Mn0.95Fe0.05O3 compound.

Zero-bias conductance peak in Dirac semimetal-superconductor devices

Abstract: The authors report the observation of a large zero bias conductance peak in junction structures of several materials, with a value close to four times that of the normal state conductance. Their analysis suggest that this can be attributed to the existence of a supercurrent between two far-separated superconducting Al electrodes.

Non-equilibrium steady state conductivity in cyclo[18]carbon and its boron nitride analogue.

Abstract: A ring-shaped carbon allotrope was recently synthesized for the first time, reinvigorating theoretical interest in this class of molecules. The dual π structure of these molecules allows for the possibility of novel electronic properties. In this work we use reduced density matrix theory to study the electronic structure and conductivity of cyclo[18]carbon and its boron nitride analogue, B9N9. The variational 2-RDM method replicates the experimental polyynic geometry of cyclo[18]carbon. We use a current-constrained 1-electron reduced density matrix (1-RDM) theory with Hartree–Fock molecular orbitals and energies to compute the molecular conductance in two cases: (1) conductance in the plane of the molecule and (2) conductance around the molecular ring as potentially driven by a magnetic field through the molecule's center. In-plane conductance is greater than conductance around the ring, but cyclo[18]carbon is slightly more conductive than B9N9 for both in-the-plane and in-the-ring conduction. The computed conductance per molecular orbital provides insight into how the orbitals—their energies and densities—drive the conduction.