Conductance Quantization Research Articles

OBSERVING CONDUCTANCE QUANTIZATION by a NOVEL MAGNETIC CONTROL SYSTEM

In this study, a novel magnetic system that allows observing quantized conductance for undergraduate and graduate laboratories is presented. Bending of a magnetic cylindrical beam, like a cantilever, was controlled by an electromagnet to provide contact between needle type electrode and a plane of conductor. It is shown that by using the beam bending, it is possible to displace an object on the beam in nanometer and micrometer scale. The measured quantized conductance results prove that the designed system can be used for demonstration of quantized conductance.

Quantized Spin Pumping in Topological Ferromagnetic-Superconducting Nanowires.

Semiconducting nanowires with strong spin-orbit coupling in the presence of induced superconductivity and ferromagnetism can support Majorana zero modes. We study the pumping due to the precession of the magnetization in single-subband nanowires and show that spin pumping is robustly quantized when the hybrid nanowire is in the topologically nontrivial phase, whereas charge pumping is not quantized. Moreover, there exists one-to-one correspondence between the quantized conductance, entropy change and spin pumping in long topologically nontrivial nanowires but these observables are uncorrelated in the case of accidental zero-energy Andreev bound states in the trivial phase. Thus, we conclude that observation of correlated and quantized peaks in the conductance, entropy change and spin pumping would provide strong evidence of Majorana zero modes, and we elaborate how topological Majorana zero modes can be distinguished from quasi-Majorana modes potentially created by a smooth tunnel barrier at the lead-nanowire interface. Finally, we discuss peculiar interference effects affecting the spin pumping in short nanowires at very low energies.

Topological field-effect transistor with quantized on/off conductance of helical/chiral dislocation states

Topology is a key ingredient driving the emergence of quantum devices. The topological field-effect transistor (TFET) has been proposed to outperform the conventional field-effect transistor by replacing the on state with topology-protected quantized conductance, while the off state has the same normal insulating characteristics and hence bears similar drawbacks. Here, we demonstrate a proof-of-concept TFET, having both on and off quantized conductance, by switching between helical and chiral topological screw dislocation (SD) states in three-dimensional topological insulators. A pair of SDs are configured, with one acting as a channel and the other as a gate controlled by a local magnetic field. Reversible field switching is achieved with the on and off conductance of $2{e}^{2}/h$ and ${e}^{2}/h$, respectively, as shown by tight-binding quantum transport calculations. Furthermore, $\mathrm{BaBi}{\mathrm{O}}_{3}$ is shown as one candidate material that has the desired topological SD states, based on first-principles calculations. Our findings open a new route to high-fidelity topological quantum devices.

Influence of the two-dimensional density of states on the temperature dependence of the electrical conductivity oscillations in heterostructures with quantum wells

In this work, the influence of two-dimensional state density on oscillations of transverse electrical conductivity in heterostructures with rectangular quantum wells is investigated. A new analytical expression is derived for calculating the temperature dependence of the transverse electrical conductivity oscillation and the magnetoresistance of a quantum well. For the first time, a mechanism has been developed for oscillating the transverse electrical conductivity and magnetoresistance of a quantum well from the first-order derivative of the magnetic field (differential) [Formula: see text] at low temperatures and weak magnetic fields. The oscillations of electrical conductivity and magnetoresistance of a narrow-band quantum well with a nonparabolic dispersion law are investigated. The proposed theory investigated the results of experiments of a narrow-band quantum well (InxGa[Formula: see text]Sb).

Optical activity and transport in twisted bilayer graphene: Spatial dispersion effects

This study investigates the mechanisms driving optical activity and quantum transport in twisted bilayer graphene systems. We demonstrate that optical activity results from spatial dispersion, making it inadequate to consider the system purely two-dimensional. Therefore, we utilize the transfer matrix method to describe the propagation of electromagnetic waves through multilayer systems, which is compatible with our treatment of spatial dispersion. Using an effective continuum model, we analyze the formation of electron states in the bilayer systems and the correlation of transverse and longitudinal motions of electrons in the two graphene layers. The chiral structure of the atomic lattices results in a finite drag component of the optical conductivity tensor due to incomplete cancellation of these correlations. We show the decisive role of this drag component for optical activity by analyzing the circular dichroism spectrum of the system. Additionally, we calculate the dc conductivity, which shows that the twisted bilayer graphene supports a quantum conductivity value proportional to ${e}^{2}/h$ at the intrinsic Fermi energy.

Exploring Conductance Quantization Effects in Electroformed Filaments for Their Potential Application to a Resistance Standard

AbstractThe ballistic conduction through narrow constrictions connecting charge reservoirs exhibits conductance quantization effects. Since the quantum of conductance is only related to fundamental constants of nature, these effects might allow the implementation of a standard of resistance, fulfilling the requirements of the 2019 revised International System of Units. Moreover, this standard would be able to work at room temperature and without a magnetic field, thus allowing its on‐chip implementation. In this work, the authors propose that breakdown filaments in thin oxide layers might be useful to this purpose. In particular, conductance quantization effects in nanolaminate Al2O3/HfO2 dielectrics are reported and the role of intrinsic values of conductance and extrinsic parasitic elements are analyzed. The fact that breakdown filaments are irreversible is an advantage due to their expected stability and to the lack of cycle‐to‐cycle variations (as compared to resistive switching devices). Although the reported sample‐to‐sample variations are still too large for a real application, there is room for improving the controlover breakdown filaments through material design and electroforming conditions. Provided that this control is achieved, an on‐chip implementation of a resistance standard for the realization of self‐calibrating electrical systems and equipment with zero‐chain traceability would be possible.

Conductance quantization in h-BN memristors

Memristive devices made of multilayer hexagonal boron nitride are attracting much attention for information storage, computation, encryption, and communication. Generating multilevel operation in these memristors would be an important added value for the field of neuromorphic computation, but the main phenomenon producing it (i.e., conductance quantization) is not well understood for this material. Here, we analyze hexagonal boron nitride memristive devices with metal electrodes operating in their quasistatic and transient regimes. Conductance quantization effects were observed both in I-V curves measured under ramped voltage stress and in the reset processes driven by voltage pulse trains. Polar and time lag plots are employed to shed light on the physics behind the device resistive switching. A model is proposed to reproduce the electrical characteristics of progressive reset processes and the discretization of the device conductance linked to quantum effects.

Theoretical proposal to obtain strong Majorana evidence from scanning tunneling spectroscopy of a vortex with a dissipative environment

It is predicted that a vortex in a topological superconductor contains a Majorana zero mode (MZM). The confirmative Majorana signature, i.e., the $2{e}^{2}/h$ quantized conductance, however, is easily sabotaged by unavoidable interruptions, e.g., instrument broadening, non-Majorana signal, and extra particle channels. To avoid these interruptions, we propose to obtain strong Majorana evidence following our novel Majorana hunting protocol that relies on dissipation introduced by disorders at, e.g., the sample-substrate interface. Within this protocol, we highlight three features, each of which alone can provide a strong evidence to identify MZM. First, dissipation suppresses a finite-energy Caroli-de Gennes-Matricon (CdGM) conductance peak into a valley, while it does not split MZM zero-bias conductance peak (ZBCP). Second, we predict a dissipation-dependent scaling feature of the ZBCP. Third, the introduced dissipation manifests the MZM signal by suppressing nontopological CdGM modes. Importantly, the observation of these features does not require a quantized conductance value $2{e}^{2}/h$.

Theory of glide symmetry protected helical edge states in a WTe2 monolayer

Helical edge states in quantum spin Hall (QSH) materials are central building blocks of topological matter design and engineering. Despite their principal topological protection against elastic backscattering, the level of operational stability depends on manifold parameters such as the band gap of the given semiconductor system in the 'inverted' regime, temperature, disorder, and crystal orientation. We theoretically investigate electronic and transport properties of QSH edge states in large gap 1-T' WTe$_{2}$ monolayers. We explore the impact of edge termination, disorder, temperature, and interactions on experimentally addressable edge state observables, such as local density of states and conductance. We show that conductance quantization can remain surprisingly robust even for heavily disordered samples because of an anomalously small edge state decay length and additional protection related to the large direct gap allowed by glide symmetry. From the simulation of temperature-dependent resistance, we find that moderate disorder enhances the stability of conductance by localizing bulk states. We evaluate the edge state velocity and Luttinger liquid parameter as functions of the chemical potential, finding prospects for physics beyond linear helical Luttinger liquids in samples with ultra-clean and well-defined edges.

Light-Triggered Reversible Tuning of Second-Harmonic Generation in a Photoactive Plasmonic Molecular Nanocavity.

The ultrasmall mode volume and ultralarge local field enhancement of compact plasmonic nanocavities have been widely explored to amplify a variety of optical phenomena at the nanoscale. Other than passively generating near-field enhancements, dynamic tuning of their intensity and associated nonlinear optical processes such as second-harmonic generation (SHG) play vital roles in the field of active nanophotonics. Here we apply a host-guest molecular complex to construct a photoswitchable molecule-sandwiched metallic particle-on-film nanocavity (MPoFN) and demonstrate both light-controlled linear and nonlinear optical tuning. Under alternating illumination of ultraviolet (UV) and visible light, the photoactive plasmonic molecular nanocavity shows reversible switching of both surface-enhanced Raman scattering (SERS) and plasmon resonance. Surprisingly, we observe more significant modulation of SHG from this photoactive MPoFN, which can be explained qualitatively by the quantum conductivity theory (QCT). Our study could pave the way for developing miniaturized integrated optical circuits for ultrafast all-optical information processing and communication.

Topological properties in an Aubry–André–Harper model with p-wave superconducting pairing

Abstract We study the topological properties of the one-dimensional p-wave Aubry–André–Harper (AAH) model with periodic incommensurate potential and transition coupling. The calculation results show that due to co-influence of the incommensurate potential and modulation phase, three topological phases arise in different parameter regions: a topologically trivial phase, Su–Schrieffer–Heeger (SSH)-like topological phase, and Kitaev-like topological superconducting phase with Majorana zero modes. By evaluating the Andreev reflection conductance, we see that in the Kitaev-like phase, the quantized conductance plateau comes into being at the zero-bias limit, due to the occurrence of resonant Andreev reflection. In addition, when the disorder effect is incorporated, the SSH-like topology is modified sensitively and the degenerate topological states split, whereas the Kitaev-like topological phase is robust to weak disorder. Finally, we find that disorder can induce topological phase transition, i.e. from the topologically trivial phase to the topological phase. Based on these results, we believe that our findings have significance for studying the topological phase transition in a one-dimensional topological superconducting system. Also, it provides a feasible scheme for clarifying different topological phases.

Self-Compliant Threshold Switching Devices with High On/Off ratio by Control of Quantized Conductance in Ag Filaments

Threshold switches based on conductive metal bridge devices are useful as selectors to block sneak leakage paths in memristor arrays used in neuromorphic computing and emerging nonvolatile memory. We demonstrate that control of Ag-cation concentration in Al2O3 electrolyte and Ag filament size and density play an important role in the high on/off ratio and self-compliance of metal-ion-based volatile threshold switching devices. To control Ag-cation diffusion, we inserted an engineered defective graphene monolayer between the Ag electrode and the Al2O3 electrolyte. The Ag-cation migration and the Ag filament size and density are limited by the pores in the defective graphene monolayer. This leads to quantized conductance in the Ag filaments and self-compliance resulting from the formation and dissolution of the Ag conductive filament.

Observation of Edge Magnetoplasmon Squeezing in a Quantum Hall Conductor.

Squeezing of the quadratures of the electromagnetic field has been extensively studied in optics and microwaves. However, previous works focused on the generation of squeezed states in a low impedance (Z_{0}≈50 Ω) environment. We report here on the demonstration of the squeezing of bosonic edge magnetoplasmon modes in a quantum Hall conductor whose characteristic impedance is set by the quantum of resistance (R_{K}≈25 kΩ), offering the possibility of an enhanced coupling to low-dimensional quantum conductors. By applying a combination of dc and ac drives to a quantum point contact, we demonstrate squeezing and observe a noise reduction 18% below the vacuum fluctuations. This level of squeezing can be improved by using more complex conductors, such as ac driven quantum dots or mesoscopic capacitors.

Effects of Quantum Recoil Forces in Resistive Switching in Memristors

Memristive devices, whose resistance can be controlled by applying a voltage and further retained, are attractive as possible circuit elements for neuromorphic computing. This new type of devices poses a number of both technological and theoretical challenges. Even the physics of the key process of resistive switching, usually associated with formation or breakage of conductive filaments in the memristor, is not completely understood yet. This work proposes a new resistive switching mechanism, which should be important in the thin-filament regime and take place due to the back reaction, or recoil, of quantum charge carriers – independently of the conventional electrostatically-driven ion migration. Since thinnest conductive filaments are in question, which are only several atoms thick and allow for a quasi-ballistic, quantized conductance, we use a mean-field theory and the framework of nonequilibrium Green’s functions to discuss the electron recoil effect for a quantum current through a nanofilament on its geometry and compare it with the transmission probability of charge carriers. Namely, we first study an analytically tractable toy model of a 1D atomic chain, to qualitatively demonstrate the importance of the charge-carrier recoil, and further proceed with a realistic molecular-dynamics simulation of the recoil-driven ion migration along a copper filament and the resulting resistive switching. The results obtained are expected to add to the understanding of resistive switching mechanisms at the nanoscale and to help downscale high-retention memristive devices.

A method to detect quantum coherent transport in memristive devices

While the size of functional elements in memristors becomes of the orders of nano-meters or even smaller, the quantum effects in their dynamics can significantly influence their transport properties, consistent with recent experimental observations of conductance quantisation in memristors. This requires the development of experimentally accessible signatures of quantum behaviour in memristive systems, such as a superposition of quantum states with different memristances. Here we discuss one such protocol. Our simulations show that periodic projective measurements induce additional spectral components in the response of quantum memristor to a harmonic input signal. Moreover, the response demonstrates a resonant behaviour when the frequency of the projective measurements commensurates with the frequency of the input. We demonstrate that observation of such harmonic mixing can be used as experimental evidence of quantum effects in memristors.Graphic abstract

Probing Fermi Sea Topology by Andreev State Transport.

We show that the topology of the Fermi sea of a two-dimensional electron gas (2DEG) is reflected in the ballistic Landauer transport along a long and narrow Josephson π junction that proximitizes the 2DEG. The low-energy Andreev states bound to the junction are shown to exhibit a dispersion that is sensitive to the Euler characteristic of the Fermi sea (χ_{F}). We highlight two important relations: one connects the electron or hole nature of Andreev states to the convex or concave nature of Fermi surface critical points, and one relates these critical points to χ_{F}. We then argue that the transport of Andreev states leads to a quantized conductance that probes χ_{F}. An experiment is proposed to measure this effect, from which we predict an I-V characteristic that not only captures the topology of the Fermi sea in metals, but also resembles the rectification effect in diodes. Finally, we evaluate the feasibility of measuring this quantized response in graphene, InAs and HgTe 2DEGs.

Double-Heterostructure Resonant Tunneling Transistors of Surface-Functionalized Sb and Bi Monolayer Nanoribbons

Zigzag nanoribbons tailored from chemically surface-modified Sb or Bi monolayers by methyl, amino or hydroxyl are investigated through first-principles electronic-structure calculations to explore their potential applications in topological transport nanoelectronics. It is verified by Dirac-point-like energy dispersion of band-edges near Fermi level that the scattering-forbidden edge-states of these nanoribbons can give a topological conductive channel with extremely high electron mobility. Accordingly, Sb/SbXHn/Sb and Bi/BiXHn/Bi nanoribbon double-heterostructures (SbXHn or BiXHn: XHn = CH3, NH2, OH) are designed as resonant tunneling transistors and modeled by bipolar transport devices with their electron transport characteristics being calculated by nonequilibrium Green’s function combined first-principles schemes. Ballistic equilibrium conduction spectra and current-voltage characteristics prove that quantum conductance currents of these nanoribbon double heterostructures originate from the electron resonant tunneling between the topological edge-states of the two constituent Sb or Bi monolayer nanoribbons through the central barrier of SbXHn or BiXHn nanoribbon segment. This renders a high resonant current peak with strong negative differential conductance, thus being competent for zero-loss and ultrahigh-frequency resonant tunneling nanotransistors.

Scanning gate microscopy in graphene nanostructures

The conductance of graphene nanoribbons and nanoconstrictions under the effect of a scanning gate microscopy tip is systematically studied. Using a scattering approach for noninvasive probes, the first- and second-order conductance corrections caused by the tip potential disturbance are expressed explicitly in terms of the scattering states of the unperturbed structure. Numerical calculations confirm the perturbative results, showing that the second-order term prevails in the conductance plateaus, exhibiting a universal scaling law for armchair graphene strips. For stronger tips, at specific probe potential widths and strengths beyond the perturbative regime, the conductance corrections reveal the appearance of resonances originated from states trapped below the tip. The zero-transverse-energy mode of an armchair metallic strip is shown to be insensitive to the long-range electrostatic potential of the probe. For nanoconstrictions defined on a strip, scanning gate microscopy allows to get insight into the breakdown of conductance quantization. The first-order correction generically dominates at low tip strength, while for Fermi energies associated with faint conductance plateaus, the second-order correction becomes dominant for relatively small potential tip strengths. In accordance with the spatial dependence of the partial local density of states, the largest tip effect occurs in the central part of the constriction, close to the edges. Nanoribbons and nanoconstrictions with zigzag edges exhibit a similar response as in the case of armchair nanostructures, except when the intervalley coupling induced by the tip potential destroys the chiral edge states.

Metal doped polyaniline as neuromorphic circuit elements for in-materia computing

ABSTRACT Polyaniline-based atomic switches are material building blocks whose nanoscale structure and resultant neuromorphic character provide a new physical substrate for the development next-generation, nanoarchitectonic-enabled computing systems. Metal ion-doped devices consisting of a Ag/metal ion doped polyaniline/Pt sandwich structure were fabricated using an in situ wet process. The devices exhibited repeatable resistive switching between high (ON) and low (OFF) conductance states in both Ag+ and Cu2+ ion-doped devices. The threshold voltage for switching was>0.8 V and average ON/OFF conductance ratios (30 cycles for 3 samples) were 13 and 16 for Ag+ and Cu2+ devices, respectively. The ON state duration was determined by the decay to an OFF state after pulsed voltages of differing amplitude and frequency. The switching behaviour is analagous to short-term (STM) and long-term (LTM) memories of biological synapses. Memristive behaviour and evidence of quantized conductance were also observed and interpreted in terms of metal filament formation bridging the metal doped polymer layer. The successful realization of these properties within physical material systems indicate polyaniline frameworks as suitable neuromorphic substrates for in materia computing.

Effect of Disorder on Magnetotransport in Semiconductor Artificial Graphene

Magnetotransport in mesoscopic samples with semiconductor artificial graphene has been simulated within the Landauer–Büttiker formalism. Model four-terminal systems in a high-mobility two-dimensional electron gas have a square shape with a side of 3–5 μm, which is filled with a short-period (120 nm) weakly disordered triangular lattice of antidots at the modulation amplitude of the electrostatic potential comparable with the Fermi energy. It has been found that the Hall resistance {{R}_{{xy}}}(B) in the magnetic field range of B = 10–50 mT has a hole plateau {{R}_{{xy}}} = - {{R}_{0}}, where R0 = h/2e2 = 12.9 kΩ, at carrier densities in the lattice below the Dirac point n < n1D and an electron plateau {{R}_{{xy}}} = {{R}_{0}} at n > n1D. Enhanced disorder destroys the plateaus, but a carrier type (electrons or holes) holds. Long-range disorder at low magnetic fields suppresses quantized resistance plateaus much more efficiently than short-range disorder.