The effect of the up-conversion of the electronic energy level occupation was earlier interpreted as an implication of the multiple scattering of the charge carriers on the longitudinal optical phonons of the lattice vibrations in a small system like a quantum dot. In this work we study the influence of this effect on the electronic motion in a nanotransistor represented by a quantum dot connected to two electric wires and a gate electrode. We show that in an asymmetric nanotransistor the up-conversion effect gives rise to a spontaneous current between the source and the drain, or to an appearance of a spontaneous voltage between these electrodes. The effect will be studied basing on the well known Datta's Toy Model of the theoretical description of the nanotransistor and on additional kinetic equations giving the multiple scattering of electrons in the quantum dot, in the self-consistent Born approximation to the electronic self-energy. We shall also briefly discuss the relation of this theoretical result to existing experiments on current-voltage characteristics in gated nanostructures.
An idea for a nanodevice in which an arbitrary sequence of three basic quantum single qubit gates - negation, Hadamard and phase shift - can be performed on a single electron spin. The spin state is manipulated using the spin-orbit coupling and the electron trajectory is controlled by the electron wave function self-focusing mechanism due to the electron interaction with the charge induced on metal gates. We present results of simulations based on iterative solution of the time dependent Schrödinger equation in which the subsequent operations on the electron spin can be followed and controlled…
We introduce a quantum Monte Carlo technique to calculate exactly at finite temperatures the Green function of a fermionic quantum impurity coupled to a bosonic field. While the algorithm is general, we focus on the single impurity Anderson model coupled to a Holstein phonon as a schematic model for a molecular transistor. We compute the density of states at the impurity in a large range of parameters, to demonstrate the accuracy and efficiency of the method. We also obtain the conductance of the impurity model and analyze different regimes. The results show that even in the case when the effe…
A solid state electronic nanodevice is an intrinsically open quantum system, exchanging both energy with the host material and carriers with connected reservoirs. Its out-of-equilibrium behavior is determined by a non-trivial interplay between electronic dissipation and decoherence induced by inelastic processes within the device, and the coupling of the latter to metallic electrodes. We propose a unified description, based on the density matrix formalism, that accounts for both these aspects, enabling to predict various steady-state as well as ultrafast nonequilibrium phenomena, nowadays expe…
A design for a quantum gate performing transformations of a single electron spin is presented. The spin rotations are performed by the electron going around the closed loops in a gated semiconductor device. We demonstrate the operation of NOT, phase-flip and Hadamard quantum gates, i.e. the single-qubit gates which are most commonly used in the algorithms. The proposed devices employ the self-focusing effect for the electron wave packet interacting with the electron gas on the electrodes and the Rashba spin-orbit coupling. Due to the self-focusing effect the electron moves in a compact wave pa…
The paper demonstrates that a two-dimensional ballistic nanodevice in which the electron gas satisfies either the Schroodinger equation (as in quantum wells in common semiconductor heterostructures) or the Dirac equation (as in graphene) is able to rectify an electric signal if the device has a non-uniform cross section, for instance a taper configuration. No p-n junctions or dissimilar electrodes are necessary for rectification.
The spin-dependent localization of electrons in quadruple quantum dots (QD's) has been studied by the configuration interaction method. We have investigated two nanodevices that consist of laterally coupled quadruple QD's. We have shown that -- in both the nanodevices with suitably chosen parameters -- the exclusive OR (XOR) logic gate can be realized by all-electrical control with the readout of output via the spin-to-charge conversion. We have determined the nanodevice parameters that are optimal for the performance of the XOR logic gate.
The electron transport through a quantum sphere with three one-dimensional wires attached to it is investigated. An explicit form for the transmission coefficient as a function of the electron energy is found from the first principles. The asymmetric Fano resonances are detected in transmission of the system. The collapse of the resonances is shown to appear under certain conditions. A two-terminal nanodevice with an additional gate lead is studied using the developed approach. Additional resonances and minima of transmission are indicated in the device.
Electronic transport in nanodevices is commonly studied theoretically and numerically within the Landauer-Büttiker formalism: a device is characterized by its scattering properties to and from reservoirs connected by perfect semi-infinite leads, and transport quantities are derived from the scattering matrix. In some respects, however, the device becomes a "black box" as one only analyses what goes in and out. Here we use the Husimi function as a complementary tool for quantitatively understanding transport in graphene nanodevices. It is a phase space representation of the scattering wavefunct…
Electric-field effect control of two-dimensional electron gases (2-DEG) has enabled the exploration of nanoscale electron quantum transport in semiconductors. Beyond these classical materials, transition metal-oxide-based structures have d-electronic states favoring the emergence of novel quantum orders absent in conventional semiconductors. In this context, the LaAlO3/SrTiO3 interface that combines gate-tunable superconductivity and sizeable spin-orbit coupling is emerging as a promising platform to realize topological superconductivity. However, the fabrication of nanodevices in which the el…
We report the room temperature observation of significant ballistic electron transport in shallow etched four-terminal mesoscopic devices fabricated on an InSb/AlInSb quantum well (QW) heterostructure with a crucial partitioned growth-buffer scheme. Ballistic electron transport is evidenced by a negative bend resistance signature which is quite clearly observed at 295 K and at current densities in excess of 10$^{6}$ A/cm$^{2}$. This demonstrates unequivocally that by using effective growth and processing strategies, room temperature ballistic effects can be exploited in InSb/AlInSb QWs at prac…
Here, we examine the influence of surface chemical reactivity toward ambient gases on the performance of nanodevices based on two-dimensional materials "beyond graphene" and novel topological phases of matter. While surface oxidation in ambient conditions was observed for silicene and phosphorene with subsequent reduction of the mobility of charge carriers, nanodevices with active channels of indium selenide, bismuth chalcogenides and transition-metal dichalcogenides are stable in air. However, air-exposed indium selenide suffers of p-type doping due to water decomposition on Se vacancies, whe…
Ballistic injection in a nanodevice is a complex process where electrons can either be transmitted or reflected, thereby introducing deviations from the otherwise quantized conductance. In this context, quantum rings (QRs) appear as model geometries: in a semiclassical view, most electrons bounce against the central QR antidot, which strongly reduces injection efficiency. Thanks to an analogy with Rutherford scattering, we show that a local partial depletion of the QR close to the edge of the antidot can counter-intuitively ease ballistic electron injection. On the contrary, local charge accum…
We present electronic transport measurements through short and narrow (30x30 nm) single layer graphene constrictions on a hexagonal boron nitride substrate. While the general observation of Coulomb-blockade is compatible with earlier work, the details are not: we show that the area on which charge is localized can be significantly larger than the area of the constriction, suggesting that the localized states responsible for Coulomb-blockade leak out into the graphene bulk. The high bulk mobility of graphene on hexagonal boron nitride, however, seems not consistent with the short bulk localizat…
Quantum dragon segments are nanodevices that have energy-independent total transmission of electrons. At the level of the single-band tight-binding model a nanodevice is viewed as a weighted undirected graph, with a vertex weight given by the on-site energy and the edge weight given by the tight-binding hopping parameter. A quantum dragon is a weighted undirected graph which when connected to idealized semi-infinite input and output leads, has the electron transmission probability ${\cal T}(E)$$=$$1$ for all electron energies $E$. The probability ${\cal T}(E)$ is obtained from the solution of …
Electronic transport through a quantum dot strongly coupled to electrodes is studied within a model with two conduction channels. It is shown that multiple scattering and interference of transmitted waves through both channels lead to Fano resonance associated with Kondo resonance. Interference effects are also pronouncedly seen in transport through the Aharonov-Bohm ring with the Kondo dot, where the current characteristics continuously evolve with the magnetic flux.
Controlling heat flow by phononic nanodevices has received significant attention recently because of its fundamental and practical implications. Elementary phononic devices such as thermal rectifiers, transistors, and logic gates are essentially based on two intriguing properties: heat diode effect and negative differential thermal conductance. However, little is known about these heat transfer properties across metal-dielectric interfaces, especially at nanoscale. Here we analytically resolve the microscopic mechanism of the nonequilibrium nanoscale energy transfer across metal-dielectric int…
We study the electronic transport properties of a dual-gated bilayer graphene nanodevice via first principles calculations. We investigate the electric current as a function of gate length and temperature. Under the action of an external electrical field we show that even for gate lengths up 100 Ang., a non zero current is exhibited. The results can be explained by the presence of a tunneling regime due the remanescent states in the gap. We also discuss the conditions to reach the charge neutrality point in a system free of defects and extrinsic carrier doping.
Nanowires have been considered for a number of applications in nanometrology. In such a context, we have explored the possibility of using ultrathin twisted nanowires as torsion nanobalances to probe forces and torques at molecular level with high precision, a nanoscale system analogous to the Coulomb's torsion balance electrometer. In order to achieve this goal, we performed a first principles investigation on the structural and electronic properties of twisted silicon nanowires, in their pristine and hydrogenated forms. The results indicated that wires with pentagonal and hexagonal cross sec…
Transition metal dichalcogenides (TMDCs) have recently shown much promise as thin layer semiconductors in application to transistor technology. TMDCs provide a unique vantage point for studying the properties of phosphorene, a highly efficient and tunable super-material. Phosphorene-TMDC heterostructure nanodevices have remained largely unexplored due to the lack of air stability observed in phosphorene under ambient conditions. This study investigates a novel nanofabrication technique that effectively enhances the air stability and practical scalability of graphene-analogous phosphorene. A ph…
We propose a nanodevice for single-electron spin initialization. It is based on a gated planar semiconductor heterostructure with a quantum well and with potentials generated by voltages applied to local gates. Initially we insert an electron with arbitrary spin into the nanodevice. Next we perform a sequence of spin manipulations, after which the spin is set in a desired direction (e.g., the growth direction). The operations are done all-electrically, do not require any external fields and do not depend on the initial spin direction.
Classical and quantum dynamics are important limits for the understanding of the transport characteristics of interacting electrons in nanodevices. Here we apply an intermediate semiclassical approach to investigate the dynamics of two interacting electrons in a planar nanochannel as a function of Coulomb repulsion and electric field. We find that charge is mostly redistributed to the channel edges and that an electric field enhances the particle-like character of electrons. These results may have significant implications for the design and study of future nanodevices.
Using local scanning electrical techniques we study edge effects in side-gated Hall nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of ~60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. Howeve…
The entropy of a system gives a powerful insight into its microscopic degrees of freedom, however standard experimental ways of measuring entropy through heat capacity are hard to apply in mesoscale and nanoscale systems, as they require the measurement of increasingly small amounts of heat. This problem calls for radically different measurement methods that do not suffer from decreasing accuracy with the decreasing size of the system. For nanoelectric devices in the state of Coulomb blockade, with only two energetically accessible charge states, two purely electric, size-independent methods o…
We demonstrate that encapsulation of atomically thin black phosphorus (BP) by hexagonal boron nitride (h-BN) sheets is very effective for minimizing the interface impurities induced during fabrication of BP channel material for quantum transport nanodevices. Highly stable BP nanodevices with ultrahigh mobility and controllable types are realized through depositing appropriate metal electrodes after conducting a selective etching to the BP encapsulation structure. Chromium and titanium are suitable metal electrodes for BP channels to control the transition from a p-type unipolar property to amb…
We present an application of a new formalism to treat the quantum transport properties of fully interacting nanoscale junctions [Phys. Rev. B {\bf 84}, 235428 (2011)]. We consider a model single-molecule nanojunction in the presence of two kinds of electron-vibron interactions. In terms of electron density matrix, one interaction is diagonal in the central region and the second is off-diagonal in between the central region and the left electrode. We use a non-equilibrium Green's function technique to calculate the system's properties in a self-consistent manner. The interaction self-energies a…
We demonstrate that in a standard thermo-electric nanodevice the current and heat flows are not only dictated by the temperature and potential gradient, but also by the external action of a local quantum observer that controls the coherence of the device. Depending on how and where the observation takes place, the direction of heat and particle currents can be independently controlled. In fact, we show that the current and heat flow in a quantum material can go against the natural temperature and voltage gradients. Dynamical quantum observation offers new possibilities for the control of quant…
Using non-equilibrium renormalized perturbation theory, we calculate the retarded and lesser self energies, the spectral density rho(omega) near the Fermi energy, and the conductance G through a quantum dot as a function of a small bias voltage V, in the general case of electron-hole asymmetry and intermediate valence. The linear terms in omega and V are given exactly in terms of thermodynamic quantities. When the energy necessary to add the first electron (Ed) and the second one (Ed+U) in the quantum dot are not symmetrically placed around the Fermi level, G has a linear term in V if in addit…
We study phonons generated by current in a Pt nanowire, by measuring resistance of another nanowire separated from the first one by an insulating spacer. For thin spacers, the resistance varies almost linearly with current at cryogenic temperatures, while an additional quadratic contribution emerges for thicker spacers. These observations suggest a non-thermal distribution of current-generated phonons that relax via strongly nonlinear dynamical processes rather than few-phonon scattering. Our results provide insight into the nonequilibrium phonon dynamics at nanoscale, which may facilitate eff…
We derive the expressions of photon energy and particle currents inside an open nanosystem interacting with light using non-equilibrium Green's functions. The model allows different temperatures for the electron reservoirs, which basically defines a photovoltaic-thermoelectric hybrid. Thanks to these expressions, we formulate the steady-state entropy production rate to assess the efficiency of reversible photovoltaic-thermoelectric nanodevices. Next, quantum dot based nanojunctions are closely examined. We show that entropy production is always positive when one considers spontaneous emission …
The fabrication of micro- and nano-scale silicon electronic devices requires precision lithography and controlled processing to ensure that the electronic properties of the device are optimized. Importantly, the Si-SiO2 interface plays a crucial role in defining these properties. While transmission electron microscopy (TEM) can be used to observe the device architecture, substrate / contact crystallinity and interfacial roughness, the preparation and isolation of the device active area is problematic. In this work, we describe the use of focussed ion beam technologies to isolate and trench-cut…
The unconventional properties of graphene, with a massless Dirac band dispersion and large coherence properties, have raised a large interest for applications in nanoelectronics. In this work, we emphasize that graphene two dimensional character combined with current standard lithography processes allow to achieve devices smaller than the Dirac electrons wavelength. In this regime, we demonstrate that the electronic properties present deep analogies with subwavelength optics phenomena. We describe the rich transport physics in graphene-based nanodevices through optical analogies: From the Beth…
Electron transport through nanodevices of atoms in a single-layer rectangular arrangement with free (open) boundary conditions parallel to the direction of the current flow is studied within the single-band tight binding model. The Landauer formula gives the electrical conductance to be a function of the electron transmission probability, ${\cal T}(E)$, as a function of the energy $E$ of the incoming electron. A quantum dragon nanodevice is one which has a perfectly conducting channel, namely ${\cal T}(E)=1$ for all energies which are transmitted by the external leads even though there may be …
Electronic transport experiments involving the topologically protected states found at the surface of Bi2Se3 and other topological insulators require fine control over carrier density, which is challenging with existing bulk-doped material. Here we report on electronic transport measurements on thin (<100 nm) Bi2Se3 devices and show that the density of the surface states can be modulated via the electric field effect by using a top-gate with a high-k dielectric insulator. The conductance dependence on geometry, gate voltage, and temperature all indicate that transport is governed by parallel s…
Inefficient screening of electric fields in nanoconductors makes electric manipulation of electronic transport in nanodevices possible. Accordingly, electrostatic (charge) gating is routinely used to affect and control the Coulomb electrostatics and quantum interference in modern nanodevices. Besides their charge, another (quantum mechanical) property of electrons - their spin - is at the heart of modern spintronics, a term implying that a number of magnetic and electrical properties of small systems are simultaneously harvested for device applications. In this review the possibility to achiev…
Ballistic transport in nanodevices based on single-crystalline Cu thin film
Pohang University of Science and Technology（浦项科技大学）；Crystal Bank Research Institute, Pusan National University（釜山大学晶体银行研究所）；Sungkyunkwan University（成均馆大学）；Copper Innovative Technology (CIT) Co.（铜创新技术（CIT）有限公司）；Ulsan National Institute of Science and Technology（蔚山科学技术院）；Mississippi State University（密西西比州立大学）；Korea Advanced Institute of Science and Technology (KAIST)（韩国科学技术院）；Massachusetts General Hospital and Harvard Medical School（麻省总医院和哈佛医学院）；Pusan National University（釜山大学）；Asia Pacific Center for Theoretical Physics（亚太理论物理中心）
We present an application of a new formalism to treat the quantum transport properties of fully interacting nanoscale junctions. We consider a model single-molecule nanojunction in the presence of two kinds of electron-vibron interactions. In terms of the electron density matrix, one interaction is diagonal in the central region and the second off-diagonal between the central region and the left electrode. We use a non-equilibrium Green's function technique to calculate the system's properties in a self-consistent manner. The interaction self-energies are calculated at the Hartree-Fock level i…
Theoretical prediction of phonon transport in modern semiconductor nanodevices requires atomic resolution of device features and quantum transport models covering coherent and incoherent effects. The nonequilibrium Green's function method (NEGF) is known to serve this purpose very well, but is numerically very expensive. This work extends the very efficient Büttiker probe concept to phonon NEGF and discusses all implications of this method. Büttiker probe parameters are presented that reproduce within NEGF experimental phonon conductances of Si and Ge between 10K and 1000K. Results of this met…
Interactions between electrons in solids are often behind exciting novel effects such as ferromagnetism, antiferromagnetism and superconductivity. All these phenomena break away from the single-electron picture, instead having to take into account the collective, correlated behaviour of the system as a whole. In this chapter we look at how tunnelling spectroscopy can be used as the experimental tool of choice for probing correlation and interaction effects in one-dimensional (1D) electron systems. We start by introducing the Tomonaga-Luttinger Liquid (TLL) model, showing how it marks a clear d…
Graphene two-dimensional nature combined with today lithography allows to achieve nanoelectronics devices smaller than the Dirac electrons wavelength. Here we show that in these graphene subwavelength nanodevices the electronic quantum transport properties present deep analogies with classical phenomena of subwavelength optics. By introducing the concept of electronic diffraction barrier to represent the effect of constrictions, we can easily describe the rich transport physics in a wealth of nanodevices: from Bethe and Kirchhoff diffraction in graphene slits, to Fabry-Perot interference oscil…
Heating Dynamics of Mesoscopic Electron Baths at High Magnetic Field
Université Paris-Saclay（巴黎萨克雷大学）；CNRS（法国国家科学研究中心）；Centre de Nanosciences et de Nanotechnologies（纳米科学与纳米技术研究中心）；University of British Columbia（不列颠哥伦比亚大学）
We demonstrated theoretically that a circularly polarized electromagnetic field substantially modifies electronic properties of a periodical chain of quantum rings. Particularly, the field opens band gaps in the electron energy spectrum of the chain, generates edge electron currents and induces the Fano-like features in the electron transport through the finite chain. These effects create physical prerequisites for the development of optically controlled nanodevices based on a set of coupled quantum rings.
By eliminating normal fabrication processes, we preserve the bulk insulating state of calcium-doped Bi2Se3 single crystals in suspended nanodevices, as indicated by the activated temperature dependence of the resistivity at low temperatures. We perform low-energy electron beam irradiation (<16 keV) and electrostatic gating to control the carrier density and therefore the Fermi level position in the nanodevices. In slightly p-doped Bi2-xCaxSe3 devices, continuous tuning of the Fermi level from the bulk valence band to the band-gap reveals dramatic enhancement (> a factor of 10) in the field-eff…
The fine structure of the Dirac energy spectrum in graphene induced by electron-optical phonon coupling is investigated in the portion of the spectrum near the phonon emission threshold. The derived new dispersion equation in the immediate neighborhood below the phonon threshold corresponds to an electron-phonon bound state. We find that the singular vertex corrections beyond perturbation theory increase strongly the electron-phonon binding energy. The predicted enhancement of the effective electron-phonon coupling can be measured using angle-resolved spectroscopy.
In this study a method for calculation of the electron-phonon scattering rate in semiconductor structures with one-dimensional electron gas is developed. The energy uncertainty of electrons is taken into account.
The influence of electron-phonon interaction on magnetotransport in two-dimensional electron systems under microwave irradiation is studied theoretically. Apart from the phonon-induced resistance oscillations which exist in the absence of microwaves, the magnetoresistance of irradiated samples contains oscillating contributions due to electron scattering on both impurities and acoustic phonons. The contributions due to electron-phonon scattering are described as a result of the interference of phonon-induced and microwave-induced resistance oscillations. In addition, microwave heating of elect…
An electron-phonon cavity consisting of a quantum dot embedded in a free-standing GaAs/AlGaAs membrane is characterized in Coulomb blockade measurements at low temperatures. We find a complete suppression of single electron tunneling around zero bias leading to the formation of an energy gap in the transport spectrum. The observed effect is induced by the excitation of a localized phonon mode confined in the cavity. This phonon blockade of transport is lifted at magnetic fields where higher electronic states with nonzero angular momentum are brought into resonance with the phonon energy.
First principle calculations of the phonons of graphene-BN heterostructures are presented and compared to those of the constituents. We show that AA and AB' stacking are not only energetically less favoured than AB but also dynamically unstable. We have identified low energy flat phonon branches of BN character with out of plane displacement and evaluated their coupling to electrons in graphene.
We develop a unified treatment of the piesoelectric coupling between two-dimensional electrons and bulk phonons in both cases of zero and strong magnetic fields, the latter corresponding to even denominator filling fractions. In contrast to the case of coupling via the deformation potential, the leading contributions due to impurity-renormalized electron-phonon vertices are not exactly cancelled by processes of inelastic electron-impurity scattering. Electron energy relaxation time, diffusion correction to the conductivity, and phonon emission rate by hot electrons are computed for realistic $…
The spectrum of electron-phonon complexes in a monolayer graphene is investigated in the presence of a perpendicular quantizing magnetic field. Despite the small electron-phonon coupling, usual perturbation theory is inapplicable for calculation of the scattering amplitude near the threshold of the optical phonon emission. Our findings beyond perturbation theory show that the true spectrum near the phonon emission threshold is completely governed by new branches, corresponding to bound states of an electron and an optical phonon with a binding energy of the order of $αω_{0}$ where $α$ is the e…
The effect of external electric field on the exchange interaction has been studied by an exact diagonalization method for two electrons in laterally coupled quantum dots (QD's). We have performed a systematic study of several nanodevices that contain two gate-defined QD's with different shapes and sizes located between source and drain contacts. The confinement potential is modelled by two potential wells with a variable range and softness. In all the considered nanodevices, the overall dependence of exchange energy $J$ on electric field $F$ is similar, i.e., for low fields $J$ increases with …
We report strong electron-phonon coupling in magic-angle twisted bilayer graphene (MA-TBG) obtained from atomistic description of the system including more than 10000 atoms in the moire supercell. Electronic structure, phonon spectrum, and electron-phonon coupling strength lambda are obtained before and after atomic-position relaxation both in and out of plane. Obtained lambda is very large for MA-TBG, with lambda > 1 near the half-filling energies of the flat bands, while it is small (lambda ~ 0.1) for monolayer and unrotated bilayer graphene. Significant electron-hole asymmetry occurs in the…
We theoretically study phonon-drag contribution to the thermoelectric power and hot-electron energy-loss rate in a Rashba spin-orbit coupled two-dimensional electron system (2DES) in the Bloch-Gruneisen (BG) regime. We assume that electrons interact with longitudinal acoustic phonons through deformation potential and with both longitudinal and transverse acoustic phonons through piezoelectric potential. Effect of the Rashba spin-orbit interaction on magnitude and temperature dependence of the phonon-drag thermoelectric power and hot-electron energy-loss rate are discussed. We numerically extra…
We construct an analytic continuum model to describe the electronic structure and the electron-phonon interaction in twisted bilayer graphenes with arbitrary lattice deformation. Starting from the tight-binding model, we derive the interlayer Hamiltonian in the presence of general lattice displacement, and obtain a long-wavelength continuum expression for smooth deformation. We show that the continuum model correctly describes the band structures of the lattice-relaxed twisted bilayer graphenes. We apply the formula to the phonon vibration, and derive an explicit expression of the electron-pho…
