Electron Waveguide Research Articles

Engineered spin-orbit interactions in LaAlO3/SrTiO3-based 1D serpentine electron waveguides.

The quest to understand, design, and synthesize new forms of quantum matter guides much of contemporary research in condensed matter physics. One-dimensional (1D) electronic systems form the basis for some of the most interesting and exotic phases of quantum matter. Here, we describe a family of quasi-1D nanostructures, based on LaAlO3/SrTiO3 electron waveguides, in which a sinusoidal transverse spatial modulation is imposed. These devices display unique dispersive features in the subband spectra, namely, a sizeable shift (∼7 T) in the spin-dependent subband minima, and fractional conductance plateaus. The first property can be understood as an engineered spin-orbit interaction associated with the periodic acceleration of electrons as they undulate through the nanowire (ballistically), while the second property signifies the presence of enhanced electron-electron scattering in this system. The ability to engineer these interactions in quantum wires contributes to the tool set of a 1D solid-state quantum simulation platform.

Bipolar electron waveguides in graphene

We show analytically that the ability of Dirac materials to localize an electron in both a barrier and a well can be utilized to open a pseudo-gap in graphene's spectrum. By using narrow top-gates as guiding potentials, we demonstrate that graphene bipolar waveguides can create a non-monotonous one-dimensional dispersion along the electron waveguide, whose electrostatically controllable pseudo-band-gap is associated with strong terahertz transitions in a narrow frequency range.

Modulation induced transport signatures in correlated electron waveguides

Recent transport experiments in spatially modulated quasi-1D structures created on top of LaAlO_33/SrTiO_33 interfaces have revealed some interesting features, including phenomena conspicuously absent without the modulation. In this work, we focus on two of these remarkable features and provide theoretical analysis allowing their interpretation. The first one is the appearance of two-terminal conductance plateaus at rational fractions of e^2/he2/h. We explain how this phenomenon, previously believed to be possible only in systems with strong repulsive interactions, can be stabilized in a system with attraction in the presence of the modulation. Using our theoretical framework we find the plateau amplitude and shape, and characterize the correlated phase which develops in the system due to the partial gap, namely a Luttinger liquid of electronic trions. The second observation is a sharp conductance dip below a conductance of 1\times e^2/h1×e2/h, which changes its value over a wide range when tuning the system. We theorize that it is due to resonant backscattering caused by a periodic spin-orbit field. The behavior of this dip can be reliably accounted for by considering the finite length of the electronic waveguides, as well as the interactions therein. The phenomena discussed in this work exemplify the intricate interplay of strong interactions and spatial modulations, and reveal the potential for novel strongly correlated phases of matter in systems which prominently feature both.

Phosphorene pnp junctions as perfect electron waveguides

The current flow in phosphorene pnp junctions is studied. At the interfaces of the junction, omni-directional total reflection takes place, named anti-super-Klein tunneling, as this effect is not due to an energetically forbidden region but due to pseudo-spin blocking. The anti-super-Klein tunneling confines electrons within the junction, which thus represents a perfect lossless electron waveguide. Calculating the current flow by applying Green’s function method onto a tight-binding model of phosphorene, it is observed that narrow electron beams propagate in these waveguides like light beams in optical fibers. The perfect guiding is found for all steering angles of the electron beam as the total reflection does not rely on the existence of a critical angle. For low electron energies and narrow junctions, the guided modes of the waveguide are observed. The waveguide operates without any loss only for a specific orientation of the junction. For arbitrary orientations, minor leakage currents are found, which, however, decay for low electron energies and grazing incidence angles. It is shown that a crossroad-shaped pnp junction can be used to split and direct the current flow in phosphorene. The proposed device, a phosphorene pnp junction as a lossless electron waveguide may not only find applications in nanoelectronics but also in quantum information technology.

Patched Green's function method applied to acoustic wave propagation in disordered media: an interdisciplinary approach

AbstractModern visualization can be formulated as inversion problems that aim to obtain structural information about a complex medium through wave excitations. However, without numerically efficient forward calculations, even state-of-the-art inversion procedures are too computationally intensive to implement. We adapt a method previously used to treat transport in electronic waveguides to describe acoustic wave motion in complex media with high gains in computational time. The method consists of describing the system as if it was made of disconnected parts that are patched together. By expressing the system in this manner, wave-propagation calculations that otherwise would involve a very large matrix can be done with considerably smaller matrices instead. In particular, by treating one of such patches as a target whose parameters are changeable, we are able to implement target-oriented optimization in which the model parameters can be continuously refined until the ideal result is reproduced. The so-called Patched Green's function (PGF) approach is mathematically exact and involves no approximations, thus improving the computational cost without compromising accuracy. Given the generality of our method, it can be applied to a wide variety of inversion problems. Here we apply it to the case of seismic modeling where acoustic waves are used to map the earth subsurface in order to identify and explore mineral resources. The technique is tested with realistic seismic models and compared to standard calculation methods. The reduction in computational complexity is remarkable and paves the way to treating larger systems with increasing accuracy levels.

Composite two-particle sources

Multi-particle sources constitute an interesting new paradigm following the recent development of on-demand single-electron sources. Versatile devices can be designed using several single-electron sources, possibly of different types, coupled to the same quantum circuit. However, if combined \textit{non-locally} to avoid cross-talk, the resulting architecture becomes very sensitive to electronic decoherence. To circumvent this problem, we here analyse two-particle sources that operate with several single-electron (or hole) emitters attached in series to the same electronic waveguide. We demonstrate how such a device can emit exactly two electrons without exciting unwanted electron-hole pairs if the driving is adiabatic. Going beyond the adiabatic regime, perfect two-electron emission can be achieved by driving two quantum dot levels across the Fermi level of the external reservoir. If a single-electron source is combined with a source of holes, the emitted particles can annihilate each other in a process which is governed by the overlap of their wave functions. Importantly, the degree of annihilation can be controlled by tuning the emission times, and the overlap can be determined by measuring the shot noise after a beam splitter. In contrast to a Hong-Ou-Mandel experiment, the wave functions overlap close to the emitters and not after propagating to the beam splitter, making the shot noise reduction less susceptible to electronic decoherence.

A guiding path for graphene circuits

Nanoelectronics The favorable optical, electronic, and mechanical properties of graphene make it a target material for nextgeneration opto-electronics. However, that graphene is an atomic layer thick, or several for bilayer and few-layer graphene, can make it challenging to pattern circuits with usual lithographic methods, especially at lateral nanometer scales. Cheng et al. show that a carbon nanotube, separated from the graphene by a thin layer of hexagonal boron nitride, creates a one-dimensional conduction path in the graphene that can be controlled by electrostatic gating. Demonstrating that the charged massless quasiparticles, Dirac fermions, can now be confined to an electronic waveguide provides a route to developing a platform for patterning complex graphene circuitry. Phys. Rev. Lett. 123 , 216804 (2019).

Guiding Dirac Fermions in Graphene with a Carbon Nanotube.

Relativistic massless charged particles in a two-dimensional conductor can be guided by a one-dimensional electrostatic potential, in an analogous manner to light guided by an optical fiber. We use a carbon nanotube to generate such a guiding potential in graphene and create a single mode electronic waveguide. The nanotube and graphene are separated by a few nanometers and can be controlled and measured independently. As we charge the nanotube, we observe the formation of a single guided mode in graphene that we detect using the same nanotube as a probe. This single electronic guided mode in graphene is sufficiently isolated from other electronic states of linear Dirac spectrum continuum, allowing the transmission of information with minimal distortion.

Strong Aharonov-Bohm quantum interference in simply connected LaAlO3/SrTiO3 structures

We report Aharonov-Bohm (AB)-type quantum interference in simply-connected devices created at the LaAlO$_3$/SrTiO$_3$ interface using conductive-atomic force microscope (c-AFM) lithography. The oscillations are multi-periodic functions of magnetic field strength, and they exhibit a substantial magnetic hysteresis with frequencies that depends on the magnetic sweep direction. The oscillation amplitude for the lowest two frequencies approaches $e^2/h$, consistent with the theoretical maximum for the AB effect, and harmonics up to third order are observable. Broadband quasiperiodic behavior is reported in a fraction of simply-connected electron waveguide devices that exhibit magnetic asymmetries. Curiously, nanoscale ring devices that are multiply-connected lack signatures of AB quantum interference. The interference phenomena are associated with an inhomogeneous magnetic landscape within the LaAlO$_3$/SrTiO$_3$ nanostructures.

Electronic transport in metal-molecular nanoelectronic networks: A density functional theory study

Electronic transport properties of nanoscale networks composed of interconnected molecules/gold nanoclusters are examined via first-principles plane wave scattering density functional theory-based simulations. Au-molecular (benzene/alkanedithiol) junctions and networks, connected in linear chains and Y-, H- and ring-shaped networks were studied. Molecular orbital calculations show HOMO (highest-occupied-molecular-orbital)-LUMO (lowest-unoccupied-molecular-orbital) gaps of chains decreased with length, indicating decreased barrier to electron transport. Orbitals near the gap of benzenedithiol networks show good delocalization whereas those of alkanedithiol molecules were more localized the metallic clusters. The transmission spectra of benzenedithiol-based networks showed increase in number/width of peaks near the Fermi energy as structures were extended due to increased orbital overlapping (spatially/energetically) with each other leading to formation of transmission pathways and electronic energy band-like properties. In contrast, lower transmission was observed for networks having localized orbitals (alkanedithiol molecular networks) with less overlap near the Fermi energy. Transmission spectra were also in good agreement with associated peaks in electronic density of states. In the case of multi-terminal networks, the location of delocalized orbitals can also shift from one branch to another, indicating potential ON/OFF switching behavior is possible. Switching elements and molecular-scale circuits (e.g., logic gates or inverters) based on Y- and H-shaped networks are proposed in analogy to electron waveguide devices and our simulations indicated that low-energy switching operation on the order of 10-50 meV may be possible. These nanoscale molecular electronic networks linked via metallic clusters provide an avenue for engineering electronics at the molecular level by using superstructures of different organic molecules and topologies.

Representation of electrons on multi-stage coupled electron waveguides by waves and particles

Representation of electrons on multi-stage coupled electron waveguides by waves and particles

Quantum regime for dielectric Cherenkov radiation and graphene surface plasmons

In this article, we propose a quantum regime for Cherenkov free-electron laser (CFEL) and surface plasmon polaritons (SPPs) excited in dielectric and multilayer graphene waveguides, respectively. This quantum regime is realized when the momentum spread induced in the interaction is smaller than the photon recoil. The discrete momentum exchange characterizing this interaction yields a significantly narrow single emission line. To determine the condition of the quantum regime, we derive an expression for the gain in the Cherenkov effect using a quantum mechanical treatment. It is assumed that the effective spread in momentum is due to the finite interaction length L (or the propagation length in the case of SPPs). For both cases, CFEL and SPPs, the effects of electron beam and waveguide parameters on the possibility of the quantum regime are studied. We conclude that the quantum regime can be basically verified at low electron beam energy (<40 keV) and at emission wavelengths in the near infrared range (<5 μm) when L is in the order of millimeters. In the case of SPPs, we also show that the feasibility to realize quantum SPPs is enhanced by increasing the chemical potential and number of graphene layers.

Coherent transport in Y-junction graphene waveguide

We performed a series of theoretical transport studies on Y-branch electron waveguides which are embedded in mid-size armchair graphene nanoribbons. Non-equilibrium Green’s function with different approximations of tight-binding Hamiltonian has been employed. Using the first nearest hopping approximation, we observed very pronounced conductance quantization, the structure of which depends on geometrical design and shows a spacing of 4e2/h, indicating the existence of valley degree of freedom. Moreover, by incorporating the third nearest approximation, we observed seminal plateaus deviated from multiples of 4e2/h conductance, suggesting the lift of valley degeneracy. Finally, Quasi-one dimensional band structure calculations have been performed to study the availability of energy channels and the role of the major geometrical parameters on the transport.

Perfectly conducting graphene electronic waveguide with curved channels

We theoretically investigate the electronic transport properties of curved graphene waveguides by employing non-equilibrium Green’s function techniques. We systematically study the dependence of the confined waveguide modes on the potential difference, the width of waveguide and side barrier. Through two-terminal electronic transport calculations, we show that the conductance of confined waveguide modes is rather robust against the bending degree of waveguide, in consistence with the band insensitivity to the side barrier. This finding of the perfectly conducting channels strongly suggests the possibility of applying the graphene waveguide in the design of low-power nanoelectronics.

Electrons in Quasi-1D Waveguides with the Spin–Orbit Interaction: Manifestations of Additional Spin Symmetry

We consider quasi-one-dimensional electron waveguides with spin–orbit interaction, which are formed in quantum wells grown along arbitrary crystallographic directions. An analytic solution to the Schrodinger equation is obtained for systems with Hamiltonians possessing additional spin symmetry. It is shown that the dispersion curves for electrons, which correspond to different size-quantization modes, can intersect only when such symmetry exists. We analyze the structure of dips appearing on the dependences of the conductance of an inhomogeneous waveguide on the energy of carriers. It is shown that the width of the dips substantially depends on the waveguide orientation in the plane of the quantum well. In particular, it vanishes when the waveguide is formed along the direction of the “magic” vector of the initial 2D system.

How a Fano Resonance Crosses the Mobility Edge in Quantum Waveguides

new scenario for the occurrence of a Fano resonance in the transmission probability of electron waveguides is investigated using a coupled-channel theory. Both a quantum dot and an antidot with either short- or finite-range interaction are embedded in the electron waveguide. Particularly, when the Fano resonance occurs close to the mobility edge (channel threshold), it is shown that Γ~U 12 4/3 , where Γ is the resonance width and U12 is the coupling strength between bound state and continuum. This is in contrast to the usual result Γ ~U 12 2 , which is valid when the resonance occurs far from the mobility edge. Furthermore, it is shown that increasing the size of both dot and antidot leads to larger resonance width.

Band-offset-induced lateral shift of valley electrons in ferromagnetic MoS2/WS2 planar heterojunctions

Low-energy coherent transport and a Goos-Hänchen (GH) lateral shift of valley electrons in planar heterojunctions composed of normal MoS2 and ferromagnetic WS2 monolayers are theoretically investigated. Two types of heterojunctions in the forms of WS2/MoS2/WS2 (type-A) and MoS2/WS2/MoS2 (type-B) with incident electrons in the MoS2 region are considered in which the lateral shift of electrons is induced by band alignments of the two constituent semiconductors. It is shown that the type-A heterojunction can act as an electron waveguide due to electron confinement between the two WS2/MoS2 interfaces which cause the incident electrons with an appropriate incidence angle to propagate along the interfaces. In this case, the spin- and valley-dependent GH shifts of totally reflected electrons from the interface lead to separated electrons with distinct spin-valley indexes after traveling a sufficiently long distance. In the type-B heterojunction, however, transmission resonances occur for incident electron beams passing through the structure, and large spin- and valley-dependent lateral shift values in propagating states can be achieved. Consequently, the transmitted electrons are spatially well-separated into electrons with distinct spin-valley indexes. Our findings reveal that the planar heterojunctions of transition metal dichalcogenides can be utilized as spin-valley beam filters and/or splitters without external gating.

Conductance in the Helimagnet- and Skyrmion-Lattice-Embedded Electron Waveguide

In this paper, we investigated the conductance properties in the helimagnet (HM)- and skyrmion lattice (SL)-film-embedded electron waveguide (WG) and found that under translation of the HM/SL layer the conductance contour as a function of the layer center demonstrates nearly identical pattern to the original spin field contour. When a WG electron is scattered by the embedded impurity, the HM/SL spin texture distorts the spin-dependent wavefunction. By interference between different quantum paths the electron experiences, the HM/SL spin texture is recorded in the conductance. The scheme also provides a possible detect of the HM/SL spin configuration by transport experiment.

A new coupling mechanism between two graphene electron waveguides for ultrafast switching

In this paper, we report a novel coupling between two graphene electron waveguides, in analogy the optical waveguides. The design is based on the coherent quantum mechanical tunneling of Rabi oscillation between the two graphene electron waveguides. Based on this coupling mechanism, we propose that it can be used as an ultrafast electronic switching device. Based on a modified coupled mode theory, we construct a theoretical model to analyze the device characteristics, and predict that the switching speed is faster than 1 ps and the on–off ratio exceeds 106. Due to the long mean free path of electrons in graphene at room temperature, the proposed design avoids the limitation of low temperature operation required in the traditional design by using semiconductor quantum-well structure. The layout of our design is similar to that of a standard complementary metal-oxide-semiconductor transistor that should be readily fabricated with current state-of-art nanotechnology.

Shubnikov-de Haas-like Quantum Oscillations in Artificial One-Dimensional LaAlO_{3}/SrTiO_{3} Electron Channels.

The widely reported magnetoresistance oscillations in LaAlO_{3}/SrTiO_{3} heterostructures have invariably been attributed to the Shubnikov-de Haas (SdH) effect, despite a pronounced inconsistency with low-field Hall resistance measurements. Here we report SdH-like resistance oscillations in quasi-1D electron waveguides created at the LaAlO_{3}/SrTiO_{3} interface by conductive atomic force microscopy lithography. These oscillations can be directly attributed to magnetic depopulation of magnetoelectric subbands. Our results suggest that the SdH oscillations in 2D SrTiO_{3}-based systems may originate from naturally forming quasi-1D channels.