Conductance Quantization Research Articles

Multistage resistive switching behavior organic coating films-based of memory devices

Memory devices were prepared by spin-coating process, and the multistage resistive switching behavior has been found in organic coating films-based of memory device. The composite of [6,6]-phenyl C61-butyric acid methyl ester and polyvinyl pyrrolidone were used as active layer materials. The as-prepared device exhibits a typical nonvolatile write-once-read-many- times storage effect, and shows multistate resistive switching behavior, and there are obvious distinctions between different resistance states. The resistance in low resistance state (LRS) and high resistance state (HRS) dependence on temperature is tested, and the resistance of LRS and HRS show metal and semiconductor characteristics, respectively. As well as, the resistance in HRS shows evident dependence on cell size. Hence, the resistive switching mechanism was attributed to the broken processes of carbon-rich conducting filament. Furthermore, conductance quantization during resistive switching processes was analyzed. This work might make it attractive for exploiting high density data storage.

Plateaus of quantized conductance with high steps in topological nodal-line semimetals

Topological nodal-line semimetals (TNLSMs) are a new state of quantum matter. It has one-dimensional energy band intersections where the conduction band and valence band cross to form one or more nodal rings. Here, based on the Landauer-B\"uttiker formula combined with the nonequilibrium Green's function method, we investigate the electron transport through TNLSM nanowires under a perpendicular magnetic field. We theoretically calculate the conductance of TNLSM nanowires and find that it is essentially different from other topological and trivial materials. The conductance of TNLSM nanowires presents the high plateaus with high steps in multiples of ${e}^{2}/h$. The height of the steps is determined by the size of the nanowire and for a TNLSM nanowire with the size of several tens of nanometers, its conductance plateaus can reach hundreds of ${e}^{2}/h$ with the step height being tens of ${e}^{2}/h$. The unique phenomenon of the high-stepped conductance plateaus originates from the peculiar energy spectrum of the TNLSMs, that is, there are a lot of subbands having the same energy minimum value. Furthermore, the high conductance plateaus and the high steps can well remain in the existence of magnetic fields, the moderate disorder, and the PT-symmetry-breaking mass term. This unique conductance characteristic can serve as a reliable signature of TNLSMs in experimental detections.

Magnesium Ions Depolarize the Neuronal Membrane via Quantum Tunneling through the Closed Channels

Magnesium ions have many cellular actions including the suppression of the excitability of neurons; however, the depolarization effect of magnesium ions seems to be contradictory. Thus several hypotheses have aimed to explain this effect. In this study, a quantum mechanical approach is used to explain the depolarization action of magnesium. The model of quantum tunneling of magnesium ions through the closed sodium voltage-gated channels was adopted to calculate the quantum conductance of magnesium ions, and a modified version of Goldman–Hodgkin–Katz equation was used to determine whether this quantum conductance was significant in affecting the resting membrane potential of neurons. Accordingly, it was found that extracellular magnesium ions can exhibit a depolarization effect on membrane potential, and the degree of this depolarization depends on the tunneling probability, the channels’ selectivity to magnesium ions, the channels’ density in the neuronal membrane, and the extracellular magnesium concentration. In addition, extracellular magnesium ions achieve a quantum conductance much higher than intracellular ones because they have a higher kinetic energy. This study aims to identify the mechanism of the depolarization action of magnesium because this may help in offering better therapeutic solutions for fetal neuroprotection and in stabilizing the mood of bipolar patients.

The role of oxygen vacancies in the high cycling endurance and quantum conductance in BiVO4‐based resistive switching memory

AbstractResistive random access memory (RRAM) has emerged as a new discipline promoting the development of new materials and devices toward a broad range of electronic and energy applications. Here, we realized a memristive device with weak dependence on the top electrodes and demonstrated the quantized conductance (QC) nature in BiVO4 matrix. The electronic properties have been investigated by the measurements of I‐V curves, where the resistive switching (RS) phenomenon with stable switching ratio and excellent long‐term retention capabilities are identified. Two more inert materials, TiN and Pd, are applied as the top electrodes to exclude the influence of electrodes on the RS states and QC behavior. The X‐ray photoelectron spectroscopy results and transport measurements reveal that the conductive filament (CF) is composed by elemental bismuth. The naturally existed oxygen vacancies in BiVO4 matrix plays as the role of catalyst in the formation and dissolution of CF in BiVO4‐based RRAM device, which is the primary cause for the observed weak dependence of switching performance in this device on the type of top electrodes. Our results clearly illustrate that BiVO4 could be a new idea platform to realize the high scalability, high cycling endurance, and multilevel storage RRAM devices.image

Existence of robust edge currents in Sierpiński fractals

We investigate the Hall conductivity in a Sierpinski carpet, a fractal of Hausdorff dimension $d_f=\ln(8)/\ln(3) \approx 1.893$, subject to a perpendicular magnetic field. We compute the Hall conductivity using linear response and the recursive Green function method. Our main finding is that edge modes, corresponding to a maximum Hall conductivity of at least $\sigma_{xy}=\pm \frac{e^2}{h}$, seems to be generically present for arbitrary finite field strength, no mater how one approaches the thermodynamic limit of the fractal. We discuss a simple counting rule to determine the maximal number of edge modes in terms of paths through the system with a fixed width. This quantized edge conductance, as in the case of the conventional Hofstadter problem, is stable with respect to disorder and thus a robust feature of the system.

Quantum interference effect in few-layered transition metal dichalcogenide

Van der Waals layered transition metal dichalcogenides (TMDCs), as atomically flat two-dimensional materials, have been studied extensively in both fundamental science and application fields in recent years. The reduced-dimensional properties of TMDCs not only provide a route for the fabricating of efficient field effect transistors and optoelectronic devices but also suggest the possibility of the devices that utilize quantum coherency. In this work, we characterize the electron transport properties of ReS2, one of the TMDCs, at both room temperature and low temperature. Of particular note, we measured strong quantum conductance oscillations as a function of the gate voltages and source-drain voltages at reduced temperature, which is evidence of quantum coherent transport. This work unambiguously establishes ReS2 as a promising candidate for future quantum materials.

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

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 $\textit{a priori}$ 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.

Coulomb blockade in monolithic and monocrystalline Al-Ge-Al nanowire heterostructures

We report the realization of Ge single-hole transistors based on Al-Ge-Al nanowire (NW) heterostructures. The formation of these axial structures is enabled by a thermally induced exchange reaction at 350 °C between the initial Ge NW and Al contact pads, leading to a monolithic and monocrystalline Al-Ge-Al NW. The 25 nm-diameter Ge segment is a quasi-1D hole channel. Its length is defined by two abrupt Al-Ge Schottky tunnel barriers. At low temperatures, the device shows a single hole transistor signature with well pronounced Coulomb oscillations. The barrier strength between the Ge segment and the Al leads can be tuned as a function of the gate voltage VG. It leads to a zero conductance at VG= 0 V to a few quantum conductance at VG= –15 V. When the gate voltage increases from –5 V to –3 V, the charging energy is extracted and it varies from 0.39 meV to 2.42 meV.

Multistage Resistive Switching Processes in Filament Conduction-Based Organic Memory Devices

The broken processes of carbon-rich conducting filament have been clarified in composite of [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) and poly(4-vinyl phenol) (PVP) based resistive memory devices. Here, the multistage resistive switching processes in Al/PCBM + PVP/Al sandwich construction were reported, which exhibits a typical nonvolatile WORM storage effect, and these several states are well separated by a resistance ratio of 1: (5.01 × 101): (2.08 × 102): (5.45 × 102): (2.42 × 103). The Al/PCBM + PVP/Al memory devices feature multistate data storage, long retention time (1 × 105 s), and large memory window (maximum ON/OFF ratio, ∼104). Furthermore, conductance quantization during resistive switching processes was analyzed. This work might promote and improve the understanding and awareness of resistive switching mechanism for organic-based memory devices.

Magnetism modulation and conductance quantization in a gadolinium oxide memristor.

The requests for higher information storage density, greater data processing power, and memory-centric computing capability in the current big data era are motivating global research interests in novel solid-state electronic devices that can unite the electron charge and spin degrees of freedom. Herein, the simultaneous realization of magnetism modulation and conductance quantization in a single gadolinium oxide memristor is reported. A remarkable enhancement of >170% in saturation magnetization at room temperature, accompanied by the emergence of a clear magnetoresistance behavior at low temperature, was obtained after setting the memristor from the initial high resistance state (HRS) into the low resistance state (LRS). By carefully resetting the memristor from the LRS into the HRS, up to 32 quantized conductance states with good repeatability and stability were observed, which could possibly allow achieving 5 bit storage in a single memory cell in the future. Moreover, the resistive switching mechanism of the memristor was thoroughly investigated with the help of temperature-dependent resistance tests and high-resolution transmission electron microscopy examination. This work could provide a powerful approach to design future multi-field modulated, high-performance information devices with integrated data storage, sensing, as well as processing functions.

Двухканальный электронный транспорт в подвешенных квантовых точечных контактах с боковыми затворами

The conductance of a suspended quantum point contact fabricated from GaAs/AlGaAs heterostructures with a two-dimensional electron gas, equipped with the in-plane side gates separated from the constriction using lithographical trenches, is studied. The conductance as a function of the gate voltages demonstrates unusual double-channel regime with independent channel’s conductance quantization: two side gates can drive the conductance of the separate channels independently. A possible electrostatic mechanism of the double-channel structure formation inside a single constriction is connected with the lateral redistribution of the low-mobility X-valley electrons contained in superlattice layers, resulting in the emergence of the potential barrier in the middle of quantum point contact, separating the conducting electrons into two channels, symmetrically shifted towards the lithographical trenches, defining the nanostructure geometry.

Synthesis and characterization of Ge-core/a-Si-shell nanowires with conformal shell thickness deposited after gold removal for high-mobility p-channel field-effect transistors

Ge-core/a-Si-shell nanowires were synthesized in three consecutive steps. Nominally undoped crystalline Ge nanowires were first grown using a vapor–liquid–solid growth mechanism, followed by gold catalyst removal in an etching solution and deposition of a thin layer of amorphous silicon on the nanowire surface using a chemical vapor deposition method. Catalyst removal is necessary to avoid catalyst melting during temperature increase prior to a-Si shell deposition. Field effect transistors based on Ge-core/a-Si-shell nanowires exhibited p-channel depletion-mode characteristics as a result of free hole accumulation in the Ge channel. Scaled on-currents and transconductances up to 3.1 mA μm−1 and 4.3 mS μm−1, respectively, as well as on/off ratios and field-effect hole mobilities up to 102 and 664 cm2 V−1 s−1, respectively, were obtained for these Ge-core/a-Si-shell nanowire FETs. The minimum subthreshold slope was measured to be 300 mV dec−1. The present work also demonstrates for the first time the conductance quantization in one-dimensional Ge-core/a-Si-shell nanowires at low temperatures. The quantization of conductances at discrete values of G0 = 2e2/h at low temperatures suggests that our Ge-core/a-Si-shell nanowires are multi-mode ballistic conductors with a mean-free-path up to 500 nm. The results provided here are relevant for the synthesis of high-quality Ge-core/Si-shell nanowires for high-mobility devices with transparent contacts to hole carriers.

Scaled conductance quantization unravels the switching mechanism in organic ternary resistive memories

Scaled conductance quantization phenomenon was observed in organic ternary memory. A new perspective to understand the nature of resistance switching in organic ternary memory devices was provided.

Study of Molecular Junctions Metal—DNA—Metal for the DNA Sequencing

The process of the DNA sequencing in solid-state nanopores is considered. The theoretical modeling of ionic, thermionic, and tunnel currents for nanopores with the gold electrodes located in bio-liquids has been carried out. It is shown that both the ionic and thermionic currents are several orders of magnitude weaker than the direct tunneling current, which reaches several hundred of nA. The differences between the values of direct tunnel currents for four DNA nucleotides, as well as for guanine-cytosine and thymine-adenine nucleotide pairs are significant, which makes it easy to identify each DNA nucleotide. The behavior of the differential quantum conductance of the corresponding molecular junctions is also analyzed. The histograms for tunnel currents and conductance for four the DNA and guanine-cytosine and thymine-adenine nucleotides are plotted.

Geometry-Assisted Topological Transitions in Spin Interferometry.

We identify a series of topological transitions occurring in electronic spin transport when manipulating spin-guiding fields controlled by the geometric shape of mesoscopic interferometers. They manifest as distinct inversions of the interference pattern in quantum conductance experiments. We establish that Rashba square loops develop weak-(anti)localization transitions (absent in other geometries as Rashba ring loops) as an in-plane Zeeman field is applied. These transitions, boosted by nonadiabatic spin scattering, prove to have a topological interpretation in terms of winding numbers characterizing the structure of spin modes in the Bloch sphere.

Quantization of spin Hall conductivity in two-dimensional topological insulators versus symmetry and spin-orbit interaction

The third-rank tensor of the static spin Hall conductivity is investigated for two-dimensional (2D) topological insulators by electronic structure calculations. Its seeming quantization is numerically demonstrated for highly symmetric systems independent of the gap size. 2D crystals with hexagonal and square Bravais lattice show similar effects, while true rectangular translational symmetry yields conductivity values much below the quantum $e^2/h$. Field-induced lifting the inversion symmetry does not influence the quantum spin Hall state up to band inversion but the conductivity quantization. Weak symmetry-conserving biaxial but also uniaxial strain has a minor influence as long as inverted gaps dictate the topological character. The results are discussed in terms of the atomic geometry and the Rashba contribution to the spin-orbit interaction (SOI). Translational and point-group symmetry as well as SOI rule the deviation from the quantization of the spin Hall conductance.

Controllable and Stable Quantized Conductance States in a Pt/HfOx/ITO Memristor

AbstractQuantum‐level manipulation of atomic configuration offers a excellent platform for the construction of exotic nanostructures that exhibit unusual solid‐state physics and electronic properties. One particular example is the memristor, in which the elaborate evolution of atomic point contact via local ionic processes and consequent stepwise device conductance quantization enable bottom‐up design of in‐memory computing with greatly increased data storage density and more efficient multi‐value logic algorithm. In‐depth understanding on the physics of atomic reconfiguration is achieved through comprehensive consideration of the thermodynamics and kinetics of nanoionics in memristors, based on which a general protocol of constructing atomic point contact structure with desired quantized conductance is established. Through energy‐driven single‐atom level oxygen manipulation in the reset process of a Pt/HfOx/ITO structure, up to 32 consecutive quantized conductance states with an interval of half conductance quantum that can be sustained for over 7000 s and tuned 500 times are demonstrated for the first time, not only allowing the physical implementation of ternary logic‐in‐memory functions, but also providing a universal methodology for building next‐generation quantum electronic devices.

Effect of quantized conductivity on the anomalous photon emission radiated from atomic-size point contacts

Abstract We observe anomalous visible to near-infrared electromagnetic emission from electrically driven atomic-size point contacts. We show that the number of photons released strongly depends on the quantized conductance steps of the contact. Counterintuitively, the light intensity features an exponential decay dependence with the injected electrical power. We propose an analytical model for the light emission considering an out-of-equilibrium electron distribution. We treat photon emission as a Bremsstrahlung process resulting from hot electrons colliding with the metal boundary, and find qualitative accord with the experimental data.

Reproducing topological properties with quasi-Majorana states

Andreev bound states in hybrid superconductor-semiconductor devices can have near-zero energy in the topologically trivial regime as long as the confinement potential is sufficiently smooth. These quasi-Majorana states show zero-bias conductance features in a topologically trivial phase, mimicking spatially separated topological Majorana states. We show that in addition to the suppressed coupling between the quasi-Majorana states, also the coupling of these states across a tunnel barrier to the outside is exponentially different for increasing magnetic field. As a consequence, quasi-Majorana states mimic most of the proposed Majorana signatures: quantized zero-bias peaks, the 4\pi4π Josephson effect, and the tunneling spectrum in presence of a normal quantum dot. We identify a quantized conductance dip instead of a peak in the open regime as a distinguishing feature of true Majorana states in addition to having a bulk topological transition. Because braiding schemes rely only on the ability to couple to individual Majorana states, the exponential control over coupling strengths allows to also use quasi-Majorana states for braiding. Therefore, while the appearance of quasi-Majorana states complicates the observation of topological Majorana states, it opens an alternative route towards braiding of non-Abelian anyons and protected quantum computation.

An ultrasensitive hydrogen fuse based on a single Pd break junction

The development of hydrogen compatibility is crucial for its being a new key energy source. A study of hydrogen fuse based on Pd break junction method is presented to explore the limit of sensitivity. The critical part of this device was fabricated from a Pd nanowire electrode (80 ± 20 nm) made by electron beam lithography (EBL) method. Subsequent electromigration technique is utilized to develop a controllable break junction on the nanowire (1–2 nm). This device shows reversible response cycles of hydrogen in an ultralow pressure range below 0.1±0.002 Pa and fuse-like response above 10 Pa at room temperature. The results show that the relationship between quantum conductance with hydrogen pressure was kept linear because of hydrogen absorption induced expansion of break junction. When the expansion exceeds the threshold value, the break junction gap is closed and becoming conductive. This method opens up a possibility to detect hydrogen under ultralow partial pressure.